Depositional and diagenetic controls on reservoir heterogeneity and quality of the Bhuban formation, Neogene Surma Group, Srikail Gas Field, Bengal Basin, Bangladesh

Abstract

This study aims to investigate the controls on reservoir heterogeneity and quality of Miocene Bhuban Formation of the Neogene Surma Group at the Srikail Gas Field, Bengal Basin, Bangladesh. The study integrates wireline log data from two wells with core description as well as petrophysical and petrographic data obtained from 24 core plug samples retrieved from three gas bearing zones. The three zones, namely (from shallower to deeper): D-upper, D-lower, and E-sands, were identified from wireline log interpretation. Based on the extracted cores, the sandstones are classified into three main facies association: 1) coarse grained facies association (CFA) composed of sandstone with parallel lamination, trough, and planar cross bedding; 2) medium grained facies association (MFA) comprises of wavy and flaser bedded sandstone; and 3) fine grained facies association (FFA) consisting of siltstone and shaly sandstone. CFA sandstones, mostly abundant in D-lower and E-sands, were deposited in a high energy distributary channel environment with strong tidal influence while the finer-grained sandstones, mainly present in D-upper sand, were deposited in a low energy distributary channel. The results obtained by integrating core plugs and wireline logs show that best reservoir quality rocks are associated with CFA sandstones which are characterized by GR below 105 API, 15–25% porosity, and 30–100 mD permeability. In contrast, FFA sandstones have lower porosity (<10%) and higher shale content (GR > 125 API). The high energy and strong tidal influence on sandstones of CFA resulted in coarser grain size (and thus larger pores), better sorting and less clay content than the finer-grained, high clay bearing, low energy sediments of FFA. Additionally, petrographic and core plug analyses revealed that mechanical compaction is an important factor that reduces the porosity and permeability with increasing proportions of ductile mineral grains. Other less important diagenetic factors include quartz overgrowth and cement. The findings of this study show that the reservoir quality of the studied field is principally controlled by both depositional environment and mechanical compaction.