GEOPHYSICAL AND SANDSTONE COMPOSITIONAL STUDY OF MIOCENE STRATA FROM THE SITAKUND ANTICLINE, SOUTHEASTERN BENGAL BASIN

The Bengal basin is located at the junction of the Indian craton to its west, the Shillong plateau and the Himalayas to the north, and the Indo-Burman ranges to the east. The basin is open to the Bay of Bengal to the south. Formed at the Indian passive margin by crustal loading during the Himalayan collision, this basin is filled by synorogenic Cenozoic sequences derived from both the eastern Himalayas and the Indo-Burman ranges. Although much of the delta remains buried, marginal uplifts expose stratigraphic sequences representing most of its depositional history. The Sitakund structure is a ~70 km long NNW-SSE trending doubly plunging asymmetric anticline located at the southeastern part of the Bengal basin.

Analysis of seismic sections and Second Vertical Derivative reveal several extensive N-S oriented faults in the area, majority of which are thrust in nature. This complex structure has been developed primarily due to east-west compression developed from the Indo-Burma convergence. Presence of thick shale is observed in well, seismic and gravity profiles. Channeling observed in the seismic sections reveals a major sediment supply route from the northeast.

The sandstones of the Lower Miocene Bhuban strata from the Sitakund structure are mostly sublitharenites and show fairly good porosities (~20%). Modal analyses of these sandstones reveal that these are dominated by monocrystalline quartz grains, feldspars and sedimentary lithic fragments. Polycrystalline quartz grains including chert fragments are also common. Plagioclase roughly equals potassium feldspars. Low-grade metamorphic fragments also equal the sedimentary lithic fragments. Upper grade metamorphic and volcanic fragments are rare. Modal composition suggests an orogenic source plotting the sandstones in `recycled orogenic' (QtFL) field of Dickinson (1985).

Gravity highs in the northern plunge of the Sitakund structure together with sandstone petrographic data suggest that the northern part of the anticline could be prospective for further exploration.