| Along with the exploration and development of Chinese continental oil fields in recent decades, oil and gas reserves from clasolite hasn’t already satisfied the need for national economic development, then the exploration and development research of carbonatite oil fields will be emphasis and focus of Chinese energy source strategies. Ordos is a famous large-scale sedimentary basin in China, whose reserves and output of oil and gas has an extremely important status. As one of important western marine basins in China, it lies in the period of marine deposit in Eopaleozoic, when Lower Paleozoic carbonate developed widely and heavily with many types. As Chinese largest one in Lower Paleozoic, Jingbian gas field shows gradually the explendid future of marine gas reservoir since its discovery in Jun.1989. Yet until now thorough awareness and research on its sedimentary system is still not enough, reservoir characteristics and distribution laws is not very clear. Hence, it is very important to study on its sedimentary facies and reservoir characteristics, so as to clarify the distribution law of reservoirs and seek abundance area of oil and gas.Lying in the western part of North China Platform and belonging to folding basin on regulatory zones of tow structural domains namely Pacific ocean in the east and Thetis in the west, Ordos basin is rich in mineral resources including natural gas. Ordos basin experienced lots of important structural events, then it can be divided into6primary tectonic units such as Yimeng upthrust, Weibei upthrust, Western Boundary obduction zone, Tianhuan depression, Yishan slope and Jinxi ply zone. The working area lies in craton depression of Ordos basin, the western side of Jingbian burial platform and Yishan slope structural zone as well.Based on the foundation of former research, adopting multidisciplinary and multi-technique comprehensive analysis methods, mixed with modern anlysis and testing such as cathode luminescence, X-ray degree of order, SEM, trace element, isotope of carbon and oxygen and fluid inclusion etc., comprehensively applying advanced theory and methods such as sedimentology, carbonate diagenesis, reservoir geology, diagenesis system research and petroleum geology etc. comprehensive study on reservoir characteristics of Ma54and Ma55in Majiagou formation Ordovician on the western side of Jingbian burial platform Ordos basin was made in this paper.According to sedimentary cycles, regional marker bed and palaeontologic characteristics etc., Majiagou formation Ordovician in the western side of Jingbian burial platform Ordos basin can be divided into6lithologic members, namely Mai and Ma6from bottom to top. Lithologic type of Ma5is mainly dolomite, dolomite gypsum, dolomite mudstone among gypsum, which can also be divided into6submembers, among them Ma54and Ma55is the research target in this paper. Ma54can be divided into three beds Ma541, Ma542and Ma543, and Ma55can be divided into two beds Ma551and Ma552.Through the study of facies markers of lithology, paleontology and Geochemistry etc., conclusion can be made that Ma54and Ma55in working area belongs to tidal flat subfacies in restricted platform facies, which can be divided into9types of tidal flat microfacies such as dolomite, limestone and dolomite, dolomite and limestone, limestone, embryonic foreland, tidal channel, gypsum contained dolomite, gypsum dolomite and dolomite gypsum. Restricted platform lies in the offshore zone of carbonate platform, due to close environment and high salineness, there are few kinds of creatures only with parts of algae and euryhalinous creatures. Since gypsum strata was found, its sedimentary environment can be judged as evaporization climate, Ma54and Ma55belongs to sedimentary models as vaporing and broad tidal flat. Carbonates includes mudcontained dolomite, mudstone dolomite, dolomite mudstone, gypsum contained dolomite, gypsum dolomite, limestone contained dolomite, limestone doloimite, dolomite limestone, macrocrystalline and crystalline powder dolomite and grain dolomite etc. The most effective reservoir rock types are gypsum contained dolomite, gypsum dolomite, macrocrystalline and crystalline powder dolomite and grain dolomite.Reservoir characteristics research shows that, Ma54weathered crust developed the following reservoir space as gypsum mould pore, crystal mould pore, and breccia pore etc. Ma55grain strand and macrocrystalline and crystalline powder dolomite reservoir developed interparticle dissolved pore and intercrystal pore. Crack-dissolved pore and cave reservoir is simlar to the Jingbian gas field and peripheral areas, their developing degree has difference. Intercrystal pore includes dolomite intercrystal pore and limestone intercrystal pore. Crack concentration is also one of the characteristics of the working area, some parts are open and semi-filled with limestone, which has important meaning in improving reservoir peameability.According to geological profiles, drilling cores, rock slice, SEM, X-ray degree of order and cathode luminescence, Ma54developed weathered crust reservoir, the most influential diagenesis types are dolomization, dissolution and corrosion, cementation and alternation, recrystallization, mineral filling and compaction pressolution. Among them dissolution and corrosion and dolomization are the most constructive diagenesis in Ma54weathered crust reservoir, which promote the formation and evolution of pores. In the working area dolomite genesis includes penecontemporaneous, burial, fresh water, hot fluid, and penecontemporaneous and burial are the main ones. In Ma54penecontemporaneous developed and In Ma55burial is the dominant factor. Compared with that of Ma54, diagenesis types in Ma55are simple, where grain strand reservoir developed, compaction pressolution, recrystallization and metasomatosis are the dominant ones. Diagenesis stages are as folloing:penecontemporaneousâ†'near earth surface shallow buryingâ†'uplift wearthering diagenesisâ†'Mid-deep burying. In the former two stages, miniature of dissolved pores and caves are formed. In the third stage, it was innovated strongly and widly. Dissolution and filling and recrystallization formed the characteristics in diagenesis stages.During the process of reservoir forming and diagenesis evolving, sedimentation is the base, and diagenesis is crucial factor. According to characteristics of reservoir and sedimentary environment and comprehensive evaluation about reservoir, plane distribution characteristics in Ma54and Ma55are depicted in detail. According to sedimentary situation, landform and diagenesis characteristics and effective reservoir etc., considering about the exploration practices,29beneficial exploration zones are selected from Ma54and Ma55in the working area, which shows the direction to the future exploration. |
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