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• The carbon isotope curve index (CICI) summarizes the isotope ratios of C 1 - C 3. • Carbon-isotope curves can be divided into Convex, Linear, Concave and Reversed. • In Normal-type gas, leakage and preferential propane biodegradation increase CICI. • In Normal-type gas, migration and mixing with microbial methane decrease CICI. The carbon isotope curve (δ13C n versus 1/n) is often used to evaluate the origin and secondary alteration of hydrocarbon gases, but the factors that influence the carbon isotope curve are not fully understood, limiting its practical application. We define a carbon isotope curve index (CICI) to quantitatively characterize the curve type. Four general curve types are defined: a Normal (δ13C 1 < δ13C 2 < δ13C 3) type and three Reversed types consisting of R 1 (δ13C 1 > δ13C 2 > δ13C 3), R 2 (δ13C 1 < δ13C 2 > δ13C 3), and R 3 (δ13C 1 > δ13C 2 < δ13C 3). CICI is defined as arctan (3Δ13C 3-2 /Δ13C 2-1) × 180/π for Normal and R 2 , –180 + arctan (3Δ13C 3-2 /Δ13C 2-1) × 180/π for R 1 , and 180 + arctan (3Δ13C 3-2 /Δ13C 2-1) × 180/π for R 3. CICI ranges from 0 to 90 for Normal, –180 to –90 for R 1 , –90 to 0 for R 2 , and 90 to 180 for R 3. Normal gases can be divided into three categories: Linear (CICI = 45), Convex (CICI 0 to < 45), and Concave (CICI > 45 to 90). The influence of kerogen type and maturity on CICI is investigated by comparing thermogenic gases generated from different kerogens in both sedimentary basins and pyrolysis experiments. Curves for low-maturity lacustrine and low-to-high-maturity coal gases are mainly Convex. In contrast, curves for high maturity to overmature marine gases are more Concave. The curve type of thermogenic gas changes from Convex to Concave with increasing maturity, and the mixing of gases generated from kerogen-cracking and oil-cracking results in the Concave type. The effect of post-generation alteration processes on CICI was elucidated using data from three field studies. In Normal-type gas, preferential biodegradation of propane and leakage increase CICI, while migration and mixing with microbial methane decrease CICI. Preferential biodegradation of propane and leakage are two other reasons for the Concave type. [ABSTRACT FROM AUTHOR]

The hydrogen isotope composition of methane is an important parameter in natural gas research and provides complementary information to that provided by carbon isotopes. The stable hydrogen isotope ratios of 313 natural gas samples from six of China’s sedimentary basins are used to evaluate the factors that influence the stable hydrogen isotopic composition of methane. An important factor is the δD of organic matter in hydrocarbon source rocks, which is influenced by the sedimentary environment and type of organic matter. Natural gases generated from sapropelic organic matter have relatively less negative δ D CH 4 , while natural gases generated from humic organic matter have relatively more negative δ D CH 4 . The other factor is thermal maturity. Increased thermal maturity leads to δ D CH 4 becoming less negative and there exists a two stage linear relationship between the δ D CH 4 and the logarithm of the vitrinite reflectance ( R o) value for natural gases generated from type III kerogen. The third factor is the environmental conditions of the aqueous medium in the original depositional environment and after sedimentation. Elemental hydrogen from water participates in biochemical processes as organisms grow and is exchanged during the sedimentation and diagenesis of organic matter as well as maturity process of kerogen to generate gases. The influence of the aqueous medium on δ D CH 4 after formation of natural gases can be ignored. Among these factors, the aqueous medium is the key constraining factor. The δ D CH 4 in natural gases can be combined with the use of stable carbon isotopic composition to identify the origins of natural gases and to aid gas-source correlations. [ABSTRACT FROM AUTHOR]