Marine controlled source electromagnetic (CSEM) data have been utilized in the past decade during petroleum exploration of the Barents Shelf, particularly for de-risking the highly porous sandstone reservoirs of the Upper Triassic to Middle Jurassic Realgrunnen Subgroup. In this contribution we compare the resistivity response from CSEM data to resistivity from wireline logs in both water- and hydrocarbon-bearing wells. We show that there is a very good match between these types of data, particularly when reservoirs are shallow. CSEM data, however, only provide information on the subsurface resistivity. Careful, geology-driven interpretation of CSEM data is required to maximize the impact on exploration success. This is particularly important when quantifying the relative resistivity contribution of high-saturation hydrocarbon-bearing sandstone and that of the overlying cap rock. In the presented case the cap rock comprises predominantly organic rich Upper Jurassic–Early Cretaceous shales of the Hekkingen Formation (i.e. a regional source rock). The resistivity response of the reservoir and its cap rock become merged in CSEM data due to the transverse resistance equivalence principle. As a result of this, it is imperative to understand both the relative contributions from reservoir and cap rock, and the geological significance of any lateral resistivity variation in each of the units. In this contribution, we quantify the resistivity of organic rich mudstone, i.e. source rock, and reservoir sandstones, using 131 exploration boreholes from the Barents Shelf. The highest resistivity (>10,000 Ωm) is evident in the hydrocarbon-bearing Realgrunnen Subgroup which is reported from 48 boreholes, 43 of which are used for this study. Pay zone resistivity is primarily controlled by reservoir quality (i.e. porosity and shale fraction) and fluid phase (i.e. gas, oil and water saturation). In the investigated wells, the shale dominated Hekkingen Formation exhibits enhanced resistivity compared to the background (i.e. the underlying and overlying stratigraphy), though rarely exceeds 20 Ωm. Marine mudstones typically show good correlation between measured organic richness and resistivity/sonic velocity log signatures. We conclude that the resistivity contribution to the CSEM response from hydrocarbon-bearing sandstones outweighs that of the organic rich cap rocks.
Bibliographical noteFunding Information:
This research is funded by the Research Centre for Arctic Petroleum Exploration, supported by industry partners and the Research Council of Norway (Grant No. 228107). PB was partly financed by the Norwegian CCS Centre, financed by the Research Council of Norway (Grant No. 257579) and industry partners. We sincerely appreciate the generous data access, particularly from the Norwegian Petroleum Directorate (DISKOS database, NPD FactPages and Svalbard exploration boreholes) and the UNIS CO2 lab (http://co2-ccs.unis.no) for access to the CO2 research boreholes onshore Svalbard. EMGS kindly provided CSEM data at eight well locations. Amando Lasabuda kindly provided digital versions of published net erosion maps. UNIS acknowledges the academic licenses of Petrel and the Blueback Toolbox generously provided by Schlumberger and Cegal, respectively. Finally, we are grateful to two anonymous journal reviewers for their constructive feedback, and Gareth Lord for proof-reading the manuscript.
© 2019 China University of Geosciences (Beijing) and Peking University
- Barents Shelf
- Source rocks