Exploration with the use of EM data in the Barents Sea: the potential and the challenges

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1 special topic Exploration with the use of EM data in the Barents Sea: the potential and the challenges Stein Fanavoll, 1* Svein Ellingsrud, 1 Pål T. Gabrielsen, 1 Raghava Tharimela 2 and Dave Ridyard 3 argue that latest marine EM technology developments, notably mapping deeper resistive structures could prove valuable in expeditng exploration of the potential opportunities offered by the Norwegian sector of the Barents Sea. T he Barents Sea has been explored for more than 30 years, yet the area remains an enigma. Early exploration proved the presence of a working petroleum system, yet despite this, there are only few significant discoveries. Electromagnetic (EM) methods have been used in the area since 2003, and resistive anomalies have been identified over all the major finds, including the three that were discovered during the past year. Still there are areas virtually unexplored exhibiting a large variety of geologic settings. The exploration challenges are substantial, and seismic data alone may not be sufficient to overcome these challenges. Historically, the major role of EM in the area has been to reduce drilling risk, and detect major potential hydrocarbon accumulations prior to leasing. However, in the future, EM may create additional value through mapping of deeper resistive structures to increase understanding of regional geology. This is especially true in an area east of the 32 meridian, earlier referred to as the grey zone, created by the 2011 treaty between Russia and Norway which opened up one of the last unexplored basins on earth. This agreement opens a new province where EM can play an important role in frontier exploration. The Barents Sea is a vast offshore area in northern Norway, spanning from the Russian border to the deep ocean margin. It comprises a wide range of geologic settings, from Tertiary plays in the west, to Triassic in the northeast, and Palaeozoic in the south. Large parts of the area in the west and northeast are still virtually unexplored, as seen on Figure 1. Even though some production licences have been awarded in these areas in the 20 th and 21 st Norwegian licensing rounds, no recent wells have been drilled. Exploration started in the early 80s and led to several promising discoveries, including Snohvit (Norway) discovered in 1984 and Shtokman (Russia) discovered in However, after these discoveries, there was a long sequence of disappointing wells in the Norwegian part of the Barents Sea. Many wells were dry but a substantial number exhibited Figure 1 Overview of Norwegian part of the Barents Sea. Areas in west and east are largely unexplored (source: NPD fact maps). Eastern part is the disputed area between Norway and Russia. the presence of non-commercial quantities of hydrocarbons, showing that there was indeed a working hydrocarbon system in most of the area. Another factor that strongly influences the exploration risk is the late uplift and erosion in the area. During the early glaciations a few million years ago, glacial movements from the mainland towards the north and west have removed up to 3 km of sediments in some areas (Riis and Fjeldskaar, 1992). This might have severe implications for exploration: removal of sediments affects the pressure in the subsurface, leading to breached seal, expansion of gas, and renewed movements along faults (Henriksen et al., 2011). Due to the long sequence of disappointing wells, there was a total stop in exploration drilling from 1994 until 2000, when the Goliat discovery was made. This led to new optimism in the area, but again, the decade from 2000 proved a disappointment to the exploration community. In summary, there has been little success in finding commercial 1 EMGS, Trondheim, Norway. 2 EMGS, Stavanger, Norway. 3 EMGS Americas, Houston, TX, United States. * Corresponding author, sf@emgs.com 2012 EAGE 89

2 special topic first break volume 30, April 2012 volumes of hydrocarbons during the history of exploration in the Barents Sea. From the wells drilled prior to 2011, a number of lessons have been learned: n There is a working hydrocarbon system in the area n Late uplift and erosion change the pressure and temperature regime, affecting reservoir and seal properties n There is a challenge to find proper reservoir quality and volume to hold commercial amounts of hydrocarbons n Seismic as a standalone tool has proven inefficient in detecting these volumes EM in the Barents Sea Since 2003, EMGS has acquired a large amount of EM data, covering many of the different geologic provinces in the Barents Sea. From 2003 to 2007, mainly exclusive 2D surveys were carried out, but in 2008 and 2010, two extensive multi-client programmes were carried out prior to the 20 th and 21 st licensing rounds on the Norwegian continental shelf, covering a total of km 2 (Figure 2). These surveys were coarse grid 3D surveys, with the main objective being to obtain resistivity information in nominated blocks prior to licence application. Sparse receiver grids could be employed in this application, because in a frontier area like the Barents Sea, the objective is to find major reservoirs worthy of development. Several oil companies have been successful in using the multi-client EM data as a strategic tool in their licence applications. Once a discovery has been made, additional EM and seismic data can be acquired to guide field development and near field exploration. During the course of EM acquisition in the Barents Sea, EMGS has acquired data in numerous locations where well data is now available. In some cases the wells were available prior to EM acquisition, and these could be used for calibration. In other places, the wells have been drilled after the EM acquisition. All these examples show a high degree of correlation between EM results and well results. One of the most important surveys leading to an increased understanding of the electromagnetic properties of the Barents Sea was acquired over the Goliat discovery. Here, the understanding of electrical anisotropy was developed in order to explain the measured data. Furthermore, it emphasized the need for developing software to handle this anisotropy correctly. As a result, EMGS has now developed full 3D, anisotropic inversion, which ensures the best possible resistivity model of the subsurface (Mohamad et. al., 2010, Kanhalangsy et. al., 2011). In 2010, a 3D dataset was acquired over the Snohvit Field as part of the EDDA consortium project. The objective of this project was to develop state of the art acquisition, processing, and interpretation workflows, as well as providing the participants full 3D controlled source EM (CSEM) datasets for their internal use. The Snohvit dataset clearly imaged the field, in addition to highlighting interesting features for further analysis. The data shows that the electrical anisotropy is very high in this area of the Barents Sea, especially for the Cretaceous and Triassic. In the Triassic, vertical resistivity reach as high as 60 ohm-m while well logs measuring horizontal resistivity show values of 10 ohm-m. Another interesting fact is the strong magneto telluric signal recorded subsea due to high latitudes combined with relative shallow water. This provides very good marine magnetotelluric (MMT) signal which gives an additional dataset to the CSEM data. The MMT data can provide valuable information to the regional understanding of resistivity and also independently support the results of the CSEM data (Figure 3). Recent exploration history In 2011, two major new discoveries were made in 20 th round licences: the Skrugard discovery in PL532 by Statoil and the Norvarg discovery in PL535 by Total. These were followed by a second successful well in PL532, the Havis discovery in In addition to the discoveries, two dry wells were also drilled in blocks where multi-client, 3D CSEM datasets were available. In both cases, the outcome of these wells was predicted correctly based on EM data. Figure 2 Multi-client 3D EM coverage in the Barents Sea. Figure 3 Comparison between CSEM and MMT 2D inversion in the Barents Sea. MMT data can provide valuable information of the regional resistivity changes and penetrates much deeper than the CSEM signal. Solid black lines are BCU and lower Triassic horizons picked from seismic data EAGE

3 special topic PL532 : Skrugard and Havis PL532 is located on a structural element known as the Polheim Subplatform. This element is characterized by a series of down stepping rotated fault blocks from the Loppa High in the east into the deep Bjørnøya Basin in the west. This setting makes the Polheim Subplatform a challenging area for EM due to rapidly changing background responses. It might then be difficult to distinguish the reservoir responses from the complex background by simply analyzing the inverted vertical resistivity model. However, by use of the apparent anisotropy attribute, it can be shown that the Skrugard reservoir is visible on multi-client EM data from The apparent anisotropy attribute is created by dividing the inverted vertical resistivity model by the horizontal resistivity model. In general, this will provide the electrical anisotropy for the different lithological layers. In addition, it will emphasise the nature of the physics where only the vertical EM field is sensitive to a thin horizontal resistor and not the horizontal field. A hydrocarbon-filled reservoir (thin horizontal resistor) will therefore show up as an anomaly with high apparent anisotropy (Figure 4). The Havis discovery is not yet covered by state of the art anisotropic 3D inversion, but preliminary analysis of the 2008 data indicates the presence of an EM anomaly (Figure 5). In 1989, a well (7219/9-1) was drilled west of Skrugard and Havis, penetrating an excellent Lower Jurassic, 110 m thick reservoir only with shows. It was obvious that the problem was a leaking fault. Today, with the presence of high quality 3D seismic, it is straightforward to identify flatspots indicating both potential gas/oil and oil/water contacts. The presence of an EM anomaly strengthens the probability of success, but may not influence decisions, as long as this play model is pursued. However, resistive anomalies can play an important role when exploring other play models in the area, such as syn-rift sedimentation in Upper Jurassic and Lower Cretaceous. PL 535 : Norvarg The Norvarg Discovery is located some 160 km northeast of Skrugard, in an area where no significant discoveries have been made, mainly due to lack of reservoir quality or hydrocarbon volume. The discovery answers an important question as to whether sufficient high quality reservoir volumes are present in this area. Knowing that almost every well in the area has reported shows or minor accumulations, the Norvarg Discovery can be the beginning of a new era for exploration in the northeastern Barents Sea. Two of the licence partners had purchased multi-client 3D EM data licences prior to submitting their licence application. Since the discovery clearly can be associated with a significant resistive anomaly, this suggests that CSEM data can provide a valuable addition to the exploration database. EM is sensitive to saturation and volume, and therefore can indicate where commercial hydrocarbon volumes are present. Figure 4 Image of the Skrugard Discovery using the apparent anisotropy attribute in a 500 m window around the Base Cretaceous unconformity, as seen on EMGS multi-client data from Outline of discovery and location of wells are taken from Norwegian Petroleum Directorate, fact maps. Figure 5 EM anomaly consistent with Havis discovery is observed on the attribute normalized magnitude vs offset. Vertical scale is source-receiver offset. Grey line indicates the approximate position of the Havis well EAGE 91

4 special topic first break volume 30, April 2012 Challenges and future potential Future exploration in the Barents Sea is likely to be focused on areas where there has been little or no exploration. Such an area is the northeastern part, where EMGS acquired data both in 2008 and No wells have been drilled so far, but the 3D EM datasets include a substantial number of interesting observations to be investigated in order to increase the understanding of EM measurements in the area (Figure 6). One of the challenges in the area is the extremely high background resistivity. In general, some places we have observed background vertical resistivity of 100 Ωm, while anomalies exhibit up to 300 Ωm. Until now, an explanation has not been forthcoming for these values. However, in this area a large amount of erosion has taken place all the way down to the Cretaceous level. It is interesting to note that the high resistivity values for the Cretaceous and Triassic corresponds well with the same stratigraphic levels found in the Snohvit area. Future drilling is expected to give valuable information to the question regarding the high background resistivities and the nature of the anomalies. The western Barents Sea is rather different from other areas in the Barents Sea, in that the area is dominated by deep Cretaceous basins, meaning that the target level is in younger sediments than other parts, mainly in Tertiary and Upper Cretaceous. In most of the area, the target depths are favourable for the use of EM, ranging from m. Also, the Tertiary overburden exhibits little complexity, which in combination with potential stratigraphic traps, make this area ideal for CSEM technology. Several interesting resistive anomalies have already been identified in the area. Some of these are likely to be drilled in the near future. An example of an anomaly in the Tertiary/ Upper Cretaceous is shown in Figure 7. Grey Zone In 2011, an agreement with Russia over the so-called Grey Zone (Figure 8) was settled. This has been a disputed area for a long time, due to disagreement on how to divide offshore areas for exploitation. The agreement has opened a totally new area for exploration. Many experts regard the area as very promising. This is mainly due to the discoveries made on the Russian side during the last decades. Discoveries such as Shtokman and Kildinskaya together with scientific Figure 6 Anomaly in high resistive background in the northeastern Barents Sea. Figure 7 CSEM anomaly at Tertiary/Upper Cretaceous level in the western part of the Barents Sea. Figure 8 Location of the disputed zone north-east of the Varanger peninsula in Norway (red outline). Discoveries on the Russian side emphasize the potential in the region (source: GEO #4, June 2010) EAGE

5 special topic mapping demonstrate the potential of this new province. As an area which has been almost completely unexplored for over 25 years, the Grey Zone represents a unique opportunity to apply modern geophysical technology to develop more efficient exploration workflows. The Norwegian Petroleum Directory (NPD) is acquiring 2D seismic in an area of 39,000 km 2 and the acquisition will be finished this summer. 3D EM data would be a logical next step in the frontier exploration process for two reasons. Firstly, sparse grid 3D CSEM could be used as a direct hydrocarbon indicator (DHI) in order to rapidly identify major potential reservoirs, and also give an early indication of the hydrocarbon potential in the area. Secondly, 3D CSEM could be combined with magneto-telluric and potential field data to map major resistive bodies at depths up to 10 15,000 m (Figure 9). Knowledge of these resistive structures could be vital in developing a rapid understanding of regional geology of the province. Summary Regarding the overall lack of success in the exploration history of the Barents Sea, we believe that new information and ideas are essential in order to improve the chance of success in the region. In our opinion, the use of EM data can increase the exploration efficiency of the province in several ways: n Sparse grid 3D CSEM can rapidly detect major reservoirs. EMGS s 2008 and 2010 multi-client programs will be complemented by new acquisition in n Higher density 3D CSEM infill of existing sparse grids can aid in development planning and appraisal. (See companion article) n CSEM and MMT can be combined to produce improved regional geologic understanding. References Henriksen, E., Bjørnseth, H. M., Hals, T. K., Heide, T., Kiryukhina, T., Kløvjan, O.S, Larssen, G. B., Ryseth, A. E., Rønning, K., Sollid, K. and Stoupakova, A. [2011] Chapter 17 Uplift and erosion of the greater Barents Sea: impact on prospectivity and petroleum systems. Geological Society, London, Memoirs January 1, 35, Kanhalangsy, C., Golubev, N., Zach, J. J. and Baltar, D. [2011] Anisotropic CSEM Inversion near the Tiger well in AC 818, Gulf of Mexico. 81 st SEG Annual Meeting, Expanded Abstract, 30, Mohamad, S. A., Lorenz, L., Hoon, L.T., Wei, T. K.. Chanola, S. K., Saadah, N., Nazihah, F. [2010] A practical example why anisotropy matters A CSEM case study from South East Asia. 80 th SEG Annual Meeting, Expanded Abstract, 29, Riis, F. and Fjeldskaar, W. [1992] On the magnitude of the Late Tertiary and Quaternary erosion and it s significance for the uplift of Scandinavia and the Barents Sea. In Larsen, R.M, Brekke, H., Larsen, B.T. and Talleraas, E. (Eds.) Special publication 1, Norwegian Petroleum Society (NPF), A NEW Performance Frontier! Dynamic Reservior Simulations > Runtime interactive Fully Utilizes Dual 8-core Intel Xeon E Cluster performance on a Desktop NO license FEES for EXTRA CORES Contact us for a FREE TRIAL Figure 9 3D MT inversion of Barents Sea salt body indicates the powerful potential to image deep resistive structures with 3D EM methods * Speciic performance gains depend on the model and may differ EAGE 93

6 EMGS: 10 th birthday observations As a postscript to the main article Svein Ellingsrud, a founder of EMGS, and Dave Ridyard, executive vice president, strategic business development, reflect on the company s roller-coaster first 10 years of pioneering a new technology. The marine EM industry has had a wild ride. In its short history the industry has featured rapid technological innovation: it is as if the last 50 years of seismic imaging development have been compressed into 10 years, because EM has moved in a short period from its initial 2D offering to today s wide-azimuth 3D capability. In these years many early adopter customers experienced immediate successes, which in some cases were followed by disappointments. However, as the technology has matured, operators and contractors have become more aware of where and how to use EM methods, and the role of EM is gradually becoming better understood and better integrated into the workflow. From the contractor perspective, all of this translated into a wave of euphoria, followed by painful consolidation. As we embark upon a new decade, EMGS has emerged as the sole surviving integrated marine EM service provider. Back in the early 90s, Geoffrey Moore wrote a seminal book on technology adoption ( Crossing the Chasm ). For those of us that have lived through the highs and lows of the EM technology adoption cycle, it is surprising, and perhaps comforting to step back and realize that the swings experienced over the years actually conform to classical technology adoption theory. Here we reflect on where marine EM is today and where it can go in the future. Commercial perspective After the early euphoria, the EM industry went through a bleak period especially in 2009 when E&P investment dollars were hard to come by and as a result this relatively new technology came under severe scrutiny. On the positive side, the experience prompted EMGS as a marine EM services provider to become more focused on delivering value to the customer. As a result we see today a much broader based demand in terms of NOCs, independents, and super majors, and it may well be that 3D EM has indeed crossed the chasm. One signal of wider industry acceptance of the technology has been the growing interest in multi-client 3D EM data. 2010/2011 witnessed some significant landmarks for the marine EM business. First EMGS acquired its main competitor (OHM), and upgraded the OHM vessels to EMGS technology and safety standards, thus significantly expanding the capacity of the company s fleet. More recently, EMGS settled its intellectual property disputes with Schlumberger enabling the two companies to enter into a collaboration agreement. Under the terms of this agreement, the parties Figure 1 EMGS revenues over the last 10 years track the classical technology adoption pattern with uncanny fidelity. are free to compete against each other where customers desire competition. However, the industry now has an additional option. Currently, no single service provider offers the perfect workflow for EM interpretation, and few oil companies have the experience and technical expertise to cherry pick technologies from multiple vendors and extract the maximum value from their EM data. The Schlumberger/ EMGS collaboration allows a customer to work with both vendors to create the best possible workflow for each unique subsurface challenge. EM technology: today and tomorrow Going forward we envisage four different applications of EM technology. Drill or drop: the traditional EM application The early dream of EM was to reduce the number of dry holes, by verifying the presence of a resistive anomaly at a proposed drilling target. In the real world, resistivity is just one more clue to build a case for or against different geologic scenarios. With over 600 projects acquired several trends are now apparent: n False negatives are very rare, and are usually avoidable with careful post inversion modelling n False positives are more common. Some are the result of non-hydrocarbon-related resistors, and others are the result of acquisition and processing artifacts. However, it is worth noting that the EM industry learns from each failure, and success rates have steadily improved over the last 10 years, due to improvements such as the movement towards high quality instrumentation, wide-azimuth 3D geometries, and anisotropic 3D inversion EAGE 95

7 n Industry publications from sources such as Shell 1 and Statoil 2 have estimated that EM can correctly predict drilling outcomes for 70 80% of EM appropriate geologic settings. The EM Risk Reduction Consortium is a recent initiative to improve the knowledge of EM reliability in a wide range of geologic settings. n With modern quantitative interpretation techniques, 3D EM surveys are now capable of significantly improving the reliability of pre-drilling resource estimation. (See Figure 2) Field development applications The emergence of meaningful inter-disciplinary quantitative interpretation techniques (Morten et al., 2011) is opening up another whole suite of applications to use EM for delineation, appraisal, well placement, and even production monitoring. The next generation of acquisition and processing technology will move these applications into the mainstream of the EM industry. Frontier applications The Barents Sea multi-client project of 2008 (and 2010/2012) has proved that low cost, sparse 3D EM grids can detect significant reservoirs with the potential to substantially reduce finding costs in frontier areas. (See Fanavoll et al. article above). Structural imaging applications Much of the early EM work focused on the detection of thin buried layers effectively a direct hydrocarbon indicator (DHI) application. However, the improvements in EM instrumentation over the past few years have led to a new set of opportunities for EM applications focused on mapping non-hydrocarbon-related subsurface resistive features such as salt, basalt, carbonates, etc. A number of commercial and research-oriented projects combining 3D CSEM and magneto-telluric data (MT) suggest that these techniques offer great value both as an independent subsurface measurement and as an aid to the construction of more accurate velocity models for seismic imaging. Summary As EMGS passes its 10 th birthday, we believe there will be plenty of late adopters who have yet to harness the value of EM technology. However, in our view the mainstream of the E&P industry is now using 3D EM where it can be shown to have value. It follows that the introduction of more applications and new technology developments that marine EM will continue to broaden its market niche over the next 10 years. Further reading Buland, A. et al. [2011] The value of CSEM data in exploration. First Break, 29(4) Morten, J.P. et al. [2011] 3D reservoir characterization of a North sea oil field using quantitative seismic & CSEM interpretation. 81 st SEG Annual Meeting, Expanded Abstract. Smit, D. [2011] Challenges to marine CSEM for hydrocarbon exploration. Marelec 2011, San Diego. DAARC500 Data Acquisition & Adaptive Aeromagnetic Real-Time Compensation Figure 2 In this example, a positive CSEM response has achieved two goals. (i) The uncertainty in Hydrocarbon volume, expressed as P10/P90 has been reduced by over 50% and (ii) the probability of encountering a field of commercial size has been increased from 30% to 50%. 2 nd Generation DAARC500 Comprehensive and flexible DAS 8 isolated RS232 channels, 1-Gbps Ethernet 32 analog inputs (16-bit) Embedded GPS receiver Up to 8 magnetometers Less than 0.1 pt internal noise Sampling rate over 1kHz Proven, extremely robust compensation algorithms Adaptive signal processing techniques Complete Solution for Aeromagnetic Surveys RMS Instruments Mississauga, Ontario, Canada Tel: (905) rms@rmsinst.com EAGE