Structural Characteristics And Genetic Model Of Lower Ordovician Carbonate Reservoirs In Tahe Oilfield

The Ordovician fractured-vuggy carbonate reservoir in Tahe Oilfield,which consists of old and deeply-buried strata,has experienced multi-period tectonism and complex diagenesis,resulting in highly complicacies of type,filling degree and distribution rule of fractures and caves.As a consequence,ascertaining the characteristics of reservoir space,controlling factors in development and evolutionary process of fractures and caves,has great guiding significance for deployment of oil and gas exploration in Ordovician fractured-vuggy carbonate reservoir in Tahe Oilfield and enhancing oil recovery.Guided by theories of carbonate reservoir geology,karst geology,carbonate petrology and other disciplines,using various methods such as observation of outcrops and cores,seismic inversion,well logging interpretation,casting thin sections,thermometry of fluid inclusions and isotope analysis,the characteristics and distribution rule of reservoir space,the controlling factors in development,the evolutionary process of fractures and caves of Ordovician fractured-vuggy carbonate reservoir in Block 6 and 7 of Tahe Oilfield are anatomized systematically,and therefore of certain research significance and innovative suggestions as follows:The large-to medium-sized karst caves are the majority of reservoir space in Ordovician fractured-vuggy carbonate reservoir in Tahe Oilfield,and the following are small-sized caves.Based on size,development position,combination relation and genetic type,the karst caves are classified into seven categories,including sinkhole,river cave,tributary cave,short-storage cave,cross-sectional seepage well and lobby cave.Nearly 90 percent of karst caves were full-to semi-filled,in which sedimentary fillings accounted for 88 percent.And it is supposed that,these sedimentary fillings made great contributions to storage properties of carbonate reservoir.There are three types of karst zones developed vertically in Ordovician fractured-vuggy carbonate reservoir in Tahe Oilfield,and the sequences is epikarst zone,seepage karst zone and runoff karst zone.Epikarst zones were the most favorable areas for fractures and caves,where river caves and tributary caves mainly developed in surface rivers,and sinkholes were generally found in slope-break belts and cliffs.Along the surface river,these caves had relatively high connectivity and thus led to better reservoir properties.In seepage karst zones,short-storage caves with large height and narrow horizontal expansion were formed by vertical seepage,and it would be easy to develop cross-sectional seepage wells in the vicinity of faults.Runoff karst zones contained underground rivers which grew parallel to the direction of formation.River caves,tributary caves and lobby caves were found out in runoff karst zones,with good connectivity along underground rivers.The formation of fractured-vuggy carbonate reservoir in Tahe Oilfield was controlled by various factors,including main-faults rifting,weathering and denudation of unconformity,diagenesis,water-level changing,ancient landform,etc.Firstly,Main-faults rifting manipulated the development positions of surface and underground rivers,and have shaped an overall karst structure of “rivers associated with faults and karst caves associated with rivers” eventually.Secondly,Ordovician strata were denuded during the Hercynian tectonic uplift,and a great quantity of fractures and caves grew near unconformity.Besides,the higher the elevation of ancient landform was,the more caves developed near unconformity surfaces.Thirdly,diagenetic fluid flowed along faults in diagenetic process,and then karst caves and fractures came into being by dissolving.Sedimentary fillings in those caves were in the uncompacted state on the back of rigid construction of cave wall,resulting in plenty of primary pores preserved.In addition,the development positions of caves in runoff karst zones were affected by the fluctuation of paleo-water level,and thus led to multilayer caves caused by underground rivers.Under the action of multi-period tectonism and complex diagenesis,fractured-vuggy carbonate reservoirs,in Ordovician,have gone through greatly complicated course of evolvement.A small amount of thrust faults formed by Late Caledonian movement had the leading function to karstification.The mass formation of fractures and caves was caused by stratigraphic unconformity and rifting during Early Hercynian movement.At this stage,sinkholes,river caves and tributary caves that related to surface rivers developed in epikarst zones;short-storage caves and cross-sectional seepage wells formed in seepage karst zones;in runoff karst zones where contained underground rivers,lobby caves,river caves and tributary caves could be found out with arenaceous and pelitic karstic cave deposits.The main-faults rifting continuously took place during Late Hercynian movement.Deep hydrothermal fluids infiltrated and upwelled through these faults,and resulted in chemical deposition and cementation in fractures and caves.After Indosinian movement,reservoirs were buried again,then experienced a series of diagenesis including cemented filling and dissolution.However,supported by the rigid construction of cave wall,there were still plenty of primary pores preserved in karst caves,thus led to good reservoir properties.