1. STRUCTURAL, PETROPHYSICAL AND GEOCHEMICAL INVESTIGATIONS OF BASEMENT OIL SEEPS FROM A RIFT BORDER FAULT SYSTEM : EXAMPLE OF THE ALBERT GRABEN (UGANDA) East Africa: From Research to Reserves, 15 April 2016, The Geological Society, London Bastien Walter (1), Yann Hautevelle (1), Yves Géraud (1) (1) GeoRessources, Université de Lorraine, France

2. Introduction Basement plays a major role for different kinds of resource: -Hydrocarbon resources -Geothermal energy -Water supplies (Source: BRGM ©) East Africa: From Research to Reserves, 15 April 2016, The Geological Society, London Basement reservoirs are particularly well developed in geological extensive settings 2 (Source: Hurricane ©)

3. Introduction East Africa: From Research to Reserves, 15 April 2016, The Geological Society, London 3 Many major oilfields discovered in basement reservoirs in extensive settings : Melut basin (South Soudan) ; Kahrir (Yemen) ; B ạ ch H ổ (Vietnam) ; North Sea (UK) ;... Example of the recent discoveries in the North Sea ; Prod. rates  15-20 kbpd Source Prospect area Heavily faulted block (Source: PESGB ©)

4. Introduction East Africa: From Research to Reserves, 15 April 2016, The Geological Society, London 4 (From Le Garzic et al., 2011) Two processes can develop reservoir capacity in the basement: - Fracturing  multi-scale contribution - Alteration Multi-scale model of fault network at regional scale (From Choi et al., 2015) Infilled fractures in crystalline basement Multi-compartments fault zone model

5. Introduction East Africa: From Research to Reserves, 15 April 2016, The Geological Society, London 5 Two processes can develop reservoir capacity in the basement: - Fracturing - Alteration (e.g. weathering, hydrothermal) Good porosity weathered granite (From Du Hung & Van Le, 2004) Example of fractured granite involved in a HC basement reservoir in the Cuu Long basin, Vietnam: (From sandatlas.org) Hydrothermal alteration and epidote mineralization

6. East Africa: From Research to Reserves, 15 April 2016, The Geological Society, London 6 (From Ring, 2008)  EARS : High hydrocarbon potential  ARS : Northern tip of the western branch of the EARS  Active rift system developing since Upper Miocene through various precambrian metamorphic gneiss, along a former proterozoic N-S to NE-SW orogenic suture  In early 2010, the Ugandan energy ministry announced discoveries of more than 6 billion bbl of oil and 100 million m 3 of gas Case study : EARS - Uganda - The Albertine rift system ( ARS ) (From Chorowicz, 2005) (From Tullow Oil)

7. East Africa: From Research to Reserves, 15 April 2016, The Geological Society, London 7  Current fluid circulation zones (HC & hydrothermal fluids) within the basement along the main rift border fault (From Ring, 2008) Case study : EARS - Uganda - The Albertine rift system ( ARS )  Atypical symmetrical basin morphology  Only the basement footwall is visible along a big fault scarp, over 50 km long (Modified from Karp et al., 2012)

8. East Africa: From Research to Reserves, 15 April 2016, The Geological Society, London 8 Main aims:  Characterization of the fault zone architecture and of its reservoir properties Identification of the key controlling factors (e.g. lithology, structural inheritance, …)  Structural and geochemical investigations of recent fluid circulations through the faulted basement Structural location of the circulations zones ? Sources ? Outlook : Analogy with other structures ? Implications for other case study ? ? (Modified from Karp et al., 2012) Objectives

9. East Africa: From Research to Reserves, 15 April 2016, The Geological Society, London 9  Analysis of a set of outcrops located along the scarp of the main rift border fault 1. The fault-related basement reservoir – geometry & properties

10. East Africa: From Research to Reserves, 15 April 2016, The Geological Society, London 10 Total measured basement fractures  Various basement lithology (granitoids, metased. rocks, dykes, …)  Complex fracture networks 1. The fault-related basement reservoir – geometry & properties Fracture network analysis

11. East Africa: From Research to Reserves, 15 April 2016, The Geological Society, London  Comparison between rock fabric data and satellite lineaments  Co-axiality between the schistosity / foliation of the Proterozoic gneissic basement and the Albertine Rift axis and associated main structures.  Tends to confirm the effect of the structural inheritance of the basement on the development of the ARS  Dominant NE-SW trending basement rock fabric (From Ring, 2008) Structural lineament sampling ; 1:25000 scale  Dominant NE-SW trending basement lineament set 1. The fault-related basement reservoir – geometry & properties Structural inheritance on the development of the ARS 11

12. East Africa: From Research to Reserves, 15 April 2016, The Geological Society, London Porosity (%)Permeability (m 2 ) P-wave velocities (m/s) Fault core (7 samples) min. : 0,55 max. : 1,15 min. : 1,14.10 -16 max. : 4,96.10 -16 min. : 5438 max. : 6060 Damaged zone (10 samples) min. : 0,68 max. : 3,41 min. : 5,45.10 -17 max. : 3,54.10 -12 min. : 3657 max. : 5625 Protolith (12 samples) min. : 1,35 max. : 2,63 min. : 3,02.10 -17 max. : 5,75.10 -16 min. : 3529 max. : 4802 CATACLASIS  Same fault zone pattern (3 compartments) identified all along the border faults Lab. petrophysical measurements : -Fault core : low matrix poro., low k  fluid barrier -Damaged zone : porosity and k increase with cracking  fluid transfer zone 1. The fault-related basement reservoir – geometry & properties Petrophysical characterization of the different fault zone compartments 12

13. East Africa: From Research to Reserves, 15 April 2016, The Geological Society, London Fault core high strain (cataclasis) erased basement pre-existing structures Fracture density is mainly controlled by the basement lithology and the structural inheritance. Relationship with distance to the fault is secondary 1. The fault-related basement reservoir – geometry & properties Structural characterization of the different fault zone compartments 13

14. East Africa: From Research to Reserves, 15 April 2016, The Geological Society, London  The multi-compartments fault-related basement reservoir : - Protolith : highly variable oriented-fracture sets, mainly NE-SW oriented, resulting from pre-rifting events. - Inherited structures erased by the major syn- rift strain along the main fault. - Fracture density is strongly controlled by the lithology - Fault core : fluid barrier zone - Damaged zone : potential high transfer and storage zone 1. The fault-related basement reservoir – geometry & properties Synthesis 14

15. East Africa: From Research to Reserves, 15 April 2016, The Geological Society, London Kibiro hot springs (From Natukunda, 2010) Kabyosi oil seep 15 2. Investigations of fluid circulation zones through the faulted basement

16. East Africa: From Research to Reserves, 15 April 2016, The Geological Society, London  Particular structural location of the circulation zones : Intersection between the main rift border fault and other secondary oblique structures 16 2. Investigations of fluid circulation zones through the faulted basement Specific structural location of HC & geothermal circulation zones Map of the Kibiro geothermal site (From Natukunda, 2010) Kabyosi HC seep area Kibiro Kabyosi 2km  Valid observation at different scales Structural lineament sampling ; 1:25000 scale

17. East Africa: From Research to Reserves, 15 April 2016, The Geological Society, London Fault breccia saturated with HC  HC seeps in basement fault zones (Late - current circulation phases) Intersection between ENE- and NNW to NNE fault zones. ~ 10m thick frac. corridors surrounding cataclastic bands, soaked with HC At the fracture/crack scale, HC appear to percolate through all the orientations (no apparent preferential orientation of percolation) 17 2. Investigations of fluid circulation zones through the faulted basement Example of the Kabyosi HC seeps Fracture corridor filled with HC

18. East Africa: From Research to Reserves, 15 April 2016, The Geological Society, London Gas chromatography-mass spectrometry analysis: 18 2. Investigations of fluid circulation zones through the faulted basement Example of the Kabyosi HC seeps UCM Hopane (biomarkers for bacterial activity) retention time (tR) intensity Pyrite (UCM : Unresolved complex mixture) Polyphased sulfur mineralization with sulfur isotopic analysis result  Standard compounds of crude oil absent (e.g. n- alkanes) ; presence of hopane highlights an intense bacterial activity  Initial molecular signatures hidden by BSR during percolation through the fault network Lacustrine source ?  Regeneration of biomarkers with pyrolysis ? highly variable δ 34S

19. East Africa: From Research to Reserves, 15 April 2016, The Geological Society, London Hypothesis : lacustrine sources for fluid circulations through the faulted basement  LT late hydrothermal carbonate mineralization (Post-max. deformation)  Temperature from δ 18O and FI micro-thermometry: 70-150°C  From δ 13C analysis, lacustrine biogenic source of carbon ? 19 2. Investigations of fluid circulation zones through the faulted basement Information from other rift-related fluid circulations Calcite veins crosscutting fault cataclasis Fluid inclusions

20. Synthesis - Conclusion East Africa: From Research to Reserves, 15 April 2016, The Geological Society, London 20  Three key elements identified along the Albert rift eastern basement border

21. Synthesis - Conclusion East Africa: From Research to Reserves, 15 April 2016, The Geological Society, London Petrological and structural inheritance of the basement Multi-compartment fault zone Fault intersections 21  Strong control of the basement petrological and structural inheritance on the development of fault & fracture networks

22. Synthesis - Conclusion East Africa: From Research to Reserves, 15 April 2016, The Geological Society, London Petrological and structural inheritance of the basement Multi-compartment fault zone Fault intersections 200-300m Damage zone properties StorageFracture-dominated porosity : 2-5% Transferk ≤ 1D (stress-dependent) 22 Architecture of the main rift faults:  Fault core : fluid barrier zone  Damaged zone : potential high transfer zone

23. Synthesis - Conclusion East Africa: From Research to Reserves, 15 April 2016, The Geological Society, London Petrological and structural inheritance of the basement Multi-compartment fault zone Fault intersections 23  Major role for fluid circulation between the different structural blocks

24. ? Synthesis - Conclusion East Africa: From Research to Reserves, 15 April 2016, The Geological Society, London  Intersections of main NE-SW rift faults & 2 ndary oblique structures allow connection between sedimentary and basement structural blocks for fluid circulations 24 (Modified from Karp et al., 2012) Basement high storage zone ?  Weathered basement ? Fluid path ?

25. Thank you for your attention East Africa: From Research to Reserves, 15 April 2016, The Geological Society, London