Characteristics and Distribution of Effective Reservoirs in the Mesozoic Clastic Rocks of the Tanhai Area, Jiyang Depression

ABSTRACT

The Mesozoic clastic reservoirs in the Tanhai area, Jiyang Depression, show low porosity, low permeability, and strong heterogeneity, making effective reservoir prediction difficult. We integrated core, log, and seismic data to build a four‐parameter evaluation model using porosity, permeability, oil saturation, and sedimentary facies. Weights were determined by combining principal component analysis and grey relational analysis. The overall minimum cutoffs for effective reservoirs are porosity greater than 7%, permeability greater than 0.2 × 10 ⁻³  µm ² , and oil saturation greater than 50%, restricted to deltaic and braided‐river sandstones and glutenites. Two types of effective reservoirs are identified. Type I reservoirs consist predominantly of glutenites developed in deltaic facies, with primary or fracture‐secondary pores; their porosity exceeds 8% and permeability exceeds 0.3 × 10 ⁻³  µm ² . Type II reservoirs consist predominantly of sandstones developed in braided river facies, with secondary‐fracture pores; their porosity ranges from 7% to 8% and permeability from 0.2 to 0.3 × 10 ⁻³  µm ² . The spatial distribution model reveals that reservoir distribution is jointly controlled by a fault—unconformity system. Effective reservoirs are concentrated along major fault zones and near unconformities at structural highs. They are continuous within fault blocks but discontinuous between blocks, and their thickness decreases with burial depth. Model validation using independent well‐test data yields a prediction accuracy of 85.71%, as shown by a confusion matrix. This study provides an effective methodological workflow for predicting strongly heterogeneous clastic reservoirs.