Porosity and gas saturation inversion models for tight gas reservoirs in the Northern Ordos Basin

Tight gas reservoirs are characterized by lithological complexity, poor physical properties, high heterogeneity, and variable pore structures, which limit their responses to conventional logging techniques. These limitations cause inaccurate calculation of reservoir parameters, especially porosity and gas saturation. Therefore, conventional models of reservoir evaluation based on logging data are unsuitable for tight and complex reservoirs. Thus, a petrophysical volume model for tight reservoirs with complex mineral composition was established based on core analysis data. An inversion model for the heterogenous porosity of tight reservoirs using optimized calculations and derivations is proposed. The tight reservoir rocks are assumed to obey a conductive parallel model, with the conductive properties of the rock components and pore fluid following a parallel law. A multicomponent gas saturation inversion model for a tight reservoir is also derived. The results of the proposed variable framework porosity and gas saturation models for tight reservoirs are consistent with actual core analysis results. The results from both the models also agree better with the theoretical considerations than those of existing reservoir models. Furthermore, both models were applied in the Northern Ordos Basin, which improved the calculation accuracy of tight reservoir parameters, helped determine good gas layers, and increased actual production returns.