Effects of Organic Matter Content and Maturity on Oil Expulsion from Petroleum Source Rocks

Longjiang Wang, Colin Barker

We have studied the effects of organic matter content and maturity on oil expulsion from source rocks. The bitumens in a source rock can be classified into two forms: (1) free bitumens in pores and (2) adsorbed bitumens on the mineral matrix and on kerogen. Presumably the adsorbed bitumens cannot migrate out of the source rock but the free bitumens can. Hence, the adsorbed bitumens in a source rock affect both oil expulsion efficiency and the composition of expelled oil. Bitumens in a rock sample were removed by solvent extraction or thermal extraction. Then, either an oil or an organic compound was adsorbed back onto the sample to simulate bitumen adsorption. We found that the effect of kerogen content and maturity on oil adsorption is significant, but the available limi ed data suggest that the effect of kerogen type on oil adsorption is minor. Oil adsorption decreases with increasing kerogen maturity but increases linearly with organic matter content up to about 10% (TOC). Above this, adsorption increases much faster with further increasing kerogen content. Based on the adsorption work, we tentatively conclude that (1) source rocks with type I and type II kerogen have higher expulsion efficiency than type III kerogen source rock, (2) expulsion efficiency increases with increasing kerogen maturity, and (3) at low organic matter content, expulsion efficiency increases with increasing kerogen content; however, when kerogen content is very high (such as coal), oil expulsion efficiency decreases with increasing organic matter content.

AAPG Search and Discovery Article #91025©1989 AAPG Midcontinent, Sept. 24-26, 1989, Oklahoma City, Oklahoma.