Tight Gas Reservoir Accumulation Model And Distribution Patterns In The Upper Paleozoic Of The East-central Ordos Basin, Northwest China

Middle east part of Ordos basin is located at middle eastern Yishan slope, with the actual exploration area of 4.7 square kilometers, it is currently probed to estimate geologic reserves of natural gas in the area of nearly 400 billion square. The study is used to analysis reservoir accumulation conditions such as formation pressure and trap type based on statistical analysis of the existing drilling logging data, core data, seismic data and previous research, and combined with the sedimentary system and sequence stratigraphy research methods in hydrocarbon source rocks cover layer. The accumulation mode of the tight sand reservoir in this area is summarized through the study of the typical gas reservoir. Based on reservoir accumulation conditions, the enrichment mode of the tight sandstone gas reservoir in this region is analyzed. At last the reserves of the natural gas are calculated and the optimal areas are selected for the exploration in the future.Through the study, high methane content are discovered in the upper Paleozoic, and the gas reservoir are mainly filled with dry gas, of which natural gas are normally coming from basin coal bed, along with a small amount of oil type gas. Coal and dark mudstone as the main hydrocarbon source rock distributed in a wide range, the peak of hydrocarbon generation is mainly occur in the middle Jurassic to early cretaceous. Distributary channel sand body and barrier island bar sand body are distributed in the reservoir with the porosity less than 10%, permeability less than 1×10-3μm2 and pore throat radius less than 1 microns. This reservoir belongs to typical tight sand reservoir. Shihezi formation is the under compaction mudstone with very good seal capability. The gas formation in Permian is underbalance formation, while the formation in Carboniferous is normal pressure. Of the area lithological trap is the main trap.According to the combination type of reservoir and hydrocarbon source rock layers, the region is divided into three ways inside source combination, proximal combination and far source combination. The main drive for gas migration is the continuous abnormal high pressure affected by reservoir pore throat size and hydrocarbon generation, migration direction are normally towards large pore throat radius and small capillary resistance of bearing. Gas reservoir accumulation period can be divided into three stages: “the Triassic, the early cretaceous, rapid subsidence stage”, “the end of early cretaceous, early tertiary, slow rise stage” and “the Himalayan stage, the stage of rapid uplift”.The upper Paleozoic gas reservoir enrichment regularities of middle-east Ordos basin are below: ① Hydrocarbon generation intensity controls the distribution of the gas reservoir. On the plane, gas reservoir is mainly located in hydrocarbon generation center or close to hydrocarbon generation; vertically, the combination in the source is as same as the region’s main gas-bearing combination. ② Reservoir sand body controls the distribution of natural gas on the lateral. Barrier Island bar sand body and distributary channel sand body are the main place of natural gas in this region. ③ The existence of mudstone seal layer makes the longitudinal differences between gas enrichment exists. Multi-layers between hydrocarbon source rock and reservoir appear while they act as not only protecting gas from releasing, but also blocking the vertical migration of natural gas, thus there are obvious content differences in longitudinal of natural gas. ④ Distributions of formation pressure and gas reservoir are with good regularity: Permian gas reservoir are mainly distributed in “low” part of the underbalance pressure of formation; and the carboniferous benxi formation gas reservoir are distributed in “high pressure” part of the normal pressure formation. ⑤ “Sweet pots”in the tight sandstone are the highly enrichment zones. Affected by structure or dissolution, they have better properties for exploration than the surrounding sand body, thus become the optimal areas to choose.Based on the hydrocarbon generation intensity method, the region’s natural gas resource is 4.5127 trillion(covers an area of 47008 square kilometers). Based on the study of the natural gas enrichment, this area could be divided into three parts: ① two optimal areas:Yanchang Area and Yan’an-Zichang-Qingjian Area. ② two prospects: West Zhidan Area and Zhidan-Ganquan Area. ③ one risk exploration area: the South Gas-bearing Area.