The Research Of Reservoir Forming Condtion Of Xujiahe Formation In The North Area Of West Sichuan Basin

Xujiahe Formation is one of the important exploration areas for the fast growth of oil and gas reserves and production in Sichuan Basin. After years’ exploration in Xujiahe Formation in northwestern Sichuan Basin, only medium and small gas fields(e.g. Yuanba Gas Field) are discovery with low degree of proved resources. Since 2009, new breakthrough has been realized in the exploration of the third member of Xujiahe Formation in the areas of Jiange and Jiulongshan, with predicted reserves incremental over a hundred billion cubic meters, indicating better exploration prospect of Xujiahe Formation in this block. And the key factor restricting the exploration in this block is how to upgrade reserves and search for more high-grade reserves. In this paper, a series of studies were performed on sedimentary facies and sandbody distribution, reservoir forming mechanisms, source rock evaluation, oil and gas reservoir dissection, and simulation experiment of migration, accumulation and reservoir formation based on gas reservoir tight sandstone gas reservoirs forming theories, after investigation was conducted on Xujiahe Formation in terms of its key reservoir forming factors. Then, the main control factors of reservoir formation and the following favorable directions and zones were proposed. This paper not only provides an important reference for the high-efficiency exploration in Xujiahe Formation, but also deepens and enriches tight sandstone gas reservoir forming theories. The achievements obtained in this paper are as follows.1. The sedimentation and development of Xujiahe Formation is a process which evolves gradually from marine facies to continental facies. Six types of sedimentary facies are identified. On the whole, Xujiahe Formation is dominantly composed of low-porosity and low-permeability tight reservoirs, with very strong heterogeneity. And in the deep part below 4000 m, sandstone reservoirs are developed with higher porosity. The formation of high-quality reservoirs is mainly controlled by rock constituents, chlorite pore-linings, rock grain grades, sorting and fractures. The reservoir in each interval is developed extensively.2. Effective source rocks are developed in all intervals of Xujiahe Formation, with the characteristics of large area, extensive coverage and continuous hydrocarbon generation. Based on recalculation, the total amount of gas generation is 148×1012 m3 and natural gas resource is 1.18×1012 m3, which are much higher than the former calculations. Obviously, it provides the resource basis for large gas fields.3. The distribution of gas reservoirs is affected by the structural characteristics of foreland basins. Five types of reservoir formation modes are established. The gas reservoirs are characterized by multiple types, various sources, multiple reservoirs, frequently high pressures, high water cut, low abundance, extensive distribution, strong heterogeneity, large difference of gas testing production and complicated water-gas relationships.4. After further study on formation mechanisms of tight sandstone gas reservoirs, three models were designed and used for large-scale 2D physical simulation experiments of natural gas migration, accumulation and reservoir formation, which considered 15 different physical properties. Quantitative analysis was performed on oil and gas migration and accumulation characteristics of tight gas reservoirs, formation mechanisms of rich gas “sweet spots” and complicated gas and water distribution characteristics of gas reservoirs. It is shown that the permeability differential between reservoirs and surrounding rocks and the charging dynamics are the key factors dominating the enrichment of natural gas in the large area of low-permeability sandstones. The experimental results are significant to the understandings on formation mechanisms and enrichment laws of tight sandstone gas reservoirs.5. Natural gas distribution and its control factors were comprehensively analyzed. Large-area distribution of natural gas is controlled by high-quality source rocks, in the form of proximal accumulation. The enrichment of natural gas is dominated by high-quality reservoirs and high yield is controlled by the development of fractures. Palaeopressure is the principal dynamic for the migration and accumulation of natural gas. Palaeostructures act as the main locations for the accumulation of natural gas.6. Alternative source rock-reservoir-cap rock assemblages are widely developed in Xujiahe Formation, and complex reservoir formation is occurred in the large area. Therefore, it presents better exploration prospects with stereo exploration potential.