Impact of Structural Style and History on Petroleum Systems of the Assam-Arakan Basin, Northeastern India
Narendra K. Verma
ONGC, Dehradun India
Assam-Arakan Basin is one of the oldest known oil provinces of the world with Digboi discovery in 1889. It is uniquely positioned between two prominent mountain chains of Himalayas and Arakan-Yoma developed on northern and southern Indian plate margins respectively. Integrated exploration studies involving reflection seismic, drilling information, extensive field mapping and satellite images by ONGC in the basin have revealed genetic evolutionary details, which basin has undergone during Eocene-Recent. The basin has multi stage structural history starting with limited imprints of initial rift faulting and Upper Gondwana sedimentation during Cretaceous followed by marginal-shallow marine Early-Late Eocene Tura-Sylhet-Kopili sedimentation. Major part of the basin constituted paleo shelf of Indian sub continent sloping southward with associated extensional faulting during Eocene-Early Oligocene, which has been modified in to foreland-Thrust belt complex as a result of Indo-Arakan plate collision. Under the influence of northward migrating Arakan thrust front-foreland assemblage, deltaic-continental mollasse sedimentation took over marine set-up in Oilgo-Miocene and thick sedimentary wedge of Barail -Tipam – Girujan - Namsang was deposited.
Almost concurrently, Himalayan collision along northern edge of Indian plate, as well as oblique Mishmi thrusting, has complicated the structural history. Thrust over movements of changing magnitude from three sides, with differential time lags have caused structural transformation from extensional and transtensional regime during Paleocene-Eocene-Early Oligocene to that of compressional and transpressional regime in Late Oligonecene-Recent and have created a mosaic of structural features in the basin with variable syn-sedimentary and post depositional imprints. Rate and timing of Arakan thrust front propagation has strongly controlled the basinal gradient and burial, which in turn has governed the source rock richness distribution in Sylhet-Kopili-Disang-Barails and its thermal maturity, as well as timing of charge release and migration route.
Strong vertico-lateral trends in the density and magnitude of structural changes controlling the hydrocarbon accumulation pattern are observed. Multilevel charging from oldest to youngest strata is evident in the frontal belt, whereas as we move away, older strata have preferential charging. Foreland depressions like Nazira low have been developed very late in the history and generally have immature source sequence, and contrary
In thrust front corridor, tip line folds, inverted structures, snake head anticlines, thrusted monoclines, duplexes and subthrust / basement culminations are common structural traps receiving the charge. In the present day foreland belt, basement highs, partially inverted anticlines, rotated fault blocks, trap-door structures formed with cross fault trends, faulted structural noses and monoclines are constituent structural entrapments.
Multiple impulses of structural adjustment have modified the structural fabric of source-reservoir-seal triplet with migrating thrust front and foreland corridor and have controlled the hydrocarbon migration and accumulation pattern, which are co relatable and have predictive value in exploration in the basin.
Presentation GEO India Expo XXI, Noida, New Delhi, India 2008©AAPG Search and Discovery