A Diagenetic Facies Model for Characterizing Yanan Formation, Ordos Basin, North Central China

Abstract

Low-resistivity pay is a popular phenomenon in many sedimentary formations, and is present in clastic reservoirs of Yanan formation, Ordos Basin of North Central China. The reservoir quality of the studied sandstones is affected positively and negatively by several diagenetic processes, our classification is based on precipitation of pyrite and feldspar and clay minerals authigenesis. The main objective of this study is to integrate lithologic and diagentic information of facies to correctly quantify the reservoir quality. Fifteen sedimentary lithofacies were described based on core description, thin section, and SEM analysis. The quantity and type of clay minerals are examined, using X-ray diffraction technique (XRD) and geochemical analysis using X-ray fluorescence technique (XRF), within the hydrocarbon bearing reservoirs. Facies are lumped together to create a composite database containing eight lithofacies associations. The calibration of log shapes and values by the core is used to establish the suite of logs that would be most suitable for the recognition and discrimination of the various types of facies and facies associations seen in cores before quantitative electro-facies analysis is carried out. Facies association model in the cored intervals is used to predict them in the un-cored intervals. BP-ANN has the most facies prediction power than discriminant function. In particular, with regard to the kaolinite- and pyrite-bearing facies Associations (FA8 and FA6), there is a >90% success rate at predicting the occurrence of them. Facies classification and clustering scheme used in this study are highly successful in prediction of porosity of reservoir rocks. The quality of the reservoirs is generally controlled by the lithologic and diagenetic behavior of rocks forming them.