OVERPRESSURE PHENOMENON ON SANDSTONE COMPOSITION AND TEXTURE: PETROGRAPHIC STUDY OF MIOCENE SEQUENCES FROM SITAKUND STRUCTURE, SE BENGAL BASIN, BANGLADESH

The Bengal basin, formed as a result of the Himalayan collision, is located at the juncture of the Indian Craton to the west, the Shillong Plateau and the Himalayan belt to the north, and the Indo-Burman ranges to the east. Sitakund structure, situated at the southeastern part of the Bengal basin, is a NNW-SSE trending, doubly-plunging and faulted asymmetric anticline. Additionally, the Sitakund structure shows overpressured zone at a relatively shallow depth of 1100 m. Compressive stresses associated with the Indo-Burmese plate convergence to the east are inferred to have contributed the most for the formation of overpressure. Compositional analysis of Miocene sandstones encountered in the drill cores of Sitakund structure documents provenance, and overpressure-induced textural and diagenetic patterns in the sandstones.

Modal analyses of sandstones of the Miocene Surma Group from the Sitakund structure suggest that these are dominated by monocrystalline quartz grains, feldspars and sedimentary lithic fragments. Polycrystalline quartz including chert grains is also common. Plagioclase roughly equals potassium feldspars. Heavy minerals and low-grade metamorphic fragments are also present. Modal composition suggests an orogenic source plotting the sandstones in `recycled orogenic' (QtFL) field of Dickinson (1985).

Petrographic study reveals presence of poikilotopic calcite cement and microfracture in sandstones from the overpressure zone. These microfractures are occupied by gray, argillaceous material, and typically follow grain boundaries producing a tortuous fracture path. Mode of formation of these microfractures is likely to be hydrodynamic due to release of fluids from compacted formations generated by overpressure. These released fluids lead to precipitation of calcite cement and subsequent diagenesis. Compaction-induced hydrofracturing commonly occurs in mud-rich sequences, as found in the Sitakund structure at a depth where overpressure has initiated.