Genesis and Distribution of Pyrite in the Lacustrine Shale: Evidence from the Es3x Shale of the Eocene Shahejie Formation, Zhanhua Sag, East China

Abstract

Pyrite is a common mineral in sedimentary rocks and is widely distributed in a variety of different morphologies and sizes. Pyrite is also widely distributed in the Es3x shale of the Eocene Shahejie Formation in the Zhanhua Sag, Bohai Bay Basin. A combination of geochemical and petrographic studies has been applied to address the formation and distribution of pyrite in the Es3x shale. The methods include thin section analysis to identify the representative samples of the shale-containing pyrite, total organic carbon (TOC) content analysis, X-ray fluorescence, X-ray diffraction, electron probe micro-analysis, and field emission scanning electron microscopy (FE-SEM) coupled with the energy dispersive spectrometer, to observe the characteristics, morphology, and distribution of pyrite in the lacustrine shale. The content of pyrite in the Es3x shale ranges from 1.4 to 11.2% with an average content of 3.42%. The average contents of TOC and total organic sulfur (TS) are 3.48 and 2.53 wt %, respectively. Various types of pyrites are observed during the detailed FE-SEM investigations including pyrite framboids, euhedral pyrite, welded pyrite, pyrite microcrystals, and framework pyrite. Pyrite framboids are densely packed sphere-shaped masses of submicron-scale pyrite crystals with subordinate large-sized euhedral crystals of pyrite. Welded pyrite forms due to the overgrowth and alteration of pyrite crystals within the larger pyrite framboids. Pyrite microcrystals are the euhedral-shaped microcrystals of pyrite. The framework pyrite is also observed and is formed due to the pyritization of plant/algal tissues. Based on the growth mechanism, the pyrites can be divided into syngenetic pyrites, early diagenetic pyrites, and late diagenetic pyrites. The presence of pyrite, especially the abundance of pyrite framboids, suggests that the environment during the Es3x shale deposition in the lacustrine basin was anoxic. Their dominant smaller size suggests the presence of an euxinic water column, which is consistent with the lack of in-place biota and high TOC contents. This research work not only helps to understand the pyrite mineralization, role of organic matter, and reactive iron in pyrite formation in the shale but also helps to interpret the paleoredox conditions during the deposition of shale. This research work can also be helpful to other researchers who can apply these methods and conclusions to studying shale in other similar basins worldwide.

Formation and Distribution of Different Pore Types in the Lacustrine Calcareous Shale: Insights from XRD, FE-SEM, and Low-Pressure Nitrogen Adsorption Analyses

“…As the distribution of pyrite framboids is abundant with the associated intercrystalline pores ( Figure 7 E,F), we can assume that only the pyrite framboids act as one of the main contributors of pore formation in the Es3x shale than any forms of pyrite. 32 …”

“… Location map and geology of the research area in the Zhanhua Depression, Bohai Bay Basin, eastern China (adapted and modified from Khan et al 32 ). …”

“… (A) Stratigraphic column of the study area and the red box is showing our study interval (Es3x shale). (B) γ-ray and SP log against the lithology of the Es3x shale in the Zhanhua Depression, Bohai Bay Basin (adapted and modified from Khan et al 32 ). …”

“…As the distribution of pyrite framboids is abundant with the associated intercrystalline pores (Figure 7E,F), we can assume that only the pyrite framboids act as one of the main contributors of pore formation in the Es3x shale than any forms of pyrite. 32 Overall, the distribution of mineral contents plays an important role to control the formation and distribution of different pore types and their structural parameters in the Es3x shale in the Zhanhua Depression (Figure 13).…”

Microscopic Reservoir Characteristics of the Lacustrine Calcareous Shale: An Example from the Es4s Shale of the Paleogene Shahejie Formation in Boxing Sag, Dongying Depression

“…Shale having TOC > 2% refers to organic-rich shale, which acts as an important hydrocarbon source, shale oil and gas reservoir, and seal rock in conventional reservoirs. , Organic-rich shale has generated oil resources, which have been stored in the organic-rich intervals or transported into adjacent, continuous organic-poor intervals . Organic-rich lacustrine shales account for nearly 20% of the conventional oil resources but also turn out to be even more important unconventional oil and gas reservoirs worldwide. − Organic-rich lacustrine shales are widely distributed all over China, for example, the Eocene Shahejie Formation in Bohai Bay Basin in eastern China, ,− the Upper Cretaceous Qingshankou Formation and Nenjiang Formation in Songliao Basin in northeast China, the Upper Jurassic Yanchang Formation in Ordos Basin in central China, and the Middle Permian Lucaogou Formation in Junggar Basin and Santanghu Basin in northwestern China . Lacustrine mudstones have restricted geographical distributions, high stratigraphic variability, mixed kerogen types, and vast total organic carbon (TOC) variations when compared to their marine counterparts.…”