Molar tooth structure(MTS)refers to a series of peculiar, ptygmatically folded and spar-filled cracks in fine-grained carbonates of the Precambrian. Particular Precambrian carbonate with MTS is called the Molar tooth carbonate(MTC). Consensuses have been achieved that the MTS is globally distributed and composed of micro-sized sparry calcite,developed during a specific time limit,early diagenetic product,and has diversified morphologies. Based on a systematical summary of the palaeogeographical distribution,development time limit,macro morphology,and the micro fabric of MTS,this paper systematically analyzes the basic arguments and limitations of the three hypotheses of MTC origin, i.e.,the physical origin,biological origin,and the biogeochemical origin hypothesis. As a globally distributed special carbonate that only appeared during the Meso-and Neoproterozoic,the focuses and hot spots of the MTC researches are the formation and disappear mechanisms. Although there are still controversies about the formation mechanism,the early diagenetic characteristics of the MTC made a growing number of geologists regard the MTC as an important recorder to reestablish the geochemical properties of the Precambrian ocean. The flourishing and(or)dismissal of the MTC has a close relationship with the evolution of paleocean properties,the supercontinent cycles,and a series of global geological events. Further researches about the MTC can go in two aspects: one is to explore the reciprocal response,promotion or inhibition relationship between the MTC and global events such as stromatolites,glacial events, etc.,and how to couple the MTC events to the assembly and breakup processes of the Columbia and Rodinia supercontinents;the other is the microscopic fabric of the microspar in the MTS, select adopt effective geochemical methods,in order to combine the researches of the occurrence,evolution,and cessation of the MTC with the evolution and changes of atmosphere,biosphere,and the Precambrian ocean properties,and finally improve the theory of the MTC formation mechanism. All these researches will help and provide clues to solve several major Precambrian scientific problems.

Taking Carboniferous KT-I and KT-II carbonate reservoirs in N oilfield in Pre-Caspian Basin as an example,the characteristics of carbonate reservoirs and evolution of reservoir properties at the early stage are systematically studied based on the core analysis,thin section,scanning electron microscopy,high-pressure mercury invasion,conventional physical property analysis and various tests. In addition,the relationship among the evolution of reservoir properties,pore throat structure and quality is further discussed. The results show that KT-I reservoir is dominated by dolomite with intergranular dissolved pores and small karst caves with good pore-throat connectivity,which belongs to high porosity-high permeability pore-cavity reservoir,followed by grainstone with intragranular dissolved pores,mold pores and biologic cavity pores that have the poor pore-throat connectivity,which belongs to the pore-type reservoir with the medium porosity and low permeability. KT-II reservoir is characterized as grainstone with intergranular dissolved pores and biologic cavities,belonging to the porous reservoir with medium-high porosity and high permeability. Further analysis shows that the thickness of single cycle layer of KT-I dolomite and limestone reservoirs is small,which is controlled by the early diagenetic karst driven by high frequency sea-level fluctuation. The thickness of KT-II layer is large,and there is no early karstification of the grain beach. There are mainly three types of reservoirs: (1)Penecontemporaneous dolomitization with stable transformation of minerals results in the dissolution of some minerals,followed by the formation of reservoirs controlled by the karstification at early diagenetic stage,and the pressure dissolution-resistance rock skeleton helps the protection of reservoir;(2)The early diagenesis karstification promoted the reservoir property→the early compaction controls the chemical fluid migration that can induce the cementation→differential cementation controls the preservation of intergranular pore and intragranular pore;(3)Primary sedimentary environment controls the formation of reservoir→early compaction and cementation control preservation of reservoir. Different reservoir formation mechanisms lead to different porosity,permeability and pore throat structure,which controls the quality of reservoirs including the dolomite and grainstone after early karstification,and the limestone that owns the primary pores. The results will provide a reference for the genetic analysis of carbonate reservoirs with similar characteristics. It also has significant karst geology meaning based on the discovery that dolomite is much easier to dissolve than limestone during the early diagenetic stage.