三塘湖盆地火山灰对沉积有机质的富集效应及凝灰质烃源岩发育模式

The enrichment effect of organic materials by volcanic ash in sediments of the Santanghu Basin and the evolutionary pattern of tuffaceous source rocks

勘探实践表明,有些凝灰质岩具有很高的产烃能力,可以作为非常好的生油岩。但有关凝灰质岩中有机质富集的原因,却一直没有得到解决。基于三塘湖盆地石炭系凝灰质泥岩岩石学和地球化学特征分析,探讨了火山灰对沉积有机质的富集效应。研究发现,三塘湖盆地石炭系凝灰质泥岩有机质丰度要比同层位地层中泥岩的有机质丰度高很多,表明火山灰对有机质的富集起到了重要作用。凝灰质烃源岩干酪根显微组分以腐泥组的矿物沥青基质和层状藻类体为主、干酪根碳同位素较轻、正构烷烃为前单高峰型(主峰碳以nC15、nC17或nC19为主)以及规则甾烷、藿烷含量丰富等均表明三塘湖盆地石炭系凝灰质泥岩有机质主要来源于藻类/细菌等低等生物。火山喷发产生的气体及飘落在海水中的火山灰物质,可以在海水中释放大量的营养元素和铁元素,促进藻类/细菌的大量繁盛。这些大量繁殖的藻类/细菌是凝灰质泥岩中有机质的重要来源,也是凝灰质泥岩中有机质富集的重要原因,据此,结合前人研究成果初步建立了凝灰质烃源岩发育模式。

Exploration practice indicates that lots of tuffaceous rocks are good oil source rocks and have a geat generative potential o! hydrocarbons, however, the reason of organic enrichment in tuffaceous rocks remains unclear. Through analyzing petrographical and geochemical characteristics of Carboniferous tuffaceous mudstones in the Santanghu Basin, we investigated the enrichment effect of organic materials by volcanic ash in sediments and found oganic materials highly enriched in Carboniferous tuffaceous mudstones of the Santanghu Basin compared with mudstones of the basin. Volcanic ash was considered as the main factor causing the enrichment of organic materials in tuffaceous mudstones. Hight amounts of mineral bituminous matrix and lamellar algainite, lighter carbon iso- topes of kerogens, concentrated short-middle chain n-alkanes （maximum at nC15 , nC17 or nC19 ） and abundant regular steranes and hopanoids all indicated that organic materials of Carboniferous tuffaceous mudstones in the Santanghu Basin are mainly derived from algae/bacteria. The results showed that the gas and volcanic ash accompanied with volcanic eruption can stimulate algal/bacterial growth through releasing high amounts of nutrient and iron elements to the sea. These algae/bacteria are the main source of organic materials deposited in tuffaceous mudstones and high amounts of algal/baterial materials are the main cause of the organic material enrichment in tuffaceous mudstones. In addition, an evolutionary pattern of tuffaceous source rocks was primarily built up based on previous researches and our new findings.