川南地区五峰组—龙马溪组页岩笔石带对比及沉积特征

摘要

The Changning and Weiyuan Blocks in southern Sichuan Basin are the main shale gas production areas in China,and their main shale gas pay zones are the organic rich shale of Upper Ordovician Wufeng Fm-Lower Silurian Longmaxi Fm,which are rich in organic shale with 20-40 m in thickness and with good correlatability.However,the organic-rich black shale in these blocks is significantly different in deposition time and rate if it is analyzed in terms of their graptolite strata.In this paper,core description and graptolite biostratigraphical classification were carried out on 8 wells in the Changning and Weiyuan Blocks by using sedimentology,biostratigraphy,geochemistry and mineralogy.It is indicated that the organic-rich shale in the Changning and Weiyuan Blocks is mainly deposited in WF1-LM4 and LM 1-LM8 graptolite zones,respectively,but their deposition time is obviously different;that the depocenter of the organic-rich shale migrated gradually northward (Weiyuan Block) under the influence of the Guangxi movement,and as it approaches the coast line in the south,the deposition of organic-rich shale ended earlier and that in the Weiyuan Block,the lower parts of Guanyinqiao Fm and parts of graptolite zones are locally missing and underwater paleouplifts (which might be above the sea surface during the Hirnantian Period) is developed,but its boundary has not been defined clearly,so the deposition of organic-rich shale is impacted,so much attention should be paid when shale gas exploration is deployed in this area.%四川盆地南部长宁与威远区块是我国页岩气主要产区之一,主要产气层上奥陶统五峰组—下志留统龙马溪组富有机质页岩厚度介于20～40 m,具有良好的可对比性,然而,从笔石地层的角度分析后发现,上述2个区块黑色富有机质页岩的沉积时间和沉积速率均存在着明显的差异.为此,应用沉积学、生物地层学、地球化学和矿物学等研究手段,开展了上述2个区块8口钻井的岩心描述和笔石生物地层划分等工作.研究结果表明:①长宁地区的富有机质页岩主要沉积于WF1-LM4笔石层段,而威远地区则主要沉积于LM1-LM8笔石层段,二者富有机质页岩的沉积时间差异较大;②受广西运动的影响,富有机质页岩的沉积中心不断向北(威远地区)迁移,越靠近南部的海岸线,富有机质页岩沉积结束的时间就越早;③威远地区局部缺失观音桥组下部和部分笔石带,发育水下古隆起(赫南特冰期或已隆出水面),其边界目前尚未明确,影响了富有机质页岩的沉积,在该区域进行页岩气勘探部署需格外谨慎.