Shale oil and gas and coalbed methane (CBM) have become major focuses of unconventional oil and gas exploration and development. However, whether they can be defined as “reservoirs” and their formation mechanisms remain unclear. Based on an analysis and summary of the concept of source-rock hydrocarbon reservoirs, this study investigates the formation mechanisms of such reservoirs and redefines them as layered sweet-spot intervals (or sweet-spot zones) formed within source rocks through micro-scale migration and accumulation or retention, with oil content (S₁) or gas content (C) reaching industrial thresholds. As indicated by the research results, source-rock hydrocarbon reservoirs are characterized by source-reservoir integration, micro-scale migration and accumulation, and continuous distribution. Their formation mechanisms involve pressure differences as the driving force, micro- to nano-scale pore-throat and fracture systems as migration pathways, self-sealing as the preservation mechanism, and heterogeneity-induced micro-scale migration and accumulation leading to the formation of sweet spots. According to rock type and hydrocarbon-generation characteristics, source-rock hydrocarbon reservoirs can be classified into two major categories: shale oil and gas reservoirs and CBM reservoirs. Shale oil and gas reservoirs are accumulations of oil and gas generated and retained in mature to over-mature mudstones and shales, as well as in thin interbeds, where dominant fabric facies and the trandenscent effect are critical to hydrocarbon enrichment and accumulation. CBM reservoirs are accumulations of natural gas generated and retained in immature to over-mature coalbeds, in which high-quality coal rocks and favorable preservation conditions are the key to hydrocarbon accumulation and productivity. Significant progress has been achieved in the exploration and development of source-rock hydrocarbon reservoirs in the Bohai Bay Basin and the Ordos Basin. In the shale oil reservoir of the 2nd member of the Kongdian Formation in the Cangdong Sag, Bohai Bay Basin, the peak daily oil production of a single horizontal well reached 208 t, and the first economically viable continental shale oil development platform in China with an annual capacity of 100 × 10³ t has been established. In the CBM reservoir of the Daning-Jixian block, Ordos Basin, the average estimated ultimate recovery (EUR) per well is expected to reach 55.67 × 10⁶ m³, and a CBM field with an annual oil-equivalent production capacity of 2.00 × 10⁶ t has been built. Source-rock hydrocarbon reservoirs are therefore expected to become an important domain for increasing oil and gas reserves and production in China.

In recent years, deep coal-rock gas has gradually become a popular target for the exploration of unconventional hydrocarbons. In China, the deep coal-rock gas exploration is concentrated in the Ordos Basin, where breakthroughs in the exploration and production pilot tests have been achieved. In contrast, the exploration of deep coal-rock gas in the Bohai Bay Basin remains in its initial stage. Using geologic data of coal seams obtained from core observations and well logging, we systematically analyze the characteristics and favorable accumulation factors of the deep Upper Paleozoic coal seams in the Dagang exploration area. Furthermore, the mechanisms underlying the gas generation, storage, and preservation in deep coal seams are explored and the developmental model of the deep Upper Paleozoic coal-rock gas is thereby clarified. The results indicate that the Dagang exploration area contains multiple favorable sweet spot intervals for the deep Upper Paleozoic coal-rock gas occurrence, where the coal seams exhibit high organic matter abundance, moderate thermal maturity, and high gas-generating intensity. In terms of pore structure, these coal seams manifest multi-scale pores and fractures, with the large-scale, intensively distributed microfractures providing effective storage spaces. The roofs and floors of the coal seams are composed predominately of mudstones, creating favorable preservation conditions. The coal seams experienced two large-scale gas-generating processes under the influence of multistage tectonic movements. Against the backdrop of complex fault-bounded depressions, the coal seams exhibit weathering-degradation zones, saturated adsorption zones,and structural hydrocarbon accumulation zones, with its deep parts characterized by supersaturated gas-bearing properties and coexistence of adsorbed and free gas. Based on these findings, we establish a developmental model for the complex accumulation of deep coal-rock gas in a complex fault-bounded basin. The theoretical research and exploration practices of the deep Upper Paleozoic coal-rock gas in the Dagang exploration area serve as a significant guide and reference for the exploration and development of deep coal-rock gas in the Bohai Bay Basin.