The British Geological Survey has concluded that the NorthFalkland Basin contains the second richest documentedsource rock in the world. They predict that as much as 100billion barrels of oil have been expelled, much of which couldstill be trapped beneath the regional seal (Richards and Hillier,2000). Despite this encouraging hypothesis, during the 50-yearexploration history of the area, only six wells have been drilled.These all lie in a small area, and to date there has been no com-mercial production, although live oil has been recovered tothe surface.

The Falkland Islands are remote, lying in the southwestAtlantic Ocean about 300 miles from Argentina. By virtue ofthis fact, the cost of any drilling in this region is likely to bemore expensive than in other parts of the world where explo-ration is more mature. Therefore, there is a greater imperativeto reduce the exploration risk as much as possible, and developa sound business case to justify the time and cost involved.

Until recently, the only way to identify whether hydro-carbons were present in an identified prospect was to drill awell. However, with as many as two-thirds of exploration wellsfailing to result in commercial discoveries, over the last fewyears, oil and gas companies have begun using controlled-source electromagnetic (CSEM) sounding as a key componentin their exploration toolkits to complement their seismic data,gain a more complete picture of the subsurface, delineatereservoirs, and reduce the risk of drilling costly dry holes.

Seismic surveying works well in many contexts; it has beeninstrumental in mapping subsurface structures and is partic-ularly effective at identifying the existence of a structural trap.However, it provides little information on the nature of thefluids within these structures and cannot easily distinguishbetween water-bearing and hydrocarbon-bearing geologicformations. In contrast, CSEM can identify, map, and imageelectrically resistive bodies in the Earth. Oil-filled reservoirsare up to two orders of magnitude more resistive than water-filled reservoirs, making resistivity structure a valuable directhydrocarbon indicator.

During a CSEM survey, electromagnetic signals are trans-mitted from a source to an array of seafloor receivers. Variationsin the received signal are interpreted using a combination offorward modeling, inversion, and imaging techniques to pro-duce resistivity cross-sections of the seafloor. Recent researchand development by OHM has indicated that combining andinterpreting seismic and CSEM data can improve under-standing of the nature of prospects and whether they arelikely to contain hydrocarbons or water. The case study pre-sented here demonstrates that results of this approach increasecertainty and informed decision-making in the explorationprocess.

Geologic setting. The North Falkland Basin is a NS-trendingAtlantic rift basin around 250 km in length and 50 km inbreadth (Figure 1). The southern parts of the basin are char-acterized by a number of transverse, NW-trending halfgrabens. The basin is dominated by lacustrine, deltaic, tur-biditic fan, and alluvial sediments comprising interbeddedsandstones and organic shale lithologies.

Six wells were drilled in the basin during 1998 by Shell,Amerada Hess, Lasmo, and IPC. The wells proved at least

two source rock intervals, with one well (now located onRockhopper acreage) drilled through a 1000-m sequence ofEarly Cretaceous source rock. A deeper source is also pre-sent in the basin, and this gave a strong gas show at 4500m in the first Shell well, the deepest in the basin. Waterdepths are 120–550 m across the area.

The North Falkland Basin has been proven petrolifer-ous, and the Rockhopper licenses include attractive struc-tural and stratigraphic prospects and leads with expectedsource, seal, and reservoir sequences. Rockhopper’s FalklandIsland acreage is split into two distinct parts within theNorth Falkland Basin. Licenses PLO32 and PLO33 lie in thenorth of the basin. Two wells were drilled by Shell in 1998in this area. One (which was drilled to 3000 m in 12 days)recovered live oil at the surface. Rockhopper is currentlyacquiring 3D seismic data over these licenses.

Licenses PLO23 and PLO24 are in the south of the basinin water depths of around 160 m. The Ernest prospect liesin PLO24, some 60 km from the nearest well. There are twopotential closures at Ernest (Figure 2), a four-way closurewith an area of 2300–2800 acres and a larger fault-boundedclosure with an area in excess of 12 000 acres. Seismic datashow localized potential flat spots within Ernest. There isalso an AVO response on the flank of the structure, and apossible gas chimney over the prospect, both of which couldbe hydrocarbon indicators. However, these data by them-

Derisking exploration prospects using integrated seismic and electromagnetic data—a Falkland Islands case studyLUCY MACGREGOR, NEVILLE BARKER, and ANDY OVERTON, OHM, Aberdeen, ScotlandSAM MOODY and DAVE BODECOTT, Rockhopper Exploration Plc, Salisbury, England

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Figure 1. Overview of the North Falkland Basin structure.

selves are of insufficient clarity to suggest the prospectshould be drilled.

The large variety of prospects and leads in this remoteacreage combined with the relatively sparse 2D seismic cov-erage makes 3D seismic or drilling an uneconomic methodof further derisking targets. As CSEM makes a completelydifferent and complementary measurement to seismic andone which is likely to provide a much more reliable directhydrocarbon indicator, it was the most economically andtechnically sensible way for Rockhopper to significantlyderisk its targets.

Survey operations and data interpretation. Survey opera-tions over the Ernest prospect took place during January andFebruary 2006. The data set comprises three-componentelectric field data from 26 seafloor receiver locations, dis-tributed along two lines (Figure 2). A subset of receivers alsorecorded the horizontal magnetic field at the seafloor. The

source tow lines are also shown in Figure 2. The sourcewaveform was designed to transmit a range of frequencies,chosen on the basis of presurvey modeling to optimize sen-sitivity of the resulting data to the structures of interest. Dataprocessing resulted in good-quality amplitude and phasedata to a source-receiver offset of 15 km in the fundamen-tal transmission frequency. For the highest frequencies trans-mitted, maximum ranges of 5–6 km are typical.

Data interpretation proceeds in stages. The first stageinvolves simple 1D reconnaissance modeling and data nor-malization. This allows gross trends in the data to be iden-tified. Such results can, however, be difficult to interpret inareas where there is a large and resistive structure in the sec-tion. This is illustrated in Figure 3, which uses a model builtto simulate the structures likely to be encountered, whichis in this case a resistive basement rock. Synthetic data gen-erated at the fundamental transmission frequency have beennormalized and plotted at source-receiver midpoints for acommon offset of 6 km. It is clear that the effect of the basin-bounding fault dominates the response.

For comparison, Figure 4 shows the equivalent plot forthe data collected along line 1. In this case the normalizingmodel is an average 1D structure derived from data collectedaway from the basin-bounding fault. Three offsets areshown: 3, 6, and 8 km. At 3-km offset, the response is dom-inated by relatively shallow structure, and there is littlevariation along the line. For the longer ranges, the signalstrength increases dramatically to the east of the basement-bounding fault. Although Figure 3 demonstrates that the tar-get has an effect on the response, in practice givenuncertainties in the background resistivity structure, thiswould be difficult to distinguish from the much larger effectof the resistive basement on the basis of a normalized plotsuch as this. Accordingly, while these normalized plots canbe a good QC tool, they are an extremely unreliable directhydrocarbon indicator, especially where any kind of geo-logic complexity exists.

A further limitation of simple normalized data plots isthat they utilize only a very small subset of the data—in thecase of Figure 4, single-frequency amplitude data, at threesource-receiver separations. In practice the data contain awealth of additional information; the full recorded electricfield comprises both an amplitude and phase component,

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Figure 2. Survey geometry over the prospect. Seafloor receivers are thewhite triangles, and the source tow lines are in red. Contours are depth inmeters to the shallow structural closure. This reflector is at the shalloweststructural level at which the closure first becomes apparent. There isobviously substantial closure at deeper mapping levels too. Blue dashedline=four-way closure defining the Ernest prospect. Yellow dashedline=larger fault-bounded closure.

Figure 3. (top) Resistivity model based on the expected structure in theregion of the Ernest prospect. (bottom) Normalized amplitude anomaliesplotted at source-receiver midpoint for a common offset of 6 km. The 1Dnormalizing structure was taken from the left side of the model. Althoughthe target reservoir has an effect on the response, in practice this would bedifficult to isolate from the much larger effect of the basement-boundingfault.

Figure 4. Normalized anomalies from line 1, plotted at three commonoffsets for the fundamental transmission frequency. The normalizingmodel was an average 1D layered structure derived from data collectedaway from the basin-bounding fault. Results are similar to the modeledcase shown in Figure 3. The effect of the basement fault dominates theresponse at large offsets.

at a range of source-receiver separations and multiple fre-quencies. To more fully utilize the data, we use a full-wave-field 2.5D inversion approach. This identifies a 2D resistivitysection that gives good agreement with the processed data.The results of inverting amplitude and phase data fromeach line to give a vertical resistivity section are shown inFigure 5, corendered with coincident seismic data.

The resistivity section along line 1 shows a large base-ment high to the east of the line, coinciding with the basinboundary fault. There is a more subdued basement high tothe west of the line. There is a laterally extensive shallowresistor at depths less than 500 m below the sea surface,beneath which more conductive material extends to thebasement. Within this conductive background is a localizedresistor; this is centered at a depth of approximately 1500 mtoward the eastern end of the line, and correlates well (in

depth and lateral extent) with theknown location of the Ernestprospect.

Along line 2 the depth to base-ment decreases slightly to the north.The shallow resistive layer andbackground structure are consistentwith the structure along line 1. Thelocalized resistive feature identifiedon line 1 is also present on line 2, andis again consistent with the extentof the Ernest prospect. There is alsoa second resistor, slightly shallowerin the section in the southern partof the large closure of Ernestprospect. Possible causes of this fea-ture are discussed next.

Discussion. The primary aim of theCSEM survey was to determinewhether there was a high-resistiv-ity zone, which could be indicativeof saturation with hydrocarbons,coincident with the mapped closureof the structure of the Ernestprospect. The CSEM survey resultsclearly show that there is such azone. A high-resistivity zone coin-cident with an identified prospectcannot unambiguously demonstratethe presence of hydrocarbons (othergeologic materials such as tight car-bonates, volcanics or coals can alsoexhibit increased resistivity).However, if that high-resistivityzone is clearly seen to be containedwithin a seismically defined struc-ture, it is hard to explain other thanby the entrapment of fluids whichare electrically more resistive thanformation water. Such a resultincreases significantly the probabil-ity that the prospect is hydrocarbon-bearing.

The secondary question iswhether the CSEM data can indicatethe extent (and therefore volume)of the possible hydrocarbon reserve.Figure 5 shows the results from thetwo survey lines with the prospectextents overlain. It appears that the

resistive anomaly on line 1 is truncated before the basinbounding fault to the east, suggesting that the larger fault-bounded closure may not be full to spill. On both lines 1and 2, the anomaly coincides well with the four-way clo-sure. The additional resistor on line 2 gives increased con-fidence that hydrocarbons may be present at least in part ofthe larger 12 000-acre closure, perhaps to the south of thecentral EW fault, an interpretation supported by AVO evi-dence.

The inversion scheme used in this work recovers smoothresistivity structures. While this regularization process isadvantageous in that it can avoid the addition of spuriousstructure, localized resistivity features and those with sharpboundaries will become smeared in the result. In the case ofthin resistive features, this means that the absolute resistiv-ity recovered in the inversion process will be lower than in

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Figure 5. Results of inversion of data from line 1 (top) and line 2 (bottom). The crossover point of thetwo lines is indicated by the white dashed line. There is a localized resistor consistent with the Ernestprospect on both lines. Mismatch between the lines is caused by the effect of 3D in the structure.

reality, whereas the integrated resistance (for a single layerthis corresponds to the resistivity-thickness product) throughthe feature will be well-constrained (MacGregor et al., 2006).The integrated resistance through the center of the Ernestresistor is 2750 Ω-m2 on line 1 and 2500 Ω-m2 on line 2.Mapping this back into a 100-m reservoir interval, this wouldcorrespond to a reservoir resistivity of 25–27.5 Ω-m, consis-tent with the range of resistivities that might be expected fora hydrocarbon-saturated reservoir.

Also of interest is the shallower resistive zone, observedon line 2 within the southern part of prospect Ernest. Althoughnot the primary target of this survey, this region of high resis-tivity lies within the survey array and is well constrained bythe data. Re-analysis of the seismic data in light of this resulthas shown that this elevated resistivity is coincident with asecond, shallower structural closure which forms part of thelarge 12 000-acre fault-bounded element of the Ernest prospect.The results of the CSEM survey, combined with the seismicindications of a flat spot within this structure and an AVOresponse on the flank, suggest the possibility of a second tar-get in the area, one that may have been overlooked based onthe seismic data alone.

Conclusions. The outcomes and analysis of the CSEM sur-vey, combined with existing seismic data, produce a morerobust and in-depth understanding of prospect Ernest thanwhat can be gained from any single geophysical measurement.Results are promising and identify a localized resistor coinci-dent with the structural expression of the Ernest prospect. Afurther resistive zone to the south of the prospect may alsowarrant further investigation.

Prior to acquisition of the CSEM data and additional 2Dseismic, prospect Ernest was a four-way dip closure with rel-

atively sparse seismic in a completely undrilled part of thebasin. There was no reason to rank this prospect higher thanother competing prospects in the basin. Although promising,there was insufficient evidence to warrant drilling Ernest.

Following the positive outcome of the CSEM survey,Rockhopper has greatly increased confidence in the prospec-tivity of the targets covered. The risk Rockhopper attaches tothe prospect has more than halved and has given sufficientencouragement to consider it a viable target for any futuredrilling campaign. The CSEM data suggest that the prospectis filled close to the spill points expected from structural map-ping from seismic data for the four-way closure. Based on thislateral extent and the expected reservoir thickness, Rockhopperconservatively views this prospect to be capable of yieldingmore than 100 million barrels of recoverable oil. Additionally,the southern part of the large fault-bounded closure at the shal-lower level is now significantly derisked by both CSEM andAVO, suggesting that the prospect can be viewed as one ofthe lowest risk opportunities in the basin.

Suggested reading. “Remote sensing of hydrocarbon layers usingSeabed Logging (SBL): Results of a cruise offshore Angola” byEllingsrud et al. (TLE, 2002). “Controlled-source electromagneticimaging on the Nuggets-1 reservoir” by MacGregor et al. (TLE,2006). “Integration of electromagnetic and seismic data to assessresidual gas risk in the toe thrust belt of the deepwater Niger delta”by Moser et al. (TLE, 2006). “Postdrilling analysis of the NorthFalkland Basin—Part 2: petroleum systems and future prospects”by Richards and Hillier (Journal of Petroleum Geology, 2000).“Remote reservoir resistivity mapping” by Srnka et al. (TLE,2006). TLE

Corresponding author: lucy.macgregor@ohmsurveys.com

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