Factors controlling accumulation of organic carbon in a rift-lake, Oligocene Vietnam

M Rizzi,D M Toan,N H Schovsbo,N T T Thuy,C Korte,H I Petersen,M B W Fyhn,J Hovikoski,J Therkelsen,M Olivarius,L H Nga,H P Nytoft,J Bojesen-Koefoed,L H Nielsen,I Abatzis

doi:10.1038/s41598-020-71829-7

M Rizzi, D M Toan + Show 13 more

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https://doi.org/10.1038/s41598-020-71829-7

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Abstract

Understanding of the processes of petroleum source rock (SR) accumulation in lacustrine rift basins and the behavior of lake systems as long-term carbon sinks is fragmentary. Investigation of an 800 m thick (500 m core and ~ 300 m outcrop), deep-lacustrine, Oligocene section in Vietnam, provides a rare insight into the controls and deposition of organic carbon (OC) and SR formation in continental rift basins. A multidisciplinary dataset, combining elemental data, inorganic and organic geochemistry with sedimentology, shows that the richest alginite-prone, sapropelic SR developed during periods of relative tectonic quiescence characterized by moderate primary productivity in a mainly dysoxic lacustrine basin. Increased rift activity and further development of graben morphology intensified water column stratification and anoxia, which hindered nutrient recycling. Sapropelic organic matter (OM) continued to accumulate, but with increasing amorphous OM content and decreasing total OC values. Periods of increased seasonality were characterized by thermocline weakening, enhanced mixing of water columns, increased primary productivity and diatom blooming. The results suggest that a change from dysoxia towards anoxia or extreme primary productivity does not necessarily enhance OC burial and SR quality. External nutrient input from a phosphate-rich hinterland is sufficient for sapropel formation, whereas the main limiting factor is methanogenesis.

Highlights

Organic carbon (OC) accumulation and development of petroleum source rocks (SR) are complex processes involving a number of interrelated factors, the relative importance of which are not fully u nderstood[1,2]
The microlaminated fabric comprises silt-lamina, which are interlaminated with micritic siderite and disseminated amorphous organic matter (AOM)
Previous studies have indicated that the deposits represent syn-rift lake sediments and that the main border fault was located a few km to the East from the study locality[47] (Fig. 1b)

Summary

Introduction

Organic carbon (OC) accumulation and development of petroleum source rocks (SR) are complex processes involving a number of interrelated factors, the relative importance of which are not fully u nderstood[1,2]. Significant insights into OC accumulation in lacustrine rift basin have been derived from the modern East African lakes[6,7,9,10,11]. These studies have revealed the importance of factors such as width-depth ratio on the lake’s mixing potential, primary productivity and a noxia[10]. The results allow an assessment of the tectonic and climatic (seasonality) controls on sedimentation, broad scale changes in water depth, evolution of rift basin morphology, changes in water column oxygenation and thermocline, effects of diagenesis as well as changes in nutrient recycling and primary productivity on source rock development. The rift basin was relatively sediment starved and intermittently, throughout its existence, received phosphate-rich sediments from a single footwall b lock[20]

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