Growth Faults in Oligocene Deltaic Sediments and Their Influence on Hydrocarbon Accumulation in Oil Fields of Upper Assam, Northeastern India

Dibakar Goswami, Carl F. Vondra

Accumulations of oil and gas have been found in the Oligocene sediments represented by the Barail Group in the Upper Assam, northeastern India. These deltaic sediments are composed of a thick sequence of an arenaceous member resting on bluish gray shale (the Eocene Kopili Formation) followed by an alternating sequence of sandstone and shale/mudstone/coal.

Seismic and subsurface geologic evidence has shown the presence of a series of growth faults in a down-basin direction partitioning the basin into a series of subparallel fault blocks. The faults, which become listric with depth, trend northeast-southwest. Some of the faults possess a slanted S-shaped profile. These faults have steep zones (50°-70°) near the top and gradually shallow at depth, with sequences on the downthrown block overthickened. Structural closures on the downthrown side of the growth faults afford an ideal condition for hydrocarbon entrapment. Multiple reservoirs are present that may consist of one or more elliptical to crescent-shaped bar sands in the arenaceous sequence or isolated sand lenses in the upper Barail Group. The structural closure of the bar ands is generally greater than that of the sand lenses.

The growth faults in the Upper Assam area may have been initiated by a combination of (1) sedimentary column of density inversion, i.e., the denser sands overlie less dense clays, (2) rapid prograding sedimentation in a deltaic environment, and (3) the effects of both incipient and pronounced plate motion that occurred during the Eocene through Oligocene.

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