ABSTRACT: Plagioclase Dissolution in Arkosic Sandstones: How Much, How Deep?

BLOCH, S., ARCO Oil and Gas Company, Plano, TX, and S. G. FRANKS, ARCO Oil and Gas Company, Lafayette, LA

The abundance and distribution patterns of secondary porosity, formed by dissolution of plagioclase, were examined in 247 samples of Eocene-Oligocene arkosic sandstone reservoirs and targets from two sedimentary basins.

Most of the plagioclase dissolution takes place during shallow diagenesis (<5000 ft of burial). This process creates, on the average, 3% of secondary porosity, mostly moldic. Plagioclase dissolution porosity, formed at depths in excess of 5000 ft, averages less than 1.5% and is predominantly intragranular. Whereas shallow leaching may enhance reservoir properties of arkosic sandstones, deep dissolution of plagioclase appears to be counterbalanced by reprecipitation of the dissolution products and provides little or no net gain in porosity. Removal of much of the aluminum released by feldspar dissolution in shallow "open system" settings and incorrect petrographic interpretations, placing extensive feldspar dissolution at deep ("late") burial conditions, may account for the controve sial aluminum source-sink imbalance.

The presence of plagioclase dissolution porosity (both moldic and intragranular) does not have a significant impact on the accuracy of empirical porosity predictions. Such predictions are based on an approach that uses a calibration data set and multiple regression analysis to determine the relationship between known, geologically significant variables, and reservoir quality. High accuracy of porosity predictions (within +/- 2% porosity of the mean measured porosity) in sandstones containing dissolved plagioclase is possible because (1) on a reservoir scale, the impact of plagioclase dissolution porosity on total porosity is generally limited and (2) preservation of secondary porosity is controlled by the same geological parameters as primary porosity and is, therefore, implicitly acc unted for by the calibration data set that provides the basis for empirical predictions.

AAPG Search and Discovery Article #91012©1992 AAPG Annual Meeting, Calgary, Alberta, Canada, June 22-25, 1992 (2009)