Depositional environmental controls on mechanical stratigraphy of Barakar Shales in Rajmahal Basin, India

IF 5.6 2区 工程技术 Q2 ENERGY & FUELS International Journal of Coal Geology Pub Date : 2024-03-07 DOI:10.1016/j.coal.2024.104477
Chinmay Sethi , Bodhisatwa Hazra , Mehdi Ostadhassan , Hem Bahadur Motra , Arpan Dutta , J.K. Pandey , Santosh Kumar
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Abstract

Understanding mechanical behaviour of shale is essential for efficient shale gas extraction, which can vary in different depositional settings. The impact of sedimentary environment on shale characteristics, such as mineralogical composition, total organic carbon content (TOC), and petrophysical properties, has been extensively researched. However, its influence on shale mechanical properties, especially in defining mechanical stratigraphy for targeting specific fracturing intervals, remains less explored. In this study, the influence of depositional environment on the mechanical properties of shale samples from the Rajmahal Basin is evaluated. Tensile strength of the samples was measured by the Brazilian splitting tensile strength and the brittleness index was calculated as a measure of mechanical properties. In addition, inductively coupled plasma optical emission spectroscopy (ICP-OES), X-ray fluorescence spectroscopy (XRF), Rock-Eval 6, and X-ray diffraction (XRD) analysis were carried out to assess geochemical characteristics of the samples from different perspectives. The results revealed that such geochemical variations that are generally controlled by the depositional environment, would impact the mechanical properties of the samples. Based on major and trace elements proxies, the depositional environment was determined to be passive continental margin, with hot and humid paleoclimatic conditions and freshwater anoxic settings. Tensile strength and brittleness index of the shale samples was observed to vary between 0.93 and 4.12 MPa and 0.71 to 3.40, respectively, while samples with the TOC exceeding 15 wt% had a strong negative correlation with tensile strength, as reasonably expected, due to weakening impact of the sedimentary organic matter on the shale matrix. Tensile strength and brittleness index correlated positively with clay mineral content, particularly their type, but negatively with the quartz content. Furthermore, samples abundant in biogenic silica exhibited reduced brittleness compared to those with lithogenic silica. Nevertheless, the variation in mechanical properties with burial depth was not substantial, and the examination of stress-strain curves indicated an overall brittle nature of the layer where the samples were retrieved from. Overall, achieving more robust conclusions regarding mechanical stratigraphy within the studied section of the Rajmahal Basin, would necessitate additional vertical sampling.

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沉积环境对印度拉杰马哈尔盆地巴拉卡尔页岩机械地层学的影响
了解页岩的机械特性对于高效提取页岩气至关重要,而页岩气在不同的沉积环境中会有不同的表现。沉积环境对页岩特征(如矿物成分、总有机碳含量 (TOC) 和岩石物理特性)的影响已得到广泛研究。然而,其对页岩力学性质的影响,尤其是在确定力学地层以锁定特定压裂区间方面的影响,仍然鲜有研究。本研究评估了沉积环境对 Rajmahal 盆地页岩样本力学性质的影响。通过巴西劈裂拉伸强度测量了样本的拉伸强度,并计算了脆性指数作为力学性能的衡量标准。此外,还进行了电感耦合等离子体光发射光谱(ICP-OES)、X 射线荧光光谱(XRF)、Rock-Eval 6 和 X 射线衍射(XRD)分析,以从不同角度评估样品的地球化学特征。结果表明,这些地球化学变化通常受沉积环境的控制,会影响样品的机械性能。根据主要元素和微量元素代用指标,沉积环境被确定为被动大陆边缘,具有湿热的古气候条件和淡水缺氧环境。据观察,页岩样品的抗拉强度和脆性指数分别在 0.93 至 4.12 兆帕和 0.71 至 3.40 之间变化,而总有机碳含量超过 15 wt%的样品与抗拉强度呈强烈的负相关,这是沉积有机质对页岩基质的削弱作用所致。抗拉强度和脆性指数与粘土矿物含量,尤其是粘土矿物的类型呈正相关,但与石英含量呈负相关。此外,与含石英的样本相比,富含生物硅的样本脆性更低。尽管如此,机械性能随埋藏深度的变化并不大,应力-应变曲线的研究表明,样本取自的地层总体上是脆性的。总体而言,要在拉杰马哈盆地的研究区段内就机械地层学得出更可靠的结论,还需要进行更多的垂直取样。
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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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