柴达木盆地三湖坳陷诺北地区第四纪生物气形成及影响因素

Formation Mechanism and Factors on the Accumulations of the Quaternary Biogenic Gas in the Nuobei Area in the Sanhu Depression, Qaidam Basin

柴达木盆地三湖坳陷东部的诺北地区目前还处于勘探初级阶段,根据岩性、古生物、元素地球化学及有机地球化学等资料,对诺北地区第四纪七个泉组气源岩特征和产甲烷菌的生存环境进行了分析,在此基础上讨论了沉积环境和地球化学条件对生物气形成的影响,预测了勘探有利区。结果表明,诺北地区七个泉组气源岩有机质丰度低,干酪根类型以Ⅲ为主,处于低成熟的热演化阶段。水体温度较低,盐度高,呈中性—弱碱性,还原性较强,有利于气源岩发育。地球化学环境影响了产甲烷菌的生长和繁育,低温和高盐抑制了产甲烷菌的活性,中性—弱碱性环境和较强还原性环境适宜产甲烷菌的生存。受地表径流影响,盐度对产甲烷菌的抑制作用在研究区内由南向北逐渐加强。对比垂向和横向运移的条件,认为诺北地区生物气以近源垂向运聚为主;中央凹陷是区内最有利的生物气勘探区,北斜坡次之。广泛发育的湖相泥岩为产甲烷菌提供了营养底物,早期成岩阶段弱压实作用提供了大量储集空间,巨厚的沉积地层和膏岩沉积增加了盖层的封闭能力。

The Nuobei area is still in the initial stage of petroleum exploration in the east part of the Sanhu Depression,Qaidam Basin.In this paper,the characteristics of the gas source rocks and the living environments of the methanogens in the Quaternary Qigequan Formation from the Nuobei area have been studied,based on a vast amount of data for lithology paleontology,elemental geochemistry,organic geochemistry etc.We discussed the sedimentary environments and geochemical conditions that influenced the formation of biogenic gas,and then to predict the favorable exploration blocks.The results show that the gas source rocks in the Nuobei area contain low organic matter content which are dominated by typeⅢkerogen and still at immature stage.The Qigequan Formation was deposited under the low temperature and alkalinity,high salinity,and strong anoxic conditions,which were favorable for the formation of the gas source rocks.The geochemical environment affected the flourish of the Methanogens,for instance,the low temperature and high salinity inhibited the activities of the Methanogens,and the neutral-alkaline and strong reducibility environment were suitable for Methanogens to flourish.The inhibition effect of salinity for the Methanogens is gradually strengthened from south to north as affected by surface runoff.The biogenic gas accumulations in the Nuobei area were dominated by vertical gas migration.According to the above analyses,we suggested that the Central Depression is the most favorable exploration area in the Nuobei area,followed by the North Slope.In these areas,the widely developed lacustrine mudstones supplied nutrient for Methanogens,the weak compaction during the early diagenetic stage provided large amounts of pores in reservoirs,and the thick sedimentary strata and gypsum deposition increased the sealing capacity of the cap rock.