Different characteristics and formation mechanisms of transitional and marine shale gas sweet spots

To provide guidance on the exploration and production of transitional shale gas, we investigate the sweet spots of transitional shale gas in the 3^rd submember of the 2^nd member of the Shanxi Formation (the Shan 2³ submember) in the Daning-Jixian block along the eastern margin of the Ordos Basin and those of marine shale gas in the 1^st sublayer of the 1^st submember of the 1^st member of the Longmaxi Formation (the Long 1¹ submember) in the southern Sichuan Basin. A combination of core and thin section observations, whole-rock and clay mineralogy by X-ray diffraction (XRD), organic geochemical analysis, scanning electron microscopy (SEM), N₂ adsorption, CH₄ isothermal adsorption, and major and trace element analyses, is applied to conduct a systematic comparative study on the characteristics and formation mechanisms of these sweet spots. The results indicate that the sweet spots of marine shale gas exhibit stable distributions and consistent development, while those of transitional shale gas show lateral discontinuities and occur across multiple layers vertically. The sweet spots of transitional shale gas feature high total organic carbon (TOC) content, medium to high maturity, and gas-prone organic matter of kerogen type Ⅱ₂-Ⅲ. In contrast, the sweet spots of marine shale gas are characterized by relatively high TOC content, high to over maturity, and oil-prone organic matter f kerogen type Ⅰ-Ⅱ₁. In the sweet spots of transitional shale gas, clay minerals are prevalent, where mesopores and macropores take a larger portion governing the occurrence of free gas. Organic matter in these sweet spots principally exhibits micropores, which contribute significantly to the specific surface area and determine the occurrence of adsorbed gas. In contrast, the sweet spots of marine shale gas display a dominance of quartz minerals. Their organic matter contains both micropores and mesopores, which serve as primary storage spaces for shale gas. The sweet spots of transitional shale gas predominantly exhibit adsorbed gas (average proportion: 66.06 %), while those of marine shale gas show predominant free gas, with adsorbed gas accounting for merely 11.15 % ~ 43.75 %. The organic matter enrichment in both types of sweet spots is governed by paleoclimate, paleoenvironment, and geologic events. Moreover, terrigenous debris input also plays a significant role in the formation of transitional shale gas sweet spots. The maximum single-well production of transitional shale gas in the Ordos Basin has been determined at up to 79,000 m³/d, demonstrating promising prospects for exploring transitional shale gas in the basin.