Geochemical Characteristics of the Upper Permian Longtan Formation from Northeastern Sichuan Basin: Implications for the Depositional Environment and Organic Matter Enrichment

Abstract

A set of organic–rich shales of the Upper Permian Longtan Formation, which is widely developed in the northeastern part of the Sichuan Basin, is a key formation for the next step of exploration and development. At present, most studies on this set of formations have focused on the reservoir characteristics and reservoir formation mechanism of the shales, and basic studies on the palaeoenvironment and organic matter (OM) enrichment mechanism have not been fully carried out. In this paper, we recovered the sedimentary palaeoenvironment by mineralogical, elemental geochemical and organic geochemical analyses, and explored the enrichment mechanism of OM under the constraints of palaeoenvironmental evolution. The shales can be divided into two stages of sedimentary evolution: compared with the shales of the Lower Longtan Formation, the shales of the Upper Longtan Formation are relatively rich in quartz, poor in clay and carbonate minerals, and the OM type changes from type III to type II2. The depositional environment has undergone a change from sea level rise, from warm and wet climate to dry and cold climate, and from oxygen–poor condition restricted to open reduction environment; the land source input has decreased, the siliceous mineral content has increased, the biological productivity has improved, and the deposition rate has changed from high to low. A depositional model was established for the shales of the Longtan Formation, reflecting the differential reservoir formation pattern of organic matter. For the Lower Longtan Formation shales, the most important factors controlling OM content are terrestrial source input and deposition rate, followed by paleoclimate and paleo‐oxygen conditions. For the Upper Longtan Formation shales, the most important controlling factor is paleoproductivity, followed by sedimentation rate. The depositional model constructed for the Upper and Lower Longtan Formation shales can reproduce the enrichment of organic matter and provide a basis for later exploration and development.

Environmental Effects on Differential Organic Matter Enrichment of the Lower Cambrian Shale, Tarim Basin, NW China

“…The enrichment of some trace elements (V, Cr, Ni, Co, etc.) in sediments is closely tied to the water redox status of the depositional environment (e.g., Jones and Manning, 1994;Algeo and Maynard, 2004;Tribovillard et al, 2006;Xiao et al, 2023). This phenomenon makes the trace elements in sediments redox-sensitive and the ratios of V/ Cr, Ni/Co and U/Th are therefore useful as proxies for distinguishing redox conditions.…”

“…Copious hydrocarbon resources have been discovered from these Cambrian organic matter-rich shales in East Siberia, Sichuan and Tarim basins (e.g., Zhao et al, 2018;. The organic matter enrichment in these shales is generally attributed to paleoproductivity, paleoredox, paleoclimate, depositional environment and terrigenous influx (e.g., Tyson, 2001;Algeo and Maynard, 2004;Brumsack, 2006;Tribovillard et al, 2006;Cremonese et al, 2013;Zhang et al, 2020;Deng et al, 2021;Xiao et al, 2023). Several geochemical indicators have been used to analyze the factors and mechanisms of organic matter enrichment (Murray et al, 1991;Algeo and Maynard, 2004;Cremonese et al, 2013).…”

Organic Matter Accumulation in the Upper Permian Dalong Formation from the Lower Yangtze Region, South China

“…The Late Permian was famous for a number of global geological events, such as rapid global warming, ocean acidification and toxicity, oceanic anoxia, marine transgression and widespread deposition of organic-rich black shales (Benton and Newell, 2014;Clarkson et al, 2015;Zhang et al, 2018;Chen Z et al, 2019;Chen J et al, 2020;Liao et al, 2020;Wu et al, 2020;Xiao et al, 2023). These black shales in South China, represented by the Dalong Formation (Fm.…”

Diagenetic Evolution Sequence and Pore Evolution Characteristics: Study on Marine-Continental Transitional Facies Shale in Southeastern Sichuan Basin

“…The Upper Permian Longtan Formation shale in the Sichuan Basin is a typical representative of MCFS in China, and it has a complex evolutionary process because of the organic-inorganic diagenesis, which has a significant impact on its reservoir space [12,15,34]. For the Longtan Formation shale in the Sichuan Basin, the related research still focuses on aspects of the sedimentary environment, reservoir distribution, reservoir-forming conditions, and hydrocarbon generation potential [35][36][37][38]. The research foundation related to pore structure and diagenetic evolution is relatively weak [7,39], and the comprehensive study of diagenesis and pore structure of MCFS in the Longtan Formation of the Sichuan Basin is still blank.…”