Research Of Source Rocks In Upper Paleozoic, Central Ordos Basin

There are many gas reservoirs in Upper Paleozoic, Ordos Basin, and the source rocks are diverse. In the northern part of the basin, it has been found after many years of development and exploration that the large gasfield in Upper Paleozoic, such as Sulige, Yulin and Daniudi gasfield, but exploration starts late in the central basin and the systematic study of hydrocarbon source rocks rocks is very weak. Although the gas reserves of Yanchang gasfield in the central basin have been increasing year by year, but the geological basic research of gas is weak and the correlation data are scarce, which greatly restricted the further exploration work in this area. However, the source rocks have obvious control effect on the formation and distribution of gas reservoir, there has not been studied comprehensively and systematically in the aspects of geochemical characteristics and hydrocarbon generation-expulsion characteristics of the source rocks in Upper Paleozoic in this area. Therefore, this paper has carried out in-depth analysis on hydrocarbon source rock distribution, geochemical characteristics and hydrocarbon generation-expulsion characteristics on the basis of a large number of core sample data of new wells, in Upper Paleozoic, central Ordos Basin. Research findings not only helps to deepen the understanding of the hydrocarbon source rock geochemical characteristics in central Ordos Basin, but also to provide reference for further study of the gas migration and accumulation characteristics and gas exploration and development.This paper is guided by the coal measure source rocks hydrocarbon theory, and through the combination method of the geochemical characteristics of source rocks, the basin thermal evolution history and hydrocarbon generation-expulsion characteristics of source rocks, a comprehensive research of geochemical characteristics and hydrocarbon generation expulsion characteristics of source rocks in Upper Paleozoic, central Ordos Basin, was carried out systematically, including definiting the organic matter abundance, type and maturity changes of source rocks, analyzing the hydrocarbon generation-expulsion characteristics and rules, and discussing the main hydrocarbon generation-expulsion stages, determinating the hydrocarbon generation- expulsion center, which provides a basis for further research on the rules of gas migration and accumulation rules of gas in this region.Based on the research of fine division and correlation of strata, the sedimentary facies of Benxi Formation, Taiyuan Formation and Shanxi Formation of the Carboniferous-Permian in central Ordos Basin were analyzed that the coal measure source rocks in Upper Paleozoic mainly consists of a set of continental and transitional phase deposited mudstone, coal and limestone. The mudstone distribution in the research area was widely distributed in the eastern region. The coal was mainly developed in Shanxi and Benxi formation. The distribution of the coal was relatively thin in the central region and the thickness of coal in the east and northwest of the research area. The limestone was mainly developed in the top of the Taiyuan formation, the thickness was thinner, and the distribution was small, only a part of the well was visible.Through the analysis of organic matter abundance, type and maturity of the coal measure measure source rocks in Upper Paleozoic in the research area, the source rocks were comprehensively evaluated. The mature coal was the most important source rocks, and the mudstone has strong hydrocarbon generating capacity, which was only inferior to coal, and the Shanxi formation mudstone was a medium-good source rocks, and the Taiyuan formation and Benxi formation mudstone were good-excellent source rocks; however, the contribution of limestone was relatively small. The contents of organic carbon in the research area of shale source rocks were generally characterized by the low north and high in the south, and the contents of organic carbon in most areas was higher than 0.5%. The kerogen types of mud source rocks in Upper Paleozoic, central Ordos Basin, was mainly type ? (humic type), and a small amount of type ? and type ?, its main source of organic matter of source rocks in terrigenous organisms and a small amount of aquatic organisms, with obvious diversity characteristics, the thermal evolution of organic matter in the research area has high maturity, generally reach over mature dry gas stage, and in the southern Fuxian-Yanchang area, the vitrinite reflectance was up to more than 3.0%, the circumference of the ring banded decreased gradually.Through the analysts of the thermal evolution stages that coal measure source rocks experienced in Upper Paleozoic, central Ordos Basin, it believes that the temperature of sets of source rocks experienced increased gradually the deposition, and in a large number of stages of hydrocarbon generation-expulsion in the middle and late Jurassic to early Cretaceous period, the hydrocarbon generation-expulsion intensity of source rocks reached its peak, after the early Cretaceous with the decrease of formation temperature, the hydrocarbon generation-expulsion rate decreased significantly and tend to stop. In Upper Paleozoic of the research area, the accumulated hydrocarbon generation intensity of the coal measure source rocks was (25?65) ×108m3/km2, and the accumulated hydrocarbon expulsion intensity was (15?60) ×108m3/km2. The hydrocarbon generation-expulsion centers were mainly located in the Suide-Ansai-Yanchang area of Northern Shaanxi, and the accumulated hydrocarbon generation intensity of the coal measure source rocks was (40?65) ×108m3/km2, the accumulated hydrocarbon expulsion intensity was (35?60) ×108m3/km2.