笔石与页岩有机质的富集关系及其贡献率——以四川盆地南缘YS118井为例

Relation and contribution rate of graptolite to organic matter enrichment in shale:A case study from Well YS118 at the southern margin of the Sichuan Basin

中国南方地区下志留统龙马溪组发育高碳富含笔石的黑色页岩,然而作为页岩重要的生物输入,目前有关笔石对有机质富集的影响、生烃机理等方面的研究则较少,并且关于笔石对有机质的富集作用也还存在着争议。为此,基于对四川盆地南缘典型井YS118井龙马溪组底部的124块页岩样品的笔石丰度、全岩总有机碳含量(TOC)、围岩TOC以及能谱对笔石和围岩的分析测试结果,分析笔石的元素组成,厘清龙马溪组底部优质页岩段纵向上笔石丰度变化特征,探讨P. persculptus带-C. vesiculosus带笔石丰度与有机质富集关系,定量表征不同笔石带笔石体对有机质的贡献率。研究结果表明:①龙马溪组页岩中笔石体碳元素含量较高(27.23%~32.25%),碳元素是笔石构成的主要元素之一;②笔石TOC大于围岩TOC,笔石较围岩更富集碳元素;③全岩TOC明显高于围岩TOC,且笔石丰度与全岩TOC相关性较好,笔石是页岩有机质的重要贡献者;④P. persculptus带和A. ascensus带中的笔石体对有机质的贡献率分别为58.5%、55.3%,笔石是页岩中有机质的主要来源;P. acuminatus带和C. vesiculosus带中的笔石体对有机质的贡献率分别为24.23%、13.65%,笔石和其他生物共同控制有机质的丰度。结论认为,笔石丰度反映了页岩中有机质富集的程度。

High-carbon content and graptolite-rich black shale is developed in the Lower Silurian Longmaxi Formation in the southern China. Graptolite acts as important biological inputs to shale, but its effects on organic matter enrichment and its hydrocarbon generation mechanisms are currently less researched. Moreover, no common agreement has been reached in terms of the effects of graptolite on organic matter enrichment. To deal with this situation, this paper analyzed the elemental compositions of graptolite based on the analysis and test results of 124 shale samples taken from the base of the Longmaxi Formation in the typical well YS118 at the southern margin of the Sichuan Basin, including graptolite abundance, whole-rock total organic carbon(TOC), surrounding rock TOC, and energy spectrum of graptolite and surrounding rock. Then, the vertical variation characteristics of graptolite abundance in the high-quality shale interval at the base of the Longmaxi Formation were elucidated. Finally, the correlation between graptolite abundance and organic matter enrichment was explored in the P. persculptus and C. vesiculosus belts to quantitatively characterize the contribution of graptolite abundance to organic matter in different belts. And the following research results were obtained. First, carbon is the main element in the graptolite of the Longmaxi Formation shale, and its content is higher, ranging from 27.23% to 32.25%. Second, graptolite has a higher TOC content and greater carbon enrichment than the surrounding rocks. Third, the whole-rock TOC is obviously higher than the surrounding rock TOC, graptolite abundance is well correlated with the whole-rock TOC, and graptolite is an important contributor to the organic matters of shale. Fourth, the contribution rates of graptolite in the P. persculptus and A. ascensus belts to organic matters are 58.5% and 55.3%, respectively, suggesting that graptolite is the primary source for the organic matters of shale, while those in the P. acuminatus and C. vesiculosus belts are 24.23% and 13.65%, respectively, and the organic matter enrichment is under the joint control of graptolite and other organisms. In conclusion, graptolite abundance can well reflect the enrichment degree of organic matters in shale.