Differential enrichment mechanism of organic matter under reduction conditions in freshwater and saline lakes

Abstract

There are different mechanisms of reduction water column for the formation and preservation of organic matter during the depositional period of the continental lake basin. Therefore, determining the nature of the water column and its influencing factors is the key factor for differential enrichment of organic matter. In this study, by analyzing the TOC, mineral types, iron speciation, elements and isotopes characteristics of typical freshwater and saline lake organic-rich shales, the water column of synsedimentary environment of different lake shale deposits was compared. Our results show that the organic-rich shale of the Chang 7 Member (Ch7) of Yanchang Formation in the Ordos Basin and the Lucaogou Formation (P2l) in the Jimsar Sag of Junggar Basin in freshwater-saline environments have an intensity of sulfate reduction (BSR) (SRI < 1.375), the shales of Fengcheng Formation (P2f) in saline lake of the Mahu Sag in Junggar Basin have strong BSR (SRI > 1.375). This indicates that in ferruginous conditions the organic matter consumed by BSR is lower than the preserved organic matter, resulting a higher TOC content; in euxinic conditions caused by excessive volcanic activity or the input of hydrothermal fluids, a large amount of organic matter was degraded and consumed, causing more iron oxides (and other iron-containing minerals) dissolve gradually to form pyrite, reducing the TOC content. This study not only enabled to understand the shale oil enrichment, but also proposed a better theoretical and practical model for hydrocarbon accumulation in shale formations, to guide future shale oil exploration efforts.