Multiple Fields within the Sequence Stratigraphic Framework of the Greater Auger Basin, Gulf of Mexico

Michael C. Dean, James R. Booth, and Bruce T. Mitchell: Shell Exploration and Production Company

Extended Abstract

The greater Auger Basin lies approximately 220 miles south southwest of New Orleans, Louisiana in the deep water Gulf of Mexico. Multiple exploration and development wells from five Shell producing fields in the greater Auger Basin have provided sequence stratigraphic information from several different basin settings and different seismic facies (e.g., Prather et al., 1998). Two of these, the proximal Auger Field and the distal Macaroni Field, have been used by Booth et al. (2000) to classify the overall seismic stratigraphic framework of the basin. Shell's producing fields in the greater Auger Basin are Auger, Cardamom, Macaroni, Oregano, and Serrano. Though these fields lie with the same basin-scale sequence stratigraphic framework, their depositional settings vary considerably. The present-day Auger salt dome, east Auger fault, and north Auger fault partition the greater Auger Basin into two sub-basins. At the time of deposition of many of the producing reservoirs, these structural features have manifested themselves in the form of the northwest-southeast trending Auger ridge. Macaroni, Oregano, and Auger Fields lie south of these structural features, in the Auger Basin proper and are primarily oil-bearing. Serrano and Cardamom lie northeast of these features in the Andros Basin and are primarily gas bearing.

Auger

Auger Field lies at the entry point to the Auger Basin. The primary reservoirs at Auger are, from top to bottom, the Yellow, Blue, Green, Golden Brown, and Pink sands. The older sands are primarily oil bearing while the younger Yellow and Blue sands are primarily gas-bearing. The gas-bearing sands tend to intersect the east Auger fault, but the oil-bearing sands pinch out before the fault. During much of Auger's depositional history, the east Auger fault and Auger dome have formed a northwest-southeast trending ridge. As sediments topped the ridge, they are deposited on the southern edge as a set of point-sourced, healing-phase slope fan systems (Booth et al., 2000). These systems manifest themselves as sheet-like deposits basinward; the more channelized features are in an updip, proximal position. Furthermore, the general stacking pattern within one depositional cycle is from more sheetlike at the base to more channelized at the top of the section. Large along-strike variations in sand thickness and quality can be seen, particularly in the proximal portions of the reservoirs.

Oregano

Oregano lies approximately six miles south-southeast of Auger. It consists of two primary reservoirs; the basal, sheet-like K3 sand topped by the more channelized K1 sand. Both sands are oil bearing and lie in the Auger Blue interval. Although the sands are probably not directly correlable, data suggest that the K1 may be in communication with Auger on a production time scale. The Oregano sands appear to be sourced from multiple directions. The K3 sand is sourced primarily from a basin entry point and is a healing-phase proximal fan (Mitchell and Dean, 2001). The updip portion of the sand consists of high energy channel features, but the downdip portion is more sheet-like. The K1, and possibly parts of the K3, are deposited not as an entry point system, but rather as a lateral margin sands.

Macaroni

Macaroni lies at the exit point of the Auger Basin and consists primarily of ponded sheet sands that have high internal correlability and are topped by more chaotic channelized features. The basin is salt bounded and the southernmost salt forms a backstop for sheet sand deposition. The more channelized nature of the upper reservoirs occurs as a result of cut and fill features as sediments overtop the basin bounding sill and exit into the Tampa Basin (Booth et al., 2000). Chaotic facies are more prevalent in the upper part of the section and in the updip, distal portions of the reservoirs. The Macaroni reservoirs are primarily oil-bearing and represent an expanded Auger equivalent stratigraphic interval.

Cardamom

Cardamom lies northeast of the east Auger fault and paleo-Auger rdge on the eastern flank of the Auger dome. The depositional history of Cardamom is the most problematic. Some of the sands at Cardamom appear to be more sheetlike and lie near a sub-basin exit point as described at Macaroni. However, there are indications that some sands could be the result of eastward spill from the Auger depositional point source or even highly proximal channelized deposits that were continuous with Auger sands prior to emplacement of Auger dome. The Cardamom reservoirs are primarily gas bearing and correlate directly to the Auger Yellow and Green reservoirs. Sands on the Cardamom side appear to be fairly channelized in the upper part of the section. Furthermore, erosional cut and fill features appear to isolate Cardamom from the equivalent interval at Serrano, to the southeast.

Serrano

Serrano lies just southeast of Cardamom, on the northeast side of the east Auger fault and paleo-Auger ridge. Serrano is primarily gas bearing and is distinct from the other fields in that most of the proven reservoirs are significantly younger than even the shallowest Auger producing zones. This is due to the significant expansion of the younger Pleistocene section on the downthrown side of the east Auger fault. The reservoirs at Serrano occur both in chaotic seismic facies (SE 55 and SE 32 sands) and in regionally continuous seismic draping events (SE 40 sand) (Dean et al., 2000). The chaotic facies consist of both sand-filled channels and sheet sands that are cut by bypass channels. In both the chaotic and draping seismic facies, the occurrence of reservoir quality sands at Serrano is largely the result of the east Auger fault and basin bounding Andros salt ridge forming a backstop for locally ponded deposition.

Summary

The Greater Auger basin is an excellent case study for variations in the deposition and preservation of reservoir quality sands in multiple basin settings. Although the mechanisms differ in different settings, productive sands can be found in proximal, distal, and lateral settings within a basin. In each setting, however, deposition is controlled both by sediment source and accommodation space. Each of these features is illustrated within the producing fields of the Greater Auger Basin.

References

Prather, B.E., J.R. Booth, G.S. Steffens, and P.A. Craig, 1998, Classification, lithologic calibration, and stratigraphic succession of seismic facies of intraslope basins, deep-water Gulf of Mexico: AAPG Bulletin, v. 82, no. 5a, p. 701-728.

Booth, J.R., A.E. Duvernay III, D.S. Pfeiffer, and M.J. Styzen, 2000, Sequence stratigraphic framework, depositional models, and stacking patterns of ponded and slope fan systems in the Greater Auger Basin: Central Gulf of Mexico slope: GCSSEPM Foundation 20th Annual Research Conference.

Dean, M.C., J.R. Booth, and K.M. King, 2000, The Present is the Key to the Plio-Pleistocene: Seafloor Analogues at Shell's GB 516 Serrano Discovery: GCSSEPM Foundation 20th Annual Research Conference.

Mitchell, B.T., and M.C. Dean, 2001, A "Healed Slope" Model for the Deposition of Turbidite Reservoirs Applied to Shell's Zia an Oregano Discoveries in the Deep Water Gulf of Mexico: GCSSEPM Foundation 21st Annual Research Conference.