Pore structures evaluation of low permeability clastic reservoirs based on petrophysical facies: A case study on Chang 8 reservoir in the Jiyuan region, Ordos Basin

The sedimentary facies, diagenetic facies and fracture facies of the Upper Triassic Yanchang Formation Member 8 (Chang 8) reservoir in the Jiyuan region were studied using core observation, thin section, logging and drilling data. On this basis, the petrophysical faices of Chang 8 oil layers were examined to evaluate the pore structure by classification and predict zones with high porosity and permeability. The petrophysical facies were divided according to the superposition and combination of sedimentary facies, diagenetic facies and fracture facies. A number of petrophysical facies of Chang 8 oil layers such as underwater distributary channel, dissolution of unstable components, high-angle fracture, were identified in this way. Four main categories of petrophysical facies were summed up according to the constructive and destructive impact of sedimentary facies, diagenetic facies and fracture facies on the reservoir property and pore structure. Since the reservoir quality index (RQI) is an optimal indicator for describing the reservoir micro-pore structure quantitatively, the value of RQI of different petrophysical facies was calculated respectively. So the classification and evaluation of Chang 8 reservoir pore structure in a single well were realized by the division of petrophysical facies combined with logging data. At last, the vertical and horizontal distribution of sedimentary micro-facies and diagenetic facies of each single layer of Chang 8 oil layers were mapped, the distribution of favorable petrophysical facies can be used to predict the zones with high porosity and permeability.