Comparison of optical and geochemical thermal maturity parameters over a stratigraphically controlled natural transect

Abstract

Thermal maturity is a fundamental parameter in understanding organic matter evolution during the burial of sedimentary rocks. To this end, numerous optical and geochemical thermal maturity parameters have been proposed, most commonly applied are reflectance or programmed pyrolysis Tmax. Reflectance of vitrinite macerals, or increasingly commonly of solid bitumen macerals, is the classical and well-standardized technique in thermal maturity studies. However, this technique can be influenced by operator bias in acquisition and interpretation. The low cost and wide availability of programmed pyrolysis data has led to the adoption of Tmax as a primary thermal maturity proxy, particularly in industrial studies. However, programmed pyrolysis was originally conceived as a screening tool and significant inconsistencies of Tmax with thermal maturation have been reported.Detailed comparison of maturity proxies is often hampered by the availability of suitable sets of samples from a maturity gradient along uniform facies. Since many of these proxies are influenced by formation specific parameters, the most valuable comparisons must be made using stratigraphically and lithologically controlled sample sets, which are relatively rare. This work examines a natural thermal maturity transect, integrating multiple methodologies to give a detailed discussion of the applicability of optical and geochemically based thermal maturity indicators in the immature to early generation range. This study utilizes a stratigraphically constrained sample set of Upper Cretaceous strata in western Canada taken from 10 wells ranging in thermal maturity from approximately 0.2 to 0.8 %VRo and is based on application of three independent methodological approaches: reflectance measurements, Tmax, and hopanoid and steroid based molecular maturity ratios.Data and correlation equations between major thermal maturity indicators demonstrates solid bitumen reflectance at pre-oil window maturity levels lies along a continuum. Reporting values within somewhat inconsistent classification schemes of bituminite and solid bitumen maceral, as suggested in some recent studies, diminishes the applicability and usefulness of this parameter. Comparison of microscopy and pyrolysis-based maturity proxies with the diagenetic evolution of hopanoids and steroids revealed a robust covariance. Individual molecular maturity parameters are unsuitable to cover the full range of thermal maturity but the wide array of proxies applicable in narrower maturity ranges allows for an optimized selection of those parameters within a given maturity interval. Pyrolysis-derived Tmax and optical determination of OM properties are effective covering a wide range of conditions ranging from the eogenetic to catagenetic stages. Fluorescence analysis is most robust in the eogenetic to diagenetic stages, whereas reflectance measurements provide higher confidence levels at elevated diagenetic or catagenetic stages. Given the limitations of individual thermal maturity parameters, thermal maturity investigations following an integrative, multi-disciplinary approach will limit the impact of uncertainty from any one parameter, improving the odds of achieving a more reliable maturity assessment.