Calculating the Total Porosity of Shale Reservoirs by Combining Conventional Logging and Elemental Logging to Eliminate the Effects of Gas Saturation

Shale reservoir exploration technology has attracted increasing attention, and total porosity is a parameter that characterizes the shale storage. Due to the complexity of mineral components and the large variety of pore types, the evaluation accuracy of total porosity of shale reservoirs is not satisfactory, at present. To address this problem, this paper proposes an evaluation method for shale reservoir total porosity based on a shale petrophysical model. We first established the petrophysical model for the calculation of total porosity and then eliminated the effect of gas saturation in the petrophysical model by combining density and neutron-porosity logging. After that, evaluations of matrix density, matrix neutron porosity, and organic matter were conducted using a combined method of elemental logging and conventional logging. Finally, the total porosity of the shale reservoir was calculated. The calculation results showed that by using the elemental logging method based on actual conditions in the research area, the shale mineral composition could be obtained, and an accurate evaluation of matrix neutron porosity and matrix density could be realized. The total organic carbon (TOC) and organic matter (OM) in the shale reservoir can be accurately calculated according to conventional logging data. The evaluation accuracy of total porosity by this method was high, wherein the predicted relative error was only 0.4. Moreover, based on theoretical deduction, it is known that the proposed method has high applicability for shale reservoirs. If the inversion effect of matrix minerals can be guaranteed, an accurate calculation of shale total porosity can be obtained. In summary, the proposed method can accurately calculate the total porosity of shale reservoirs, which provides a reference for the exploration and exploitation of shale reservoirs.