Potential hydrocarbon reservoirs of Albian–Paleocene age in the Nuussuaq Basin, West Greenland

The onshore Nuussuaq Basin in West Greenland is important for hydrocarbon exploration since many of the key petroleum systems components are well exposed and accessible for study. The basin has thus long served as an analogue for offshore exploration. The discovery of oil seeps on Disko, Nuussuaq, Ubekendt Ejland, and Svartenhuk Halvø (Fig. 1) in the early 1990s resulted in exploration onshore as well. In several wells, oil stains were observed in both the siliciclastic sandstone and in the volcanic series. An important aspect of any petroleum system is a high quality reservoir rock. The aim of this paper is to review petrophysical aspects of the reservoir potential of key stratigraphic intervals within the Nuussuaq and West Greenland Basalt groups. Reservoir parameters and porosity–permeability trends for potential siliciclastic and volcanic reservoirs within the relevant formations of the Nuussuaq Basin are discussed below. Geological setting

The onshore Nuussuaq Basin in West Greenland is important for hydrocarbon exploration since many of the key petroleum systems components are well exposed and accessible for study. The basin has thus long served as an analogue for offshore exploration. The discovery of oil seeps on Disko, Nuussuaq, Ubekendt Ejland, and Svartenhuk Halvø (Fig. 1) in the early 1990s resulted in exploration onshore as well. In several wells, oil stains were observed in both the siliciclastic sandstone and in the volcanic series. An important aspect of any petroleum system is a high quality reservoir rock. The aim of this paper is to review petrophysical aspects of the reservoir potential of key stratigraphic intervals within the Nuussuaq and West Greenland Basalt groups. Reservoir parameters and porosity-permeability trends for potential siliciclastic and volcanic reservoirs within the relevant formations of the Nuussuaq Basin are discussed below.

Geological setting
The Nuussuaq Basin formed in the Cretaceous-Palaeogene as part of a complex system of linked rift basins that developed along West Greenland during the opening of the Labrador Sea and Baffin Bay (Oakey & Chalmers 2012). As a result of Neogene uplift, the sediments and overlying Palaeogene volcanic rocks are exposed on Disko, Nuussuaq, Upernivik Ø and Svartenhuk Halvø (Fig. 1, Dam et al. 2009). The sedimentary succession is interpreted as deposited in fluvial, delta, shelf and deep marine environments, and is divided into ten formations forming the Nuussuaq Group (Fig. 2). The overlying West Greenland Basalt Group (WGBG) includes subaerial lava flows and hyaloclastite breccias (Larsen et al. 2016). A companion paper to this contribution (Sørensen et al. 2017, this volume) provides additional information regarding the structural development and potential source rock distribution of the Nuussuaq Basin.

Lithology and distribution of relevant formations
The Atane Formation is known from the eastern part of the Nuussuaq Basin east of the Kuugannguaq-Qunnilik fault, (KQ fault, Figs 1, 2). The formation is up to 800 m thick in individual outcrops and consists of delta deposits, which include laterally extensive sandstone sheets (Dam et al. 2009). The mudstone-dominated, marine Itilli Formation ( Fig.  2) is known from northern and western Nuussuaq (west of the Ikorfat fault) and is more than 2.5 km thick (Sønderholm & Dam 1998). The Umiivik Member crops out in northern Nuussuaq between the Ikorfat and the Itilli faults ( Fig. 1), whereas the Anariartorfik Member crops out west of the KQ fault.
The up to 438 m thick marine Kangilia Formation ( Fig.  2) crops out on northern Nuussuaq between the Ikorfat and Itilli faults ( Fig. 1) and has been drilled in the GANT-1 and GRO-3 wells. It has also been measured in an outcrop on southern Nuussuaq. Mudstones dominate in the outcrops while sandstones dominate in the wells. The Annertuneq Conglomerate Member mainly comprises conglomerates and sandstones (Dam et al. 2009).
The sub-marine to marine Agatdal Formation (Fig. 2) is known from the Agatdal area on central Nuussuaq west of the Ikorfat fault as well as from the GRO-3 well west of the K-Q fault (Fig. 1). The formation is up to 148 m thick and consists of mudstones, sandstones and conglomerates (Dam et al. 2009).
The volcanic Vaigat Formation (Fig. 2) is up to 1600 m thick in western Nuussuaq and northern Disko and increases to at least 5 km in thickness on Ubekendt Ejland (Larsen et al. 2016). The lower part of the formation is dominated by hyaloclastic breccias that are overlain by lava flows.

Methods
Ten wells and eight sedimentary outcrop successions located in the Nuussuaq Basin and described by Sønderholm & Dam (1998) and Dam et al. (2009) were analysed to quantify the sandstone component of the siliciclastic formations of the Nuussuaq Group. All sandstone intervals were defined as potential sandstone reservoirs and expressed as the cumulative sandstone thickness of a formation. Potential reservoir content was defined as the cumulative sandstone thickness divided by formation thickness. The cumulative sandstone thickness in outcrops and wells was estimated from sedimentological logs and core descriptions. Mudstones and heteroliths were classified as non-reservoir lithologies and included in the term 'shale'. In the uncored GRO-3 well, sandstone content was determined from wire-line logs and implies a shale content of 0-15%. Five wells containing volcanic successions of the WGBG were analysed to identify potential reservoir sections within the hyaloclastite successions. These were quantified as the cumulative thickness of hyaloclastite breccias. Lava flows were classified as non-reservoirs. Porosity-permeability trends were established for the GANE-1/1A, GANT-1 and Marraat-1 wells based on core analysis data. The porosity of the clastic formations in GRO-3 was calculated from the density wire-line log. Sandstones with porosities >10% are referred to as porous sandstones. A porosity log of the GANE-1/1A well was generated from gamma-ray and density logs obtained from core scans.

Reservoir properties of formations containing potential hydrocarbon reservoirs
Potential reservoirs were identified within sandstones of the Atane, Itilli, Kangilia, and Agatdal formations and hyaloclastite breccias of the Vaigat Formation (Fig. 2). The main reservoir parameters including cumulative reservoir thickness (CRT), potential reservoir content (PRC) and porosity-permeability data are summarised below, with details in the Electronic Supplement (ES). Atane Formation. CRT is in the range 26-360 m corresponding to a PRC of 45-80% ( Fig. 3 and ES). The reservoir quality is well developed in central Nuussuaq with sandstone porosities of 5-25% (mean 17%) and air permeabilities of 0.5-150 mD (Appel & Joensen 2014).
Itilli Formation. CRT is in the range 21-458 m corresponding to a PRC of 2-51% ( Fig. 3 and ES). The relatively low content of potential sandstone reservoirs compared to the Atane, Kangilia and Agatdal formations reflects significant variations within and between the members of the Itilli Formation. In the GRO-3 well, the individual sandstone units are up to 100 m thick and average porosity is 4%.
Kangilia Formation. CRT is in the range 0-134 m corresponding to a PRC of 0-72% indicating significant lateral variation in depositional environments ( Fig. 3 and ES). In the mudstone-dominated successions, the Annertuneq Conglomerate Member constitutes a potential reservoir. In the GRO-3 well, porosities range between 5-17% (average 6%).
Agatdal Formation. CRT is in the range 71-148 m corresponding to a PRC of 50-67% ( Fig. 3 and ES). Porosities of 6-21% and permeabilities up to 9 mD were measured in the GANE-1 well (Fig. 4), and in the GRO-3 well, an average formation porosity of 9% was assessed.
Vaigat Formation. Several oil shows have been encountered in the hyaloclastites and lava flows. Only the hyaloclastites seem to possess the necessary permeability to constitute a reservoir despite indications of lower average porosities (6%) than the lava flows (8%). For identical porosity values, the permeability is 30 times higher for the hyaloclastites compared to the lava flows (Fig. 4). The CRT of the hyaloclastites is in the range 80-671 m corresponding to a PRC of 84-100% ( Fig. 2 and ES).

Porosity-permeability relations of potential reservoirs
The regional distribution of porosity and permeability in the Nuussuaq Basin deposits is poorly known due to the scarcity of core measurements or studies concerned with the effects of diagenesis on reservoir quality (Kierkegaard 1998). One relatively swift way of improving the porosity database is by using core scans to generate a porosity log (Pedersen et al. 2013). This is shown for the Agatdal Formation in GANE-1/1A in Fig. 5. The core log-generated porosity curve fits nicely with the core measurements and the core log-derived lithology corresponds well with the lithological log from Dam et al. (2009).
Core analysis data from the Marraat-1, GANE-1/1A, and GANT-1 wells were used to outline porosity-permeability trends within the siliciclastic and volcanic successions (Fig. 4). In the siliciclastic reservoirs, the porosity and permeability data show a high degree of scatter and no consistent trend can be identified. The large range in porosity and permeability values probably reflects variations in grain size and diagenesis. A large part of the porosity in the GANT-1 sandstones is secondary and related to dissolution of detrital feldspar grains (Kierkegaard 1998).
The hyaloclastite samples show a relatively high permeability-porosity ratio (trend A in Fig. 4) compared to the lava flows (trend B in Fig. 4). At 10% porosity, the expected permeability is 5 mD for hyaloclastites, but only 0.8 mD for lava flows, a tendency assumed to reflect textural control on permeability. In hyaloclastites, a network of connected pores ensures fluid or gas flow, whereas the isolated pore systems in lava flows strongly impede permeability, even at high porosity.

Conclusions
The onshore Nuussuaq Basin in West Greenland contains potential hydrocarbon reservoirs within the siliciclastic Atane, Itilli, Kangilia and Agatdal formations, and within the hyaloclastite intervals of the Vaigat Formation. The siliciclastic reservoirs occur in a wide range of geological environments from fluvial over deltaic to slope and marine settings. The potential reservoir sandstone content is generally more than 50% for the sections studied from the Atane, Kangilia and Agatdal formations, but significantly lower in the Itilli Formation. The cumulative sandstone thickness is mostly >100 m for all formations, including the Itilli Formation.
Porosity and permeability data suggest that sandstone and hyaloclastite reservoirs may be of good quality with porosities up to 20%. Permeabilities are mostly below 10 mD. However, porosity and permeability data are restricted to the western part of Nuussuaq and the diagenetic control on the reservoir quality is poorly understood regionally.