ABSTRACT

This paper describes the establishment of shale-gas play risk workflow and discusses the major geologic risks for the lower Cambrian shale-gas play on the Yangtze platform. The shale-gas play risk is assessed by considering the probability of occurrence (Po) of hydrocarbon charge, reservoir, and retention (HCRR) of a shale-gas play. The Po of each of these three factors is determined according to geological understanding and data availability. The risk of these factors is categorized into high risk (Po ≤ 0.3), relatively high risk (0.3 < Po < 0.5), relatively low risk (0.5 < Po < 0.7), and low risk (Po ≥ 0.7). Multiplication of these probabilities generates the chance of geologic success (COGS). The COGS values determine the shale-gas play risk. This workflow provides a method for transferring geological understanding and data availability to quantitatively assess the probability of HCRR. The ranking process allows a focus on the assessment unit of the lowest risk, which is most likely to deliver high-quality prospects.

The shale-gas play risk assessment was used to evaluate the major geologic risks of the lower Cambrian shales in each assessment unit and the range of uncertainties resulting from the available database. Essentially, extremely high thermal maturities (vitrinite reflectance equivalent > 3.5%) and poor hydrocarbon retention are the key geologic risks of the lower Cambrian shale-gas play. Uncertainty is inherent in the play risk assessment owing to sparse well data coverage. Understanding the play risks and uncertainties warrants further study of the impact of extremely high thermal maturities and burial history and how it facilitates decision-making for exploration investment.

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