AAPG Bulletin, V. 83 (1999), No. 5 (May 1999), P. 727-756.

Primary Migration by Oil-Generation Microfracturing in Low-Permeability Source Rocks: Application to the Austin Chalk, Texas¹

Robert R. Berg and Anthony F. Gangi²
 

©Copyright 1999.  The American Association of Petroleum Geologists.  All Rights Reserved
 

¹Manuscript received March 11, 1997; revised manuscript received March 5, 1998; final acceptance October 2, 1998.
²Department of Geology and Geophysics, Texas A&M University, College Station, Texas 77843-3115; e-mail: [email protected]

We are grateful to Roger Sassen and Joel Watkins who reviewed an early draft and made helpful comments for its improvement, and to AAPG reviewers Alton Brown, Ray Fletcher, Jon Olson, and Stephen Laubach. 
 

ABSTRACT

Fracturing of low-permeability source rocks is induced by pore-pressure changes caused by the conversion of organic matter to less dense fluids (oil and gas); these fractures increase the permeability and provide pathways for hydrocarbon migration. An equation for the pressure change is derived using four major assumptions. (1) The permeability of the source rock is negligibly small (0.01 µd; 10^-20 m²) so that the pore-pressure buildup by the conversion is much faster than its dissipation by pore-fluid flow. (2) The stress state is isotropic so that horizontal and vertical stresses are equal. The source rock fails when the pore pressure equals the overburden pressure. (3) The properties of the rock, organic matter, and fluids remain constant during oil generation. This assumption is valid when the change in depth (i.e., pressure and temperature) is small. (4) Only two reaction rates are required for the conversions, a low-temperature reaction rate for the kerogen/oil conversion (E approx. = 24 kcal/mol, A approx. = 10¹⁴/m.y.) and a high-temperature reaction rate for oil/gas conversion (E  approx. = 52 kcal/mol, A approx. = 5.5 ´ 10²⁶/m.y.).

The equations for generation rate and pressure change are applied to the Austin source rock by adjusting the several variables to fit geochemical data, core saturations, and observed levels of oil and gas production. This application demonstrates that the equations are easily applied in calculating depths of primary migration for low-permeability source rocks.