Carboniferous Sedimentary Fill Evolution Of Dajing Depression At The Estern Junggar Basin

With the development of the oil and gas exploration in the eastern of junggar basin, the Oil and gas reservoirs of carboniferous volcanic rock gradually become the exploration focus. Studying the sedimentary environment and sedimentary fill characteristics of carboniferous in Dajing depression, it can not only reflect the hyperplasia- transformation process of central Asian orogenic belt and has important scientific significance; but also help volcanic oil and gas exploration and development in the future in our country. On the basis of the carboniferous strata and sedimentary analysis, drilling core as the main research object, the integrated use of petrological and geochemical test and analysis methods, structural analysis as auxiliary means, combining previous research achievements, this paper establish the carboniferous stratigraphic system, reconstruct the carboniferous sedimentary fill characteristics and set basin structures and properties. This thesis mainly made the following results and understanding:According to the chronology test results and seismic section features, in combination with characteristics of Paleontological strata, logging curves and the lithology contrast, the carboniferous strata of Dajing depression can be divided into five sets of sequence: Lower Carboniferous Dishuiquan Formation, The lower volcanic rocks section in the Batamayineishan Formation, The clastic sedimentary rocks in the Batamayineishan Formation,and The upper volcanic rocks section in the Batamayineishan Formation(Upper Carboniferous), Shiqiantan Formation.According to the seismic p-wave velocity, combined with the feature of magnetic anomalies, it shows that basin basement has the characteristics of nonuniformity. In addition, the positive εHf(t) value of the carboniferous volcanic rocks indicates that the basement is young crust, at least indicates that the region does not have the large-scale Cambrian crystalline basement.Volcanic rock samples in Batamayineishan Formation are mainly basic volcanic rock,and the samples are characterized by enrichment in LREEs and LILEs and depletion in HFSEs(e.g. Ta, Nb). The tectonic background of east Junggar changed from convergent island-arcs to post-collision. In the early Early Carboniferous, the east of the region became to post-collisional extension settings and then the west of the region became to post-collisional extension settings from late stage of early Carboniferous or early stage of Late Carboniferous.Early Carboniferous the study area has the characteristics of littoral and neritic facies sedimentary environment. In the early Early Carboniferous, sedimentary thickness ranges from 100 m to 500 m, and the study area develop the offshore sand-shale alternating layer and fan delta facies. Then Volcanic eruption did great effect on most part of the study area, and sedimentary thickness ranges from 500 m to 1000 m. The lithofacies mainly include explosive facies and flowing facies from basic to intermediate, and to acid. Late Early-Carboniferous, sedimentary thickness ranges from 300 m to 500 m, and the successions comprise sandstone and shale and subordinate volcanic rock.Late Carboniferous has the characteristics of the marine-terrigenous facies and continental sedimentary environment. The early of Late Carboniferous is mainly composed of the clastic rocks of littoral neritic facies and transitional facies, and sedimentary thickness ranges from 300 m to 1000 m. This stage develops volcanic activity and the lithofacies mainly include volcanic breccia, basalt and andesite. At the latest stage, the most part of the study area was subjected to erosion. Sedimentary thickness ranges from 200 m to 850 m and sedimentary environment is mainly composed of continental facies, such as the alluvial fan facies in Shiqiantan sag.