Study On Depositional Facies And Evaluation On Reservoir For Chang 6～4+5 Member Of Yanchang Formation In Western Mahuangshan

Triassic Yanchang formation is the main oil bed of western Mahuangshan area which is located in the west of Ordos basin. In the area, the fault system is complicated, the type of sedimentary facies and the distribution of sand body is not very clear, the late diagenesis is intense, characteristics of the reservoir is still unknown, which affects the recognition of enrichment regularity of oil and gas and restraints the exploratory development. Wherefore, fine analysis of sedimentary microfacies, the division of diagenetic stage, the study of reservoir characteristics and identification of the oil-water distribution are done in order to provide a reliable geologic basis to develop the reservoir reasonably and efficiently.On the basis of the theories of high-resolution sequence stratigraphy, reservoir sedimentology and reservoir physics, etc, through the core, logging, thin slice, mercury penetration, relative permeability and so on, using the technology of sand contrast oriented by sedimentary microfacies, thin slice identification, analysis of reservoir diagenetic evolution and the classification and comprehensive evaluation of reservoir, we start sedimentary microfacies and reservoir studies in Chang4+5 and Chang6 oil beds of western Mahuangshan area. The following results are obtained in the thesis:Firstly, the high-precision isochronous stratigraphic framework of the study area is established and the plane distribution of deposition system and characteristics of vertical evolution of intended intervals is analyzed in the isochronous stratigraphic framework. Then the depositional models of delta front are summarized and the mechanism of sandy debris flow is discussed. The study area is mainly delta front, in the front of which sandy debris flow deposition is developed. Underwater distributary channel and distributary mouth bar are the dominant sedimentary types. From bottom to top , the distal bars and the distributary mouth bars decrease gradually, while the underwater distributary channels increase. On Plane, from the Chang6 to Chang4+5, two small delta front lobes gradually merge into a single large body and the distribution of sand body extends.Secondly, secondary dissolution porosity is the main pore type and diagenetic fissure which is along the direction of phyllite is the favorable accumulational place of oil and gas. There are three secondary porosity zones in the vertical direction, mainly for the dissolution of calcite and clay minerals. Chang6 to Chang4+5 periods is in the medium diagenetic phase A and B stage. Combined with the buried history, we analyze the main characteristics of every diagenetic stage, summarize the evolution model of diagenesis and explain the reasons of the low porosity and permeability of reservoirs. Thirdly, sedimentation is the material condition for the formation of low-permeability reservoir, diagenesis is an important factor in the transformation of the reservoir, faults and fractures control the hydrocarbon migration and accumulation. It is the middle of the underwater distributary channel and the top of the mouth bar where the feldspar dissolutes strongly, the cementation of calcium is not very serious, which is also the beneficial place of hydrocarbon accumulation.Last, with the research of microfacies and reservoir characteristics, we discuss the causes and characteristics of sand bodies. Then, we take physical property, the parameters of mercury penetration and the casting thin slice information to evaluate the reservoir comprehensively. At last, we propose the beneficial exploration areas.