A Geologic Review of the Mahogany Subsalt Discovery: A Well That Proved a Play*
(The Mahogany Subsalt Discovery: A Unique Hydrocarbon Play, Offshore Louisiana**)
Holly Harrison1, Dwight ‘Clint’ Moore2, and Peggy Hodgkins3
Search and Discovery Article #60049 (2010)
Posted April 28, 2010
*Adapted from presentation at AAPG Annual Convention, 1995, and from **extended abstract prepared for presentation at GCSSEPM Foundation 16th Annual Research Conference, “Salt, Sediment and Hydrocarbons,” December 3-6, 1995.
Extended abstract used with permission of GCSSEPM Foundation whose permission is required for further use.
Appreciation is expressed to GCSSEPM Foundation, and to Dr. Norman C. Rosen, Executive Director, for permission to use it in this adaptation.
1 Phillips Petroleum Company, Bellaire, TX; currently BP, Houston, TX ([email protected])
2 Anadarko Petroleum Corporation, Houston, TX; currently ION Geophysical Corporation, Houston, TX ([email protected])
3 Amoco Production Research, Tulsa, OK; currently Veritas Hampson-Russell, Calgary, AB ([email protected])
The Mahogany subsalt discovery of Phillips Petroleum Company, in partnership with Anadarko Petroleum Corporation and Amoco Production Company, is the petroleum industry's first commercial subsalt oil development in the Gulf of Mexico. Located 80 miles offshore Louisiana on Ship Shoal Blocks 349 and 359, the Mahogany #1 (OCS-G-12008) was drilled in 375 ft of water to a depth of 16,500 ft and tested both oil and gas below an allochthonous salt sheet. The discovery well tested 7256 BOPD and 7.3 MMCFD on a 32/64" choke at 7063 PSI flowing tubing pressure (FTP). The #2 delineation well (OCS-G-12008) was drilled from the same surface location to a depth of 19,101 ft MD (18,572 ft TVD). A different zone in this well was tested in July, 1994, and flowed 4366 BO and 5.315 MMCFD on a 20/64” choke at 6287 PSI FTP. These flow rates suggest that high sustainable production rates can be expected, and they are confirmed by rock property studies and detailed well log analysis. A third well (OCS-G-12010 #2) was spud in September, 1994.
The primary subsalt reservoir is a high-pressured oil sand with high permeability and porosity and has tremendous deliverability. The field is located 80 miles offshore Louisiana on Ship Shoal South Additions blocks 349/359. The structure is interpreted as a faulted anticline overlain by allochthonous salt. Prestack depth-migrated 3-D seismic data was integrated into a regional geologic model that was based on 2-D time-migrated data. Regionally, the area is characterized by multiple salt sheets, which form a salt canopy sutured east of Mahogany, and several older and deeper sheets are also identified. Structural and rheological aspects of the thick salt sill have been addressed using selected examples of rotary sidewall cores and data on an anomalous "gumbo" shale immediately below the salt which contributes to the understanding of lateral variations at the base of the allochthonous salt.
Subsalt depositional fairways can be approximated by mapping relative salt-induced paleo-bathymetry. Deepwater sand fairways are closely related to salt movements and extend under the salt sheets. Depositional environments and reservoir parameters in productive sandstone intervals have been defined using whole core and well log imaging.
Exploration for hydrocarbons below salt in the Gulf of Mexico is not a new idea. Overhangs and shoulders on salt diapirs have been known and drilled since the 1920s. What is new however, is the concept of drilling through laterally continuous salt sheets that extend vertically and horizontally from deep feeder stocks. The subsalt play in the Gulf of Mexico has been active since the early 1980s (Moore and Brooks 1995). New techniques in seismic processing, such as 3-D prestack and poststack depth migrations, allow better imaging of the subsalt section. Advances in the geologic understanding of allochthonous salt sheet emplacement and deformation coupled with new seismic processing and drilling technology has lowered the risk of exploration below salt sheets in the Gulf.
In 1990, Exxon announced a subsalt discovery in water depths over 4300 ft deep in Mississippi Canyon block 211 named Mickey. Although this was the first discovery of the subsalt play, it was the Mahogany discovery in 1993 that sparked the play to new heights because it was the first commercial discovery (Figures 1 and 2). The field's commerciality is enhanced by its proximity to existing shelf infrastructure and moderate (370 ft) water depths.
This article discusses the regional geologic setting of the subsalt reservoirs and the relationship of depositional fairways to paleo-bathymetric geometry as a function of paleo-salt distribution (Figures 3, 4, 5, 6, 7, and 8). Integration of the regional 2-D seismic data with the much more localized 3-D prestack depth-migrated data was critical to mitigating risk for the prospect. Pressure and temperature gradients found at Mahogany were predicted from regional well control and other subsalt well analogs, and the results were generally typical of subsalt wells. The low-resistivity character of the primary target sand is due to fine laminations of sand and shale which are illustrated by Formation Micro Imaging logs and is also observed in other reservoirs deposited in deepwater environments (Shew et al., 1994, Darling and Sneider, 1993).
Mahogany is on trend with Bullwinkle, Boxer, and Green Canyon 18 fields near the edge of the shelf/slope break (Figure 2). The field is below a large tabular salt sheet that converges with another salt sheet farther to the east at approximately equivalent depths. The Ewing Bank Thrust (Figure 2) is located along the leading edge of the eastern salt sheet and is one of the first thrust faults to be documented in the Gulf (Huber, 1989). Previous drilling in the Mahogany area was for seismic amplitude anomalies and structural closures above the salt, but no significant reserves were found. The Mahogany discovery was drilled about 6 miles from the southern edge of the salt sheet. Currently there are more subsalt penetrations through this salt sheet than in any other tabular salt sheet in the Gulf.
Prior to the emplacement of the allochthonous salt sheets, the deep salt roots that ultimately fed the shallow salt sheets probably had local bathymetric relief updip (north) of the Mahogany area. Seismic data show that there may be several generations of salt sheets at different levels within the sedimentary column near Mahogany. For example, Green Canyon 18 field (Figures 7 and 8) is located south of the Mahogany field in a depositional fairway which loaded and deformed a salt sheet that is now buried much deeper than the Mahogany salt sheet. Geologic models of salt sheet emplacement (Fletcher et al., 1995; Wu et al., 1990) and biostratigraphic data in the study area demonstrate that the Mahogany allochthonous salt sheet probably formed within the lower bathyal to abyssal environment. Sand fairways in slope environments are primarily controlled by local and regional bathymetric variations. Basin depocenters were also variable as multiple generations of salt sheets impacted sand fairway distributions. At Mahogany, it was possible to map the older depositional fairways and extend them under the salt sheets once the deeper salt features were identified. The strike orientation of salt and basin distribution is critical to fairway prediction. After sand deposition, allochthonous salt flow at shallow depths below the sea floor was triggered and spread laterally about 8-12 miles downslope, blanketing the area.
The structure at Mahogany is interpreted as a 3-way dipping anticline (Figures 9, 10, 11, 12, and 13). The closure on the northwest flank may be due to faulting. Further seismic processing will aid in resolving this portion of the structure. Smaller faults have been identified in the wells which complicate the structure. The discovery well drilled in Ship Shoal 349 encountered very little sand above the salt and had suprasalt temperatures and pressures typical for the area (Figure 14). The well drilled a continuous salt section over 3500-ft thick which contained minor sedimentary inclusions, several of which exhibited oil and gas shows (Figures 14 and 15). Rotary sidewall cores were taken in the salt and analyzed for viscosity and strain rates. These data were used to help engineer casing designs and evaluate salt creep.
Although salt is an incompressible rock and therefore pore pressures are constant, the mud weight during drilling was increased in the salt interval to control the gas liberated from the sediment inclusions and also in anticipation of drilling higher pressures below the base of salt. The sedimentary section immediately below salt is a high-pressured "gumbo" with pore pressures that may exceed 17 pounds per gallon mud-weight pressure gradients (0.88 PSI/ft). The pressure gradient in this subsalt layer regresses with depth until a more regional gradient is achieved, although still geopressured (Figure 16). Salt has a high thermal conductivity and the temperature gradients within the salt are low (0.26 degree F/100 ft). There is a low temperature gradient zone below the salt, but temperature gradients gradually increase again with depth.
The primary subsalt pay sand was flow tested in two stages. DST 1 from perforations of a low resistivity interval flowed 3700 BO and 550 MCFD (Figures 17, 18, and 19). Perforations at the base of the sand (from coarser, thicker bedded subunit) were then added, and the commingled flow rate was 7256 BO and 7.3 MMCFD on a 32/64" choke at 7063 PSI flowing tubing pressure (Figures 17, 18, 19, 20, and 21).
The low resistivity log response of the pay sand is due to fine interlaminations of sand and shale (Figures 20 and 21). The Formation MicroImaging log and percussion sidewall cores reveal individual sand laminae less than 0.25 in. thick; yet the overall laminated sequence is capable of significant flow rates.
Overall, the sand has a fining-upward texture with a more coarse-grained and thicker bedded layer at the base (Figures 22 and 23), overlain by an extensive rippled and highly laminated, low-resistivity sand and silt interval (Figures 24, 25, 26, 27, and 28), with channel and laminated sands at the top (Figures 29 and 30).
Phillips Production Company and partners Anadarko Petroleum Corporation and Amoco Production Company have drilled three oil wells at Mahogany-a straight-hole discovery well, and two directional wells are drilled-to the northeast and to the southwest. A fourth delineation well was spudded in May, 1995. Development plans include installing a platform with a production capacity of 45,000 BOPD and 100 MMCFD. First production is scheduled in December, 1996, with anticipated initial flow rate of about 22,000 BOPD and 30 MMCFD, making it the first subsalt oil development in the Gulf of Mexico.
The major conclusions about the Mahogany Subsalt Discovery are:
Darling, H.L., and R.M. Sneider, 1993, Productive low resistivity well logs of offshore Gulf of Mexico: causes and analysis, in Moore, D.C., ed., Productive low resistivity wcl110gs of the offshore Gulf of Mexico: New Orleans Geological Society and Houston Geological Society Special Publication, p. B-1 - B-26.
Fletcher, R. C., M. R. Hudec, and I. A. Watson, 1995, Salt glacier and composite sediment-salt glacier models for the emplacement and early burial of allochthonous salt sheets, in M. P. A. Jackson, D. G. Roberts, and S. Snelson, eds., Salt tectonics: a global perspective: AAPG Memoir 65, p. 77-108.
Moore, D.C., and R. Brooks, 1995. The evolving exploration of the sub-salt play in offshore Gulf of Mexico: GCAGS Transactions, v. 20. p. 20.
Huber. W.F., 1989, Ewing Bank thrust fault zone Gulf of Mexico and its relationship to salt sill emplacement: GCAGS Transactions, v. 39. p. 60-64.
Shew, R.D., D.R. Rollins, G.M. Tiller, C.J. Hackbarth, and C.D. White. 1994, Characterization and modeling of thin-bedded turbidite deposits from the Gulf of Mexico using detailed subsurface and analog data: Gulf Coast Section Society of Economic Paleontologists and Mineralogists (GCSEPM)15th Annual Research Conference. p. 327-334.
Wu, S., A.W. Bally, and C. Cramez, 1990, Allochthonous salt, structure and stratigraphy of the north-eastern Gulf of Mexico, Part II: Structure: Marine and Petroleum Geology, v. 7. p. 334-370.
Used with permission of GCSSEPM Foundation whose permission is required for further use.
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