Turbidite Feature And The Study Of Reservoir Forming Condition Of Chang6in South Fuxian County Of The Ordos Basin

On the basis of collecting and summarizing previous researches, by comprehensive application of theories of sedimentology, petroleum geology, reservoir geology, and logging geology and the latest progress, on the full use of drilling, well logging, and dynamic data of production,based on regional geological background of the Paleozoic of Ordos Basin, focusing on Chang6formation of Southwest Fuxian County of the Ordos Basin, this article comprehensively analysises the Yanchang formation, the sedimentary system, and reservoir characteristics of turbidity. On this basis, taking basic analysis of reservoir forming conditions as the means, this article undertakes the study of hydrocarbon source rock, cap rock and reservoir cap assemblage, reservoir forming time and transport conditions and other aspects, finally puts forward the factors of controlling Chang6oil and gas enrichment. These provide theoretical basis for guiding the further oil and gas exploration and the other similar oil and gas reservoirs" discovery and forecasting in other regions.Chang6formation are subdivided into3subgroups, on this basis.this article focuses on the subgroups’research of sedimentary facies, subfacies, microfacies, deeply discusses the characteristics and the time and spatial distribution. Turbidity sand is fine, with incomplete BaoMa series, slot mode, channel mode and flame structure can be seen in it. Turbidity sand has grain size cumulative probability curves of two sections type or single section type and C-M diagrams paralleling to the C=M baseline, all these constitute the main features of Chang6typical turbidite sedimentary. In this zone, Chang6only develops middle fan subface, furtherly divided into3microfacies which are turbidite channel, the zone between turbidite channels, the transition zone of middle fan. In the plane, it mainly develops two turbidite channels on south to north.Each reservoir subgroup turbidite channels swings and changes slightly. Chang63and Chang62belong to early to middle constructing period of the turbidite fan development, developing turbidite channels, which sand body has a wide range and is thick. Chang61belongs to late period of the turbidite fan development. Its turbidite channels gradually retreat.The study on reservoir shows that Chang6reservoir is depositing sand body in middle fan turbidite channels of the delta front slump turbidity. The lithology is mainly fine-grained feldspathic sandstone, the size is generally small, the structure maturity and composition maturity are relatively low. The pore types are mainly intergranular pores, which belong to the very low porosity and very low permeability reservoir. Due to the differences of sedimentary microfacies and diagenesis, Chang6turbidity reservoir has strong macroscopical and microcosmic heterogeneity. The extensive development of crack not only improves the reservoir property, but also provides good channels for the oil and gas migration.The forming conditions of Chang6reservoir shows that Chang7source rocks has wide distribution, large thickness and high abundance of organic matter, the kerogen types belong to I Ⅱ1type, the kerogen has higher maturity and has entered oil peak, belongs to effective hydrocarbon source rocks of this area; oil and source correlation analysis shows that the crude oil of Chang6oil groups is come from Chang7hydrocarbon source rocks. The mudstone of Chang4+5oil groups and Chang6oil groups lateral phase change or variation of performance form the oil and gas shielding layer.The study of Chang6oil and gas enrichment regularity show that:①the rich source rocks of Chang6reservoir formation provided corporeal basis;②the sedimentary microfacies and the sand body develop degree are the determinant factors of controlling the distribution of oil and gas;③the good sealing and sheltering conditions are the key to the reservoir formation;④the local nose-like uplift has certain relationship with the reservoir formation. Chang6reservoir distributes in the most developed area of the fan turbidite channels sand body, in the plane, it’s mainly spread out from the north to east or from south to west along the turbidite channels, which has the characteristics that oil and sand body distribution has small area and the connection is poor. But in the local area, as the sand thickness is thick, many periods of turbidite channels sand body are staggered and superposed, the reservoir distribution are also exhibiting staggered and superposed characteristics, so these can form large scale oilfield. Looking for the better performance turbidity channel sand body and good cap rocks, and to strengthen the study on cracks become the key to the research and exploration in the next step.