黔北下志留统龙马溪组烃源岩有机地球化学特征及其古环境意义

Organic geochemical signatures and palaeo-environmental implications for the source rocks from the Lower Silurian Longmaxi Formation in northern Guizhou

关键词:  龙马溪组  有机地球化学  古环境  剖面对比

Key words:  Longmaxi Formation  organic geochemistry  palaeo-environment  profile correlation

摘要:

通过对黔北地区下志留统龙马溪组烃源岩的有机碳、类异戊间二烯烃、萜烷及甾烷等有机地球化学的研究,发现具有以下特征:①正构烷烃分布以单峰分布为主,主峰碳为nC₁₈~nC₂₀;②Pr/Ph比值在0.16~1.24之间,普遍具有植烷优势;③伽马蜡烷指数>0.10;④Ts/(Tm+Ts)比值较高,在0.42~0.61之间,且分布十分稳定;⑤C₂₇/C₂₉甾烷比值在0.65~1.53之间,多数样品具C₂₉甾烷分布优势;⑥有机碳含量普遍大于0.5%,且自下向上呈规律性递减;⑦δ¹³C_org<-28‰,普遍偏轻。这些特征均指示研究区龙马溪组的有机物产率较高,同时具有良好的有机质保存环境。通过对比岩性及有机地球化学剖面,将龙马溪组烃源岩划分为3个发育阶段。

Abstract:

The organic geochemical signatures such as organic carbon, isoprenoid alkanes, terpanes and steranes in the source rocks from the Lower Silurian Longmaxi Formation in northern Guizhou are generalized in the following, including (1) single-peak distribution of normal alkane, with the main peak carbon varying between nC₁₈ and nC₂₀; (2) Pr/Ph ratios ranging from 0.16 to 1.24, with a general advantage of phytane; (3) gammacerane index>0.10; (4) Ts/(Tm + Ts) ratios ranging from 0.42 to 0.61; (5) C₂₇/C₂₉ ratios ranging from 0.65 to 1.53; (6) organic carbon contents generally more than 0.5%, with a gradual decrease from bottom to top, and (7) δ¹³C_org <-28‰. The organic geochemical signatures outlined above indicate higher organic carbon productivity and reduction environments in the Longmaxi Formation. The source rocks in the Formation may have experienced three stages of evolution, including (1) higher organic carbon productivity and highly reduced environment in the basal part; (2) moderate organic carbon productivity and reduced environment in the middle part, and (3) lower organic carbon productivity and slight redox environment in the upper part.