Study On Reservoir Characteristics And Gas Content Of Marine Shale Of Longmaxi Formation In Northern Yunnan-Guizhou

The Paleozoic shale gas resources in South China were mainly enriched in the southern part of Sichuan basin and its periphery of the middle-upper Yangtze platform.This paper took the Longmaxi formation marine shale in northern Yunnan-Guizhou on the front edge of Daloushan tectonic belt as the research object.Based on literature research,regional tectonic background,field production data,field geological survey and laboratory testing,sedimentary control factors of organic matter enrichment,pore structure characteristics of shale reservoir,organic geochemistry and gas content characteristics,and influence of preservation conditions on gas content were explored.The following conclusions and understandings had been obtained.(1)Sedimentary control factors of organic matter enrichment.The Longmaxi formation was divided into 6 medium term cycles(SQ1-SQ6)and 22 short term cycles(L1-L22)using 396kyr eccentricity long cycle and 102kyr eccentricity short cycle.The climatic fluctuations modulated by 102kyr eccentricity short cycle controlled the accumulation of organic matter.Combined with the sedimentary-tectonic environment,the sedimentary factors controlling the accumulation of organic matter in the Longmaxi formation in the undercompensated retention environment of the progressive basin-mountain structure foreland basin in the northern Yunnan-Guizhou were analyzed based on the palaeontological productivity,redox condition,sea level eustacy and organic carbon content.Four enrichment models were established from bottom to top:high palaeontological productivity anaerobic and oxygen-poor environment deep water enrichment model in semi-closed shallow sea,medium palaeontological productivity oxygen-poor and oxygen-rich environment deep water enrichment model in semi-closed shallow sea,low palaeontological productivity oxygen-rich environment deep water enrichment model in semi-closed shallow sea,and low palaeontological productivity oxygen-rich environment shallow water enrichment model in closed inland sea.Among them,the first two enrichment models were favorable to the formation of organic-rich shale.(2)Pore structure characteristics of shale reservoir.The shale mineral constituent was mainly clay minerals and quartz,followed by calcite and dolomite,and a small amount of feldspar and pyrite.The clay mineral constituent was mainly illite,followed by illite/montmorillonite mixed layer,and a small amount of chlorite,montmorillonite and kaolinite.With increasing depth,quartz content and brittleness index increased gradually.The types of reservoir space were poresand fractures.The pore structure characteristics and influencing factors of overmature(Ro>2.4%)shale reservoir were discussed.It was found that the specific surface area was negatively correlated with the average pore size.The pore types of shale reservoirs were mainly mesoporous,with relatively few macropores and micropores.Total pore volume was negatively correlated with depth,positively correlated with clay content,and negatively correlated with quartz content.Moreover,clay minerals were favorable for the formation of micropores and mesopores,while quartzs were favorable for the formation of macropores.In the constituent of clay minerals,total pore volume was positively correlated with the content of illite/montmorillonite mixed layer and chlorite.and negatively correlated with the illite content.In addition,illite/montmorillonite mixed layer and chlorite were favorable to form micropores and mesopores,while illite was favorable to form macropores.Total pore volume was negatively correlated with TOC and Ro.(3)Organic geochemistry and gas content characteristics.The organic carbon content of Longmaxi formation shale reservoirs in northern Yunnan-Guizhou was generally high(0.08%～7.39%,with an average of 1.91%).The maturity of organic matter was relatively high(1.16%-4.21%,with an average of 2.54%),which was in the over-mature diagenetic stage and mainly produced dry gas.The organic kerogen types were type Ⅰ and type Ⅱ1.Longitudinally,the climate fluctuation was controlled by the short-cycle regulation of 102KYR eccentricity,and the organic carbon content in SQ1 sequence was the highest(0.30%-7.39%,2.42%on average).On the plane,the Jianwu syncline and Xuyong syncline in the north of study area,which were close to the sedimentary center,have the largest organic carbon content and organic matter maturity,which are conducive to the formation of shale gas reservoirs.Field test of shale core in Longmaxi formation showed low gas content.Longitudinally,the SQ1 had the highest gas content,and the gas content decreased from the bottom to the top.The Jianwu syncline and Xuyong syncline near the sedimentary center had the highest gas content in the plane.The carbon isotope inversion or reversal of Longmaxi formation shale gas was caused by the mixture of coal-derived gas and oil-type gas,the mixture of "same type and different source" gas and the mixture of"homologous and different period" gas.Shale reservoirs were highly heterogeneous.Its adsorption capacity was the result of the interaction of many factors.The larger the mesoporous volume of shale,the higher the organic carbon content,the more clay minerals,the more illite/montmorillonite mixed layers,the more chlorite,the lower the temperature and the higher the pressure,the stronger the adsorption capacity and the higher the gas content of shale.The type of kerogen and the maturity of organic matter did not directly affect the gas adsorption capacity of shale,but indirectly by changing the characteristics of pore structure.(4)Influence of preservation conditions on gas content.The larger thickness of the shale cap,the wider distribution area,the stronger capillary sealing ability,the better toughness,the better preservation conditions,and the more conducive to the formation of shale gas reservoir.As for the roof and floor,the larger continuous deposition thickness,the more stable distribution,the more compact lithology,the greater breakthrough pressure,the stronger sealing ability,and the better preservation conditions are,the more conducivc to the formation of shale gas reservoir.The fault of Huangjinba gas field in northern Yunnan-Guizhou was explained by 3D geological modeling.6 main marker reflectors and 4 auxiliary marker reflectors were selected for tracking.The faults were classified and counted(a total of 138).Among them,93 faults cut off the bottom of the Longmaxi formation.The formation uplift would lead to the reduction of the formation depth,and even denudation of the exposed surface of the gas reservoir,resulting in the complete destruction of the preservation conditions,formation pressure and gas content.Large-scale faults(Tongtian faults)did great damage to the preservation conditions,compressive reverse faults with less or smaller fault development and gentle dip did little damage,and faults that cut through the Longmaxi formation but did not extend to the surface did little damage.The Jianwu syncline in the north and the Xuyong anticline in the northeast were the target areas for shale gas,which had high formation pressure,good preservation conditions and high gas content.