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Tobin, Rick C.1 
(1) BP America, Inc, Houston, TX

ABSTRACT: Effectiveness of Grain Coatings on Preserving Reservoir Quality in High-Temperature, Deep Burial Settings

Grain-coating clays and early diagenetic cements help to preserve reservoir quality in sandstones by forming barriers to quartz cement nucleation. Reservoir quality modeling software are useful for quantitatively predicting the effect of grain coating completeness on porosity preservation in high-temperature, deep burial settings. The effect of grain coating completeness was evaluated for various formations in the deep gas trend in the Gulf of Mexico. Results demonstrate that grain coatings (mainly chlorite) are more effective at retarding quartz cementation at low temperature than at high temperature. When formation temperatures are high (>200° C), grain coatings must be nearly complete to significantly retard the rate of cement growth. Cement nucleation is effectively suppressed with less complete coatings when temperatures are low. 
Model simulations and natural observations further indicate that the minimum degree of grain coating completeness required to retard quartz cementation also depends on grain size and framework composition. For sandstones buried to equivalent temperatures, the degree of grain coating completeness required to retard cementation is greater for sandstones that are rich in detrital quartz than for lithic or feldspathic varieties. Similarly, fine-grained sands require more complete grain coatings to retard quartz cement than coarse-grained sands. 
A comparison of natural sandstones in various high-temperature basins worldwide indicate that porosity may be preserved through the retardation effect of grain coatings to temperatures well in excess of 200° C. However, reservoir quality risk analysis in these settings must take into account the degree of grain coating completeness along with textural and compositional variability.


AAPG Search and Discovery Article #90026©2004 AAPG Annual Meeting, Dallas, Texas, April 18-21, 2004.