A Kineticaliy Controlled Fluid Pressure and Migration Model
Per Arne Bjorkum and Paul H. Nadeau
New concepts of silicate diagenesis and their effects on rock properties have major implications for predictive models of geopressure and migration scale / efficiency. At temperatures greater than approximately 70°C, precipitation rate controlled internally sourced quartz cementation and neoformation of diagenetic clay become important porosity and permeability reduction factors, respectively. In shales, clay diagenesis rapidly reduces permeability, rendering them prone to overpressure. The shale/source rock capillary pressure is increased dramatically and later onset of hydrocarbon expulsion is facilitated by microfractures, which allows for greatly increased expulsion and lateral primary migration efficiency. Quartz cementational porosity loss in sandstones and sil stones proceeds mainly independent of pressure and hydrocarbon saturation, which can result in hydrofracturing of the overlying seals/shales. The effect of hydrofracturing results in a coupling of thermally controlled porosity reduction processes, fluid overpressure, and episodic expulsion. In areas of thick shales, the probability of long range, km scale, vertical migration increases such that hydrocarbon migration to shallower sequences increases. Correspondingly, the preservation of economically attractive hydrocarbon accumulations at depth is reduced, except in reservoirs with mitigating geologic factors. The underlying thermal controls of silicate diagenetic and organic maturation processes result in 'self-organized' basin scale fluid migration systems which optimizes the occurrence of economic hydrocarbon accumulations to certain stratigraphic/structural levels within any given basin.
AAPG Search and Discover Article #91019©1996 AAPG Convention and Exhibition 19-22 May 1996, San Diego, California