Bulletin of the Geological Society of Malaysia, Volume 41, Dec. 1997, pp. 139 – 150
School of Science & Technology, Universiti Malaysia Sabah, Locked Bag 2073, 88999 Kota Kinabalu, Sabah
Abstract: Some quantitative studies were carried out on nine wells in the Baram Delta Field. Sedimentary cycles boundaries were determined using wireline logs and seismic data. Thicknesses of sand, shaly sand, and shale facies were determined for each well based on gamma ray and spontaneous potential logs. Results showed that the field is sandstone dominated (42.3%). Thicker reservoir sand occurs in Cycle VI Lower which is about 46%. The thickness of this reservoir sand increases toward the north and northwest directions. This reflects that the subsidence of the area increases in these directions. The porosity decreases with depth due to an increase in compaction with depth. The surface porosity is about 60% reaching 20% at a depth of 2,440 m. The shale volume percentage varies from 11.5 for poorly stratified sandstone (Sps), 19 for laminated sandstone (Sl), 33.5 for heterolithic sandstone (Sm) and 77 for mudstone (M) facies. The percentage of density/sonic porosity ranges from 23 for Sps, 22.55 for Sl, 21.45 for Sm and 19.10 for M facies. Rapid sedimentation rates led to the formation of overpressured zone at depth of about 2,100-2,300 m. The geothermal gradient of the area shows an average of 1.75°C/100 m and increases to an average of 2.75°C/100 m at the top of the overpressured zone, a depth of about 2,250 m. The overpressured zone is unfavourable habitat for hydrocarbons.