Paleodepth of Burial: Case History of Exposed Paleozoic Carbonates in Arbuckle Mountains, Oklahoma
Scott J. Glash, Gerald M. Friedman
Using several reliable geothermometers, such as fluid-inclusion analysis, vitrinite reflectance, ^dgr18O, illite crystallinity, and percent magnesium concentration in solid-solution, data obtained on formation temperatures of calcite cements from Paleozoic carbonates and shales in the Arbuckle Mountains of southern Oklahoma indicate that these temperatures can only be attained assuming a stable-platform geothermal gradient in a range of 23°C/km-25°C/km, at depths exceeding 2.0 km. These temperatures imply that these strata were once buried and are exposed today.
Homogenization temperatures of fluid-inclusions found in calcite cements and veins in the Paleozoic carbonates indicate precipitation at depths where temperatures exceed surface temperatures. Temperatures ranged from 108°C in the Pennsylvanian to 315°C in the Cambrian. Depths calculated ranged from 5.5 km (Pennsylvanian) to 8.3 km (Cambrian). An average freezing temperature of -4.09°C correlates with 5% NaCl. Brines from which cements precipitated had almost twice as much NaCl as normal seawater. The relationship between freezing and homogenization temperatures shows that as the homogenization temperatures increased, the salinity of the subsurface waters increased.
Vitrinite reflectance values for samples from the Woodford Shale (Devonian) and Sycamore Limestone (Mississippian) yield a mean value reflectance of 0.55%. LOM values determined from mean reflectance correspond to 7.8 (Devonian) to 8.0 (Mississippian). These values correspond to a maximum temperature of 98°-100°C for a linear relationship between LOM and depth. For a nonlinear relationship of LOM vs. depth, maximum temperatures of 50°-52°C are inferred. These temperatures (50°-52°C) are based on a nonlinear curve of vitrinite data from a well in Beckham County, Oklahoma. Both sets of temperatures (98°-100°C and 50°-52°C) are within the diagenetic stage boundary between immature and mature sediments, and are at the edge of the oil windo according to the stages of petroleum generation. Reflectance values from shale samples imply a maximum depth of burial of 4 km based on a 25°C/km geothermal gradient.
AAPG Search and Discovery Article #91031©1988 AAPG Eastern Section, Charleston, West Virginia, 13-16 September 1988.