Fine Description And Sweet Spot Of Coal Reservoir In Zhengzhuang Block Of Qinshui Basin

The main purpose of this paper is the geological characteristic study of coal reservoir in Zhengzhuang Block,Qinshui Basin.The whole research process is based on the theory of petroleum geology,sequence stratigraphy,sedimentary petrography,development geology and reservoir engineering.Geological data,seismic data,core data,well test data,log information,experimental data and dynamic document are analysed in the study.Ultimately,with methods of layer correlation,multiple regression,multilevel fuzzy synthetic discrimination and modern reservoir description,acquires structural characteristics,sedimentary facies distribution and heterogeneity of coal reservoir;based on which,classification and evaluation of coal reservoir are conducted and favorable blocks are predicted.On the basis of the previous studies and logging data,the sublayers of Shanxi formation in Zhengzhuang Block are divided with the theoretical principle and method of sequence stratigraphy and sedimentary petrography,which are upper Shanxi formation,3~#coal seam and lower Shanxi formation.Based on the sublayers division sesults,microtectonics are analysed by a comprehensive analysis of core data,logging data and seismic data,which lead to the depiction of faults and microtectonics distribution.Based on sedimentary evolution analysis and data of core and log,the sedimentary facies and microfacies of Shanxi formation are studied.It is concluded that the sedimentary facies are typical shallow sea to delta facies and the sedimentary microfacies are mainly consist of inter distributary bay and peat swamp.The inter distributary bay microfacies,distributed locally in the shape of strip and lenticle,are favorable for the preservation of coalbed methane.The roof and floor lithology of 3~#coal,which are mainly consist of mudstone,bass and argillaceous sandstone,are also favorable for the preservation of coalbed methane.Based on the previous studies of coal reservoir properities and by the contrast of core with log response,it is discovered that coal seam has the log response characteristics of high resistance,high neutron,high acoustictime,low bulk density,low litho density,and low gamma.So the coal seam can be distinguished in the well log,according to which the key section of coal seam constructed,then the coal seam can be distinguished in the whole area from line to surface.According to the thickness distribution of coal seam in the study area,it is found that the coal seam distribute stably on the whole and has the tendency of thinning from east to west.After the recognition of coal seam,the research method of coal characteristics are established,including quantitative identification of coal structure and characterization of physical property of coal reservoir.It is found that the log response characteristics of original texture coal are high density,high resistance,low gamma,low acoustictime and low borehole diameter through constrasting the coal texture of core with log response;on the contrary,the tectonic coal has the characteristics of low density,low resistance,high gamma,high acoustictime and high borehole diameter.Through combination of all these kinds of log curves,the prediction formula of coal structure was established by which the coal structure index“k”can be calculated.For tectonic coal,the value of“k”is over 1;and for original texture coal,the value of“k”is below 1.It is found that there is mainly original texture coal in Zhengzhuang Block after the application of the prediction formula and this is favorable for the formation of well reservoir.Besides,there is dual porosity system for coal reservoir of which the coal matrix porosity is so low that it is mainly the cleats contribute for the porosity and permeability of coal reservoir.So the degree of cleat development can be characterized by coal porosity;meanwhile,the cleat permeability can be calculated by the relationship between coal porosity and permeability acquired from permeability experiment.On this basis,taking into consideration of the influence of matrix and mud content for the reservoir permeability and using the multivariate regression method,the quantitive explanation model of coal porosity and permeability is established;then this quantitive explanation model is demarcated by the geological data,production performance and experiment results and analysed through large scale of well data;finally,a formula model for porosity and permeability explanation applicable to Zhengzhung block is equated.Through applying this formula model in Zhengzhang block,acquires the porosity and permeability of original texture coal and tectonic coal distribution respectively.Most of the cleats are filled up by analysing the distribution of porosity and permeability.After all,the heterogeneity of coal reservoir is depicted from aspects of coal seam thickness,coal structure and physical property.After a comprehensive analysis of structure,sedimentary and heterogeneity characteristics of coal reservoir,ten evaluation parameters like porosity,permeability,sedimentary microfacies,buried depth,et al,which are closely related to the coal reservoir property,were selected for the classification and evaluation of coal reservoir by multilevel fuzzy synthetic discrimination technique.As a result,acquires the favorable zone distribution of original texture coal and tectonic coal.Ultimately,Zhengzhuang Block is divided into three categories of district,namely?????,by a combination of these two kinds of favorable zones.This study can provide guidance and criteria for the following development of Fanzhuang block.