For the past two decades our understanding of the evolution of passive margins has been greatly enhanced. Research by academic consortiums and petroleum companies, using field studies and a wealth of new well and seismic data, has yielded a much clearer understanding of crustal architecture and the rifting mechanics of many of our continental margins. In unravelling the complexities of the enigmatic area between oceanic and continental crust and revealing the processes relating to extreme crustal thinning and breakup, a new exploration domain has been created, increasing the global footprint for future exploration.
While the observations made by the industry and academia have been around for decades, exploration has been slow to act on these insights. With the exception of the brave few, adoption of these learnings into play-based exploration workflows and capturing acreage has been measured. However, times are changing. In the industry we are now witnessing a wider acceptance of these insights. More and more companies are positioning themselves in the distal domains of passive margins with large acreage grabs poised to unlock new plays (Figure 1).
This article will highlight some of the key learnings from the latest research, the implications for play-based exploration and how we can move forward to become predictive to unlock the prize.
Old vs New
Determining subsidence history and paleo heat flux is critical to unlocking the remaining plays of our continental margins. The only way to successfully achieve this is to understand the mechanisms and timing of rifting. In the past we have made relatively simple assumptions. One of those assumptions dictated that rifting is of the same age across a margin and that breakup can be defined by the breakup unconformity. A simple yet profound consequence of this assumption was to incorrectly project observations made at proximal margins into the distal domain.
Holding a model on margin evolution is critical for play-based exploration. Historic rifting models often invoked systematic thinning of the continental crust towards the ocean continental boundary, which generally increased perceived risk moving from proximal to distal domains. In particular the risk associated with source rock maturity was often a barrier to exploring the distal domain. The assumption was that the primary source of heat flux is from radiogenic minerals present in the continental crust and sediment pile. Thinning in the distal domain and thus reducing radiogenic heat production introduced a significant perceived risk on source maturation.
It is now known that historic views of a margin development are overly simplistic and are not supported by observations. Rather than systematically thinning from the proximal to distal domain, crustal thickness has been observed to vary greatly along continental margins. Interpretation of seismic data has highlighted many occurrences of thick basement highs formed by mantle, continental crust or magmatic additions, found far outboard of the limit of stretched and thinned continental crust (Figure 2). The zone between thinned continental and oceanic crust is now widely accepted to be extremely variable and complex in nature along both strike and dip directions. The zone can be associated with extreme crustal thinning, thick zones of exhumed and extruded mantle, proto-oceanic crust and true oceanic crust. We define this zone as ‘the distal domain’. The subsidence and paleo heat flux of this domain varies greatly and the extent of the variation is in many ways dictated by the magmatic budget available during rifting and its presence after rifting. These insights challenge much of our preconceived ideas on potential plays in the distal domain and prompt us to rethink the prize.
Recognising that the distal domain is complex and accepting these complexities as opportunities offers additional real estate along our passive margins. We now understand that basement highs can exist in the distal domain. These highs are surrounded by stretched and thinned continental crust (e.g. Santos/ Sao Paulo plateau; outer Campos high; Padouck High – see Figure 3). In addition to observing these on deep seismic reflection lines and from gravity data, we are slowly building a global well database on the subsidence and heat flux history of these highs.
Such basement highs are often attractive exploration targets. To unravel their potential, care needs to be taken to place these distal highs in relation to the proximal domain, the necking zone and the amount of stretching and thinning surrounding the high. In doing so we can unravel crustal composition, timing of rifting, and available magmatic budget during and after rifting, which allows us to be predictive on paleo heat flux, subsidence history and reservoir quality. Fluids associated with the magma may play an important role in early diagenetic evolution of the sediments and must be considered when determining reservoir play risk. The integration of regional and high resolution magnetic and gravity data coupled with deep seismic data is critical. Offset well data needs to be carefully evaluated to determine if it bears any relation to the stratigraphy to be tested. High-level observations regarding the character of these highs on gravity, seismic and from well data can then be made (Table 1).
The fundamental lessons learnt when drilling a basement high is that the subsidence and heat flux is very different to that of the surrounding basin, with the paleo heat flux often higher and the subsidence less. The basement thickness of these fragments dictates that they remained relatively buoyant throughout much of their post-rift history and therefore post-rift sediment cover is thin and condensed due to non-deposition and erosion. In play-based exploration, where previous exploration wells can influence the perception of play risk, particular care should be taken when using wells drilled on distal basement highs, as they often do not represent the stratigraphy, subsidence and heat flux of the surrounding basins. Likewise, a well drilled on very attenuated crust does not preclude the presence of an outboard thicker continental fragment.
A discussion on exploring in the distal domain would not be complete without a note on magmatic budgets. The magmatic budget available for rifting can be very variable and is not directly related to ‘ß values’, as often assumed in classical models. There are many occurrences in the literature which describe magma-poor and magma-rich zones. However, these are end-member models. The industry now accepts that the available budget can be very variable as, for example, many margins globally exhibit magma-poor rifting in the proximal domain but magma-rich rifting in the distal domain.
The magmatic budget fundamentally dictates the thermal and subsidence history of the sedimentary section. In a magma-poor rift, breakup occurs in a deepwater environment resulting in deepwater sediments. In magma-rich margins breakup is often subaerial to shallow marine and subsequently the sediments represent this paleo environment. To complicate matters, in a hybrid margin exhibiting both magma-rich and magma-poor rifting, the volume of magma appears to change rapidly across transform faults and the subsidence history can show extreme variations within a few kilometres. The magmatic additions (intrusives plus extrusives) can also thicken the crust and can be mistaken for ‘original’ continental crust, leading to incorrect estimates of radiogenic heat flux and estimates of heat flow.
The volume of magma is dictated by the position of the asthenosphere under the sediment pile and the duration of its perturbation. In areas of hyperextended crust, heat flux from the mantle becomes a more important contributor than heat flux from radiogenic elements present in the crust and sediment pile. A shallow asthenosphere for a prolonged period of time has the ability to introduce high paleo heat fluxes into the overlying sediment pile. Recent wells drilled on hyperextended continental crust demonstrate that paleo heat flow here can be high. The data also shows that heat flow does not decay but remains high long after the period of rifting even to the present day. This is a more surprising and complex observation and is the subject of ongoing research. Several wells drilled along the distal domain of the Atlantic margin record present-day geothermal gradients in excess of 40° C/km. These elevated paleo and present day temperatures create an environment for maturation of both rift and drift related source rock facies in the distal domain. This is in stark contrast to our perception of immature source rocks inherited from the simplistic rifting model.
How do we Unlock the Distal Frontier?
As an industry we need to understand that collaboration will be the key to innovation. A perfect work programme utilises the bigger industry brain. Maximising the information from the experience of others, coupled with new data from potential fields, wells and seismic, will be required to unlock the prize.
We now recognise the complexities of the distal domains and no longer perceive these as risks but rather as opportunities for frontier exploration. The most fundamental lesson we can take going forward in exploring this new frontier is that we cannot simply project the lessons learnt in the proximal domain into the distal, but treat it as a new and distinct play fairway.