Models for dynamic evaluation of mudstone/shale cap rocks and their applications in the Lower Paleozoic sequences, Sichuan Basin, SW China

The estimation of the sealing capability of cap rocks dynamically is one of the fundamental geological problems that must be resolved in petroleum exploration. In this paper, a porosity–capillary pressure (ϕ–Pc) method was used to estimate the sealing capacity during cap rock burial and a permeability–capillary pressure (K–Pc) plus over-consolidation ratio (OCR) method to estimate the sealing capacity during uplift. Based on 120 capillary pressure–porosity measurements, a ϕ–Pc mathematical model has been established for the burial phase. By analyzing the permeability data measured from 12 mudstone/shale samples from different stratigraphic intervals under step loading conditions, a permeability–confining pressure (K–P) mathematical model has been constructed. Moreover, capillary pressure–permeability data measured from 141 mudstone or shale cap rocks have been used to establish a K–Pc mathematical model for seal capacity evaluation during uplift. Additionally, the OCR of mudstone or shale was calculated from the results of reconstructed burial and uplift histories. These mathematical models were used in case studies in the Sichuan Basin, South China. The results indicate that capillary pressure (Pc) is inadequate in evaluating sealing capacity of cap rocks but the combination of parameters Pc and OCR works well. The agreement between calculated and measured values shows the effectiveness and reliability of the models for dynamic estimation of the sealing capacity of mudstone/shale cap rocks established in this study.