OBrienEtAl2009 PleistoceneSubmarineFan GardenBanks GOM

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Integrated 3D Seismic, Core, and Well Log Studyof an Upper Pleistocene Submarine Fan Reservoir

in the Garden Banks Area, Northern Gulf of Mexico

Sean OBrien, M. Royhan Gani, and Abu K. M. Sarwar

Department of Earth and Environmental Sciences, University of New Orleans,2000 Lakeshore Dr., New Orleans, Louisiana 70148

ABSTRACT

The Upper Pleistocene slope-apron to submarine fan deposits in the northern Gulf

of Mexico have proven to be valuable targets for hydrocarbon exploration and produc-

tion. The Field 236 of the Garden Banks area, located 170 mi (273 km) southwest of

Lafayette, Louisiana, has producing wells in these deep marine sandstones. Although

this field has been in production since the late seventies, there are no published data on

the specific depositional setting of the reservoir sandstone.

Encompassing six blocks of Field 236 with an area of 54 mi2(140 km2), this study

used a combination of proprietary and public-domain data, including 3D seismic data,

wireline logs, velocity surveys, and side-wall cores. Isopach and seismic amplitude maps

reveal that the reservoir rock of this field is part of a submarine channel-lobe complex

where individual paleogeomorphic units like meandering channel, crevasse splay, and

lobe are identifiable. The deposition took place in an east-west trending and east-sloping

minibasin, the formation of which is likely linked to deformations related to salt bodies

surrounding the area. This study also identifies a potential exploration/drilling target to

the northeast of the study area in block 193.

INTRODUCTION

Hydrocarbon discoveries have proven to be a major economic engine throughout the world including South-ern Louisiana. From the inner-continental shelf to the deepwater plays, South Louisianas geology and sedimen-tological styles have provided many avenues for hydrocarbon traps. The Upper Pleistocene slope apron and sub-marine fan deposit in the northern Gulf of Mexico is one of the best hydrocarbon plays stretching from the EastBreaks to the Grand Isle area. This play has proven reserves of 185.466 MMboe (million barrels of oil equiva-lent) of oil and condensate and 1.776 Tcf (trillion cubic ft) of gas from 128 sandstone bodies (Hentz et al., 1997).

This paper focuses on the Garden Banks area (Fig. 1), specifically Field 236 (Fig. 2), which has producinghydrocarbon wells in the Upper Pleistocene delta-fed slope-apron deposits (Galloway et al., 2000). Althoughvarious data were published on this area including stratigraphic and structural settings, there is no published res-ervoir-scale sedimentological mapping of the area using 3D seismic data. The main objective of this paper is to

investigate, using a combination of proprietary and public 3D seismic and well data, depositional elements andpotential drilling targets for continued exploration of Field 236.

OBrien, S., M. R. Gani, and A. K. M. Sarwar, 2009, Integrated 3D seismic, core, and well log study of an Upper Pleisto-cene submarine fan reservoir in the Garden Banks area, northern Gulf of Mexico: Gulf Coast Association of GeologicalSocieties Transactions, v. 59, p. 563-571.

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Integrated Study of an Upper Pleistocene Submarine Fan Reservoir in the Garden Banks Area, Northern Gulf of Mexico

By the early Cenozoic, regionally-extensive shallow water to continental siliciclastic depocenters developedin the northwestern Gulf Basin, reflecting the drainage of sediments from the Laramide Orogeny in the westernUnited States (Weimer et al., 1998; Martin, 1978). A lowering of relative sea level enhanced the amounts ofmaterial removed from the continent and inner shelf by river incision and transport (Bouma, 1981; Mossa, 1996).The sediments deposited during this time have formed important reservoirs in both offshore and onshore basins.These depocenters switched during the Miocene in such a way that the center portion of North America wasdraining through Louisiana into the north-central Gulf of Mexico Basin. Sediment loading during the Cenozoiccaused the Jurassic salt to become allochthonous having varying geometries across the Basin (e.g., Nelson, 1989).Coeval bathyal sediments were deposited in a series of minibasins on top of and between allochthonous salt bod-ies. These deep-marine systems contain the reservoirs for the northern deepwater Gulf subprovince, where trapshave formed commonly due to salt tectonics (Weimer et al., 1998).

STUDY AREA

The Garden Banks Field 236 is located roughly 170 mi (273 km) southwest of Lafayette, Louisiana in anaverage water depth of 700 ft (200 m). The field is comprised of seven blocks: 141, 191, 192, 193, 235, 236, and237 (Fig. 2), where each block is 9 mi2(23 km2). The study area, covering the entire field except block 141 (Fig.2), is 54 mi2(140 km2). The known reservoirs of this field reside in the Upper Pleistocene progradational delta-fed apron deposits (Galloway et al., 2000).

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Figure 2. Present study focuses on six blocks of Garden Banks Field 236 encompassing 54 mi2(140

km2). Black circles are well locations. Note the coverage of 3D seismic data that was shot for all six

blocks, and the location of seismic line shown in Figure 3. Side-wall cores of well 2_236 were used in

this study.


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DATASET AND METHODS

This research was conducted using a combination of proprietary and public domain data. Proprietary 3Dseismic data provided by Diamond Service Corporation covers the entire six blocks in the study area (Fig. 2).The data acquisition parameters are as follows: acquisition bin, 25 ft x 150 ft (8 m x 46 m); record length, 9.6sec; and nominal fold, 34. The public data acquired from Minerals Management Service consist of wireline logs(gamma and resistivity), check-shot velocity surveys from ~25 wells, geology reports, side-wall core descrip-tions, and some biostratigraphic reports.

Seismic and well data were loaded into GeoGraphix, Landmarks geological-interpretation software. Forthe Upper Pleistocene study interval, we mapped several horizons, generated isopach maps, and correlated welllogs, seismic, and side-wall core data. Velocity surveys of the wells were used to generate interval-velocity mapsfor the 3D survey that were instrumental in converting isochore maps to isopach maps. Techniques of seismicgeomorphology were also used to identify paleogeomorphic units by producing seismic attribute maps.

RESULTS

Five horizons, each represented as distinct reflectors, were picked in 3D seismic data (Fig. 3). Horizon 1(H1) and 5 (H5) correspond to the top and bottom of our study interval, respectively. A detailed investigation

was conducted between H3 and H4, which is a distinct package with high-amplitude reflectors and believed torepresent a potential channel-lobe complex.

Figure 3. Seismic inline 11094-1 showing five picked horizons (H1 through H5) in the Upper Pleisto-

cene study interval. Using velocity survey, side-wall core litholog of well 2_236 was correlated with the

seismic section. The sandy package between H3 and H4, the producing reservoir in well 2_236, is the

focus of this study. For location of the inline, see Figure 2.

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The package between H3 and H4 was penetrated completely by well 2_236 (i.e., well #2 in block 236), forwhich wireline logs, velocity data, side-wall core data, and geology reports are available to integrate into ourseismic observations. A sand-shale litholog was generated for well 2_236 based on the descriptions of the side-wall cores (Fig. 4). The interval between 5154 ft (1570 m) and 5430 ft (1655 m) is mostly very fine (to fine)sandstones with thinner mudstone interbeds. When correlated with gamma and resistivity logs of this well, theside-wall cores show a good lithological match (Fig. 4). Using the velocity survey for well 2_236, the litholog

was also correlated with seismic inline 11094_1 that crosses this well. The sandy interval of the litholog(between 5154 and 5430 ft [1570 and 5430 m]) nicely correlates with the package between H3 and H4 in theseismic section as shown in Figure 3.

Figure 4. Correlation of interpreted lithologic column from side-wall cores with wireline logs of well

2_236, showing a good lithological match. The top and bottom of the producing 4500 ft sand are at

depths of 5152 ft (1570 m) and 5430 ft (1655 m), respectively. For a seismic-to-well tie, see Figure 3.


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The second isopach map between H3 and H4, representing mostly sandstones (Fig. 3), can be interpreted asshowing an east-west trending channel complex at the northern side (Fig. 5B). In this isopach map, a lobe com-

plex is also interpreted to form at the eastern end of the channel complex, indicating an eastward channel-flow(i.e., minibasin slope). The existence of channel-lobe complex is also supported in a total amplitude map slicingthrough the middle (and parallel to H4) of the package between H3 and H4 (Fig. 6). This amplitude map is inter-

preted to show a meandering to straight channel with a distinct crevasse splay in the middle, and a lobe complex

in the north. This channel and lobe are also discernable in a related seismic section (Fig. 6).

Figure 6. Total amplitude map (upper image) slicing through the middle (and parallel to H4) of the

package between H3 and H4 (lower image), revealing a meandering to straight channel with a distinct

crevasse splay, and a lobe complex. This channel and lobe can also be seen in the corresponding

crossline (lower image).


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DISCUSSION AND CONCLUSIONS

Although the Field 236 in Garden Banks has been in production since the late seventies, there is no pub-lished data on the specific depositional setting of the reservoir sand. The field is mentioned in theAtlas of North-ern Gulf of Mexico Gas and Oil Reservoirs, volume 2, where the deposits are broadly identified as an UpperPleistocene submarine fan sandstone (Hentz et al., 1997). The channel-lobe complex identified in this study isknown as the producing 4500 ft sand in the geological report of well 2_236. According to this report, the 4500ft sand is an Upper Pleistocene sand-package found in the TrimosinaA faunal zone. In well 2_236, the top and

bottom of the 4500 ft sand are at depths of 5152 ft (1570 m) and 5430 ft (1655 m), respectively.Using public well data and proprietary 3D seismic data, we were able to map the producing 4500 ft sand of

the Field 236 and interpet its submarine channel-lobe depositional elements. We feel that this approach is effec-tive, yet quick, for mapping and verifying depositional environments of reservoir rocks in producing fields. Withrecent technical advances in 3D seismic acquisition, processing, and interpretation, the cost of these data has de-creased substantially and the usage, particularly in the field of seismic geomorphology (Posamentier, 2003; Saw-yer et al., 2007), has increased noticeably in the past few years.

This study also reveals a potential exploration/drilling target to the northeast of the study area in block 193.As shown in Figure 5B, a thick channel-lobe sandbody in this area has not been tested by drilling activity. Afuture extension of this study in block 193 could provide more definitive information on the extent and nature ofthis channel-lobe sandbody and could rank drilling targets.

ACKNOWLEDGMENTS

We would like to thank Diamond Service Corporation for the donation of the 3D seismic data used in thisstudy. Thanks are also due to Landmark for the donation of the Geographix software to University of New Or-leans that was used to interpret seismic and well log data.

REFERENCES CITED

Bouma, A. H., 1981, Depositional sequences in clastic continental slope deposits, Gulf of Mexico: Geo-Marine Lettersv. 1, p. 115-121.

Buffler, R. T., 1991, Early evolution of the Gulf of Mexico Basin, inD. Goldthwaite, ed., An introduction to centralGulf Coast geology: New Orleans Geologic Society, Louisiana, p. 1-15.

Galloway, W. E., P. E. Ganey-Curry, X. Li, and R. T. Buffler, 2000, Cenozoic depositional history of the Gulf of Mex-ico Basin: American Association of Petroleum Geologists Bulletin, v. 84, p. 1743-1774.

Hentz, T. F., S. J. W. Seni, and E. G. Wermund Jr., eds., 1997, Atlas of northern Gulf of Mexico gas and oil reservoirs:Volume 2, Pliocene and Pleistocene reservoirs: Texas Bureau of Economic Geology, Austin, Texas, 203 p.

Martin, R. G., 1978, Northern and eastern Gulf of Mexico continental margin: Stratigraphic and structural framework:American Association of Petroleum Geologists Studies in Geology, v. 7, p. 21-42.

Mossa, J., 1996, Sediment dynamics in the lowermost Mississippi River: Engineering Geology, v. 45, p. 457-479.

Nelson, T. H., 1989, Style of salt diapirs as a function of the stage of evolution and the nature of the encasing sedi-

ments: Proceedings of the 10th Annual Gulf Coast Section of the Society of Economic Paleontologists and Miner-alogists Foundation Research Conference, Houston, Texas, p. 109-110.

Posamentier, H. W., 2003, Depositional elements associated with a basin floor channel-levee system: Case study from

the Gulf of Mexico: Marine and Petroleum Geology, v. 20, p. 677-690.

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Sawyer, D. E., P. B. Flemings, R. C. Shipp, and C. D. Winker, 2007, Seismic geomorphology, lithology, and evolutionof the late Pleistocene Mars-Ursa turbidite region, Mississippi Canyon area, northern Gulf of Mexico: AmericanAssociation of Petroleum Geologists Bulletin, v. 91, p. 215-234.

Weimer, P., M. G. Rowan, B. C. McBride, and R. Kligfield, 1998, Evaluating the petroleum systems of the northerndeep Gulf of Mexico through integrated basin analysis: An overview: American Association of Petroleum Geolo-

gists Bulletin, v. 82, p. 865-877.


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OBrienEtAl2009 PleistoceneSubmarineFan GardenBanks GOM