Overpressure in the Eastern Bengal Basin, Bangladesh, and Its Relation to Compressional Tectonics
Type of Degreethesis
Geology and Geography
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Proximity to the Indian craton, the Himalayas, and the Indo–Burman ranges and rapid orogenic sedimentation have resulted in the generation of overpressure in the Bengal basin. In eastern Bengal basin, overpressure zones frequently have been reported from the Miocene sequences in exploratory wells. The majority of the wells drilled so far in the eastern Bengal Basin encountered overpressure zones at depths ranging from less than 1 km (Patharia–5) to 4.5 km (Muladi–1). The average geothermal gradient in the eastern Bengal basin area ranges from 15.8 to 30° C/km. In the western stable shelf area, the average geothermal gradients range from 21.1 to 31.6° C/km. The depth–pressure profile indicates that formation pressure gradient increases from 0.12 atm/m in the west and north to 0.18 atm/m in the east and southeast. The depth to the top of the overpressure zone is shallower where the amplitude of structures is lower and depth to the top of Miocene Bhuban Formation is shallower. Therefore, the depth to the top of overpressure becomes shallower toward the east. Miocene Bhuban sediments exhibit transformation of smectite into illite through intermediate illite/smectite mixed layer clay. The most important change is the loss/gradual decrease of the illite/smectite mixed layer clay, which indicates gradual diagenetic changes with increasing burial depth. Compaction–induced hydrofracturing and clay injection have been identified in the Fenchuganj–2 well below the overpressure zone. Feldspar dissolution has played an important role in secondary porosity development in the Bengal basin. 40Ar/39Ar analysis of detrital muscovite from the Kamta–1 well provides muscovite cooling ages between ca 15 Ma and 554 Ma. However, the youngest age cluster in all three samples shows average cooling ages ranging from #15 Ma to 16 Ma. Considering the average cooling age and depth of the samples, rate of deposition is estimated to be 0.96 mm/year to 1.88 mm/year. Incomplete dewatering of fine–grained sediments, smectite dehydration, increase in pressure gradients toward the east, and high rates of sedimentation contributed the development of overpressure in the eastern Bengal basin.