Experimental determination of effective porosity in terrestrial shale reservoirs and logging characterization methods： A case study of shale of Es4UC in Boxing Subsag

The exploration of shale oil and gas in Jiyang Depression has made breakthroughs since 2020. It is urgent to report the scale of shale oil reserves accurately. The accurate acquisition of effective porosity in the laboratory and the fine logging characterization are key factors restricting the reporting of shale oil reserves. The analysis and characterization of effective porosity in shale reservoirs were carried out based on core experiments， nuclear magnetic resonance （NMR） logging， and conventional logging data to solve the problem of evaluating the effective porosity of shale reservoirs. In the core experiment， the experimental procedures of the liquid measurement method and two-dimensional （2D） NMR method were optimized， and many methods were compared and selected. In the logging characterization， the 2D NMR logging frequency and temperature were calibrated based on the core calibrating logging concept， and the sensitivity was analyzed to establish a conventional logging evaluation model of effective porosity. Based on the above research， the experimental measurement and the fine logging characterization of effective porosity were carried out in the shale reservoir in the Upper Chunhuazhen Submember of the 4th Member of the Econe Shahejie Formation （Es4UC） in Boxing Subsag， and the experimental test method was determined， which is mainly liquid measurement method and supplemented by the 2D NMR method. The effective porosity T2 cutoff value was clarified in the 2D NMR experiment， and the NMR logging-based effective porosity characterization method was innovatively formed based on the frequency and temperature correction. Two conventional logging evaluation models were established， containing lithology indicators （natural Gamma） and physical property indicators （interval transit time and neutron porosity）. The calculation results have high accuracy， and the relative errors are all below ±12%， which meets the requirements of reserve estimation regulation， achieving accurate characterization of effective porosity of shale reservoirs， and the models were successfully applied in reserve reporting.