Thrust Faults and Pressure of Overpressure Formations in the South Junggar Thrust-and-Fold Belt, China

Wang, Lining; Yang, Geng; Li, Wei; Wang, Xiaobo; Li, Benliang

The South Junggar thrust-and-fold belt, which has abundant oil and gas resources, is one of the key exploration regions in Xinjiang, China. However, the structural complexity and poor imaging make it difficult to interpret the seismic profiles accurately. In this study, we focus on the Manas anticline, of which the mid-shallow part of 2-D and 3-D seismic data are very poor, especially in the axial part and the forelimb of the anticline and in the Paleogene Anjihaihe Formation demarcated by drilling wells. The mid-shallow thrust faults are developed along the mudstones and shale of the Anjihaihe Formation which has higher abnormal overpressure than other formations. So based on the limited dip log data, we cannot determine the locations of thrust faults in the structural thickening of the Anjihaihe Formation which has higher abnormal overpressure than other formations. In this study, the outcrop locations of the thrust faults are projected to the seismic profile, and the location of the thrust fault breakpoints are determined by both of the seismic profiles and logging data. After careful comparison of the formation pressure (i.e., changes of formation pressure coefficients monitored by wells) and the locations of thrust faults, we have found that the formation pressure gradients are closely related with the locations of thrust faults. This relationship can also be presented and supported by the formation pressure variation diagram projected to the seismic profile. In addition, Modular drill stem test (MDT) data shows that two pressure systems exist in the Ziniquanzi Formation and are separated by a thrust fault. The slope change points of the pressure-depth curve indicate the locations of thrust faults. To conclude, 1) the Manas structure is a thrust fault belt composed of a series of imbricate fault fans; 2) The variation of formation pressure is controlled by the development of thrust faults and 3) Structural contraction serves as the main reason for abnormal overpressure in this area. Our results also demonstrate that formation pressure gradient can be used to determine the locations of thrust faults for the seismic interpretation in the tectonic-induced overpressure area. This method could help us better understand the reservoirs controlled by tectonic stress and faults in thrust and fold belt, with potential implications for future oil and gas exploration and development in the areas with similar geological background.

AAPG Search and Discovery Article #90163©2013AAPG 2013 Annual Convention and Exhibition, Pittsburgh, Pennsylvania, May 19-22, 2013