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Parameters Controlling Distribution of Diagenetic Alterations within Fluvial and Shallow Marine Sandstone Reservoirs: Evidence from the Libyan Basins
Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Mineralogy Petrology and Tectonics.
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.
Series
Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology, ISSN 1651-6214 ; 1375
Keyword [en]
Diagenesis, Structural setting, Depositional facies, Basin thermal history, Ther- mal/Hot basinal brines, Fluid inclusions, Raman Spectrometry, Stylolites, Hydro-carbon migration
National Category
Geology
Research subject
Earth Science with specialization in Mineral Chemistry, Petrology and Tectonics
Identifiers
URN: urn:nbn:se:uu:diva-284581ISBN: 978-91-554-9581-7 (print)OAI: oai:DiVA.org:uu-284581DiVA: diva2:920642
Public defence
2016-06-14, Hambergsalen, Villavägen 16, Uppsala, 14:00 (English)
Opponent
Supervisors
Note

Errata: Felaktigt disputationsdatum på spikbladet.

Available from: 2016-05-20 Created: 2016-04-18 Last updated: 2016-06-15Bibliographically approved
List of papers
1. Impact of structural setting on diagenesis of fluvial and tidal sandstones: The Bahi Formation, Upper Cretaceous, NW Sirt Basin, North Central Libya
Open this publication in new window or tab >>Impact of structural setting on diagenesis of fluvial and tidal sandstones: The Bahi Formation, Upper Cretaceous, NW Sirt Basin, North Central Libya
2012 (English)In: Marine and Petroleum Geology, ISSN 0264-8172, E-ISSN 1873-4073, Vol. 38, no 1, 211-231 p.Article in journal (Refereed) Published
Abstract [en]

This petrographic, mineral chemical, fluid inclusion, and stable isotopic study shows that the distribution of diagenetic modifications and their influence on reservoir quality and heterogeneity in tidal and fluvial sandstones of the Upper Cretaceous Bahi Sandstones in the rift Sirt Basin, NW Libya varies systematically along a series of closely-spaced, dominantly normal faults between the basin margin and more basinward-located areas. Shallow eogenetic modifications resulting from the percolation of meteoric waters, include infiltration of grain coating clays, kaolinitization of detrital silicates, and cementation by dolomite and K-feldspar overgrowths. Mesogenetic alterations (>70 °C, >2 km) include feldspar albitization, illitization of infiltrated clay and kaolinite, conversion of kaolinite into dickite, and cementation by quartz overgrowths (T h 112 °C-134 °C), barite (T h 145 °C-158 °C) and Fe-carbonates. The restriction of barite and Fe-carbonate cements to the basinward-located sandstones suggests formation by hydrothermal fluids along the faults. Extensive feldspar dissolution and formation of moldic pores in sandstones from the basin margin were probably caused by deep percolation of meteoric waters. Results from this study regarding the structural control on the spatial distribution of diagenetic alterations have implications for constraining the flux of pore fluids and, by extension, reservoir quality in analogous epicratonic rift basins.

Keyword
Diagenesis, Hydrothermal fluids, Libya, Reservoir quality, Sandstone, Sirt Basin, Structural setting, Barite, Cementing (shafts), Cements, Chemical modification, Feldspar, Kaolinite, Mineralogy, Quartz, Sedimentology, Silicates, Solvents, Structural dynamics, Cretaceous, hydrocarbon reservoir, hydrothermal fluid, structural control, Libyan Arab Jamahiriya, Sirte Basin
National Category
Earth and Related Environmental Sciences
Research subject
Earth Science with specialization in Mineral Chemistry, Petrology and Tectonics
Identifiers
urn:nbn:se:uu:diva-184894 (URN)10.1016/j.marpetgeo.2011.05.006 (DOI)000310668100016 ()
Available from: 2012-11-20 Created: 2012-11-15 Last updated: 2017-12-07
2. Open versus closed mesogenetic systems in Cretaceous fluvial and tidal sandstones, Sirt Basin, Libya
Open this publication in new window or tab >>Open versus closed mesogenetic systems in Cretaceous fluvial and tidal sandstones, Sirt Basin, Libya
2014 (English)In: GeoArabia, ISSN 1025-6059, ISSN ISSN 1025-6059, Vol. 19, no 4, 113-140 p.Article in journal (Refereed) Published
Keyword
Open versus closed mesogenetic systems, Flux of hot baisnal fluids, Cretaceous sandstones, Sirt Basin, Libya
National Category
Geology
Research subject
Earth Science with specialization in Mineral Chemistry, Petrology and Tectonics
Identifiers
urn:nbn:se:uu:diva-280985 (URN)
Available from: 2016-03-16 Created: 2016-03-16 Last updated: 2017-11-30
3. Impact of depositional facies on the distribution of diagenetic alterations in the Devonian shoreface sandstone reservoirs, Southern Ghadamis Basin, Libya
Open this publication in new window or tab >>Impact of depositional facies on the distribution of diagenetic alterations in the Devonian shoreface sandstone reservoirs, Southern Ghadamis Basin, Libya
2015 (English)In: Sedimentary Geology, ISSN 0037-0738, E-ISSN 1879-0968, Vol. 329, 62-80 p.Article in journal (Refereed) Published
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.

Keyword
Shoreface sandstones, Diagenesis, Reservoir quality, Ghadamis Basin, Libya
National Category
Geosciences, Multidisciplinary
Research subject
Earth Science with specialization in Mineral Chemistry, Petrology and Tectonics
Identifiers
urn:nbn:se:uu:diva-270638 (URN)10.1016/j.sedgeo.2015.09.003 (DOI)000364883200006 ()
Available from: 2016-01-01 Created: 2016-01-01 Last updated: 2017-12-01Bibliographically approved
4. Quartz and Fe-dolomite cements as records of shift in formation-water chemistry: Devonian shoreface sandstones, the Ghadamis Basin, Libya
Open this publication in new window or tab >>Quartz and Fe-dolomite cements as records of shift in formation-water chemistry: Devonian shoreface sandstones, the Ghadamis Basin, Libya
Show others...
(English)In: Journal of Petroleum Geology, ISSN 0141-6421, E-ISSN 1747-5457Article in journal (Refereed) Submitted
Keyword
Shoreface sandstones, Ferroan dolomite, Fuid inclusion, Raman Spectometry, Hydrocarbon migration, Ghdamis Basin, Libya
National Category
Geology
Research subject
Earth Science with specialization in Mineral Chemistry, Petrology and Tectonics
Identifiers
urn:nbn:se:uu:diva-281016 (URN)
Available from: 2016-03-16 Created: 2016-03-16 Last updated: 2017-11-30

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