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AAPG Bulletin

Abstract


Volume: 77 (1993)

Issue: 9. (September)

First Page: 1488

Last Page: 1501

Title: Preservation of Shallow Plagioclase Dissolution Porosity During Burial: Implications for Porosity Prediction and Aluminum Mass Balance

Author(s): Salman Bloch, Stephen G. Franks (2)

Abstract:

The abundance and distribution patterns of secondary porosity formed by dissolution of plagioclase were examined in 216 samples of middle Eocene-Oligocene shallow marine to nonmarine arkosic sandstones from the southern San Joaquin basin, California. The proportion of plagioclase intragranular porosity and plagioclase moldic porosity is depth dependent. For purposes of this study, pores containing less than approximately 40% of remnant plagioclase were counted as "moldic" plagioclase porosity. Pores within skeletal plagioclase grains or mostly enclosed by skeletal grains were counted as intragranular plagioclase porosity. Moldic porosity is most significant at shallow (< 5000 ft or 1500 m) depths, whereas the relative importance of intragranular porosity increases belo approximately 5000 ft (1500 m). These trends are most likely caused by gradual, preferential collapse, with increasing burial, of moldic porosity generated predominantly at shallow depths. Intragranular porosity in more deeply buried sandstones is partly inherited from shallow depths.

Feldspar dissolution porosity formed in shallow, "open-system" settings typically is not accompanied by aluminosilicate (e.g., kaolinite) by-products. This porosity, either moldic or intragranular, can be preserved during burial, particularly in sandstones with high percentages of nonductile detrital grains. Removal of much of the aluminum released by feldspar dissolution in shallow "open-system" settings coupled with incorrect petrographic interpretations, placing extensive feldspar dissolution at deep (late) burial conditions, may account for the controversial aluminum source-sink imbalance.

The presence of plagioclase dissolution porosity (moldic and intragranular) is important, but it 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 the presence of secondary porosity is implicitly accounted for by independent variables (e.g., composition, grain size, sorting, burial history) in the calibration data set that provides the basis for empirical predictions.

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