Formation mechanisms and exploration practices of source-rock hydrocarbon reservoirs

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.