| In recent years,the test production of some wells was high in the Middle Permian Qixia-Maokou Formation in Northwestern Sichuan basin,indicating that this area has great potential for natural gas exploration.But the region has complex geological conditions and low level of oil and gas exploration,which have restricted the further exploration.It is urgent to summarize the controlling factors for differential accumulation of deep marine carbonate rocks and to establish a natural gas accumulation model.By making full use of outcrop,core,seismic,logging and geochemical data,this paper analyzed the accumulation elements of the Middle Permian in Northwestern Sichuan,and determined the origin and source of natural gas.Through the anatomy of key oil-gas-bearing structures,the main controlling factors of natural gas accumulation are summarized,and the typical Middle Permian natural gas dynamic accumulation model was established.The main conclusions are as follows:The Lower Cambrian and Middle Permian are the best source rock intervals in Northwestern Sichuan Basin,which is widely distributed and thick,the Lower Silurian source rock is more limited in distribution.The development of source rocks is controlled by sedimentary facies and upwelling under specific paleogeographical frameworks.The abundance of organic matter in source rock has reached good level,the origin kerogen types mianly consist of type I and type II,the maturity of all sets of source rock is in the high-over-mature stage with great potential for hydrocarbon generation.The Middle Permian reservoirs are mainly Qixia Formation beach facies pore-type dolomite reservoirs and Maokou Formation beach-edge fractured limestone reservoirs in Northwestern Sichuan basin,the porosity is mostly 0.09%～2.87%,the permeability is concentrated in 0.01×10-3μm2～0.9×10-3μm2,reservoirs are mostly low porosity and permeability,with relatively medium-high porosity and permeability locally,the petrophysical properties are significantly affected by structural fracture.The Middle Permian natural gas is high-over-mature dry gas in Northwestern Sichuan basin,which mainly belongs to crude oil cracking gas,the overall distribution of methane carbon isotopes is between-35.7‰and-27.3‰,proving mostly came from mixed sources.In conclusion,the natural gas mainly comes from the Middle Permian marine hydrocarbon source There are differences between different structures,and some structures have the contribution of the Lower Cambrian or the Lower Silurian.There are two modes of natural gas generation and storage in Northwestern Sichuan basin,including inself-generation self-storage model and lower generation and upper storage model.And the transport system is dominated by faults.The temperature measurement result of the inclusions indicated that the reservoirs generally have two/three-phase hydrocarbon migration,with the main peak temperatures ranging from 130°C to 140°C and 150°C to 170°C.The two critical accumulation periods are indicated as the Indosinian movement period（accumulation of paleo-gas reservoirs）and the Yanshan-Himalayan movement period（the adjustment and enrichment of natural gas reservoirs）.The frontal zone offold-thrust belt in Northwestern Sichuan basin has moderate tectonic activity and good post-preservation conditions,making it the most favorable exploration target. |
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