Oil Enrichment Regularity And Target Area Forecast Of Chang2Reservoir In Youfangzhuang Area, Ordos Basin

The Youfangzhuang region is located in the Ordos basin in Shanbei slope’s central, which is rich in Mesozoic petroleum, but the whole exploration and research of Mesozoic petroleum is still in a low level. Based on research and observation of the cores, data of this area such as costing thin sections, SEM(scanning electron microscopy), physical property, size analysis, porosity image, mercury source rocks, oil color-qualitative analysis, well logging and oil testing is comprehensively used in this paper. With methods and theories of stratigraphy, sedimentology, reservoir geology, logging geology and petroleum geology, the stratum of Chang2formation is divided and sublayers are compared; Analysis of the sedimentary facies and sand body distribution characteristics, study of the reservoir characteristics, carried out the hydrocarbon source rocks characteristics and oil-source correlation. On above basis, the oil enrichment regularity of Chang2formation(oil-bearing) in Youfangzhuang area is summarized and the favorable target area is finally forecasted.According to the traditional division method, this paper divided Chang2formation into Chang21, Chang22, Chang23, using main and auxiliary symbol layer of this region, cyclical control and depth reference methods. Among them, the Chang21is the main oil-bearing formation, according to the sedimentary cycle is further divided into Chang211and Chang212two sand group. And several connecting-well sections were established to make strata correlation. On the basis of marker beds have been identified, Selected four backbone profiles were compared in the study area:①Hu130-Xin1-A26-A34;②Yuan72-A24-All-A15-A19-A35.③A40-A46-A16-A22-A17-A6-A5;④Ding117-Ding608-Ding22-Hu108-Hu112-A27。The lithological association of the study area shows this area is a delta deposition, the first combination is distributary channel deposit of delta, the second combination is mouth bar deposition of delta front.According to the types, characteristics, structural characteristics and deposition of the rock and log interpretation, determined Chang2reservoir group is delta plain sub-facies. On the basis of regional sedimentary background, combination of geological features and logging phase analysis, determined the main sedimentary micro facies in this study is: distributary channel micro facies, plain distributary channel flank micro facies and interdistributary estuary in delta front microfacies.Research and analysis of data shows that, the mainly reservoirs of Chang2in the study area is medium to fine grained arkoses, flour-fine grained feldspar sandstone, sandstone grains smaller, and sorting is good, the main reservoir is the porosity, the main reservoir is the intergranular pores, feldspar dissolution porosity, low porosity and permeability are the main characteristics. Porosity and permeability of Chang2reservoir have a good positive correlation, and penetration increases with increasing porosity. The main pore types of Chang2are Sheet throat or curved sheet throat. Reservoir throat of Chang2is divided into thick throat(>10μm), medium throat (5～10μm), fine throat (1～5μm), micro throat(0.01～1μm). According to the above criteria, the most throat of Chang2sand sets is thin throat, accounting for68%, and crude throat accounting for17%.In fact, the formation and accumulation of oil and gas reservoirs in the Triassic Yanchang formation is controlled by channel sand body, nose-like bulge background, migration channels, hydrodynamic, and other factors.1) Distributary channel sand body is the basis of geological reservoir formation;2) The upper sand body in Chang2of Yanchang formation is the point of hydrocarbon migration;3) Favorable sandstone body is the direction of hydrocarbon migration and accumulation;4) The main reservoir trap conditions in Chang2is compaction construct nearly north-south distribution interdistributary bay (depression) and the distributary channel sand body which is formed on the background of west-dipping monoclinic structure;5) Nasal fold structure has a certain control on the role of the regional oil and gas accumulationOil and gas enrichment patterns of Yanchang group Chang2reservoir in the study area can be summarized as follows:The depositional environment has an important control role in the formation of the reservoir, distributary channel sand body is the main place of oil and gas accumulation, the compaction construct provides a good trap for the formation of reservoir, the region where the formation water alternation of relative stagnation in the Jurassic is conducive Chang2reservoir’s formation.Favorable area assessment is carried out in combination with the main factors which affect hydrocarbon accumulation、hydrocarbon migration and accumulation. Forecasting to the favorable area is divided into three categories:Class Ⅰ predict favorable areas is rolling development block, directly provide the basis to build capacity of the rolling development. Class Ⅱ predict favorable area is a rolling evaluation block is based on evaluation of reservoir exploitation value and proven reservoir scale, ready to build capacity for the purpose. Class Ⅲ predict favorable areas that scroll exploration blocks, the purpose is to evaluate the oil content of the different blocks, the discovery of new reservoirs, which can provide the basis for prospect and disposition.According to the selection and evaluation criteria, this study preferred to two Class I favorable block, three class Ⅱ favorable blocks, one Class III favorable block, and provides more fully geological basis for next exploration and development of Chang2reservoir in survey region.