Abstract: Enhanced Interpretations of Thermal Maturity Using Rotational Reflectance of Dispersed Vitrinite, Overmature Strata of the Arkoma Basin and Ouachita Thrust Belt

David W. Houseknecht, C. M. B. Weesner

Rotational reflectance of dispersed vitrinite, measured photometrically using automated polarizer rotation, can provide objective and direct measurement of reflectance anisotropy on individual particles of dispersed vitrinite on which otherwise only random reflectance can be determined. Using this method to analyze of Paleozoic shale samples provides data that characterize the thermal maturity of the Arkoma-Ouachita region as anchizonal through incipient greenschist metamorphic. Measured vitrinite reflectance extends to >5% R_rot (rotational reflectance), which corresponds to >6% R'_max (apparent maximum reflectance). Significantly, commercial natural gas production extends to (approx.) 4% R_rot, and the prolific nature of production from strata at his thermal maturity level contradicts the concept of an abrupt deadline for commercial natural gas occurrence.

The use of rotational reflectance data also can enable the interpretation of relative timing between thermal maximum and thrust faulting. In the Arkoma basin of Oklahoma, a zone of bed-parallel decollement is suggested in one well by an interval of samples containing biaxial vitrinite amidst a sequence of samples containing uniaxial vitrinite. In the Ouachita frontal thrust belt of Oklahoma, the predominance of uniaxial vitrinite in a complexly thrusted area suggests that the thermal maximum post-dated thrusting. In contrast, a similar structural setting in Arkansas yielded a predominance of biaxial vitrinite at comparable levels of thermal maturity, suggesting that the thermal maximum and thrusting overlapped in time. The inferred differences in timing between thermal and structural vents corresponds to deterioration of economic natural gas production, suggesting that the rotational reflectance technique may help to explain the distribution of economic natural gas resources on a regional scale.

AAPG Search and Discovery Article #90986©1994 AAPG Annual Convention, Denver, Colorado, June 12-15, 1994