海-陆过渡相与海相页岩气“甜点段”差异特征与形成机理

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

张琴 邱振 赵群 董大忠 刘雯 孔维亮 庞正炼 高万里 蔡光银 李永洲 李星涛 林文姬

Qin ZHANG Zhen QIU Qun ZHAO Dazhong DONG Wen LIU Weiliang KONG Zhenglian PANG Wanli GAO Guangyin CAI Yongzhou LI Xingtao LI Wenji LIN

为了指导海-陆过渡相页岩气的勘探开发，以鄂尔多斯盆地东缘大宁—吉县地区二叠系山西组2段3亚段（山2亚段）海-陆过渡相页岩气甜点段与四川盆地南部地区志留系龙马溪组一段1亚段（龙一亚段）1小层海相页岩气甜点段作为研究对象，开展系统的岩心观察、薄片鉴定、全岩-黏土X射线衍射、有机地球化学分析、扫描电镜观察、N低温吸附、CH等温吸附以及主、微量元素分析实验，系统对比研究了海-陆过渡相与海相页岩气甜点段的特征与形成机理。研究结果表明：① 海相页岩气甜点段稳定分布与发育；海-陆过渡相页岩气甜点段横向不连续，纵向多层段发育。② 海-陆过渡相页岩气甜点段总有机碳含量（）高，处于中-高成熟阶段，有机质以Ⅱ-Ⅲ型为主；海相页岩气甜点段较高，处于高-过成熟阶段，有机质以Ⅰ-Ⅱ型为主。③ 海-陆过渡相页岩气甜点段矿物组成以黏土矿物为主，黏土矿物孔隙以介孔和宏孔为主，它们控制游离气的赋存；有机质以发育微孔为主，它是比表面积的主要贡献者，控制吸附气的赋存。海相页岩气甜点段以石英矿物为主；页岩有机质同时发育微孔和介孔，它们是页岩气赋存的主要储存空间。④ 海-陆过渡相页岩气甜点段以吸附气为主，平均占66.06 %；海相页岩气以游离气为主，吸附气占11.15 % ~ 43.75 %。⑤ 海-陆过渡相和海相页岩气甜点段有机质富集均受到古气候、古环境以及地质事件控制，但陆源碎屑输入对海-陆过渡相页岩气甜点段有机质富集具有重要控制作用。⑥ 鄂尔多斯盆地海-陆过渡相页岩气单井最高产量达到7.9×10 m/d，研究和勘探表明海-陆过渡相页岩气具有良好的勘探前景。

To provide guidance on the exploration and production of transitional shale gas， we investigate the sweet spots of transitional shale gas in the 3 submember of the 2 member of the Shanxi Formation （the Shan 2 submember） in the Daning-Jixian block along the eastern margin of the Ordos Basin and those of marine shale gas in the 1 sublayer of the 1 submember of the 1 member of the Longmaxi Formation （the Long 1 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）， N adsorption， CH 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 （） content， medium to high maturity， and gas-prone organic matter of kerogen type Ⅱ-Ⅲ. In contrast， the sweet spots of marine shale gas are characterized by relatively high content， high to over maturity， and oil-prone organic matter f kerogen type Ⅰ-Ⅱ. 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 m/d， demonstrating promising prospects for exploring transitional shale gas in the basin.

10.11743/ogg20240514