Recovery method for total porosity of shale based on porosity origin: A case study of the lower part of the third member of Shahejie Formation in the Dongying Sag, Bohai Bay Basin

There are many types of pores in shale, which provide an important permeable-storage space for oil and gas enrichment. The restoration of shale pore evolution is the key to the evaluation of shale oil and gas resources and the optimization of zone, and it is also an important basis for the study of hydrocarbon expulsion process. The compaction of mud shale which reduces pore, hydrocarbon generation which forms pore, and tectonic effect which forms fracture are important mechanisms of shale pore generation. Based on these three kinds of pore forming mechanisms, a mudstone compaction model was used to calculate the porosity change of shale at different depths. The relationship between organic pore and organic carbon content during geologic evolution was modeled according to the measured data such as vitrinite reflectance, organic carbon content and physical properties of shale. After modeling the evolutionary trend of different pore types, a the multi-type shale pore recovery method based on pore formation was established based on comprehensive tectonics, facies, diagenesis and stress field characteristics. The evolution of three pore types in the lower part of the third member of Shahejie Formation in the Dongying Sag was restored by this method, and the spatial distribution of pores of different genetic types was predicted. The distribution of the total porosity of shale was determined. The forecast results are in good agreement with the real drilling data.