Study On The Clastic Rock Depositional Characteristic And Diagenesis Of Upper Triassic Xujiahe Formation In Midwest Of Sichuan Basin

The diagensis character of Xujiahe formation in Upper Triassie in Midwest of Sichuan was studied in the research, which was guided by the theory of sedimentology and geology of reservoir. According to the references, the field data, indoor analysis and data of outcrops observation was combined to discuss the main type, characteristics and effects on the reservoir. Based on that, a model of form and evolvement of reservoir was setup. It was used to forecast the avail area for reservoir upgrowth.In the research about the strata sedimentary characteristic, the regularities of distribution and the characteristic of sedimentary facies unfolding were summarized based on manoeuvre-research and exploration of the research section. Form the second to sixth member, the sediment of Xujiahe formation were basically continental deposit, which could be divided into alluvial fan, fan-delta, braided river delta and lakes facies. The second member was mainly lithic feldspar sandstone and arkose quartzite, while the fourth member was mainly arkose quartzite, feldspathic lithic sandstone and lithic feldspar sandstone and the sixth formation is mainly feldspathic lithic sandstone and lithic sandstone. The rock was high in structural maturity and relatively lower in compositional maturity. From the second member to sixth, sandstone of quartz and feldspar content and compositional maturity decreased but lithic content gradually increases. As the distance from the source area increases, the structure maturity within each member gradually increases form north to south and from west to east. The physical property of reservoir was poor because it was low-porosity, low-permeability and especially low-porosity, especially low-permeability.The diagenesis of Xujiahe reservoir was complicated. Compaction and cementation of siliceous and carbonate reduced the porosity, while dissolution and tectonic disruption increased porosity. The diagenetic stage of Xujiahe formation was at mid diagenesis A to B substage. With core observation and thin layer analysis, combined with the result of scanning electron microscope was x-ray diffraction, the division of diagenetic lithofacies of Xujiahe reservoir was carried out. In terms of reservoir physical property, chlorite diagenetic cementation lithofacies were the best. The dissolution and kaolinite diagenetic cementation lithofacies were the second best. The silicic diagenetic cementation lithofacies, calcitic diagenetic cementation lithofacies and the strong-compacted lithofacies were the worst. The physical property of reservoir were multiply influenced by original components of sediment, deposition, diagensis and tectonism.Based on the research above, favorable reservoir development areas were divided into two classes. The second and the forth members of Xujiahe formation in the Jinhua region and Bajiaochang area wereⅠ-class area, which were surrounded by grandⅡ–class areas. The second member in the area of Nanchong-Yingshan and Suining in the southern had distribution ofⅡ–class area,. The fourth member in the area of southern Nanchong, Baimiaochang and Moxi had distribution ofⅡ–class areas. The sixth member in the area of Chongxi-Yingshan, Suining-Longnvsi and Guangan-Longgang was found with larger distributions ofⅡ–class area,Ⅰ–class area only distributed in the north of Xiandu-1 Well and the south of Ying-23 Well at small scale.