Stratigraphic Characteristics and Sand Distribution of the Hackberry Wedge (Middle Oligocene, Frio Formation), Southeast Texas and Southwest Louisiana: A Sand-Rich Slope-Fan Complex

DIMARCO, MICHAEL J., Shell Western Exploration and Production, Houston, TX, and R. CRAIG SHIPP, Shell Development Company, Houston, TX

The Hackberry wedge has been long recognized by Gulf Coast geologists by its anomalous deep-water fauna, rapid lateral variation

in sand thicknesses, and prominent basal erosional unconformity. Hackberry sandstones also serve as major hydrocarbon reservoirs in many fields discovered to date. A vertical succession through the Hackberry typically shows the basal erosional surface overlain by a variably sandy interval, informally termed the lower Hackberry sands, and capped by a thick sand-lean, deep-water shale interval, the Hackberry shale.

High-quality seismic data indicate that the Hackberry has a distinctive seismic signature. This seismic signature permits the interpretation of Hackberry lithologic characteristics in a sequence stratigraphic framework. Key elements of the seismic signature include (1) a series of half-graben-like slumps, marking the updip limit of the Hackberry wedge and representing the failure of an immediately preexisting shelf edge, (2) a basal erosional surface, in some places channelized as deep as 1800 ft and cutting into the Eocene, representing a prominent sequence boundary upon which the Hackberry was deposited, and (3) a pronounced downlap surface with well-developed suprajacent clinoform geometries, representing a maximum flooding surface within the Hackberry shale.

Most Hackberry sand is confined to the lower Hackberry sand interval immediately above the sequence boundary. The Hackberry sand isopach shows numerous linear to tortuous sand thicks separated by areas of little or no sand. In some places, linear thicks can be related to eroded and channelized slope paleotopography that is discernible seismically. Elsewhere, linear sand thicks are not associated with a clear erosional expression on seismic records and may represent broadly linear, aggradational turbidite fills in paleolow positions. Other, more irregular to ovoid-shaped sand patterns represent ponded aggradational deposits in intraslope paleolows and basins. Paleontologic data strongly suggest that this system was deposited at the time of the largest middle Oligocene sea level lowstan shown on the Haq et al. (1987) coastal onlap curve. Deposition of lower Hackberry sands commenced when fluvio-deltaic systems bypassed the foundered shelf edge and sediment-gravity flows ensued through a tortuous network of upper slope channels, gullies, and other paleolows. Collectively, these deposits represent the upper and middle parts of a sand-rich slope-fan complex.

AAPG Search and Discovery Article #91006 © 1991 GCAGS and GC-SEPM Meeting, Houston, Texas, October 16-18, 1991 (2009)