Spatial variations in diagenetic facies in tight sandstones from sublacustrine-fan deposits: Upper Triassic Yanchang formation, Ordos Basin, China

Sublacustrine-fan, tight sandstones in the Yanchang Formation of the Ordos Basin are one of the most important tight-oil producing intervals in China and are reported here the key diagenetic facies. Five diagenetic facies are recognized in the Chang 6 tight sandstone interval, including relatively extensive dissolution (ED), relatively moderate dissolution with grain-coating illite (GCI), pore-filling illite cemented (PFI), extensive carbonate cemented (ECC), and tightly compacted (TC) diagenetic facies. A back-propagation (BP) neural network method is used to build an identification model of diagenetic facies in wells by correlating the various diagenetic facies with conventional well logs for which the identification accuracy can reach as high as 84.97%. Moreover, a novel method of interaction analyses of a large number of horizontal wells and small-spacing development wells and multidimensional constraints by the sedimentary architecture distribution is proposed to predict the distribution of diagenetic facies between wells, and a spatial variation model showing the stacking patterns and geometric features of diagenetic facies in tight sandstones from sublacustrine-fan deposits is established. Diagenetic facies varies more significantly in the direction perpendicular to the paleoflow than that in the direction parallel to paleoflow. The GCI diagenetic facies is most widely distributed within the lobe sandstone bodies as elongated lenses, usually irregularly surrounding the ED diagenetic facies or adjacent to PFI diagenetic facies. From the PFI, ED to GCI diagenetic facies, the average of width and length and the maximum thickness tend to increase. Furthermore, variations in thickness and proportion of various diagenetic facies are closely associated with lake-level cycles.