The bitumen formation and Re-Os characteristics of a CO2-rich pre-salt gas reservoir of the Kwanza Basin, offshore Angola
The bitumen formation and Re-Os characteristics of a CO2-rich pre-salt gas reservoir of the Kwanza Basin, offshore Angola
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DOI：
10.1016/j.marpetgeo.2022.105786
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发表时间：
2022-06
影响因子：
4.2
通讯作者：
Junjie Liu;Honggang Zhou;M. Pujol;D. Selby;Jie Li;H. Tian
中科院分区：
文献类型：
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作者：
Junjie Liu;Honggang Zhou;M. Pujol;D. Selby;Jie Li;H. Tian
本文对安哥拉南部宽扎盆地盐下富CO2气藏中沥青的形成过程和Re-Os系统进行了研究。岩浆作用产生的CO2可能从储层顶部向下扩展，在储层内部形成CO2浓度梯度和温度梯度，导致沥青质沉淀和热裂解梯度。由于与周围岩石的热交换，热锋可能位于CO2锋之后。因此，沥青质沉淀可能在热裂化之前发生。这一沥青形成过程与沥青含量和岩石评价Tmax随深度降低的特征以及全岩和沥青的Re、Os含量特征相一致，5个间距较近的沥青样品的Re-Os年龄为116 ± 29 Ma，与CO2充注时间一致，可能指示了沥青的形成时间。沥青质存款的不均匀的187 Os/188 Os和缺乏流动性，以及相对较低的热裂解程度（Tmax为425-470 °C）可能是沥青初始187 Os/188 Os缺乏均一化的原因。1.2-1.8），并有有限的187 Os/188 Os长入。从潜在源岩（Barremian-Aptian Red和Grey Cuvo地层）沉积到沥青形成的1500万年，源岩应具有高放射成因的初始187 Os/188 Os（>1）。这与共轭巴西边缘盆地中的相当湖相地层的一致。因此，Os同位素组成表明，湖相红色和灰色Cuvo地层可能是所研究的南宽扎盆地富CO2盐下气藏的沥青源岩。
Here we present a study on the formation process and Re–Os systematics of the bitumen discovered in a CO2-rich pre-salt gas reservoir, south Kwanza Basin, offshore Angola. The magmatism-derived CO2may have expanded downwards from the top of the reservoir, creating CO2concentration and temperature gradients within the reservoir, and leading to the gradients of asphaltene precipitation and thermal cracking. The heat front may have been behind the CO2front due to the heat exchange with surrounding rocks. Thus, asphaltene precipitation has likely occurred before the thermal cracking. Such bitumen formation process is consistent with the observed decreasing bitumen content and Rock-Eval Tmax with depth, and also the characteristics of the whole rock and bitumen Re and Os concentrations.Five closely spaced bitumen samples define a Re–Os age of 116 ± 29 Ma that is identical to the timing of CO2charging and thus may indicate the timing of bitumen formation. The heterogeneous187Os/188Os and lack of mobility of the asphaltene deposit, and the relatively low extent of thermal cracking (Tmax of 425–470 °C) may account for the lack of homogenization of the bitumen initial187Os/188Os.Given the highly radiogenic initial187Os/188Os (ca. 1.2–1.8) of the bitumen and limited187Os/188Os ingrowth within the ca. 15 million years from the deposition of potential source rock (Barremian-Aptian Red and Grey Cuvo formations) to the bitumen formation, the source rock should possess a highly radiogenic initial187Os/188Os (>1). This is consistent with that of the equivalent lacustrine strata in the conjugated Brazilian marginal basins. Thus, the Os isotope composition indicates that the lacustrine Red and Grey Cuvo formations could be the source rock for the bitumen of the studied CO2-rich pre-salt gas reservoir in the south Kwanza Basin.