Diagenetic controls on the reservoir quality of Neogene sandstones in the offshore area of the Bengal Basin, Bangladesh

Abstract

The Neogene sandstones offer hydrocarbon reservoirs in the offshore area of the Bay of Bengal (Bangladesh), but their reservoir quality is not well known. Understanding the impact of depositional and diagenetic controls on reservoir quality is crucial to hydrocarbon exploration and production in reservoir sandstones. This study aims to investigate the depositional and diagenetic controls on reservoir quality of the Neogene Surma Group from the Sangu Gas Field (the only offshore gas field), in the Bay of Bengal, Bangladesh using a combination of petrographical, petrophysical, X‐ray diffraction, scanning electron microscope fitted with energy dispersive spectra and core analysis data. Diagenetic modifications to the sandstones include mechanical and ductile grain compaction followed by clay cementation (dominantly chlorite, illite–smectite and minor kaolinite), poikilotopic calcite cement and subordinate amounts of quartz cement, as well as partial dissolution of feldspars and lithic fragments. Sandstones with primary porosities ≥20% have average intergranular volume value of 33%, although permeability in most sections of the reservoir falls below 8–10 mD. There has also been localized creation of secondary porosity due to dissolution. The average intergranular porosity lost by compaction is 14% (ranges from 3% to 37%), and the average porosity lost by cementation is 10% (ranges from 3% to 37%). This study shows that the reservoir quality is predominantly controlled by depositional environment (ductile grain supply, clay content), a diagenetic processes (compaction followed by clay cement and early to intermediate poikilotopic calcite cement) and will provide insights into the reservoir quality evaluation in the offshore areas of the Bengal Basin.