| The marine carbonate rocks of Ordos Basin develops widely and thickly. It is rich in natural gas resources. Majiagou formation of Ordovician is the key strata for oil and gas exploration and development. As the exploration extends, the new gas-bearing formation are found in the Majiagou formation near the centre palaeo-uplift. For example, the industrial gas flow has been found from dolomite of the M56sub-member of Majiagou formation located in the well Su51area. Further study to the sedimentary microfacies distribution and reservoir controlling factors of the main gas reservoirs in the east side of middle portion of the centre palaeo-uplift(to the west of the Jingbian gas field) are significant for further revealing the developmental rule of the dolomite lithologic reservoirs in this area. According toabundant data of the cores observation, test analysis and detection logging, etcetera, the characteristics of the petrology, sedimentary microfacie, reservoir,diagenesis and control factors of reservoirs (M55-10sub-members of Majiagou formation) were investigated in this paper.On the basis of the sublayers comparison and facies signs, the sedimentary facies of drilling cores were analysed, and the results show that the sedimentary environment is the restricted-open platform,which could been devided into5facies including dolomitic tidal flat, peritidal beach, limy&dolomitic lagoon, limy lagoon and gypsum lagoon.The sedimentary microfacies distributions of M55-10sub-members of Majiagou formation revealed the evolution process of sedimentary environment. The results show that marine regression evaporation environment include M56, M58, M510. The reservoir rocks are mainly gypsum rocks and dolomites which contains much shale. The M55, M57, M59were marine transgression environment. The reservoir rocks are rich in limy content and poor in shale levels. In the paper, the peritidal-beach sedimentary model was established. The depth of the water gradually rised from palaeo-uplift to the east portion of Ordos basin, and the distribution of facies was in the shape of north-south banding. Because of the gentle topography, the sea level fluctuation, which could influence the sedimentary environment remarkably, made the dolarenite beach migrating for a long distance, forming the sheet beach distribution with a large area.In the study area, the reservoir rocks mainly contained grain dolomite, residual dolarenite, dolomitic limestone/limy dolomite and karst brecciaous dolomite. The dolomite reservoir is characterized by low porosity and low permeability(the porosity range from0.5%to5%, the permeability range from0.01mD to5mD), whose reservoir space mainly contained intracrystalline pores, residual intergranular pores, dissolved pores and dissolved fracture. The correlation coefficient is0.29which indicate that the correlation between porosity and permeability was poor, so the reservoirs have a strong heterogeneity. With the microscopic section of the reservoirs analysed, it was found that the diagenesis contained mainly dolomitization, dissolution, compaction&pressolution, cementation, recrystallization, metasomatism and cavity filling, etcetera. There are four phases in the diagenesis:1)the penecontemporaneous-early diagenesis stage which is the shallow burial,2)the supergene stage whose environment is meteoric water,3)the middle diagenesis stage which is the shallow-middle burial,and4) late diagenesis stage which is the middle-deep burial.The reservoirs are controlled obviously by peritidal beach sedimentary microfacies, so the peritidal beach sedimentary microfacies are the best reservoirs in the study area. The reservoirs have experienced a complex densification in diagenesis evolution process. The burial dolomitization, bedding karstification and fracture are key factors for the formation of the dolomite reservoirs which have high quality in the peritidal beach sedimentary microfacies of the study area. The high quality of reservoirs are controlled by peritidal beach and constructive diagenism facies which include dolomitization-dissolution diagenesis facies, degypsification-dissolution diagenesis facies, strong-dissolution diagenesis facies and dolomitization diagenesis facies. A prediction on favorable exploration area of M55-10sub-members of Majiagou formation in the study area has been presented in the paper. The highest favorable reservoirs distribution as following:Taol7-Su379well area, Shan370well area, Su127-Lian30well area, which are the superposition areas between the peritidal beachs microfacies and the dolomitization-dissolution diagenesis facies, degypsification-dissolution diagenesis facies, strong-dissolution diagenesis facies and dolomitization diagenesis facies. |
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