皖南地区上二叠统大隆组页岩沉积环境与有机质富集机理

Sedimentary environment and organic matter enrichment mechanisms of the Upper Permian Dalong Formation shale, southern Anhui Province, China

丁江辉 张金川 石刚 申宝剑 唐玄 杨振恒 李兴起 李楚雄

Jianghui Ding Jinchuan Zhang Gang Shi Baojian Shen Xuan Tang Zhenheng Yang Xingqi Li Chuxiong Li

皖南地区上二叠统大隆组富有机质页岩不仅是中国下扬子区重要的烃源岩，也是目前中国页岩气勘探的重要层位之一。为研究其沉积环境与有机质富集机理，对港地1井进行系统取样，开展了有机地球化学测试和元素地球化学分析等工作。结果表明：皖南地区大隆组页岩沉积时期处于暖湿型气候，具有高的生物生产力和沉积速率，早-中期伴随着区域性海平面上升，沉积水体加深且滞留程度增强，整体处于贫氧-厌氧环境；到了晚期区域性海平面下降，水体变浅且滞留程度减弱，整体处于氧化环境，且在二叠纪末期伴随有明显的火山热液活动。大隆组富有机质页岩的形成，不是由单一因素所决定，而是由古气候、水体氧化还原性质、生物生产力、沉积速率等多个要素相互配置与耦合的结果。皖南地区大隆组早-中期区域性海平面上升把深水的营养物质带到浅海的透光带，促进表层水体微生物大量繁殖，产生了高的生物生产力，且贫氧-厌氧的水体环境有利于有机质保存，形成了偏还原条件下的有机质富集模式；大隆组晚期沉积水体整体处于氧化状态，虽然含氧水体不利于有机质保存，但二叠纪末期大规模的火山热液活动蕴含了丰富的营养物质，促进生物大量繁殖，这样一方面可以形成高的生物生产力，另一方面生物的呼吸也可以加剧水体中氧的消耗，而且高的沉积速率也可以缩短有机质在水体中被氧化分解的时间，使得有机质快速埋藏和保存，形成了偏氧化条件下的有机质富集模式。

The organic-rich shale in the Upper Permian Dalong Formation in southern Anhui Province is one of the major source rocks in the Lower Yangtze region and also the important shale gas exploration target in China.Its sedimentary environment and organic matter enrichment mechanisms were studied through organic geochemical tests and elemental geochemical analyses on samples from Well Gangdi-1 in the area.Results show that the shale was deposited in a warm and humid paleoclimate in which biotic productivity was high and sedimentary rate was relatively fast.The sedimentation experienced an environmental transition from an overall dysoxic-anaerobic setting with regional rising sea level that enhanced sediment retention during the early and middle stages to an oxydic setting with falling regional sea level (accompanied by regional volcanic hydrothermal activities at the end of the Permian) that reduced sediment retention during the later stage.In other words, the Dalong Formation shale, rather than a child of a single factor, is the result of a mutual configuration and coupling of multiple factors such as paleoclimate, paleoredox, biotic productivity, and sedimentary rate.The rising regional sea level during the early sedimentary stage took with it abundant nutrients from deep up to the surface, leading to bacteria, algae and phytoplankton booms that significantly enhanced the biotic productivity and a dysoxic-anaerobic water body that aided in the preservation of organic matter in a partially reducing environment.The falling sea level that formed an oxydic environment during the late stage of sedimentation was unfavorable in terms of organic matter preservation.However, the organic matter was still well preserved when the Permian volcanic hydrothermal activities brought in rich nutrients that promoted the mass reproduction of microorganisms (both the source of high biotic productivity and the reason of a rapid consumption of oxygen), accelerated sedimentary rate to shorten the exposure time, and fasten the burial and preservation process of organic matter.

10.11743/ogg20210114