沉积物和原油中重排藿烷的生物来源和成因机制

Discussion on Biological Origin and Formation Mechanism of Rearranged Hopanes in Sediments and Crude Oils

重排藿烷逐渐在各类沉积物和原油中被检测,但其生物来源与形成转化机制等尚不清楚.在中元古界下马岭页岩和侏罗系湖相原油系统地鉴定了四类重排藿烷.基于各类重排藿烷相对丰度的变化关系和形成转化路径,明确了研究区各类重排藿烷的相对丰度和生成速率主要受控于生源构成或甲基重排反应过程.其中,早洗脱重排藿烷和21-甲基-28-降藿烷的形成过程比17α(H)-重排藿烷系列更为复杂,使得二者在沉积物和原油中分布相对局限.18α(H)-新藿烷不仅可源自细菌藿多醇,还可源自里白醇和里白烯,故其在地质样品中分布更为广泛.重要的是,虽然富含重排藿烷的烃源岩和原油屡见报道,但它们可能并非源自无处不在的细菌,而更可能仅存在于局限环境下的少数细菌种属.

Rearranged hopanes have gradually been reported in sediments deposited in various sedimentary environments and related crude oils,but their biological origins and formation mechanisms are still controversial.In this paper it systematically identified four types of rearranged hopanes in the Mesoproterozoic Xiamaling black shales and Jurassic lacustrine crude oils.Based on the relationship between the relative abundance of rearranged hopanes and their biosynthetic reaction schemes,it is proposed that their relative abundance and generation rate are mainly controlled by the biological origins and times of methyl rearrangement reaction.The formation process of early eluting rearranged hopane and 28-nor-spergulanes is more complicated than that of 17α(H)-diahopane,which makes their distribution in sediments and crude oil more limited.In addition,18α(H)-neohopane can not only be derived from the bacteriohopanetetrol of prokaryotic organisms,but also from diplopterol and/or diploptene,so it is widely distributed in geological samples.Importantly,although sediments and crude oils rich in rearranged hopanes are frequently reported,they may not originate from ubiquitous prokaryotic bacteria,but may only exist in a small number of bacterial communities living in a specific environment.