Stale Johansen

Late Palaeozoic structural development of the South-western Barents Sea

Abstract A regional grid of multichannel seismic reflection profiles records the Late Palaeozoic structure and tectonic development of the south-western Barents Sea. A 300 km wide rift zone, extending at least 600 km in a north-easterly direction, was formed mainly during Middle Carboniferous times. The rift zone was a direct continuation of the north-east Atlantic rift between Greenland and Norway, but a subordinate tectonic link to the Arctic rift was also established. The overall structure of the rift zone is a fan-shaped array of rift basins and intrabasinal highs with orientations ranging from north-easterly in the main rift zone to northerly at the present western continental margin. The structural style is one of interconnected and segmented basins characterized by halfgraben geometries. A less prominent north-westerly fault trend abuts against the main rift zone from the south-east. From the beginning of Late Carboniferous times, the tectonic development was dominated by regional subsidence, and the entire Barents Sea region gradually became part of a huge Permian-Triassic interior sag basin. This development was interrupted by renewed Permian-Early Triassic rifting and formation of north trending structures in the western part of the rift zone. The tectonic link between the northeast Atlantic and Arctic rifts, initiated in the Middle Carboniferous, then became the primary locus of deformation. The tectonic relationship of north-east Atlantic-Arctic rifting to the development of Late Palaeozoic basins, which dominate the structure of the eastern Barents Sea, remains poorly understood. The rapid Late Permian-Early Triassic subsidence of these earlier fault-controlled basins also affected the western Barents Sea. This suggests possible influence on rifting in the Barents Sea by active-margin processes operating at the eastern Barents Sea margin during subduction of the Uralian Ocean floor. Strong control on the Late Palaeozoic structural development by zones of weakness in the basement is interpreted to be inherited from three major compressional orogens-Baikalian, Caledonian and Innuitian-converging and partly intersecting at a major tectonic junction in the south-western Barents Sea. Local observations indicate that the Barents Sea Caledonides were affected by a Devonian phase of late-orogenic extensional collapse.

Subsurface hydrocarbons detected by electromagnetic sounding

Seismic imaging techniques can readily detect potential hydrocarbon (HC) traps but discriminating between the presence of water or hydrocarbons in such traps has remained a challenge. Detection of subsurface hydrocarbons by an active source electromagnetic (EM) sounding application, termed seabed logging (SBL), has recently shown very promising results, but has until now not been fully demonstrated. Here, we present SBL data from the Troll West Gas Province (TWGP), offshore Norway, providing irrefutable evidence for direct detection of a deeply buried hydrocarbon accumulation by electromagnetic sounding. A powerful horizontal electric dipole (HED) source induced up to 170 % increased subsurface returned signals above the gas accumulation. This result opens a new frontier in hydrocarbon exploration. Introduction Remote sensing techniques record variations in petrophysical parameters such as acoustic or electric properties. Seismic sounding is by far the most common of such tools and typically uses acoustic waves to map boundaries between layers with contrasting acoustic properties. Seismic data can provide detailed information about layering but is not very well suited for direct detection of pore fluid composition. Given detection of a structural geometry that may have allowed accumulation of HC within porous sedimentary rocks, the main remaining uncertainty is therefore whether the pore space is filled with saline water or HC. For this reason only 10-30% of exploration wells penetrate commercial oil or gas reserves in many areas. Electromagnetic sounding uses EM energy transmitted by an HED source to detect contrasts in subsurface resistivity. Resistivity variations in rocks are generally controlled by the interplay between highly resistive minerals (1011-1014 Ωm) and pore fluids including low resistive saline water (0.04-0.19 Ωm) and/or infinitely resistive hydrocarbons (Rider, 1996). Tight crystalline rocks such as oceanic crust typically show high resistivities (100-1000 Ωm) with variations mainly controlled by saline fluids in fracture networks. Sedimentary rocks can exhibit a wide range of resistivities (0.2-1000 Ωm) mainly controlled by variations in porosity, permeability and pore connectivity geometries in addition to pore fluid properties and temperature (Rider, 1996; Schlumberger, 1987). The high resistivity of hydrocarbon filled reservoir rocks (30-500 Ωm) compared with reservoirs filled with saline formation water (0.5-2 Ωm) makes EM sounding a potential tool for detection of subsurface HC. Although EM techniques have been used for many years, EM sounding has until recently not been applied in offshore HC exploration. A full scale EM sounding test offshore Angola in 2000 indicated that a new application of EM sounding, SBL, had a promising potential for direct detection of deeply buried hydrocarbons (Ellingsrud et al., 2002; Eidesmo et al., 2002). Until now the interpretation of SBL data has been hampered by the lack of statistically significant calibration data demonstrating that deeply buried HC accumulations were detectable by the SBL method. However, recent development of a new powerful HED source has opened the way for improved acquisition, processing and interpretation of SBL data. In this study we present SBL data across the TWGP, offshore Norway. Increased EM retur n signals over TWGP are caused by reflection and refraction of EM energy from a high resistivity HC accumulation situated ca 1100 m below the seabed. These data are in accordance with modelling results and provide the first evidence for direct detection of a deeply buried hydrocarbon accumulation by subsea EM sounding.

Continuation of the Caledonides north of Norway: seismic reflectors within the basement beneath the southern Barents Sea

Abstract The offshore continuation of the Caledonides to the north of Norway is poorly understood, and the seismic signature of these rocks is unknown. Indeed, the seismic signature of basement rocks is in general poorly documented from conventional seismic data. This paper discusses an area in the southern Barents Sea — the ‘Gjesvaer low’ — where inferred Caledonian basement rocks have been studied from conventional seismic data. The Gjesvaer low, which has not been described previously as a separate structural element, is defined on image-processed gravity data. The main processing steps were directional filtering and principal component analysis. The interpretation of the low as a feature within the inferred basement rocks is based on well data, the calculated depth to magnetic basement, seismic signature and velocities. In addition, two-dimensional gravity modelling shows that density variations within the basement rocks may explain the observed gravity anomaly. Although the following model may be simplistic, distinct seismic reflectors within the low may be interpreted as originating from tectonic boundaries within a thrust system. In the most likely evolutionary scenario, the Gjesvaer low and the south-western part of the Nordkapp Basin are interpreted as having been formed as continuous Caledonian structures whose continuity has survived until the present. Subsequent Late Palaeozoic erosion may have removed more than 10 km of Caledonian rocks in the area of the low. Carboniferous rifting reactivated the Caledonian structures and the south-western Nordkapp Basin was formed. The Nordkapp Basin, which was decoupled from the Gjesvaer low in Carboniferous times, subsided while the low was again eroded.

High-resistivity anomalies at Modgunn arch in the Norwegian Sea

Typical seismic data provide information about subsurface stratigraphy and structure. Formation characteristics, such as lithology and fluid content, can also be predicted from seismic data. Well-log data can verify seismically extracted formation characteristics. However, well drilling is relatively expensive and the success rate of commercially viable exploration wells, depending on the seismic data, is only about 10–30% (Johansen et al., 2005). Additional remote sensing methods for the detection of subsurface formation properties (e.g. resistivity) can be used to minimize the uncertainties associated with drilling. The recently developed SeaBed Logging (SLB) method shows a very promising potential for the detection of deeply buried highresistivity layers (Eidesmo et al., 2002). Resistivity contrasts in the subsurface strata make SBL a potential tool for the detection of high-resistivity hydrocarbon reservoirs or other high-resistivity lithologies, such as salt domes, volcanic rocks or igneous sills. The first fullscale SBL calibration survey was conducted offshore Angola in 2000 (Ellingsrud et al., 2002), opening a new frontier in hydrocarbon exploration. Subsequently, several surveys were performed over known hydrocarbon fields offshore Norway. SBL calibration surveys from Ormen Lange and Troll Western gas province have been presented by Rosten et al. (2003) and Johansen et al. (2005), respectively. In this article, we present SBL data acquired across the Modgunn arch, which is located in the Norwegian Sea. The SBL data interpretation aims at finding the resistivity distribution within the seismically interpreted subsurface strata. The Modgunn arch is characterized by strong seismic anomalies, which may partially correspond to high-resistivity anomalies. The SBL data of this area, in parts, show strata with high resistivity. SBL data analysis can predict the presence of the high-resistivity layers and rocks, but due to low resolution, it is difficult to determine the exact geometry of the resistivity structure from the SBL data alone. To establish the quantitative relationship between the seismic anomalies and the resistivity distribution within the strata, SBL and seismic data interpretation play complementary roles. The integrated approach of seismic and SBL data interpretation provides a realistic subsurface resistivity distribution with fewer uncertainties. An interpretation study, based on electric field magnitudes taken from the same data set, has been presented by Bhuiyan et al. (2005).

Remote sensing methods in offshore exploration

Abstract The aim of modern remote sensing methods is to enhance variations in digital data sets by applying various numerical filtering techniques and statistical calculations in order to enhance features of interest in the available data. However, a necessary condition for this approach is the availability of an advanced and user-friendly remote sensing system with the possibility to perform interactive processing and “on-the-screen” interpretation. In the present work, a remote sensing system from International Imaging Systems (I2S) was used to interpret geophysical data sets from the Barents Sea region (both offshore and onshore). Features interpreted from the data sets are integrated with other data sets in order to obtain information about the co-variation of features having geological significance. These gravimetric data were used to identify Palaeozoic structural elements in the Barents Sea. Gravity data based on satellite altimetry is also presented and compared with conventional gravity data. The present results indicate that main structural elements can be defined more precisely from the processed data than from ordinary gravity contour maps. An additional benefit of such processing is the detection of subtle trends in the gravimetric field.