Awaken your structural geologist!
An understanding of tectonic phases will help regional exploration, facilitate modelling and de-risk leads.
Offshore Côte d‘Ivoire is a part of the wider West African Transform Margin, a structurally complex area bounded by major tectonic lineaments that formed during its separation from the Equatorial Margin of Brazil during the early Aptian. It exhibits syn-rift sub-basins that are bounded by later large right-lateral transform movements that drove the inversion of earlier basins and structures.
Tectonic phases
In the sub-Saharan passive margins of Africa there is a distinct syn-rift phase with extensional basins providing new accommodate space, followed by a post-tectonic phase that sees sediments deposited during rising sea- levels in a period of tectonic quiescence. In contrast to this, the West African Transform Margin has a more complex tectonic evolution, with multiple extensional and compressional tectonic phases that exhibit interaction and remain active present day.
In order to constrain regional exploration and facilitate the modelling and de-risking of identified leads, these multiple tectonic phases are being interpreted and corroborated on regional datasets. Four controlling geological phases can be recognised, defined by tectonic events and constrained by plate tectonic reconstructions and gravity modelling.
The controlling phases, that may not be exhaustive, are:
- Pre-rift: indeterminate Palaeozoic to Early Mesozoic sandstones and mudstones within intracratonic basins, undrilled in Côte d’Ivoire.
- Syn-rift: pull-apart movement resulting in coastal and shallow marine sands of Aptian and lower Albian age. These sandstones provide the reservoir for most structural plays in Côte d’Ivoire.
- Syn-transform: right lateral strike-slip transpressional movement of late Albian to Cenomanian age, resulting in the inversion of pre-rift and syn-rift basins and witnessing the deposition of shallow marine sands.
- Post-transform: deeper water clastic deposits that remain influenced by transform related inversion and faulting.
These four tectonic phases can be interpreted on seismic and in well data across offshore Côte d’Ivoire and provide a tectonostratigraphic framework to constrain the interpretation of sea-level controlled depositional sequences. Furthermore, the influence that late tectonic movements have on reservoir distribution and hydrocarbon trapping in the post-transform is becoming increasingly apparent and the results of recent wells when integrated with regional seismic data suggest the recognition of two distinct plays.
Key tectonically influenced plays
The two post-transform plays, influenced by late tectonic movements, are transform channel and fan sandstones that onlap against syn-depositional inversion structures that provide an up-dip sealed pinch-out; and post-transform channel and fan sandstones that are truncated up-dip by late-stage faults, which are either inversion-derived or late transform movement faults, providing a fault-seal.
Many exploration wells have targeted viable channel or fan sandstone reservoirs and drilling results
have shown the presence of good quality reservoir sandstones charged with hydrocarbons. Assessment of these successes suggests a strong indication that there are associated late-stage tectonic movements that have contributed to the success of the play, with syn- depositional inversion providing a trap and/or late-stage fault truncation providing an up-dip seal.
Whilst such late-stage tectonic activity can provide traps and fault-seals that contribute to the success of plays, the activity can also cause plays to fail. Inversion movements can reposition reservoirs, resulting in trap leakage, and can also cause channels to become more erosive, incising into underlying sandstones and breaching their top seals. Furthermore, late-stage faulting or fault reactivation after hydrocarbon charge can cause trapped hydrocarbons to leak through fault breach.
Role of tectonism in key wells
The well-publicised Jubilee Field, discovered in 2007 by Kosmos Energy in offshore Ghana, is an example of a post- transform sandstone reservoir that is truncated up-dip by late-stage faulting that provides the seal to hydrocarbon migration (Richmond Energy Partners, 2016). Similarly, in the nearby cluster of fields known as TEN (Tweneboa, Enyenra and Ntomme), hydrocarbons are understood to be trapped by lithological barriers within stacked sand channels, draped over an inversion structure, with additional sealing provided up-dip by late-stage faulting similar to that at Jubilee (Tullow, 2008).
In the deep waters offshore Benin, the Houmelan-1 well, drilled in 2013, tested a Senonian-age post-transform sandstone reservoir with good rock qualities. The well had oil shows, demonstrating a working petroleum system (Shell, 2013) but the hydrocarbons were believed to have migrated through the reservoir due to a lack of up-dip trap.
In Côte d’Ivoire, the South Grand Lahou-1 (SGL-1) well, drilled in 2009, targeted similar mid-Cretaceous age sandstones that onlap against an inverted syn-rift structure. Analysis of the reservoir target using multiclient 3D seismic data shows that although the sandstone body exhibits a pronounced onlap in dip direction, in strike direction the body is clearly uplifted over the top of an inversion structure and as such likely had its trapping potential disrupted (Figure 1).
The Kosrou-1 well, drilled in Block CI-116 in 2012, tested a sandstone channel system (Offshore Energy Today) that onlaps an inverted syn-rift structure, known as the East Grand Lahou High. This well encountered oil shows in sandstone channels with good reservoir qualities but are understood to have lacked top seal due to incision of younger channels, resulting in hydrocarbons migrating up dip. Further analysis is required on the available multiclient 3D data to understand the relationship of this incision with the timing of inversion events (Figure 2).
The Buffalo-1X and Capitane East 1-X wells, drilled in Côte d’Ivoire in 2011 and 2014 respectively, tested post-transform mid-Cretaceous channel and fan sandstone bodies. Both wells encountered hydrocarbon shows and sub-economic accumulations and the failure of the plays is interpreted as being attributable to the timing of late-stage tectonic events. At the Buffalo-1X well, the sand body appears to have depositional continuity over the inversion structure, suggesting that the inversion may not have been active enough to cut off the sand system and provide a trap. At the Capitane East 1-X well, although the pinch-out against the inversion structure appears trapping in dip direction, the upper part of the reservoir sandstone body is interpreted to have depositional continuity around the flank of the inversion structure (Figures 3 and 4).
Robust tectonic framework
With a robust tectonic framework in place, corroborating the different observed rift and transform phases together with the timing of inversion, explorers can begin to model and de-risk the influence of late-stage tectonism on post-transform leads. The relationship of the timing of inversion to sand body deposition should help de-risk the trap integrity of pinch-outs against structural highs; where inversion is weak or late the risk of sand continuity up-dip is likely greater. Additionally, where inversion is syn-depositional and more pronounced, the risk of channel incision into the top of the reservoir resulting in top seal breach is greater. Finally, where late-stage faults are seen to cut through the reservoir body, either driven by inversion or simply the reactivation of earlier rift faults during transform movement, the timing of the faulting is critical. Where the fault cuts pre-date charge they can provide a critical seal for hydrocarbons but where faults post-date charge (Figure 4) they can rupture top seals, resulting in leakage.
