Geochemical characteristics and origin of natural gas from Wufeng-Longmaxi shales of the Fuling gas field, Sichuan Basin (China)
Geochemical characteristics and origin of natural gas from Wufeng-Longmaxi shales of the Fuling gas field, Sichuan Basin (China)
复制标题
DOI：
10.1016/j.coal.2016.12.003
复制
发表时间：
2017-02
影响因子：
5.6
通讯作者：
Rui Yang;Sheng He;Q. Hu;Dongfeng Hu;Jizheng Yi
中科院分区：
文献类型：
--
作者：
Rui Yang;Sheng He;Q. Hu;Dongfeng Hu;Jizheng Yi
涪陵气田作为我国第一个大型页岩气田，近年来被视为四川盆地天然气勘探和生产的重要地区之一。本文以涪陵气田5口页岩气威尔斯井共24个气样的分子和稳定碳同位素组成为基础，对涪陵气田上奥陶统五峰组和下志留统龙马溪组页岩底部天然气的成因进行了研究，并对气藏的成藏和勘探前景进行了展望。分析了地球化学特征和天然气成因，探讨了地球化学异常（碳同位素反转）的原因。分子组成分析结果表明，涪陵气田天然气为干气，主要成分为甲烷（97.9-98.9%），乙烷（C2 H6）、丙烷（C3 H8）和非烃类气体（主要为CO2和N2）含量极低。这些干气属于油伴生气，主要来源于二次裂解。由于缺乏从未成熟到晚成熟的成熟梯度样品，涪陵气田WL气的δ 13 C2值与湿度值之间的演化趋势不明显，但均位于同位素反转带。气态烷烃的碳同位素显示出完全的同位素反转（δ 13 C1> δ 13 C2> δ 13 C3），这表明相对较高的热成熟度，并与实测镜质体反射率和模型值（~ 3.0%Ro）相一致。WL气体中观察到的完全碳同位素逆转是由几种机制的组合引起的，其中高温下的同位素交换是主要的控制因素。其他次要因素包括C2 H6和C3 H8的Rayleigh型分馏、二次裂解以及不同热成熟度水平气体的气体扩散混合。
As the first giant shale gas field in China, the Fuling gas field has recently been regarded as one of the most important regions for natural gas exploration and production in the Sichuan Basin. However, the origin of natural gas from Upper Ordovician Wufeng and the bottom of Lower Silurian Longmaxi (WL) shales in the Fuling gas field is poorly understood to limit a comprehensive understanding of gas generation, accumulation and exploration.In this work, based on molecular and stable carbon isotopic composition of a total of 24 gas samples from five shale gas wells in the Fuling field, we analyzed the geochemical characteristics and gas origin, and discussed the cause for the geochemical anomalies (carbon isotopic reversals). Molecular composition results show that gases from the Fuling gas field are dry and mainly composed of methane (97.9–98.9%), with a very low level of ethane (C2H6), propane (C3H8) and non-hydrocarbon gases (mainly CO2and N2). These dry gases are classified as oil-associated gas and mainly derived from secondary cracking. Due to the lack of gas samples across a maturation gradient from immature to late mature, the WL gases in the Fuling field show an unclear evolution trend between the δ13C2and wetness values; however, all these samples are located in the isotopic reversal zone. Carbon isotopes of gaseous alkanes clearly display full isotopic reversals (δ13C1> δ13C2> δ13C3), which is indicative of a relatively high thermal maturity and consistent with the measured vitrinite reflectance and modeled values (~ 3.0% Ro). The observed complete carbon isotopic reversals in the WL gases are caused by a combination of several mechanisms, in which isotope exchange at high temperature is the primary controlling factor. Other secondary factors include Rayleigh-type fractionation of C2H6and C3H8, secondary cracking and gas diffusion mixing of gases at different thermal maturity levels.