Study On Sedimentation Simulating Experiment Of He-8Period In Ordos Basin

Ordos Basin is the second largest sedimentary basin in China and is rich in oil, gas, coal and other mineral resources. The general tectonic features show a anisomerous syncline of widely slow eastern side and precipitously narrow west, running north to south. The neotectonic movement in the region was mainly vertical movement with less regional faults. Based on the basement nature, geological evolution and tectonic characteristics, the regional tectonic can be divided into six secondary tectonic units, including Yimeng uplift, Weibei uplift, Jinxi fault-fold belt, North Shaanxi slope, Tianhuan depression and western margin thrust belts.The Shihezi formation of middle permian series can be divided into lower Shihezi formation and upper Shihezi formation. Lower Shihezi formation can be divided into five gas-layers from base to top, lower He8, upper He8, He7, He6and He5. Upper Shihezi formation can be divided into four gas-layers from base to top, He4, He3, He2and Hel. The He8is the main horizons studied in the paper and is further divided into six layers from sedimentary cycles.As China’s the second largest oil and gas accumulation Basin, Ordos Basin has the characteristics of multiple oil and gas bearing sequences, broad exploration prospect and so on. The wide-spread sandstone and a higher degree of gas reservoirs are formed in upper Paleozoic in Ordos Basin, and many gas fields has been found including Yulin gas field, Sulige gas field. As one of the main gas producing pays, reservoir sandbodies of He8are developed particularly. With the gas field exploration and development, plenty of research works have done on characteristics and formation of sandbody and so on, but lack of understanding of the key factors of control large area distribution of sandstone and vertical superimposition. So it has theoretical and practical significance to carry out sedimentation simulation experiment of braided river delta.This thesis takes He8section in Ordos basin as the research object. Based on investigating regional geological background, tectonic evolution, sedimentary characteristics and establishing geological model of sand deposition process on the basis of the similarity theory of deposition, directed by similarity theory, physical model is determined with the experimental apparatus, designing detailed experiment scheme, according to change parameters of flow, mud and sand, boundary conditions, the forming process of braided river delta of shallow-water slope are simulated. The systematic study on the main controlling factors of the formation and evolution of the braided fluvial delta is completed, and the formation mechanism of large area distribution of sandstone under the background of the gentle structural is revealed, vertical overlay of sandstone is analysised, the characteristics of the sedimentary micro-facies and sedimentary structures are discussed.The quantitative recovery of the palaeotopography and slope of He8period shows that the terrain slopes gently and the slope is less than1°. Provenance analysis show that during the deposition of He8session, the clastic sediments were from northern and northeastern basin, multi-source supply control the lithological characteristics of reservoir sandbody in different regions, lithic sandstone and lithic quartz sandstone in east and quartz sandstone in west. The reservoir sandstone of He8session is mainly of medium-coarse grain, coarse grain distributed from the north to the south. The sandbody of He8session stacked continuously in vertical direction, and they stacked in shape of blanket and distributed in large area, forming the braided river delta of shallow-water slope depositional model.According to sedimentation simulating experiment, it is considered that the controlling factors of the formation and evolution of the braided fluvial delta include basement subsidence, gradient, flow, lake level, sediment flux and so on. The basement subsidence control the slope and accommodation. Flow dominate the development of distributary channel and types of sandbody, and the importance decreased gradually from floodwater to low-flow to middle-flow. Lake level affect the development of distributary channel and delta. Sediment flux control the sedimentation rate of braided fluvial delta.The results of sedimentation simulating experiment show that gentle landscape, frequent changes of the lake level, strong source supply, high sand, high velocity stream, multi-water intersected are conducive to widespread sandstone. Gentle landscape is based, water flows with little speed, distributary channels are developed and lateral migrated continuously, the sandbody assemble in horizontal direction; Frequent changes of the lake level is important controlling factor. The alternation of relatively stable lake level and lacustrine regression is the important reason for the widespread sandtone; Strong source supply is material basis, which accelerates the deposition process, sandbody migrated continuously and assembled; High velocity stream is considerable driving force, makes coarse particles long range transportation, distributary channels migrated continuously, which made channel mouth bar assemble in vertical direction, are beneficial to extend sandstone in vertical and lateral direction, coarse particles distributed widely. Intersected muti-water is an important way, catchment bodies expand widespread sandstone area. Sandbody compose affect the distribution characteristics, they usually stacked continuously in vertical direction and distributed in large area with high sand, and the continuity of sandbody is poor with high mud. The matching between basin subsidence and deposition rate ensures development of glutenite.Based on experimental and section analysis, this paper summarizes vertical superimposition of sandbody, including interaction cutting of fluvial channel and sandbar, interfingering under the multi-source conditions. The extent of sandbody superimposition is closely related to the change of relative lake level, migrating of distributary channel, river downcutting, interacting of multi-source.Under the conditions of this experiment, braided river delta plain develop and braided delta front are rarely seen. Distributary channel, longitudinal bar, diagonal bar, mouth bar, abandoned channel, subaqueous distributary channel, interdistributary estuary in delta front can be divided, transverse bar is not developed. In the process of this experiment, longitudinal bar and diagonal bar are interchangeable, and mouth bar also can transform to longitudinal bar and diagonal bar.From the fine description of the three-dimensional shape of braided fluvial delta sandbody, it is considered that the bedding structure is developed. The lateral accretion cross-beddings are observed in cross section, parallel bedding, cross-bedding and foreset are also observed in vertical section.