Quantitative evaluation of the transporting capacity of unconformities: A Case Study from the Zhongguai Area, Junggar Basin, NW China

Unconformities are special geologic bodies with a three-layer structure including the rock above the unconformity, a weathered clay layer, and a leached rock. The Upper Wuerhe (Upper Permian) bottom unconformity in Northwest Junggar Basin is the main pathway system linking the Zhongguai (ZG) lift with hydrocarbon source kitchens. Based on the log response characteristics of the unconformity's vertical structure and core calibration, a principal component analysis (PCA) was applied to extract and identify the evaluation parameters of the unconformity's vertical structure. On the basis of Darcy's Law, the quantitative characterization formula of the unconformity's transport capacity with an actual geological significance was established after taking several factors into consideration. The migration probability of the unconformity was used to evaluate the possibility that the unconformity acted as a migration pathway. Consequently, a relationship between the migration probability and the evaluation parameters was developed to analyze the effectiveness of these parameters in characterizing the transporting capacity of the unconformity. This study shows that the quotient of the two principal components extracted from the conventional logging curves (Pc1/Pc2) have different reference values in the leached and parent rocks. The distribution of thicknesses of the weathered clay layer and the leached rock is well matched with the distribution of paleogeomorphology. The parameters representing only a single geological factor are not enough to determine whether the unconformity acted as a migration pathway. The flow coefficient (Cf) containing multiple factors can effectively and clearly identify the transporting capacity of the unconformity, considering the positive correlation between the parameter Cf and the migration probability of the unconformity. The oil and gas shows of the area with a parameter Cf greater than 10 × 10−4 cm2/s are abundant, which is the superior transporting area of the unconformity, and there is a good matching relationship between the distribution of the superior transporting area of the unconformity and that of the hydrocarbon distribution. •Unconformities are special geologic bodies with a three-layer structure.•The boundary of leached rocks and parent rocks can be effectively identified by the PCA from geophysical log data.•We establish the quantitative evaluation parameter of transporting capacity of unconformities.•Geoparameters pertin