Reservoir Characteristics And Hydrocarbon Accumulation Regularity Of Chang8in Huanxian Area,Ordos Basin

Ordos basin is the second largest sedimentary basin on land in China which possesses abundant oil and gas resources and has bright ospects in oil and gas exploration. It has been becoming one of the important tactical area in oil and gas exploration in China. The amout of oil and gas resources and the degree which it is proven will directly influence the development of China petroleum Industry.The Huanxian district is located in the southwest margin of Ordos basin. The target intervals is member8of the Yanchang Formation of Upper Triassic(Hereinafter referred to as Chang8reservoir) which is below a lacustrine sedimentation called Chang7reservoir and on top of Chang9reservoir. In recent years, with great progress in the petroleum exploration in Chang8reservoir,it shows good exploration and development prospects.The reservoir characteristics in the reaserch area can also be able to deeply studied.It is found that the Chang8reservoir with low porosity and permeability is controled by various factors, the reservoir development is considered to use fracturing technology, so this paper tries to realize the lithology、physical properties and pore structure characteristics based on the analytic data of casting thin section, scanning electron microscope, X-ray diffraction,to provide the theory basis for the next step in exploration and development.The article continues researches under the guidance of sedimentary petrography and reservoir sedismentology,and its research object is Chang8oil layer of Yanchang Formation in Ordos basin.This study needs geology、well-logging、geochemical as a basis,and starting from stratigraphic correlation and sedimentary framework,we have studied the sedimentary provenance、reservoir characteristics and we have get clear on the sedimentary characteristics and the exhibition of the sand body.Finally,this paper summarized the sedimentary model and reservoir evaluation.According to the above research,the main achievements and cognitions are obtained as follows:1、Through the comparative study repeatedly of the Chang8oil-bearing strata of171wells in Huanxian area,the k1mudstone layer are considered as the main marker beds in the study area.Under the control of marker bed、the principle of cyclic comparison tracking the adjacent wells and the similar strata thickness tuff,the main oil-bearing strata Chang8sand group is divided into6sub-stratums. 2、Through observing the cores,the color of Chang8sandstone is gray and dark gray. The rock types of study area can be divided into sandstone、siltstone、mudstone and some transitional types.During its deposition,shallow water delta was developed.The shallow water environment markers,such as coal seams,plant leaves and foots,worm holes,and so on,were widely distributed in the cores indicating the relatively shallow water during its deposition (several meters to over ten meters).The results indicated that the study area belong to the delta inner front deposition,which could be identified into underwater distributary channel,underwater distributary interchannel, Underwater crevasse-splays,mouth bar.The primary is the former two kinds of micro-facies;3、The research of reservoir characteristics indicates that the lithology are middle-fine grained feldspathic litharenite sandstone,lithic arkose sandstone with low component maturity and structure maturity.The main interstitial material is authigenic clay mineral and carbonate cement. Quartz overgrowth is uncommon. The pore type is dominated by residual intergranular pores,corrosion pore space in feldspars and detritus solution pores. The pore type is dominated by residual primary intergranular pore, intergranular vulgar pore, interquartz micropore and fracture with the reduced-neck, silm, fine tube and cluster pore throat occurring as the main throat type. It is a low porosity and low permeability reservoir. The examination of the controls of the reservoir rocks shows that the component and structure size of the sandstone are important conditions which affect the reservoir’s performance.The diagenesis of the matrix is the dominant factor which affects the resevrior’s performance.During diagenesis process,the main factor destroying porosity is function of compaction and cementation. Solution is beneficial to formation’s development,especially secondary pore formed by feldspar dissolution have become the important factor to affect reservoir property.The sedimentary microfacies is the principal geological factor controlling the development and distribution of the reservoir rocks in the study area. Microfissuring can improve the reservoir permeability.4、Based on the previous research results, low permeability and low porosity actual condition in Ordos Basin, according to the sedimentary characteristics, reservoir characteristics of Chang8, the following parameters are choosed to classify Chang8reservior.The main parameters include sedimentary characteristics,interstitial material, physical property characteristics and pore types, pore structure characteristics, mainstream larynx way radius, effective porosity, oil saturation, etc, as the main parameters of evaluation.Among them,the sedimentary microfacies types and interstitial material reflect the macro-characteristics of Chang8reservoir. Reservoir pore structure parameters include porosity, permeability. According to the above parameters,the reservoir evaluation standard is finally worked out.5、Based on the above reaserch,a systematic analysis has also been done to the enrichment factors and distributions of oil-gas in study area. Analysis shows that the Chang8reservoir have a very good hydrocarbon source condition.distributed channel sand bodies in study area provide nice accumulation conditions,superimposed distributary channel sandstone and the microcrack systemare the dominant migration pathway. Combining with the above research,the main reservoir-forming control factors are finally summarized.