New insights from a regional and reservoir level interpretation in the mature Central North Sea
With the upcoming 30th UK Offshore license round centered on mature areas of the UK Continental Shelf, the challenge for hydrocarbon explorers is to find and develop new prospects in areas that have already been extensively explored.
In this article we revisit some working reservoir units in the portion of the Central North Sea that spans the Witch Ground Graben, Fisher Bank Basin, Forties-Montrose High, Central Graben, Ula/Gyda Terrace and Auk Ridge areas. Most of this vast region in the Central North Sea is covered by 23,650 km2 of conventional PGS multiclient 3D seismic and its pre-stack derivatives, MegaSurveyPlus, which is a regional, fully migrated 3D dataset, imaged using a modern processing sequence, with the ability to obtain an improved and consistent regional geological perspective. Several recent 3D broadband dual-sensor towed streamer surveys (GeoStreamer®) are included in the study area and have been integrated into this analysis.
This region of the Central North Sea is characterized by a high well density (54 exploration wells per km2), yielding a total of 17.4 Bboe total recoverable reserves (IHS Edin, 2016). Nevertheless, the Central North Sea is still expected to contain around 4.5 Bboe of yet-to-find hydrocarbon resources (DECC, 2015), mostly locked in near-field pools that have been challenging to characterize and are potentially located in deeper reservoirs.
New Exploration Insights
The MegaSurveyPlus dataset has been used to directly highlight sediment distribution of all Paleogene reservoir units including potential leads. To undertake this analysis, extracted stratal slices were chronostratigraphically calibrated by integrating well and field data and then a maximum amplitude extraction was performed through chronostratigraphic windows, highlighting possible sandstone- versus mudstone-prone areas (Figures 1A-C).
Figure 1: Paleogene sandstones in the Central North Sea: (A) 3D view of the Maureen Fan. (B) Millennium Atlas-derived map showing distribution of Tay Sandstone interval; (C) Maximum amplitude extraction showing the Tay Sandstone interval; (D) Structuralstratigraphic lead within the Cromarty interval. (PGS MegaSurveyPlus)
Figure 1: Paleogene sandstones in the Central North Sea: (A) 3D view of the Maureen Fan. (B) Millennium Atlas-derived map showing distribution of Tay Sandstone interval; (C) Maximum amplitude extraction showing the Tay Sandstone interval; (D) Structuralstratigraphic lead within the Cromarty interval. (PGS MegaSurveyPlus)
Figure 1: Paleogene sandstones in the Central North Sea: (A) 3D view of the Maureen Fan. (B) Millennium Atlas-derived map showing distribution of Tay Sandstone interval; (C) Maximum amplitude extraction showing the Tay Sandstone interval; (D) Structuralstratigraphic lead within the Cromarty interval. (PGS MegaSurveyPlus)
Figure 1: Paleogene sandstones in the Central North Sea: (A) 3D view of the Maureen Fan. (B) Millennium Atlas-derived map showing distribution of Tay Sandstone interval; (C) Maximum amplitude extraction showing the Tay Sandstone interval; (D) Structuralstratigraphic lead within the Cromarty interval. (PGS MegaSurveyPlus)
Closures have been observed within the sandstone-prone areas, identifying further untested leads and, where possible, pre-stack derivatives (relative Vp/Vs and relative acoustic impedance) were utilized to characterize the elastic reservoir behavior of some of these leads (Figure 1D). It was found that all likely sandstone-prone areas of each chronostratigraphic interval mirror the sediment distribution maps of the Millennium Atlas (2003), further ground-truthing this seismically driven methodology (Figure 1B-C).
Furthermore, MegaSurveyPlus has been utilized as a consistent regional screening tool to locate areas where Mesozoic and Paleozoic plays are visible, and to compare them to existing deep discoveries. Subsequently, a pre-stack seismic simultaneous inversion of the 3D dual-sensor data has been performed over some fields and identified leads in order to better understand the reservoir-scale elastic properties distribution thanks to the pre-stack broadband AVO-compliant dataset (Figures 2 and 3).
Figure 2: (A) Full stack MegaSurveyPlus, Top Chalk horizon clearly visible. (B) Full-stack GeoStreamer, Top Chalk horizon is also clearly visible, but the greater bandwidth shows significant uplift, increasing intra-chalk reflectivity. (Source: PGS)
Figure 2: (A) Full stack MegaSurveyPlus, Top Chalk horizon clearly visible. (B) Full-stack GeoStreamer, Top Chalk horizon is also clearly visible, but the greater bandwidth shows significant uplift, increasing intra-chalk reflectivity. (Source: PGS)
Reliable Scanning Tool
Figure 3: (A) GeoStreamer seismic section through Cod and Jackdaw fields, showing two undrilled Jurassic structures; (B) Relative acoustic impedance of the Jackdaw structure. The tilted Jurassic fault blocks are well imaged as are the Jurassic sandstone intervals. The sandstones have a high relative IP, while the shales have a low relative IP, with the sand acoustic impedance softening in the presence of hydrocarbons (oil) (red in well stick). (Source: PGS)
This work has shown that new exploration insights at both reservoir and regional scale relating to leads, prospects, plays and migration pathways can be realized even in a highly mature exploration setting such as the Central North Sea.
Such new insights have been enabled by utilizing a fully migrated 3D regional dataset (MegaSurveyPlus) as a consistent and reliable regional scanning tool (particularly in the Paleogene section) and combining it with the AVO-compliant pre-stack attributes of dual-sensor broadband GeoStreamer 3D datasets.
This has led to a significant improvement in subsurface understanding for both near-field exploration and field development, for both the Paleogene and deeper reservoirs.
Unless otherwise stated, all image by PGS.
