Sedimentary System Of Xujiahe Formation In Santai-Yanting Area, Sichuan Basin

In this paper, material provenance and sedimentary system features of Xujiaheformation in Santai-Yangting area are studied based on detailed core observation andunderstanding of geological setting with well data, logging and seismic interpretation.Sedimentary patterns were established, isoline graph of sandstone thickness andpercent content, and distribution characteristics of sedimentary facies were plotted.With regional material component characteristic, it confirmed that in Xujiahe Fmin Santai-Yangting area was located in multiple sources convergence area, based onthe analysis of thin section analysis, heavy minerals and palaeocurrent. Thesedimentation was effected some large provenance from Longman Mountains’northern sector and Motianling ancient land in the northwest, Micang Mountain in thenortheast, and northern Guizhou ancient land in the southeast at the same time. Thenorthern part of the study area (P area) was mainly influenced by the provenance fromLongmen Mountains’northern sector in the northwest and Micang-Daba MountainMountain in the northeast. The southern part of the study area (G area) was mainlycontroled by northern Guizhou ancient land in the southeast, and carbonate detritus inthe Xujiahe Fm are mainly from Longmen Mountains’northern sector and Motianlingancient land. Provenance from Micang Mountain and Motianling ancient land werecharacterized by higher content of siliceous detritus.Formation lithology of studied area in Xujiahe formation was mainly acombination of dark gray mudstone, siltstone, and grey fine-coarsest sandstoneinterbedded, with local coal streak and gravel layer. Rock grain sizes of Xu-2andXu-4members were relatively large, and mainly a combination of thick stratifiedsandstone, thin stratified mudstone and siltstone with coal streak. However, Xu-3andXu-5members were characterized by a large set of mudstone and siltstone withthin-medium sand layer, thin seam and coal. Sedimentary structure caused by tractioncurrent including wedge cross bedding, tabular cross bedding and trough crossbedding mainly developed in sandstone of Xujiahe formation, and liquefactiondeformation structure and wave-generated cross beddings mainly developed insiltstone and argillaceous siltstone. As to the sedimentary facies type, Xu-1member mainly developed marine braided river delta sedimentary, except for the tidal beddingdeveloped in the limy siltstone of it reflecting the connection of studied area and thesea in the early Xujiahe. Moreover, Xu-2and Xu-4member were lake delta facies,and could be determined as shallow braided river delta sedimentary system, based onhigher sand/clay ratio of the sectional structure. Xu-3and Xu-5(including Xu-6)members with lower sand/clay ratio were mainly meandering river delta-shallow lakesedimentary system.