© 2011, Weatherford Laboratories All rights reserved.
the Origin of Hydrocarbon Gas Shows and Gas Seeps
(Bacterial Gas vs. Thermogenic Gas) Using Gas Geochemistry
Knowing whether a
natural gas show is biogenic gas or thermogenic gas can have critical
implications for the presence of liquid hydrocarbons in a basin. As described
below, geochemical analyses can reveal the origin of a gas show or seep,
and can reveal the presence of an effective petroleum system in a basin.
Types of natural
Two distinct processes
produce hydrocarbon gas: biogenic and thermogenic degradation of organic
matter. Biogenic gas is formed at shallow depths and low temperatures
by anaerobic bacterial decomposition of sedimentary organic matter. In
contrast, thermogenic gas is formed at deeper depths by:
(1) thermal cracking
of sedimentary organic matter into hydrocarbon liquids and gas (this
gas is co-genetic with oil, and is called "primary" thermogenic gas
(2) thermal cracking
of oil at high temperatures into gas ("secondary" thermogenic gas) and
Biogenic gas is very
dry (i.e., it consists almost entirely of methane). In contrast, thermogenic
gas can be dry, or can contain significant concentrations of "wet gas"
components (ethane, propane, butanes) and condensate (C5+ hydrocarbons).
Why does the type
of gas matter?
As described above,
biogenic gas is unrelated to the processes that form oil. As a result,
if a gas seep or show is bacterial in origin, then the presence of the
gas says nothing about the likelihood of an effective petroleum system
existing in the basin. Hydrocarbon liquid could be present in the area,
but, if it is, then its presence is fortuitous: it is unrelated to the
bacterial gas show. Similarly, if a gas seep bubbling from a lake bottom
is found to be bacterial gas, then the presence of the seep says nothing
about the presence of an underlying petroleum system.
In contrast, if a
gas show is found to be thermogenic in origin, then the possibility exists
that the gas derives from a gas cap overlying a down-dip oil leg. That
possibility can be further evaluated from specific aspects of the composition
of the thermogenic gas (see, Predicting Down-dip
Oil, and Basin
Apart from the oil-exploration
implications, the gas type also affects the gas value. The value of gas
is enhanced by the content of wet-gas components. Biogenic gas is typically
dry (except in situations where it has been in contact with thermogenic
liquid hydrocarbons); therefore, biogenic gas can be of significantly
lower value than thermogenic gas.
Can biogenic gas
be an economic gas resource?
In numerous cases,
biogenic gas occurs in sufficient quantities to be economically produced
solely for the gas value. In fact, biogenic gas accounts for as much as
20% of the world's natural gas resource (Rice, 1993). However, accumulation
of biogenic gas in commercial quantities requires relatively unusual geologic
conditions, including formation of stratigraphic or early structural traps,
formation of adequate early seals, and rapid sedimentation rates. The
traps must form early, because biogenic gas can only accumulate when it
can migrate as a free gas phase. A free gas state results when biogenic
gas generation exceeds the gas solubility in the pore fluid or when gas
exsolution from pore water is caused by reduction of the hydrostatic pressure.
Exsolution of gas can be a consequence of falling sea level or uplift
and erosion. Gas saturation of formation waters (and consequent formation
of a migrating free gas phase) can only occur at shallow depths (<4000-6500
ft.). At deeper depths, the increased solubility of gas in formation water
prevents the formation of a free gas phase that can migrate and accumulate
as a biogenic gas accumulation (see Rice, 1993 for a detailed discussion).
Using gas geochemistry
to distinguish biogenic gas from thermogenic gas
Gas geochemistry readily
reveals whether a gas is biogenic or thermogenic. Furthermore, the composition
of a thermogenic gas reveals the thermal maturity of the source rock that
generated the gas. Specifically:
(1) Biogenic methane,
on average, contains isotopically lighter carbon (i.e., is more depleted
in 13C) than thermogenic methane. Biogenic gas is also drier
than many thermogenic gases. Ranges of gas composition corresponding
to (i) bacterial gas, (ii) thermogenic, oil-associated gas, (iii) dry,
post-mature thermogenic gas, and (iv) gas of mixed biogenic/thermogenic
origin have been defined, facilitating the interpretation of gas compositional
data (e.g., Schoell, 1983, 1988; Faber et al., 1992; Whiticar, 1994).
In cases where gas seeps may be derived from biogenic decomposition
of anthropogenic wastes ("landfill" gases), other tracers, such as 14C
and tritium can be used to distinguish landfill
gases from older biogenic gases unassociated with anthropogenic
waste (Coleman, 1995).
gas components (methane, ethane, propane) generated at a given thermal
maturity contain, on average, isotopically heavier carbon than do the
corresponding gas components generated at a lower thermal maturity.
Relationships between gas isotopic compositions and source maturity
have been calibrated, allowing the vitrinite reflectance equivalent
(VRe) of the gas source to be estimated from the gas geochemistry (Faber,
1987; Berner and Faber, 1988; Berner, 1989).
Using gas geochemistry
to solve exploration and development problems
Sampling gas for analysis
(whether it be mud gas, produced gas, or a gas seep) is a straightforward
process (see sampling techniques).
At OilTracers, we integrate gas geochemical data with engineering
and geological information to solve a variety of exploration and development
problems. For example, mud gas geochemical logging
can be used to identify pay zones, assess hydrocarbon type and determine
reservoir continuity. In addition, gas geochemistry of produced gases
can be used to reveal gas type and maturity, information that can be combined
with basin modeling
to better define the petroleum system in a basin. Furthermore, we use
gas geochemistry to distinguish landfill gas
from other types of biogenic gas. For more information on applications
of gas compositional data, or to discuss a specific project, e-mail us
at firstname.lastname@example.org, or call
us at (214) 584-9169.
Berner, U., 1989,
Entwicklung und Anwendung empirischer Modelle fur die Kohlenstoffisotopenvariationen
in Mischungen thermogener Erdgase: Ph.D. thesis, Technische Universitat
Berner, U., and E.
Faber, 1988, Maturity related mixing model for methane, ethane and propane,
based on carbon isotopes: Org. Geochem., v. 13, p. 67-72.
Coleman, D. D., C.-L.
Liu, K. C. Hackley, and S. R. Pelphrey, 1995, Isotopic Identification
of Landfill Methane: Environmental Geosciences, v. 2, p. 95-103.
Faber, E., 1987,
Zur isotopengeochemie gasformiger Kohlen wasserstoffe: Erdol Erdgas Kohle,
v. 103, p. 210-218.
Faber, E., W. J.
Stahl, and M. J. Whiticar, 1992, Distinction of bacterial and thermogenic
hydrocarbon gases, in R. Vially, ed., Bacterial Gas, Paris, Editions Technip,
Rice, D. D., 1993,
Biogenic gas: controls, habitats, and resource potential, in D. G. Howell,
ed., The Future of Energy Gases - U.S. Geological Survey Professional
Paper 1570, Washington, United States Government Printing Office, p. 583-606.
Schoell, M., 1983,
Genetic characterization of natural gases: AAPG Bulletin, v. 67, p. 2225-2238.
Schoell, M., 1988,
Multiple origins of methane in the earth: Chemical Geology, v. 71, p.
Whiticar, M. J.,
1994, Correlation of natural gases with their sources, in L. B. Magoon,
and W. G. Dow, eds., The Petroleum System, From Source to Trap, AAPG,