Relationship Between Lithological Combination And Gas-containing In Upper Paleozoic Of Southern North China

The Southern North China Basin is located in the southern part of the North China Plateform.The Late Paleozoic basin is in the water receded environment.The Taiyuan Formation is epicontinentally deposited,and the Shanxi Formation is deltaic sediments.There are eight sets of coal-seam formations in the Carboniferous-Permian of the Southern North China Basin.Because of rapidly lithological changes,it can form a variety of lithologic combinations.The gas content of different lithologic combinations is significantly different.In transitional facies,the vertical overlap of sandstone,mudstone,coal and limestone can form a variety of lithologic combinations.Considering the cap rock and the source rock,combineing the sedimentary system of the Southern North China Basin,the lithologic composition can be classified into four categories: limestone cap rock,sandstone cap rock,mud shale cap rock and coal bed cap rock.According to whether includes coal seam or not,the lithologic composition can be further divided into six classes: limestone cap rock including coal seams,limestone cap rock without coal seams,sandstone cap rock includes coal seams,sandstone cap rock doesn't include coal seams,mud shale cap rock and coal bed cap rock.The Southern North China Basin mainly contains limestone cap rock including coal seams,sandstone cap rock including coal seams,sandstone cap rock that doesn't includes coal seams,limestone cap rock that doesn't include coal seams.By comparing the organic geochemical characteristics of different mud shales from different lithological combinations,it was found that the organic type in the study area was mainly type III,with a small amount of type II2;The Ro of shales were all above 3%,and all in a mature stage.These parameters in different lithological combinations has no significant differences,but the TOC of mud shales from different lithological combinations has a significant difference.The TOC of mud shales which include coal seams is significantly higher than that doesn't include coal seams.This phenomenon was mainly caused by the depositional environment.By comparing the porosity and permeability of shales from different lithological combinations,it was found that the porosity of mud shales is better than that doesn't include coal seams.The porosity and permeability of different lithologic is also quite different.Overall,the shale is the best,the porosity and permeability of limestone is the smallest,followed by sandstone,which provides good conditions for limestone and sandstone as cap rock.By analyzing the correlation between organic parameters and gas-containing properties,it was found that TOC,the total pore volume positively correlated with gas content in the Southern North China Basin,the thickness and mineral content of shale were poorly correlated with gas content.By considering the TOC,the porosity,permeability,the total pore volume of mud shale from different lithology combinations,the porosity and permeability of the cap rock,it was found that the limestone cap rock including coal seams is the most favorable type for shale gas accumulation.The limestone cap rock including coal seams form the ideal combination of limestone-sandstone-mudstone-coal.This type of combination is not only beneficial to the storage and preservation of shale gas,but also beneficial to exploit the coal-bed gas and shale gas together;the gas-containing of limestone cap rock with coal seams is the best,followed by the sandstone cap rock with coal seams,the limestone cap rock without coal seams and the sandstone cap rock without coal seams is the worst.