Feature And Thermal Evolution History Of Paleozoic Source Rocks In South-eastern Ordos Basin

Characteristics of source rocks and their thermal evolution history in South-eastern Ordos basin are studied systematically, on the basis of research to features of source rocks, tectonics and thermal history of Paleozoic in the area. Ability of gas generation and change of gas producrion of Paleozoic source rocks in different thermal evolution stage through geological history were disscussed.Gas-generation intensity and hydrocarbon-generation center or source rocks kitchen of the areas were determined in order to prvides the basic information for gas generation, migration and accumulation in South-eastern Ordos Basin.The South-eastern Ordos basin consists of two tectonic units—Yishan slope and Weibei uplift.Paleozoic of Weibei uplift was divided into two sub-units by Jinghe fault, Shanbei paleo-depression and South margin paleo-depression.Paleozoic was distributed in South-eastern Ordos basin widly and its sedimentary faces change quickly. It can be divided into four blocks according to sedimentary faces, which are central paleo-uplift, Shanbei paleo-depression, South margin paleo-depression and Tongchuan-hancheng area. Ordovician distributed mainly in Shanbei paleo-depression and South margin paleo-depression but default in part of central paleo-uplift. Carboniferous and Permian distributed mainly in the north-east of central paleo-uplift. Thickness of later Paleozoic depositions in central paleo-utplift and the south part of the area is thin.Ordovician source rocks consists of marine mudstones , mud-limestones and mud-dolostones.The sedimentary center of Ordovician marine mudstones and mud-carbonate rocks lie in South margin paleo-depression and Shanbei paleo-depression.Evaluations show that mudstone, mud-limestones and mud-dolostones of Majiagou Formation in Shanbei paleo-depression and Pingliang Formation in South margin paleo-depression are middle-good soruce rocks. Source rocks of Majiagou Formation in South margin paleo-depression and Qingyang paleo-uplift are middle-poor source rocks.They belong to I and II kinds of organic matter.Thermal degree of the source rocks is wet gas-dry gas phase.The upper Paleozoic source rocks distributed in the east part of paleo-uplift, consist mainly of dark mudstones, coal seam and carbonate rocks in Shan 2 member of Permian Shanxi Formation to Benxi Formation of Carboniferous.The imput of organic matter of upper Paleozoic source rocks are various and types of kerogens are diversity but is humic-sapropel organic matter in general.Thermal degree of the upper Paleozoic source rocks is dry gas phase.Based on the researches to Paleo-geothermal degree using R₀-TTI method and analysis to thermal evolution history using the curve of sedimentary-tectonic evolution, Paleozoic source rocks in South-eastern Ordos basin was thought to be the phase of gas generation at later Triassic Yanchang period, gas generation and hydrocarbon expulsion peak at later Jurassic period followed migration and accumulation phase.Studies of Ordovician gas-generation intensity show that there were two gas generation center, one is Shanbei paleo-depression whose center lies in Shan 139 well, another is South margin paleo-depression whose center lies in Chuntan 1 well.The Shanbei paleo-depression periphery is the most favorable areas for gas exploration, South margin paleo-depression periphery is second.There are three gas generarion center in the upper Paleozoic, they are the Shanbei paleo-depression, the South margin paleo-depression and the later Palezoic center in Qingyang. Shanbei paleo-depression periphery is the most favorable areas for Paleozoic gas exploration, and South margin paleo-depression is second.The prospect for Paleozoic gas exploration in later Paleozoic Qingyang gas generation center and its periphery is need to be studied further.