Lower limits and controlling factors of the petrophysical properties of coal-measure sandstone oil reservoirs: A case study of the Jurassic Yan’an Formation in the Huanjiang area, Ordos Basin, China

Coal measure sandstone oil reservoirs of the Jurassic Yan'an Formation in the Ordos Basin, China have strong heterogeneity. Reservoir quality is the core factor that determines the large-scale accumulation of hydrocarbons. Taking the Yan’an Formation in Huanjiang area, Ordos Basin an example, we calculated the lower limits of petrophysical properties based on empirical statistics, porosity and permeability intersections, mercury injections and irreducible water analysis, oil tests, oil bearing property analysis and distribution functions. In addition, we investigated the main controlling factors of effective reservoirs in the Yan'an Formation. The results show that the lower limits of petrophysical properties of effective reservoir in the Yan'an Formation showed a gradually decreasing trend as the depth increased. Furthermore, we obtained the functions of the lower limits of the petrophysical properties and the depth. Sedimentation and diagenesis have a significant control effect on the quality of reservoirs in the Yan'an Formation. High-quality reservoirs were mainly developed in the sedimentary microfacies of point sand bar and distributary channel sand bodies. The strong compaction and cementation in the diagenesis process were the internal reasons for the poor petrophysical properties of the Yan'an Formation reservoirs. Overall, the effect of compaction on the petrophysical properties of the reservoir was stronger than that of cementation; while the secondary dissolution pores significantly improved the petrophysical properties of the reservoir.