Sedimentary Characteristics Research On Carbonate Platform Margin Of Changxing Age In Northwest Sichuan Basin

Along with breakthrough of Puguang, Yuanba, Longgang areas in northeastern Sichuan basin,more attention had been attracted on the distribution of oil and gas resources in northwest Sichuan Basin. People now have great controversy in sedimentary characteristics in this area. this dissertation was primarily focused on the upper Permian Changxing formation in northwest Sichuan basin. Based on lots of outcrops, combining some drilling and logging data, this dissertation studied on rock association, palaeontologic assemblages, deposition fabric, section, Mechanism of deposition with multiple methods. The principle of Sedimentology, Sequence Stratigraphy, Paleontology, Geophysics and Tectonics and some other traditional geology theories were used as the instructions.Two sedimentary system has been recognized that is carbonate platform system and deep-water basin system. Carbonate platform system was composed by restrctied platform, open platform, reef-bank on the edge of the platform and marginal slope. It is mainly distributed in the south of Jiange ,the east of Anxian,and the west of Yilong,most located in the area of Cangxi,Langzhong and Jiangyou. Carbonate platform system margin was the primary sedimentary environment in survey region. It can be subdivided into two faces as platform margin reef-bank and platform margin slope which is mainly distributed in the area of Bechuan-Shuigentou in Jiangyou , Yudongliang-Yongningpu in Cangxi-Nanjiang. From platform margin to north is the carbonate- silica basin. Is mainly composed of a thin layer of silicalites with dark grey and ash black,siliceous limestone and very thin layer of mudstone folder.Combined with the regional tectonic background , making third-order sequences for the mapping unit ,the characteristics of sequence stratigraphy of the Changxing formation and Wujiaping formation are analyzed,two sequence-palaeogeography maps compiled as middle-late Wujiaping stage (PSQ2+PSQ3) and Changxing stage(PSQ4+PSQ5). The map compilation show that in middle-late Wujiaping stage,the study area showing the pattern of deep and shallow ramp,Changxing stage was totally deposited as carbonate platform. From the view of structural point,deep faults of different periods and the basement faults controlled the deposition pattern of basin. Their presence not only accelerated the transformation of ramp mode in the Late Permian Wujiaping to rimmed carbonate platform mode in Changxing stage, but its distribution pattern and relative down movement had a significant role in the control of the local Upper Permian sedimentary faces distribution. From the impact of sea level change, the deposition of carbonate plus caused by continuing transgression will facilitate the transformation of ramp to the platform. And it would have a significant effect on the distribution of sedimentary faces.