Reconsideration of the pre-1970 model for the timing of oil expulsion from source rocks

Abstract

The current model for oil expulsion occurring at 120-150°C by anhydrous pyrolysis is not consistent with either analyses of the total organic carbon (TOC) content of subsurface source rocks or the chemical attributes of shallow oils. New data from North Sea Upper Jurassic source-rock samples show no decline in TOC between 120 and 150°C, which is a key prediction by the current model for oil expulsion. Instead, it is proposed here that oil expulsion occurs mainly at around 30°C at depths <1 km, similar to models advocated prior to 1970. Expulsion is suggested to be facilitated by CO2 gas generated from decomposing organic matter. Migration out of the source rock and into nearby high-permeable pathways occurs in the direction towards decreasing overburden via horizontal fractures that originate from swelling of load-bearing organic laminae. The thermally immature (heavy) oil is then converted to light oil within reservoirs by hydrogenation at 60-70°C. Hydrogen gas is common in subsurface fluids and is released by coalification of organic matter in mudstones in amounts sufficient to hydrogenate the reservoired heavy oil volumes. In this model, most heavy oil accumulations are interpreted as immature rather than the products of low-temperature biodegradation or partial evaporation of mature oil.