Abstract:

The distribution characteristics of carbon isotopes in sedimentary organic matter are often used as indicators for oil-source correlation, determination of organic matter origins, and paleoenvironmental analysis. However, when organic matter reaches the over-mature stage, cracking processes can lead to isotopic fractionation, resulting in anomalies such as carbon isotope reversal. This phenomenon, particularly in deep and ultra-deep petroleum systems, has long been a key and challenging issue in hydrocarbon geology research. This study examines the carbon isotopic distributionsof various solid and liquid organic materials from source rocks and reservoirs in the Anyue Gas Field, the central Sichuan Uplift, Sichuan Basin, based on existing data and relevant research. The findings indicate that paleo-oil reservoirs experienced thermal alteration, causing the early-formed liquid hydrocarbons in both source and reservoir layers to crack into gaseous hydrocarbons, residual liquid hydrocarbons, and pyrobitumen. The pyrobitumen can be categorized into in-situ pyrobitumen in source rocks and reservoir pyrobitumen. In present-day source and reservoir layers, an overall inversion is observed where kerogen has lower δ¹³C values than liquid hydrocarbons, along with a localized inversion in which saturated and aromatic hydrocarbons show higher δ¹³C values than non-hydrocarbon components and asphaltenes. Additionally, the δ¹³C values of reservoir pyrobitumen are lower than those of both kerogen and liquid hydrocarbons. Comprehensive analysis of relevant data and simulation experiments indicates that carbon isotopic enrichment during hydrocarbon cracking is the primary cause of the observed isotopic inversions in both source and reservoir samples. After high-temperature cracking, the residual liquid hydrocarbons derived from original liquid hydrocarbons in source rocks and paleo-reservoirs exhibit an overall increase in δ¹³C values of approximately 4‰. Due to varying thermal exposure in source rocks and paleo-reservoirs, the extent of δ¹³C increase differs among various group components of the residual liquid hydrocarbons. Although reservoir pyrobitumen largely inherits the isotopic signature of the original crude oil, thermochemical sulfate reduction (TSR) during its formation may also contribute to its anomalously light carbon isotopic values. In summary, the carbon isotopic inversions observed in the source and reservoir layers of the study area are primarily attributed to carbon isotopic fractionation during liquid hydrocarbon cracking—a phenomenon that may be common in deep and ultra-deep petroleum reservoirs that have experienced high temperatures.

Key words: carbon isotope, carbon isotopic reversal, organic matter, paleo-oil reservoir, Anyue Gas Field

摘要：

沉积有机质的碳同位素分布特征常被用作油源对比、有机质成因及古环境分析的依据,但当有机质进入过成熟阶段后，裂解作用会导致同位素分馏，使其碳同位素分布特征出现倒转等变化。特别是在深层和超深层的油气地质研究中，有机质碳同位素倒转的现象一直以来都是重点和难点。本文基于现有数据和相关研究成果揭示了四川盆地川中隆起安岳气田烃源岩、储层中各类固体及液体有机质的碳同位素分布特征，探讨了其成因机制。研究表明，由于古油藏经历了热事件的改造，烃源层和储层中早先形成的原始液态烃均遭裂解并形成了气态烃、残余液态烃和焦沥青三种产物，其中焦沥青可分为烃源层原位焦沥青和储层焦沥青。在现今的烃源层和储层中，出现了干酪根碳同位素值低于液态烃的总体倒转现象，同时也出现了饱和烃、芳烃碳同位素值高于非烃和沥青质的局部倒转现象；储层中的焦沥青碳同位素值分别低于干酪根和液态烃。综合相关数据和模拟实验结果分析可知，裂解过程中的碳同位素富集效应是烃源岩和储层样品中观测到的同位素倒转现象主要原因。烃源层和古油藏储层中的原始液态烃经过高温裂解后形成残余液态烃的碳同位素值整体升高了约4‰，由于烃源层和古油藏储层的受热程度不同，其残余液态烃中不同的族组分的碳同位素值升高幅度不同；储层焦沥青虽然主要继承了储层原油的同位素特征，但在其形成过程中发生的TSR作用可能也对其碳同位素值的异常偏轻有一定的积极作用。总之，导致研究区烃源层和储层中出现碳同位素倒转现象的主要原因是液态烃裂解过程中的碳同位素分馏效应，这可能是经历过高温的深层和超深层油气藏中的普遍现象。

关键词: 碳同位素, 碳同位素倒转, 有机质, 古油藏, 安岳气田

CLC Number: 

• P618.13

• TP18