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Abstract:
The assessment of the gas generation potential of highly to overmature shale is key for shale gas evaluation, but it is always fraught with challenges. In this study, immature shale samples with type-I kerogen were investigated for their oil expulsion efficiencies (EEs) by means of pyrolysis in sealed gold tubes to investigate the influence of EEs on the gas generation process. Experimental results were then applied as an analogue to evaluate the gas generation potential of the Lower Cambrian Niutitang shale reservoir in South China. The results demonstrate that the gas yields of C1–C5 alkanes obviously decrease with increasing EEs and are a result of thermal cracking gas caused by a decrease of the oil retention. The change of the gas drying index (i.e., C1/C1–5) indicates that EasyRo of 1.81% calculated from pyrolysis temperature indicates the start of a significant cracking of heavy hydrocarbon gases (C2–5). Before, the generated gas is mainly wet gas, whereas at higher maturities, C2+ alkanes are substantially cracked into methane. The iC4/nC4 and iC5/nC5 ratios exhibit a gently increasing trend, which may result from the catalytic role of clay minerals. Considering the semiquantitative statistical analysis of solid bitumen, the EEs of the Lower Cambrian Niutitang shale in South China were calculated. The gas generation potential of the Niutitang shale of two selected wells is in the range of 4.21–37.62 m3/t and 2.53–11.15 m3/t, respectively, whereas the gas contents of these two wells are low. This mismatch indicates that the gas accumulation in the Niutitang shale reservoir is related to preservation conditions rather than based exclusively on the generation potential of the shale. This study offers a conceptual approach to evaluate the gas generation potential of organic-rich shale after hydrocarbon expulsion and enhances the accuracy of the prediction of highly mature and overmature shale gas resources in South China.
