Reservoir-forming characteristic of marine coal-measure gas and its favorable development segments：Taking the Carboniferous Xiangbai Formation in Well Longcan 1 in Weining,North Guizhou as an example

The Lower Carboniferous Xiangbai Formation in Northern Guizhou is a set of marine coal-measure sedimentary strata,which is one of the important strata for shale gas (SG) exploration and development.In this paper,the Longcan test well 1 from Weining Longjie syncline in the north of Guizhou Province is taken as an example to evaluate the coal-measure gas (CMG) accumulation characteristic and favorable development segments.The results show that the spatial distribution of Xiangbai Formation reservoirs has segmented feature.The thin coal seams are developed in the lower section,with a large number of coal seams,and interbedded coal-shale-sandstones.The shale are developed in the upper section,with a large continuous thickness of a single layer.The TOC content ranges from 0.77% to 8.94%,with an average of 2.01%,which increases with the increasing depth of the layer.The upper member of Xiangbai Formation is mainly type II1,and the lower member is mainly type II2.The average Ro,max of shale is 2.54%.The mineral components of shale are mainly clay minerals and quartz.The average content of quartz is 38.52%,the average content of clay is 48.99%,and the average brittleness index is 48.02%.The brittleness index generally increases with the increasing depth of the layer.The shale reservoir has an average porosity of 4.77% and an average permeability of 0.002×10-15 m2,which is a low-permeability reservoir.The physical properties of shale reservoirs are close to those of non-coal-bearing marine facies,and are better than coal-bearing marine-continent transitional facies.The SG content ranges from 0.27 to 1.48 m3/t,with an average of 0.57 m3/t.The SG composition results show that it is still in the weathered zone below 860 m.The average Ro,max of the coal seam is 2.92%.The coalbed methane (CBM) content of coal seams ranges from 3.34 to 10.66 m3/t,the average CBM content is 5.88 m3/t,and the CBM saturation is low.Overall,it shows that the potential of marine CMG resources is mainly characterized by SG,supplemented by CBM,and it is speculated that deep areas have more exploration prospects.Furthermore,using coal-measure SG as the main body of evaluation,based on the key parameters of single layer thickness,TOC,gas content,brittleness index,etc.,the principal component analysis method is used to evaluate the favorable development segment of CMG in the upper and lower member of the Xiangbai Formation.The comprehensive research suggests that the favorable development segments are mainly located in the middle parts of the upper member and lower parts of lower member in the Xiangbai Formation.