湘西北地区五峰—龙马溪组页岩物质组成特征与页岩气潜力分析

Material composition and hydrocarbon potential of the shale of Wufeng-Longmaxi Formation in northwestern Hunan

为了研究湘西北龙山—永顺地区上奥陶统五峰组—下志留统龙马溪组页岩气资源勘探潜力,利用野外典型露头地质剖面资料,通过对典型剖面页岩样品的系统观测、采样、利用有机地球化学和矿物组成特征研究,评价研究区目标地层页岩的成烃成藏物质基础。研究结果表明,研究区五峰—龙马溪组黑色泥页岩普遍具有有机碳含量高、残留可溶有机质氯仿沥青"A"低、生烃潜能S1+S2指数略低。分子生物标志化合物分析显示,正构烷烃无明显奇偶优势,受一定程度的微生物降解影响,多表现为双峰型(C_(18)/C_(25)),∑C_(21-)/∑C_(22+)值为0.45～1.11,显示轻烃组分缺失,重烃组分占绝对优势;Pr/Ph值范围在0.27～0.44,具有植烷优势,显示原始有机质发育于较强还原沉积环境。烃源岩样品饱和烃组成相对丰度五环三萜烷>三环萜烷>四环萜烷,三环萜烷中C_(21)、C_(23)、C_(24)呈倒V字形分布,Ts/Tm为0.39～0.66。C_(27)规则甾烷含量略大于C_(29)规则甾烷,显示沉积母质以浮游生物及海洋型自养菌为主,亦含有部分藻类为主的有机质输入。镜质组反射率Ro均在2.57%～3.01%,普遍小于3%,已经达到了过成熟阶段。全岩矿物组成成分分析显示,页岩中脆性矿物为主要组成成分,其次为黏土矿物。综合页岩有机和无机物质组成分析结果,提出研究区湘西北五峰—龙马溪组页岩具备页岩气成烃成藏的物质条件基础,有机质含量高、热演化程度适中、脆性和吸附性较好,为深入开展页岩气勘探目标区优选奠定了工作基础。

In order to investigate source material and sedimentary environment of Lower Paleozoic Wufeng-Longmaxi Formations shale in northwestern Hunan, the authors studied the organic matters, biomarkers and mineral composition. The results show that they have the following characteristics: No odd-even carbon dominance appears;N-alkanes mostly have bimodal performance (C18/ C25), and the ratios of ΣC21-/ΣC22+ are 0.45-1.11, indicating the dominant role of heavy hydrocarbon components;the average ratios of Pr/Ph vary from 0.27 to 0.44, showing reduction conditions. The tricyclic terpane values of C21, C23 and C24 show inverted Vshaped distribution, and the ratios of Ts/ Tm are between 0.39 and 0.66. The related content of C27 sterane is slightly higher than that of C29 sterane. Based on these features of biomarker, it is believed that the shales of Wufeng-Longmaxi Formation were formed in a reduction environment in shallow water in the Early Paleozoic, with high degree of thermal evolution of organic matter, and the main sources were bacteria, algae and other aquatic organisms. The vitrinite reflectance equivalent values vary from 2.57% to 3.01%, suggesting that all of the shale samples have reached the mature stage. The mineral compositions of the shale samples indicate that quartz and clay minerals dominate the mineral composition, indicating a good brittleness index and adsorption capability. It is concluded that exist favorable geological conditions for shale gas enrichment in the marine shale in the present study area: organic- rich intervals, moderate thermal evolution, good brittleness index and adsorption capability. The results obtained by the authors have laid an important basis for preference of shale gas exploration target area and further gas exploration in this area.