东非裂谷Albertine盆地上中新统—下上新统烃源岩地球化学特征与沉积环境

Albertine盆地是东非裂谷系重要的裂谷盆地, 油气资源丰富, 上中新统-下上新统烃源岩是盆地重要的生烃岩系, 目前研究相对薄弱。综合利用地质学、岩石热解、元素地球化学等资料, 对Albertine盆地上中新统和下上新统的烃源岩进行了评价, 并分析了晚中新世-早上新世时期的古环境特征。研究表明, 两套烃源岩处于成熟阶段, 但有机质丰度和类型存在差异, 下上新统烃源岩有机质丰度更高, 属于中等-好烃源岩, 干酪根类型主要是Ⅱ₁~Ⅱ₂型; 上中新统烃源岩有机质丰度略低, 属于差烃源岩, 干酪根主要是Ⅱ₂~Ⅲ型。沉积环境是造成烃源岩差异的重要原因。晚中新世-早上新世时期, 研究区属于还原性较强的淡水湖盆, 水体逐渐加深, 气候越来越温暖, 细菌及微生物繁盛, 有机质类型变好, 同时, 湖泊生产力提高, 沉积速率有所加快, 有机质能够更有效地保存, 烃源岩的有机质丰度更高。研究结果对于Albertine盆地的油气勘探与开发具有重要的理论意义和实际意义。 

Geochemical characteristics and sedimentary environment of upper Miocene-lower Pliocene source rocks in the Albertine Basin,East African Rift Valley

The Albertine Basin is an important rift basin in the East African Rift System, enriched in oil and gas resources. The upper Miocene to lower Pliocene hydrocarbon source rocks are important source rocks in the basin, but the research is relatively limited. In this work, we evaluate the source rocks of the upper Miocene and lower Pliocene in the Albertine Basin by integrations of geological, rock pyrolysis and elemental geochemistry data and analyses the paleoenvironmental evolutions of the late Miocene to early Pliocene. These results show that the two sets of source rocks are under the mature stage, but the abundance and type of organic matter are different. The lower Pliocene source rocks have higher organic matter abundance and belong to middle-good source rocks, yielding type Ⅱ₁ -Ⅱ₂ kerogen. The upper Miocene source rocks have lower organic matter abundance and belong to poor source rocks, yielding type Ⅱ₂-Ⅲ kerogen. The sedimentary environment is an important controlling factor for the difference in source rocks. During the late Miocene to early Pliocene, the study area was a freshwater lake with strong reducibility, gradually increasing water depth, a warmer climate, flourishing bacteria and microorganisms, and better organic matter types. At the same time, lake productivity is elevated, the sedimentation rate is accelerated, and organic matter can be preserved more efficiently, so the abundance of organic matter in source rocks is higher.