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Structural Evolution of the Northern Assam Basin: A Case for a Dynamic Hydrocarbon System Model

W. Norman Kent
Kent GeoScience Associates, Richmond, TX

The upper Assam basin is an intermountain basin surrounded on three sides by fold and thrust belts. The area is known as one of the most seismic active areas of the world. It has experienced two of the largest earthquakes in recorded history. Upper Assam basin is also the home to two of the world's largest oil and gas fields. These two superlative characteristics of the basin suggest the temporal nature of oil and gas accumulations and underline the significance of viewing petroleum systems as dynamic processes as opposed to static entities.

An integrated regional study was conducted part of a petroleum exploration program of the Jaipur anticline area. In this study, structural modeling and sequence stratigraphy techniques were used to define the stratigraphic and structural evolution of the basin and to relate that geologic history to timing of maturation and migration of hydrocarbons.

During the study, regional unconformities, recognizable on geologic maps, seismic profiles and well logs, were identified. Mapping truncation of formations at each of the unconformities and construction of isopach maps provided insight into the development of regional structural features. Maturity data for the source rocks in the basin indicate that hydrocarbon generation began as early as mid-Oligocene, with a major pulse occurring in Late Miocene. These data also show that the source rocks are now at peak maturity for oil generation in parts of the basin. This interval of hydrocarbon generation includes nearly the entire time in which the region evolved from an extensional plate margin to an intermountain basin between two orogenic belts. Sustained hydrocarbon generation throughout a complex structural history implies a complex history for migration and accumulation of hydrocarbons in the basin.