The pores evolution of lacustrine shale induced by smectite-to-illite conversion and hydrocarbon generation: upper Triassic Yanchang Formation, Ordos Basin, China
The pores evolution of lacustrine shale induced by smectite-to-illite conversion and hydrocarbon generation: upper Triassic Yanchang Formation, Ordos Basin, China
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DOI：
10.1016/j.petrol.2021.108460
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发表时间：
2021-02-20
影响因子：
--
通讯作者：
Wang Dailun
中科院分区：
文献类型：
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作者：
Deng Hucheng;Fu Meiyan;Wang Dailun
湖相页岩（例如沉积在湖中的页岩）是石油和天然气工业的重要商业生产目标。页岩孔隙网络对天然气、石油的赋存状态及储量计算具有重要影响。勘探实践表明，湖相页岩的孔隙网络与海相页岩有很大不同。尽管许多研究提出湖相页岩孔隙网络受粘土矿物的影响，但湖相页岩孔隙的起源、演化和保存仍不清楚。本研究旨在揭示鄂尔多斯盆地上三叠统延长组长7段湖相页岩同步生烃和蒙皂石转化对孔隙形成和演化的影响。 C7段的中下部被称为“张家滩”。页岩，是近年来我国油页岩勘探的重点。结合分析技术来研究与粘土矿物相关的孔隙演化。丙七段页岩中粘土矿物主要为伊利石-蒙脱石混层矿物、伊利石、高岭石、绿泥石等，占全岩的25.9%～59.1%。密度泛函理论（DFT）孔隙体积与粘土矿物含量之间存在正相关关系。 DFT方法适用于所有类型的孔隙，特别是微孔。页岩样品中以中孔（2~50 nm）为主，有机质中仅存在少量微孔（2 nm）。研究表明，中孔的发育受蒙皂石-伊利石转化的控制。随着蒙脱石层转化为伊利石，中孔体积和表面积将减小。然而，由于矿物体积收缩和超压，大孔（50 nm）会随着伊利石层的形成而增加。在中等热成熟度下，蒙皂石向伊利石的转化有利于孔隙网络。蒙脱石向伊利石的同步转化和生烃是湖相页岩孔隙形成和保存的关键机制。
The lacustrine shales (e.g. shales deposited in the lake) are important commercial production targets for the oil and gas industrial. The pore network of shales has a significant effect on the occurrence state of natural gas&oil and reserve calculation. The exploration practice has indicated that the pore network of lacustrine shale is very different from that of marine shale. Although many researches have proposed that the pore network of lacustrine shales is influenced by clay minerals, however, the origin, evolution and preservation of pores in the lacustrine shales are still obscure. This study aims to reveal the effect of synchronous hydrocarbon generation and smectiteillite conversion on the formation and evolution of pores in lacustrine shales from the Chang #7 (C7) Member of the Upper Triassic Yanchang Formation in Ordos Basin, China. The middle and lower part of C7 Member is known as the ?Zhangjiatan? shale, which is focused by oil shale exploration recently in China. The combination of analytical techniques was used to investigate the pore evolution related to clay minerals. The clay minerals within shales from the C7 Member consist of illite-smectite mixed layer minerals, illite, kaolinite and chlorite, accounting for 25.9?59.1% of the whole rock. There is a positive relationship between density functional theory (DFT) pore volumes and the content of clay minerals. DFT method is appropriate for all types of the pore, especially micropores. The mesopores (2?50 nm) dominated in the shale samples, while the organic matter only hosted a small amount of micropores ( 2 nm). In the study, it?s shown that the development of mesopores is controlled by the smectite-illite conversion. The mesopores volume and surface areas will decrease with the smectite layers converting into illite. However, the marcopores ( 50 nm) can increase with the formation of illite layers, caused by the minerals volume shrinks and overpressure. The smectite-to-illite conversion is favorable for the pore network under the moderate thermal maturity. The synchronous smectite-to-illite conversion and hydrocarbon generation is a critical mechanism for the formation and preservation of pores in the lacustrine shale.