Porosity Trends of Nonreservoir and Reservoir Sandstones, Anadarko Basin, Oklahoma
HESTER, TIMOTHY C., and JAMES W. SCHMOKER, U.S. Geological Survey, Denver, CO
The porosity of nonreservoir sandstones in Caddo County, Oklahoma, is determined using compensated-neutron and formation-density logs.
Our preliminary data set represents more than 3000 net ft of Pennsylvanian and Permian age sandstones from 12 well locations. These porosity data and the average porosities of sandstone oil and gas reservoirs within the Anadarko basin of Oklahoma are each compared to a broad, composite set of porosity data from numerous basins that represent sandstones in general, and they are also compared to each other.
The porosity of nonreservoir sandstones in Caddo County declines predictably as a power function of increasing thermal maturity for vitrinite reflectance [R(0)] of 0.5 to 1.3%. The rate of porosity decrease with increasing thermal maturity is more rapid than that of the average porosity-R(0) trend of the composite set, but is still within the porosity-R(0) envelope of sandstones in general.
Hydrocarbon reservoir sandstones of the Anadarko basin, however, follow a different pattern. Their rate of porosity loss is much slower than that of both sandstones in general, and nonreservoir sandstones of Caddo County. This slow rate of porosity decline with increasing R(0) could be due to inhibiting effects of early hydrocarbon emplacement on diagenesis and (or) to the bias of economic selection. In any case, as R(0) increases beyond about 1%, the porosity of Anadarko basin reservoir sandstones is anomalously high compared to both nonreservoir Anadarko basin sandstones and sandstones in general.
AAPG Search and Discovery Article #91004 © 1991 AAPG Annual Convention Dallas, Texas, April 7-10, 1991 (2009)