Sedimentary microfacies in the Chang-6₂ to Chang-6₃ oil reservoirs of the Upper Triassic Yanchang Formation in the Huachi-Qingyang region, Ordos Basin

A detailed study is conducted, on the basis of core examination, grain-size analysis and well logs, to the lithology, grain size, sedimentary structure, well logging response, biomarker and contact relation of the Chang-6₂ to Chang-6₃ oil reservoirs of the Yanchang Formation in the Huachi-Qingyang region, Ordos Basin. The sedimentary facies in the study area may be classified into two sedimentary subfacies including the semi-deep lake to deep lake and delta front subfacies, and six sedimentary microfacies including the semi-deep lake mud, turbidite, sandy debris flow, subaqueous distributary channel, interdistributary bay and sheet sand microfacies. During the deposition of the Chang-6₃ oil reservoirs of the Upper Triassic Yanchang Formation, the shallow-lake delta front deposits were well developed in the semi-deep to deep lake sedimentary environments in the northeastern and southwestern parts of the study area, where the sandy debris flow deposits may provide excellent sites for the oil-gas accumulation in the deep lake area. The results of research in this study may be significant to the exploration and development of the Chang-6 oil reservoirs as the key exploration horizons of the Upper Triassic Yanchang Formation in the Huachi-Qingyang region.