The albitization of K-feldspar is a common diagenetic process that has thus far received little attention and is not fully understood in fine-grained sedimentary rocks. To better understand the albitization of K-feldspar, studies in organic- and silt-rich fine-grained rocks of the lower Cambrian Qiongzhusi Formation in the southwestern Upper Yangtze region, China, were carried out via X-ray diffractometry (XRD) and field emission scanning electron microscopy (FE-SEM). The results show that five types of albitized K-feldspar textures have developed: microcrystal albite replacement, irregular blocky replacement along margins, cleavage planes or microcracks of K-feldspars, complete pseudomorphic replacement, albite overgrowth, and albite pore filling. Organic- and silt-rich fine-grained rocks differ from sandstones and mudstones in terms of the rock structure and mineral assemblage, which results in differences in the textures and degree of albitization of K-feldspar. Illitization of clay has an impact on the albitization of K-feldspar. In provenance analyses using feldspar, fine-grained rocks, especially those that underwent mesogenesis, should be treated with caution because detrital feldspars have been destroyed. Theoretically, the albitization of K-feldspar could increase the porosity of reservoirs, although, from our observations, most of the related secondary pores are cancelled out or became isolated pores due to other diagenetic processes (compaction, cementation, etc.) in organic- and silt-rich fine-grained rocks.

The Wufeng Formation-Longmaxi Formation (O₃w-S₁l₁) black shales within the Yangtze foreland basin (South China) hold significant potential as unconventional oil and gas resources. However, several challenges, including sluggish sedimentation rates, thin sedimentary layers, and notable homogeneity, hinder a comprehensive grasp of the organic matter enrichment (OME) mechanism within these shales. These challenges impede the exploration of unconventional oil and gas resources. This study aims to identify the O₃w-S₁l₁ black shales in the Weiyuan area through a combined approach of organic and inorganic geochemical analyses. By doing so, it delves into the controlling mechanism behind OME in the black shales of the Yangtze foreland basin. The findings of this research reveal that the O₃w-S₁l₁ black shales primarily consist of sapropelic kerogen and exhibit signs of over-maturation, with TOC content ranging from 0.43% to 8.21%. These shales are classified as organic-rich, mainly composed mixed and siliceous lithofacies. The presence of silica in the shales originates mainly from biogenic sources, and the highest TOC content coincides with a Si_xs contribution of around 30% to the total silica content. During the late Katian, a global sea level drop led to an oxic paleoenvironment, transitioning into a euxinic paleoenvironment as sea levels rose in the early Rhuddanian. Notably, TOC content exhibited a significant correlation with paleoceanographic conditions (e.g., V/Cr, Ni/Co) and paleoproductivity levels (e.g., P/Al, Ba_bio), while displaying a negative correlation with paleoclimate conditions (e.g., CIA, C-value), terrigenous detrital input intensity (e.g., Si_xs, Ti/Al), and sedimentation rate ((La/Yb)_N). Specifically, favourable conditions for OME encompass robust reducing seawater conditions, high paleoproductivity, a humid climate, reduced influx of terrigenous debris, and relatively elevated sedimentation rates. Regional tectonic movements (Guangxi movement) and fluctuations in relative sea levels exert influence over the deposition of O₃w-S₁l₁ black shales. This study provides a valuable case study for exploring and developing shale gas resources within the Yangtze foreland basin.

This study uses logging data, mineral component content, total organic carbon (TOC) content, and microscopic characteristics of the organic-rich shales in the Wufeng and Longmaxi Formations, as well as data reported by other researchers, to demonstrate that upwelling has played an important role in the organic matter enrichment. The results show that (1) the organic-rich shales of Well N211 in the Upper Yangtze region are located in the Wufeng Formation and the lower Longmaxi Formation, with a burial depth between 2308–2357 m. (2) The organic-rich shales are enriched in biogenic silica. (3) Based on paleogeographic location and characteristics of organisms, this study determines that upwelling occurred during the deposition of the organic-rich shales in the Wufeng and Longmaxi Formations, promoting the enrichment of organic matter in the shales. (4) The upwelling intensity gradually increased from the sedimentary period of the organic-rich shales in the mid-lower Wufeng Formation to the sedimentary period of the Guanyinqiao Member, and then decreased gradually from the sedimentary period of the Guanyinqiao Member to the sedimentary period of the organic-rich shales in the Longmaxi Formation, and leads to the different enrichment of organic matter in the vertical direction. The different developments of upwelling led to the coexistence of both high and low TOC contents in the Guanyinqiao Member along the vertical direction.