Distinguishing the source oft l l ti ithnaturalgas accumulations withacombined gas and co-producedformation watergeochemicalapproach: A case study from theAppalachian BasinBy:Stephen Osborn, Duke UniversityPost-doctoral research associatedoctoralresearchassociate(stephen.osborn@duke.edu)andJennifer McIntosh,Universityof Arizonay(mcintosh@hwr.arizona.edu)EPA Technical Workshop, February 24-25 2011Acknowledgements:2Oil and gasoperators in thestudy areaRobert Jackson(Duke))Avner Vengosh (Duke)Nathaniel Warner (Duke)Introduction•Strong interest in Devonian age organic-rich shales of thenorthern Appalachian basinnorthernAppalachianbasin.“Appalachian Basin’s Marcellus – the new target”-AAPG Explorer (March 2008)AAPGExplorer(March2008)“There’s Gas in Those Hills” -N.Y. Times (April 2008)S th t ti l i t l i t t•Some concern onthe potentialenvironmentalimpacttoshallow groundwater•Distinguishing the source of natural gas may provide someinsight into understanding mechanisms associated withoil/gas operations,if any.p , yObjective•Discuss the use of gas and co-produced formation watergeochemistry for identifying the source of natural gas•Presentgasgeochemistryfor the northern Appalachian Basing y pp“Chemical and isotopic tracers of the contribution of microbialChemicalandisotopictracersofthecontributionofmicrobialgas in Devonian organic-rich shales and reservoir sandstones,northern Appalachian Basin”O b S G d M I t h J C A li d G h i t 25-Osborn,S.G. andMcIntoshJ.C.,AppliedGeochemistry,25, pp.456-471.”RegionalGeologicSetting•Microbial gas is activelygenerated at theshallow margins of thel i t d Mi hi dglaciatedMichigan andIllinois basinsNorthern AppalachianNorthernAppalachianBasin:•Similarglacial historyyand litho-stratigraphicrelationships•I t t diff ?Importantdifferences?(modified from McIntosh and Martini, 2007)Research Area(modified from Shaver et al., 1986; Laughrey and Baldassare, 1998)Quantity and Quality of Organic MatterOrganic-rich shalescototaontainlrganiccarbon (TOC) up to18%(ModifiedfromRepetskietal.,2006)Origin of MethaneOriginofMethaneLimit of LastGlacial MaximumGlacialMaximumCarbon Isotopes C1-C40C4C3C2C1CarbonIsotopesC1-C4‐1038069MalloryMichaiak‐30‐20ClarkRdTorrey2CircleDr 

# δ 13 

# C‐40 

# (‰)‐50‐60Reciprocalcarbonnumber1/4 1/3 1/2 1Carbon Isotopes C1-C40C4C3C2C1CarbonIsotopesC1-C4‐10‐30‐20ClarkRd 

# δ 13 

# C‐40 

# (‰)‐50‐60Reciprocalcarbonnumber1/4 1/3 1/2 1Hydroen Isotopes of Water and Methaneg pEvidence for Microbial Methanogenesisand Sulfate ReductionandSulfateReductionSummary ofR ltesusDominantly thermogenic gas throughout the study area•highδ13C-CH4values,Some Microbial Methane•lowδ13C-CH4values (-74 to -57‰)Mixed gas, though still dominantly thermogenic•slightly negativeδ13C-CH4(<-50‰)•low methane to ethane and propane ratios,•negative to slightly positiveδ13C-DIC values with low alkalinities•relatively high salinity•high methane to ethane and propane ratios,•some positiveδ13C-DIC and increasing alkalinities•low to moderate salinity.•low methane to ethane and propane ratios with increasing trend•some positiveδ13C-DIC values•low to moderate salinityTake Home MessageG l i di t fth i•Generalindicators ofermogenic gas:•Presence of higher chain hydrocarbons (ethane, propane, butane)greater than 5% combined with,Cb i l f h h50‰•Carbonisotope values ofmethane greater than ~-•Correlation between reciprocal carbon number and carbon isotopevalues of higher chain hydrocarbons•Genetic and post-genetic considerations:•Organic matter source (effects carbon isotopic value)•Oxidation and Diffusion (fractionation of isotopes and gas composition•Mixing of different thermogenic sources•General indicators of biogenic gas:•Relatively Low salinity (less than~2-4 MCl)RelativelyLowsalinity(lessthan•Low concentration of alternative electron acceptors (SO4>~1mM)•Highδ13C-DIC values (>~+10‰) and high Alkalinity (>~10 meq/Kg)•Correlation ofδDof methane and co-produicedwaterCorrelationofofmethaneandcoproduiced