FR.P. Langford and J.D. Grigsby
University of Texas, Bureau of Economic Geology
W.E. Howard
ResTech, Inc.
ABSTRACT
Two high-resolution well logs may be used with core to improve estimates of porosity and permeability. The microresistivity log from the high-resolution dipmeter and the enhanced-resolution bulk density log can differentiate portions of core cemented with different minerals in gas reservoir sandstones from the Oligocene Vicksburg Formation. Quartz-overgrowth- and calcite-cemented horizons are distinguishable from chlorite-cemented horizons using the microresistivity log; chlorite-cemented horizons present a smoother, less serrated curve. High-density calcite-cemented horizons are distinguishable from quartz-overgrowth-cemented horizons on a high-resolution density log. Because the cement facies have different ranges of porosity and permeability and different porosity/permeability relationships, better predictions of BB reservoir properties may be made. Typical 6 in. sampling rates for density logs do not image thin high-density calcite-cemented horizons and may underestimate or overestimate porosity thickness of individual sandstones by up to 60 percent. Permeability thickness estimates made from log-derived porosities using a regression equation of only calcite- and quartz-overgrowth-cemented data are four times higher than the estimates made using a regression equation from only chlorite-cemented data. Flagging the different diagenetic facies and making separate permeability-porosity regressions for chlorite-and calcite/quartz-overgrowth-cemented intervals allow a more consistent and accurate estimate of permeability than using a simple regression of all log and core data.