Distinguishing the source of 

nat l l ti ith tural gas accumulations with a 

combined gas and co-produced formation water 

geochemical 

approach: A case study from the Appalachian Basin 

By: Stephen Osborn, Duke UniversityPost

-doctoral research associate doctoral research associate

(stephen.osborn@duke.edu)and Jennifer McIntosh, y Universit

y of Arizona

(mcintosh@hwr.arizona.edu)

EPA Technical Workshop, February 24-25 2011
Acknowledgements:

Oil and gas

operators in the 

study area

Robert Jackson 

( ) Duke

)

Avner Vengosh (Duke)Nathaniel Warner (Duke)
Introduction

• Strong interest in Devonian age organic-rich shales 

northern Appalachian basin northern Appalachian basin. 

“Appalachian Basin’s Marcellus – the new target” 

-AAPG Explorer (March 2008) AAPG Explorer (March 2008)

“There’s Gas in Those Hills” -N.Y. Times (April 200

• S th t ti l i t l i 

Some concern on the potential environmental impac

shallow groundwater 

• Distinguishing the source of natural gas may provide 

insight into understanding mechanisms associated woil/gp , y as operations, if any.
Objective

• Discuss the use of gas and co-produced formation w

geochemistry for identifying the source of natural ga

• Present g g y pp as geochemistry for the northern Appalach

“Chemical and isotopic tracers of the contribution of m

Chemical and isotopic tracers of the contribution of m

gas in Devonian organic-rich shales and reservoir sannorthern Appalachian Basin” 

-O b S G d M I t h J C A li d G h i t 

Osborn, S.G. and McIntosh J.C., Applied Geochemist

456-471.”
Regional 

Geologic 

Setting

• Microbial gas is actively 

generated at the shallow margins of the gl i t d Mi hi d lacia

t

e

d Mi

chigan an

d

Illinois basins

Northern Appalachian Northern Appalachian

Basin:

• Similar 

g y lacial histor

y

and litho-stratigraphic relationships

• I t t diff ? Impor

tan

t differences

?

(modified from Mc
Research Area

(modified from Shaver et al., 1986; Laughrey and Baldassare, 1998)
Quantity and Quality of Organic Ma

Organic-rich shales contain total organic

carbon (TOC) up to 18%

(Modified fro
Origin of Methane Origin of Methane
Limit of Last 

Glacial Maximum Glacial Maximum
Carbon Isotopes C

1

-

C

4

0

C

4

C

3

C

2

C1

Carbon Isotopes

C

1

-

C

4

38069 ‐10MalloryMichaiak

‐30‐20 Clark Rd

Torrey2CircleDr

‐40‐50‐60

Reciprocal carbon number

1/4 1/3 1/2 1
Carbon Isotopes C

1

-

C

4

0

C

4

C

3

C

2

C1

Carbon Isotopes

C

1

-

C

4

‐10‐30‐20 Clark Rd‐40‐50‐60

Reciprocal carbon number

1/4 1/3 1/2 1
Hyg p dro en Isotopes of Water and Met
Evidence for Microbial Methanogen

and Sulfate Reduction and Sulfate Reduction
Summary of 

R lt esu s

Dominantly thermogenic gas throughout the stud

• high δ13C-CH4 values,

Some Microbial Methane 

• low δ13C-CH4 values (-74 to -57‰)

Mixed gas, though still dominantly thermoge

• slightly negative δ13C-CH4 (<-50‰)

• low methane to ethane and propane ratios, • negative to slightly positive δ13C-DIC values with low 

• relatively high salinity

• high methane to ethane and propane ratios, • some positive δ13C-DIC and increasing alkalinities

• low to moderate salinity.

• low methane to ethane and propane ratios with incre• some positive δ13C-DIC values

• low to moderate salinity
Take Home Message

• G l i di t f General indica

tors o

f th i ermogenic gas:

• Presence of higher chain hydrocarbons (ethane, propan

greater than 5% combined with,

• Cb i l f h h Car

bon isotope values o

f met

hane greater t

han ~-50

‰

• Correlation between reciprocal carbon number and carb

values of higher chain hydrocarbons

• Genetic and post-genetic considerations:

• Organic matter source (effects carbon isotopic value)• Oxidation and Diffusion (fractionation of isotopes and ga• Mixing of different thermogenic sources 

• General indicators of biogenic gas:

• Relatively Low salinity (less than Relatively Low salinity (less than

~

2

-4 M Cl

)

• Low concentration of alternative electron acceptors (SO

• High 

δ13C-DIC values (>~+10‰) and high Alkalinity (>~1

• Correlation of Correlation of

δ

D of methane and co of methane and co

-produiced produiced water