Study On Reservoir Characteristics Of Ma5₄¹ Member In North Jingbian Gas Field, Ordos Basin

The Ma5₄¹ member of Ordovician is one of the main producing reservoirs in the north Jingbian Gas Field, Ordos Basin. It is very important to research on reservoir characteristics in order to explore and develop natural gas in this area. The research object of this paper is Ma5₄¹ member of north Jingbian Gas Field. Based on previous study, and guided by the theory of sedimentology and reservoir geology, author elaborated the sedimentary microfacies, diagenesis and reservoir characteristics etc. in this paper, by detailed describing the core of 40 wells, identifying rock thin section, cathodoluminescence, SEM observation, mercury intrusion analysis, logging, database of core porosity and permeability, etc. There are six primary views are achieved in this study as following:First, Sedimentary microfacies analysis shows that the depositional environment of Ma5₄¹ member in the north Jingbian Gas Field is generally in the tidal flat environment. On the basis of ptrologic characteristics, sedimentary structure, profile structure, etc. three subfacies can be divided, including supratidal zone, intertidal zone, and subtidal zone. Among them, supralittoral zone can be divided further into supratidal muddy—dolomite ground, supratidal gypseous—dolomite ground, supratidal dolomite ground, supratidal lime—dolomite ground, and intertidal zone can be subdivided into intertidal dolomite ground, intertidal dolomitic—limestone ground, intertidal limestone ground. The sedimentary microfacies characteristics are also discussed in detail. Then, according to the conception of "dominant fades", the planar distribution map of sedimentary microfacies for Ma5₄¹ member is drawn, and the sedimentary microfacies pattern is given.Secondly, secondary porosity is the main reservoir spaces in Ma5₄¹ member, north Jingbian Gas Field, and can be classified into two main types: pore and fracture, and total nine subtypes. Among them, intracrystalline pore, intercrystalline solution pore, solution pore and cavity, mould pore, dissolution fractures are main reservoir space types and their reservoir qualities are quite good. Interbreccia pore or fracture, structural fracture have big variation of reservoir qualities. Only when infilling acts weak, they can form good reservoir. Suture shrinkage joint have less probability of appearance, they are dominated by ineffective fracture. Therefore, have the worst reservoir qualities.Third, the main diagenesis undergone by the Ma5₄¹ member in north Jingbian Gas Field can be divided into three types. First type is the main constructive diagenesis include dolomitization, gypsumization, dissolution, degypsumization, fracturing; Second types is the main destructive diagenesis consist of compaction, dedolomitization, metasomatic—filling; Third type is recrystallization and pressure solution have double effects. Using Nomination Principle of direct—diagenesis, three diagenetic facies can be recognized, including degypsumization—calcite filling diagenesis facies, dedolomitization—degypsumization—calcite filling diagenesis facies, clay filling diagenesis facies. The classified evaluation of each diagenesis facies is also made. Meanwhile, based on different pore evolution phase, five diagenetic phase can be divided in sequence of early diagenesis phase, shallow buried diagenesis phase, supergene ancient karst diagenesis phase, post shallow buried diagenesis phase, post deep buried diagenesis phase.The fourth section, reservoir of Ma5₄¹ member in north Jingbian Gas Field belongs to low porosity and low permeability reservoir types, and has good relativity between physical parameters. Mercury injection data shows the main throat types are micro—fine throat and fine throat, the secondly for coarse throat. Thus, the pore structure is good—medium, and the relativities between pore structure parameters and porosity are good except for the relativity between core porosity and sorting coefficient. Meanwhile, due to the influences under different sedimentary environment, diversity and multiple phases of diagenesis, nonuniformity of fracture development, structure and physical properties of reservoir rock show strong heterogeneity. On the one hand, rhythm model of permeability manifested as diversity in longitudinal development. Furthermore, three rhythmic patterns can be recognized: abnormal rhythms, random rhythms, homogeneous rhythms. On the other hand, average porosity and permeability presented as nonuniformity in lateral development. However, the isoline map of porosity is coincident with that of permeability from the overall, and both distribution patterns shows characteristics of high northwest and the low southeast.Fifthly, on the basis of specific study through porosity and permeability of reservoir rock, effective thickness, open flow capacity, pore structure parameters, four reservoir types can be divided in the research area: class I includes pore and cavity—fracture type, which has good reservoir qualities; class II consists of pore type, which has good—medium reservoir qualities; class III is compose of micro pore—fracture type, which has poor reservoir qualities; class IV can not be considered as reservoir, which contains tight micro pore type.Finally, based on the mentioned above parts from sedimentary microfacies, porosity types, diagenesis, reservoir characteristics and classified evaluation, three distribution of favorable reservoir belts can be forecast: block 1 is located in the north study area which has good reservoir qualities, and mostly belongs to supratidal dolomite ground and karst slope, block 2 belongs to supratidal dolomite ground and supratidal lime—dolomite ground, block 3 belongs to intertidal dolomite ground and supratidal lime—dolomite ground and also situated in karst slope, however, its reservoir physical property take second place with limited distribution.