The Genesis And Environmental Significance Of High Hydrocarbon Groundwater In Shallow Triassic Aquifer Of Northern Zhijin, Guizhou, China

Original high hydrocarbon groundwater represents a kind of groundwater in which hydrocarbon concentration exceeds 0.05mg/L(Environmental quality standards for surface water grade ? for central drinking water source).Based on different sources,high hydrocarbon groundwater can be divided into secondary and original high hydrocarbon groundwater.Different from secondary high hydrocarbon groundwater,the original high hydrocarbon groundwater forms in natural condition.It will significantly reduce the environment capacity of hydrocarbon and lead environmental problems.For the past five years,this research had carried out for a long-term monitoring of groundwater in shallow Triassic aquifer in north Zhijin,Guizhou.In the area,it found the concentration of petroleum hydrocarbon was always above 0.05mg/L.The trace-level secondary hydrocarbon pollution could not produce high hydrocarbon groundwater in the area,which indicated original high hydrocarbon groundwater in the area.By using inside and field experiments,this research carried out a comprehensive study in researching the ‘recharge-runoff-discharge' characteristics of the original high hydrocarbon groundwater and strategically assessing the environmental impact of the original high hydrocarbon groundwater in shallow Triassic aquifer.The main achievements had been concluded as follow:1.‘Recharge' – The origin of high hydrocarbon groundwater in shallow Triassic aquiferTriassic strata could emit hydrocanbon into shallow groundwater in the area.By analyzing sedimentary environment,paleontology,chemical composition,hydrocarbon potential and biomarker characteristics from Yongningzhen formation(T1yn)to Xujiahe formation(T3x),this research carried out the whole Triassic period developed a transgressive-regressive process with multi sedimentary cycles.The results determined that most strata in the area can generate hydrocarbon,but their potential are different.The upper Yongningzhen formation(T1yn4)and upper Guanling formation(T2g3)dolomite and calcite dolomite had higher potential than others.The hydrocarbon generated from carbonate rocks contains lower aquatic with terrigenous detrital organic matter contribution deposited under marine conditions.The middle Guanling formation(T2g2)limestone had normal hydrocarbon generation potential.The organisms were derived from lower aquatic with terrigenous detrital organic matter contribution deposited under marine conditions.The middle-upper Yongningzhen formation(T1yn3),lower Guanling formation(T2g1)and Falong formation(T2f)had relative low hydrocarbon generation potential.And Xujiahe(T3x)was mainly continental sedimentary stratum with extremely low petroleum hydrocarbon generation potential.2.‘Runoff' – Hydrocarbon emission theory by water-rock-hydrocarbon interactionHydrocarbon in shallow groundwater and rock developed from the same source.Moreover the hydrocarbon could migrate into groundwater by corrosion emission and extraction emission.Baesd on the results of hydrogeological condition and hydrocarbon generation potential,the research took rock and groundwater samples at different aquifers and depth.By using chemical constituents,isotope analysis,secondary hydrocarbon pollution and comparing biomarker characteristic in groundwater and rocks,it determined the mineral elements in shallow groundwater mainly from strata.Additionally there was relative strong water-rock interation in the area.The distribution of heavy saturated carbons in rock and groundwater was similar.Additionally based on the physicochemical property,the saturated carbon with less carbon would harder to dissolve in shallow groundwater.3.‘Discharge' – Main driving force of hydrocarbon emission by hydrocarbon generation strataThe main driving force of hydrocarbon emission would be corrosion emission.Moreover extraction emission would improve this process.The results of physical model and corrosion experiment indicated the hydrocarbon emission ability relate to degree of rock corrosion.The higher rock corrosion always had better hydrocarbon emission.In T1yn4,the relation between rock corrosion and hydrocarbon emission showed as a sudden changing,which indicated T1yn4 would likely to be hydrocarbon reservoir.Otherwise in T2g3,it showed as gradual changing,which indicated T2g3 would likely to be hydrocarbon source strata.Even though petroleum hydrocarbon concentration in extraction emission model was higher than in corrosion emission model,the hydrocarbon concentration would extremely low in nature groundwater,which means the effect of extraction emission would very weak.To calculate the concentration of petroleum hydrocarbon in Dalongjing-Yichonggou groundwater system,this research setted several assumptions,such as 1.This research calculated the concentration of petroleum hydrocarbon in the independent groundwater system.2.The water-abundance in the system was homogeneous.3.There were no karst tunnels in the system.4.The source of all the mineral elements in groundwater was from strata.5.The petroleum hydrocarbon would not migrate with the groundwater.6.All the petroleum hydrocarbon would be emitted into groundwater and distributed homogeneous.The result of the concentration of petroleum hydrocarbon by using the method of the concentration of Ca2+ and Mg2+ showed as 0.05-0.50 mg/L which matched well with the field data.Determining this method could be applied to evaluate the concentration of petroleum hydrocarbon in karst groundwater in these assumptions.Moreover,the result of the concentration of petroleum hydrocarbon by using the method of field corrosion experiment showed as 4.60 mg/L,which much higher than field data.Indicating this method could be used only if the corrosion was controlled by cracks and the samples developed representational cracks.4.‘Application' – Original high hydrocarbon groundwater potential area division in shallow Triassic aquifer in Jinsha zone36.92% of Jinsha zone was Triassic.The main hydrocarbon generation strata were T1yn4 and T2g3.To divide the original high hydrocarbon groundwater potential area in Jinsha zone,this research setted several assumptions,such as 1.This research calculated the concentration of petroleum hydrocarbon in the independent groundwater system.2.The source of Ca2+ and Mg2+ in groundwater in discharge zone was from strata.3.The degree of water-rock interaction only depended on the contact time between water and rock.4.There were no karst tunnels in the system.5.It considered that there was original high hydrocarbon groundwater in the gourndwater system when the concentration of petroleum hydrocarbon reached 0.04 mg/L.Incorporated with the local geography of Jinsha zone,the result indicated the original high hydrocarbon groundwater mainly distributed in the south,middle,northwest and northeast of Jinsha zone.Especially in the limestone area with gentle terrain,and the dolomite area with clastic rocks,when the concentration of Ca2+ and Mg2+ reached 9.15mg/L and 5.45mg/L,respectively,it would likely to exist original high hydrocarbon groundwater.