ACCUMULATION MODEL OF SHALE GAS IN COMPLEX SYNCLINE --A CASE STUDY ON SANGZHEPING SHALE GAS FIELD, SICHUAN BASIN, CHINA

With increasing demand for natural gas, greater emphasis is being placed on the exploration of shale gas and the analysis of shale fracture systems. Although fracture systems are advantage for the accumulation of shale gas, they are disadvantage for gas preservation when a large number of fractures are connected with faults.

This research focused on the gas accumulation model of complex fracture systems which is one of the important factors for improving shale gas production from complex synclines, especially for the ones which are reconstructed by multistage tectonic activities. In this study, integrating well logs, outcrop and seismic data, it is demonstrated that the distribution of shale gas in Sangzheping Syncline of southeastern Sichuan Basin is mainly controlled by fractures and faults. Combined with geological feature and well logs in this area, seismic techniques including seismic continuity, curvature analysis, spectral decomposition and spectral attenuation, are used for analyzing and mapping fracture and fault systems. Based on characterization of fracture and fault systems, the complex syncline is divided into three parts: synclinal limbs, synclinal core and transition zone between limbs and the core. Then a new combination of inversion technology and gas content data from four wells, located in different parts of the syncline respectively, is applied for identifying the relationship between the distribution of gas content and shale fractures.

The results show that the lateral variation of gas-bearing layers is controlled by syncline structure: low gas content (average value 1.06 cm³/g) at transition zone between limbs and core due to good connectivity between developed fractures and faults and high gas content (average value 2.18 cubic cm³/g) at syncline limbs and cores with little fractures. Therefore, the accumulation model of shale gas controlled by fractures and faults is established, which should be helpful for better understanding the shale gas accumulation characteristics in other similar geologic setting area around the world.