The Upper Jurassic Blokelv-1 cored borehole in Jameson Land, East Greenland – an introduction

The Geological Survey of Denmark and Greenland (GEUS) successfully drilled the fully cored Blokelv-1 borehole in the central part of the Jameson Land Basin in East Greenland, targeting the Upper Jurassic, rich source-rock interval of the Hareelv Formation. The borehole achieved 100% core recovery from 1.72 m to a total depth of 233.8 m; the recovered Hareelv Formation section consists of interlayered black, laminated organic-rich mudstones, massive sandstones and heterolithic sandstone–mudstone intervals of the Katedralen Member, and amalgamated massive sandstones of the Sjællandselv Member. The core is of very high quality and has been subjected to an extensive sampling and analytical programme focused particularly on petroleum geological aspects, as presented in the following eight papers in this volume. This bulletin describes an important, previously poorly documented member of the ‘Kimmeridge Clay’ family of prolific petroleum source rocks in the North Atlantic area.

The Blokelv-1 borehole was drilled by the Geological Survey of Denmark and Greenland (GEUS) in Jameson Land during the summer of 2008. It was the first of a series of fully cored boreholes planned as part of a collaborative project between GEUS and a number of sponsoring oil companies entitled Petroleum Geological Studies, Services andData in East andNorth-East Greenland (Bojesen-Koefoed et al. 2009, 2014). The borehole is situated in the central, deep part of the Jurassic Jameson Land Basin near the Blokelv river, after which the borehole is named, c. 35 km west of the Constable Pynt airport (Fig. 1).
The borehole targeted a poorly exposed part of the Upper Jurassic Hareelv Formation that represents a mid-Kimmeridgian sea-level highstand (Surlyk 2003) and is a correlative of the well-known Kimmeridge Clay Formation (sensu lato) of Northwest Europe; it was thus expected to include a rich petroleum source-rock interval. The North-East Greenland part of the Circum Arctic Resource Appraisal (CARA; Christiansen et al. 2006;Gautier 2007;Gautier et al. 2011) pointed out that the quality of an Upper Jurassic -Lower Cretaceous source rock in North-East Greenland should be considered a major risk element in assessing the petroleum potential of the region. Data based on outcrop sampling, however, have shown surprisingly low petroleum potential. A drilling programme that included the Blokelv-1 borehole was thus designed to provide as complete coverage as possible of the entire Oxfordian to Ryazanian succession onshore East and North-East Greenland in order to better understand the geological context of the source rocks and to provide fresh and unweathered samples for new and comprehensive analyses of the entire succession.
The succession cored by the Blokelv-1 borehole covers the interval from the Middle Oxfordian to the Lower Volgian and in general terms the analytical programme included: • Detailed sedimentological description and analysis, including interpretation of depositional environments represented by the deposits • Detailed biostratigraphic analysis, based on both palynomorphs (dinoflagellate cysts) and macrofossils (ammonites) • Detailed analysis of the petroleum potential of mudstones, including richness, thermal maturity, biomarker and stable carbon isotopic fingerprints of source rocks and oil stains • Detailed analysis of the reservoir properties of sandstones, including diagenetic studies • Chemostratigraphy based on bulk geochemical analysis and provenance analyses of sandstones based on dating of detrital zircons • Analysis of uplift history based on apatite fission track analysis (AFTA) • Analysis of magmatic intrusions penetrated.
The results of the analytical programme have been previously reported to the group of sponsoring oil companies ). Following the expiration of the 5-year confidentiality clause on 1 January 2015, the data were released for exclusive use by GEUS, and this bulletin presents the key results in a series of papers, each dealing with different aspects of the analyses.

Drilling operations
The drilling operation was organised by GEUS and comprised a camp of six persons, including two geologists, two technicians, one of whom also undertook catering, and two drillers. The drilling was part of a major field-

Blokelv-1
work programme, and a helicopter was chartered for the full season. Two days of reconnaissance prior to drilling ensured selection of the optimal location for the drill site, remote from exposed major sills and dykes. The disassembled drill-rig parts were flown as sling loads from the Constable Pynt airport to the Blokelv-1 drill site at 70°45.305´N / 23°40.430´W, at an elevation of 181 m above sea level (Fig. 1). The drill rig was assembled directly from the helicopter sling (Fig. 2). Mobilisation of the drill rig and camp took three days, with six tons transported in helicopter sling and cabin loads.  installed in the hole down to 184 m. Demobilisation and down-hole logging took two days. Drilling demanded a daily water consumption of 10 m 3 , with 80 m 3 used in total. Water was supplied from the nearest river at about 300 m lateral distance and 75 m vertical distance; this was achieved using a petrol-driven water pump and inflatable hoses to supply two 5000-litre basins (Fig. 3). In addition, a 3000 litre basin of saline water was used to circulate through the borehole at night thereby avoiding freezing of the drill string in contact with the permafrost. Permafrost was present from a few decimetres below the surface down to about 100 m. Weather conditions were characteristic of the region being predominantly calm and sunny with temperatures up to c. 20°C during the day, interrupted by a few days of fog or low clouds and one day with strong winds.

Drilled succession
The low plateaux in the terrain around the Blokelv-1 drill site consist of sandstone of the Upper Jurassic (Lower Volgian) Sjaellandselv Member underlain by poorly exposed alternating black mudstone and grey -yellowish grey sandstones of the Katedralen Member of the Hareelv Formation (Figs 4, 5).
The core section has been dated by biostratigraphy, the zonation being based on combined ammonite and dinoflagellate data (Alsen & Piasecki 2018, this volume). The upper part (0-10.08 m) is assigned to the Sjaellandselv Member of the Hareelv Formation and consists of frac-tured and structureless, marine gravity-flow sandstones with a few thin very dark grey mudstone beds that contain fossils indicating the lowermost Volgian P. elegans Chronozone. The interval from 10.08 m to TD at 233.8 m is assigned to the Katedralen Member of the Hareelv Formation, spanning the middle Oxfordian to lowermost Volgian C. densiplicatum -P. elegans Chronozones. The cored Katedralen Member comprises very dark grey to black laminated mudstones (54%), sandstone/mudstone heteroliths (12%), and weakly stratified -structureless or intrusive sandstones (34%) (Figs 6, 7). The base of the Hareelv Formation was not reached in the cored interval according to the lithological characteristics, but the lowermost part of the recovered core is assigned to the C. tenuiserratum and C. densiplicatum Chronozones, which is time equivalent to the upper part of the Olympen Formation (Larsen & Surlyk 2003;Bruhn & Surlyk 2004

Logging
A full wireline logging programme was planned to include conductivity, spectral gamma and sonic logs. The borehole was unstable at certain intervals, however, probably due to drilling-induced fractures in homogeneous sandstone. In-hole logging therefore only included    6. Simplified lithological log of the Blokelv-1 core showing the conductivity log and gamma-ray log measured in the field. Note that the wireline gamma-ray log was limited to the lowermost c. 45 m due to technical problems; these data were subsequently supplemented by a core spectral gamma-ray log. Note the marked conductivity response of the three thin igneous intrusive bodies. cps: counts per second. mS/m: millisiemens per metre. a complete conductivity log and a partial gamma log (232-184 m depth; Fig. 6). To compensate for this, a high-resolution spectral gamma log and density log of the complete core were later measured in the core laboratory at GEUS (Bjerager et al. 2018a, this volume). The temperature in the borehole was measured 25.5 and 40.5 hours after drilling ceased at depths of c. 10 m, 35 m, 85 m and 184 m. After 40.5 hours, the temperature stabilised and the estimated depth of the base of the permafrost is at 100 m (Table 1). Wire and sensors were left in the borehole for later measurements when temperature conditions are in full equilibrium.

Sampling and analytical programme
At the drill site, 79 full core samples for gas analysis were collected immediately from the bottom of each recovered core (spacing of 3 m) and stored in sealed metal cans. In addition, eight samples were collected for preliminary and 'express' biostratigraphic studies immediately after the drilling operation. Three samples of observed bitumen were collected in the cores, including liquid bitumen internally in a belemnite, and solid bitumen along fractures (Bojesen-Koefoed et al. 2018, this volume). Core photographing was conducted under moist and surface-dry conditions. The core was sedimentologically logged at a scale of 1:20 for detailed facies analysis and re-drafted at scales of 1:50 and 1:500 for overview logs (Bjerager et al. 2018a, this volume).
An extensive sampling programme was subsequently conducted in the laboratories at GEUS. Biostratigraphic studies are based on 50 mudstone samples for palynology and 42 levels with ammonites (Alsen & Piasecki 2018, this volume). Chemostratigraphic and diagenetic studies are based on 42 samples for bulk chemistry, 27 thinsections for petrography, and 22 XRD samples for bulk sample and clay mineralogy (Bjerager et al. 2018a, this volume;Olivarius et al. 2018a, this volume). Provenance studies are based on six samples for identification of heavy minerals and zircon ages (Olivarius et al. 2018b, this volume). Reservoir properties were evaluated on 25 plugs and comprise measurements on grain density, porosity and permeability (Bjerager et al. 2018a, this volume). Petroleum geological evaluation is based on 139 samples for source rock quality and organic maturity with 42 samples for biomarker analysis, 22 samples for stable carbon isotope analyses (Bojesen-Koefoed et al. 2018, this volume). Two samples of igneous intrusions were subjected to major and trace element analysis (Larsen 2018, this volume). Uplift studies were focused on two samples for apatite fission-track analysis (AFTA; Green & Japsen 2018, this volume).
Depth/m Temperature Temperature after 25.5 hours after 40.5 hours