Evaluation Of Reservoir Properties And Gas Content For Key Shales In Guizhou Province

As one of the China's marine shale gas exploration potential favorable zones,Guizhou Province which is located in the southwest of Upper Yangtze Plate has excellent shale gas accumulation conditions.With emphasis on the lower Cambrian Niutitang Formation and the Lower Silurian Longmaxi Formation of Marine facies as well as Upper Permian Longtan Formation of transitional facies in Guizhou,this paper aims to reveal the reservoir conditions and gas-bearing characteristics of the three sets of gas-bearing shale formations in the study area.Niutitang Formation belongs to the open shelf deposit model overall;Longmaxi Formation belongs to the limited retention shallow sea deposit;Longtan Formation shale enriching of organic matter belongs to tidal-flat and paludal subfacies deposits.Affected by shallow marine to the deep water shelf sedimentary environments,kerogen types of shale section rich in organic of Niutitang Formation and Longmaxi Formation are given priority to ???,kerogen of Longtan Formation shale of transitional facies given priority to ? for its tidal-flat to swamp facies material sources;In addition,thickness and organic carbon content of three sets of series of strata also controlled by sedimentary facies,Niutitang Formation is greater than the shallow deposition of Longmaxi Formation,but Longtan Formation is relatively higher for its coal bearing strata.Greatly influenced by buried depth,three sets of series has the characteristics of high-over maturity,and declines in turn.Three sets of series all have the characteristics of high brittleness minerals,high specific surface,low porosity,ultra-low permeability,relative development of micro-pores and fractures,but there are differences between the reservoirs.Niutitang Formation has the highest brittleness mineral content,Longtan Formation relatively enriches in clay minerals,Longmaxi Formation in the middle position;Longtan Formation is superior to the others at porosity and permeability in the whole,Longmaxi Formations is close at porosity with Niutitang Formation,but the permeability is higher than that of Niutitang at least one order of magnitude.The structure of shale reservoir of Niutitang Formation is compact,rarely the micro-pores and micro-cracks can be found under the electron microscope,but Longmaxi Formation are relatively development of these.Pore structure parameters are also showed that Longtan Formation as a whole is better than that of the Longmaxi Formation,better than Niutitang Formation.Development of natural fracture has a certain coupling relationship with the fault system,Longmaxi Formation has more than two periods structural fractures,most of which are high angle seam;natural fracture of Niutitang Formation is also more high angle seam,but are more calcite filling.The ground stress field simulation can better predict fracture development scale.Longmaxi and Longtan Formations have higher gas content,surpassing that of Niutitang formation remotely.Gas content of three sets of shale formations have the increasing trend with the increase of the buried depth,which plays a positive role to Niutitang Formation;Free gas of Niutitang Formation accounts for larger,but Longtan Formation relatively enriches in adsorbed gas up to 75%,Longmaxi Formation is centered.The contents of organic carbon,clay mineral and quartz are the main controlling factors of shale gas content,but for Guizhou province under the background of complex tectonic zone and high mature marine shale,preservations especially the structural and faults development are likely to be the chielf conditions determing the exitence of shale gas.The results in a certain extent,can solve the basic geological problems in the shale gas exploration and development at present stage in Guizhou,and provide direction guidance for optimization of the shale gas favorable districts as well as the establishment of the demonstration zone.