鄂尔多斯盆地长7_(3)亚段泥页岩黄铁矿发育特征及其地质意义

Characteristics and geological significance of pyrite in Chang 7_(3) sub-member in the Ordos Basin

黄铁矿作为泥页岩中广泛发育的一种矿物类型,其在沉积环境判别及页岩油气富集规律分析方面具有重要意义。结合岩心观察、扫描电镜薄片鉴定、X-射线衍射数据分析等方法,以鄂尔多斯盆地延长组7段3亚段(长7_(3)亚段)泥页岩样品为例,开展黄铁矿宏观及微观特征分析,探讨其在环境指示及页岩油富集方面的意义。研究区长7_(3)亚段泥页岩中黄铁矿平均含量为16%,最高达50%。岩心上以沉积期黄铁矿结核、团块为主,微观上可观察到草莓状黄铁矿、自形晶和黄铁矿交代充填生物体共3种类型。草莓状黄铁矿具2种外部形态:一种具明显的球状轮廓;另一种为簇状、连片集合体,主要为沉积期产物。自形晶以孤立或顺层分布的八面体、立方体为主,属成岩早期形成;局部可见黄铁矿交代充填藻类格架,多在晚成岩阶段形成。长7_(3)亚段草莓状黄铁矿粒径均值为6.5~12.3μm,标准偏差为1.8~5.3μm,指示沉积期水体具含氧特征;黄铁矿含量与有机质丰度成正比,反映黄铁矿对有机质富集具重要意义,但其对油气富集的作用仍待深化研究;黄铁矿对储层物性存在双向影响,应结合不同类型黄铁矿特征具体探讨其赋孔特征。

Pyrite is widely developed in shale reservoirs.It is of great significance in the identification of sedimentary environment and the analysis of shale oil and gas enrichment.Combined with core observation,scanning electron microscopy identification,X-ray diffraction analysis and other methods,taking the shale samples in Chang 7_(3) sub-member of Yanchang Formation in the Ordos Basin as an example,macro and micro characteristics analysis of pyrite is carried out.At the same time,its significance on environmental evolution and shale oil enrichment is discussed.The results show that there is a large amount of pyrite in the Chang 7_(3) sub-member shale,with an average content approximately of 16%,up to 50%.There are mainly nodules and clumps sedimentary pyrite in core.Three types of pyrite including strawberry-like shapes,automorphic crystals,and metasomatic filling organisms are observed.Strawberry-like pyrite has two external forms,one with obvious spherical outline,and the other as clusters and contiguous aggregates,which are the products of the sedimentary period.The self-shaped crystals are mainly octahedrons and cubes that are isolated or distributed in the layer.They are formed in the early diagenesis.Pyrite metasomatism can be seen filling the algae framework locally,which was formed in the late diagenetic stage.The average particle size of pyrite in Chang 7_(3) sub-member is 6.5-12.3μm,and the standard deviation is 1.8-5.3μm,indicating the characteristics of oxygen content in the water body during the sedimentary period.The content of pyrite is directly proportional to the abundance of organic matter,reflecting that pyrite is of great significance for the enrichment of organic matter and hydrocarbons.Strawberry-like shaped pyrite crystallites have a protective effect on the development of internal organic matter pores.Pyrite has a two-way influence on the physical properties of the reservoir,the pore-forming characteristics of different types of pyrite should be discussed in detail.