Semi-quantitative study on reservoir configuration in grey theory—A case study of H3 sand unit of Huagang Formation in A Structure, Xihu Sag

Using grey theory, we calculated quantitatively the comprehensive evaluation index IRE of the interlayer in Huagang Formation (Fm) of the A Structure in the Xihu Sag, China Sea, from which the type of interlayer in thick-bedded sandstone was identified and the channel period was divided. Combing logging data and lithofacies associations, we clarified the strength of hydrodynamic force, divided the reservoir types, and then defined influencing factors of high-quality reservoir development. In addition, based on single channel identification and classification, the average thickness of single interlaced strata was determined, and the river width-thickness ratio and sand-soil ratio were calculated to semi-quantitatively characterize the river connectivity and distribution characteristics. Based on the influencing factors and channel distribution characteristics of high-quality reservoirs, the development area of high-quality reservoirs was predicted. Results show that there are two types of interlayers in Huagang Fm of the A Structure, namely, desilting stratum and mudstone interlayer. The desilting stratum with low IRE was 24～45, and that of the mudstone layer was 51～110, much greater, thus the three sets of single channel sand bodies in 10 stages were identified in the H3 sand unit of Huagang Fm. The width-thickness ratio of single channel in the work area was 38.87, and the range of channel distribution in each period was calculated to be 1.1～2.3 km. The channel sand bodies are almost overlapped. Under the guidance of seismic composite microfacies, the early channel of Huagang Fm was identified and tracked. Combined with the distribution characteristics of sedimentary microfacies, coarse-grained facies zoning, and diagenesis development, we considered that the favorable reservoirs are more developed in the south area of the A Structure.