Provenance Analysis And Its Oil-gas Significance Of Jurassic Shaximiao Formation And Suining Formation In The Middle Part Of West Sichuan Depression

The West Sichuan Depression is in Sichuan Basin. The middle part of West Sichuan Depression is areas which includes Anxian-Mianyang to the south, Dayi-Longquanyi to the north, Longmen mountain front to the east and Fenggu-Zhongjiang to the west. Voluminous sedimentary materials were transited into the basin and deposited during the process of the continent-continent collision between Yangtze plate and Qiangtang block. Best sedimentary records were reserved due to the multistage tectonic evolution, weathering and erosion. Based on outcrop observations, profile measurements, core observations, sample measurements and indoor analysis, such items as the provenance directions, types of sedimentary facies, distribution of sedimentary facie and its influence on reservoir property of Jurassic Shaximiao Formation and Suining Formation were studied in detail.According to outcrop and core observations, sedimentary structures, log responses and seismic responses analysis, the sedimentary facies as alluvial facies, fluvial-alluvial plain facies, delta facies and lacustrine facies were distinguished of Jurassic Shaximiao Formation and Suining Formation in the middle part of West sichuan depression. The sedimentary facies were further divided into 9 subfacies and 19 microfacies. The deposits indicated a variable trend of coarse-fine-coarse-fine upwards. The distribution of sedimentary facies was from alluvial -fluvial plain facies to delta facies in the northwest, and shore lacustrine facies in the southeast.Through the light mineral and heavy mineral analysis, it's found that rich feldspar and poor quartz compositions were showed in the light mineral combinations of the Shaximiao Formation. The heavy mineral assemblages were usually garnet- epidote-zircon-tourmaline in the Shaximiao Formation. In the Suining Formation, poor feldspar and rich quartz compositions of light mineral assemblages, and zircon- tourmaline-garnet of heavy mineral assemblages were identified. The palaeocurrent was mainly NW-SE when the Shaximiao Formation deposied. The palaeocurrent was only NW-SE when the Suining Formation deposited. With the test of quartz cathodeluiminescence, quartz grains of Shaximiao Formation showed the blue violet and brown rays and that of Suining Formation showed mostly blue and seldom brown rays. According to the synthesized researches of sedimentary facies and the light mineral and heavy mineral analysis, it was concluded that the provenances of Shaximiao Formation were from Longmen mountains to the northwest and Micang mountains to the northeast. The parent rocks of Shaximiao Formation were metamorphic rocks and predominantly granites. The sources of Suining Formation were from the northwest and the parent rocks were predominantly granites.The properties of parent rocks, degree of weathering and erosion were in control of the properties of terrestrial clastic. The compositions of parent rocks were closely linked with the properties of sandstone reservoir. The sandstones which contained more feldspar grains, and better sorting and grinding roundness developed in the grains, would have better reservoir properties. The distribution of sedimentary facies was a macroscopical factor which directly controlled the development of sand bodies. The delta deposition were widely developed when the Shaximiao Formation and Suining Formation deposited in the middle part of the West Sichuan Depression. The sandbodies of deltaic distributary channels were with better physical properties of reservoirs. Coarse grain, well sorted, low shaliness, more unstable mineral components and more secondary porosities generally developed in such sandstones.