Research On Sedimentary System Of Yanchang Formation Of Triassic Period In Xunyi Area,Ordos Basin

Recently, the exploration and development of tight oil & gas have been continuously increasing. It makes us to elaborate more detailed study of sedimentary characteristics and sedimentary system. Meanwhile, the explanation of thick sandstone in deep-water is generally not precise enough. This paper is aiming at researching sedimentary characteristics, sedimentary facies and sand body space distribution of Yanchang formation in Xunyi area, under the guidance of theory system including Sedimentology, tectonic sedimentology, sequence stratigraphy and so on. On the basis of the above study, sedimentary system and evolution are been analyzed deeply and comprehensively from two aspects of micro-characteristics and macro-transformation.The climates in Xunyi area was warm and wet, plants and aquatic animals were thriving with the effect of indo-movement through the whole Yanchang period. According to sedimentary background, ten oil reservoirs are stable as a whole and the north stratum are thicker by the methods of identifying marker beds, sedimentary cycle correlation and so on. Also each interval is gradually increasing at the east of Zhuanjiao. But the later tectonic uplift made the formation low in the north and west. At the same time, denudation are increasingly serious towards south-west.Xunyi area are located in the south of Ordos, and kinds of semi-quantitative experiments suggest that sediments came from northern Qinling Mountains, a part of stratum in the west part of Xunyi were effected by provenance from the north-west. Generally, the ability of supplying sediment from provenance was becoming weak during the period of Chang73, and then enhanced gradually. After the highest capacity inChang6, sedimentary source decreased till the last of basin evolution, except the small scale's strength during Chang3 period.It developed proximal braided river delta-lake sedimentary system, which was dominated by sedimentary climates and provenance. During the periods of Chang1011 and Chang 1, main sand distributary of braided river delta plain facies were distributary channel and banded. Braided river delta front-shallow lake deposits appeared during Chang9, Chang8, Chang3 and Chang 2. Underwater distributary channel sand occupied the most part of sand body. It became into the deep lake deposits during Chang7-Chang4+5. Meanwhile, The thick sandstone with high density in deep-water was not only turbidity current but also sandy debris flow, slump sedimentary sand body.Ordos basin was beginning to expand and the shoreline became wider from Chang 10 to Chang8. At the same time, climate was turning to warm and wet. Water deepened, so the environment got more reductive and paleosalinity of water decreased obviously. In initial stage of Chang7, the basin sink strongly. The research area changed into the deep lake facies environment. In this period, volcanic ash from frequent volcanism in the south of Ordos basin deposited with abundant organic matters, so the high-quality source rocks were developped with thin layer of tuff interlayers accompany. After Chang6 sedimentary period, the whole basin declined gradually and deep-lake line shrinked to the north of study area. With the basin silting and diminishing until Chang 1 period, the area turned into overwater environment.