Abstract

Coal has been suggested as an important source for liquid hydrocarbons in Sumatra. The concept that coal can generate significant amounts of liquid hydrocarbons can not be lightly dismissed, nor quickly accepted without more convincing geochemical information. In addition to hydrocarbon generation, it is critical to consider the expulsion efficiency of coals. To better evaluate the liquid hydrocarbon potential of coal, hydrous pyrolysis experiments were completed using Tertiary lignites (Ro < 0.35%) from the Bukit Asam region (Muara Enim Formation) of South Sumatra and North Dakota (USA). The Bukit Asam lignite is liptinite-rich (32% of total maceral content); resinite is the most abundant liptinite maceral (13.7%). The North Dakota lignite is vitriniterich (93%) and liptinite-poor (3%). These lignites have Rock Eval Hydrogen Index values of 483 and 123 mg HC/g OC, respectively.

The difference in oil-pyrolysate yield and composition, and experimental temperature of maximum generation for these two lignites is directly related to the type and amount of liptinite and vitrinite macerals. A maximum of 48 and 153 mg/g OC of oil-pyrolysate was generated and expelled from the North Dakota and Bukit Asam lignites, respectively. Maximum generation and expulsion occurs at 340°C for the North Dakota lignite and 360°C for the Bukit Asam sample. Although the overall composition of the generated products from the two lignites are similar, the distribution of these products is significantly different.

Geochemical characteristics of these artificially generated oil-pyrolysates are similar to some non-marine oils, but display distinct differences compared to lacustrine and marine sourced oils. Comparison of these oil-pyrolysates with naturally occurring Sumatran oils demonstrate that "coaly" organic matter contributes various amounts to these oils.

Compared to most petroleum source rocks, significant amounts of liquid hydrocarbons are assimilated by the vitrinitic matrix of coal prior to expulsion. This limits the amount of liquid hydrocarbons available for migration and reservoir accumulation. The following factors are essential to evaluate the potential of coal to be a significant source of liquid hydrocarbons: (1) maceral composition, (2) quantity and composition of generated products, (3) expulsion efficiency, (4) detailed oil/source correlation, and (5) stratigraphic occurrence.