A Physical Model for Primary Expulsion of Hydrocarbons From Petroleum Source Rocks

Abstract

It is nearly 50 years since the first publication of a thermally driven model for generation of hydrocarbons from organic debris contained in sedimentary rocks, and in that time there has been continuous development and refinement of our understanding of the chemical processes involved, and their application to exploration. During that time there has also been substantial development of our understanding of the processes of migration of hydrocarbons through the sedimentary sequence, but there has been little or no comparable development of our understanding of the physical expulsion process – how the oil and gas is initially expelled from the source rock. Following is a simple pressure driven model which explains the process. Under normal burial conditions, potential petroleum source rocks undergo progressive compactive dewatering in an orderly manner, and are normally pressured and of relatively low permeability when petroleum generation begins. Upon reaching thermal maturity, kerogenous material begins to generate a mix of hydrocarbons; in general the initial products are gases and light oils. Since clay dewatering begins at approximately the same temperature, bound water is also released into the available porosity. This relatively rapid release of mixed light hydrocarbons and clay-derived water into the available porosity results in a rise of pore pressure within the source rock. However, since each unit volume of source rock is enclosed by like volumes undergoing a similar process, within any large volume of source rock, no pressure differential arises to drive expulsion. As the source rock continues to generate fluids, pore pressure rises and the rock becomes increasingly unstable. Ultimately, the rock is fractured by a transient seismic wave; pressurised pore fluids immediately expand and flow into the fracture. Once flow begins, exsolving gases give effective pressure maintenance, and flow may continue, only ceasing when pressure equilibrium is restored. When flow ceases, the fracture system closes and seals. Depending on the source richness, generative potential may remain in the kerogen. As temperature is unaffected by the process, generation continues while source potential remains. The process repeats – possibly multiple times. From cycle to cycle, removal of some of the more mobile kerogen components causes the chemistry of the expelled mix to change. This model has implications for many aspects of petroleum system studies.

AAPG Datapages/Search and Discovery Article #90217 © 2015 International Conference & Exhibition, Melbourne, Australia, September 13-16, 2015