Exploring the Mid North Sea high

By Gregor Duval and Matthew Dack, CGG

Published at: December 7, 2015

The Mid North Sea High is one of the last remaining underexplored areas of the UKCS. Away from the main Kimmeridge Clay source rock kitchen it is believed that a Carboniferous petroleum system could be in place but it has not yet been made evident by successful drilling. Supporting the exploration efforts in this region, CGG has acquired a new broadband marine seismic survey over the southern parts of Quadrants 29 and 30, where until now only 2D seismic data have been accessible. This 3D dataset is therefore the first of its kind in this area. It covers approximately 5,600 km2 around and south of the Auk field and it has already revealed some interesting geological features which help to better understand the geology of the Mid North Sea High.

The interpreted seismic line (below) is oriented south-west to north-east. It cuts perpendicularly through the major bounding normal fault defining the southern margin of the Central Graben. The Auk field is found in Lower Permian-aged Rotliegend sandstones trapped in the footwall of the South Central Graben fault and perched on a Devonian paleo-high. The low-frequency content of the data helps to image deep features such as the geometry of the mid-Devonian Kyle Limestone.

Hunting the carboniferous play

New regional broadband 3D seismic data reveals more information about the geology of this poorly understood region.

CGG recently undertook the acquisition of a large 3D broadband seismic survey around the Auk field area and south of it onto the Mid North Sea High. This survey involved the use of CGG’s broadband technologies, BroadSeis™ and BroadSource™, pushing the boundaries of broadband seismic imaging even further, with usable frequencies reaching as high as 200 Hz in the shallow section to image the thinnest formations and identify potential shallow hazards, while reliable low frequencies reached as low as 2.5 Hz to image deep features. Early results of the processed preSTM data are giving some interesting clues about the existence of a pre-Permian petroleum system in the area.

Probable gas, possible oil plays

Following the work of Hay et al. in 2005 and R. Milton- Worssell in 2010, the existence of a Carboniferous hydrocarbon play on the Mid North Sea High is more commonly acknowledged now. At present, only a few of the wells drilled within this part of the North Sea penetrated below Permian-aged strata to give useful information about the Carboniferous section, and in fact many of these wells were drilled on the Auk-Flora palaeo-high where the Carboniferous is absent.

Figure 1: Shallow RMS amplitude map showing potential gas escape features over the Mid North Sea High.

Five of the wells that did encounter a Carboniferous section found source rock potential within coal measures (both Dinantian and Westphalian in age), namely: 44/2-1, 36/13-1, 36/23-1, 36/26-1 and 38/16-1. Maturity studies of these coal measures over the southern part of Quadrant 29 (Hay et al., 2005) have proved the potential for the expulsion of large volumes of gas. This is also verified by the fact that two wells found gas shows within Permian and Carboniferous reservoir intervals. In addition, hydrocarbon indicators and gas chimneys are clearly observed on the fast-track volume of the new 3D seismic data (see foldout image above and Figure 1). A dozen of these gas escape features have been identified within the 5,600 km² area covered by the new broadband seismic survey, correlating with the maturity model of Hay et al. Coincidently, gravity modelling (R. Milton-Worssell et al., 2010) and interpretation of the deep sub-Permian section on seismic data demonstrate the presence of a deep Devono-Carboniferous basin around the southern parts of Quadrant 29 and northern part of Quadrants 37 and 38, bounded to the north-east by the Auk-Flora High in Quadrant 30.

Besides the Carboniferous gas play, the south-west margin of the Central Graben and the northern part of the Mid North Sea High offers potential for Kimmeridge Clay oil to migrate into traps within Palaeozoic and Mesozoic reservoirs. Interpretation of the fast-track BroadSeis seismic data reveals the large vertical throw of the main north-west to south-east bounding fault near Auk and the absence of salt along this trend. This means that mature Kimmeridge Clay source rock intervals in the hanging wall can be found adjacent to Devonian, Carboniferous and Permian reservoir rocks in the footwall, allowing oil sourced from deep in the Central Graben to migrate into reservoirs in the Mid North Sea High region. This is exactly how the Auk structure has been filled with oil. Finding oil sourced from the Kimmeridge Clay formation over the rest of the Mid North Sea High relies on long, near-horizontal migration pathways from the kitchen area in the north-east. Oil shows found within Jurassic Fulmar sands in wells 30/28-1 and 30/28-2 have proved this to be possible.

Furthermore, there is an indication that oil may have been generated locally from Carboniferous lacustrine source rocks. A study of oil samples retrieved from the Rotliegend sandstone interval in well 29/20-1, located immediately west of the Auk field, shows that it has probably been sourced from such source rocks (PA Resources UK, 2010), hence highlighting the possible presence of a deeper oil play over the Mid North Sea High.

Reservoirs and undrilled closures

A number of fields located in the southern part of the Central Graben have Permian or older-aged reservoirs, including the Auk, Flora, Alma (formerly Ardmore) and Innes (formerly Argyll) fields. Reservoir intervals can be found from Carboniferous-aged deltaic and fluvial sandstones through the Lower Permian Rotliegend aeolian dune sands to the Upper Permian Zechstein dolomite or even the Triassic Bunter sands (see stratigraphic chart in Figure 2). Where Zechstein salt is present and thick enough, it acts as a regional seal for sub-Permian reservoirs, but where it is absent it provides a window for hydrocarbons to migrate into shallow reservoirs such as the Triassic Bunter sands or maybe other untested intervals in the Jurassic or in the Tertiary. Above the Zechstein salt, trapping is mainly provided by draped sediments over underlying salt diapirs.

Figure 2: Generalised stratigraphic chart for the North Sea area highlighting the main source rocks and reservoirs to be found over the Mid North Sea High (modified from The North Sea Millennium Atlas).

At depth, interpretation of the Base Zechstein Salt horizon on the new seismic data has revealed several large undrilled closures over this part of the Mid North Sea High. Two examples are shown in Figure 3, one covering over 20 km² in areal extent. Some of the more subtle sub-Permian closures seen in the Two-Way-Time (TWT) seismic domain would need to be converted to depth to validate the potential. The presence of salt diapirs and their inherent high seismic velocities creates artificial pull-up structures on the TWT seismic data. For this reason, the new 3D broadband seismic data is being processed with preSDM using the latest multi-layered tomography technology for more accurate velocity model building.

Figure 3: Two examples of undrilled structural closures identified at Base Zechstein Salt level within the new 3D broadband seismic data coverage. (Source: CGG Data Library).

A promising area

Interest in the potential hydrocarbon plays to be found over the Mid North Sea High has clearly seen a recent uptake through the allocation of over 20 exploration blocks in the area following the 27th UKCS Licensing Round. As a result, operators have committed to the drilling of four firm wells and two contingent wells within this area to target the plays briefly mentioned in this article. In the case of the Carboniferous play, some source rock maturity studies indicate possible total volumes of expelled hydrocarbons of up to 28 Bbo of oil and 156 Tcfg from this region alone. Depending on retention and recovery factors, the actual quantity of recoverable hydrocarbons is still highly speculative, but the potential is certainly high enough to attract the interest of oil and gas explorers.

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