Active Fault Participation in the Diagenetic Modification of Sandstone Reservoir Properties

BURLEY, STUART D., Clastic Diagenesis Group, Manchester, United Kingdom, and JOHN WALSH and JUAN WATTERSON, Fault Analysis Group, Liverpool, United Kingdom

In sedimentary basins undergoing regional strain, faults have a potential for influencing subsurface fluid flow by providing some of the driving energy for fluid movement. Variable displacement on faults in the slip direction results in systematic volume changes in the surrounding sedimentary rocks. Compressed and dilated volumes are distributed according to position relative to the fault. Intermittent seismic slip produces rapid pressure changes and high hydraulic gradients capable of causing movement of large fluid volumes or of maintaining pressure differentials if pore fluid migration is obstructed. Aseismic creep can sustain such pressure differentials throughout the active life of the fault.

As a result of the hydraulic gradients generated by individual faults, subsurface fluids may either be transferred between formations juxtaposed across the fault or vertically transported along the fault. Faults may thus provide the means of mixing of subsurface fluids, and are potentially zones of intense diagenetic modification. Lateral fluid movement across faults may establish chemical disequilibrium, but where vertical fluid transport is rapid along permeable fault zones and the migrating fluid has minimal opportunity for reaction with host rocks, conditions are optimal for the transport of both thermal and chemical disequilibrium. Depending upon the specific local conditions of changes in pressure, temperature, and composition, dissolution of detrital grains and both precipitati n and/or dissolution authigenic mineral components may occur.

An appreciation of fault-influenced diagenetic modification is particularly pertinent to an understanding of the heterogeneity of sandstone reservoirs where pore water and hydrocarbon migration from source to reservoir rocks are an integral part of the hydrocarbon accumulation process. Mineralogical and fabric modifications to rock components may result in either significant enhancement of porosity or extensive cementation and compaction that overprints regional burial diagenetic assemblages. Inactive or cemented faults may behave as hydrocarbon seals. The potential contribution of faults to sandstone reservoir heterogeneity should always be considered in models of burial diagenesis and hydrocarbon migration.

AAPG Search and Discovery Article #91007© 1991 AAPG International Conference, London, England, September 29-October 2, 1991 (2009)