--> Models of Natural Gas Origin – A Short History

AAPG Hedberg Conference, The Evolution of Petroleum Systems Analysis

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Models of Natural Gas Origin – A Short History


Since the 1960s, investigators have observed that trends in the molecular and isotopic compositions of natural gases are very similar throughout the world. These natural gas compositions are generally explained by the transition with increasing depth from biogenic gas origin to mixtures of increasing thermogenic component. This transition concept has evolved from a series of models developed over the past 40+ years. One or more of these models is used to determine the origins of natural gas in current natural samples. Galimov (1969) graphically distinguished several zones of methane generation in the sediment, while modelling the theoretical curve corresponding to isotopic equilibrium of the methane‐carbon dioxide system at the given temperature. The defined zones were: I. Biochemical zone, II. Catalytic zone, and III. Thermal zone. Stahl (1974) developed a graphic model explaining the isotopic and molecular compositions of natural gases. The model demonstrated trends in 13C values and C1/Cn ratios with increasing age and maturity. Light isotopic ratios and very high C1/Cn ratios are indicative of a biogenic gas origin. Zones of higher temperature where chemical processes such as hydrolysis, cracking, and hydrogen disproportion become increasingly important cause isotope ratios of methane to increase and gases to become wetter. Finally, with extreme maturity an overcooked stage of is described, where virtually all of the available carbon has been converted to dry methane gas. Bernard (1978) studied the origins and concentrations of light hydrocarbon gases in marine sediments and marine seeps. The two parameters that he used to characterize the origins of natural gases in a graphic model, which came to be known as a “Bernard Plot,” were 13C values of methane and C1/(C2+C3) molar ratios. Molar ratios of the light hydrocarbons expressed in this manner yielded values which varied over 4 orders of magnitude, and were deemed to be more distinctive than Stahl's C1/Cn values. Biogenic gases consist almost exclusively of methane, having C1/(C2+C3) ratios greater than 1,000 and 13C values of methane more negative than –60o/oo. Thermogenic hydrocarbon gases generally have C1/(C2+C3) ratios smaller than 50 and 13C values more positive than –50o/oo. His geochemical model based on these two parameters was used to show that natural gas compositions can be altered due to mixing of gases from the two sources as well as by microbial action and migration through sediments. Schoell (1980) used a genetic diagram in which C and H stable isotopes of various natural methanes were related to each other. This plot has become known as a “CD Diagram.” He extended this concept by adding a crossplot of the fraction of C2+ hydrocarbons vs. 13C values of methane similar to that of Stahl. These Schoell Plots were designed to serve as a guide to interpretation not only of biogenic and thermogenic gases, but also of oil‐associated and mixed gases and those of even more complex origins. Schoell differentiated two stages of thermogenic gas: (1) during or immediately following oil formation, which results in gases associated with crude oils (associated gas), and (2) following the principal stage of oil formation, which results in dry or deep dry gases (non‐associated gas). Faber (1987) developed a plot illustrating trends in the carbon isotopic compositions of methane, ethane, and propane in natural gases from around the world. In general, the isotopic ratios of carbon in these three components of a thermogenic natural gas trend together, each getting isotopically heavier with increasing maturity of the their responsible source material. The graph is commonly called a “Faber Plot” and can be used to differentiate natural gases that are derived from a single source from gases that are mixtures derived from two or more sources. A plot of the carbon isotopic ratio of methane vs. ethane, or of propane vs. ethane, that falls on this set of empirical lines supports a singly sourced natural gas, whereas a plot away from the lines suggests that the gas accumulation is a mixture of gases. Chung (1988) developed a graphic model to differentiate natural gases that are derived from a single source from gases that are mixtures derived from two or more sources. The method is based on plotting the isotopic composition of methane, ethane, propane, n‐butane, and n‐pentane as a function of the reciprocal of the carbon number of the gas. A linear trend supports a cogenetic origin, whereas a non‐linear fit suggests that the gas accumulation is a mixture of gases, a chemically altered gas, or a gas derived from a structurally heterogeneous carbon source. This plot is commonly known as a “Natural Gas Plot” or a “Chung Plot.”