Expulsion of oil from petroleum source rocks: inferences from pyrolysis ofsamples of unconventional grain size

Oil expulsion efficiencies from different lithologies have been investigate d 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, empl oy finely ground samples (< 0.25 mm). Although this facilitates the escape of indigenous and generated hydrocarbons for analytical purposes, it change s rock texture and largely precludes much information about expulsion. It s eems far from natural. Accordingly, we performed Rock-Eval pyrolysis and op en-anhydrous programmed-temperature pyrolysis gas chromatographic (Py-GC) a nalyses, on five grain-size splits of samples - up to 8 mm. This does revea l 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 pyrolys is results suggest that neither the release of liquid hydrocarbons from ker ogen nor the movements of these hydrocarbons within source rock, is the maj or limiting factor on the expulsion of oil from H-rich kerogen-containing s ource rocks (torbanite, lamosite, marinite, liptinite-rich humic coals). Th ermovaporization of indigenous liquid (S1) hydrocarbons from tight carbonat es 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 conta ining H-poor kerogen (e.g. brown coal, humic coal) are not good expellers o f liquid hydrocarbons probably because both the release of oil from the ker ogen and the movement of oil within a coal are limiting factors on expulsio n. However, pyrolysis results on liptinite (sporinite)-rich immature humic coal (HI > 300 mg HC/g C-org) show that liquid range hydrocarbons are expel led from coal when the generated oil exceeds the coal sorption capacity. Th is suggests that expulsion of oil from liptinite-poor, or in general H-poor , humic coals is primarily limited by the release of generated liquid hydro carbons from kerogen because such coals, generally, do not generate suffici ent amounts of liquid hydrocarbons to overcome the coal sorption capacity. (C) 1998 Elsevier Science Ltd. All rights reserved.