川西坳陷南段天然气来源与碳同位素地球化学研究

Carbon Isotopic Geochemistry and Origin of Natural Gases in the Southern Part of the Western Sichuan Depression

川西坳陷南段是四川盆地主要的天然气产区之一。前人对该区天然气的来源有多种见解。为了更清楚地认识此地区天然气的来源与运移,本文分析了平落坝、大兴西和白马庙气田16件天然气样品的C1—C4烃及CO2 组份的碳同位素组成。所获得的同位素数据结合化学成份和地质资料表明,3个气田的烃类完全是热解成因的,都来源于气田下面的上三叠统烃源岩。这些气田的甲、乙烷碳同位素组成随深度呈不同规律的变化,这些变化归因于烃源岩生烃的热解过程和烃类运移的动力学过程。平落坝气田中侏罗统气藏的烃类大部份形成于烃源岩低成熟和成熟的早期阶段并受到晚期成熟气体的不断补给。平落坝和大兴西气田多数气藏的烃类被认为是从源区垂直向上运移通过上伏地层而进入气藏的,白马庙气田的烃类被认为是沿断裂通道向上侏罗统气藏聚集的。平落坝和大兴西气田的δ13 Cco2 值有很宽的分布范围(- 10 .7‰～- 0 .7‰) ,这表明气田的CO2 由来自基底的海洋碳酸盐岩无机碳成份和沉积地层的有机碳成份混合而成。这些气田的[He/CH4] [N2 /CH4]值之间和δ13 Cco2 δ13 C1值之间的相关性表明,非烃气体在进入气藏前已同烃类很好地混合,并被CH4为主流相的气流携带着向气藏运移。

The southern part of the Western Sichuan Depression is one of the major gas-producing provinces in Sichuan Basin. There previously existed several different opinions on the gas origin of this area. In order to get a better understanding of the source and migration of natural gases in this area, the authors analyzed carbon isotopic compositions of C 1-C 4 hydrocarbon and CO 2 components of these gases. Natural gas samples were collected from 16 producing wells of well- known Pingluoba, Daxingxi and Beimamiao gas fields. Together with chemical composition and geological information, the isotope data indicate that hydrocarbons of all three gas-fields seem to be entirely thermogenic and are derived from the Upper Triassic source rocks at a relatively great burial depth. Methane and ethane of these gas fields display different trends of isotopic compositional variations with depth. These changes can be attributed to the thermal cracking process during the formation and the dynamic process during the secondary migration of these hydrocarbons. The hydrocarbons in Pingluoba and Daxingxi gas fields are assumed to have migrated from deeply buried source rocks to shallower reservoirs. In addition, the hydrocarbons of Middle Jurassic reservoirs in the Pingluoba gas field have greater wetness and lighter isotopic composition than those of Upper Triassic reservoirs, which is believed to be the result of their generation from early mature source rocks and their mixing with late mature gases. Having high wetness and heavy isotopic composition, the Beimamiao hydrocarbons are supposed to have been migrating into the Late-Jurassic reservoirs along faults and fractures.