Abstract

The Hechuan gas play of the second member of the Xujiahe Formation (Xu-2) in Sichuan Basin of China has low porosity and permeability and is a typical tight sandstone gas resource play. During 2008, 64 wells were drilled in the Hechuan gas play in our study area, of which 34 wells tested gas with commercial flow. The proven reserves in the Hechuan block (HC1) are 4.1 TCF over an area of 505.2 km². The probable reserves in the Tongnan block (TN2) are 4.2 TCF in an area of 437.3 km². To forecast the resource potential and geographic distribution of the tight gas in the Hechuan play, we used a numerical method to characterize the continuous tight gas accumulations. Unlike the existing models for conventional resource estimation, the genetic method proposed here is based on the principles of a unique trapping mechanism and pooling dynamics of the tight gas accumulations. We assume that the over-pressure associated with hydrocarbon (gas) generation in the source rocks is the primary driving force for gas migration and accumulation in tight reservoirs. This approach takes into account the interrelationships among the free-gas pressure in the source rocks, the reservoir formation water pressure, the capillary pressure, the compaction history, and diagenetic and hydrocarbon generation history and processes. A numerical model is established to represent the equilibrant processes of the various pressure components and the charging height of the free-gas column. The simulation results on both the resource potential and geographic location are consistent with what has been discovered in the target blocks (HC1 and TN2) and the model also predicted that about half of the total tight gas resource remains undiscovered. Geographically, the remaining resources are distributed in the northeastern areas of the HC1 block, and in the north and southeast of the TN2 block.