Paleoenvironment of the Lower–Middle Cambrian Evaporite Series in the Tarim Basin and Its Impact on the Organic Matter Enrichment of Shallow Water Source Rocks

Mingyang Wei,Xiting Xu,Zongfeng Li,G Harrison,Hua Zhang,Kai Lu,Zheng Shen,Wei Liu,Axel Munnecke,Demin Zhang,Zhidong Bao

doi:10.3390/min11070659

Abstract

Just as in deep-water sedimentary environments, productive source rocks can be developed in an evaporitic platform, where claystones are interbedded with evaporites and carbonates. However, the impact of the paleoenvironment on the organic matter enrichment of shallow water source rocks in an evaporite series has not been well explored. In this study, two wells in the central uplift of the Tarim Basin were systematically sampled and analyzed for a basic geochemical study, including major elements, trace elements, and total organic carbon (TOC), to understand the relationship between TOC and the paleoenvironmental parameters, such as paleosalinity, redox, paleoclimate, paleo-seawater depth, and paleoproductivity. The results show that the Lower–Middle Cambrian mainly developed in a fluctuating salinity, weak anoxic to anoxic, continuous dry and hot, and proper shallow water environment. The interfingering section of evaporites, carbonates, and claystones of the Awatag Fm. have higher paleoproductivity and higher enrichment of organic matter. Paleosalinity, redox, paleoclimate, paleo-seawater depth, and paleoproductivity jointly control the organic matter enrichment of shallow water source rocks in the evaporite series. The degree of enrichment of organic matter in shallow water source rocks first increases and then decreases with the increase in paleosalinity. All the samples with high content of organic matter come from the shallower environment of the Awatag Fm.

Highlights

People’s understanding of the relationship between evaporites and hydrocarbons can be traced back to biblical times when people began to discover and use asphalt near theDead Sea [1]
Anhydrite is supposed to be formed by dehydration of gypsum precipitated from seawater, which occurs during a long period of burial diagenesis
Dolomites are the most widely distributed Lower–Middle Cambrian rocks in the Tarim Basin. They occur in various formations in most areas of the basin and can be subdivided into stromatolitic dolomite, grain dolomites, micritic dolomites, crystalline dolomites, and dolomites mixed with gypsum or claystone, in which the proportion of dolomite is more than 50%

Summary

Introduction

People’s understanding of the relationship between evaporites and hydrocarbons can be traced back to biblical times when people began to discover and use asphalt near theDead Sea [1]. People began to pay attention to the influence of evaporative environments on the hydrocarbon generation potential of source rocks. Some scholars have focused on the lacustrine evaporite series of the Lower Cretaceous in the south Atlantic Ocean [10] and the salinized lacustrine basins in China [11]. They think the evaporitic environment may be beneficial to the formation of saline lacustrine source rocks. Studies on the impact of the paleoenvironment on the organic matter enrichment of shallow water source rocks in marine evaporite series are still lacking [12,13,14,15,16,17]

Methods

Results

Discussion

Conclusion