Geomechanical properties of the Permian black shales in the southern main Karoo Basin: lessons from compositional and petrophysical studies

IF 1.2 4区 地球科学 Q2 GEOLOGY
C. Geel, S. Nolte, E. M. Bordy
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引用次数: 2

Abstract

Permian black shales from the lower Ecca Group of the southern main Karoo Basin (MKB) have a total organic carbon (TOC) of up to ~5 wt% and have been considered primary targets for a potential shale gas exploration in South Africa. This study investigates the influence of shale composition, porosity, pressure (P) and temperatures (T) on their geomechanical properties such as compressive strength and elastic moduli. On average, these lower Ecca Group shales contain a high proportion, ~50 to 70 vol%, of mechanically strong minerals (e.g., quartz, feldspar, pyrite), ~30 to 50 vol% of weak minerals (e.g., clay minerals, organic matter) and ~0 to 50 vol% of intermediate minerals (e.g., carbonates), which have highly variable mechanical strength. Constant strain rate, triaxial deformation tests (at T ≤100°C; P ≤50 MPa) were performed using a Paterson-type high pressure instrument. Results showed that the Prince Albert Formation is the strongest and most brittle unit in the lower Ecca Group in the southern MKB followed by the Collingham and then the Whitehill Formation. Compressive strength and Young’s moduli (E) increase with increasing hard mineral content and decrease with increasing mechanically weak minerals and porosity. On comparison with some international shales, for which compositional and geomechanical data were measured using similar techniques, the lower Ecca Group shales are found to be geomechanically stronger and more brittle. This research provides the foundation for future geomechanical and petrophysical investigations of these Permian Ecca black shales and their assessment as potential unconventional hydrocarbon reservoirs in the MKB.
卡鲁盆地南部二叠纪黑色页岩的地质力学性质:成分和岩石物理研究的经验教训
来自南部主要Karoo盆地(MKB)下Ecca群的二叠纪黑色页岩的总有机碳(TOC)高达~5 wt%,被认为是南非页岩气勘探的主要目标。该研究考察了页岩成分、孔隙度、压力(P)和温度(T)对其抗压强度和弹性模量等地质力学特性的影响。平均而言,这些较低的Ecca组页岩含有较高比例的机械强度强矿物(如石英、长石、黄铁矿),约占50 ~ 70 vol%的弱矿物(如粘土矿物、有机质)和约占0 ~50 vol%的中间矿物(如碳酸盐),这些矿物的机械强度变化很大。恒应变速率、三轴变形试验(T≤100℃;P≤50 MPa),采用帕特森式高压仪进行。结果表明,在MKB南部的下Ecca组中,Prince Albert组是最强和最脆的单元,其次是Collingham组,然后是Whitehill组。抗压强度和杨氏模量(E)随硬矿物含量的增加而增大,随力学弱矿物和孔隙度的增加而减小。通过与国际上一些页岩进行对比,发现下Ecca组页岩在地质力学上更强、更脆。该研究为今后二叠系Ecca黑色页岩的地质力学和岩石物理研究以及作为MKB潜在非常规油气藏的评价奠定了基础。
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来源期刊
CiteScore
3.60
自引率
5.60%
发文量
18
审稿时长
>12 weeks
期刊介绍: The South African Journal of Geology publishes scientific papers, notes, stratigraphic descriptions and discussions in the broadly defined fields of geoscience that are related directly or indirectly to the geology of Africa. Contributions relevant to former supercontinental entities such as Gondwana and Rodinia are also welcome as are topical studies on any geoscience-related discipline. Review papers are welcome as long as they represent original, new syntheses. Special issues are also encouraged but terms for these must be negotiated with the Editors.
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