Sedimentary Facies And Depositional Environment Of Middle Permian Pingdiquan Formation,Huoshaosan Area,Northeast Of Junggar Basin

Huoshaoshan area is located in northeastern Junggar Basin, with northern part adjacent to Karamaili Mountain and south boundary restrained by Shaqi uplift. In this area, Middle Permian Pingdiquan Formation (P2p), which characterized as gray, gray-black, and greyish-green mudstones, sandstones, sandy conglomerates, microcrystalline dolomite, and limestones, is an important hydrocarbon-bearing reservoir, as well as it is in whole Junggar Basin. However, many unusual minerals, such as aegirine, aegirine augite, batisite, analcime, calcite, dolomite, and pyrite etc., were found in sedimentary rocks of Pingdiquan Formation by the study of mineralogy and petrology. Meanwhile, a special type of sedimentary rock, named "porphyritic" dolomite or "porphyritic" shale was also found in Pingdiquan Formation. This rock has the volcanic porphyritic texture, containing "phenocrysts" and "matrix". Phenocrysts are often composed of coarse-grained calcite, dolomite, batisite, and pyrite. But the surrounding rock "matrix" was microcrystalline dolomite, black mudstone, or dolomitic mudstone, rich in organic matter.According to the mineral assemblages, rock associations, sedimentary structures, fossils, and geophysical features, four types of sedimentary facies and eight sub-facies and microfacies were recognized in Pingdingquan Formation, including lacustrine facies, fan-delta facies, magmatic-hydrothermal exhalative facies and gravity flows. Lacustrine facies was basic sedimentary facies of Pingdingquan Formation. Two sub-facies were recognized, including coastal shallow lake subfacies in lower and middle Pingdiquan Formation, and semi-deep lake sub-facies in upper Pingdiquan Formation. Fan-delta facies (only fan-delta front recognized), was distributed mainly salty water. The negative trend of ?18O could be affected by temporary temperature rises, causing by intermittent hydrothermal exhalations. Major elements and their ratio show that research area was probably in a humid environment at that time. Meanwhile, trace and rare elements reflect a relative high salinity, anoxic, weak reduction, and partial alkaline depositional environment in Pingdiquan peroid.In north part of research area in lower and middle Pingdiquan Fm., lacking of fan-delta plain sub-facies. Magmatic-hydrothermal exhalative facies, with the feature of "porphyritic" dolomite, unusual mineral assemblages, was usually found in deep-lake facies, and was thought to be related to the activities of magmatic and hydrothermal exhalations in early and middle Pingdiquan period. In lower Pingdiquan Fm, it was found in northern part of Huoshaosan area. In middle Pindiquan Formation, it distributed in the center area. Two gravity flows, debris flow and turbidity, were also found in Pingdiquan Fm. Turbidites in middle Pingdiquan Fm. are very good hydrocarbon reservoirs in research area.Analysis of geochemical data indicates that Pingdiquan Fm. was formed in a quite high temperature and water. The negetive trend of ?180 could be affected by the temporary rise in temperature caused by the intermittent activity of hydrotherm. The major elements and their ratio indicate that the study area was a humid environment. The trace and rare elements infer that the major environment of research area is high salinity, hypoxia, weak reduction, lacustrine facies during the sedimention period of Pingdiquan formation. In combination with the large tectonic background, the study area is mainly the warm and humid, high salinity, hypoxia, and the reduction of the inland lake sediments in the condition of the reduction of the continental rift.