Expulsion of oil from petroleum source rocks: Inferences from pyrolysis of samples of unconventional grain size

Oil expulsion efficiencies from different lithologies have been investigated and compared employing pyrolyses of samples of unconventional grain-size (from <0 106 mm up to 8 mm) in the expectation of getting a little closer to natural systems. Standard (regular) pyrolyses, including Rock-Eval, employ finely ground samples (< 0.25 mm). Although this facilitates the escape of indigenous and generated hydrocarbons for analytical purposes, it changes rock texture and largely precludes much information about expulsion. It seems far from natural. Accordingly, we performed Rock-Eval pyrolysis and open-anhydrous programmed-temperature pyrolysis gas chromatographic (Py-GC) analyses, on five grain-size splits of samples up to 8 mm. This does reveal more information about expulsion and permits inferences about subsurface natural expulsion. The expulsion of oil from a source rock depends upon the release of the generated oil from kerogen and movement (primary migration) of the released oil within the source rock. Thermovaporization and pyrolysis results suggest that neither the release of liquid hydrocarbons from kerogen nor the movements of these hydrocarbons within source rock, is the major limiting factor on the expulsion of oil from H-rich kerogen-containing source rocks (torbanite, lamosite, marinite, liptinite-rich humic coals). Thermovaporization of indigenous liquid (S1) hydrocarbons from tight carbonates and liptinite-poor humic coals is adversely affected by increasing grain size of the rock, suggesting that primary migration of liquid hydrocarbons is relatively restricted. Retention of liquid hydrocarbons may lead to oil to gas cracking, Py-GC analyses support this inference. Source rocks containing H-poor kerogen (e.g. brown coal, humic coal) are not good expellers of liquid hydrocarbons probably because both the release of oil from the kerogen and the movement of oil within a coal are limiting factors on expulsion. However, pyrolysis results on liptinite (sporinite)-rich immature humic coal (HI > 300 mg HC/g C(org)) show that liquid range hydrocarbons are expelled from coal when the generated oil exceeds the coal sorption capacity. This suggests that expulsion of oil from liptinite-poor, or in general H-poor, humic coals is primarily limited by the release of generated liquid hydrocarbons from kerogen because such coals, generally, do not generate sufficient amounts of liquid hydrocarbons to overcome the coal sorption capacity.