1. Objectives and implications 2. Geological setting of ... · Fig. 1 Namibe Basin geographic...
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South Atlantic Research Manchester E-mail: [email protected] Thanks to NERC CDT in Oil and Gas for funding this project. BP and Equinor are also thanked for providing additional funding, samples and logistic support. Reconstructing fluid circulation pathways in volcanically influenced settings: a case study from the Namibe Basin (Angola) Fiordalisi Edoardo 1 , do Couto Ramos Pereira, G. 1 , Rochelle-Bates, N. 1 , Marchegiano, M. 2 , John, C. 2 , Dixon, R. 3 , Sharp, I. 4 , Schröder, S. 1 1: University of Manchester; 2: Imperial College London; 3: BP Exploration; 4: Equinor, Exploration Research and Technology Fig. 4 Depositional model, with creation of slope terraced morphologies in a subaerial environment (stage 1), while vent pinnacle morphologies are formed following lake level rise (stage 2). 1. Objectives and implications 3. Facies and depositional model • Carbonate spring mound systems mostly occurring along faults, characterised by distinctive travertine vent & slope facies and lacustrine deposits (Fig. 3). • Mixed subaerial/lacustrine environment, possibly evolving into a fully lacustrine setting (Fig. 4). Fig. 3 Continental carbonate occurrence and facies. A) Multiple spring mound systems (white arrows) developing at various distances from the main fault; B) Vent/slope transition; C) Vent pinnacle morphologies; D) Coquinas within lacustrine facies. Vents Slope Creating a depositional and diagenetic model for the continental spring carbonates in the Namibe Basin in order to provide a better understanding of plumbing system geometries, fluid sources, fluid interaction with surrounding sediments and mechanisms leading to carbonate precipitation in volcanically influenced settings. 2. Geological setting of the Namibe Basin • Developed as part of the South Atlantic rifting (Fig.1). • Overall syn-rift to sag continental/lacustrine setting vs. post-rift marine setting (Fig. 2). • Non-marine carbonate setting temporarily re- established in the post-rift after renewed tectonism and magmatism (study interval). Fig. 1 Namibe Basin geographic location (red box). 10° E 15° E 20° E 25° E Angola Namibia Zambia Democratic Republic of Congo Botswana 5° S 10° S 15° S 20° S + + Basement Volcanics Conglomerates Sandstone s Mudstones Evaporites Carbonates Marine fauna Study interval Fig. 2 Namibe Basin onshore stratigraphy Syn-Rift Rift sequence Precambr. Neocom. Aptian Albian Cen./Tur. Coniac./ Sant. Camp. Maastr. Pre-Rift Sag Phase Post- Rift + + + + + + + + Age Sedimentary Log Lacustrine carbonates Travertine Lacustrine marls Rippled sand Conglomerates Volcanics 200 m 30 m 4. Fluid sources and fluid evolution 5. Conclusions -8 -7 -6 -5 -4 -3 -2 -1 0 1 2 3 -6 -5 -4 -3 -2 -1 0 1 2 δ13 C ‰PDB δ18 O ‰PDB 0,702 0,704 0,706 0,708 0,71 0,712 0,714 0,716 -7 -6 -5 -4 -3 -2 -1 0 1 2 δ18 O ‰PDB 87 Sr/ 86 Sr • Pervasive matrix dolomitisation (fabric preserving) and dolomite cement overgrowths (Fig. 5). • δ 13 C PDB (Fig. 6A) reflect infiltrated groundwater undergoing degassing at surface. • δ 18 O PDB (Fig. 6A) reflect temperature decrease away from vent facies and meteoric/slightly saline waters ( 47 temperatures between 55 and 32 C and δ 18 O SMOW between -1.4 and 2.6). • 87 Sr/ 86 Sr suggest fluid circulation through surrounding magmatic deposits, which could have acted as Mg source for dolomitisation (Fig. 6B). • Intense fracturing and silicification postdating dolomitisation (Fig. 7). FI on silica suggest temperatures between 70 and 260 C, which might relate to a later stronger magmatic pulse. A B C D 6. Acknowledgements Fig. 6 Plots of dolomite matrix C/O stable isotopes (A) and Sr isotopes (B) B C 0.00 2.86 D 0.00 0.79 E Fig. 5 Dolomitised matrix and dolomite cement. A, B) Transmitted light; C) CL; D, E) EPMA maps of Sr (D) and Mn (E) wt.% concentrations. A 500 μm Fig. 7 Silica occurring as matrix replacement (A) and cement in fractures (B). A 500 μm B 500 μm Degassing Temperature decrease • Plumbing system mostly fed by infiltrated and heated groundwater. • Faults represent the main conduits for upward fluid circulation. • Depositional and diagenetic fluids probably exploited the same plumbing system. • Fluid circulation was probably fairly local. Stage 1 Stage 2 Volcanics Conglomerates Spring carbonates Vent Slope Lacustrine Coniacian/Santonian seawater Early Cretaceous volcanics (87Sr/86Sr) Late Cretaceous volcanics (87Sr/86Sr) A B
Transcript of 1. Objectives and implications 2. Geological setting of ... · Fig. 1 Namibe Basin geographic...
South Atlantic
Research
Manchester E-mail: [email protected]
Thanks to NERC CDT in Oil and Gas for funding this project.
BP and Equinor are also thanked for providing additional funding, samples and logistic support.
Reconstructing fluid circulation pathways in volcanically influenced settings: a case study from the Namibe Basin (Angola)Fiordalisi Edoardo1, do Couto Ramos Pereira, G.1, Rochelle-Bates, N.1, Marchegiano, M.2, John, C.2, Dixon, R.3, Sharp, I.4, Schröder, S.1
1: University of Manchester; 2: Imperial College London; 3: BP Exploration; 4: Equinor, Exploration Research and Technology
Fig. 4 Depositional model, with creation of slopeterraced morphologies in a subaerial environment(stage 1), while vent pinnacle morphologies areformed following lake level rise (stage 2).
1. Objectives and implications
3. Facies and depositional model• Carbonate spring mound systems mostly occurring along faults, characterised by distinctive
travertine vent & slope facies and lacustrine deposits (Fig. 3).
• Mixed subaerial/lacustrine environment, possibly evolving into a fully lacustrine setting (Fig. 4).
Fig. 3 Continental carbonate occurrence and facies. A) Multiple spring mound systems (whitearrows) developing at various distances from the main fault; B) Vent/slope transition; C) Ventpinnacle morphologies; D) Coquinas within lacustrine facies.
Vents
Slope
Creating a depositional and diagenetic model for the continental spring carbonates in the Namibe Basin in order to provide a better understanding of plumbing system geometries, fluid sources, fluid interaction with surrounding sediments andmechanisms leading to carbonate precipitation in volcanically influenced settings.
2. Geological setting of the Namibe Basin
• Developed as part of the South Atlantic rifting(Fig.1).
• Overall syn-rift to sag continental/lacustrinesetting vs. post-rift marine setting (Fig. 2).
• Non-marine carbonate setting temporarily re-established in the post-rift after renewedtectonism and magmatism (study interval).
Fig. 1 Namibe Basin geographiclocation (red box).
10° E 15° E 20° E 25° E
Angola
Namibia
Zambia
Democratic Republic of
Congo
Botswana
5° S
10° S
15° S
20° S ++ Basement
Volcanics
Conglomerates
Sandstones
Mudstones
Evaporites
Carbonates
Marine fauna
Study interval
Fig. 2 Namibe Basin onshorestratigraphy
Syn-Rift
Rift sequence
Precambr.
Neocom.
Aptian
Albian
Cen./Tur.
Coniac./Sant.
Camp.
Maastr.
Pre-Rift
Sag Phase
Post-Rift
+
++
+
++
+
+
Age Sedimentary Log
Lacustrine carbonates
Travertine
Lacustrine marls
Rippled sand
Conglomerates
Volcanics
200 m
30
m
4. Fluid sources and fluid evolution 5. Conclusions
-8
-7
-6
-5
-4
-3
-2
-1
0
1
2
3
-6 -5 -4 -3 -2 -1 0 1 2
δ1
3 C
‰P
DB
δ18 O ‰PDB
0,702
0,704
0,706
0,708
0,71
0,712
0,714
0,716
-7 -6 -5 -4 -3 -2 -1 0 1 2
δ18 O ‰PDB
87Sr
/86Sr
• Pervasive matrix dolomitisation (fabricpreserving) and dolomite cementovergrowths (Fig. 5).
• δ13CPDB (Fig. 6A) reflect infiltratedgroundwater undergoing degassing atsurface.
• δ18OPDB (Fig. 6A) reflect temperaturedecrease away from vent facies andmeteoric/slightly saline waters (47
temperatures between 55 and 32 C andδ18OSMOW between -1.4 and 2.6).
• 87Sr/86Sr suggest fluid circulation throughsurrounding magmatic deposits, which couldhave acted as Mg source for dolomitisation(Fig. 6B).
• Intense fracturing and silicificationpostdating dolomitisation (Fig. 7). FI on silicasuggest temperatures between 70 and 260C, which might relate to a later strongermagmatic pulse.
A
B C D
6. Acknowledgements
Fig. 6 Plots of dolomite matrix C/O stable isotopes (A) and Sr isotopes (B)
B C
0.00
2.86
D
0.00
0.79
E
Fig. 5 Dolomitisedmatrix anddolomite cement.A, B) Transmittedlight; C) CL; D, E)EPMA maps of Sr(D) and Mn (E)wt.%concentrations.
A
500 μm
Fig. 7 Silica occurring as matrixreplacement (A) and cementin fractures (B).
A
500 μm
B
500 μm
De
gass
ing
Temperature decrease
• Plumbing system mostly fed byinfiltrated and heatedgroundwater.
• Faults represent the mainconduits for upward fluidcirculation.
• Depositional and diageneticfluids probably exploited thesame plumbing system.
• Fluid circulation was probablyfairly local.
Stage 1
Stage 2Volcanics
Conglomerates Spring carbonates
Vent
Slope
Lacustrine
Coniacian/Santonianseawater
Early Cretaceousvolcanics (87Sr/86Sr)
Late Cretaceousvolcanics (87Sr/86Sr)
A
B
