--> Migration of Petroleum in Source Rocks: Quantitative Constraints and Driving Forces, by J. K. Michelsen and G. K. Khorasani; #90986 (1994).

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Abstract: Migration of Petroleum in Source Rocks: Quantitative Constraints and Driving Forces

Johan K. Michelsen, Ganjavar Khavari Khorasani

Petroleum in actively generation source rocks resides in three different phases dependent on the pressure and temperature of the source rock. At low temperatures and pressures, the petroleum molecules resides in one or two phases. Before the kerogen is saturated with respect to petroleum, the petroleum resides in solution in kerogen only. At this stage, primary migration do not take place by any mechanism. During progressive generation, the kerogen becomes saturated with petroleum and the petroleum resides both in solution in kerogen and in a free petroleum liquid. At higher temperatures and pressures, the petroleum species resides in a supercritical fluid and in solution in kerogen. At most pressure and temperature conditions of oil generating sediments, the amount of petroleum molec les in a free petroleum phase is measured to be negligible compared to the amount in solution in kerogen, i.e., source rocks efficiently expells most of the free petroleum phase. Mass balance calculations for worldwide source rocks, supports the high expulsion efficiency, the expulsion efficiency increasing with increasing maturity. Diphasic flow in the water-wet pore network of the source rock do not occur because there is never available sufficient amounts of petroleum for immiscible displacement to take place. Only two possible transport mechanisms therefore remains: diffusion in kerogen and monophasic flow in a petroleum-wet microfracture network. Several petroleum provinces in the world contain petroleum reserves of a magnitude and with timing constraints that require petroleum flux s much larger then can take place by diffusion. Monophasic flow in a petroleum-wet micro-fracture network is therefore a requirement. Many earlier workers has expressed skepticism of monophasic flow as the main flow mode because microfracture generation by petroleum potential build up appeared unreasonable for most source rocks. However, in these studies the kerogen was assumed to initially be under a pressure corresponding to the overall fluid pressure in the rocks. Petrographic evidence strongly support that in fact most of the kerogen in source rocks is load bearing. This implies that indeed there exists an efficient, generally applicable mechanism for mico-fracture generation where pressure gradients in the kerogen itself drives the fracture generation. However, even though the petro eum will flow in an oil wet environment, the flow pattern will be controlled by the water potential. By discussing the source rock both in terms of the thermodynamics of the organic matter, and the rock mechanical properties, both observations in the natural setting and various laboratory experiments can be explained.

AAPG Search and Discovery Article #90986©1994 AAPG Annual Convention, Denver, Colorado, June 12-15, 1994