1997
1.U.S. Geological Survey
2. Colorado School of Mines
A) Mosaic, Netscape, or other World Wide Web (WWW) network browser software is necessary to view and link the HTML (Hypertext Markup Language) text and GIF (Graphic Interchange Format) images located on this CD-ROM. All computer software on this CD-ROM are present in the "software" directory. Mosaic software are located in the "Mosaic" folder in each of the "mac" (Macintosh), "pcwindows" (PC Windows systems), and "unix" (UNIX) folders. Please refer to the readme files in these folders for information on how to load the software onto your system. Image viewer (such as Lview and JPEGView) software can be used to display the large-scale images. These software programs are commonly accessed by the network browsing software using text and image hooks, an invisible process to users.
B) Movie player software (MoviePlayer, SimplePlayer, Fast Player, Sparkle, and others) is required to open and animate the single-fork stand-alone movies located in the movies directory. Our efforts to get license releases for Apple Computer, Inc. freeware came to naught. Apple QuickTime software for Macintosh and PC/Windows computers is necessary to animate movies. The QuickTime package, which includes MoviePlayer software, is located at the following Web site: http://quicktime.apple.com . An ftp.support.apple.com location is for those of you using File Transfer Protocol. Should these sites change, try searching the WWW using "quicktime software," or similar phrase. Instructions for copying the software from this and other sites are also explained. We recommend copying the packages during evenings or the weekend because of a very limited number of seats. Other movie player software are also available on the WWW. Try searching the web using "movie software" , movieplayer, or other text strings.
C) The home page (homepage.htm) for this document is located in both the root ("USGS_3D") directory and text ("ssx_txt") subdirectory. All HTML text files have ".htm" endings. Non-HTML text files display ".txt" endings.
This CD-ROM publication contains raw and interpreted data, maps, and other graphic displays linked to the geological history and characteristics of the Upper Cretaceous-age Sussex Sandstone of the House Creek oil field, Powder River Basin, Wyoming. Data were statistically analyzed and mapped in 2-D and 3-D, and included as ASCII-formatted, space-delimited data files. Files included on this CD-ROM can be retrieved for use in other applications, including graphics editing and display, mapping, and statistical programs. Graphics are stored as Graphics Interchange Format (GIF) and TARGA (TGA) images (see glossary for explanations of acronyms and words).
World Wide Web (WWW) network browsing software is required to integrate the graphics images and text located on this CD-ROM. While included HTML (Hypertext Markup Language) documents (in the "ssx_txt" directory) can be viewed with standard word-processing software, the interactive capabilities of Web browsing software link text, images, movies, and data. Numerous graphics software programs can view and edit individual GIF and TARGA files which are located in the "ssx_gif," "ssx_tga," and several other directories.
This CD-ROM was produced in accordance with both the ISO 9660 and Macintosh HFS standards. Contained data and text files can be read by platforms supporting either of those standards. Also required is a CD-ROM drive and associated software. Provided graphics and movies are designed for use under DOS/Windows on IBM or 100 percent compatible personal computers (PC's) and the Apple Macintosh family. The stand-alone EarthVision faces file is a 3-D model intended for use on Silicon Graphics Corporation IRIS workstations. This ssxporos.faces file displays porosity distribution and sand-ridge boundaries for the House Creek field. The ssxporos.demo.Z and evview.Z compressed files are located in the "IRIS" subdirectory; these files are intended primarily for archive, but they can be loaded onto an IRIS workstation to create a stand-alone EarthVision 3-D model.
All brand and product names are trademarks or registered trademarks of their respective companies. The tm notation is not appended to these product names, largely because the HTML superscript notation did not work with all World Wide Web (WWW) browsers. The authors primarily use Mosaic as the reference for accessing the information on this CD-ROM. The Mosaic programs located on this CD-ROM are software that are written and upgraded at the National Center for Supercomputer Applications (NCSA) at the University of Illinois in Urbana-Champaign. Included with computer programs located in the "software" directory are general directions to copy Mosaic and other applications for your platform. Other network browsing programs are available and have been used to view this publication.
Mosaic and associated software are available through anonymous FTP (file-transfer protocol) from NCSA. The ftp location is ftp.ncsa.uiuc.edu. Due to the number of people that use, and overload, the network, you may need to make several attempts to access and copy the files from NCSA. We recommend linking with the NCSA network early or late in the day. You are utilizing FTP on UNIX computers; remember that UNIX is case sensitive.
The file-naming convention on this publication is primarily eight dot three. This indicates the file has a maximum of eight characters to the left of and three to the right of the ".". An asterisk (*) is used in this publication as a symbol to match all characters. It commonly indicates a longer file name, for example Mosa*.hqx would access all .hqx files that start with Mosa and end with .hqx, such as Mosaic.hqx.
While this is dependent upon your system and software, stand-alone movies located in the "movie" directory can be opened by 1) starting your movie viewer and opening the movie file, 2) double-clicking on individual movie names or icons, or 3) accessing the movies with imbedded links in the HTML documents, which is the slowest option of the three. The *.MOV movies are designed to run on Macintosh and PC/Windows computers. The movies were created using the QuickTime and GraphicConverter software programs on a Macintosh IIsi. Our experiments with MPEG formatted movies produced lower quality images and are not included with this publication. File names displayed on each movie image are saved as separate GIF and (or) TARGA files; single 3-D images can also be displayed from the "3-D images and movies" subsections of the table of contents.
Files located on this CD-ROM are accessed locally and linked by the WWW browsing software using the File command heading and Open File or Open Local subheadings (generally). For that reason, MacTCP software is not required to view the files. MacTCP v. 2.0.2 (or later) Apple software is needed to connect to a remote host (also known as cruising the Web). MacTCP now comes bundled with System 7.5. Your software needs may be different, depending upon the network browser software utilized.
Each of the below software programs has its own folder(s) within the "mac," "pcwindow," and (or) "unix" subdirectories. Due to the eight-dot-three file-naming convention, most names were shortened. File and software names, and directory locations are listed below. Please refer to the readme files associated with most of the software for information on how to load the programs onto your computer. We recommend opening the readme files using word-processing programs, however, should you access readme files through the below links, return by using arrows, back, or other options on your network browsing software.
GIFConverter 2.3.7 (486 KB) (shareware) - GIFConverter is used to open and read numerous graphic file formats. Formats include GIF, TIFF, RIFF, PICT, JPEG (JFIF), MacPaint and Thunderscan. It can write these formats, as well as black-and-white EPS (encapsulated PostScript). GIFConverter is used to convert files for use in other programs, to view files, and to print them out. The readme.txt file is saved in the "GIFConv" subdirectory as readme.txt. The compressed application program is named GIFConv.SEA .
StuffIt Expanderª for Windows expands files compressed in the following formats: StuffIt (.sit), ZIP (.zip), ARC (.arc), Arj (.arj), and gzip (.gz). StuffIt Expander will also decompress files in uuencoded (.uue), BinHex (.hqx) and MacBinary (.bin) formats. Self-extracting archives created by StuffIt, ZIP, and Arj can also be expanded using StuffIt Expander. StuffIt.EXE binary executable is located in the "/software/pcwindow/stuffit" subdirectory.
QuickTime software of Apple Corporation is required to view the movies. Also needed is movie player software, most of which is freeware or shareware. Try searching the WWW using "movie+software" . The current WWW site for information on Apple QuickTime freeware is http://quicktime.apple.com. QuickTime and MoviePlayer software for Macintosh computers and PC/Windows computers can be downloaded from links at http:/www.apple.com, or can be copied using File Transfer Protocol (ftp) with the following command: ftp ftp.support.apple.com . Unfortunately, this anonymous-user ftp site has a small quota, even with an additional ftptoo.support.apple.com site. Because of a limited number of seats, we don't recommend accessing the site during business hours.
Movies can be called up from the text, but access is much quicker directly from the movies folder. Should you have trouble displaying images, try increasing available memory by closing all unnecessary windows and software programs. Shrinking the size of the movies may also aid in their display. We recommend playing with the menu bar located at the top of the movies and the bounding scroll bars; both to improve the image display and to familiarize yourself with program capabilities, including print and file export.
The six movies located in the "movies" folder display 3-D distribution and volumes of porosity and permeability for the Upper Cretaceous age Sussex "B" sandstone across the southern two-thirds of the House Creek oil field, Powder River Basin, eastern Wyoming. Information on data sources, and technical and research methodology are in the techinfo.htm file located in the "ssx_txt" directory. A glossary is included (glossary.htm) in the "ssx_txt" subdirectory for definitions of computer and geologic terms.
Figure 1. Structure contour map on the top of the Upper Cretaceous Sussex Sandstone Member of the Cody Shale, and approximate age equivalents, Powder River Basin, Wyoming and Montana. Location of the House Creek field is shown. Areal extent of the Sussex "B" sandstones is delineated by black-colored fields. Shaded fill outlining the basin are outcrops of Lance Formation and Fox Hills Sandstone. The rectangle located next to the 30-miles scale bar defines the area of studied outcrops of the Sussex and Shannon Members, much of the Salt Creek anticline, and the towns of Midwest and Edgerton. Contour interval is 500 ft. Datum is mean sea level. A detailed (176 KB) version is named indexfig.gif.
Vertical exaggeration of the 3-D images ranges from 20 to 60 times. The Sussex "B" sandstone is less than 60 ft (18 m) thick and would look like a razor's edge without the exaggeration. The House Creek oil field is about 1 mi (1.6 km) wide and, combined with the small Porcupine field at the southern boundary, is about 36 mi (58 km) long. Reservoir-grade intervals in images are shown on the porosity (left) color bar by green through red (8 to 21 percent porosity); low-porosity non-reservoir intervals are purple and blue (1 to 8 percent porosity). Movies show:
Figure 20. East-west oriented vertical slices of porosity (left, percent) and permeability (right, mD). This image (porpermv.gif) shows the highly variable porosity (left) and permeability (right) using core data. Porosity values/colors are reservoir grade (green through red) of 8 through 21 percent, and below-reservoir-grade low-porosity (blue) of less than 8 percent. Permeability ranges are: purple and blue, up to 5 mD; green, 5 to 16 mD; and yellow through red are greater than 16 mD. Average permeability of Sussex "B" reservoir sandstones is 15 mD. The close-spaced 3-D grid essentially treats each well as a separate set of points for the purpose of showing the variation in porosity and permeability. The various diamond shapes and streaks of low through high porosity and permeability illustrate the interbedding of thin beds of reservoir- and non-reservoir-grade sandstone and mudstone. These diagrams include the eastward and upward increases in porosity and permeability, and stacking of sand ridges shown in previous images. This slice is located in the northwestern half of the field. Yellow dots are (left) core and well-log and (right) core sample locations. Slices are 1.8 mi (2.8 km) wide and 50 ft (15 m) high. Vertical exaggeration is 30x. Respective names of the east-west porosity and permeability slide-image files are 3dew_img.htm and 3dvk_img.htm .
Individual GIF files used to generate the movies are stored in the below directories (bounded by parentheses); GIF files are displayed in the text portion of this publication as single images, and as slide shows using an external viewer, such as JPEGView and LView.
All movies are stored in the movies directory;
This document explains how to access, use, and copy the various software, image, and text files located on this CD-ROM. The following terms and abbreviations are explained in the glossary that is included on the CD-ROM. NCSA (National Center for Supercomputing Applications) Mosaic and other World Wide Web (WWW) software allow the viewer to read and display individual image and HTML text files. Included in the readme1.htm file are directions for copying Mosaic and other software to your PC/Windows, Macintosh, and (or) UNIX platforms. readme1.txt is the standard text version of readme1.htm. Stand-alone movies are linked graphic interchange format (GIF) images for viewing on Macintosh and PC Windows computers. Because the MPEG format movies we created had poor visual quality, we decided not to include them on the CD-ROM. The EarthVision stand-alone model showing 3-D porosity for the Sussex "B" sandstone is designed to run on IRIS workstations. Your computer's ability to view the images may be limited by its random access memory (RAM); it may help to close any unnecessary programs and open windows.
Insert the CD ROM in your CD-ROM reader. Text and images are viewed using network browsing software. Text files are stored in the "ssx_txt" directory; links among these HTML files are shown in the table of contents and included in most displays. While many are saved as text files that can be opened using most word processing software, the WWW browsing display abilities result in much cleaner viewing. The movies, located in the "movie" directory, display 3-D porosity and permeability across the House Creek field. These are animated by clicking on the *.MOV icons or file names. Movies can also be accessed from text hooks in some of the HTML files. The stand-alone 3-D porosity model will only run on IRIS workstations and on SUN workstations that possess a special graphics template. The (zipped) ssxporos.demo.Z 3-D model is located in the "IRIS" directory and can be started by opening the evview.Z file (zipped EarthVison View program).
The Home Page, or start of the Mosaic document (or other network browsing software) can generally be invoked by double-clicking the Mosaic name or icon. Once the browsing software is initiated, start this 3-D CD-ROM publication by opening the homepage.htm file under the open local or other file access command. These are commonly located under the File banner located at the top of your computer screen. We recommend that you peruse these menu items so that you will know your viewing, printing (also under File), and file-saving options. Depending on your computer system, size of the open window can be adjusted using the arrow key.
This CD-ROM contains geologic and geochemical data collected as part of the Reservoir Characterization and 3-D Reservoir/Basin Characterization for CD-ROM Projects of the U.S. Geological Survey. Information presented here is intended to address the national plans of Energy Information Transfer and Energy Environmental Investigations and Assessments. This publication contains raw and interpreted data, maps, and other graphic displays for the Upper Cretaceous Sussex Sandstone in the House Creek oil field, Powder River Basin, Wyoming. Location and geologic explanations are included in the Geology of the Sussex Sandstone and other sections.
Data sources include 1) core data from Kerr-McGee, Meridian Oil, and Woods Petroleum Corporations, 2) bore-hole history and lease and field production information from Petroleum Information Corporation, and 3) geologic, geophysical, and geochemical data generated and interpreted by USGS scientists.
This publication is intended to serve a broad audience. An important purpose is to explain geologic and geochemical factors that control petroleum production from the House Creek field. This information may serve as an analog for other marine-ridge sandstone reservoirs. The 3-D slide and movie images are tied to explanations and 2-D geologic and geochemical images to 1) visualize geologic structures in three dimensions, 2) explain the geologic significance of porosity/permeability distribution across the sandstone bodies, and 3) tie this to petroleum-production characteristics in the oil field. Digital publications are an important medium to archive maps, figures, and large volumes of geologic and geochemical data that would otherwise be lost. Movies, text, images including scanning electron photomicrographs (SEM), thin-section photomicrographs, and data files can be copied from the CD-ROM for use in external mapping, statistical, and other applications.
Data has been statistically analyzed and mapped in 2-D and 3-D and included in the "data_set" directory as ASCII-formatted, row- and column-delimited data files. Files located on this CD-ROM can be read on UNIX, Macintosh, and PC/Windows computers. Information on how to access and read the contents is located in the following files: readme1.htm and .txt (readme first), and techinfo.htm (technical information on data sources).
The Upper Cretaceous Sussex "B" sandstone was deposited as a probable transgressive-marine sand-ridge complex in a mid-shelf position. The "B" sandstone is bounded by upper and basal disconformities and encased in mudstones and low-porosity and low-permeability sandstones of the Cody Shale. Reservoir characteristics are controlled primarily by depositional and diagenetic heterogeneity at megascopic (field), macroscopic (well), and microscopic (rock sample) levels. To simplify, this means production of oil is controlled by stacking and interbedding of sandstone and mudstone beds and by geochemical changes through time that affect flow of fluids through the rock.
More than 24.8 million barrels of oil (MMBO) have been produced from the Sussex "B" sandstone in the House Creek field, Powder River Basin, Wyoming. Greatest oil production, porosity, and permeability, the thickest reservoir sandstone intervals, and best lateral continuity of the primary reservoir facies are all located parallel and proximal to field axes. Decrease in reservoir quality west of the axes is due to greater heterogeneity from interbedding of low- and moderate-depositional-energy facies, with associated drop in porosity and permeability. Decrease in production east of the axes results primarily from a combination of seaward thinning of the primary reservoir facies and non-deposition of sand ridges.
The House Creek field has two axis orientations; these are related to depositional patterns of the four sand ridges. Deposition of the "B" sandstone began in the southeastern corner of the field with sand ridge 1; axis orientation is about north 20 degrees west. Later-deposited sand ridges 2 through 4 are located west and north of sand ridge 1; their axis orientations are approximately north 32 degrees west. Progressive northward deposition of later sand ridges is probably concurrent with uplift of the northeast-trending Belle Fourche arch. Movement along the arch and of lineaments may have caused topographic highs that localized Sussex and Shannon deposition proximal to the arch.
Three-dimensional images and stand-alone movies reveal porosity and permeability distribution within, and stacking patterns of sand ridges. These models show:
Images and movies displaying 3-D slices of the porosity and permeability models of the Sussex "B" sandstone are tied to descriptions of Powder River Basin and Sussex "B" sandstone geology, statistics on porosity and permeability distribution across the field, and images of core, outcrops, and thin-section and SEM photomicrographs. The purposes are to characterize relative influence of depositional and diagenetic processes on distribution of hydrocarbons in the House Creek field and to visualize these processes. Technical information on geologic, geochemical, and computer geologic methods of analysis are detailed in Technical information on data sources (techinfo.htm). Diagenetic and depositional environment research (Higley, 1994) is based on petrographic analysis of about 60 thin sections, carbon and oxygen isotopic analysis of about 20 samples of carbonates, cathodoluminescence, SEM, and X-ray diffraction studies, and core, outcrop, and well-log studies. Well-logs for approximately 500 boreholes in the field area and 120 within the House Creek and Porcupine oil fields were used to map locations of the Ardmore bentonite, tops and bases of the Sussex "B" sandstone and included sand ridges, and well-history data. Sussex and Shannon Sandstones Members core porosity and permeability data was derived for 37 wells; this information for 13 Sussex "B" sandstone intervals augmented well-log porosity data for an additional about 90 wells.
As detailed in readme First (readme1.htm), links among text, images, and movies are accomplished with pointing and clicking with computer mouse buttons. An asterisk (*) indicates a longer or variable file name. Most image files are saved in separate directories in Targa (*.tga and *.TGA) and GIF (*.gif and *.GIF) graphic formats. Images within this Geology of the Sussex Sandstone section and subsections are located in the "ssx_gif" and "ssx_tga" directories and are generally saved as full-size and scaled-down images. The "scaled down" images are half or less the dimensions and file size of the full-scale ones; they are accessed more quickly on this CD-ROM then are the large images. File names and sizes accompany figures.
The Powder River Basin of Wyoming and Montana is an asymmetric Laramide-age structural basin with a steeply dipping western flank and a gently dipping eastern flank. The Upper Cretaceous Cody Shale, including the Sussex Sandstone Member and its age equivalents, underlies most of the Powder River Basin and outcrops along the western flank, west of the outcrops of Lance Formation and Fox Hills Sandstone shown as shaded fill on figure 1. The informally named Sussex "B" sandstone unit of the Sussex Sandstone Member has a limited areal extent that is approximated by the outlines of the House Creek and Porcupine oil fields shown on figure 1.
The House Creek field is located on the gently-dipping eastern flank of the Powder River Basin in Wyoming. The field trends about N. 40 degrees W., approximately parallel to present-day basin structure contours. It is about 1 mi (1.6 km) wide and, combined with the small Porcupine field at the southern boundary, is about 36 mi (58 km) long. A narrow zone of non-oil-productive, low-permeability marine mudstone separates these two fields. Oil from both fields is produced from the Upper Cretaceous Sussex "B" sandstone. Depths of the Sussex "B" sandstone in the fields ranges from about 7,000 to 9,000 ft (2,100 to 2,700 m), and in thickness up to 52 ft (16 m); average thickness of the reservoir interval totals about 20 ft (6 m).
Figure 1. Structure contour map on the top of the Upper Cretaceous Sussex Sandstone Member of the Cody Shale, and approximate age equivalents, Powder River Basin, Wyoming and Montana. Location of the House Creek field is shown. Areal extent of the Sussex "B" sandstones is delineated by black-colored fields. Shaded fill outlining the basin are outcrops of Lance Formation and Fox Hills Sandstone. The rectangle located next to the 30-miles scale bar defines the area of studied outcrops of the Sussex and Shannon Members, much of the Salt Creek anticline, and the towns of Midwest and Edgerton. Contour interval is 500 ft. Datum is mean sea level. A detailed (176 KB) version is named indexfig.gif.
This reservoir characterization of the House Creek oil field integrates depositional energy and environments, facies distribution, and burial diagenesis. These factors influence petroleum production by controlling;
The informally named Sussex "B" sandstone is one of as many as 12 separate sandstone bodies that are located at different vertical and lateral positions within the Sussex Sandstone Member (Anderman, 1976; Crews and others, 1976). Figure 2 is a generalized stratigraphic section for Cretaceous and Tertiary sediments in the basin, including major unconformities.
Figure 2. Stratigraphic nomenclature for Late Cretaceous and Tertiary rocks, Powder River Basin (stratprb.gif, 32 KB).
The Cretaceous epicontinental seaway connected the Gulf of Mexico with the Arctic Ocean during deposition of the Upper Cretaceous Cody Shale, as shown on figure 3 (Brenner, 1978, Gill and Cobban, 1973). The western margin of the seaway was marked by a tectonically active thrust belt that extended from Alberta to southern Nevada and included the actively growing Rocky Mountains. The eastern margin of the region was a relatively stable cratonic hinge zone that extended from Minnesota to eastern Kansas (Witzke and others, 1983).
Figure 3. Map showing Cretaceous epicontinental seaway paleogeography during Campanian time (Late Cretaceous) (modified from Brenner, 1978, reprinted by permission, Gill and Cobban, 1973) (wseaways.gif). The large rectangle is Wyoming, the small one shows location of the Powder River Basin.
Late Cretaceous time was marked by episodic transgression and regression of the Cretaceous epicontinental sea (Weimer, 1984), which resulted in deposition of primarily marine sediments in the northern part of the seaway. Kauffman (1977) lists as many as 10 transgressive-regressive cycles for the Western Interior during Cretaceous time. This list includes marine transgression during the time of deposition of the Sussex Sandstone Member, followed by regression of the sea. These cycles are correlative around the world and were based on biostratigraphy and radiometry (Vail and others, 1977).
Distinct eastward changes in shelf, slope, and basin morphology have been mapped in the Powder River Basin by Loutit and others (1988), who believe that a eustatic sea level rise in latest Maastrichtian time, about 67 million years ago (Ma), significantly affected sedimentation patterns on continental margins; this event roughly correlates with the start of the Laramide orogeny (Tweto, 1975, 1980). Lillegraven and Ostresh (1990), however, postulate that Upper Cretaceous time in the seaway is marked by a very slow regional regression of the shoreline that is consistent with global lowering of sea level for this time period; their data are based largely on mapping and analysis of ammonite zones. Their evidence indicates that shoreline position during Late Cretaceous time was relatively static and considerably west of Sussex "B" sandstone deposition.
During Campanian time (74-84 Ma) most of the Powder River Basin was part of a broad muddy shelf. Pierre and Cody Shales deposition is characterized by mainly marine muds punctuated with relatively thin sands (Brenner, 1979). During deposition of the Cody Shale, regional morphology consisted of a distinct shelf, slope, and basin configuration; relief on the shelf may have been about 1,000 ft (300 m) in the eastern part of the Powder River Basin (Asquith, 1970). The abrupt eastern termination of the Sussex Sandstone corresponds with thinning and eventual pinchout of the Sussex interval (Brenner, 1979).
The Upper Cretaceous Sussex "B" sandstone was deposited in a probable mid-shelf position in the Cretaceous epicontinental seaway (Higley, 1994). Basin morphology during deposition of Sussex and Shannon shelf sandstone beds in the Powder River Basin is illustrated on figure 4 (shelfmod.gif) (Tillman and Martinsen, 1984).
Figure 4. Basin morphology during deposition of shelf sandstone beds in the Powder River Basin (Tillman and Martinsen, 1984 reprinted by permission). The House Creek field is located about 10 miles (16 km) east of and approximately parallel to Hartzog Draw field. Oil in the Hartzog Draw and Heldt Draw fields is produced from the Upper Cretaceous Shannon Sandstone.
Actual proximity of the Sussex "B" sandstone relative to shoreline is unknown. The difficulty with sand-ridge models lies with transporting and reworking sediments this far from shore. This Sussex "B" and other Sussex and Shannon Sandstone mid-shelf sand ridges had the following sources of sand and sediment transport:
One source of sand for the Sussex Member and underlying Shannon Member is erosion of precursor marine and possible nonmarine strata during transgression, with redeposition in shoreface to mid-shelf environments as marine-ridge sandstones.
Deposition of shelf sands may also result from local and regional tectonic activity in the region. Kauffman (1977) believed the eastern platform of the Powder River Basin showed minimal localized tectonic activity, as reflected in a broad uniform pattern of marine sedimentation. Topographic highs on the Cretaceous shelf could have developed as a result of;
An ordered sand-ridge field, such as the Shannon Sandstone in the Hartzog Draw area (figure 4), located about 10 mi (16 km) west of the House Creek field, develops from an initial, low-relief, highly variable topography (Gaynor and Swift, 1988). Tillman and Martinsen (1984), stated that "Detailed subsurface correlations of the Shannon sand ridges throughout the Powder River Basin, using well-developed bentonite markers, show the reservoir facies to "rise and fall" parallel to their elongation, indicating that the ridges were not deposited in layer-cake fashion." Sandstone ridges in the Sussex "B" sandstone rise to the north and to the west. These stratigraphic patterns of development of the higher energy shelf-ridge facies are interpreted to reflect sea-floor topography during their deposition. If basin structural features resulted in topographic highs during deposition of the Sussex "B" and other offshore sandstones, they could have resulted in concentration of sand and (or) sand ridges on the muddy shelf.
Holocene shelf sand ridges on the Texas shelf and the Georges Bank - Nova Scotia - Newfoundland - Labrador shelf system (Slatt, 1984) were examined and compared to sand-ridge deposits from the Cretaceous epicontinental seaway. Ridges in the transgressive Holocene sand sheets are associated with topographically high banks. Sediments here are derived from reworking of underlying substrates with contemporaneous deposition of mud in adjacent low areas. Oceanic currents play a strong role in localizing sand ridges, in grain sorting efficiency, and in areal distribution and stacking patterns of sands and muds.
Lithostratigraphic and biostratigraphic data suggest Sussex Sandstone depositional intervals east of the House Creek field are compressed rather than missing (Gill and Cobban, 1966; Asquith, 1970; Brenner, 1979). This indicates a reduced sediment accumulation rate east of the field and greater rates of compaction for mudstone than sandstone. This also indicates the area of the House Creek field was a paleo-topographic high. Thickness of the "B" sandstone ranges from several feet along and east of the seaward margins of the field to almost 60 ft (18 m) along the western margin; this is a lateral distance of 1 to 1.5 miles. Sediments along the western field margin consist mainly of moderate- to low-depositional energy sandstone and mudstone. This wedge shape also indicates oceanic currents which sculpted the Sussex "B" sandstone were a fractionation mill. Sediments were eroded from the front of the marine ridges and re-deposited landward (westward). This is illustrated by figure 5. This north-south-oriented vertical slice of the Sussex "B" sandstone is located near the northern boundary of the study area. It was mapped using the basal disconformity surface at the datum; the actual shape of the "B" sandstone is probably wedge-shaped with a lower surface that curves downward to the left (landward). This is an oblique slice across the field, the wedge shape is even more pronounced when cut perpendicular to the field axes.
Figure 5. This north-south-oriented vertical slice of the 3-D porosity model (3dns_img.htm) shows upward and westward (landward) increase in sediment thickness, and progressive westward deposition of later sands in the Sussex "B" sandstone (wedgessx.gif). Blue on the 3-D model is primarily mudstone and sandstones containing less than 8 percent porosity, green is mainly fine- to medium-grained sandstones exhibiting 8 to 12 percent porosity, and yellow contours are mainly medium-grained trough-cross-bedded sandstones that contain greater than 12 percent porosity. Respective average porosity of the "B" sandstone and of reservoir intervals are about 8 percent and 12 percent. Slice width and thickness are about 2.5 mi (4 km) and 50 ft (15 m). Vertical exaggeration is 60x. Yellow dots are well sample locations. Index contour map shows cumulative oil production per well; contour interval is 40,000 barrels.
Tectonic influences may have altered sea floor relief and slope of the shelf during deposition of the Sussex "B" sandstone. This would provide sources of sediment through changes in relative sea level and oceanic currents, and influence areas of sediment deposition. Slack (1981) believes the Belle Fourche arch ( fig. 1), and other associated northeast-trending structural lineaments in the northern two-thirds of the Powder River Basin controlled localization of all significant Shannon and Sussex productive sandstones. Sussex and Shannon production is concentrated along the arch in this area of the basin. The axis of the arch crosses the northern quarter of the House Creek field. Location of the arch is based on gravity data; structures are difficult to identify from conventional subsurface structure maps. Uplift and erosion throughout Wyoming prior to deposition of the Teapot Sandstone Member of the Mesaverde Formation (72 Ma) (fig. 2) probably obliterated evidence of additional transgressive and regressive phases of deposition during the latter part of the Late Cretaceous (Gill and Cobban, 1966).
The Belle Fourche arch probably formed from subtle variations in regional structural patterns which are precursors to the Laramide orogeny (Slack, 1981). Subtle uplift along the arch occurred from Lower Cretaceous Muddy through Upper Cretaceous Sussex deposition, a period of about 35 m.y. The Belle Fourche arch was slightly positive during deposition of the Sussex Sandstone (Slack, 1981), and marine ridge deposition over structural highs may have generated topographic highs along the arch.
Other suggestions of greater topographic relief and subtle uplift in the House Creek field area are 1) deposition of the Sussex "B" sandstone began in the southeastern corner of the field and progressed to the north, toward the arch, 2) the thickest section of the Sussex "B" sandstone, and many intervals of Shannon Sandstone, are located near the axis of the arch, 3) in fact, most of the productive Sussex and Shannon intervals are along this trend, and 4) the thin, areally discontinuous Sussex "A" sandstone overlies the "B" sandstone along the western field boundary. The "A" sandstone, located just below the Ardmore bentonite, produces oil from the Triangle U field. This field is located on the axis of the Belle Fourche arch in T. 46 N., R. 75 W.
The House Creek and Porcupine oil fields are delineated by a major marine-ridge system that is composed of complexly interbedded thin-bedded mudstone and sandstone of the Sussex "B" sandstone. The informally named Sussex "B" sandstone is one of as many as 12 separate sandstone bodies that are located at different vertical and lateral positions within the Sussex Sandstone Member (Anderman, 1976; Crews and others, 1976). Core and outcrop studies were combined with well log and analytical data, plus published research, to assign depositional environments for the Sussex "B" sandstone. Sandstones and mudstones were mapped across the study area to evaluate scales, types, and timing of reservoir heterogeneities, including lateral and vertical continuity and extent of bedforms, interbedding of reservoir and non-reservoir (seal) units, and depositional changes across the field related to variations in facies thickness, depth, and distribution of permeability boundaries.
The "B" sandstone was deposited in a probable mid-shelf position in the Cretaceous Epicontinental seaway. Four separate sand ridges comprise the "B" sandstone in the field area. Transgression during deposition created westward (landward) backstepping of later deposited sand ridges. Sand-ridge development began in the southeastern corner of the field and progressed to the northwest, based on stacking patterns and correlation of sand-ridges and contained mudstone-sandstone beds, Sussex "B" upper and basal disconformities, and the overlying Ardmore bentonite.
Figure 6 shows thickness (in feet) of the Sussex "B" interval between the upper and basal disconformities. Landward stacking of sandstones is evident from the westward increase in sediment thickness. When mapped using either the overlying Ardmore bentonite or the bounding disconformities as datums, the Sussex "B" sandstone is wedge-shaped. The Cody Shale interval between the bentonite and "B" sandstone also is wedge-shaped; however, the open end is toward the ocean. The bentonite is a time-equivalent layer of volcanic ash that was chemically altered to clays during burial. While this trend is clear within the House Creek and Porcupine fields, each reservoir must be evaluated separately for stacking patterns. The top of the Sussex "B" reservoir sandstone in the study area is located approximately 20 to 60 ft (6 to 18 m) below the Ardmore bentonite, based on correlation of well-logs and core studies (Higley, 1994). Variation in thickness of the Ardmore-Sussex "B" interval results from 1) influence of depositional environment, depositional energy, and sediment supply on localization of sediments, 2) greater rates of compaction for shale-rich versus sandstone beds, and 3) local or basinal structural movement and its association with rates of deposition.
Figure 6. Isopach map of the Sussex "B" sandstone showing thickness in feet of the interval between bounding disconformities; contour interval is 4 ft (1.2 m). Map scales are meters. The small index contour map is cumulative oil production; contour interval is 40,000 barrels. The thin red line outlines the boundary of the House Creek field. Thin white lines in the southern half of the field are limits of sand-ridge 1 and overlying sand-ridge 2. The large-scale 64 KB image is named ssxiso.gif .
Based on thin-section, core, outcrop, well-log, and production data, the highest quality Sussex "B" reservoir intervals are fine- to medium-grained, glauconitic, thinly bedded, trough- and planar cross-bedded sandstones. These facies contain greater reserves, porosity, and permeability and exhibit fewer laterally continuous permeability barriers and baffles than the fine-grained massive, ripple-laminated, and wavy-bedded sandstones that also produce oil.
Transport of shoreface and shelf sediments across this seaway resulted from complex interaction of oscillatory and unidirectional currents, created primarily by intermittent along-shelf, southerly storm flows (Gaynor, 1991). Sandstone ridges are aligned obliquely to the prevailing transport direction, as is shown on figure 7. Figure 8 is an idealized vertical slice across three stacked sand ridges. Due to the great spatial scales (width and length) of Shannon and Sussex sand ridges, the ridges must be obliquely oriented with respect to fluid flows (Gaynor and Swift, 1988), as opposed to the flows being perpendicular to ridge boundaries. Cross-ridge flow accelerates on the up-current flank of the ridge; consequently, it is the up-current flank that undergoes erosion (Gaynor and Swift, 1988). This cross-ridge acceleration and down-ridge frictional deceleration results in a rotation of flow over the ridge that causes erosion of the up-current flank, aggradation of the crest of the ridge, and growth of the sand ridge (Gaynor and Swift, 1988).
This model also explains thinning of the Sussex "B" sand ridges proximal to the open ocean, aggradation of the reservoir intervals along the crests of the sand ridges, and decrease in grain size, sorting efficiency, and associated porosity and permeability of sandstones west of axes of individual sand ridges and for the entire "B" sandstone. Directions of maximum fluid flow for this sand-ridge model are parallel to the axes of each trough-cross-bedded bedform and also parallel to field axes. These flow directions result from depositional patterns of the trough-cross-bedded sandstones, and concentration along field axes of the thickest and most porous and permeable units.
Figure 7. Plan view of lithofacies distribution on a sand ridge, and generalized flow streamlines (srplanvs.gif). Flow streamline arrows are the directions of currents over the sand ridge. Scale is approximate. Click sand Ridge - plan view (modified from Gaynor and Swift, 1988, reprinted by permission) to see the detailed (srplanvw.gif) image (208 KB).
Figure 8. Idealized east-west-oriented vertical section across three stacked sand ridges (ssxflowu.gif). General facies types are listed. No vertical or horizontal scales.
Figure 8 shows upward and seaward increase in depositional energy (yellow dotted and green areas), which is tied to increases in porosity and permeability. The basal disconformity (wavy line) is the horizontal datum for the 3-D porosity and permeability models. The wedge shape of the Sussex "B" interval results from reworking by currents of seaward margins of sand ridges, and landward redeposition of sediment. The blue-lined areas are basal and landward low-depositional-energy facies; these exhibit low porosity, permeability, and petroleum production.
The disconformity at the top of the Sussex "B" sandstone is generally marked by a thin chert-pebble sandstone (figure 9A). Shading variation of the quartz (figure 9B) results from fracturing of the grain in this cross-nicols photomicrograph view (light is transmitted differently due to rotation of the crystal axes). Quartz grains that were incorporated from underlying sand-ridge sediments commonly exhibit early stages of diagenesis within marine environments, primarily chamosite overgrowths under the quartz overgrowths. Grain-to-grain contacts within this facies are mainly point with lesser long-straight contacts.
Figure 9. A. Chert-pebble-lag core photograph, and B. crossed-nicols photomicrograph of a chert-pebble lag, No. 14-9 Federal well. Chert pebble lag core links the detailed (576 KB) ctlagc.gif core image. The bimodal grain distribution (mix of large and small grains) is visible on both images. Chert pebble thin section shows the large-scale (736 KB) ctlagts.gif photomicrograph image. Labels are Q quartz, CT chert, and C calcite. Quartz grains incorporated from underlying central-ridge facies sandstones commonly "float" in a matrix of (blue stained) ferroan carbonate cement.
Figures 10, 11, and 12 are thin-section, outcrop, and core photographs of trough-cross-bedded sandstones. These sandstones are the primary reservoir facies; facies are further divided based on relative decrease in depositional energy into central ridge, high-energy ridge margin, and low-energy-ridge margin (Tillman and others, 1985).
Figure 10. This transmitted-light thin-section image is from 8,014 ft (2,443 m) depth in the Empire-Federal "C" oil well. Deformation of glauconite (G) pellets and other soft sediments in trough-cross-bedded sandstones form microscopic and macroscopic permeability baffles to fluid flow. Note also that quartz (Q) grains surrounded by glauconite do not exhibit the later diagenetic quartz cementation. Mudstone (brown) and glauconite (green) are concentrated along the lower depositional energy laminae of trough-cross-bedded sandstones. Late-diagenetic dolomite cement is stained dark blue (D). Medium-blue epoxy fills pore spaces (ellipse with a line through it). Glauconite is 1.4 MB thin section (glauts.gif) image.
Figure 11. Photograph of an outcrop of Sussex Sandstone Member low-energy-ridge-margin (LRM) sandstone; outcrop is located in NW. corner of sec. 7, T. 40 N., R. 78 W., Natrona Co., Wyoming. LRM thickness is about 1 ft (0.5 m). Greater dispersal of glauconite, clays, and mud in lower depositional energy facies decreases porosity and permeability. This thin trough-cross-bedded sandstone, and the bounding burrowed and bioturbated sandstone/mudstone facies, contain mudstone and glauconite laminae and beds that are preferentially eroded, causing greater relief. Blue and purple lines in this small-scale image separate bedding planes. Trough-cross-bedded sandstone 1 MB (ssxcrp2.gif) outcrop image.
Figure 12. Core photograph of trough-cross-bedded medium-grained sandstone from 8,169-ft depth in the No. 1 Mandel-Federal well. Depths are in feet. Arrow points up. Siderite (S) clasts are concentrated on the bedding planes. Trough-cross-bedded sandstone 512 KB (trs8169.gif) core image.
Figures 13 and 14 show inter-ridge depositional environments that consist mainly of burrowed and intact mudstone laminae, and interbedding of mudstone with fine-grained sandstone. Abundant permeability barriers result in low levels of petroleum production. These facies, concentrated toward the base of sand ridges, compartmentalize the reservoir intervals.
Figure 13. Core photograph of wavy-bedded fine-grained sandstone from 8,194-ft depth in the No. 1 Mandel-Federal well. Dark lines are mudstone laminae that segregate the thin, discontinuous sandstone beds. A siderite drape (arrow) is located on a ripple reactivation surface. Core width is 3.5 inches (9 cm). Arrows within core point up-hole. Wavy-bedded sandstone - 688 KB core (wss8194.gif) image.
Figure 14. Core photograph of a bioturbated very fine grained sandstone from 8,038-ft depth in the No. 1 Empire Federal well. This facies exhibits very low porosity and low permeability and is a reservoir seal. Black arrow points to a mudstone lamina. Gold arrow shows a burrow trace. Core width is 2 inches. Bioturbated siltstone - 800 KB core (bss8038.gif) image.
Heterogeneity (essentially nonuniformity) in sandstone reservoirs is controlled by the following factors; (modified from Weber, 1986; Schenk, 1988, 1992):
Reservoir quality in the Sussex "B" sandstone is a function of sedimentologic and petrologic heterogeneity. This and the following section discuss the relative influences of the above factors on porosity and permeability distribution and development. Faulting, aside from minor fracturing as viewed in core, is undocumented for the Sussex "B" sandstone.
Initial porosity in the Sussex "B" sandstone in the basin was probably 35-40 percent by volume for trough- and planar-cross-bedded facies. Because of poor sorting (more clay and mud) finer grained sandstones exhibit lower initial porosity and permeability. The porosity range is based on examination of 54 thin sections and is tied to analogs. Pryor (1973) analyzed distribution of porosity and permeability in unconsolidated beach sands in the Gulf Coast; he found that initial porosity of 39 to 56 percent decreased rapidly with burial. Wilson and Sibley (1978) determined that the Cambrian Galesville Sandstone, of Wisconsin, exhibited initial porosity of 45 to 50 percent; this decreased during burial to about 33 percent. This decrease resulted mainly from rearrangement of grains during early burial. Further reduction to 24 to 33 percent was due to later quartz pressure solution (dissolution of quartz grains and associated closer packing of grains), and emplacement in pores of dissolved quartz, kaolinite (clay), iron hydroxide, and adularia (a feldspar mineral).
Sussex "B" sandstone porosity and permeability range up to about 21 volume percent and 200 millidarcies (mD), respectively, for core tests from 15 widely scattered boreholes. Geometric mean of permeability for this 791 core-plug data set is 0.62 mD.
Core plus well-log average porosity is 8.8 percent for the entire interval of the Sussex "B" sandstone, and about 13 percent for reservoir intervals. These 2,600 core plus well-log points have median porosity of 8.4 percent. Porosity values were calculated from bulk density well-log traces for approximately 100 wells; values were recalibrated to a specific grain density of 2.68 g/cm3 (Higley, 1994). Sonic log traces were used for porosity calculations for several wells. These porosity/permeability data files are located in the data_set directory.
Figure 15. Graph of core and well-log porosity versus depth for 2,600 Sussex Sandstone Member points. The average 8.8 percent porosity of the Sussex "B" sandstone and of reservoir intervals (13 percent) is also plotted. Scatter in data is considerable.
Published reports of reservoir permeability and porosity in the House Creek field vary; Bauder (1981) listed a geometric average permeability of 6.9 mD for the Sussex "B" sandstone. Sabel (1985) reported mean porosity and permeability of 13 percent and 15 mD, respectively, for reservoir sandstones in the House Creek field.
Figure 16. Volume percent porosity and permeability for 791 core plug samples for 30 wells, Sussex Sandstone Member (Higley, 1994). Greater porosity correlates with permeability increase. Scatter in data is considerable. Dotted green lines are geometric averages for this data of 8.8 percent porosity and 0.62 mD permeability. Dashed red lines represent average porosity and permeability of 13 percent and 15 mD for reservoir intervals of the "B" sandstone. Yellow circles surround average porosity and permeability values for sand-ridge facies determined from core examination of 8 wells. Sandstone facies are: chert-pebble lag, C; central-ridge, R; high-energy ridge-margin, H; low-energy ridge-margin, L; wavy-bedded, W; inter-ridge sandstone, S; and burrowed inter-ridge sandstone and mudstone, b.
Sussex "B" sandstone facies and mineralogy vary greatly at megascopic (field scale), and even more so at macroscopic (well and rock), and microscopic (thin-section) scales. The most porous, permeable strata are the moderate- to high-depositional energy trough-cross-bedded medium-grained sandstones; these are primarily concentrated towards the tops of sand ridges. Increase in porosity and permeability is tied to increase in depositional energy. This results from:
Amounts of calcite cements in 17 trough-cross-bedded (H, R, and L in the above porosity/permeability figure) sandstone samples range from 2.3 to 34 percent, and average 8.8 percent. Conversely, 7 samples of inter-ridge burrowed mudstone to fine-grained sandstone (b, S, W) only averaged 4.2 percent calcite cement. Resulting average minus-cement porosity is calculated at only 18 percent. Extensive loss of porosity and permeability due to compaction and prior to calcite cementation, is indicated by the low percentage of calcite cement and large volume of mudstone and other soft sediments (about 27 percent).
Percent volumes of calcite cement and intergranular porosity for 54 thin sections are shown on figure 17; microporosity within clays and grains is not recorded. Thirteen percent porosity is average for reservoir intervals; 8.8 percent is average for the entire Sussex "B" sandstone, based on core porosity/permeability of 791 points. Intervals of greatest porosity generally contain less than 3 percent calcite cement; calcite cement in this mainly trough-cross-bedded-sandstone facies is commonly spotty in thin sections, with evidence of extensive dissolution of cements.
Figure 17. Plot showing volume percentages of core porosity and calcite cement for the Sussex "B" sandstone. Low-porosity sandstones associated with greater than 10 percent calcite cement commonly contain no effective porosity. This occurs mainly during the earliest stage of calcite cementation. The region with low porosity and minimal calcite cementation results from early diagenetic compaction. These samples are primarily burrowed and bioturbated sandstone and mudstone.
Flow units are reservoir zones that exhibit similar fluid flow, porosity/permeability, reservoir potential, and production characteristics. Ebanks (1987) defined flow units as "volume of rock subdivided according to geological and petrophysical properties that influence the flow of fluids through it." Flow unit characteristics of the various Sussex "B" sandstone facies are assigned and detailed in Higley (1994); they are included below to show porosity/permeability/facies types for each reservoir and non-reservoir group of rock types.
Sussex "B" sandstone fluid flow potential is rated E(xcellent), G(ood), F(air), and P(oor). These categories are general indicators of the reservoir favorability. Divisions are based on porosity and permeability characteristics of central ridge and other facies, probable directions of maximum fluid flow in the field, vertical and lateral continuity of reservoir and non-reservoir flow units (based on core, outcrop, and geophysical well-log examination), presence and number of permeability barriers and baffles, and petroleum production characteristics of wells. Table 3 lists six flow units; these consist of three reservoir (E, G, F) and three low-permeability, non-reservoir (P5, P6, P7) units. Flow unit E exhibits excellent reservoir and fluid flow properties; G contains good reservoir properties, F exhibits marginal to fair reservoir properties, and the non-reservoir P units (P5, P6, P7) are permeability baffles and barriers bounding and interbedded with E, G, and F flow units. Determination of thickness and distribution of flow units across the field was based on examination of core from 20 wells, well-log data for more than 120 bore holes, and on outcrop studies.
FLOW-UNIT RATING, PERMEABILITY (mD), POROSITY (PERCENT), AND PETROLOGIC CHARACTERISTICS
An example of a Good to Excellent flow unit is the trough-cross-bedded medium-grained sandstone shown in figure 18 (bedding planes are outlined in blue and purple in this snncrp1s.gif image). This is a non-productive probable lower-shoreface (near the beach) sandstone of the Shannon Sandstone Member. Even though this is a very porous sandstone, porosity, permeability, and fluid flow are all decreased by the thin sandstone beds (bounded by purple) that are highly cemented by quartz. Flow is influenced by orientations of trough-cross-bedding planes. Primary direction of maximum fluid flow is perpendicular to this image, and along the bedding planes of the individual cross-bedding laminae. Note also (for future reference) that individual beds have general lensate and tabular forms. The thin, highly cemented, low-permeability sandstone layers are massive in appearance, compared to the porous cross-bedded intervals that erode more readily.
Figure 18. This outcrop of a Shannon Sandstone Member trough-cross-bedded sandstone is located at Shepperson Ranch, west of Midwest and Edgerton, Wyoming (figure 1). The thin, low-porosity, quartz-cemented beds are outlined in purple in this small-scale view. Porous and permeable intervals are bounded by a blue line. The 1.2 MB image is named snncrp1.gif .
Sussex "B" sandstone heterogeneity in the House Creek field results mainly from the following factors. Many of these factors are discussed in other sections, for example, the interbedding characteristics at macroscopic scales are detailed in the Depositional Model section.
Figure 19. Illustrated are the lensate form of the reservoir, and of individual reservoir sandstones, in this vertical east-west slice showing 3-D distribution of porosity across sand ridge 1 and overlying sand ridge 2. Yellow dots are locations of well-log porosity measurements. Blue and purple areas are primarily non-reservoir intervals. These are basal and landward low-depositional-energy sandstones and mudstones of individual sand ridges (porosity ranges up to 8 percent). Green areas are reservoir-grade porosity of 8 to 12 percent. The breakover point between green and yellow is 12 percent; this approximates the 13 percent average porosity of reservoir intervals; yellow through red (20 percent porosity) are the best quality reservoir intervals. These are largely the medium-grained trough-cross-bedded sandstones, although even here there is considerable interbedding of low- through high-depositional-energy facies. Width of the reservoir at this slice is about 2 mi (4 km). Vertical exaggeration is 60x. This is 3dewlens.gif, slice 10 of the 3-D distribution of porosity model, as shown in the ewporosv.MOV movie.
The reservoir sandstone intervals (green, yellow, orange, and red) are essentially lens shaped. These are composed of many thin, laterally discontinuous, interbedded sandstone and mudstone beds. The sandstone intervals pinch out along the eastern (seaward) margin and grade into non-reservoir (blue) and lower quality reservoir facies along the western field boundary. Datum is the basal disconformity (erosional surface). Using the overlying Ardmore bentonite as the datum, the field actually has a wedge shaped base and top. The same bulge (lens) is visible along the field axis, and reservoir intervals pinch-out to the east. A thin interval of high porosity (red) capping the field in some images is a remnant of the gridding process; a thin, areally discontinuous porous zone is located in several geophysical well logs at the top of the "B" sandstone.
This 3-D distribution of porosity slice (figure 19) reveals;
Core porosity represents point sources, whereas well-log porosity is commonly smoothed over intervals of a few feet. Illustrating the high variability in porosity and permeability over short horizontal and vertical distances requires 3-D images of core data, rather than porosity and permeability calculated from geophysical well logs (data sources and analytical procedures are detailed in the Technical information on data sources section of the Table of Contents).
Figure 20. East-west oriented vertical slices of porosity (left, percent) and permeability (right, mD). This image (porpermv.gif) shows the highly variable porosity (left) and permeability (right) using core data. Porosity values/colors are reservoir grade (green through red) of 8 through 21 percent, and below-reservoir-grade low-porosity (blue) of less than 8 percent. Permeability ranges are: purple and blue, up to 5 mD; green, 5 to 16 mD; and yellow through red are greater than 16 mD. Average permeability of Sussex "B" reservoir sandstones is 15 mD. The close-spaced 3-D grid essentially treats each well as a separate set of points for the purpose of showing the variation in porosity and permeability. The various diamond shapes and streaks of low through high porosity and permeability illustrate the interbedding of thin beds of reservoir- and non-reservoir-grade sandstone and mudstone. These diagrams include the eastward and upward increases in porosity and permeability, and stacking of sand ridges shown in previous images. This slice is located in the northwestern half of the field. Yellow dots are (left) core and well-log and (right) core sample locations. Slices are 1.8 mi (2.8 km) wide and 50 ft (15 m) high. Vertical exaggeration is 30x. Respective names of the east-west porosity and permeability slide-image files are 3dew_img.htm and 3dvk_img.htm .
Variable distribution of porous zones is illustrated in figure 21 A. and B. Shown are the respective 3-D distribution and volumes of porosity for 8 to 21 percent, and 12 to 21 percent porosity. These views were chosen along the eastern margin of the field because deposition of the Sussex "B" sandstone began here, and upward and landward (westward) backstepping of later-deposited sand ridges is better displayed with this view than for other orientations. Average porosity of the Sussex "B" sandstone is about 8.8 percent; average porosity of reservoir intervals is about 13 percent. 3-D distribution of intervals of porosity are explained and illustrated in the text (volporf.mov) sections of the table of contents.
A. 
B. 
Figure 21. A. This (por8vol.gif) 3-D view of 8 to 21 percent porosity exhibits a total volume of 358.6 million cubic meters. This is about 70 percent of the total volume of 525.5 million cubic meters of the Sussex "B" sandstone within the field boundaries. The 64 KB full-size image is named eside10f.gif . B. This (por12vol.gif) 3-D image of 12 to 21 percent porosity has a total volume of 115.9 million cubic meters, which comprises less than 25 percent of the total volume of the "B" sandstone in the field. In comparison, the 13 to 21 percent porosity range is about 62.2 million cubic meters; this is about 12 percent of the field volume. The 64 KB full-size image is named eside10j.gif . Green on the figures is 8 to 12 percent porosity. Yellow through red are 12 to 21 percent porosity. Blue arrow marks the boundary between sand ridge 1 and 2. The thin white line marks the upper disconformity surface of the "B" sandstone, except for the seaward margin of the House Creek field, which is outlined in red. The white arrows and line separate the northwestern and southeastern ridge systems; the gap in reservoir-grade porosity units is apparent. X and Y scales are shown in meters around the base of the block. Maximum thickness of the "B" sandstone is about 60 ft (18 m). North arrow is included. Vertical exaggeration is 80x.
Macroscopic and Microscopic (Well and Sample) Scales
Processes of porosity and permeability destruction from pervasive early cementation by calcite and (or) quartz, precipitation of clays, or by compaction are shown in figure 22 (cmts4sm.gif).
Figure 22. Thin-section photomicrographs of four samples of the Sussex "B" sandstone. Q is quartz, CT shows the chert grains, K is kaolinite, C is calcite, F shows feldspar grain (gray in the crossed-nicols view), MS are mudstone pore-fill and clasts, Oil coats grains and fills pores. Blue epoxy fills pore spaces. cmts4.gif is the full large-scale 2.4 MB image. Clockwise from upper left, images show;
The 3-D computer model images, including figures 21 and 22, show reservoir heterogeneity at megascopic (field) and macroscopic (well and rock) scales. Macroscopic heterogeneity includes stacking of sand ridges, lateral and vertical variation in porosity and permeability, upward and seaward increase in these properties, the lensate and tabular shapes of sandstone beds, and interbedding of high- and low-depositional energy facies. Microscopic heterogeneity, as shown by thin-section and SEM photomicrographs, results mainly from highly variable amounts and distributions of cements and authigenic and detrital clays; this porosity and permeability signature is highly facies related.
Porosity in the Sussex "B" sandstone is mainly primary intergranular (pore volumes remaining from the time of deposition; intergranular means located between the grains). Secondary porosity results from corroded and dissolved feldspar and other lithic grains, which enhances porosity and permeability. Microporosity is present within pore-filling clays, largely kaolinite, and within some corroded chert and other grains. Microporosity is not effective porosity because the small pores are poorly connected. Intergranular plus intragranular porosity in thin sections is as much as 17.5 percent. Average porosity for all thin sections is 5.2 percent; 3.4 percent of this is primary intergranular and as much as 1.9 percent is secondary and intragranular porosity. Average thin-section porosity values for Sussex "B" facies are listed in table 2; this is also located in the ssx_txt directory as the Ssxtable2.txt file. Format is tab-delimited, file is saved as an Microsoft Word text document. Tables located within the HTML documents are ASCII file formats.
Thin-section porosity values are lower than core and average reservoir porosity for two main reasons. First, microporosity is not included in thin-section estimates but is measured during core analysis. The amount of microporosity may be considerable since clays comprise an average 12 percent by volume of the Sussex "B;" clays are the primary permeability barrier at a microscopic scale. The percentage of microporosity should decrease up-section because the amounts of suspended-load sediment and most authigenic and detrital clays decreases upward. Second, three-dimensional effects of thin sections influence point-count results. These 3-D effects include Sussex "B" grain-size sorting and distribution (which are rarely homogenous in sandstones), presence of horizontal laminae and drapes of glauconite and mudstone, orientation of grains and clasts, and other factors that influence heterogeneity.
Sussex "B" sandstone primary and secondary porosity, and microporosity are shown on the following thin-section photomicrograph and SEM images. Pore spaces in all photomicrographs are filled with blue epoxy. Ferroan carbonates are stained blue (ferroan dolomite has dark blue crystals). Unless otherwise indicated, photomicrographs are with transmitted light. Scale bars on most images are 0.1 mm. Porosity in the thin section is partially decreased by early diagenetic cementation of quartz grains. The original rounded quartz grains are outlined by green-colored chlorite crystals. Early diagenetic cementation preserves porosity and permeability in the Sussex "B" sandstone by supporting the grain framework and decreasing porosity loss through compaction.
Figure 23. A. Thin-section photomicrograph from 8,012 ft (2,442 m) depth, No. 1 Empire Federal "C" well (fdissols.gif). Dissolution of the large feldspar (F) grain, located near the center of the image, results in secondary porosity. The crystal structure and outline of the feldspar grain is still apparent. Primary (depositional) porosity borders this and other grains. Quartz grains are labeled Q. Microporosity results primarily from breakdown of feldspar into kaolinite (K on large-scale view) and other clay minerals; this causes a mottled appearance of the blue epoxy that fills pore spaces. MS is a mudstone clast. Scale bar is 0.1 mm. The full-scale 1.5 MB thin-section photomicrograph of feldspar dissolution is fdissol.gif. B. Scanning electron microscope (SEM) image of kaolinite shows partings between booklets and minor amounts of "wispy" illite-smectite clay from the No. 1-23 House Creek Federal well (semkaosm.gif). Scale bar, below the illite-smectite label, is 4 micrometers. The 656 KB SEM full-scale image of kaolinite booklets is semkao.gif.
The Sussex "B" sandstone exhibits a complex diagenetic history that strongly influenced porosity and permeability preservation, loss, and enhancement. These factors in turn affect emplacement, trapping, and production of petroleum. The Sussex "B" sandstone is a litharenite to feldspathic litharenite, based on the classification of Folk (1970). The early, middle, and late diagenetic stages are shown in the following figure 24. High initial rates of compaction occur mainly in muds, and in sands that were not lithified, or supported, by early quartz and (or) calcite cementation. Rates of compaction decrease with time. The dashed lines are time periods of dissolution of grains and cements, which develops secondary porosity and permeability. Diagenetic stages were derived from examining the following; 1) cores of 10 wells, 2) 54 thin sections (including cathodoluminescent thin sections), 3) numerous SEM images, and 4) X-ray fluorescence (XRF) and diffraction (XRD) charts.
Figure 24. Generalized paragenetic sequence of the Sussex "B" sandstone (diagenes.gif). Dashed lines indicate times of dissolution of cements or grains. Yellow zone is period of sediment compaction. Medium blue zone is period of migration of hydrocarbons from shale source rocks. Dark blue zone is time of oil emplacement into the House Creek and Porcupine fields.
Precipitation of chlorite, pyrite, and alteration of fecal pellets to glauconite results from reducing conditions in marine pore fluids; this occurs soon after burial. Accumulation of bacterial end-products in pore fluids often results in precipitation of carbonate minerals and iron sulfides (Clayton, 1992). These end products affect pore water pH, Eh, and ionic strength.
The earliest diagenetic event is compaction. This is most significant for sediments with a high mud content; sand-sized grains floating in the mudstone matrix exhibit few and minor diagenetic changes. This results from isolation of these grains from pore fluids that affected cementation and dissolution. Rate of compaction of sediments decreases through time; later diagenetic cementation of sandstones by calcite and quartz stabilizes the grain framework; this decreases porosity and permeability loss through rearrangement of grains.
Even though three periods of calcite cementation are shown on figure 24, there is actually an additional stage of very early diagenetic calcite cementation. Because it is very limited in scope and distribution, it was not added to the figure. The following transmitted (rfca1sm.gif) and plane-polarized (rfcasm.gif) light images are radial-fibrous ferroan calcite. Radial-fibrous and planar-laminar calcite precipitates rapidly from pore fluids that are highly supersaturated in the carbonate ion. Bounding quartz grains exhibit peripheral dissolution but do not contain the later diagenetic chlorite rims or quartz overgrowths.
Figure 25. A. Transmitted-light view of very early diagenetic radial-fibrous calcite (C) that displaced quartz (Q) grains and glauconite. Calcite precipitated within the glauconite pellet and blasted it apart. Grains float in the matrix of calcite cement and mudstone (MS). Rfca1.gif is the full 752 KB transmitted-light image. B. The star-pattern of the calcite is apparent on this plane-polarized view. Rfca.gif is the full 736 KB plane-polarized image. Blue color results from staining the thin section for ferroan calcite.
An early-diagenetic stage of grain dissolution and replacement by calcite cement (alteration of lithics on figure 24) is indicated by standard and cathodoluminescent (CL) examination of several thin sections. The Figure 26 (clumcmts.gif) photomicroscope image of a chert-pebble lag shows the following: 1) Chert, quartz, and feldspar float in a matrix of ferroan calcite (orange) cement, 2) the "bright" spots or ghosts on the sample are not grains; they are calcite cement that is optically continuous with surrounding calcite cement. These are called ghosts because they show locations of grains that were replaced by the calcite. Ghosts are invisible using standard petrographic methods, except when floating grains and fragments suggest their origin. Because they are invisible, calculated minus-cement porosity volumes would be too great for these samples (sediment porosity near the time of deposition). Visual estimation of volume of grains and fragments removed during this stage is 1 to 5 percent.
Figure 26. Cathodoluminescent photomicrograph of ferroan calcite cement (orange and bright orange) within a chert-pebble lag. Calcite is optically continuous across this view. Grains of chert (CT), quartz (Q), and feldspar (striped grain) float in a matrix of calcite cement. The feldspar grain is partially replaced by this very early diagenetic calcite. Bimodal grain distribution and point contacts between grains are also shown. clumcmt.gif is the full-scale 736 KB image. Quartz overgrowths are black (visible on the full-scale image). Scale bar is 0.1 mm.
Bimodal grain distribution results from erosion of underlying ridge sediments and incorporation of grains into the chert lag. Quartz grains exhibit very minor (meniscal in appearance) quartz cementation, that in turn has peripheral dissolution by the early diagenetic calcite cement. Quartz grains commonly have coatings of early-diagenetic chamosite. The style of early diagenesis represented in this lag and the underlying sandstone is characteristic for marine pore fluids; the sediments were probably not exposed to fluctuating ground water in a vadose zone. The meniscal (concavo-convex) occurrence of authigenic quartz results mainly from peripheral dissolution of the poorly developed overgrowths.
Rims of chlorite grow perpendicular to quartz grain surfaces and appear to be intergrown with the quartz cement, as shown on the figure 27 SEM image (semchlsm.gif). Quartz displays well-developed overgrowths. X-ray diffraction analysis (Higley, 1994) indicates that most of the Sussex "B" chlorite is ferroan chlorite, or chamosite. Fuchtbauer (1983) postulates that chlorite, chamosite, and glauconite are found almost exclusively in marine environments.
Figure 27. Scanning electron microscope (SEM) images of early-diagenetic chlorite rims (bladed crystals) that coat quartz grains and are intergrown with quartz overgrowths, No. 14-9 Federal well. Brown box shows location of the zoomed-in image. A pore space is labeled (ellipse with a line through it). Semchl.gif is the full-scale 768 KB image.
Precipitation of (middle diagenesis) calcite cement prior to cementation by quartz is illustrated by the figure 28 cross-plot of depth in feet from surface versus volumes of a) calcite cement, b) quartz cement, and c) core porosity and d) permeability. Sussex "B" sandstone average porosity and permeablity of 13 percent and 15 mD are shown, as well as Sussex core and Shannon outcrop exposures that approximate core facies described in Higley (1988, 1994). Porosity and permeability exhibit general upward increase, which correlates with increase in depositional energy (trough-cross-bedded sandstone), and decrease in amounts of calcite cements. Twin spikes of about 13 percent quartz cement are mirrored by low percentage of calcite cement.
Figure 28. Shown are 1) 20 to 0 percent calcite (CO3++) and 14 to 4 percent quartz (QTZ) cements, 2) 20 to 0 percent core porosity (POR), 3) 100 to 0.01 mD permeability (K), and 4) photographs showing facies for the Sussex "B" sandstone, Empire Federal "C" oil well. TRSS is trough-cross-bedded sandstone intervals; these are concentrated near the top of the "B" sandstone and are the primary reservoir facies. "THIN-BEDDED VF-F SS, MS LAMINAE" refers to the thin-bedded horizontally stratified fine-grained sandstone and interbedded mudstone. Bioturbated mudstone is the blue-gray-colored rock located mainly at the base and top of this section. Heavy vertical bar on the right side of the figure is the perforated zone. The full-scale 144 KB image is empfedsr.gif.
Early diagenetic alteration of the hash is indicated by presence of the competent siderite clasts on Sussex "B" trough-cross-bedded sandstones (figure 29). The initial stage of calcite cementation in the Sussex "B" sandstone is probably contemporaneous with alteration to siderite of foraminiferal tests and other depositional carbonate hash (figure 30).
Figure 29. Core photograph (trs8167s.gif) from 8,167-ft depth in the Mandel Federal "C" well shows siderite clasts that were ripped up by high-energy currents and redeposited on the trough-cross-bedded sands of submarine dunes. Original horizontal bedding is shown in clasts by the stringers of quartz grains. The full-scale trs8167.gif image is 992 KB.
Figure 30. Thin-section photomicrograph of a siderite clast in a chert-pebble lag, No. 14-1 Federal well, Porcupine field (foramsm.gif). Foraminifera tests in the clast have been partially replaced by siderite (brown). Evidence for the marine origin of the siderite clasts includes alteration to siderite of Haplophragmoides and Reophax foraminiferal tests. The full-size foram.gif image is 768 KB.
A number of diagenetic stages are shown on the figure 31A, B transmitted-light and crossed-nicols photomicrographs (pyrca1sm.gif, pyrca2sm.gif). Pyrite (P) framboids (similar in appearance to raspberries) are black in transmitted-light and crossed-nicols views. These late-diagenetic pyrite crystals are commonly associated with the dolomite and may result from changes in chemical composition of pore fluids resulting from migration of hydrocarbons into the Sussex "B" sandstone.
Figure 31. A. Transmitted-light and B. crossed-nicols thin-section photomicrographs of pyrite framboids (P) within ferroan dolomite (D) from a trough-cross-bedded sandstone at 8,014-ft (2,443 m) depth in the Empire Federal "C" oil well. Quartz (Q) and feldspar (F) grains bound optically-continuous calcite (C) cement. G is a glauconite pellet. Medium blue is epoxy and mottled blue results from kaolinite clay (transmitted light view), dark blue on both views is the late-diagenetic ferroan dolomite. Full-size transmitted-light image is 752 KB pyrca1.gif. Full-size crossed-nicols view is 784 KB pyrca2.gif
