Semi-quantitative optimal evaluation of favorable lithological zones under the background of river-lake interaction: a case of the SQ3 sequence of the Huagang Formation in the Middle-South of Xihu Depression

In recent years, significant breakthroughs have been made in the structural lithological composite trap of oil and gas in the Xihu Depression, and the exploration focus on one of its main oil and gas enrichment layers, the Huagang Formation, has gradually shifted towards lithological exploration. However, the exploration targets for the lithology of the Huagang Formation are currently unclear. We analyzed the sedimentary characteristics of the SQ3 sequence of the Huagang Formation in the Middle-South of Xihu Depression. On the basis of clarifying the distribution of sedimentary facies zones, a semi-quantitative method of grid division was used to evaluate the development potential of lithological targets, identify favorable development areas of sand body, search for favorable lithological bodies, and provide new method for finding lithological targets in the future. It is suggests that: ① The sedimentary facies of the SQ3 sequence developed mainly river facies, shallow water delta facies, and shallow lake facies, showing obvious alternation in both horizontal and vertical directions from river-dominated facies, river-lake interactive facies, to lake-dominated facies. ② Based on existing drilling statistic data, sequences of sedimentary, structural, and lithological parameters affecting lithological targets and oil and gas injection were established, and a set of multi-dimensional semi-quantitative evaluation methods for the selection of optimal favorable zones in the Middle-South of Xihu Depression was established using grid unit structure sedimentation. ③ The distribution of key areas and dominant sedimentary facies were identified as the delta front composite sand bars, river channel composite sand bars, and estuary bars.