f Early generated hydrocarbons: origin and significance for coaly source rocks

Petrographic investigation under normal reflected white light and blue light excitation were performed on a number of coals from the Sarawak basin and from the Batu Arang basin of Peninsula Malaysia. These are oil-prone coals that contain significant amounts of hydrogen-rich liptinitic macerals. The thermal maturity of the samples, based on vitrinite reflectance, ranges from Ro 0.35 to 0.50% and they are therefore thermally immature for significant oil and gas generation. The coal samples from the Sarawak basin, however, show microscopic features of early hydrocarbon generation. These include the presence of exsudatinite, oil droplets, and oil haze all of which occur in close association with the macerals bituminite, suberinite and phlobaphinite. In contrast, no distinctive oil generative features were observed in the Batu Arang coals even though they are of the same maturity as the studied Sarawak basin coals. This is therefore considered to be a consequence of the maceral composition of the Batu Arang coals, specifically the absence or trace occurrence of the macerals bituminite and suberinite.<br>The early generated hydrocarbons expelled by the oil-prone coals of the Sarawak basin were observed to impregnate the coal fabric and give rise to plastic behaviour in certain macerals1,2. These “oil-like” materials originate mostly from the maceral suberinite. The generated hydrocarbons impregnated the coal fabric as evidenced by the suppressed vitrinite reflectance values. Saturation of pores and surfaces of coal fabric eventually lead to a continuous bitumen network and subsequently fractured the coal fabric as generated materials lead to elevated pore pressure (Figure 1). This is envisaged as an effective means of expulsion of hydrocarbons from coaly source rocks. Coals from wells of offshore Balingian Province show similar oil-generative features as those observed in outcrops of onshore Sarawak. These features, however, are only distinct in early mature to mid mature (0.45-0.75%Ro) coals and are no longer distinct in the latter part of the oil window. It is therefore suggested that early generated materials have a significant role in hydrocarbon generation and expulsion from coaly source rocks.