Study On Sedimentary And Sand Body Distribution Of The 10th Member Of Yanchang Formation Of Zhijing-Ansai Area In Ordos Basin

The Ordos basin is very rich in natural gas resources,and its oil and gas exploration has a history of more than 30 years.The main object of basin exploration is formation — lithologic gas reservoir.What's more,the gas and oil reservoir is characterized by low porosity,low permeability,low abundance and thin reservoir,etc.This paper mainly discusses about the Zhijing—Ansai region of the Ordos basin.The area starting at Jingbian County in the north and ending at Yongning County is about1.2 x 104 km2.The east of Zhijing—Ansai area is Ansai County and Wuqi County is in the west.The main investigative layer in this area is Chang10 oil group of Yanchang Formation,in which the Chang 101 oil reservoir is the prior oil layer under research.In this paper,through the field outcrop as well as the core observation description of the key wells,we studied the characteristics of sedimentary rocks of Chang 10 oil group in color,lithology,sorting,rounding and bedding plane structure in Zhijing-Ansai area.In the meantime,by integrating diverse analyses and assay data like thin section analysis,trace elements and other appropriate analytical methods,it's more clear to define the geological features of sedimentary rocks.On this basis,single composite histogram of the sedimentary facies of Chang10 in the study area can be drawn out,with the previous collection of logging data codified by integrating the features of rock electricity,particle size,cycle analysis.According to the principle,from dot to line and then to plane,the sedimentary connected well profiles and plans of sedimentary facies were formed,which was based on selecting a total of 11 main sections that were parallel or perpendicular to the source in the work area.By systematically analyzing the sedimentary system of Zhijing-Ansai area.the final formed sedimentary facies model makes the evolution mode in study area distinct.In accordance to the matter-source supply on the basis of the research of sedimentary facies,the research work on the sand body structure clarifies its distribution range and characteristics,laying a solid foundation for the subsequent oil and gas development.According to the distribution characteristics of light minerals,heavy minerals and lithic components of sandstone,it comes to the conclusion that the provenance of study area were mainly derived from the NNE Yinshan Mountains orogenic belt and Lvliang uplift.Based on the marked bed in the study area and taking the features of electric petrophysical response and sedimentary cycle rule and the principle of strata thickness,the important layers,Chang 1011 oil group,can be divided into three parts: Chang 1011,Chang 1012 and Chang 1013,from top to bottom.As for the key sand group Chang1011,where most oil wells belong to,can be further divided into Chang 1011-1 and Chang1011-2.And 11 stratigraphic contrast sections were made to study them accurately.There is a research to show that the total thickness of the strata of Chang 101 oil group is about 110 m and Chang 1011,Chang 1012,Chang 1013 layer generally have few changes in the study area.The small changes just exist in some districts.In the whole study area,the average thickness of the strata of Chang 101 oil group is the largest,about 50 m.Two internal small layers,Chang 1011-1 and Chang1011-2,as well as Chang 1011 and Chang 1012 are all about 20-35 m.According to the research on the core observation,logging curve,casting sheet,trace elements and grain size analysis,in the backdrop of regional sedimentary,it is considered that the Chang 101 segment in the study area belongs to the delta plain sub sedimentary facies,developing distributary channel,interdistributary lowland,distributary channel lateral margin and other sedimentary microfacies,and the Chang10 phase sedimentary model has been established.Through analyzing the research,the Chang10 phase is mainly based on the high construction framework sand body that deposited in the micro phase of the diversion channel,which extends from the northeast to the southwest.The lithology of sand body is relatively coarse,primarily including packsand and medium-grained sandstone.It develops groove,block,plate and other large-scale cross bedding and the reservoir space is developed in terms of being a favorable reservoir body.It is considered that the research area's source mainly comes from the northeast by the comprehensive study on light mineral analysis,heavy mineral assemblage characteristics and the ancient flow.The study area in Chang 101 is distributary channel microfacies in delta plain mayor phase,with an advantage in deposition conditions and sand development generally,has a strong scalability in horizontal and widely distribution,and its longitudinal thickness is large.The sediment grain size in distributary channel microfacies is relatively coarse and is profitably transformed by river and lake waves during deposition.With good sediment grain sorting,favorable formative reservoir conditions,the distribution of source rocks and tectonic and other controlled conditions,it's advangeous to develop oil and gas reservoir,so oil and gas are mainly reserved in these areas in Chang 101 oil group.By doing research on the thickness and distribution rules of the sand,a detailed anatomy of sand vertical structure and plane distribution rules were able to be carried out.The sand body in Chang 101 oil group are developed,river diversions intersect into a planar mesh distribution of sand body,multi-stage longitudinal channel sandstone stacked to form a thick layer sandstone with an average thickness of 23.2m.These thick layer sandstone provide favorable reservoir for the hydrocarbon charging.