四川盆地五峰组-龙马溪组页岩储层中黄铁矿特征及地质意义

Characteristics and geological significance of pyrites in Wufeng and Longmaxi Formation reservoir shale in Sichuan Basin, China

黄铁矿是页岩储层中含量最多的重矿物,其质量分数普遍在1%~4%之间。为了研究海相页岩储层中黄铁矿与沉积环境、有机质的关系,采用野外剖面、光学显微镜和扫描电子显微镜等手段在宏观和微观尺度下进行黄铁矿类型及特征研究。黄铁矿的主要产状类型有黄铁矿结核、黄铁矿条带、草莓状和立方体黄铁矿及其集合体、几丁虫状黄铁矿及其集合体等生物交代黄铁矿,在野外剖面中还存在黄铁矿条带和结核伴生、黄铁矿与方解石沿裂缝充填、黄铁矿结核内部存在裂隙等现象。对扫描电镜下草莓状黄铁矿和立方体黄铁矿的粒径进行统计分析,发现四川盆地海相页岩储层中草莓状黄铁矿直径与其单晶直径之间存在正相关关系,草莓状黄铁矿平均粒径集中在2.5710~6.2393μm,草莓状黄铁矿单晶平均粒径分布在0.3188~0.7037μm,而图像分析显示黄铁矿粒径集中在2~6μm。五峰组-龙马溪组下段页岩储层草莓状黄铁矿平均粒径分布稳定,平均值为4.8309μm,沉积水体环境为硫化和次氧化环境;而草莓状黄铁矿与有机质含量存在正相关关系。页岩中与黄铁矿相关的孔隙类型主要有草莓状黄铁矿单晶间的有机质孔、晶间孔及黄铁矿与周围有机质之间的孔隙,有机质孔隙较小,但有利于储层中天然气吸附。

Pyrite is one of the most abundant heavy-minerals in shale formation and its mass fraction ranges from 1%to 4%generally.In the article,the types and characteristics of pyrites are studied through outcrop observation and by optical microscope and scanning electron microscope in order to unravel the relationship between pyrites and sedimentary environment,organic matter in marine shale formation.It reveals that the pyrites generally occur as nodules,bands,framboidal and cubic and cubic aggregations as well as bio-metasomatic pyrites,such as chitinozoa pyrites and their aggregation.In the field section,it is found that the banded pyrites are associated with nodule pyrites,pyrites and calcites are filled in the cracks,and cracks occurred in the pyrites.Statistics of framboidal and cubical pyrites by SEM reveals that there is a positive relationship between diameter of framboidal pyrites and monocrystal pyrites in marine shale-gas formation in Sichuan Basin.The average diameter distribution of framboidal pyrites is 2.5710~6.2393μm,and the average diameter of monocrystal framboidal pyrites ranges from 0.3188 to 0.7037μm in SEM analysis,and 2 to 6μm in CIAS analysis.The average particle size distribution of framboidal pyrites in Wufeng-Lower Longmaxi Formation is stable,with an average of 4.8309μm.Its sedimentary water is considered as vulcanized and suboxidized environment.There is positive correlation between the content of framboidal pyrites and organic matter.The pore types related to pyrites in shale mainly include organic matter pores between monocrystal pyrites,intercrystalline pores,and pores between pyrites and surrounding minerals.Organic matter pore is small,but is favorable for gas adsorption in shale formation.