Sedimentary Environment And Reservoir Capacity Of Ordovician Palaeokarst Chemical Fillings In Outcrop Northwest Of Tarim Basin

The carbonate rocks developed in the Ordovician in the northwestern Tarim Basin have experienced a long period of structural evolution and paleokarst,forming a set of complex fracture-cavity reservoirs.This type of reservoir is filled by various types of fillings,and chemical filling of fractures and caves is an important factor that affects the karst fractures and caves,oil and gas migration channels,and the quality of fractured-cavity reservoirs.On the basis of full investigation of previous studies,the main research object is the chemical fillings filled in the fractures and caves of the ancient karst outcrops in the Keping-Bachu area of the northwestern Tarim Basin.Comprehensive using outcrops,oilfield cores,logging data and geochemical analysis and testing methods,studying the types,distribution,genesis,sedimentary environment and storage performance of chemical fillings in fractures and caves.Based on the petrological characteristics of the fracture-cavity filling material of Keping-Bachu outcrop in the northwest of Tarim Basin,The chemical fillings in the outcrop area are divided into seven types,these are travertine,calcite,fluorite,siliceous,pyrite,sulfur and gypsum.Described the production status,geometric form and rock and mineral characteristics of various chemical fillings in detail.At the same time,the chemical filling of core fractures and caves in Tahe Oilfield is divided into three types:gaint crystalline calcite,siliceous rock and calcareous cemented glutenite.Then summarized the logging response characteristics of different fracture-cavity chemical filling sections.The chemical fillings for the outcrop in the northwest of the Tarim Basin,By X-ray diffraction,major elements,trace elements,rare earth elements,carbon,oxygen,sulfur isotopes and other geochemical methods,the results show that the chemical sediments in the Liuhuanggou and Xikeer areas are mainly influenced by tectonic activity and hydrothermal fluid,the chemical sediments in the Wudaoban and Yijianfang are mainly influenced by climate and diagenetic environment.The formation environment and genetic model of travertine filling in the outcrop in the northwest of the Tarim Basin are further discussed.The controlling factors for the formation of travertine are climate environment,hydrogeological conditions,physical and chemical conditions of water bodies,biological activities and tectonic activities.Then The travertine deposits in the northwest area of Tarim Basin are compared with those in other countries in terms of sedimentary environment,sedimentary model and sedimentary rate.In addition,the karst cave sedimentary model was established,and the sedimentary characteristics of travertine were summarized.Through comparison,it is found that the travertine filled in the northwest of the Tarim Basin has developed pores and good connectivity,and is closely related to tectonic movement,fluid properties and biological activities.The storage space types of ancient karst fracture-cavity chemical fillings are divided into pores,fractures and pores.It is found that the porosity of chemical backfill is mainly concentrated in 2%-6%,the permeability is generally and distributed in the range of15×10^-3?m².It may be affected by diagenesis that the originally well-developed pores were filled due to gravel collapse or calcareous cementation.Secondly,the existence forms and oil-bearing conditions of various chemical fillings in different storage spaces were studied.Based on the analysis of morphology,genesis,distribution and reservoir properties of outcrop travertine,compared the characteristics of the core and chemical fillings identified by logging in the Tahe Oilfield.It is found that the reservoir capacity of fracture-cavity chemical filling in the overburden area of Tahe oilfield is poor.However,the outcrop and cover area are quite different,it is judged that the reasons are low core extraction rate,difficult logging identification,uneven distribution of chemical filling,mineral transformation and recrystallization in the burial process,etc.The comprehensive analysis shows that most of the chemical fillings except travertine have no reservoir capacity,but the sedimentary distribution of travertine is relatively limited,its ability to form large oil and gas reservoirs is limited,too.