长江三角洲启东地区全新统气源岩有机地球化学特征及生物气形成的控制因素

摘要

根据钻井岩心、古生物、有机岩石学观察及有机地化测试数据分析,结合区域地质背景资料,对长江三角洲启东地区ZK01井全新统地层中两套气源岩的沉积特征、有机质丰度、有机质类型、有机质成熟度等有机地球化学特征进行了系统研究,探讨控制研究区浅层生物气生成的主要因素.结果表明,全新统河口湾—河漫滩相灰色黏土及粉砂质黏土中有机碳含量在0.20％～0.65％之间,平均0.46％,氯仿沥青"A"平均值71.71×10-6;浅海相深灰色淤泥质黏土有机碳含量在0.19％～0.52％之间,平均0.34％,氯仿沥青"A"平均值65.69×10-6;前者有机质丰度略高于后者,生气能力略强于后者.两套气源岩中有机质来源均由低等水生生物和陆源高等植物两部分组成,属于混源输入,且以陆源高等植物有机质为主,有机质类型为含草本腐殖型、含草本腐泥腐殖型.两套气源岩镜质体反射率(Ro)在0.50％～0.65％之间,正构烷烃奇偶优势明显,热解最大峰温峰多数低于435℃,表明有机质处于未成熟阶段.长江三角洲启东地区具有有利于浅层生物气生成的基础地质条件、物质条件、地球化学条件,这些因素相互作用,相互制约,相互补充.%Based on a relatively systematic study of core lithology, paleontology, organic petrology and organic geochemistry, combined with regional geological background, the abundance, types and maturity of organic matter in the two sets of gas-generating source rocks of the ZK01 well, Qidong area, Changjiang (Yangtze) River delta, were investigated. Controlling factors for the formation of shallow biogenic gas in this study were further addressed. Results show that TOC value of the estuary-floodplain facies is between 0.20％and 0.65％with an average of 0.46％, and the chloroform bitumen content averages at 71.71 × 10-6. The TOC value of the shallow marine facies is between 0.19％and 0.52％with an average of 0.34％, and the chloroform bitumen content averages at 65.69 × 10-6. Thus, the organic matter abundance of estuary-floodplain facies is higher than that of shallow marine facies. This implies a higher gas productivity for the source rocks in the estuary-floodplain facies floodplain. Terrigenous organisms take dominance in organic matters of the Holocene gas-source sediments, and lower aquatic organisms are also present. The organic matter types are herbaceous sapropel-humic and herbaceous humic. Organic matters are still in immature stage, with the Ro ranging between 0.50％and 0.65％, an odd even predominance of n-alkanes, and Tmax value below 435℃. The formation of the shallow biogenic gas is mainly controlled by three factors, i.e., geological conditions, material conditions and geochemical conditions. The interaction, influence and supplement of these three factors are conducive to generate and accumulate shallow-biogenic gas.