Calculation of the original abundance of organic matter at high-over maturity: A case study of the Lower Cambrian Niutitang shale in the Micangshan-Hannan Uplift, SW China

The evaluation of shale source rocks is one of the most basic and important processes during shale gas exploration to estimate the hydrocarbon genetic potential, especially for shale source rocks at high-over maturity. This study focuses on the geochemistry of Niutitang shale at high-over maturity to identify the practicability of organic matter abundance calculation. Ninety-eight Niutitang shale samples from outcrops in the Micangshan-Hannan Uplift were selected and tested. The geochemical characteristics of the Niutitang shale source rocks including the total organic carbon (TOC), pyrolysis data, thermal maturity, kerogen types and clay mineral composition, were investigated using tests of C/N elemental analysis, Rock-Eval, bitumen reflectance (R-b), organic carbon isotope (delta C-13(org)) and X-ray diffraction (XRD). Then, the formulas for calculating the coefficient of original TOC (Kc) and original genetic potential S-1 + S-2 (Ks) were deduced using the pyrolysis data and the results before and after calculation were analyzed. The result of the present work imply that it is necessary and reasonable to calculate the original abundance of organic matter in marine shale source rocks at high-over maturity and that a TOC of at least 1.0% may be appropriate to predict favorable zones of marine shale at high-over maturity in China.