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Impact of depositional facies on the distribution of diagenetic alterations in the Devonian shoreface sandstone reservoirs, Southern Ghadamis Basin, Libya
Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Mineralogy Petrology and Tectonics.
Petr Inst, Dept Petr Geosci, Abu Dhabi, U Arab Emirates..
2015 (English)In: Sedimentary Geology, ISSN 0037-0738, E-ISSN 1879-0968, Vol. 329, 62-80 p.Article in journal (Refereed) PublishedText
Abstract [en]

The middle Devonian, shoreface quartz arenites (present-day burial depths 2833-2786 m) are important oil and gas reservoirs in the Ghadamis Basin, western Libya. This integrated petrographic and geochemical study aims to unravel the impact of depositional fades on distribution of diagenetic alterations and, consequently, related reservoir quality and heterogeneity of the sandstones. Eogenetic alterations include the formation of kaolinite, pseudomatrix, and pyrite. The mesogenetic alterations include cementation by quartz overgrowths, Fedolomite/ankerite, and illite, transformation of kaolinite to dickite, illitization of smectite, intergranular quartz dissolution, and stylolitization, and albitization of feldspar. The higher energy of deposition of the coarser-grained upper shoreface sandstones combined with less extensive chemical compaction and smaller amounts of quartz overgrowths account for their better primary reservoir quality compared to the finer-grained, middle-lower shoreface sandstones. The formation of kaolin in the upper and middle shoreface sandstones is attributed to a greater flux of meteoric water. More abundant quartz overgrowths in the middle and lower shoreface is attributed to a greater extent of stylolitization, which was promoted by more abundant illitic clays. This study demonstrated that linking the distribution of diagenetic alterations to depositional facies of shoreface sandstones leads to a better understanding of the impact of these alterations on the spatial and temporal variation in quality and heterogeneity of the reservoirs.

Place, publisher, year, edition, pages
2015. Vol. 329, 62-80 p.
Keyword [en]
Shoreface sandstones, Diagenesis, Reservoir quality, Ghadamis Basin, Libya
National Category
Geosciences, Multidisciplinary
Research subject
Earth Science with specialization in Mineral Chemistry, Petrology and Tectonics
URN: urn:nbn:se:uu:diva-270638DOI: 10.1016/j.sedgeo.2015.09.003ISI: 000364883200006OAI: oai:DiVA.org:uu-270638DiVA: diva2:890213
Available from: 2016-01-01 Created: 2016-01-01 Last updated: 2016-05-26Bibliographically approved
In thesis
1. Parameters Controlling Distribution of Diagenetic Alterations within Fluvial and Shallow Marine Sandstone Reservoirs: Evidence from the Libyan Basins
Open this publication in new window or tab >>Parameters Controlling Distribution of Diagenetic Alterations within Fluvial and Shallow Marine Sandstone Reservoirs: Evidence from the Libyan Basins
2016 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

This thesis demonstrates that geological setting, depositional facies, open system flux of hot basinal brines and descending of shallow waters have a strong impact on the distribution of the diagenetic alterations within continental and paralic/shallow marine sandstones which in turn control the quality and heterogeneities of the reservoirs.

Geological setting controls the mineralogical and textural maturity of sandstone, whereas depositional facies control the pore water chemistry (marine, brackish or meteoric), sedimentary texture and sand body geometry. Eogenetic alterations in the fluvial deposits are dominated by precipitation of infiltrated clays, kaolinitization of detrital silicates, whereas the shallow marine deposits are dominated by precipitation of early calcite and kaolinite. Conversely mesogenetic alterations are dominated by clay minerals transformation, quartz overgrowths and Ferroan- carbonates, barite and anhydrite. Flux of hot basinal brines is evidenced by precipitation of mesogenetic minerals that lack of internal sources (e.g. barite, anhydrite and ferroan carbonate cements), which is evidenced by: (1) restricted occurrence of these minerals in downthrown blocks. (2) The high fluid inclusion homogenization temperatures (Th) of quartz overgrowths (Th > 110-139°C), and carbonate cements (T > 80-140°C), which also have light δ18OV-PDB(-17.6‰ to -6.7‰). Flux of hot basinal brines is further evidenced by occurrence of saddle Fe-dolomite along stylolites. Fluid inclusion microthermometry further revealed a dramatic shift in pore- water chemistry from NaCl dominated brines during precipitation of quartz overgrowths to NaCl-CaCl2 dominated brines during cementation by Fe-dolomite. Presence of mixed brine (NaCl+CaCl2) systems in the fluid inclusions suggests flux of descending waters, which have circulated in the overlying carbonate-evaporite successions. The restricted occurrence of oil- filled inclusion to quartz overgrowths and methane to Fe-carbonate cements suggest migration of oil during precipitation by quartz and migration of methane during precipitation by Fe- carbonate cements. The extensive mesogenetic cements in the down thrown blocks is attributed to flux of basinal brines along deep seated faults, i.e. open system diagenesis.

Integration of fluid inclusion microthermometry, isotopes, Raman spectrometry and thermal tectonic evolution of basins are essential techniques for unraveling the evolution of basinal fluids, cementation conditions and relative timing of hydrocarbons migration.

Place, publisher, year, edition, pages
Uppsala: Acta Universitatis Upsaliensis, 2016. 62 p.
Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology, ISSN 1651-6214 ; 1375
Diagenesis, Structural setting, Depositional facies, Basin thermal history, Ther- mal/Hot basinal brines, Fluid inclusions, Raman Spectrometry, Stylolites, Hydro-carbon migration
National Category
Research subject
Earth Science with specialization in Mineral Chemistry, Petrology and Tectonics
urn:nbn:se:uu:diva-284581 (URN)978-91-554-9581-7 (ISBN)
Public defence
2016-06-14, Hambergsalen, Villavägen 16, Uppsala, 14:00 (English)

Errata: Felaktigt disputationsdatum på spikbladet.

Available from: 2016-05-20 Created: 2016-04-18 Last updated: 2016-06-15Bibliographically approved

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Khalifa, Muftah Ahmid
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