Analyses Of The Provenance And The Depositional Systems Of Xujiahe Formation In Northeast Of Sichuan Basin

Northeast of Sichuan is the main gas source area of " Sichuan-to-East gas transmission project ", After marine facies carbonate strata exploration success, the terrestrial clastic rocks strata have also made great breakthrough in recent years. Nevertheless, the research extent of terrestrial clastic rocks formation is still low. For the actual needs of oil or gas exploration and on the basis of the theories of petrology ,sedimentology,tectonics and the principle of basin analysis,this article focus on several problems which troubled oil and gas exploration of clastic rocks strata in Northeast of Sichuan basin. These problems are: (1) Rock stratigraphic characteristics of the Late Triassic Xujiahe Formation from different depositional period (2) The evolutionary relationship of orogenic belt uplift-episodes and the the provenance direction (3) Late Triassic sedimentary system, especially how do the changes of provenance and direction control the sedimentary system in Northeast of Sichuan basin? The thesis has gained the following achievements:(1) Xujiahe Formation can be divided into five members and the 2nd member can be divided into upper and lower part on the basis of the division and correlation of the Xujiahe Formation of upper Triass. The distribution of Xujiahe 1st member and the lower part of the 2nd member stratigraphic are restricted, which mainly distributed in the West and yuanba region, however, missing in the northeast of study area and Tongnanba region. The other members are widely distributed in the study area. After each member partition according to rock types ,it is found that lithic sandstone reduce and quartz sandstone increase from the basin margin to basin.(2) The relationship of provenance and basin margin orogenic belt was researched by means of multiple debris analysis,sandstone debris analysis, heavy minerals and gravel characteristics analysis. It was found that, the tectonic was reversed from extrusion background to stretching background in Sichuan Basin after Indo-Chinese movement of the early scenes in the middle Triassic. so after the collision between South China block and North China block,Qinling orogenic belt was generated at the north of study area in Late Triassic. This orogenic belt in the late Triassic provided sustainability much substance to north-northeast of study area. The substance which is from sinian stratum -silurian stratum in south qinling area,included shallow metamorphic rock, siliceous rock, sedimentary and volcanic rocks, etc . The ancient Longmenshan orogenic belt which located in northwest of study area, started to uplift at the 2rd member depositional period early and overall uplift at at the 4th member depositional period. It provided much carbonate substance to northwestern of study area, which is from Devonian-Triassic strata in western of Sichuan basin. The Research show that Qinling orogenic belt is the mainly Provenance at xu1 depositional period, because of Longmenshan orogenic belt do not have uplift. However, the north part of longmen mountains begin to uplift and provide much carbonate substance to northwestern of study area at the 2rd member depositional period, but qinling orogenic belt is still mainly provenance; After Anxian movement at 3rd memeber depositional late period , longmen mountains overall uplift and is the mainly Provenance. Qinling orogenic belt is the secondary Provenance during the 4th-5th member depositional period. Moreover, luzhou - open jiang is uplift during the 1st-2rd member depositional early period. Obviously, it is one of provenance directions of study area.(3) The study area was divided into①continental system group: which mainly including alluvial fan sedimentary system, the river sedimentary systemand lake delta sedimentary system;②sea-land transition system group: which including delta sedimentary system;③marine systems group: which including coastal sedimentary system. The thesis described their characteristics and finally summed up the two types of Late Triassic depositional model: the depositional model of the sea-land transition during during the1st-3rd member depositional period, the continental alluvial - lacustrine depositional model during xu4 and xu5 depositional period.(4) The paper combines provenance evolution with the depositional systems of different depositional periods and draw lithofacies paleographic maps of different depositional periods. It is found that the time during the 1st-3rd member depositional period is marine facies depositional period ,when the ancient Qinling Mountains provides sediments continually from the North - North East to the south, which making sedimentary facies southward migration, such as the marine delta southward migration. The uplift and eastward migration of the northern section of Longmen Mountains after the 2nd member depositional early period make sedimentary facies eastward migration.The time during the 4th-5th member depositional period is continental facies depositional period .when the 4th depositional period, there are many alluvial fans at basin margin.some directly put into the lake as fan delta, other connect with alluvial plain. Because of Indo-China movement late scenes, it is erosion area in the northern of the study area, but the other area is the alluvial plain sediments at the 5th member depositional period.