鄂尔多斯盆地天环坳陷北段奥陶系烃源岩特征及成熟演化史

The geochemical characteristics and thermal evolution of Ordovician sourcerocks in the north of Tianhuan depression, Ordos basin

为研究天环坳陷北段下古生界天然气成藏规律,利用探井资料和有机地球化学方法,评价了天环坳陷北段奥陶系烃源岩特征并恢复其成熟演化史。天环坳陷北段奥陶系乌拉力克组和克里摩里组烃源岩有机碳含量和生烃潜量相对较高,烃源岩有机质丰度以中等—好为主,少部分为差烃源岩;三道坎组和桌子山组烃源岩以差为主。干酪根组分鉴定表明,天环坳陷北段奥陶系烃源岩主要为Ⅱ1~Ⅰ型干酪根,热演化程度高,处于生干气阶段。成熟演化史模拟显示,古生代—中生代早期地温梯度低,地层埋藏浅,烃源岩成熟演化程度低。燕山期,由于构造热事件,地温梯度迅速升高,早白垩世烃源岩经历的最高温度超过200℃,达到生气高峰期,早白垩世是油气成藏的主要时期。早白垩世之后,鄂尔多斯盆地整体抬升,地温梯度减小,生烃作用停止。

In order to study the gas accumulation mechanism of lower Paleozoic in the north of Tianhuan depression, using exploratory well data and organic geochemical methods, the characteristics of Ordovician source rocks were evaluated and their maturity evolution history was restored. Organic carbon content and hydrocarbon potential of source rocks from the Wulalike and the Kelimoli Formations are relatively high, the source rocks are mainly medium-good and a few are poor. The source rocks of the Sandaokan and the Zhuozishan Formations are mainly poor. The organic matter types of source rocks are mainly kerogen type Ⅱ1~I, with high degree of thermal evolution, in the dry gas stage. Simulation of mature evolutionary history shows that the Palaeozoic-early Mesozoic geothermal gradient was low, the burial depths of the Formations were shallow, and the degree of maturity and evolution of source rocks was low. In the Yanshan period, due to tectonic thermal events, the geothermal gradient increased rapidly, and maximum temperature experienced by the early Cretaceous source rocks exceeded 200 ℃, reaching the peak of gas generation. Early Cretaceous is the main period of hydrocarbon accumulation. After Early Cretaceous, Ordos basin uplifted, geothermal gradient decreased and hydrocarbon generation stopped.