Thermal maturity Organic matter Porosity Second White Specks Formation Belle Fourche Formation (original) (raw)

This study presents approaches for evaluating hybrid source rock/reservoirs within tight-rock petroleum systems. The emerging hybrid source rock/reservoir shale play in the Upper Cretaceous Second White Specks and Belle Fourche formations in central Alberta, Canada is used as an example to evaluate organic and inorganic compositions and their relationships to pore characteristics. Nineteen samples from a 77.5 m-long core were analyzed using organic petrography, organic geochemistry, several methods of pore characterization, and X-ray powder diffraction (XRD). The lower part of the studied section includes quartzand clay-rich mudrocks of the Belle Fourche Formation with low carbonate content, whereas the upper portion contains calcareous mudrocks of the Second White Specks Formation. Strata are mineralogically composed of quartz plus albite (18e56 wt. %), carbonates (calcite, dolomite, ankerite; 1e65 wt. %), clays (illite, kaolinite, chlorite; 15e46 wt. %), and pyrite (2e12 wt. %). Petrographic examinations document that organic matter represents marine Type II kerogen partly biodegraded with limited terrestrial input. Vitrinite reflectance R o (0.74e0.87%), T max values (438e446 C) and biomarkers indicate mid-maturity within the oil window. The relatively poor remaining hydrocarbon potential, expressed as an S2 value between 2.1 and 6.5 mg HC/g rock, may result from an estimated 60e83% of the original kerogen having been converted to hydrocarbons, with the bulk having migrated to adjacent sandstone reservoirs. However, the presentday remaining total organic carbon TOC pd content remains relatively high (1.7e3.6 wt. %), compared with the estimated original TOC o of 2.4e5.0 wt. %. The calculated transformation ratio of 60e83% suggests that the remaining 17e40 wt. % of kerogen is able to generate more hydrocarbons. The studied section is a tight reservoir with an average Swanson permeability of 3.37$10 À5 mD (measured on two samples) and total porosity between 1.7 and 5.0 vol. % (3 vol. % on average). The upper part of the sandy Belle Fourche Formation, with slightly elevated porosity values (3.5e5 vol. %), likely represents the interval with the best reservoir properties in the studied core interval. Total pore volume ranges between 0.0065 and 0.0200 cm 3 / g (measured by a combination of helium pycnometry and mercury immersion). Mesopores (2e50 nm B) are the most abundant pores and occupy 34e67% of total porosity or a volume of 0.0030e0.0081 cm 3 /g. In comparison, micropores (<2 nm B) cover a wide range from 6 to 60% (volume 0.0007e0.0053 cm 3 /g), and macropores (>50 nm B) reach up to 57% with the exception of some samples failing to indicate the presence of this pore fraction (volume 0.0000e0.0107 cm 3 /g). Macroporosity is mostly responsible for variations in total porosity, as suggested by macroporosity's strongest correlation with total porosity within the section. The relatively narrow ranges of TOC and minerals contents among measured samples limit our ability to further deconvolute factors that influence changes in total porosity and pore size distribution.