| The Ordovician Ma5 Member of Ordos basin in the Lower Palaeozoic is mainly composed of carbonate rocks. Jingbian gas field have been found for Shancan 1 well tested a large gas flow from Ma5 Member in 1989. There were long and old evolution in carbonate reservoir of Ma5 Member, so it experienced multiphase complicated diagenetic evolutions and acquired strong heterogeneity. Secondly gas and water distribution is more complex, that accumulation process study needs further deepening. Therefore it is important to study sedimentary facies, paleogeomorphology, reservoir characteristics, gas-water distribution and gas accumulation mechanism in the lower Paleozoic group. There are important theoretical significance and practical application value to predict favorable reservoirs and carbonate reservoir exploration and development.The sediment facies, paleogeomorphology and ancient karst development characteristics are studied by use of the data of well logging, cutting logging, core observation, combined with casting thin sections, scanning electron microscope and cathode luminescence analysis. By means of capillary pressure curve, gas and water phase permeability, nuclear magnetic resonance, the main trace elements, rare earth elements, Sr and C/O isotopes, the features of the carbonate reservoir are comprehensively researched. The controlling factors of gas and water distributionIt are summarized based on the gas testing and water geochemistry analysis. On the basis of fluid inclusion analysis, combined with gas accumulation conditions, the forming mechanisms of gas accumulation are discussed.It is showed that the sedimentary environments are considered as supratidal zone and intertidal zone in Ma 5 member with five types of sedimentary microfacies. Water was mainly inflow from the direction of northeast by east, southwest and southeast. The karst highland in the northwest part was locally developed with remnant hills and depressions, and karst slope in the central and east parts was developed with gentle hills and shallow depressions, five ancient erosional grooves were developed stretching nearly from west to east. Carbonate reservoirs have experienced the stages of syngenetic karst, supergene karst and burial karst, it is concluded that the gentle hill in the karst slope and regions beside erosional grooves are favorable areas for gas enrichment. Rock types of reservoir rocks are mainly composed of dolostone, there are good physical properties in samples containing cracks and holes. Dissolution and cementation were essential to reservoir property. There were well physical properties respectively in Ma51 sub-member with dolomite-quartz-kaolinite cementation and dissoved pore types, in Ma52 sub-member with dolomite-quartz-calcite cementation and dissoved pore types, and in Ma541 sub-member with dolomite-quartz-kaolinite cementation and dissoved pore types. So four kinds of pore structure were formed respectively, i.e the type of solution pores-intergranular pores (dissolved) (?class), intergranular pores (dissolved)-fracture (? class), fracture-micro pores (? class) and micro pores (? class). Dolomitization were occurred mainly in shallow-middle buried period ahead of oil and gas generation. Calcite veins were mainly formed in middle-deep burial period after the hydrocarbon expulsion. The macroscopic distribution of gas and water is mainly controlled by the ancient karst and diagenesis, and its microscopic distribution is mainly affected by pore structure and reservoir connectivity. The first hydrocarbon charging was happened in the early of late Jurassic stage and a kind of small gas reservoir were formed. The second was occured in the late of early Cretaceous stage. After that the gas reservoirs were gathered in the remnant hill, gentle hills and the part in both sides of the erosion grooves after being adjusted. |
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