Geologic Model of a Small, Intraslope Basin: Garden Banks 72 Field, Offshore Louisiana

R. A. Kolb, J. N. Tuller, M. H. Link, G. Shanmugam

Garden Banks 72 field is 115 mi off the Louisiana coast and lies near the shelf-slope break in water depths ranging from 450 to 800 ft. During the middle Pleistocene, the area was the site of a small, restricted basin on the upper slope, into which turbidite sandstones were deposited. These sandstones have been slumped, uplifted, and faulted, forming oil and gas traps in the field.

Mobil and partners AGIP and Kerr-McGee leased block 72 in 1984. Three wells and two sidetracks have been drilled, discovering oil and gas in middle Pleistocene sandstones. A total of 650 ft of core was cut in two wells. Geologic data in the block have been supplemented by 2-D and 3-D seismic surveys.

Trapping mechanisms in the field are both structural and stratigraphic. The structural high is on the southwest flank of a northwest-southeast-trending shale/salt ridge. The middle Pleistocene reservoir sandstones trend northeast, and their seismic signature consists of discontinuous, hummocky reflections; the presence of hydrocarbons in these sandstones causes anomalous seismic responses. Amplitude terminations often cross structural contours, implying stratigraphic pinch-outs.

Data from electric logs, seismic, and cores demonstrate that the middle Pleistocene reservoir sandstones are the result of deposition by turbidites into a small, restricted basin. Associated facies identified include channels, levees, and possible sheet (lobe) sandstones. Postdepositional activity has included slumping and reworking by bottom currents (contour currents?). The resulting depositional model for this field can be applied to many of the recent discoveries in the Flexure trend.

AAPG Search and Discovery Article #91022©1989 AAPG Annual Convention, April 23-26, 1989, San Antonio, Texas.