Lundin (Norway)

Abstract The Gohta discovery well 7120/1-3, drilled on the Loppa High in the SW Barents Shelf in northern Norway encountered a meteoric karst system hosted by the late Permian lower Røye Formation that formed during a period of sub-Triassic exposure. The karst system is preserved as a series of interbedded collapse breccias representing formerly open palaeocaves, matrix-rich breccias that represent palaeocave fills with in situ and reworked stalactites and stalagmites. Layers of ‘Swiss cheese’ texture are interpreted as zones of dissolution in marine-meteoric mixing zones that may have controlled the levels of palaeocave development within the Gohta structure. These other cavities and the matrix of karst breccias were infilled by layered fine doloarenite matrix interpreted as an internal fill generated by collapse. This was followed by a later matrix of dark grey argillaceous mudstone that is interpreted as an external fill that pre-dates the overlying Klappmyss Formation. The karst system has undergone complex early diagenesis that includes extensive dissolution, early leaching of silica, the precipitation of pyrite on internal surfaces and sediment and the precipitation of carbonate and silica as speleothems and vadose cement. A borehole imaging log (BIL) was run across the lower Røye and Triassic Klappmyss formations, including the cored interval that provides a record of inclination and azimuth of the karst fills and in situ Røye Formation. The core provides information about the origin and timing of the different karst fills, and the BIL provides information about their azimuth and inclination. This integrated use of core and BIL data allows the incremental tilting history of the Loppa High structure during exposure to be reconstructed. At present, the Gohta structure has a finite tilt of 5–10° to the SE. The plunge of in situ stalagmites and stalactites suggests that the karst system was initiated while the Røye Formation was slightly tilted to SE. The dip of karst components and the host stratigraphy indicate several phases of incremental tilting during deposition of the doloarenite and argillaceous mudstone karst matrix. There appears to have been a reversal of tilt of the Gohta structure prior to its onlap by the Klappmyss Formation.

The Loppa High Composite Tectono-Sedimentary Element is a mature exploration area in the southwestern Norwegian Barents Sea hosting Skrugard (in 2012) and the Late Paleozoic Gohta (in 2013) discoveries. The exploration of the Norwegian Barents Sea started in 1980 and the Loppa High in 1985. Today the Loppa High is one of the most studied areas of the Barents Shelf. Subsurface mapping is based on dense coverage of modern 3D and 2D seismic data and drilling results of over 40 exploration wells. This chapter summarizes the tectonosedimentary and subregional development, and the petroleum geology of the Loppa High.

The core cleaning approaches aim to remove native crude oil, mud filtrates and evaporated salts which leads to strongly water-wet cores. In the standard core cleaning method, the solvent(s) injection continues until no more oil is observed in effluent. This is generally confirmed by visual examination, rather than by analyzing the composition of effluent, simply because it is expensive. In this work, we selected two preserved rock sections from an oil reservoir in North Sea. 1.5-inch diameter core plugs were cleaned by two different flush cleaning programs (so-called Program-1 and 2). In Program-1, samples were flushed first with 75% toluene + 25% methanol and then followed by alternate of toluene-chloroform-methanol until the samples were deemed cleaned from visual examination. In Program-2, samples were flushed by alternating toluene-tetrahydrofuran-chloroform for 8 cycles. Oil effluents from Program-2 were collected for composition GC analyses. The efficiencies of two flush cleaning programs were compared by measuring the wettability of the samples by combined Amott-USBM wettability method. The USBM index shows samples after cleaning program-2 are more water wet. Samples were then aged in crude oil at reservoir conditions for 4 weeks. Drainage and imbibition capillary pressure using centrifuge and porous plate and the end-point liquid permeabilities were then measured on restored samples. End-point properties after aging process, showed wettability being restored to intermediate water-wet as is expected from field production history. This study shows that standard cleaning does not fully clean these samples. Meanwhile, the elaborate Program-2 is often impractical, takes too long, is expensive and limited by laboratory capacity.

Seismic data acquired at different times can provide insight into changes in an oil/gas reservoir. We apply time-lapse AVO inversion methodology to the Edvard Grieg field in the North Sea. The methodology we use inverts for discrete lithology-fluid classes (LFCs), where different fluid fillings and/or pressure effects in the monitor are represented by different LFCs. Our focus here is on fluid substitution. We only look at amplitude effects, and use monitor time-lapse data that is time aligned with the base seismic.

This paper presents a case study from the Edvard Grieg field in the North Sea. PP and PS ocean bottom cabled (OBC) seismic data have been used to build high-frequency Vp- and Vs- velocity models using full waveform inversion (FWI). Since the start of production from the field in November 2015, three vintages of time-lapse (4D) OBC data have been acquired, in 2016, 2018 and 2020. In all instances, both PP-and PS- data have been processed and imaged to extract as much 4D information as possible. However, the PS data have been challenging to use in an image sense, as the quality of the images have been low, especially when compared to the PP data. A new PS-FWI method including PS-reflection data has been proposed to update and build a new high-frequency Vs velocity model. This includes both the low-wavenumber background model, as well as the high- frequency perturbation model, to bring the Vs model all the way up to 30Hz. The new 30Hz Vs model combined with a 65Hz Vp model is demonstrated to produce superior images of the reservoir target, both using RTM and FWI derivative image algorithms (FWI Imaging). The method shown here, using processed radial PS data components, is a key milestone in ongoing work to utilize raw PS component data, which will ultimately lead to a full elastic FWI solution.

Summary This study focuses on limitations in imaging of fault geometries explored by combined seismic interpretation and modelling. The challenges and pitfalls related to interpreting normal faults are addressed by evaluating objective and subjective uncertainties. Geological models with different architecture were made based on fault interpretations on 2D seismic section by a test-panel of 20 geoscientists. More detailed and realistic fault architectures, based on published knowledge from outcrop studies including damage zones and fracture corridors, are included in refined geological models. The geological fault models, both with and without damage zones, were explored with 2(3)D point-spread function (PSF)-based convolution seismic modelling to investigate the potential of seismic data to image detailed fault architectures and associated fluids. The high-resolution seismic data show more details that combined with seismic modelling can add confidence to interpretation of conventional data. Synthetic seismic sections from the refined geological models are used to address detection thresholds for structural details in seismic data, both conventional and high-resolution. The results show that: (1) The fault is characterized by brighter reflection(s) when adding a damage zone; (2) There are stronger amplitudes for the models with CO2-filled fractures; and (3) Fracture corridors are clearly visible as dipping reflections crossing continuous horizons/layers.

Summary Additional insight into the interaction between pressure and fluid saturation changes within the reservoir can be extracted through discrimination of time lapse (4D) signal from PP and PS data. Previous processing of the 4D PP dataset over the Edvard Grieg field from 2016 to 2022 yielded interpretive saturation related 4D changes due to production and injection. The PS radial data followed, centred on a co-processed up/down deconvolution (referred to here as radial/down deconvolution for PS) workflow in alignment with the PP route. The result was a unique 4D PS response to that of PP due to changes being pressure related. The UDD workflow involved spectral division of the radial with the downgoing wavefield in the 3D tau,p,q domain. The output was shaped to a user-defined wavelet and 3D deghosted. Although the condensed and optimised deconvolution process requires minimal processing, the result is sensitive to instrument calibration, source variability, and spatial sampling. Performing joint PP-PS tomography provided an uplift in depth-domain event alignment, which helped to correlate structural boundaries with the extracted 4D attributes. The 4D PS result correlates well with the predicted model and the low frequency response was observed to be significantly improved over the legacy processing route.

Summary The 3700km2 Nordkapp Basin area, Barents Sea, was recently acquired with a wide-spread source-over-spread design. With its 6 sources sitting on top of 18 multi-sensor streamers, one sail-line can record a dense carpet of 108 sublines separated by only 6.25m. By thinly sampling the near offset over the full azimuth, this new source-over-spread setup is particularly well suited to image the steeply dipping salt flanks that extend up to the water-bottom. After application of a dedicated processing sequence carefully designed to honour the full resolution of the recorded data, the obtained high-resolution image is able to distinguish even small-scale geological features. This large 3D volume was also compared with an NFH image. Using the full benefits of the hexa-source, a high-end processing sequence was applied to the NFH data to overcome the usual weak signal-to-noise ratio of such records. The comparison between the two final images confirms the high-resolution quality of the source-overspread volume, which includes enhanced lateral resolution, especially along the crossline direction, and access to AVO and RMO information. On the other hand, the very thin vertical sampling of the NFH data extends the recorded bandwidth by two octaves, making it a possible complement to the source-over-spread image.

Summary Multi-component data recording from ocean-bottom seismic (OBS) surveys captures both PP and PS (converted wave) events. Processing such data can produce superior images compared to those obtained from conventional streamer acquisitions. In addition, PP and PS images can provide valuable insights into reservoir properties. However, PS imaging needs high-quality and high-resolution P- and S-wave velocity models in depth. While full-waveform inversion (FWI) for P-wave velocity model building is well established, an equivalent tool for updating the S-wave velocity (Vs) is still a challenge. A recent FWI methodology based on PS reflection data (PS-RFWI) has been proposed for the Vs model building. Updates from this technique are typically low wavenumber in nature. In this abstract, we show an application of PS-RFWI to OBS data from the Central North Sea and demonstrate an approach to update the high-wavenumber Vs components. Our real data application produces a high-quality 30 Hz Vs model that reduces the image undulations at the reservoir level and allows to generate a subsequent high-resolution Vs FWI Image.