Print this pageInfluence of Synsedimentary Tectonics on Hydrocarbon Systems: Three Examples from Baluchistan Province, Pakistan
ADAMS, R. D. and J. M. L. CATER
Three outcrop examples of sediments deposited during collision between the Indian and Afghan plates showcase the effects of coupled tectonics and sedimentation on potential source, seal and reservoir rocks.
The Paleocene Dunghan Formation, deposited during the initial collision, is predominantly dark, fine-grained, middle-to-outer neritic to bathyal carbonate. An argillaceous interval, present only in mid-formation in the western part of the outcrop belt, was apparently derived from the incipient collision zone to the west and not from the Indian plate to the east. Increased rates of subsidence lead to a deep restricted basin and preservation of organic material. The western siliciclastic source may have diluted the source potential of the Dunghan Formation.
The Late Paleocene Khargani Fm., positioned atop the Dunghan Fm., is composed of laterally discontinuous deposits of either a matrix-poor conglomerate or a matrix- rich conglomerate. Distribution of the two types of conglomerate may reflect original distal (matrix-poor) and proximal (matrix-rich) positions relative to siliciclastics shed off the Indian craton. Both types of conglomerates are interpreted as olistostromes derived from a shallow-water platform west of the outcrop belt that was uplifted, exposed and eroded. The matrix-poor conglomerate is a potential reservoir if primary porosity is preserved. The matrix-rich conglomerate has terrestrial organic matter and be a potential source.
The Oligocene/Miocene Nari and Gaj Fms., deposited during the latter part of the collision, are predominantly marine and non-marine siliciclastics with some carbonate units. There is a pronounced northward thinning of the formations related to forebulge migration. Correlations of members and paleosols restrict timing and placement of the forebulge. In turn, forebulge migration influenced geometries of potential reservoirs and seals within the formations and affected the thermal history of potential source rocks.