| Significant amount of coalbed methane(CBM)has been found in the vast expanse of exploration area in eastern Ordos basin,China,however,the geological controlling factors are also variable.Based on the analysis of the dynamic variation of the geology conditions along temporal evolution,the CBM accumulation and reservoring rules are concluded in fourthfold:the horizontal extention about the sedimentation evolution;vertical variation on different burial depth;temporal evolution of the CBM composition;and substances of petrology heterogeneity.Furthur,the coupling effect of the four aspects,which showing the CBM accumulation and prolific production trend,are detailed studied with an insight into the CBM geological controlling rules in eastern Ordos basin.The north 38° structural belt was clarified as the the "seesaw" point during the structural rise and subsidence in the north and south,with the coalbearing strata showing a thickness and lithology balancing variation through the point.The sediment sources in north showed an effect at the beginning of Taiyuan Stages,while the south sources played its roles obviously since the Shanxi Stage even if it can be seen in middle of Taiyuan Stage.The low curve meandering river played an import role in the formation of the Jinci,Qiaotou and Beichagou sandstones in the north.The top of the main coal in Taiyuan Formation was chosen as the sequence top,and the sedimentation of the coalbearing strata was detailedly illustrated.The in-situ stress,geothermal and reservoir stress fields of the coal seams were studied with a discussion of the relationship with burial depth.Most of the coal reservoirs are underpressured,and the geothermal field shows a north low and south high station with the Liulin area working as a hedge.The in-situ stress shows a positive relationship with the burial depth generally,with the 800～1100 m being of the transition zone.However,it cannot be ignored that different areas show different transition depth,as the Liulin of 750 m while Hancheng of 1000 mThe geological explanation to the lightening phenomena of the carbon isotope of methane in CBM was concluded.Through combination analysis of the relationship between δ13CPDBand the relative effectors,paramount in determining CBM carbon isotope being lighter in the eastern Ordos Basinare the genesis ofsecondary biogas in the north,water soluble effects in the active hydrodynamic areas in the middle,desorption fractionation effect promoted by tectonic evolution in the south,and sudden warming hydrocarbon fractionation accelerated by igneous activity in some areas.The development mechanism and heterogeneity of the physical property of the coal reservoir were discussed.Generally,from north to south,as well as from low to deeper strata,the degree of coal metamorphism increases,with an increase in gas content.As the maximum of vitrinite reflectance(Ro,max)of coal varies between 0.4%—1.2%,the 800—1000 m depth shows a marked turnaround as a beginning of the gas content decrease.While for Ro,max of 1.2%—2.6%,1400 m is shown as the turnaround point.The heterogeneity of coal reservoirs were caused by the structural difference,and the cross section difference was caused by the sedimentation variation,and both the above worked to the heterogeneity of coal from nm—km scales.As for the CBM accumulation and reservoiring rules,the effects of lithology trap,gas-pressure trap,hydro-pressure trap and adsorption trap in CBM reservoirng have been classified.And the upper limit for reservoir is the methane oxidation belt,while the lower limit is the in-situ stress variation belt.Furthur,the CBM reservoir types are named as the structural,lithology and hydrogeology trap dominanted reservoirs,with the typical reservoirs detailedly presented. |
