SEISMIC PREDICTION OF EFFECTIVE RESERVOIRS IN THE SECOND MEMBER OF XUJIAHE FORMATION (T_3x_2) IN HEBAOCHANG STRUCTURE, THE SOUTHERN SICHUAN BASIN

T3x2 Sandstones in Hebaochang low-amplitude structure are excellent in combinations of source-reservoir-cap rocks and are rich in petroleum resources. Exploration practices show that the distribution of reservoir quality is non-uniform and commercial gas flow can be tested only when effective reservoirs with porosity over 8% and single layer thickness over 10 m are penetrated, thus prediction of effective reservoirs is the key to the description of gas reservoir in T3x2 of Hebaochang structure. The Xujiahe Formation in the study area is divided into 4 seismic sequences by using the principles and methods of sequence stratigraphy. The effective reservoirs in T3x2 are mainly developed in the low system tract of sequence 2. The geological, logging and seismic features of T3x2 in the low system tract of sequence 2 are analyzed, and this sequence is determined to be mouth bar microfacies by using seismic attributes and waveform grouping techniques. Meanwhile, the distribution of karst fossil residual hills on the Middle Triassic erosion surface is determined through studying the thickness of sequence 1. The distribution area of the mouth bar microfacies and karst fossil residual hills on the erosion surface control the distribution of effective reservoirs in T3x2. Based on a qualitative prediction of the favorable distribution areas of the effective reservoirs, quantitative prediction of the effective reservoirs are performed by using high resolution seismic inversion technique. Drilling results are in good coincidence with the predictions and commercial gas flows are tested in all the wells drilled.