Carbonate Microfacies Characteristics and Reservoir Heterogeneity of the Ordovician Weathering Crust(O_1ma51－4 )of the Zhongbu Gasfield,Ordos Basin, Northwest China

Key words: 
• Zhongbu gasfield /
• carbonate rock /
• microfacies /
• MF association /
• reservoir

Abstract: the Ordovician weathering crust (O_1ma51－4)is a major gas-bearing stratum of the Zhongbu gasfield in central Ordos Basin. 24 types of microfacies with different and abundant pores have respectively been recognized according to their mineral,texture,structure, diagenesis, geochemistry and petrophysics features. They were genetically reorganized into 8 MF associations. The table (below) shows the main characters of the MF associations.The main factors which control the characteristics of the reservoir are distribution character of the sedimentary factes (belts) and their MF associations, diagenesis, succeeding tification and tectonic movement.Sedimentary factes and their MF associations are the basic controlling factors. They control the regional strike and general distribution of the reservoir. The MF associations are the material foundation of the succeeding processes, which have direct strategic significance in guiding the petroleum exploration. The microfacies study results indicate that, for one thing the difference between the MF associations leads to the reservoir heterogeneity and, for another,the favourable microfacies have a very high proportion vertically in every well district and good continuity laterally, esp. in each section of O_1ma51,the porportion of the narrowly distributed unfavourable micro factes is very low. The succeeding processes(including diagenesis,karstification and tectonic movement) determine the specific distribution patterns of the effective pore space and fvourable reservoir bodies, which has the most actual significance in directing the development.