A complex case of thermal maturity assessment in a terrigenous sedimentary system: The Northwestern Black Sea basin

IF 5.6 2区 工程技术 Q2 ENERGY & FUELS International Journal of Coal Geology Pub Date : 2024-03-19 DOI:10.1016/j.coal.2024.104496
Hamed Sanei , Oskar Hagelskjær , Henrik I. Petersen , Arka Rudra , Søren B. Nielsen , François Lorant , François Gelin
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Abstract

An integrated approach combining fluorescence spectrometry, pyrolysis geochemistry, and dispersed organic matter (DOM) maceral analysis has been used to determine the thermal maturity of Oligocene source rocks in two neighboring offshore wells within the complex deep-sea system of the Northwestern Black Sea. The samples contain predominantly immature Type II kerogen, including a mixture of marine liptinite (alginite, liptodetrinite), degraded liptinite, primary vitrinite, and reworked vitrinitic macerals. The occurrence of several vitrinite reflectance (VRo) populations in the samples is attributed to a high influx of reworked vitrinite transported to the marine basin from multiple terrestrial sources with diverse degrees of degradation. The presence of multiple VRo populations complicates the application of VRo for determining the thermal maturity of the organic matter. Further, the VRo measurements on the primary vitrinite population show only a subtle increasing trend with depth. A significantly improved thermal maturity resolution was obtained using the red to green quotient (R/G). R/G was measured on the autochthonous unicellular Tasmanites-type alginite under UV-excitation. In contrast to VRo the results show a significant correlation between increasing R/G values with increasing depth. This indicates that the R/G quotient is a better maturity proxy for the studied low maturity marine system with a large influx of sediments. The beginning of the downward declining trend in the temporal variation of liptinite is regarded as the onset of catagenesis. This occurs at a burial depth of 2.5 km and a R/G value of 0.50, corresponding to a VRo equivalent (VRo Eq) of 0.44%. At a burial depth of 3.2 km and a R/G value of 0.57 (VRo Eq = 0.57%), the onset of the oil window is marked by an increase in the amount of solid bitumen (initial oil solid bitumen). The observed alterations in maceral composition with burial depth align with the thermal maturity of the wells. However, the data suggest that the thermal evolution of the organic matter is out of sync with the surrounding formation temperature, possibly due to rapid burial induced by a high sedimentation rate.

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陆相沉积系统热成熟度评估的复杂案例:黑海西北盆地
在黑海西北部复杂的深海系统中,采用荧光光谱仪、热解地球化学和分散有机物(DOM)宏观分析相结合的综合方法,确定了两口相邻海上油井中新世源岩的热成熟度。这些样本主要含有不成熟的第二类角质,包括海洋褐铁矿(藻褐铁矿、褐铁矿)、降解褐铁矿、原生褐铁矿和再造褐铁矿大分子的混合物。样本中出现多个玻璃光泽反射率(VR)种群的原因是,从多个陆地来源运输到海洋盆地的再造玻璃光泽大量涌入,而这些玻璃光泽的降解程度各不相同。由于存在多个椭偏荧光团,应用椭偏荧光团来确定有机物的热成熟度变得更加复杂。此外,对原生钙钛矿群的 VRo 测量结果仅显示出随着深度的增加而微妙增加的趋势。利用红绿商(R/G)可以大大提高热成熟度的分辨率。R/G 是在紫外线激发下对自生单细胞海藻进行测量的。与 VR 不同的是,结果显示 R/G 值的增加与深度的增加之间存在显著的相关性。这表明,对于所研究的大量沉积物涌入的低成熟度海洋系统来说,R/G 商数是一个更好的成熟度代表。锂辉石的时间变化开始呈下降趋势,这被认为是催化作用的开始。这发生在埋藏深度为 2.5 千米、R/G 值为 0.50、VR 当量为 0.44% 时。在埋藏深度为 3.2 千米、R/G 值为 0.57(VR = 0.57%)时,石油窗口的出现标志着固体沥青量(初始石油固体沥青)的增加。随着埋藏深度的增加,观察到的宏观组成变化与油井的热成熟度相一致。然而,数据表明,有机质的热演化与周围地层温度不同步,这可能是由于高沉积率引起的快速埋藏造成的。
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来源期刊
International Journal of Coal Geology
International Journal of Coal Geology 工程技术-地球科学综合
CiteScore
11.00
自引率
14.30%
发文量
145
审稿时长
38 days
期刊介绍: The International Journal of Coal Geology deals with fundamental and applied aspects of the geology and petrology of coal, oil/gas source rocks and shale gas resources. The journal aims to advance the exploration, exploitation and utilization of these resources, and to stimulate environmental awareness as well as advancement of engineering for effective resource management.
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