渝东南地区五峰组-龙马溪组页岩草莓状黄铁矿形成机制、生成环境及对储层的影响

Formation mechanism and formation environment of framboidal pyrite in Wufeng Formation-Longmaxi Formation shale and its influence on shale reservoir in the southeastern Chongqing,China

为探讨草莓状黄铁矿的形成机制、生成环境及对页岩储层的影响,以渝东南地区五峰组-龙马溪组页岩为研究对象,对岩心样品进行氩离子抛光-场发射环境扫描电镜实验观察、草莓状黄铁矿粒径统计分析和页岩中有机质孔隙圆度的统计分析等研究。结果显示草莓状黄铁矿最初生长的位置在有机质表面,由于有机质的吸附作用较强,沉积水体中的铁离子在有机质表面不断聚集生长,在缺氧的硫化环境中逐渐演化为草莓状黄铁矿。彭水地区五峰组-龙马溪组的页岩形成于缺氧的硫化环境中,对有机质的保存十分有利;草莓状黄铁矿往往与有机质混杂堆积在一起,反映草莓状黄铁矿与有机质相伴生的特点;五峰组页岩的有机碳含量较高,而统计视域内黄铁矿数量较少,但是单个草莓状黄铁矿颗粒往往较大,其颗粒的大小可能与沉积速率有关;黄铁矿颗粒的球形结构特征和抗压强度大,能够对黄铁矿颗粒内部有机质孔隙起到一定的保护作用。

The framboidal pyrite in Wufeng Formation-Longmaxi Formation shale in southeast Chongqing was studied and the grain size and pore roundness of the organic matter in framboidal pyrite were analyzed by means of experimental observation of argon ion polishing-field emission environmental scanning electron microscopy,in order to discuss its formation mechanism and generating environment.The results showed that the initial growth location of framboidal pyrite was on the surface of organic matter.Due to the strong adsorption of organic matter,iron ions in the deposited water constantly gathered and grew on the surface of organic matter,and gradually evolved into framboidal pyrite in the environment of hypoxia vulcanization.The Wufeng Formation-Longmaxi Formation shale in Pengshui area was formed in anoxic vulcanization environment,which was very beneficial to the preservation of organic matter.Framboidal pyrite was often mixed and piled together with organic matter,which reflects the concomitant characteristics of framboidal pyrite and organic matter.The TOC in the Wufeng Formation shale is relatively high,while the amount of pyrite in the statistical field is relatively small.However,the single framboidal pyrite particle is usually larger,and the size of the particle may be related to the deposition rate.The spherical structure and high compressive strength of pyrite gains can protect the internal organic pores in pyrite grains.