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Abstract: Detrital and Geochemical Control on Chlorite Grain-Coats in Sandstones

HARRIS, NICHOLAS B.

Chlorite grain coats may preserve anomalously high intergranular porosity in sandstones, often at great depths. In the Gulf of Mexico, Tuscaloosa sandstones with porosities of greater than 20% produce gas from fields at depths of 20,000 feet. These sandstones may therefore present an attractive exploration target in deep parts of sedimentary basins if their occurrence can be predicted. Chlorite grain coats affect log readings, yielding underestimates of porosity and irreducible water saturation. Also chorite in sandstones may react with acid well treatments, severely damaging the reservoir, unless treated with anti-oxidants and chelating agents. Chlorite-rich sandstones may therefore contain bypassed pay, either due to misinterpreted well logs or faulty well completions.

Recent studies, particularly in the North Sea, suggest that many chlorite grain coats form by recrystallization of early Fe-rich marine clay, deposited near river mouths. The coincidence of several factors is probably required: weathering of a terrane with sufficient iron-bearing minerals; and chemical conditions in the marine environment to prevent sequestering of iron by carbonates or sulfides. Applications to sandstones in the Appalachian and Illinois basins will be considered.

AAPG Search and Discovery Article #90939©1997 AAPG Eastern Section and TSOP, Lexington, Kentucky