Bulletin of the Geological Society of Malaysia, Volume 41, Dec. 1997, pp. 85 – 94
Department of Geology, Univerisity of Malaya, 50603 Kuala Lumpur
Abstract: The onshore and offshore Tertiary sedimentary sequences of Sarawak contain numerous coal seams of Oligocene and Lower Miocene age. The offshore extensions and equivalents of these onshore basins contain oil that is considered to be possibly sourced from these coals and related terrestrial-derived organic matter. Despite this, very little information has been published on the geochemical and petrographic characteristics of the Sarawak coals. To rectify this, all the major Tertiary coal deposits of Sarawak have been visited and samples collected. In this preliminary study, results from two areas i.e. the Bintulu area and the Stapang block of the Merit-Pila coalfield will be presented emphasizing on the type of common liptinitic constituents of the coals and their relation to oil generation and expulsion. A number of petrographic features that are commonly considered to indicate oil generation and expulsion from coals were observed. Such features include the occurrence of exsudatinite; oil droplets and oil haze; changes of fluorescence intensity; and development of micrinite. The precursors for the exsudatinite in the Bintulu coals is mainly bituminite and that from Stapang, Merit-Pila is mainly suberinite. The development of the exsudatinite is taken to represent an early phase of liquid hydrocarbon generation and its expulsion took place at about 0.4%Ro for the Merit-Pila coals and 0.45%Ro for the Bintulu coals (earlier than the generally considered “oil-window” of 0.5% vitrinite reflectance). It has been possible to recognize the order of liquid hydrocarbon generation for a number of the common liptinitic constituents present in these Tertiary coals of Sarawak.