--> Abstract: Baldpate Field Exploration History, Garden Banks 260, Central Gulf of Mexico, by A. Okuma and B. Boslaugh; #90923 (1999)

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OKUMA,ANGELO, and BRIAN BOSLAUGH*,Amerada Hess Corporation, Houston,TX

Abstract: Baldpate Field Exploration History, Garden Banks 260, Central Gulf of Mexico

Baldpate field is situated 195 miles southwest of New Orleans. Louisiana, in 1650 feet of water, in the Central Gulf of Mexico. Proved and probable reserves are estimated at over 100 MMBOE covering OCS blocks 259 and 260 (Figure 1). Baldpate field is in the Plio-Pleistocene flexure trend in northeastern Garden Banks, where regional structure is dominated by elongate salt withdrawal basins and sub-horizontal salt bodies. Primary exploration targets in the area are lower Pleistocene and upper Pliocene sandstone reservoirs deposited in a slope channel/basin floor fan deepwater setting.Typical production in this trend is volatile liquids with high GOR.The seismic response of these hydrocarbon reservoirs. both modeled and observed is high amplitude zero offset negative reflections that increase with offset.

Baldpate prospect was originally identified as a seismic amplitude anomaly on a single   spec 2-D seismic line. Subsequent interpretation on a 1/4-mile grid of lines extracted from a spec 3-D seismic survey showed the prospect to be a combined structural-stratigraphic trap on the southwest flank of a large tabular salt feature (Figure I ). Since the full 3-D seismic volume would be acquired only after making a significant discovery, long cable (6000 m) 2-D seismic data was acquired on a proprietary basis over the prospect for amplitude and AVO analysis. Regional rock properties modeling matched the observed seismic character to that of a hydrocarbon reservoir.

Amerada Hess and Oryx Energy Company drilled the Garden banks 260 #1 discovery well in 1991 to a total depth of 18,322 ft. and penetrated 180 ft. of net pay.Two major reservoirs having 104 ft. and 64 ft. were production tested at a combined rate of 6,496 BOPD and 24,525 MCFGD.The GB 260 #1 sidetrack-1 was drilled to evaluate the down-dip limit of the seismic anomaly and encountered 243 ft. net pay in the same two zones.The sidetrack-1 well tested at a slightly higher combined rate of 8,144 BOPD and 18,804 MCFGD.The GB 260 #1 sidetrack-2 was drilled to test the up-dip limit of the seismic anomaly and also evaluate possible gas phase inferred from wire-line interpretation of the discovery well (Figure 2). Unfortunately. this well drilled into unexpected salt and found the reservoir sands truncated by salt (Figure 3).The GB 260 #3 was drilled in a non-amplitude supported sub-salt area southeast of the discovery well to test the idea of potential reserves that might not be imaged beneath the salt due to complex salt geometry.This well drilled into salt and emerged underneath to find the reservoir section truncated.The GB 260 #3 sidetrack-1 drilled entirely outside of salt and penetrated a full 180' net pay.The results of three successful reservoir penetrations, including conventional core recovery indicated Baldpate reservoir sands were basin floor sheet sands. having excellent lateral continuity with average porosity of 29 % and permeability of 500 milidarcies. Even though the Baldpate reservoirs are visible on 2D and 3D seismic, two of the four wells were unsuccessful, proving the misleading nature of time migration and the justification for 3D pre-stack depth migration when exploring near complex salt bodies (Figure 3).

With the successful drilling results at Baldpate, our exploration efforts continued to evaluate two satellite structures north and northeast of Baldpate prospect. Baldpate North was discovered in 1995 by the Amerada Hess operated well, Garden Banks 215 #2 which encountered 200 feet net pay in lower Pleistocene reservoirs.This satellite accumulation is fault separated from Baldpate field by a down-to-the-north fault and reserves from this field will be.drained by the pre-drilled Garden Banks 259 #3 well. In 1996, Amerada Hess drilled the Garden Banks 216 #2 well to evaluate a seismic amplitude anomaly on the northeast flank of Baldpate salt (Figures l and 2).This well penetrated 150 net feet of pay in lower Pleistocene reservoirs. Reserves from this field will be sub-sea completed to Baldpate tower.

Amerada Hess and 50% partner Oryx Energy Company. are developing Baldpate field and its satellite accumulations with a compliant tower. Baldpate tower stands 1,900 feet high, making it operationally the world's tallest freestanding man-made structure. First oil came on stream in September 1998. Current daily production is 37,000 BOPD and 132,000 MCFGD from four wells. Peak production is expected to exceed 60,000 BOPD and 200 MMCFGD.
 
 

Fig. 1. Composite structure map for major reservoirs in Baldpate (Big Sand), Balpate North (Lentic BLK3), and Penn State (Lentic UL1) fields. Maps derived from seismic depth converted with checkshot velocities and fit to well control. Geologic cross section and seismic line are shown. Only the initial Exploratory wells are shown..

Fig. 2. Seismic line from original contractor time migration 3D survey showing amplitude anomalies and salt outline. Note that wells 260#1 and 260#1st1 penetrated 180' + pay thickness at the main amplitude level while well 260#1st2 drilled into salt (see geologic cross section Fig. 3). The Penn State discovery well 216#2 is shown.

Fig. 3. Geologic structure showing major pay interval (Big Sand) in 260#1 and 260#1st1. Note that even though 260#1st2 evaluated updip amplitude, the well drilled into salt instead. Suggests the seismic amplitude was over migrated into salt.

AAPG Search and Discovery Article #90923@1999 International Conference and Exhibition, Birmingham, England