South Atlantic

Research

Manchester E-mail: edoardo.fiordalisi@manchester.ac.uk

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