Effect of Sandstone Detrital Composition on Reservoir Quality Assessment and Prediction

Salman Bloch

Initial composition of sandstones is a key control on reservoir quality. In weakly-cemented sandstones (<10% cement), rigid grain content, rather than grain size, is the main parameter affecting permeability, particularly in the 5,000-10,000 ft depth range. The rigid grain content can be predicted from provenance and depositional-facies calibration data.

In most reservoirs, with the exception of highly cemented sandstones and sandstones with a low rigid grain content, there is a discernible reIationship between initial (Beard-Weyl) and present-day permeability. For a giver burial history, in weakly-cemented sandstones containing more than 85%, rigid grains, facies-controlled permeability contrasts that existed in the original sediments are enhanced. Increasing ductile grain abundance progressively worsens reservoir quality. For example, in moderately-sorted sandstones with more than 15-20% of altered volcanic rock fragments (VRFs permeability is reduced to < 0.1 md, even following a favorable burial history Samples with 10-15% VRFs have permeabilities <5md, whereas samples with <10% VRFs can have high permeability, depending n grain sizes Chemical diagenesis, the effects of which can destroy, preserve or enhance reservoir quality, is also controlled by detrital components, interacting with pore fluids.

A newly-developed systematic approach, based on empirical and "hybrid" models (e.g., Exemplar^TM), allows in many instances to accurately predict or assess, prior to drilling, compactional effects, the type and extent of cementation, and occurrence of porosity-preserving coats on detrital grains.

AAPG Search and Discover Article #91019©1996 AAPG Convention and Exhibition 19-22 May 1996, San Diego, California