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.

As an important pilot target for shale gas exploration and development in China, the Longmaxi Formation shale in the Dianqianbei Area is characterized by high content of nitrogen, which severely increases exploration risk. Accordingly, this study explores the genesis of shale gas reservoir and the mechanism of nitrogen enrichment through investigating shale gas compositions, isotope features, and geochemical characteristics of associated gases. The high-nitrogen shale gas reservoir in the Longmaxi Formation is demonstrated to be a typical dry gas reservoir. Specifically, the alkane carbon isotope reversal is ascribed to the secondary cracking of crude oil and the Rayleigh fractionation induced by the basalt mantle plume. Such a thermogenic oil-type gas reservoir is composed of both oil-cracking gas and kerogen-cracking gas. The normally high nitrogen content (18.05%–40.92%) is attributed to organic matter cracking and thermal ammoniation in the high-maturity stage. Specifically, the high heat flow effect of the Emeishan mantle plume exacerbates the thermal cracking of organic matter in the Longmaxi Formation shale, accompanied by nitrogen generation. In comparison, the abnormally high nitrogen content (86.79%–98.54%) is ascribed to the communication between the atmosphere and deep underground fluids by deep faults, which results in hydrocarbon loss and nitrogen intrusion, acting as the key factor for deconstruction of the primary shale gas reservoir. Results of this study not only enrich research on genetic mechanism of high-maturity N₂ shale gas reservoirs, but also provide theoretical guidance for subsequent gas reservoir resource evaluation and well-drilling deployment in this area.