Study On Structural Features And Natural Gas Preservation Conditions Of The Lower Paleozoic In The Southern Ordos Basin

Recent years,with a series of major discoveries of Marine carbonate exploration in Sichuan Basin,Tarim Basin and Ordos Basin in China,Marine carbonate oil and gas exploration has received more and more attention.The southern part of Ordos was in the developmental position of marine deposits in the Early Paleozoic,with strata well developed strata of the early Paleozoic and the thick deposition thickness.It is of great significance to study the structural evolution and gas preservation conditions of the ancient Marine strata for oil and gas exploration.Based on previous studies,this research carried out systematic field observation,profile measurement,and sample collection in the southern Ordos Basin.Deep research was conducted on the structural characteristics,deformation stages and evolution history of the southern basin.Besides,a variety of methods were applied to analyze caprock and its preservation conditions of the Lower Paleozoic.On this basis of the above research,This paper evaluates the natural gas exploration prospects of the Lower Paleozoic in the study area by integrating the strength of the Lower Paleozoic structure reformation,reservoir development degree and caprock distribution conditions of the Lower Paleozoic in the study area.The main understandings obtained from the research are as following:(1)The southern part of the basin develops faults,and the strikes are mainly near east-west,northeast-north east,followed by north west-north west west,And from south to north,the scale of the tectonic deformation increased,the strength of the tectonic deformation decreased.The faults were mainly formed in the Yanshanian,followed by the Caledonian and Himalayan.(2)The southern part of the basin has undergone the superimposition and transformation of multiple tectonic movements.The Yanshanian period is the strongest,mainly manifested as a series of anticlines and associated fault structures.It is the main structural deformation period of the southern Ordos Basin,which established the structure framework of the Weibei Uplift.the Caledonian was strong in the western part of the southern margin of the basin,mainly manifested as a series of thrust faults dipping southward and northward;at the end of the Late Triassic,the Indosinian movement was relatively strong in the southwestern margin of the basin,manifested as compression Uplifting and denudation;Himalayan movement is dominated by uplifting and denudation,with normal faults developed.(3)There are mainly two early uplift and cooling events in the southern part of the basin,which are the late Early Cretaceous from 100 to 110 Ma and the Cenozoic since 30 to 40Ma.Late features.It has experienced intense denudation with a large uplift since the Early Cretaceous.The denudation amount is large in the south with intense transformation.The intensity of the transformation and denudation amount is reduced from south to the north,and the transformation intensity is stronger in the periphery than the inside the basin.(4)The regional caprocks in the southern part of the basin are mainly mudstones of Shihezi Formation and Shiqianfeng Formation,the Upper Paleozoic.The Ordovician of the Lower Paleozoic develops tight limestone and marl,and gypsum salt is mainly developed in The northeast,used as a local and direct capping layer.The southern section of the study area(south of Longxian County)is missing regional cap rock and the preservation conditions are poor.The salinity of formation water is high in the north and low in the south,indicating that the natural gas preservation conditions in the south and west are relatively poor,and in the interior of the basin is better.(5)The Upper Paleozoic Shihezi Formation mudstone is thick and has a high breakthrough pressure of up to15MPa,strong sealing ability,and good caprock quality;the breakthrough pressure of the bauxite in the Benxi Formation is greater than 15MPa,and the porosity and permeability are low,and the quality of the caprock is good.The Ordovician gypsum salt rock has the highest breakthrough pressure,which can reach 20MPa,the porosity is 0.15%,the permeability is 0.8×10^-7um²,the sealing performance ablity is strong,and the caprock quality is good.The Marl has the second highest breakthrough pressure,with a breakthrough pressure up to 12MPa,which is a good inner caprock.(6)The breakthrough pressure of the Upper Paleozoic mudstone in the southern Ordos Basin has the characteristics of gradually decreasing from west to east and from north to south.Most of the mudstone breakthrough pressures of the lower Shihezi,Shiqianfeng and Shanxi formations are greater than 15MPa,with good sealing performance.(7)The preservation conditions of natural gas in southern Ordos Basin can be divided into three categories.All in all,the preservation conditions have improved from south to north,and the Ordovician strata of the southernmost part of the basin is exposed on the surface,The central paleo-uplift and its evolution play an important role in controlling the distribution of source-reservoir-caprock and natural gas accumulation in the study area.The central paleo-uplift and its evolution play an important role in controlling the distribution of sourcing-reservoir-cap and gas accumulation in the study area,which predicts the favorable areas for gas exploration in the Lower Paleozoic.The Fuxian-Yichuan area is considered to be the most favorable area for natural gas exploration,and Binxian-Chunhua is a more favorable area.