Spectral Gamma Ray (SGR) Analysis of Sedimentary Environments in Early Continental Rift Basins: Implications for Petroleum Exploration
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Alshawib, Almahdi M.M, Spectral Gamma Ray (SGR) Analysis of Sedimentary Environments in Early Continental Rift Basins: Implications for Petroleum Exploration, Trinity College Dublin.School of Natural Sciences, 2022Download Item:
Abstract:
Continental rift basins are structures of global geoscientific significance, as they hold over 30% of the world?s giant oil fields and serve as modern day examples of unique biodiversity sites, typically operating as self-contained tectonic systems that generate their own climate, hydrology, and sedimentation. However, many ancient rift basins lie offshore in the subsurface. The depth of water and associated high costs of drilling are among the greatest challenges facing hydrocarbon exploration in these offshore rift basins. Further, their sedimentological modelling must rely solely on limited geophysical and well log data. In order to test whether well log data could, in the future, yield a more detailed interpretation of original sedimentary depositional environments over time, this study conducted detailed examination of high-resolution onshore analogue examples of continental syn-rift sedimentary lithofacies architecture and depositional styles for their spectral gamma-ray (SGR) response.
Two continental rift basins selected for this study, the Lusitanian Basin, Portugal and the Lake Albert Rift Basin, Uganda, are ideal choices as onshore analogues. These two rift basins exhibit all of the classic tectonic, sedimentological, geomorphological, and hydrological features found in the early stages of continental rifting. Moreover, these basins display high-resolution sedimentary fluvial, deltaic, and lacustrine/marine deposits and their SGR responses indicate that they developed during early- to mid-continental rifting. A third basin, the Clare Basin in Ireland, was also studied for its representation of varied transitional sediments.
In this context, the key framework of this project was the application of the handheld SGR instrument, a cost-effective technique of identifying depositional, pedogenic, and hydrological conditions during fluvio-deltaic to marginal lacustrine/marine sedimentation and early burial palaeo-processes.
Two methods of SGR interpretation were developed for this thesis to help interpret the datasets in more detail: the SGR Percentage KUTh ternary plots (SGR Ternary plot) and Rift Basin Index (RBI). The SGR Ternary plot method, originally introduced from earlier research, was applied in this research as a means of calibrating the method for future applications. The RBI method is newly proposed by this research, developed as a means of relying on proportional potassium(K)?uranium(U)?thorium(Th) relationships in sediments to differentiate between terrestrial, transitional, and lacustrine/marine depositional environments. The respective calibration and application of these two methods helped not only to determine the depositional environments of outcrops from the different basins, but were also used to analyse SGR data from a subsurface well in the Lake Albert Rift Basin (Til-1 well), thereby enabling markedly detailed interpretation of critical palaeo-environmental conditions during sediment deposition and early burial for that basin.
From this study, the application of SGR has proven effective for the interpretation of data from both the field outcrops and the subsurface well, indicating a correlation between the SGR results and the interpolated depositional environments. Specifically, from the results, high K concentration was found to reflect coarse fluvial sediments, high U concentration suggests either the presence of organic matter or a deep aqueous environment such as lacustrine/marine, and high Th concentration indicates clay sediments, implying a deep marine depositional environment.
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Irish Centre for Research in Applied Geosciences (iCRAG)
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https://tcdlocalportal.tcd.ie/pls/EnterApex/f?p=800:71:0::::P71_USERNAME:ALSHAWIADescription:
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Author: Alshawib, Almahdi M.M
Advisor:
Nicholas, ChristopherPublisher:
Trinity College Dublin. School of Natural Sciences. Discipline of GeologyType of material:
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