What are the keys to revealing the remaining potential of the UK Lower Cretaceous?
The current very low level of exploration drilling in the UK North Sea should not signal that we are entering the final exploration phase. A popularly held view is that all the obvious sizeable structures have been drilled and the Paleocene has been largely exploited. So where are the widely accepted ‘yet to find’ resources hidden? Azinor Catalyst, a relatively new technology lead arrival, strongly believes that most of the remaining potential is probably to be found in the Lower Cretaceous and Upper Jurassic deepwater turbidite plays as well as the deeper Pre-Cretaceous high pressure/high temperature (HPHT) reservoirs.
For decades the potential of these Lower Cretaceous and Upper Jurassic deepwater turbidite sands has fascinated and intrigued geologists. Their proximity to mature world-class source rocks like the Kimmeridge Clay Formation, and proven trap and seal combinations (ultimate Chalk Group top seal), means that much of the geological risk remains primarily with finding effective reservoirs. Whilst the sands have been found and shown to work in a number of areas, they continue to prove elusive throughout much of the region.
Azinor Catalyst believe that their technical team of experts holds the key to unravelling the predicted potential of the Lower Cretaceous and Upper Jurassic deepwater plays. Through a regional understanding and the ability for largescale interpretation, it is developing new geological and geophysical insights into the significant remaining potential of these exploration plays.
The Britannia field, Central North Sea. With resources of 3-9 Bboe yet to be found, the sun is yet to set on the UKCS. (Source: ConocoPhillips)
Past Results and Perspective
Figure 1: UK Central North Sea Lower Cretaceous and Upper Jurassic turbidite reservoir fields by discovery date.
These deepwater turbidite plays often lend themselves to being a playground for structurally guided stratigraphic traps. Limited sediment supply results in the development of mini-basin ‘fill and spill’ depositional systems focused on the basin-axis, with turbiditic sands being deposited within hemipelagic shales.
These geological models are proving themselves globally, particularly in Atlantic Margin settings; even within the UK North Sea a significant track record of exploration success has been established, with examples such as the Scapa, Goldeneye, Blake and Britannia fields in the Lower Cretaceous, and the likes of Buzzard, Golden Eagle and Brae in the Upper Jurassic. It should be noted that all of these fields have a significant stratigraphic trapping mechanism (Figure 1).
Figure 2: UK Central North Sea turbidite reservoir field creaming curves.
Reserves of over 1.8 MMboe have been proven in the Lower Cretaceous reservoirs, with almost all of this generated from the fields of the Moray Firth in the UK Central North Sea. Reserves are dominated by the early successes of four fields in the mid- 1970s, in particular the Britannia and Captain fields. Since then more recent discoveries, such as Goldeneye and Blake in the late 1990s, have had an impact on maintaining the gradient of the creaming curve (Figure 2).
It has to be acknowledged that there has been a relative lack of targeted exploration of the Lower Cretaceous turbidite plays historically in comparison to some of the other major North Sea plays at least. One of the key factors that may be driving this is a lack of obvious structures. Azinor Catalyst is in no doubt that, through targeted and forensic exploration efforts, the Lower Cretaceous can be shown to hold significant ‘yet to find’ potential within the UK North Sea.
Progressing Geological and Geophysical Understanding
Figure 3: Moray Firth gross Lower Cretaceous isopach map overlain with palaeogeographic drainage network, UK Central North Sea.
Whilst the potential of the Lower Cretaceous has been acknowledged, a different and new perspective is required to push our play understanding forward. From a geological point of view, the use of high fidelity regional mapping has enabled the accurate reconstruction of palaeobathymetric surfaces and sediment drainage networks. Combining these outputs with more conventional techniques, such as isopach and structural mapping, has helped to pinpoint where restricted deepwater mass-flow sediments are likely to be found (Figure 3). Furthermore, the development of highly calibrated basin models have provided a more detailed understanding of the hydrocarbon charge and source rock characteristics, enabling the industry to push further onto the margins of the basin where many stratigraphic trapping opportunities are located.
In order to support the understanding of these geological models, Azinor Catalyst has invested in licensing over 26,000 km2 of regional 3D broadband seismic data to help provide further quantitative geophysical calibration. This latest generation of seismic data provides broad-bandwidth, high signal-to-noise datasets that allow more subtle prospectivity to be delineated through robust AVO techniques and anomalies. The clarity of seismic, in conjunction with the UK North Sea’s huge wealth of well control, allows the full characterisation of the rock properties, which enables the geophysicists to determine what response is caused by oil, what is water and what is background shale (Figure 4). Understanding how both the reservoir and non-reservoir rocks from different formations vary spatially and vertically makes is possible to unravel the related seismic response.
Integrating solid geological models and testing them through these advanced geophysical techniques allows for a cohesive and evidence-based approach for screening and high grading exploration opportunities, thus potentially ticking both geological and geophysical boxes.
Figure 4: Rock property trends – acoustic impedance vs. depth, and impedance analysis.
In the past, the Lower Cretaceous has proved difficult to pin down in terms of seismic behaviour, since both the sands and shales have similar acoustic impedance values. This very low acoustic impedance contrast at the top and base reservoir interfaces results in a weak seismic response (Law, 2000), therefore introducing significant uncertainty into conventional seismic-based mapping techniques. This issue is associated with the Britannia, Goldeneye and Rochelle fields to name but a few. All these fields have excellent reservoir and fluid properties but are difficult to understand seismically using AVO and inversion. If, however, we look at the regional rock property trends we can see that this is not the case for all Lower Cretaceous reservoir rocks. It appears that there is a ‘sweet spot’ as the Lower Cretaceous reservoirs get shallower, maintaining higher porosities as a result of less chemical and physical alteration. This can be diagrammatically shown by a simple depth trend (Figure 4). In the shallow section shales are softer than sands, but with increased depth and physical compaction they quickly become harder than them. This then places the sands in a ‘sweet spot’ depth interval where we expect to see acoustic brightening associated with hydrocarbon presence. Stepping deeper again, the sands have undergone chemical alteration/compaction leading to cementation that influences the rock properties and stiffens the rock, making the sands harder than their surrounding shales once again.
Whilst this can be considered overly simplistic, in practice it highlights that there is a window where Lower Cretaceous rocks can be differentiated seismically. Encouragingly, when looking at known Lower Cretaceous fields and analogues this concept appears to hold together well. The likes of Britannia and Goldeneye represent the deeper interval, while Scapa resides in the ‘sweet spot’ section and the Captain field is found in the shallower interval. It is within the ‘sweet spot’ interval that seismic and AVO techniques can be used with the largest degree of confidence in exploration in the UK North Sea.
Using rock property and impedance analysis we see that it is possible to geophysically separate sands and fluids from background non-reservoir rocks like shale and tight sands. So, while we might not be within the shallower and softer rocks of the Paleocene, reservoir sands and hydrocarbon pay can be indicated in the Lower Cretaceous if the seismic signal-to-noise ratio is high enough. The separation of seismic AVO response from the background trend will be less than that observed in younger rocks, but it is still detectable, as illustrated by the example lines through the Lower Cretaceous Scapa field and Azinor Catalyst’s Partridge prospect (Figure 5).
Figure 5: AVO-generated extended elastic impedance of the Scapa field and Partridge prospect.
Time to Focus…
Only 5% of all UK Continental Shelf fields and discoveries are known to be in pure stratigraphic traps; 12% in combination structural/ stratigraphic traps and the rest in structural ones (Munns, 2003). In contrast, 60% of all Lower Cretaceous deepwater turbidite fields have either combination or stratigraphic trapping mechanisms. The majority of these fields have been found by accident or as a secondary target, so perhaps now is the time to focus the industry’s efforts on the extensive yet-to-find potential of these stratigraphic plays.
Despite only limited amounts of discovered volumes being delivered in recent years, since the likes of Goldeneye and Blake (Lower Cretaceous) and Buzzard and Golden Eagle (Upper Jurassic), the focus for today’s explorers might usefully remain in the pre-Chalk Group stratigraphy. We should not be unduly scared of targeting stratigraphic-based traps as long as these opportunities are based on well-grounded geological models and supported by robust seismic analysis.
Whilst this article has focused on the Lower Cretaceous, the same principles apply to the Upper Jurassic. Leveraging regional knowledge will provide insight and understanding and hopefully lead to significant value generation via further North Sea success.
Jasmine field, Central North Sea. (Source: ConocoPhillips)
References
Law, A., et al, 2000. The Kopervik fairway, Moray Firth, UK. Petroleum Geoscience, 6, 265-274.
Munns, J., Stoker, S., 2003 – Stratigraphic trap potential lightly tested, United Kingdom continental shelf open to frontier thinking. Offshore Magazine.
