深部铅锌热液流体作用下的原油地球化学特征

Geochemical characteristics of crude oils under alteration of deep Pb-Zn-bearing hydrothermal fluids

中国南方盆地深部热液流体活跃区分布广泛,热液流体类型各有不同,滇西北兰坪地区幔源铅锌热液成矿流体尤为显著,形成了典型的超大型陆相沉积浅成矿床,其中古油气藏遗迹明显,深部铅锌热液流体活动对生烃及油藏热蚀变的影响显著。通过分析热蚀变后铅锌矿中伴生原油及固体沥青中生物标志物、烃类及杂环化合物的地球化学特征,发现随着深部热液流体作用增强,生物标志物(如正构烷烃、甾烷、藿烷等)相对含量逐渐降低,未分离复杂混合物(UCM)相对含量逐渐增加,弱偶碳数优势明显(CPI≤1.0),多环芳烃(PAHs)相对含量逐渐降低,而链状苯基化合物(如联苯)与含S化合物(如噻吩类)相对含量逐渐增加。样品不存在明显的生物降解、蒸发分馏、硫酸盐热化学还原(TSR)及原油裂解等次生作用。兰坪地区铅锌矿中伴生原油和沥青的分子地球化学特征及其特殊的变化规律,可能是由于沉积有机质或古油藏与富催化剂的幔源铅锌热液流体发生热液催化反应导致的。本研究揭示了深部热液流体作用和分子地球化学特征的耦合关系,识别并提出了热液流体作用下杂环化合物形成的机制和原油热蚀变的指标。

Active deep hydrothermal fluids of different types are widely distributed in many areas of the basins in South China.The Lanping area,northwest Yunnan Province,is one of such places where mantle-sourced Pb-Zn-bearing hydrothermal fluids are quite typical and had led to the formation of ultra-large terrestrial sedimentary epithermal deposits with residual paleo oil and gas reservoirs showing hydrocarbon generation and reservoir alteration obviously dented by the deep Pb-Zn-bearing hydrothermal fluid activities.By analyzing the geochemical characteristics of biomarkers,hydrocarbons and heterocyclic compounds in crude oils and solid bitumen after hydrothermal alteration in Pb-Zn ore from the area,the study shows that,with increasing hydrothermal alteration,the relative abundance of biomarkers(e.g.,n-alkanes,steranes,hopanes,etc.)gradually decrease,the relative content of UCM increases,the weak even carbon number predominance gets more obvious(CPI≤1.0),the relative content of polycyclic aromatic hydrocarbons(PAHs)gradually decreases,and the relative abundance of phenyl derivatives(e.g.,biphenyl)and Scontaining compounds(e.g.,thiophenes)progressively increase.The samples show no signs of influence of secondary effects such as biodegradation,evaporative fractionation,thermochemical sulfate reduction(TSR)and oil cracking.The change of geochemical characteristics of molecular compounds in the associated crude oils and bitumen of Pb-Zn ore samples is possibly due to the hydrothermal catalytic reaction of sedimentary organic matter or paleo-oil reservoirs with catalyst-rich mantle-sourced Pb-Zn-bearing hydrothermal fluids.This study reveals the coupling relationship between the deep hydrothermal alteration and the molecular geochemical characteristics,identifies the genetic mechanisms of heterocyclic compounds and proposes indicators for hydrothermal alteration of crude oils.