Characteristics Of Lower Paleozoic Source Rocks In Micangshan Uplift Area, Sichuan Basin

Micangshan uplift develops on the margin at the north in yangtsensis platform, and extends Sichuan province and Shanxi province. The west of Micangshan uplift is Bikou microblock and Longmenshan thrust belt, and at the north of it is Qinling orogenic zone, and to the east is Dabashan thrusted belt, while at the south is Sichuan basin.The aim of research is to study lower Palaeozoic strata in this region, collect samples to analyse and discern the geologic features of source rocks. Lower Cambrian Qiongzhusi formation and upper Ordovician- lower Silurian are the main source rocks in Micangshan uplift area.The source rocks of Qiongzhusi formation are much more integrated and better in northwest and southeast than in southwest and northeast. At the ease of this formation is the black median laminaed shale and some silty shale, with silicious lunker.The lower-middle of it is grey-yellow calcareeous siltstone, laminated which is much conspicuous after weathered. The middle-upper of it is yellow-black median laminaed mudstone and some argillaceous siltstone. The upper is grey-yellow medium siltstone with limestone lentes.The effective thickness of Source Rocks is not more than eighty metres.The Source Rocks of the upper Ordovician- lower Silurian is few in the northwest, while outcroping well in the south. The first fraction of this formation is black median lamina shale with the siliceous rocks of radiolarite. The bottom of it is yellow-green median laminaed silty shale. The middle of it is a black thin shale. The upper of it is a black thin shale, horizontally stratified. The second fraction of this formation is grey-yellow median lamina shale and pelitic siltstone. The bottom of it is yellow-green shale with silty bands,and the middle is green- grey shale. The upper of it is the deeply grey thin shale with rheoglyphs. The effective thickness of it is usually from dozens to one hundred metres.The average TOC of the lower Cambrian Qiongzhusi formation is 2.29 percent, which means that this formation could be good source rocks. The latent capacity of pyrolysed hydrocarbon in more than 70 percent of these samples is less than 0.1mg/g, and even the largest one is no more than 0.3mg/g. The average value of the samples is only 0.07mg/g, which can not represent the latent capacity.The average TOC of upper Ordovician- lower Silurian is 0.11 percent, which is very low. The latent capacity of pyrolysed hydrocarbon is very low and 64 percent of these samples are between 0.1 and 0.2mg/g, which is also not the criterion of the latent capacity.The index number of organic material in source rocks of the lower Cambrian Qiongzhusi formation is more than 90 percent, most samples of which are no less than 97 percent. According to this data, we can see that the type of organic material in Qiongzhusi formation belongs to type I.The vitrinite reflectance of more than 50 percent samples is over 2 percent in the lower Cambrian Qiongzhusi formation. This means that the formation is over matured. The maximal temperature of thermal decomposition in Qiongzhusi formation is higher than 490 degrees centigrade, which shows that the maturity of organic material is in dry gas phase.The upper Ordovician- lower Silurian is over matured, as the peak temperature thermolysis is moer than 460 degree. This formation mainly produces combination gas and dry gas.On the chromatogram pictures of satisfied hydrocarbons, most of the samples show unimodality and the main number of carbon is no less than 22. The main number of carbon in source rocks is between 18 and 24, and 16 in samples of bitumen. This feature is connected with post mature peroid inseparablely.The OEP and CPI of those samples in Micangshan uplift are much close with each other, which are almost 1, except one sample(QT-64). This reflects that the maturity of organic material in this region is in upper peroid. The Pr/Ph of all samples is very low ,which is between 0.24 and 0.93. This indicates that phytane gains ground, which indicates that the depositional feature is oxygen free environment.According to analyse the ratio of tricyclic terpane to hopanoid, the ratio of gammacerane to hopanoid and the ratio of diasteranes to regular gonane in source rocks and bitumen in Sinian Dengying formation, we see that the bitumen in Sinian is relative to source rocks in Cambrian, but not from upper Ordovician- lower Silurian.The types of regular gonane are'V'and angular'L'. The prominent feature is that the number of carbon is 27 in preponderance. This indicates that the organism producing the source rocks were mainly from ordinary aquatic life and in reducing environment with some solt.The reservoir Sinian Dengying formation bitumen in Micangshan uplift has mang advantage features, such as good source rocks, excellent sealing gland, superior reservoir strata and effective conservation. So Sinian Dengying formation is worthy of exploration.The Cambrian Qiongzhusi formation shale in Micangshan uplift has similar characteristic to the Barnett shale in Fort Worth basin in America. It is suggested that the Qiongzhusi formation shale gas pool may have good future.