Experimental study on late gas generation characteristics of the Middle Devonian sapropelic source rocks of in Northwestern Sichuan Basin

The Paleozoic source rocks in southern China contain mainly Type Ⅰ-Ⅱ kerogen and are currently overmature with vitrinite reflectance (R_o) greater than 2.0 %. Accurate evaluation of the gas generation potential of Type Ⅰ-Ⅱ kerogen after oil generation and expulsion and geochemical characteristics of over-mature gases are of great significance to natural gas exploration of deep and ultra-deep Paleozoic gas pools in southern China. In view of this, the mature Devonian source rock samples (R_o≈1.1 %) collected from the Guanwushan Formation in Northwestern Sichuan Basin are applied for gold tube pyrolysis experiments, and the gas generation potential of Type Ⅰ-Ⅱ kerogen at the highto-over mature stage and the geochemical characteristics of over-mature gas generated are investigated in combination with existing documents. The results show that the residual oil content of the studied samples is 140 mg/gTOC and has a gas generation potential of 220 mL/gTOC at present, indicating high gas-generation potential. Furthermore, the amount of gas from kerogen cracking is at least 140 mL/gTOC and that of gas from residual oil cracking is at best 80 mL/gTOC, indicating that kerogen-cracking gas is the main source of late gas generated from the sapropelic source rocks under high oil expulsion efficiency. Meanwhile, the geochemical characteristics of the late gases generated from the sapropelic source rocks are obviously affected by oil expulsion efficiency. For example, the dryness coefficient of the gases is high, and the methane carbon isotope grows heavier rapidly with increasing methane yield. At R_o≈3.50 %, the carbon isotope of methane is close to that of the original kerogen (with a mere carbon isotopic fractionation difference of 0.5 ‰). The results obtained above will be of scientific basis to the evaluation of natural gas exploration potential and the identification of natural gas sources in the deep and ultra-deep Paleozoic reservoirs in southern China.