thermogenic vs biogenic methane, page-2

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 gas

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 ), and

(2) thermal cracking of oil at high temperatures into gas ("secondary" thermogenic gas) and pyrobitumen.

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 Modeling).

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).

(2) Thermogenic 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, L.L.C., 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 [email protected], or call us at (214) 584-9169.

REFERENCES

Berner, U., 1989, Entwicklung