George D. Klein1 and Kenneth R. Chaivre2
Search and Discovery Article #10045 (2003)
*Adapted from presentation by the senior author at East Texas Geological Society TECH 2003, April, 2003, and an earlier version published in GCAGS Transactions, 2002 (Klein and Chaivre, 2002).
1SED-STRAT Geoscience Consultants, Inc., Sugar Land, TX, 77479-6238 ([email protected])
2ConocoPhillips Company, Houston, TX
The Bossier Formation of the western East Texas basin consists of two sequences separated by a major sequence boundary (SB-2). Its base and its top are defined by two sequences, SB-1 and SB-3, respectively. The former separates the Bossier from the Gilmer Lime (Cotton Valley Lime), and the SB-3 separates the Bossier from the overlying Cotton Valley Sand. SB-2 was identified in seismic sections by tracing seismic reflectors and geometries representing basin floor and slope fans. This boundary on the shelf is below a series of stacked deltaic sands. In well log sections, basin floor fan log patterns were traced laterally into slope fan and stacked delta log signatures. The fans represent a lowstand systems tract; the lower Bossier, a transgressive systems tract; and the upper Bossier, a prograding complex.
Analysis of burial history suggests that the lower Bossier accumulated when the East Texas basin was underfilled. Lowering of sea level associated with SB-2 is related to a major climate shift from tropical to cooler conditions that favored a rapid influx of sands from the ancestral Mississippi, Ouachita, and Red River systems. These sands formed within prograding deltaic packages, outer incised valley fill stacked deltas, and submarine fan systems. The stacked deltas and basin fan sand systems represent prospective gas plays.
Figure 1. (Left) Bossier study area on map of East Texas Basin showing regional tectonic features (after Montgomery and Karlewicz, 2001). (Right) Isopach map of Bossier Formation, in seven county area and location of Well Log Section 6-6’ (Figure 5). Arrows show interpreted primary route of sand feeder systems. (0–1,000' isopach—Shelf; 1,000–1,500' isopach—Slope; >1,500' isopach—Basin Floor).
Figure 4. Stratigraphy of Bossier Formation: Two approaches. (Left) Columnar section showing intertonguing interpretation of upper Bossier contact (after Montgomery and Karlewicz, 2001). (Right) Sequence stratigraphic model for Bossier Formation (from Williams and Mitchum, 1997).
Figure 5. NW-SE dip-line Well Log Cross-Section 6–6’, Bossier Formation, Freestone and Leon Counties, Texas (Figure 1 (Right) for location). Division of the Bossier into lower and upper units by recognizing sequence boundaries SB-1, SB-2, and SB-3. Dark Reddish Orange--basin floor fan; Green--levee-channel fan complex; Yellow--deltas.
Figure 6. Interpreted, updip part of NW-SE seismic dip line. Segpolar display (published with permission of Seismic Exchange, Inc). Division of Bossier Formation is shown, along with shelf edge, salt features, and seismic stratigraphic features.
Figure 7. Interpreted, downdip part of NW-SE seismic dip line. Segpolar display (published with permission of Seismic Exchange, Inc). Division of Bossier Formation is shown, along with seismic stratigraphic features.
Figure 10. Map of depositional systems of upper Bossier Formation (SB-2 to SB-3), East Texas Basin, Texas (after Ewing, 2001, Figure 3). Orange (sandy deltaic or fluvial), yellow (sand-rich shorelines), light blue (carbonate ramps), gray (basinal shales), olive (submarine fan sands), light blue green line (updip limit of Jurassic).
Figure 11. Map of seven county area, East Texas Basin, showing depositional units defining prospective plays in the Bossier Formation and location of well log cross-sections in red) used in this study.
Figure 12. Burial history and tectonic subsidence curves, depocenter, East Texas Basin, Texas. Curve derived from stratigraphic sonic log markers at southeast end of dip-oriented seismic line (Figure 7).
Figure 13. Average global temperature during the Phanerozoic and part of the Precambrian, with one of the changes from warm to cool during the latter part of the Jurassic (from Scotese, 2000; www.scotese.com)
Figure 16. East Texas Bossier play, as of 2002, showing fields, discovery wells, pipeline, and Anadarko Petroleum Corporation acreage (from J.N. Seitz, Banc of America, Security Energy and Power Conference, with permission of Anadarko Petroleum Corporation).
1). Establish sequence stratigraphic framework.
2). Identify major sand facies.
3). Map sand facies and identify potential exploration fairways.
4). Tie stratigraphy and facies to basin history.
• Detailed interpretation of well-log cross-sections (13 lines, 151 wells)
• Detailed analysis of 13 seismic lines (~840 Mi.)
• Identify all major sequence boundaries and systems tracts (methods of Mitchum et al., 1995; Van Wagoner et al.,1990).
• Map log facies and seismic facies.
• Identify best prospective fairways.
Regional to Global Setting (Figures 1, 2, and 3)
Characteristics of time during deposition of Bossier Formation include:
• Global high stand of sea level (Kimmeridgian)
• Time of generation of major source beds
• Tropical/humid paleoclimate
• Deep weathering - shale-dominated system; Bossier - Low net/gross sand
The Bossier Formation is divided into two sequences by a mid-Bossier sequence boundary (SB-2) (Figures 5, 6, 7, 8, and 9).. The base and the top of the Bossier are also delineated by sequence boundaries (SB-1 and SB-3, respectively).
The lower Bossier Formation (below SB-2) formed during rapid subsidence when mud was the dominant sediment deposited in the basin. On the other hand, the upper Bossier was deposited after a drop of seal level. The basin then was characterized by areas of deep water fan sedimentation as well as shelf to shelf-edge, stacked deltas, which together represent the fairways of the Bossier play (Figures 9, 10, and 11).
• Bossier was deposited during time of rapid mechanical subsidence.
• Deposition could not keep up with subsidence until sea level dropped (SB-2).
• PGC (prograding complex) formed during time when deposition kept up with or exceeded subsidence rate.
• SB-2 may signify possible climate change favoring influx of sand (Figure 13).
In mud-dominated systems, a drop in sea level will extend fluvial systems basinward. An associated change in climate is the driver that disperses sand into such settings.
The exploration potential of this finding opens up many new sandy reservoir possibilities in mud-dominated systems.
Within the shelf region (mostly in Freestone County), the upper Bossier Formation is characterized by sandy deltaic units that organized as combined progradation, aggradational, stacked units composing a major gas producing fairway system.
Within the slope, well developed basin floor fans, slope fans, and channel/levee complexes define a potential deep-water gas-producing fairway system.
In the East Texas basin, deposition of the Bossier occurred during a time of relatively rapid fault-controlled subsidence, modified by salt movement. These developments, when combined with a lowering in sea level and moderate climate change resulted in the distribution of sand reservoirs in the shale-prone Bossier system.
Some of the factors in determining the future of the Bossier play are:
• Gas price: > $4.00 to $4.50
• Acreage Costs
• Engineering at depths > 19,000 ft.
Figure 16 shows the occurrence of major gas fields of the Bossier Play in the East Texas basin, as of 2002. Our analysis shows that the Nan Su Gail, Dowdy Ranch, Mimms Creek and Dew Fields occur within Highstand Systems Tracts, whereas the Bear Grass field occurs in a Transgressive Systems Tract. Bald Prairie field represents a Lowstand Systems Tract field. Application of the sequence stratigraphic creaming curve (Figure 15) suggests that the current status of the Bossier play is presently at the nick point between the Transgressive Systems Tract and the Lowstand Systems Tract. Likely, the Bossier Play has an estimated four to five years left.
ConocoPhillips Company (Phillips Petroleum Company at the time of this study [David R. Fox, Scott McCullough, Lisa Ashabranner, Campbell Huntington]).
John Havens, Seismic Exchange Inc.
Doug Barnhouse, G.A.S Unlimited.
C.R. Scotese (www.scotese.com).
Robert W. Stancil, Anadarko Petroleum Corporation.
Norman C. Rosen, GCSSEPM Foundation.
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