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 (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 (3Δ¹³C_3-2/Δ¹³C_2-1) × 180/π for Normal and R₂, –180 + arctan (3Δ¹³C_3-2/Δ¹³C_2-1) × 180/π for R₁, and 180 + arctan (3Δ¹³C_3-2/Δ¹³C_2-1) × 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.

碳同位素曲线（δ ¹³ C _n与 1/n）常用于评价烃类气体的成因和次生蚀变，但影响碳同位素曲线的因素尚未完全了解，限制了其实际应用。我们定义碳同位素曲线指数（CICI）来定量表征曲线类型。定义了四种通用曲线类型：正态 (δ ¹³ C ₁  < δ ¹³ C ₂  < δ ¹³ C _{3 ) 类型和由 R 1}组成的三种反转类型(δ ¹³ C ₁  > δ ¹³ C ₂  > δ ¹³ C ₃ ) 、R ₂ (δ ¹³ C ₁  ＜δ ¹³ C ₂  ＞δ ¹³ C ₃ )、R ₃ (δ ¹³ C ₁  ＞δ ¹³ C ₂  ＜δ ¹³ C ₃ )。对于正态和 R ₂ , –180 + arctan (3Δ ¹³ C _3-2 /Δ ¹³ C _2-1 ) × CICI 定义为arctan (3Δ ¹³ C _3-2 /Δ ¹³ C _2-1 ) × 180 /π R ₁为 180/π ，R ₃为180 + arctan (3Δ ¹³ C _3-2 /Δ ¹³ C _2-1 ) × 180/π 。_{对于正常，CICI 范围为 0 到 90；对于 R 1} ， CICI 范围为 –180 到 –90；对于 R ₂ ， CICI 范围为 –90 到 0 ；对于 R ₃，CICI 范围为 90 到 180。正常气体可分为三类：线性（CICI = 45）、凸面（CICI 0 至 < 45）和凹面（CICI > 45 至 90）。通过比较沉积盆地和热解实验中不同干酪根产生的热成因气体，研究了干酪根类型和成熟度对 CICI 的影响。低成熟度湖相和低成熟度煤气的曲线以凸型为主。相比之下，高成熟度到过成熟海洋气体的曲线更加凹。随着成熟度的增加，热成因气体的曲线类型由凸型变为凹型，干酪根裂解和石油裂解产生的气体混合，形成凹型。使用来自三项实地研究的数据阐明了后代改变过程对 CICI 的影响。在普通型气体中，丙烷的优先生物降解和泄漏增加了 CICI，而迁移和与微生物甲烷的混合降低了 CICI。丙烷的优先生物降解和泄漏是凹型的另外两个原因。