Kim Senger

The importance of natural fractures in a tight reservoir for potential CO2 storage: a case study of the upper Triassic–middle Jurassic Kapp Toscana Group (Spitsbergen, Arctic Norway)

Abstract In the Longyearbyen CO2 laboratory project, it is planned to inject carbon dioxide into a Triassic–Jurassic fractured sandstone–shale succession (Kapp Toscana Group) at a depth of 700–1000 m below the local settlement. The targeted storage sandstones offer moderate secondary porosity and low permeability (unconventional reservoir), whereas water injection tests evidence good lateral fluid flow facilitated by extensive fracturing. Therefore, a detailed investigation of fracture sets/discontinuities and their characteristics have been undertaken, concentrating on the upper reservoir interval (670–706 m). Datasets include drill cores and well logs, and observations of outcrops, that mainly show fracturing but also some disaggregation deformation bands in the sandstones. The fracture distribution has a lithostratigraphical relationship, and can be subdivided into: (A) massive to laminated shaly intervals, offering abundant lower-angle shear fractures; (B) massive to thin-bedded, heterogeneous, mixed silty–shaly intervals, with a predominance of non-systematic, pervasive bed-confined fractures; and (C) massive to laminated, medium- to thick-bedded, fine- to coarse-grained sandstones with a lower frequency of mostly steep fractures. These domains represent pseudo-geomechanical units characterized by specific fracture sets and fracture intensity, with indicated relationships between the bed thickness and fracture intensity, and with domains separated along bedding interfaces. We discuss the impact of these lithostructural domains on the fluid flow pathways in the heterolithic storage unit.

Effects of igneous intrusions on the petroleum system: a review

Igneous intrusions feature in many sedimentary basins where hydrocarbon exploration and production is continuing. Owing to distinct geophysical property contrasts with siliciclastic host rocks (e.g., higher Vp, density and resistivity than host rocks), intrusions can be easily delineated within data sets including seismic and CSEM profiles, provided igneous bodies are larger than the detection limit of the geophysical methods. On the other hand, igneous bodies affect geophysical imaging in volcanic basins. Recent analyses of 3D seismic data, supported by field observations and lab-based experiments, have provided valuable insights into the prevailing geometries of intrusions, i.e. (1) layerdiscordant dykes, (2) layer-parallel sills and (3) saucer-shaped intrusions. Where emplaced, intrusive bodies affect all five principal components of a given petroleum system: (1) charge, (2) migration, (3) reservoir, (4) trap and (5) seal. Magmatic activity may positively or adversely affect any of these individual components, for instance by locally enhancing maturation within regionally immature source rocks, typically 30-250% of the intrusion thickness, or by causing compartmentalization of source and reservoir rocks. Site-specific evaluations, including the timing and duration of the magmatic event are needed to evaluate the overall effect of intrusions on a given sedimentary basin’s petroleum system, and these are highlighted by case studies from different volcanic basins.

From field analogues to realistic seismic modelling: a case study of an oil-producing andesitic sill complex in the Neuquén Basin, Argentina

Interpretation of seismic data has played a major role in recent advances in the studies of igneous sill complexes. Seismic modelling studies based on field analogues represent a promising tool to close the scale gap between observations from outcrops and seismic data and support seismic interpretation. Virtual outcrop models are commonly used to include high-resolution geological structures in models of seismic-scale field analogues. However, realistic seismic modelling requires not only detailed structural input, but also well-constrained elastic properties and an adequate seismic modelling technique. Here, we present a seismic modelling study of oil-producing andesitic sills in the Neuquén Basin, Argentina, which implements all modelling elements at high accuracy by combining virtual outcrop models, well data and a 2(3)D filtering method. Our results indicate that the modelled seismic signatures of intrusive bodies observed in field analogues are characterized by frequency-dependent interference and strong amplitude variations owing to highly variable elastic properties of both host rock and sills. We demonstrate that detailed waveform patterns observed in real seismic data can be linked to intrusive bodies below the traditionally assumed limit of resolution via realistic seismic modelling. This illustrates how an integrated modelling approach based on field analogues can aid seismic interpretation.

Integrated Characterization of an Organic-rich Caprock Shale, Svalbard, Arctic Norway

Both thermogenic and biogenic gas were encountered during scientific drilling on Svalbard, Arctic Norway. The thermogenic gas has been encountered in an interval at 650-703 m depth, spanning both the lower part of the caprock, an organic-rich shale unit with subordinate siltstone intervals, and the upper part of the siliciclastic reservoir targeted for CO2 storage. Both water injection tests and gas flow tests were conducted to establish the formation injectivity and production capability of this interval. In this contribution, we investigate the organic rich shale interval in detail, integrating well data with direct observations on outcrop analogues, to present a conceptual model of the reservoir-cap rock interface.

Fracture-related fluid flow in sandstone reservoirs: insights from outcrop analogues of south-eastern Utah

Fault- and fold-related fractures influence the fluid circulation in the subsurface, thus being of high importance for CO2 storage site assessment, especially in terms of reservoir connectivity and leakage. In this context, discrete regions of concentrated sub-parallel fracturing known as fracture corridors are inferred to be preferential conduits for fluid migration. We investigate fracture corridors of the middle-late Jurassic Entrada and Curtis formations of the northern Paradox Basin (Utah), which are characterized by discoloration (bleaching) due to oxide removal by circulating CO2- and/or hydrocarbon-charged fluids. The analyzed structures are located in the footwall of a km-scale, steep normal fault with displacement values on the order of hundreds of meters. They trend roughly perpendicular and subordinately parallel to the main fault direction, and define a systematic network on the hundreds of meters scale. The fracture corridors pinch- and fringe-out laterally and vertically into single, continuous fractures, following the axial zones of open fold systems related to the evolution of the main fault. Based on the presented data we hypothesize that such fracture corridors, connecting localized reservoirs at different stratigraphic levels up towards the surface, represent preferred fluid migration pathways rather than the main faults.

CO2 storage in the high Arctic: efficient modelling of pre-stack depth-migrated seismic sections for survey planning

ABSTRACT The sequestration of CO2 in subsurface reservoirs constitutes an immediate counter‐measure to reduce anthropogenic emissions of CO2, now recognized by international scientific panels to be the single most critical factor driving the observed global climatic warming. To ensure and verify the safe geological containment of CO2 underground, monitoring of the CO2 site is critical. In the high Arctic, environmental considerations are paramount and human impact through, for instance, active seismic surveys, has to be minimized. Efficient seismic modelling is a powerful tool to test the detectability and imaging capability prior to acquisition and thus improve the characterization of CO2 storage sites, taking both geological setting and seismic acquisition set‐up into account. The unique method presented here avoids the costly generation of large synthetic data sets by employing point spread functions to directly generate pre‐stack depth‐migrated seismic images. We test both a local‐target approach using an analytical filter assuming an average velocity and a full‐field approach accounting for the spatial variability of point spread functions. We assume a hypothetical CO2 plume emplaced in a sloping aquifer inspired by the conditions found at the University of Svalbard CO2 lab close to Longyearbyen, Svalbard, Norway, constituting an unconventional reservoir–cap rock system. Using the local‐target approach, we find that even the low‐to‐moderate values of porosity (5%–18%) measured in the reservoir should be sufficient to induce significant change in seismic response when CO2 is injected. The sensitivity of the seismic response to changes in CO2 saturation, however, is limited once a relatively low saturation threshold of 5% is exceeded. Depending on the illumination angle provided by the seismic survey, the quality of the images of five hypothetical CO2 plumes of varying volume differs depending on the steepness of their flanks. When comparing the resolution of two orthogonal 2D surveys to a 3D survey, we discover that the images of the 2D surveys contain significant artefacts, the CO2‐brine contact is misplaced and an additional reflector is introduced due to the projection of the point spread function of the unresolvable plane onto the imaging plane. All of these could easily lead to a misinterpretation of the behaviour of the injected CO2. Our workflow allows for testing the influence of geological heterogeneities in the target aquifer (igneous intrusions, faults, pervasive fracture networks) by utilizing increasingly complex and more realistic geological models as input as more information on the subsurface becomes available.

Characterization and modelling of a naturally fractured reservoir-caprock unit targeted for CO2 storage in arctic Norway

Successfully storing CO2 underground requires a good understanding of the subsurface at the storage site, and its robust representation in geological models. Geological models, and related simulations, provide important quantitative information on critical parameters for the optimal utilisation of the subsurface, such as storage capacity, fracturing pressure, optimal injection rates and drilling strategy. In the majority of cases, such models are constructed on the basis of seismic and well data, and history matched using production and injection data. On the Arctic archipelago of Svalbard, however, a siliciclastic unit ca. 700-1000 m deep is considered for CO2 storage, and its outcrop equivalents are exposed 15-20 km from the planned injection site. These outcrops provide an important insight into the structural and sedimentological heterogeneity of the target reservoir. The use of modern tools such as photogrammetric digital outcrops enhances our ability to obtain relevant quantitative data for the geomodel. We here present an integrated characterization of the UNIS CO2 project target reservoir, combining well, core, seismic, EM and outcrop data, to build a realistic model of the planned CO2 storage site.

Investigation of a Mid-crustal Conductor in the North Gjallar Ridge, Offshore Norway, Inferred from Magnetotelluric Data

In this case study, we present the results of the processing, analysis and 2D inversion of Magnetotelluric (MT) data acquired in the North Gjallar Ridge (NGR), in the outer Voring basin (Norway), in July 2014. Although the primary objective of the survey was to collect Controlled Source Electromagnetic (CSEM) data for hydrocarbon exploration purposes, a good quality MT signal was extracted alongside on 120 EM receivers. MT data in the NGR reveal an unusual drop in apparent resistivity values at long periods. Such behavior was already observed in the outer Voring basin and the Exmouth plateau (offshore Australia), both are volcanic passive margins. Indeed, 2D inversion of MT data shows a consistent recovery of a conductor at mid-crustal depth (8-12 km) along the axis of the ridge. Imaging seismically crustal structures in the NGR has long been challenging due to the presence of a dense network of volcanic sills and dykes. On the contrary, MT signal can diffuse beneath them and sense deep geo-electrical structures. We tentatively interpret the conductor, which correlates with a positive Bouguer anomaly, as related to the presence of deep non-intruded sediments.

Structural Characterization of the Longyearbyen CO2 Lab Reservoir-caprock Succession

This baseline study on fracture populations affecting the Mesozoic sedimentary succession of central Spitsbergen (Svalbard) has been performed to characterize the reservoir-caprock system explored for potential subsurface CO2 storage by the Longyearbyen CO2 Lab project. Integrating structural and stratigraphic analyses of outcrop and borehole data, we identified recurrent litho-structural and structural units (LSUs and SUs, respectively) on the basis of their fracture associations, lithologies and dominant sedimentary facies. A principal fracture set trending approximately E-W (J1) and a subordinate fracture set trending approximately N-S (J2) have been recognized. Subordinate systems of shear fractures (S1) trending roughly NE-SW and NW-SE, and a secondary low-angle, fracture set (S2) striking E-W to NW-SE have been observed. Their origin is interpreted as related to the far-field stress of the Paleogene West Spitsbergen fold-and-thrust Belt. The identified units are thought to influence the local hydrogeologic regime due to the intrinsic variations in the matrix and fracture network properties. The architecture of the reservoir-caprock succession is segmented, with the vertical alternation of intervals characterized by 1) fracture porosity and permeability, 2) microfracturing-related matrix porosity, and 3) preferential subsurface fluid flow pathways.

High-resolution Fracture Characterization of a Siliciclastic Aquifer Targeted for CO2 Sequestration, Svalbard, Norway

The target siliciclastic aquifer investigated by the Longyearbyen CO2 Lab as a possible test-scale CO2 storage unit is a dual-permeability reservoir characterized by fractured, tight lithologies. By integrating borehole and outcrop data, the reservoir section has been subdivided in intervals defined by 5 litho-structural units (LSUs), each one characterized by different lithologies and fracture sets interpreted to represent pseudo-geomechanical units. Due to their contrasting features, these LSUs are believed to have a crucial influence on subsurface fluid migration. Our results indicate that fractured shale intervals control lateral fluid flow (predominance of low-angle fracture) whereas sandy and coarser intervals seem to control vertical fluid flow (predominance of high-angle fractures), locally enhancing the contribution of the matrix porosity. Horizontal and vertical high permeability conduits can be found at the LSUs’ interfaces, along the chilled margins of igneous sills and dykes, and along the damage zone of mesoscopic faults, due to the localized enhanced fracturing (fracture corridors). A large database containing structural data on fractures has been acquired and analyzed in order to extrapolate calibrated parameters for numerical modeling and flow simulations. These in turn allow reservoir volumetric calculations, assessment of seal integrity and forecasting of vertical/lateral connectivity of the reservoir.