The Voltaian Basin. A New Look at its Prospectivity

Transcription

1 The Voltaian Basin A New Look at its Prospectivity James Yamoah Geoscientist GNPC ( ) 1

2 Content Project Background & Objectives Regional Neoproterozoic setting Voltaian Basin geology Surface geology Lithostratigraphy Neoproterozoic glaciations and source rocks Why missing Bambouaka Group interval from 950 to 620 Ma is important? Four possible models for missing interval at out-crop Structure Future work Provisional seismic locations Work Forward Conclusions 2

3 Voltaian Basin covers 40% of Ghana s land mass. Equivalent basins in age (Neoproterozoic) produce hydrocarbons elsewhere in the world (e.g. Siberia, Oman) Very little is known about the deeper parts of the Voltaian Basin. Exploration of this Neoproterozoic basin carries higher risk than most conventional petroleum plays because of its age and probable geological history. From an exploration viewpoint, it needs to be explored and better understood Background 3

4 Objectives Undertake studies and acquire new data to demonstrate the hydrocarbon prospectivity of the Voltaian Basin. Prove-up or provide evidence to support the presence of working petroleum system in the basin by: Seismic Well samples Geochemistry Maximise current data to produce a geological model of the basin in terms of stratigraphy, structure and petroleum geology Produce a comprehensive and organized data base and set of evaluations that can be used to justify further investments 4

5 Proterozoic Basins Craig, J, Thurow, J, Thusa, B, Whitham, A, and Abutarruma, Y,

6 Late Neoproterozoic Palaeogeography Volta Basin 6

7 Regional Structure Pan African fold-belt or Trans-Saharan mobile belt Peripheral passive margin / foreland basin on SE edge of West Africa Craton. Affected by Pan African tectonics Could position on edge of craton and close to epi-continental ocean favour the development of passive margin marine sediments in the Volta Basin? West fold-belt Dahomeyide thrust front 7

8 Voltaian Basin Geology 8

9 Volta Basin Surface Geology & Stratigraphy Extensively studied at outcrop and data available from 48 wells Possibly depositional-environment comments (fluvial, lacustrine, glacial, marginal marine, etc) Tripartite stratigraphic division for Voltaian Super-group: Bambouaka-Afram Oti-Penjari Obosum Lithofacies appear to be dominantly continental. Minor carbonates, tillites and shallow marine indications Significant missing section at outcrop between Oti-Penjari & Bambouaka = 350 Ma unconformity gap - coincides with a major regional unconformity. Could part of this missing Bambouaka Group be present in deeper basin to east? Could it contain SRs? Could some of the Oti-Penjari mudstones at outcrop become potential SRs further in the basin i.e. become more marine to the east? 9

10 Affaton s view on basin structure shown in cross section 10

11 WSW-ENE Schematic Section 350 Ma hiatus at unconformity between Oti and Bambouaka/Afram Postulated projected position and penetration of Premuase-1 well Thick Oti-Penjari SG develops in foredeep in front of developing Dahomeyide mountains in Ediacaran Oti Approx 5 km to basement Bambouaka This basin model by Affaton assumes the Ma interval is absent (eroded?) throughout basin. There is no additional stratigraphic interval in the Bambouaka assumed in the basin centre just some minor basinward thickening. 11

12 350 Ma hiatus at unconformity between Oti and Bambouaka/Afram Alternative Possibility Thick Oti-Penjari SG develops in fore-deep in front of developing Dahomeyide Mountains during Ediacaran Oti Bambouaka developed in local rift-basins, which are seen in other basins Younger Bambouaka sub-crops the base Oti unconformity and is not seen at outcrop 12

13 Same model as Affaton, thickening shown in Oti Megasequence in foreland basin but Bambouaka Ms shows relatively consistent thickness. Rb-Sr isochron dating of 660 Ma & K-Ar of 620 Ma Unconformity hiatus from circa Ma Rb-Sr isochron dating of Ma From Deynoux et al (2006) 13

14 NEOPROTEROZOIC (Ediacaran or Vendian) NEOPROTEROZOIC (Tonian or Late Ripean) ~542 Ma Base Cambrian Missing section ~620 to 970 Ma ~970 Ma (South & West area) Informal Oti-Penjari foreland sequence ~970 Ma (North & North-West area) Mudstones, tillites possible SR intervals ~620 Ma MESOPROTEROZOIC (Early Ripean) Ma Volta Basin Stratigraphy 14

15 SUMMARY STRATIGRAPHY A generally tripartite subdivision established (Affaton, Bohzcko, Shell GGS, BGS, Fugro etc) Neoproterozoic Voltaian Super-Groups Obosum Group Oti-Penjari Bambouaka and Kwahu Each group is separated from the underlying one by an unconformity Basal Oti-Penjari beds and the underlying Bambouaka/Kwahu beds represents a time hiatus,( 970 to 620 Ma) according to zircon dating LITHOLOGY Upper Voltaian: sandstones, dominantly in the northern and northwestern part of the basin Middle Voltaian: fine, shallow marine clastics, siliceous rocks, carbonates Lower Voltaian: sequence of clastics (Afram shales)

16 Why is this missing interval from 950 to 620 Ma important? Time of significant glacio-eustatic sea level rises that produce organic rich source rocks in Neoproterozoic It includes the Cryogenian period of major glaciations Snowball Earth period ( Ma) E.g. Source rocks for the Lena Tunguska Siberian oil fields It also includes the period when organic rich stromatolitic carbonates and marginal marine shales developed in Taoudenni Basin during Pre-Snowball period Atar Group ( Ma) is interval of source rocks and reservoirs in Taoudenni Basin and provides evidence of marine incursion far onto the craton thought to be because of intra-cratonic subsidence 16

17 Neoproterozoic Glaciations & Source Rocks 17

18 VB CORRELATED TO REGIONAL CHRONOSTRATIGAPHY

19 Stratigraphic period missing in Volta Basin based on outcrop geology 19

20 Viscous black bitumen in core in Oti Series sands Significant gas shows Oil shows Viscous black bitumen in Bongo Da carbonates cores Viscous black bitumen in core in Oti Series sands Bitumen in sands in core at top of Lower Series (Afram) 20

21 Kwahu & Bambouaka Groups 100m thick Abetifi-Damongo 21

22 Four possible models for missing Bambouaka section at out-crop 1. Regional uplift during Pan African Orogeny and total erosion circa 620 Ma in the Voltaian Basin Broad epeirogenic uplift and erosion that maintains relative flat beds No major angular unconformity 2. Area was prominent with non-deposition between 970 to 620 Ma 3. Section is partly eroded at basin edge but a younger interval remains in the central parts of the basin and subcrops the Oti Unconformity towards west. 4. Possible basin margin onlap and pinch-out Seismic data is the best way to evaluate this. 22

23 Structural Features Early structures affected lower interval i.e. Bambouaka/Kwahu Passive margin setting (marine, fluvial) E-W & NW-SE rift faults Horst and graben features possible NE-SW arcuate faults Syn-sedimentary faulting probable Rifting ceased by 620 Ma as did not affect Oti- Penjari Middle Series Intermittent uplift and deformation In phase 1 report, BGS speculate that there was significant uplift and erosion at about 620 Ma. Not mentioned in Phase 5 report. Mid-Late structures E-W & NE-SW pervasive faults Termination of rifting by 586 Ma and onset of compression and thrust faulting from the east Thrust front brought about foreland basin deposition of thick molasse Gravity indicates NW-SE rift basin in the north Possible E-W or ENE-WSW rift basin in south Possible basement highs associated with gravity highs 23

24 Questions remain Where are outcrop samples? At GGS? And the GIS project? Are mudstones observed at outcrop of marine origin? Are they potential SRs? Could they be deeper marine to east? What is basin architecture prior to thrusting? Passive margin with marine shelf to east? What are facies types and GDE* Can we expect shallow marine sands and carbonates? Is there an additional stratigraphic interval in the basin centre that s missing at outcrop? Could this provide a SR? Will it be over-mature and in gas window? Has whole basin been uplifted and eroded at ~620 Ma with total removal of the Ma interval? Or just partially eroded? What causes the gravity data contrasts? Are there basement faults and, if so, do they penetrate higher into the overlying sediment? What trap types exist? More likely fault closures than dip-closures? What is basin s burial history? Tight sands at Premuase indicate significant burial * GDE Gross depositional environment 24

25 Work Options 25

26 Provisional seismic locations seismic Premuase-1 approx loc Acquire 6 or more long regional 2D seismic lines Dependent on budget Main objectives Delineate basin structural form Confirm gravity interpretation Calibrate gravity depth to basement Identify any stratigraphic interval not seen at surface that may be a potential source rock interval, i.e. onlapping or truncated intervals Increase understanding of basin history Will help define depths to potential source rocks, in lower Oti-Panjari and Bambouaka/Afram Use to locate slim-hole wells proposed to sample source rocks if required Cross major gravity features, which could become prospective leads Pass 1 or 2 lines through the Premuase-1 well Run lines through as many of the waterwells and SGST wells as possible without compromising positions Pass lines through wells with oil shows 26

27 Work Forward GIS project (provided by Fugro 2009?) layers to show: Latest GGS/BGS surface geology map Faults and geological features All wells (i.e. water-wells, SGST, Premuase-1) All sample locations for geological outcrop data on a GIS layer All gravity, magnetic and geological maps from 2009 Report by Fugro Construct series of stratigraphic well correlation panels incorporating data from all wells Produce chronostratigraphic chart summarising stratigraphy and major tectonic/glacial events/marine incursions/identify potential SR intervals Build up lithostratigraphic framework from the correlations Insert best-guess lithostratigraphy units Acquire Seismic To understand the stratigraphy at different Target depths 27

28 Conclusions The presence of a petroleum system in the basin is the single most important factor for exploration More technical evaluations are currently carried out: studies, geochemistry, GIS compilation Oil indications in some wells suggests a petroleum system but these may be insufficient to justify further investment Potential source rocks may be too deep towards the east except in shallower western area. Acquisition of several regional seismic lines will provide data to evaluate the basin in terms of stratigraphy, structure and petroleum geology 28

29 29

30 Premuase-1 30

31 ZIRCON DATING SUMMARY Samples (2&3) > 1000Ma (l. Voltaian, Kwahu-Bambouka Group) Samples (21, 24 &26) Ma (m. Voltaian, Oti Penjari &u. Voltaian Obusum Groups) Samples (5, 7, 10, 12 & 13) Age similar to Kwahu Banbouka Group (Buem & Togo Sandstones) Arnold K. Akah (GGS) Two samples analysed from Bambouka Group Youngest zircon grains about 1000Ma Most Zircon Grains (Mesoproterozoic) Sst. Samples Geological map of the Volta Basin and surroundings, simplified after Affaton et al (1980) with sample localities.

32 Infracambrian/Neoproterozoic succession in Abolag-1 is organically lean, based on geochemical analysis of 170 samples Only one sample yielded TOC content of 0.95%. SOURCE ROCK POTENTIAL OVERVIEW TAOUDENI BASIN - MAURITANIA TAOUDENI Nevertheless Abolag-1 flowed Mscfd from fractured dolomites However: Neoproterozoic type-1 hydrocarbon source rock with TOC (17-20%) found in a water well to the Northwestern side of basin Brecciated stromatolitic limestone from the Abolag-1 Hydrogen Index of 800 meaning good oil generating potential S. LU NING ET AL., 2009 Infracambrian hydrocarbon source rock potential and petroleum prospectivity of NW Africa