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

Abstract

The carbon isotope curve (δ13Cn 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 (δ13C1 < δ13C2 < δ13C3) type and three Reversed types consisting of R1 (δ13C1 > δ13C2 > δ13C3), R2 (δ13C1 < δ13C2 > δ13C3), and R3 (δ13C1 > δ13C2 < δ13C3). CICI is defined as arctan (3Δ13C3-2/Δ13C2-1) × 180/π for Normal and R2, –180 + arctan (3Δ13C3-2/Δ13C2-1) × 180/π for R1, and 180 + arctan (3Δ13C3-2/Δ13C2-1) × 180/π for R3. CICI ranges from 0 to 90 for Normal, –180 to –90 for R1, –90 to 0 for R2, and 90 to 180 for R3. 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.