火山灰对页岩油气成藏的重要作用及其启示

An important role of volcanic ash in the formation of shale plays and its inspiration

全球主力页岩油气层大多伴生火山灰层,剖析火山灰的类型、成分、成因和成岩演化作用,探索火山灰与富有机质页岩的形成关系,有利于明确火山灰对页岩油气层的作用机制。通过对我国主要产页岩油气盆地的野外考察和页岩岩心观察,应用场发射扫描电镜和能谱元素分析等技术,结合生产实践成果,研究得出了4点认识:①火山灰以中酸性为主,大多来自爆炸式火山喷发,其表面附着易溶薄层盐膜,入水后可迅速释放铁盐等营养物质,有利于促进藻类勃发;②藻类勃发不仅是优质烃源岩的主要物质来源,而且还可促进碳酸盐岩矿物沉积,形成藻类与碳酸盐岩纹层状互层,利于发育层间孔缝,提高页岩油气层的物性和脆性;③在以缺氧环境为主的深水区,不仅厘米—毫米级火山灰层数与富有机质泥页岩厚度呈正相关关系,而且频繁的微米级火山灰沉降也能形成大面积厚层优质烃源岩;④板块运动形成的造山带与板块俯冲带具有良好的对应关系,是爆炸式火山喷发的发育带,其周边的古湖泊和海洋是形成大面积厚层优质烃源岩的有利区。结论认为:四川盆地及其周边的下古生界海相页岩为页岩气现阶段最可行的勘探领域,除鄂尔多斯等主要含油气盆地外,银额、民和、二连和海拉尔等中小盆地也具有较大的页岩油(致密油)勘探潜力。

Volcanic ash layers are discovered in many main producing shale oil ＆ gas formations all over the world. The analysis of the types, composition, origin and diagenetic evolution of volcanic ash and studies on the relationship between volcanic ash and organic-rich shales are essential to determine the effects of volcanic ash on shale oil/gas layers. Based on the observation of shale outcrops and cores of the main producing shale basins in China, the advanced technologies such as field emission scanning election microscope and energy spectrum analysis were used to study the role of volcanic ash in the formation of shale plays, and the following conclusions were obtained. （1） Most of the volcanic ash is of intermediate-acidic, and originate from explosive volcano eruptions. The ash particles are commonly covered with soluble salt coatings, which can rapidly release a large amount of nutrients, such as iron-containing salt, resulting in plankton bloom. （2） Planktonic algae bloom can not only supply organic matters for the formation of high quality source rocks, but improve the deposition of carbonate minerals. The interlaminated algae and carbonates are favorable for the development of interlayer pores and fractures and also for the enhancement of physical properties and brittleness. （3） The number of volcanic ash layers in centimeter to millimeter scale has a positive correlation with the thickness of organic-rich shales deposited in commonly anoxic deepwater environments. In addition, the frequent deposition of volcanic ash in micrometer scale can even form extensive thick high-quality source rocks. （4） The orogenic belts resulted from plate movement generally have a good corresponding relationship with subduction zones and are favorable for explosive volcano eruption. The surrounding ancient lakes and sea are likely to form extensive thick high-quality source rocks. On this basis, the marine shales of the Lower Paleozoic in the Sichuan Basin and its periphery are predicted to be the most promising targets for shale gas exploration at present. In addition to the major petroliferous basins such as Ordos, the medium-sized and small basins such as Yinre, Minhe, Erlian and Hailar, also show good shale oil （tight oil） potentials.