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Abstract

The Rocky Mountain Oilfield Testing Center (RMOTC) has recently completed a test of an airborne microgravity and electric field sensing technology developed by Electro-Seise, Inc. of Fort Worth, Texas. The test involved the use of a single engine airplane to gather data over the Teapot Dome oil field along a tight grid spacing and along thirty (30) survey lines. The resultant gravity structure maps, based on the field data, were found to overlay the known structure of Teapot Dome. In addition, fault maps, based on the field data, were consistent with the known fault strike at Teapot Dome. Projected hydrocarbon thickness maps corresponded to some of the known production histories at RMOTC. Exceptions to the hydrocarbon thickness maps were also found to be true.

TABLE OF CONTENTS

TABLES

Introduction

The Rocky Mountain Oilfield Testing Center (RMOTC) has recently completed a test of an airborne microgravity and electric field sensing technology developed by Electro-Seise, Inc. of Fort Worth, Texas. The test involved the use of a single engine airplane to gather data over the Teapot Dome oil field along a tight grid spacing and along thirty (30) survey lines. The Teapot Dome oil field, also known as the Naval Petroleum Reserve No. 3 (NPR-3), is located thirty-five (35) miles north of Casper, Wyoming (See Figure 1). During the testing process, RMOTC witnessed the airborne data gathering capabilities as well as the processing and interpretation of raw data resulting in final contour maps with three dimensional effects.

Background

The use of gravity measurements as an exploration tool in the oil and gas industry dates back many decades. Ander and Chapin1 present a concise summary of the current gravity methods and uses including advantages and disadvantages. Gravity methods assist the explorationist in identifying the size, shape, and depth of anomalous masses. Ander and Chapin state, “Gravity has advantages over other methods … Fast, inexpensive tool for evaluating large areas..… Can distinguish sources at exploration depths. Disadvantages of gravity methods include gross imaging of structures, resolution deteriorates with depth and does not provide a structural cross section without additional input. Faults can be identified on gravity maps through steep gradients or truncation of trends.”

Geologic Setting

The Naval Petroleum Reserve No. 3 (NPR-3) is located in the southwest portion of the Powder River Basin, approximately twenty-seven (27) miles north of Casper, Wyoming. (See Figure 1). NPR-3, often commonly referred to as Teapot Dome, is a large northwest-southeast trending anticline. The anticline is an extension of the larger Salt Creek anticline to the north and is a doubly plunging, asymmetrical anticline. The structure drops much more rapidly on the west flank than on the east side of the structure. See Figure 6 for an illustrative structure map based on the lowest producing formation found at NPR-3.

Production from Teapot Dome commenced in the 1920’s with full development activities beginning in 1976 after the effects of the first Arab oil embargo. Production has been from nine (9) productive horizons with the Shannon, Steele and Niobrara Shales, Second Wall Creek, and Tensleep Formations being the most productive. Figure 2 below illustrates the stratigraphy present at Teapot Dome.

Figure 2. Geologic Column of Teapot Dome Field

Figure 3.NPR-3 Conceptual Cross Section

Structural Setting

Figure 3 is a conceptual cross section of Teapot Dome. The top of the Shannon Sandstone lies approximately 250 feet below the surface at the top of the anticline. At the edge of the field, the Shannon top is approximately 1000 feet below the surface. From a gravity survey viewpoint, the basement rock, referred to as granite, will be approximately 750 feet closer to the surface at the top of the anticline than at the edges.

Figure 4. Reprocessed 2D Seismic Data with Basement Rock

The density contrasts in the overlying formations are essentially the intrusion of the higher density granite basement rock as compared to the marine shales and sandstones of the producing formations Figure 4 is an older seismic line that has been recently reprocessed. The intrusion of the basement rock is shown graphically with editing of the image. Table 1 lists some average porosities and reservoir properties for the sandstone producing formations at NPR-3.

Assuming average rock properties for each formation, the bulk densities are calculated and shown in Table 2.

Data Gathering

RMOTC witnessed the initial data gathering operations aboard the single engine airplane. The first of six (6) survey lines were recorded at night to reduce interference. The data gathering aboard the aircraft involved a proprietary sensor developed by Electro-Seise, Inc. using a Differential Global Positioning System (DGPS) and a laptop computer interfaced with a data recording module. Using minimal information from RMOTC, Electro-Seise designed a grid of thirty (30) survey lines spaced 200 meters apart. The survey grid is shown in Figure 5.

Review of Data

After the raw data was collected over the thirty (30) survey lines, Electro-Seise personnel spent several weeks at RMOTC processing the raw data including the necessary corrections. One such correction, cited by Ander and Chapin, is the EÖTVÖS correction necessary for gravity collected on moving platforms. The correction is for the platform’s velocity and heading – data easily obtained through the GPS system and instrumentation. From the processed data numerous maps were generated by Electro- Seise, including maps with three-dimensional aspects. The three-dimensional presentation was possible with use of special plotting routines and the use of Chromatek Inc. glasses.

Exploration Tool vs. Field Development Tool

The produced maps would generally be used in an exploration environment to identify possible structures or anomalous masses such as ore bodies or salt deposits. The use of the maps in a producing field environment, such as Teapot Dome with over a thousand wells drilled, is not the usual application of this technology. The objective of this report is to show the overall correspondence between the Electro-Seise data as processed and interpreted by Electro-Seise, and the known geologic structure of Teapot Dome.

The known geologic structure of Teapot Dome is a function of depth and producing formation. The shallow Shannon formation has been extensively drilled in the southern half of the field. In the northern half, north of a major fault in sections 27 and 35, the Shannon is non-productive. In addition, the western and southern flank of the anticline is also not productive so well control is also sparser in those areas. In general, the deeper the producing horizon, the less well control is possible. For example, the deepest producing formation at RMOTC is the Tensleep sand at approximately 5500 feet below the surface. The Tensleep has been penetrated twenty-eight (28) times resulting in approximately a dozen commercial wells. The Shannon, however, has had over five hundred (500) wells produced with the majority being economic due to its shallow depth.

The use of an exploration tool on a developed field makes some comparisons difficult. It is the intent of this report to show the similarities and differences of the exploration data and field data. Due to the lower well spacing in the deeper formations, it is not always possible for a one-to-one comparison since log or core data doesn’t exist. Recent 3D Seismic data (January, 2001) may provide additional insight and comparisons after the data is processed and interpreted in the coming months.

Maps Generated

Based on the interpreted and contoured data, a series of maps were generated for RMOTC. Electro- Seise worked in an AutoCad format with multiple layers and images on each map. Base maps of wells were provided by RMOTC along with scanned images of older reservoir maps. Electro-Seise Technicians handled the overlay of new and old maps producing a series of new maps for review and analysis.

The following is an analysis of the maps as presented by Electro-Seise. The focus of the analysis is primarily on the structure maps produced by Electro-Seise. For presentation purposes of this report, the maps were modified slightly to produce presentation quality graphics on a small-scale format. The contour lines were closed and other minor changes made. In general, RMOTC separated the individual contour levels for each horizon as presented by Electro-Seise. Simplified versions of the structure maps were also developed by hand drawn overlays in AutoCad. The isolation of the individual closed contours and the overlay of the structure maps are shown in the following sections for each horizon mapped by Electro-Seise.

Figure 6. Tensleep Horizon shown with all contour levels.

Figure 6 shows the Electro-Seise contours for the Tensleep horizon. The Tensleep is the deepest oil producing reservoir at Teapot Dome with the sands at approximately 5400 feet below ground level. The Tensleep is approximately 1500 feet above the basement intrusive rock.
The contour values presented by Electro-Seise are negative. The negative contour values that are shown are -100, -120, -150, and -200. RMOTC separated the above map into individual contour levels to illustrate the overlay on structure of the individual values. Figures 7-10 illustrate this analysis and the corresponding graphical comparison. The values are derived from the micro-gravity measurements taken from the airborne surveys. Since the values are believed to be relative, absolute units are not given.

The structure map shown is a hand drawn Autocad overly of a RMOTC scanned structure map. The map was simplified for presentation purposes by removing the minor faults shown on the original map. Recent 3D Seismic data (January, 2001) may change the above map slightly.

Figure 7. Tensleep Horizon shown with the -100 contour level. The contour (-100) is shown in Figure 7. Four (4) of the closed contours are close to, or on, the anticlinal axis. From an exploration viewpoint, the contours would indicate the possibility of an underlying structure in a northwest by southeast trending direction. The other three (3) local highs are in the southern portion of the anticline. The southern three (3) local highs are on the nose of the anticline and have not been previously mapped. The placement of the highs in the southern portion has resulted in steep microgravity gradients. Ander and Chapin1 state, “ Faults can be identified through either steep gradients or truncation of trends.”

Figure 8. Tensleep Horizon shown with isolated contour level of - 120. The contour (-120) is shown in Figure 8. Similar to the closed contour level of –100, four (4) of the closed contours are close to, or on, the anticlinal axis. The closed contour level of –120 is fairly close to the closed contour level of –100 and similar conclusions could be made about the possible presence of an underlying structure. At this contour level there are two (2) local highs in the southern portion of the anticline. The southern two (2) local highs are on the nose of the anticline and have not been previously mapped.

Figure 9. Tensleep Horizon shown with isolated contour level of - 150. The contour (-150) is shown in Figure 9. The closed contour level continues to expand. Two (2) nodes cover the top of the anticline with smaller nodes developing along the flanks of the anticline. The two (2) nodes along the southern portion of the anticline also continue to expand with a smaller node developing. The close spacing of the three (3) contour levels ( -100, -120, and -150) result in steep gradients which indicate possible faulting. The presence of minor and major faulting at Teapot Dome is well documented and exists on every major horizon. Fault displacement can range from just a few feet to over 100 feet depending upon the reservoir depth and location.

Figure 10. Tensleep Horizon shown with isolated contour level of - 200. The contour (-200) is shown in Figure 10. The -200 closed contour level is the last level mapped by Electro-Seise for the Tensleep horizon. At this level, the two (2) nodes on top of the anticline have merged with a single node covering the top of the structure and draping over the eastern and western flanks. The two (2) southern nodes continue to remain separate possibly due to a projected fault. See Figure 10. From an exploration viewpoint, the top of the structure is not as well identified as previous contour levels indicated. The northwestsoutheast trending nature of the structure is still evident.

Figure 11. Shannon Horizon shown with all contour levels.

Figure 11 shows the Electro-Seise contours for the Shannon horizon. The Shannon is the shallowest oilproducing reservoir at Teapot Dome, with the sands at approximately 250 feet below surface at the top of the anticline. Along the eastern flank, Shannon is approximately 1000 feet deep and is approximately 6600 feet above the basement intrusive rock.

The contour values presented by Electro-Seise are negative. The negative contour values that are shown are -200, -250, -275, and -300. RMOTC separated the above map into individual contour levels to illustrate the overlay on structure of the individual values. Figures 12-15 illustrate this analysis and the corresponding graphical comparison. The values are derived from the gravity measurements taken from the airborne surveys. Since the values are believed to be relative, absolute units are not given.

The structure map shown is a hand drawn Autocad overlay of a RMOTC scanned structure map. The original structure map was based on existing productive acreage. The map was not constructed for areas of the field where the Shannon is not productive. Many of the minor faults within the Shannon were retained to show the relative frequency of faulting and the general fault direction. The recent 3D Seismic data (January, 2001) will not change the interpretation of the Shannon because at this shallow depth seismic data was not possible.

Figure 12. Shannon Horizon shown with the -200 contour level. The contour level (-200) is shown in Figure 12. The two (2) closed contour levels essentially straddle the top of the anticline shown at the Shannon horizon. The two (2) closed contours are close to or on the anticlinal axis. The southern closed contour does drape over the western flank. From an exploration viewpoint, the contours would indicate the possibility of an underlying structure in a northwest by southeast trending direction. The top of the structure would be isolated to a small portion of the field based on this contour level.

Figure 13. Shannon Horizon shown with the -250 contour level. The contour level (-250) is shown in Figure 13. The two (2) closed contour levels have merged into a single node. This closed contour essentially straddles the top of the anticline and drapes over the western and eastern flanks. The contour has also extended down the southern nose of the anticline. From an exploration viewpoint, the contour would indicate the possibility of an underlying structure in a northwest by southeast trending direction. The top of the structure would be coincidental with the major axis of the hatched area.

Figure 14. Shannon Horizon shown with the -275 contour level. The contour level (-275) is shown in Figure 14. This separate contour is very similar to the previous due to the close spacing of the lines. From an exploration viewpoint, the contour would indicate the possibility of an underlying structure in a northwest by southeast trending direction. The top of the structure would be coincidental with the major axis of the hatched area.

Figure 15. Shannon Horizon shown with the -300 contour level. The contour level (-300) is shown in Figure 15. The –300 closed contour level is the last level mapped by Electro-Seise for the Shannon horizon. At this level, a single node covers the top of the structure and drapes over the eastern and western flanks. From an exploration viewpoint, the top of the structure is not as well identified as the –200 level contour indicated. The axis of the structure would be coincidental with the major axis of the hatched area.

Figure 16. Second Wall Creek Horizon shown with all contour levels.

Figure 16 shows the Electro-Seise contours for the Second Wall Creek Horizon. It is essentially at middepth of the producing reservoirs at approximately 2800 feet below surface, and is approximately 4000 feet above the basement intrusive rock.

The contour values presented by Electro-Seise are negative. The negative contour values that are shown are -75, -100, -120, -150, and -200. RMOTC separated the above map into individual contour levels to illustrate the overlay on structure of the individual values. Figures 17 to 21 illustrate this analysis and the corresponding graphical comparison. The values are derived from the gravity measurements taken from the airborne surveys. Since the values are believed to be relative, absolute units are not given.

The structure map shown is a hand drawn Autocad overly of a RMOTC scanned structure map. The original structure map extended off the field, north towards the larger Salt Creek Anticline. The projected faults of the Second Wall Creek were retained to show the relative frequency of faulting and the general fault direction. The recent 3D Seismic data (January, 2001) may change the structure map slightly when mapping is complete.

Figure 17. Second Wall Creek Horizon shown with the -75 contour level. The contour level (-75) is shown in Figure 17. The single closed contour is v ery close to the structural top in the southern portion of the field. From an exploration viewpoint, the closed contour would indicate near top of an underlying structure. Based on this high contour level, the areal extent of the structure would not be delineated.

Figure 18. Second Wall Creek Horizon shown with the -100 contour level. The contour level (-100) is shown in Figure 18. The closed contour levels essentially straddle the axis of the anticline shown at the Second Wall Creek Horizon. The closed contours give a better indication of the areal extent of the anticline than the previous (-75) contour level. From an exploration viewpoint, the above contours present one of the best correlations seen with Electro-Seise data. The top of the structure is clearly identified both in the Northern Second Wall Creek and the Southern Second Wall Creek.