SHIPP, R. C., Shell Development Company, Houston, TX; and M. J. DIMARCO, Pecten International Company, Houston, TX

ABSTRACT: The Mid-Oligocene Hackberry Depositional System: A Sand-Rich Slope-Fan Complex

The Hackberry depositional system in southeast Texas and southwest Louisiana is recognized for its significant hydrocarbon reservoirs, anomalous deepwater fauna, and rapid lateral variation in sandstone thickness. A typical vertical succession within the Hackberry interval consists of a basal unconformity overlain by an interbedded sandstones and shales, informally termed lower Hackberry interval, which is capped by a downdip-thickening, fine-grained upper Hackberry interval, the deepwater Hackberry shale. High-quality seismic data display a distinctive seismic signature for the middle of the Oligocene, which includes the Hackberry system. Components of this signature are: (1) retrogradational rotational slides representing failure of the pre-existing shelf edge; (2) a prominent uncon ormity that is the basal Hackberry sequence boundary; and (3) a pronounced downlap surface on top of the lower Hackberry interval with well-developed suprajacent clinoform geometries.

Most of the sandstones in the Hackberry system are confined to the lower Hackberry interval. An isopach map of net sandstone thickness shows numerous linear to ovoid-shaped thick areas, separated by little or no sandstone deposition. Commonly, dip-oriented linear depocenters are related to eroded and channelized slope paleotopography on seismic data. Ovoid-shaped depocenters represent ponded aggradational deposits in intradomal minibasins. Deposition of lower Hackberry sandstones commenced, when remobilized fluvio-deltaic sediments bypassed the rotationally failed shelf edge. Sediment-gravity flows were focused through a tortuous network of upper slope fairways and interdomal basins. Collectively, these deposits represent the upper to middle portions of a sand-rich slope-fan complex. ower Hackberry sandstones appear to become more laterally continuous basinward, particularly in the present-day downdip portion of the onshore southwest Louisiana trend.

AAPG Search and Discovery Article #90987©1993 AAPG Annual Convention, New Orleans, Louisiana, April 25-28, 1993.