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Comprehensive Prediction of Favorable Gas Reservoir in the Large-Scale Tight Sandstone of Upper Triassic Xujiahe Formation in Hechuan Area of Sichuan Basin, China

Xu, Anna; Wang, Zecheng; Bian, Congsheng; Xu, Zhaohui; Li, Yonggen; Zhai, Xiufen

Hechuan area is located in the gentle slope of Central Sichuan Uplift in Sichuan Basin. It belongs to a gas-bearing structure of gentle anticline; the major gas layer is Triassic Xujiahe Member 2 sandstone, which is consisted of multistage stacked braided river and delta sandbodies. The characteristic is the wide spread of sandbody and the good lateral connection. But the porosity and permeability is low, mostly between 2~7% and 0.001~1mD. The gas reservoirs with porosity more than 4% only developed in very limited local areas, with severe heterogeneity and poor lateral connectivity. Geologic, seismic, and well logging data are used in this paper to predict the distribution of large scale tight gas reservoirs by three steps and three methods. Firstly, the core, casting slices, well log data and production performance test results are studied to determine the controlling factors and limit values of gas reservoirs. The results show that the micro-facies and diagenesis are the dominant factors controlling the distribution of favorable gas reservoirs. Fractures can improve local petrophysical properties. Higher porosity and permeability can be found in coarse sandstone of channels, with dissolved vugs and fissures, which means favorable gas reservoir zones in large area of tight sandstone. The lower limit values of permeability, pore-throat size, and porosity for gas-bearing reservoirs in Xujiahe Member 2 sandstones are respectively 0.05mD, 0.2μm, and 4.5%. Secondly, considering micro-facies, diagenetic facies, and fractures as three key factors in controlling development of reservoirs, three methodologies are used in comprehensive prediction of gas reservoirs, which are the facies controlled stochastic simulation method, the logging lithological facies interpretation method, and the fractures indication technique by the stress-strain relation of rocks. The thickness of gas-bearing sandstones and the distribution of higher porosity and permeability are predicted, while the thickness of dissolved sandstone, and general distribution of fractures are predicted. Thirdly, applying the above results, the images obtained through these three methods are stacked to synthetically predict and evaluate the distribution of favorable reservoirs. The results show that the gas-bearing reservoirs in Member 2 Xujiahe Formation are located in the structural axis or tectonic hinges, and areas where main channel or sand bars micro-facies, and fractures are well developed. In summary, the prediction of large scale tight gas sandstone reservoirs should be conducted following major controlling factors, and comprehensive research should be carried out using optimized prediction technique methods, which are applicable in the geological setting of the studied area.


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