Organic Geochemistry of Oligocene-Miocene Deposits of the Bengal Foredeep, Bangladesh

The argillaceous rocks of Miocene age which are not rich in organic matter (TOC= 0.3-0.6%), contain predominantly humic type (terrestrial) organic matter (OM) with fair to moderate oil sourcepotential (H/C=0.7-Q.97) and. at an early stage of thermal maturation (R°=0.4-0.6%). are regarded as a possible source rock for gas/condensate accumulated in the nearby Bokabil and Bhuban sandstones. Oligocene sediments, on the other hand, are more rich in OM (TOC=O.7-1 .5%), contain predominantly mixed type of OM (humic-sapropelic) with good oil source potential (H/C= 1), are located at present within the oil window maturation limit (R°=0.65-1.2%) and are responsible mainly for oil generation in the region. The beginning of oil window has been traced at a depth between 3.6 to 5.0 km and at a temperature between 104° to 140°C. The great depth and high temperature at the beginning of the oil window are caused by low geothermal gradient and shortage of geological time. The study of deep catagenetic zonation of OM of rocks with relation to vertical zonality of hydrocarbon formation has enabled distinguishing three hydrocarbon generation zones in the sedimentary cover of the Bengal Foredeep:

(i) the upper catagenetic zone of gas generation (R°=0.4-0.6%);

(ii) the middle catagenetic zone of oil generation (R°=0.6-1.15%); and

(iii) the lower catagenetic zone of gas generation (R° >1.15%).

The quantitative measurement of liquid and gaseous hydrocarbon based on material balance method suggests that the Oligocene-Miocene shales arc capable of generating enough hydrocarbon for commercial accumulation not only for gas, but also for oil.