Reservoir Quality of Neogene Sandstones from Bengal Basin, Bangladesh

Islam, Md. Aminul
Department of Petroleum Geoscience, University of Brunei Darussalam, Bandar Seri Begawan, Brunei Darussalam.

This study has been carried out to find out the reservoir quality and its possible controls with the help of petrographic and petrophysical information derived from thin section study and well log analysis. In this study, UltraPoreTM 300 Helium Pycnometer System has been used to determine the grain volume, pore volume and porosity. Mercury porosimeter PASCAL 440 also has been used to determine pore radius and their distribution. Permeameter of Core-Lab has been used to determine the permeability. In all cases, 40 samples from four studied wells have been considered for measurements. Quantitative calculations of petrophysical properties have been carried out using the postulated formulas in the oil industries

Core analysis result from Bengal Basin reveals that the gas reservoirs are characterized by good quality reservoir. The important reservoir properties are porosity, permeability and pore radius. Average core porosity, permeability and pore throat values are 20%, 209 mD and 44020 Å respectively. Core porosity values usually exceed thin section porosity (8.7%). Different types of cross-plot provide good clues regarding the controlling factors of reservoir properties. Permeability of the reservoir sandstones is largely dependent on porosity. Permeability and porosity are also dependent on textural parameters like size, sorting and matrix of the reservoir sandstones. It reveals from thin section study and different cross plots that diagenetic cements are the main controlling factors of the reservoirs. Quantitative determination of the volume of cements shows that cements have negative correlation with porosity and permeability.

Four gas wells of Bengal Basin exhibits 23 gas zones with variable thicknesses. The study confirms the measurements of log-derived petrophysical parameters including shale volume, porosity, water and petroleum saturation, permeability, moveability index and bulk volume of water that range from 12-32%, 18-31%, 18-36%, 64-82%, 28-323 mD, 0.13-0.52 and 0.03-0.07, respectively. The average values of the mentioned parameters are 20%, 22%, 26%, 74%, 110 mD, 0.28 and 0.05, respectively. The study indicates that log porosity exceeds core porosity and log permeability significantly lagging behind core permeability. The study also reveals that all the gas zones possess good quality reservoirs except few moderate quality reservoirs of Bakhrabad gas field.

AAPG Search and Discovery Article #90155©2012 AAPG International Conference & Exhibition, Singapore, 16-19 September 2012