The evolutional process of palaeoceanic environment and its effect on the accumulation of organic matter during the Ordovician–Silurian transition in Lower Yangtze region has been overlooked compared to that in Upper Yangtze region of South China, although their paleogeographic settings were expected to be discrepant. This paper documents the marine depositional environment, paleoclimate, and sediment supply changes, and discusses their roles in controlling the organic matter enrichment in sedimentary rocks within the Ordovician–Silurian transition of the Lower Yangtze region, using the latest geochemical data of the continuous drilling core. The stratigraphic framework of the Ordovician–Silurian transition in the Lower Yangtze region is composed of two third-order sequences, each of which can be subdivided into a lower TST (transgressive systems tract) and an upper RST (regressive systems tract). TST1 represented an evident depositional transition stage which was marked by the ending of the underlying carbonate sediments and the initiation of the terrigenous clastic-dominated sediments. Geochemical proxies indicate that the relatively low productivity, dysoxic water column condition, and high sediment supply flux collectively resulted in inadequate organic matter hosted in deposits of the TST1. During the depositional period of RST1, the global sea level declined due to the Hirnantian glaciation age. The icehouse also caused the decrease in overall river flux and, thus, the terrigenous clastic sediment supply. The icehouse also strengthened the upwelling that occurred in the Lower Yangtze sea. The upwelling boosted the marine algae explosion through the delivery of abundant nutrients, which not only enhanced paleoproductivity but also led to an anoxic environment by oxygen consumption. Such high paleoproductivity, anoxic water column environment, and low sediment supply flux caused the deposition of organic-rich shale. The sea level rose during the TST2 due to the ending of an ice age. The relatively large water depth and high paleoproductivity associated with volcanic eruptions are the main factors that caused the enrichment of organic matter during this stage. During the deposition of RTS2, the increase of sediment supply flux resulted in a decrease in accommodation space and water depth and the dilution of organic matter in deposits, which was the primary constraint of organic matter accumulation.

In recent years, Ordos Basin has been the largest petroleum field in China and represents significant potential for gas exploration in the Lower Paleozoic carbonate reservoirs. Taking the central-eastern Ordos Basin as an example, this work studies the reservoir characteristics, origin, and controlling factors of the crystalline dolomites of the Sanshanzi Formation by means of petrological and geochemical studies. The Sanshanzi Formation dolostones comprise three kinds: crystalline dolostone with grain-shadow (Rd1), very fine–fine crystalline dolostone (Rd2), and medium-coarse crystalline dolostone (Rd3). The pore spaces include intercrystalline pores and intercrystalline solution pores, residual intergranular pores, vugs, and fractures. The medium-coarse crystalline dolostone is the best reservoir. The average porosity is 4.61%, and the average permeability is 0.91 mD. The sedimentary environment in the upper Cambrian was a large area of tidal flats. Under the influence of penecontemporaneous meteoric water leaching, dissolution developed at the top of the dolomite flat–lagoon depositional cycle, with vugs developing in the very fine–fine crystalline dolostone. Penecontemporaneous dolomitization enhanced the compaction resistance and effectively preserved the pore space of the reservoir. In addition, recrystallization improved the crystalline texture, turning the intergranular pores into residual intergranular pores and intercrystalline micropores into intercrystalline pores. In this case, the very fine–fine crystalline dolostone was transformed into medium-coarse crystalline dolostone. The results of this study provide a case for the in-depth understanding of the development and preservation mechanism of ancient dolomite reservoirs and also point to the research pathways for oil and gas exploration in the Cambrian in the Ordos Basin in the future.

The Late Permian strata of the Xuanwei Formation in the eastern Yunnan region exhibit extensive diverse morphological features within siderite deposits. These variations in siderite deposits suggest potential differences in their formation processes. In this study, fieldwork and comprehensive indoor studies revealed four distinct forms of siderite deposits: stratiform-laminated, lens-like nodule, sandstone cementation, and fracture filling. The stratiform-laminated siderite, varying in color from bluish-grey to dark grey, is composed of uniformly sized microcrystalline to fine-grained siderite along with detrital matter, displaying precise layering and banding structures that suggest direct deposition from cyclic iron-rich seawater under reducing conditions. Lens-like-nodule siderite, which appears grey-yellow, is composed of mud microcrystalline siderite, medium to coarse-grained pseudo-ooids, and glauconite. It shows conformable distribution characteristics resulting from the diagenetic differentiation of iron-rich sediments under reducing conditions during the diagenetic and early diagenetic periods. Siderite as sandstone cementation exhibits a yellow-brown color and consists of dispersed colloidal siderite and cemented siderite clumps that fill intergranular pores of detrital particles. It precipitated under reducing conditions within those intergranular pores. Siderite filling fractures typically appear as vein-like or network-like structures intersecting bedding at large angles. They exhibit grain structures with significant variations in size. These siderite deposits exhibit exceptional purity and result from siderite dissolution during sedimentary periods, followed by reprecipitation within regional extensional fractures during the diagenetic phase. The primary occurrence of siderite deposits in the study area is within coal-bearing strata, as revealed by the integration of sedimentary profiles and sedimentary facies analysis. The coal-bearing strata, influenced by the Emeishan large igneous province, underwent iron enrichment during and after volcanic eruptions while developing a reducing environment, which was facilitated by abundant vegetation. Consequently, geological processes led to siderite layers, lens-like siderite nodules, and siderite cementation. The Yanshan orogeny induced extensive high-angle fracture development in epigenetic coal-bearing strata, facilitating fluid circulation and the redistribution of soluble siderite. This geological activity resulted in the formation of vein-like structures composed of siderite.