渝南奥陶-志留纪之交黑色笔石页岩的时空分布

The late Ordovician to the early Silurian is a critical time interval during which great biological and local tectonic events happened. On the Yangtze Platform, two series of black shales, the Wufeng Formation and the Lungmachi Formation deposited widely during the exact time interval. They are also layers where shale gas was yielded. Graptolite is the dominant fossil inside the two series of black shales. Its biostrtigraphic division and correlation have been taken as an efficient tool in precisely locating the geographic distribution problems of black shales. In the south part of Chongqing, near the south boundary of the Sichuan Basin where is extremely promising in shale gas exploration, Shale gas from the late Ordovician-early Silurian black graptolitic shale sediments are ready for exploration. However, till now there is little detailed studies on the biostratigraphic and geographic distribution of the Ordovician-Silurian Wufeng and Lungmachi black shales in the south part of Chongqing area. This applying project chooses six sections in the south part of Chongqing area near around the covered Sichuan basin and aims at doing some studies on taxonomy and biostratigraphic division and correlation on the graptolites from the Wufeng and Lungmachi Formation as well as working out the biostratigraphic and geographic distribution of the two series of black graptolitic shales. With the assistance of borehole data, this project will study the relationship between graptolitic biostratigraphy and shale gas gathered layers. By taking advantage of this, this project will work out the biostratigraphic and geographic distribution of shale gas gathered layers inside the two series of black shales under the Sichuan Basin, especially in the Fuling area inside the Basin which yield shale gas. This applying project is in hope of making usage of graptolitic biostratigraphy on the gas and oil 3D distribution, as well as detection and exploration array.

在重大生物事件和地区性构造事件频发的奥陶-志留纪之交，扬子区广泛发育了两套黑色笔石页岩——五峰组和龙马溪组，它们正是页岩气产出的重要层位。笔石作为其中占主导地位的生物化石，其生物地层分带和对比已成为精确研究黑色页岩时空分布的有效手段。在极具页岩气勘探前景的四川盆地南缘的渝南地区，五峰和龙马溪组黑色页岩中的页岩气正待开发。然而，目前该区尚缺乏运用笔石生物地层手段解决黑色页岩页岩气有利层段时空分布的研究。本项目拟选择渝南地区贴近四川盆地覆盖区的6条奥陶-志留系剖面，研究五峰、龙马溪组内笔石的分类学及生物地层划分和对比，获得在笔石带时间框架下渝南地区两套黑色页岩的时空分布。结合井下资料，探究笔石带与页岩气赋存有利层段之间的对应关系，并以此指导和推断临近覆盖区，特别是已发现页岩气藏的涪陵地区盆内五峰、龙马溪组含气有利层段的分布范围，以期把笔石生物地层研究用于指导和推断油气的时空分布和勘探布局上。

本项目选取重庆石柱漆辽村木厂沟剖面，通过多次野外工作采集了大量的奥陶-志留纪之交的笔石化石标本。对于采集的标本开展了系统古生物学研究。根据笔石动物的重要性状以及始端发育型式鉴定了笔石属种、确立笔石分类单元在木场沟剖面的延限，并识别出木场沟剖面内的笔石带序列。自下而上共识别出了Dicellograptus complexus带、P. pacificus (Lower Subzone) 带、Tangyagraptus typicus带、Dicerarograptus mirus带、M. extraordinarius 带、M. persculptus带、A. ascensus带、P. acuminatus带、C. vesiculosus带、C. cyphus带，共10个笔石化石带。在木厂沟剖面开展的地球化学研究显示，有机碳同位素曲线在pacificus带和persculptus带出现了正飘移。正好对应了界限附近的海平面下降和上升过程。通过和多个油气部门的合作研究，本研究将木厂沟露头剖面的笔石带序列作为奥陶系、志留系黑色页岩的划分和对比标准，完成了黑色笔石页岩的井下生物地层划分。通过将笔石生物分带和井下资料结合，确立五峰组、龙马溪组内页岩气赋存的有利层段（TOC含量较高层段）所对应的笔石带范围大致为LM1—LM5，建立了笔石带与页岩气赋存有利层段的关系。从多口页岩气井的岩芯资料可以看出，渝南地区从五峰组到龙马溪组笔石带发育齐全，具有较好的页岩气潜力。有利的页岩产气层段LM1—LM5地层厚度具有较大差异性。黑色页岩厚度最大的主要分布在四川綦江、长宁、重庆涪陵、巫溪一带。