Geologic Structure And Evolution Of The South-centralsegments Of West Ordos Basin

The West Ordos is a SN trending narrow belt which is north to Table Mountain and south to Pingliang area. Its west is bounded by Helanshan、Liupanshan arc structural belt and east to Tianhuan depression. It is located between two different tectonic domains of China. Its structure is extremely complex, so does its lithological changes. The study of basic geology and petroleum exploration is early, but there are still many questions to be solved from now on. The reason is that its tectonic is very complex.So seismic imaging is very difficult. In the early days, people couldn’t get highly accurate seismic data because of technology limited. In recent years, technology and seismic data processing improved rapidly. Changqing oil field began the petroleum exploration of West Ordos again in 2012.Both the anew proceed early seismic data and new collection highly accurate seismic data provide the basic of study of real characteristics of geology structure. In addition, the study of geology structure of West Ordos need to be conducted by new theory and systemic and comprehensive analysis. Therefore, we form more accurate recognition to the real geology structure from the results and it will have more scientific significance. In the meantime, it can conduct the petroleum exploration of West Ordos and has the production of symbolic meaning.This paper is conducted by fault related folding theory and so on. And this paper uses the latest drilling data and highly accurate seismic data to structural interpretation with limited by shallow geology and chronology information for quantify and qualitative analysis of its characteristics of geology structure. On this basis, this paper proposes new segment division scheme and summarize the characteristics of geology structure and evolution of each segment and quantify discuss its difference of each segment. At last, this paper proposes the favorable exploration areas according to the basic petroleum geology characteristics and get the main research progress as follows:1.The West Ordos conforms with the pattern of inverted multi-decollement tectonic deformation system of early tension and late extrusion. Late Proterozoic, early Carboniferous and early Cenozoic(partial area) are tension periods. But the late Triassic and Jurassic and so on are extrusion periods. The study area shows complex tectonic evolution of multi-stage and multi-phase. There are many detachment layers, such as the mud shale of the bottom of Cambrian, coal of Carboniferous and Permian, mud shale of Triassic and coal of Jurassic. Therefore, it is an inverted tectonic deformation system with multi-decollement layers.2.The major of West Ordos is a tectonic belt of Yanshan stage. This paper make sure its tectonic deformation phrase according to the analysis of unconformity,growth strata and the method of Low-temperature thermochronology. On this basis,this paper propose that there are difference uplift among different areas, and it appeared to extrusion at late Triassic, but its amplitude was not huge,the real time of major over-thrust phrase is late Jurassic and keep going from Cretaceous to now to make the structure of early phrase modified and complexed. Different areas have difference in geology structure result in the different directions, intensity of stress and lithological changes.3.The West Ordos has a obvious characteristic of segment division. And different segment has different characteristic of geometry and kinematics. It can be divided into two regions: the thrust belt of West Ordos and West margin rift belt. And different from previous studies, to the thrust belt of West Ordos, this paper divided it into 5 segments with the elaboration structural interpretation for highly accurate seismic data using the method of analysis of axial surface: Ciyaobao segment, Majiatan segment, Huianbao segment, Shajingzi segment and Pengyang segment. Each segment has the characteristic of over-thrust, but there are obvious differences among them. To the West margin rift belt, it performed over-thrust before Cenozoic, and strata were uplifted and corroded. But it turned into tension environment and performed a half graben structure. The tendency of fault of Hengshanbao segment is different from other segments. So, this paper speculate that it should be the back thrust belt of the area of Yinchuan garben and be modified at last.4. The West Ordos has multi-source rocks, multi-reservoirs and multi-seals. It has the same characteristic of petroleum with Ordos basin and form multireservoir-cap assemblages. The time of tectonic development has fine matching with it of the generation and expulsion of hydrocarbon of source rocks, and it is benefit to oil-gas accumulation to early or simultaneous traps. The major hydrocarbon accumulation time is middle and late of early Cretaceous. Later tectonic movement manifest as transformation or destruction. Therefore, there are two petroleum exploration areas: Yandunshan target and Shajingzi target, they have the potential of forming lithologicl hydrocarbon reservoir and structural- lithologicl reservoir.