Fault Seal Mechanisms in Contractional and Transpressional Settings: Examples From the Ventura Basin, Southern California

James W. Handschy, Bryan Jolley, Greg J. Mc Rae

Hydrocarbon traps which require fault seals have been documented in many different geologic settings worldwide. However, the actual mechanisms which cause the faults to seal are often difficult to constrain because of sparse subsurface sampling. Extensive coring programs m the Ventura Basin during the 1950's provided an extremely rich petrophysical data set and allow high resolution study of fault seal mechanisms. The Oakridge fault Where it separates Saticoy field and the Bridge pool from the Main pool of South Mountain field plus several faults within Ventura Avenue field have been identified as sealing faults through a combination of conventional trap analysis techniques (structural mapping, Allan type fault plane sections, etc.), pressure history studies and geochemis ry. Core analyses are used to quantify bulk reservoir properties and fault zone properties. Four basic sealing mechanisms are identified in subsurface samples: 1) juxtaposition of reservoir against non-reservoir; 2) clay gouge within fault zones; 3) cataclastic grainsize reduction in fault zones; 4) ductile deformation of labile grains in fault zones (strain induced formation of "pseudomatrix" and occlusion of pore space). A fifth fault seal mechanism, fault zone cementation, was not observed in core samples, but is common m some fault outcrops and may also be important in the subsurface. The relative sealing capacity for each mechanism (based on capillary pressure estimates) is quite variable. For some faults column heights and/or pressures correlate well to measured fault zone properti s, whereas for other faults the correlation is poor. This suggests that the material properties of the fault may vary laterally.

AAPG Search and Discovery Article #91020©1995 AAPG Annual Convention, Houston, Texas, May 5-8, 1995