Genetic significance of carbon isotope curve types of methane, ethane, and propane in natural gas.

• 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]