Controls on Organic Matter Accumulation of the Triassic Yanchang Formation Lacustrine Shales in the Ordos Basin, North China.

Xiaoliang Chen,Bin Zhang,Zhiguo Mao,Haiping Huang

doi:10.1021/acsomega.1c02993

Abstract

Lacustrine shales in the third submember of the Chang7 (Chang73) of the Triassic Yanchang Formation have the highest oil and gas generation potential in the Ordos Basin, North China. To unravel factors governing organic enrichment within this submember, Rock-Eval pyrolysis, major and trace elemental analyses, and molecular composition of extractable organic matter were applied for redox condition, paleosalinity, dilution effect by terrestrial input, paleoproductivity, and paleoclimate condition investigation. The total organic carbon (TOC) contents of the Chang73 organic-rich lacustrine shales show a tripartite feature and can be divided into the upper organic-rich section (UORS, average TOC 6.8 wt %), the middle organic-lean section (MOLS, average TOC 3.5 wt %), and the lower organic-rich section (LORS, average TOC 6.7 wt %). The variation of the productivity-related paleoclimate is likely the main driving force leading to the change of organic richness within the Chang73 submember. The MOLS was deposited under a relatively hot and arid climate (high Sr/Cu but low Rb/Sr values) with lower paleoproductivity (low Porg/Ti and Porg values). Additionally, clastic dilution may further reduce the TOC content to a certain extent in the MOLS. The UORS and LORS, however, were deposited under a warm and humid climate, which leads to enhancement of chemical weathering (high Ln(Al2O3/Na2O) values), increased nutrient input, and elevated paleoproductivity. Furthermore, paleoproductivity of UORS and LORS was further boosted by additional key nutrients, such as Fe and P2O5, provided by syn-depositional volcanic ash. Both paleoredox (U/Th, Corg/P, and Pr/Ph) and paleosalinity (Sr/Ba, gammacerane index) proxies suggest no noteworthy variation of redox and salinity conditions throughout the Chang73 interval.

Highlights

Conventional petroleum resources have been partly overshadowed by unconventional oil and gas resources, among which organic-rich shale sourced resources, shale oil and shale gas, have gained great attention for their worldwide distribution in marine environments and in lacustrine successions.[1−4] The controlling factors of organic matter enrichment in marine environments have been extensively investigated, which can be attributed to the productivity model and the preservation model.[1−3] In the productivity-driven model, the flux of organic carbon sinking to the seafloor is the leading factor controlling the organic accumulation, while the defining factor of organic matter accumulation in the preservation-driven model is the redox conditions of the water column, with reducing conditions strengthening the preservation of organic matter and oxidizing conditions weakening the preservation of organic matter
The upper and lower sections of the Chang[73] submember show a higher total organic carbon (TOC) content (4.2−8.8 wt %, average 6.8 and 4.7−8.6 wt %, average 6.7 wt %, respectively) than that in the middle section (1.2−5.7 wt %, average 3.5 wt %) (Figure 2), which can be further divided into the upper organic-rich section (UORS), the middle organiclean section (MOLS), and the lower organic-rich section (LORS) (Table 1 and Figure 3)
Factors that may lead to notable difference of organic matter abundance in study sections, including paleosalinity, clastic dilution, terrestrially derived nutrients, and paleoproductivity, were strongly controlled by the paleoclimate, which exerts a major impact on organic matter accumulation within the UORS, MOLS, and LORS

Summary

Introduction

Conventional petroleum resources have been partly overshadowed by unconventional oil and gas resources, among which organic-rich shale sourced resources, shale oil and shale gas, have gained great attention for their worldwide distribution in marine environments and in lacustrine successions.[1−4] The controlling factors of organic matter enrichment in marine environments have been extensively investigated, which can be attributed to the productivity model and the preservation model.[1−3] In the productivity-driven model, the flux of organic carbon sinking to the seafloor is the leading factor controlling the organic accumulation, while the defining factor of organic matter accumulation in the preservation-driven model is the redox conditions of the water column, with reducing conditions strengthening the preservation of organic matter and oxidizing conditions weakening the preservation of organic matter. Previous studies[4] had revealed that deposition of lacustrine sediments differs from marine ones in scale and in being more sensitive to changes in climate and clastic supply, resulting in the complexity of organic accumulation in lacustrine systems. Controls on organic matter enrichment in these shales have been investigated by previous studies. Some studies suggested that the strongly reducing bottom-water conditions control the organic matter enrichment of the Chang[7] shales,[8,9] whereas others argued that other factors but not redox condition of the bottom-water play major roles in organic enrichment of the section as the redox condition could be dominant by oxic− suboxic during the Chang[7] period.[10,11] Seawater invasion

Objectives

Results

Discussion

Conclusion