Mechanism Of Formation And Differential Preservation For The Storage Space Of The Ordovician Tongzi Formation In The Southeastern Sichuan Basin

The Sichuan Basin is one of the key regions for marine carbonate hydrocarbon exploration of China.This basin is rich in hydrocarbon resources,and a series of large marine carbonate gas fields have been found in the Sinian-Middle Triassic marine strata.In recent years,the carbonates of Cambrian Longwangmiao Formation,Canglangpu Formation,and Xixiangchi Group have continuously achieved new discoveries and breakthroughs,effectively revealing the broad exploration prospects of the Lower Paleozoic marine carbonate layers in the Sichuan Basin.At present,the Ordovician hydrocarbon exploration in the Sichuan Basin has not made a substantial breakthrough,whereas the previous studies have found that the hydrocarbon of Ordovician Tongzi Formation carbonate is well displayed,especially in the southeastern Sichuan Basin where the Tongzi Formation dolostone is thicker and develops shoal facies deposits,indicating a good basic condition for the formation of high-quality reservoirs.However,the overall study degree of the Tongzi Formation in southeastern Sichuan Basin is poor,and its geological conditions of hydrocarbon have yet to be clarified.Finding high-quality reservoirs is essential to achieve breakthroughs in hydrocarbon exploration of Tongzi Formation.Based on this,this paper focuses on the carbonate rocks of the Tongzi Formation in southeastern Sichuan Basin,playing close attention to the “formation and differential preservation of storage space”,comprehensively using the theories of sedimentology,reservoir geology,isotopic geochemistry,etc.,and making full use of outcrop data,the features of sedimentary,storage space,and diagenesis are primary analyzed by means of thin identification,scanning electron microscopy,cathodoluminescence,fluid inclusions,carbon-oxgen-strontium isotopes,and major and trace elements.Then,the effects of cyclic sedimentation,dissolution and dolomitization of the development of the storage space are emphatically analyzed,the fluid property and origin of the cements(infills)of pores,vugs,and fractures are discussed detailedly,and the evolution models of storage space are established under the sedimentary-diagenesis system.Finally,the mechanism of formation and differential preservation for the storage space is clarified and the relevant control factors are identified,and proposes a favorable exploration direction for the Tongzi Formation.This study shows:(1)The Tongzi Formation has complex lithology,which is divided into 29 types of rock microfacies by rock microfacies analysis.Taking the microfacies combination used as a indicator of facies to explain the sedimentary environment.It is indicated that the Tongzi Formation has 6 types of sedimentary environments,including lagoons,tidal flats,mixed tidal flats,shoals,intershoal seas,and intra-platform depressions.The Tongzi Formation has 7 types of typical sedimentary sequence combinations are constituted by lagoons-tidal flats,lagoons-shoals-tidal flats,intra-platform depressions-lagoons-tidal flats(shoals),intra-platform depressions-intershoal seas-shoals,intershoal seas-tidal flats,shoals-lagoons,and shoals-intershoal seas-intraplatform depressions.(2)The Tongzi Formation develops two complete third-order sequences and one third-order sequence ascending semi-cycle.The sedimentary environment within the three-level sequence framework has undergone a cyclic evolution of open platform(SQ1TST)?restricted platform(SQ1HST?SQ2HST)?open platform(SQ3TST).The distribution regularity of facies has been clarified and a shallow sea carbonate platform deposition model of “depressions and lagoons inheritance,shoals and tidal flats interaction,and shoals form into belts” of Tongzi Formation is established.It is pointed out that surrounding intra-platform depressions and lagoons,the sendmentary environment is dominated by tidal flats that overlapped by shoals,and the intra-platform shoals are mainly distributed in the regions of Lichuan-Shizhu-Wulong-Nanchuan-Xishui-Gulin.(3)The Tongzi Formation develops various types of storage space,which consists of intercrystalline pores/intercrystalline dissolved pores,residual intergranular pores/intergranular dissolved pores,intragranular dissolved pores/moldic pores,fenestral pores/gypsum moldic pores,vugs,caves,diagenetic fractures,tectonic fractures,and dissolution fractures.Dissolution,dolomitization,tectonic fracturing,and hydrocarbon charging play a constructive role in the formation and evolution of the storage space;compaction and pressure solution,burial cementation,overdolomitization,and recrystallization lead to the destruction of storage space.Intercrystalline pores,intercrystalline dissolved pores,residual intergranular pore,intergranular dissolved pores,vugs,and caves are the main effective storage spaces of the Tongzi Formation,while the frequency of fractures is relatively low,and most of them are infilled,therefore its storage performance is relatively poor.(4)The Tongzi Formation develops syn-sedimentary dissolution,eogenetic dissolution,burial dissolution,and telogenetic dissolution.The syn-sedimentary-eogenetic dissolution is driven by the relative sea level decline related to the high-frequency sequence and the third-fourth-order sequence,respectively.It mainly carried out the dissolution alteration for the sediments in restricted platform shoals and tidal flats,resulting in multiple types of dissolved pores,vugs,and caves that are the main source of the storage space of the Tongzi Formation.The burial dissolution is related to the acidic fluid formed by hydrocarbon generation of organic matter and TSR reaction and the deep hydrothermal fluid,which are mainly inherits and adjusts the previous storage space,without causing a significant net increase of the storage space.Meteoric water dissolution in the telogenetic period occurred late and small in scale,therefore its formed storage space has no meaning for the storage of hydrocarbon.(5)The Tongzi Formation has various dolostone/dolomite types,including five types of matrix dolostone/dolomite that consist of the micrite dolostone(Dol-1),silty crystalline dolostone(Dol-2),silty fine crystalline dolostone(Dol-3),(residual)granular dolostone(Dol-4),and silty-fine crystalline dolomite in limestone matrix(Dol-5),as well as two types of dolomite cements(infills)that consist of silty-fine crystalline dolomite(Cd-1)and heterogeneous dolomite(Cd-2).Based on dolostone/dolomite texture characteristics,crystal types and geochemical parameters,it is indicated that the fluid properties of matrix dolostones/dolomites are similar,and their dolomitization fluids are derived from the coeval seawater,which suggesting that the Dol-1?Dol-4 are formed from evaporative concentration-seepage reflux dolomitization,and the Dol-5 is the product of burial dolomitization.Combined with fluid inclusion data,it is proposeded that the diagenetic fluid of Cd-1 and Cd-2 is sea-sourced fluids sealed in the Tongzi Formation and formed during the normal burial warming process without being affected by abnormal thermal events.(6)The formation of the storage space of the Tongzi Formation depends on cyclic sedimentation,dissolution,dolomitization and tectonism.Cyclic sedimentation is the basis for the development of the storage space,determines the component and texture of rock and the favorable facies,and controls the scope and intensity of early dissolution and dolomitization;dissolution is an important mechanism in the formation of storage spaces,the syn-sedimentary-eogenetic dissolution is a key factor in the development of the storage space;dolomitization is of positive significance for improving the storage performance of crystalline carbonate rocks;the multi-stage fractures formed by tectonism are important conditions of the reconstruction or the optimization of storage space.The differential preservation of the storage space mainly depends on the intensity of burial cementation,the degree of dolomitization and the degree of hydrocarbon charging.The cements(infills)formed by the burial cementation is an important reason for the destruction of the storage space;dolomitization effectively inhibits the later cementation and enhances the ability of the rock to resist compaction and pressure solution,which is a key factor for the preservation of the early storage space;hydrocarbon charging is conducive to the preservation of the early storage space and determines the final effective development degree of the storage space.(7)The porosity and permeability conditions of carbonate rocks in the Tongzi Formation are generally poor,but relatively high-porosity and high-permeability reservoir segments can be developed by constructive diagenesis,the prospects for exploration therefore are generally considerable.The exploration targets of the Tongzi Formation in southeastern Sichuan Basin should be based on the deposits in restricted platform shoals and tidal flats.Class I favorable zones are located in the regions of Gulin–Chishui–Qijiang–Xishui,and Class II favorable zones are located in the regions of Fuling–Shizhu–Nanchuan.