Geophysical Prospecting, 2015, 63, 12841310 doi: 10.1111/1365-2478.12278

Field test of sub-basalt hydrocarbon exploration with marinecontrolled source electromagnetic and magnetotelluric data

G. Michael Hoversten1, David Myer2, Kerry Key1, David Alumbaugh3,Oliver Hermann4 and Randall Hobbet41Chevron Energy Technology Company, 6001 Bollinger Canyon Road, K1010 San Ramon, CA 94583, USA, 2Blue Green Geophysics,3Scripps Institution of Oceanography, and 4Chevron Norge AS

Received January 2014, revision accepted January 2015

ABSTRACTThe recent use of marine electromagnetic technology for exploration geophysics hasprimarily focused on applying the controlled source electromagnetic method for hy-drocarbon mapping. However, this technology also has potential for structural map-ping applications, particularly when the relative higher frequency controlled sourceelectromagnetic data are combined with the lower frequencies of naturally occurringmagnetotelluric data. This paper reports on an extensive test using data from 84marine controlled source electromagnetic and magnetotelluric stations for imagingvolcanic sections and underlying sediments on a 128-km-long profile. The profileextends across the trough between the Faroe and Shetland Islands in the North Sea.Here, we focus on how 2.5D inversion can best recover the volcanic and sedimentarysections. A synthetic test carried out with 3D anisotropic model responses showsthat vertically transverse isotropy 2.5D inversion using controlled source electromag-netic and magnetotelluric data provides the most accurate prediction of the resistivityin both volcanic and sedimentary sections. We find the 2.5D inversion works welldespite moderate 3D structure in the synthetic model. Triaxial inversion using thecombination of controlled source electromagnetic and magnetotelluric data provideda constant resistivity contour that most closely matched the true base of the volcanicflows. For the field survey data, triaxial inversion of controlled source electromag-netic and magnetotelluric data provides the best overall tie to well logs with verticallytransverse isotropy inversion of controlled source electromagnetic and magnetotel-luric data a close second. Vertical transverse isotropy inversion of controlled sourceelectromagnetic and magnetotelluric data provided the best interpreted base of thevolcanic horizon when compared with our best seismic interpretation. The structuralboundaries estimated by the 20-m contour of the vertical resistivity obtained byvertical transverse isotropy inversion of controlled source electromagnetic and mag-netotelluric data gives a maximum geometric location error of 11% with a mean errorof 1.2% compared with the interpreted base of the volcanic horizon. Both the modelstudy and field data interpretation indicate that marine electromagnetic technologyhas the potential to discriminate between low-resistivity prospective siliciclastic sed-iments and higher resistivity non-prospective volcaniclastic sediments beneath thevolcanic section.

Key words: Electromagnetic, Sub-basalt.

E-mail: hovg@chevron.comNow at: NEOS Geosolutions

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INTRODUCTIO N

Seismic imaging for hydrocarbon reservoirs located beneathbasalt is often challenging due to the high velocity and extremeheterogeneity of basalt flows. When assessing the prospectiv-ity beneath basalt, a critical factor is the presence of sedimentswithin a depth range that will accommodate hydrocarbons. Ifsediments are present, the depth extent and structure of theoverlying basalt is critical to constructing accurate velocitymodels for migration of seismic data and to develop drillingplans. Recent drilling results based on interpretation of thebasalt thickness from seismic data on the Norwegian NorthAtlantic Margin (NAM) have shown misinterpretation of thebasalt thickness in excess of 1 km in some cases. Explorationwells in the NAM cost in the range of hundreds of millionsof U.S. dollars, so there is a significant motivation for usingnew technology to improve base basalt location. In this paper,we demonstrate that a combination of controlled source elec-tromagnetic (CSEM) and marine magnetotelluric (MT) datacan provide inversion models of the basalt section along withunderlying sediment and structures that can be used to dis-criminate between differing seismic interpretations and hencereduce the risk for drilling predictions and improve migrationvelocity models.

Early application of marine MT data focused on imagingthe base of resistive salt bodies (Constable et al. 2000;Hoversten, Morrison, and Constable 1998; Key, Constable,and Weiss 2006), whereas early application of MT andCSEM for imaging the base of basalt was first performed onland. Prieto et al. (1985), Warren and Srnka (1992), Witherset al. (1994), Morrison et al. (1996), and Smith et al. (1999)used MT data to image the base of massive basalt flows inthe Columbia River Basin. More recently, Strack and Pandey(2007) and Colombo et al. (2011) have presented work usingon-shore MT and CSEM for subbasalt imaging and Jegen etal. (2009) combined gravity and MT data.

While there are a large number of exploration and pro-duction wells drilled on the NAM, relatively few have beendrilled into basalt, and even fewer have penetrated the base.Only a few cases have been reported where marine CSEM andMT data were acquired over such well penetrations. In 2012,Chevron undertook a calibration survey of CSEM and MTdata on a profile that spans the FaroeShetland Trough withthe Faroe Islands to the northwest and the Shetland Islands tothe southeast. This profile connected two wells that had pene-trated the base of massive basalt flows: Brugdan to the north-west and Rosebank to the southeast. While the Rosebank wellpenetrated through basalt flows and underlying volcaniclastic

into prospective siliciclastic sediments, the Brugdan well hittotal depth in volcaniclastic sediments beneath the massivebasalt flows. This test line was selected because we had twowell penetrations of the volcanic section. Furthermore, theseismic definition of the base of the basalt was considered tobe of high quality, at least around the Rosebank well. Thequality of the seismic image of the base of the volcanic sectionis far worse in many other exploration plays.

S U R V E Y G E O M E T R Y A N D D A T APROCESS ING

The calibration survey is a combination of a 128-km-long re-gional 2D line with 86 CSEM/MT receivers connecting thetwo wells, and two small 1212 km 3D surveys surroundingthe Brugdan and Rosebank wells (Fig. 1). Of the 138 deploy-ments planned for the survey, 135 returned usable CSEM dataand 133 returned usable MT data, resulting in 84 successfulstations along the 2D profile. The receiver spacing on the re-gional 2D line that ties the two wells is 1.5 km, whereas thereceiver spacing of the orthogonal lines that make up the mini-3D surveys around the wells is 2 km. In this paper, we onlyconsider the regional data from the 2D profile.

The survey spans the FaroeShetland Trough with re-ceivers positioned at seafloor depths varying from 240 m to1180 m (Fig. 2). Continuous measurements of water con-ductivity by the CSEM towfish, augmented by depth versusconductivity profiles measured using expendable bathyther-mographs, show the water conductivity varying from 3.70S/m near the surface to 2.94 S/m at depth. To account forthis variation, we built a stratified water model using the av-eraged conductivity in 100-m-thick layers extending to belowthe thermocline, where the water conductivity stabilizes.

Instruments were deployed using the standard free-falltechnique and most landed within 100 m of their plannedlocations. Since no instrument carried a compass, instrumentorientations were derived by post-processing the CSEM data.The contractor uses multiple, semi-independent methodswhose details are held private as intellectual property. There-fore, we independently validated the reported orientationsusing the orthogonal Procrustes rotation method outlinedin Key and Lockwood (2010). The differences betweenour solutions and the contractors have an approximatelyGaussian distribution with a mean of zero and standarddeviation of about 2. Perturbation analysis shows that thislevel of orientation uncertainty corresponds to a relativeuncertainty in the modelled CSEM and MT responses that isless than 1% for the inline receivers along the 2D profile.

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1286 G. M. Hoversten et al.

Figure 1 Base map showing the 2D regional CSEM/MT line with 1.5 km receiver spacing running from northwest to southeast over theBrugdan-1 and Rosebank-1 wells. Existing oil fields (green) and gas fields (red) are shown. Small 3D surveys are laid out around each well with2-km receiver spacing on 5 towlines that are orthogonal to the 2D regional line.

MT impedance was derived using the robust, multi-variate errors-in-variables method described by Egbert (1997),where data from multiple stations are used to discriminatecoherent MT signal from incoherence noise. Because robustmethods can sometimes return poor results if there are toomany noise sources in the data (e.g., Weckmann, Magunia,and Ritter 2005), we used local coherence and amplitude mea-sures to limit the input data to that with high-quality signal(i.e., retaining data with high coherence and omitting datawith low amplitude). This greatly improved the quality of theresulting impedance.

The CSEM survey included eleven towlines, one acrossthe Trough and five orthogonal tows over each 3D patch. Inall, 395 receiver plus towline pairings were processed, and

six frequencies were obtained: 0.2 Hz, 0.4 Hz, 0.6 Hz, 1.0Hz, 1.4 Hz, and 2.6