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ARTICLE Hot Rocks and Oil: Are Volcanic Margins the New Frontier?

ASSESSING THE CHALLENGES AND OPPORTUNITIES OF FINDING AND EXTRACTING NATURAL RESOURCES FROM A VOLATILE ENVIRONMENT By Dougal A. Jerram Hot rocks and oil: Are volcanic margins the new frontier?

VISITING MODERN VOLCANOES: LEARNING FROM PAST AND PRESENT Volcanoes evoke a classic image of a cone-shaped mountain exploding ash, or rivers of molten rock. The lure of the is almost primordial. Looking back into the Earth’s past volcanic events—including some of nature’s most beautiful volcanoes and eruptions in the present (e.g., Bárðarbunga, ; , ; Pu`u `O`o, - see Figure 1)—can shed light on the active cooling planet and the role volcanoes played in Earth’s evolution1.

Figure 1: Understanding modern volcanic systems; a) recent volcanic activity at Bárðarbunga with fissure eruptions which fed flows (Sept 2014), b) sampling at the front of the advancing lava where it is diverting a meltwater river (Sept 2014), c) sampling lava flows at Hawaii1, d) the volcano Stromboli erupting at sunset1.

Every new volcanic eruption is a real-time opportunity to capture exactly what happens over time and allows researchers to examine the resultant lava flows, explosive ash horizons and other deposits. The spectacular Bárðarbunga, for instance, provided examples of the erup- tion entering and diverting a modern river system that came down from the ice cap (e.g. Figure 1b). This eruption also became the largest in Iceland for over 200 years with the lava field covering 85 km², and an erupted volume of 1.4 km³ of new lava2. Older rock masses also provide clues of where ancient volcanic episodes occurred, their timing and their relationship to surrounding rocks and Earth history. Modern systems provide the key to understanding the past volcanic events, and geoscientists use the ancient dissected systems to help them understand what happens within and beneath volcanoes as they build volcanic constructions.

2 Examining modern volcanoes and the ancient volcanic past can offer insights into the relationships between volcanic rocks, the sediment/petroleum systems associated with them and the exploration potential in volcanic basins – between hot rocks and oil. Seeing how modern sedimentary systems interact with volcanic eruptions and the resultant deposits provides a direct view of how both competing processes act on one another. Studying the ancient deposits preserved in onshore segments of volcanic systems also allows analogues to be drawn with offshore exploration targets3. These insights, when applied with other research findings, can improve the understanding of exploration plays, reduce risk and lead to better outcomes. The next stage is to investigate the role of volcanic margins in present and future natural resource exploration.

WHAT ARE VOLCANIC MARGINS AND WHY MIGHT THEY BE IMPORTANT FOR NATURAL RESOURCE EXPLORATION? Many of the sedimentary basins that have accumulated the rocks that can create, trans- port and store oil and gas in the subsurface have also been the sites of major volcanic episodes in Earth’s history. At punctuated stages in the past there have been large volcanic eruptions, termed ‘flood ’, which have been responsible for millions of cubic kilometres of erupted rocks, and similar amounts of frozen , stored in the plumbing systems beneath these ancient volcanoes. These events leave footprints on the Earth’s surface known as large igneous provinces (LIPs)4. These flood basalts and their magma feeding systems are often found intricately associ- ated with some of the major sedimentary basins. A look at the world map (Figure 2) with indicated basins and volcanics paints this picture5, and helps geoscientists define areas where exploration of the sediments will be affected by volcanic rocks along ‘volca- nic margins’. Hydrocarbons, as well as reservoirs, occur in many of these prospective volcanic basins. It is believed that many more exist and are yet to be discovered.

Figure 2. World map highlighting the main mafic large igneous provinces, sedimentary basins and the volcanic basins where there is a good association of the LIPs with sedimentary basins4, 5.

3 ISSUES WITH VOLCANIC ROCKS AND EXPLORATION In order to get an oil or gas prospect one needs what is known as the ‘5 steps to heaven’. 1. Reservoir – the rock mass that has void space to host the hydrocarbon. 2. Seal – a rock that is impermeable to the migration of hydrocarbons. 3. Trap – a structure/orientation of rock strata that will trap the migration of hydrocarbons to the surface. 4. Source – an organic-rich sediment that will mature with burial and lead to the production of hydrocarbons (e.g. organic shales, coal, etc.). 5. Timing/migration – the set of events that allows the maturation and migration of the hydrocarbons to occur after the trap is in place so that they are successfully fed into the reservoir.

How these ‘5 steps to heaven’ are found in a volcanic basin will depend on the timing of the volcanic events, the intrusion of the magma through the sediments and the development of traps prior to any charging of hydrocarbons. Hot magma can, in principle, act as a heat source to mature the petroleum system. For the most part, thick igneous rocks are thought to have reasonably good sealing properties. It is also known from exposed rock sequences that volcanic rocks can have a variety of shapes and geometries that create traps. Volcanic rocks, however, are unlikely to be source rocks as they do not contain organic material. So the volcanic system—from intrusions through the basins to the eruptions at the sur- face—can contribute multiple scenarios that can lead to a working hydrocarbon prospect6,7. The most common volcanic margins are those found at rifting settings. This setting can be used to further explore what types of exploration targets may be available. A schematic vol- canic rifted margin is presented as a 3D model and in cross-section in Figure 3 (with various aspects of the potential petroleum system highlighted in Figure 3b)5. Mixed sequences of hydro volcanic and lava flows are found along with associated sills and dykes which form the plumbing systems of the volcanic margins underneath. These systems develop where the rifting apart of continents coincides with flood and creates LIPs. With continued rifting, the volcanic deposits are preserved on either side of the rifted continents. Examples of these are the North and South Atlantic margins with the North Atlantic Igneous Province (NAIP) and the Parana-Etendeka Province (see map in Figure 2)8. In the rifted volcanic margin model, hydrocarbon reservoirs can be found in common styles of trap (e.g. structural, stratigraphic, etc.) in structures and sediments that are above (supra) or below (sub) the volcanics. The volcanic rocks might not have influenced the prospect, but may now cause imaging and drilling concerns. The emplacement of the volcanics through their plumbing systems of dykes and sill can create fluid migration pathways where hydro- carbons are channelled. This can create structures where hydrocarbons may be trapped. The intrusion of the hot magma into organic rich sediments may also affect and enhance the maturation, leading to the generation of hydrocarbons. A picture, therefore, develops where the presence of significant volcanic material can both hinder and be advantageous depending on the circumstances. There is a certain asymme- try to the distribution of the volcanic and intrusive rocks in this rifted margin model. The amount and thickness of the units change from very minor — in parts of the basins where only the sills are found — to much thicker, where the thick lava sequences and associated sills are found (e.g., Figure 3b).

The existing and potential exploration into these thicker zones, however, can prove prob- lematic. Technical issues include imaging within and beneath the volcanics and the prospect of drilling through thick lava flows.

4 Figure 3. Volcanic rifted margins; a) schematic 3D cartoon of the development of a typical volcanic margin, b) the resultant cross-section of a typical volcanic rifted margin (e.g. NE Atlantic Margin), with volcanic facies and possible hydrocarbon traps and migration pathways indicated5.

IMAGING BENEATH VOLCANICS In order to confidently explore the subsurface, successfully imaging possible hydrocarbon targets is important. This is where volcanic rocks provide a specific problem, often termed the ‘Sub-Basalt Imaging’ problem9. Volcanic intervals in the subsurface, usually thick sequences of basalt but can also include relatively thin horizons, often masks the rock units below. When imaging the Earth’s interior with seismic waves, the volcanic units contain high velocity horizons, a massive range in high to low velocity in repeated cycles within the volcanic stratigraphy. They can have markedly irregular surfaces and geometries compared to simple flat sediments. All of these factors can reduce the transmission of seismic energy below, provide increased noise and also lead to ‘false’ layers/structures (multiples) in the data. Oil and gas companies looking to drill in these challenging areas need as much ‘visible’ data as possible before deciding whether to engage.

5 Two possible solutions to the Sub-Basalt Imaging problem are to: 1. Find better ways to process existing data to emphasize the signal that one gets in the ‘sub-basalt’ domain. 2. Better design and plan new acquisitions where thick volcanic sequences are known to present a challenge.

Improvements in acquiring new data include: 1. The position and recording of the seismic source at specific depths in the water. 2. The types of source fired. 3. The types and position of the receivers used. For example: long offset arrays of receivers, bespoke 3D seismic surveys and even ocean bottom situated seismometers.

These are all designed to reduce the amount of noise and enhance low frequency data, which is known to penetrate through the volcanics. Imaging enhancement requires ana- lysing the already available data, which includes enhancing key frequencies and reducing noise and any multiples that are generated in the original acquisition. This approach can yield vast improvements to existing data (e.g., Figure 4)10. But even with higher quality imaging, drilling must be performed —­­ which may lead to other potential pitfalls.

Figure 4. Improvements in sub-basalt imaging, with innovative processing of existing data10 (images courtesy of TGS).

6 DRILLING THROUGH VOLCANICS After identifying a target, the second major issue is drilling through the volcanic rocks themselves. This may not seem like such a big issue. The volcanic rocks will be quite hard and competent, providing a slow but predictable medium to plan a drilling campaign. Volcanic rocks can contain variations of rock properties such as velocity within a single flow spanning almost the entire range of rock types11. It’s all down to bubbles, cracks and weathering, as well as certain key volcanic facies types. As the lava erupts and is emplaced, it degasses. These bubbles rise towards the top of the flow and can be trapped in the upper parts as it cools and solidifies. The upper crust of the flows is invariably fractured as the lava flow is further emplaced and cooled, and is often weathered between eruption events. Basalt lava flow velocities can subsequently range from 6-2 km/s due to this massive internal heterogeneity (e.g., Figure 5)11, 5. Differing volcanic facies (e.g., lava flows or hydrovolcanic rocks formed in water) have different rock properties and variability12 as well. The thought of drilling through a kilometer or more of lava flows and other facies can be much more daunting.

Figure 5. Inside the lava flow. A single lava flow can contain as much rock-property variation as seen in the whole of the sedimentary rocks in the basin. These flows can be repeated many times, developing a thick sequence of rocks with variable geophysical responses11,5.

A key issue is predicting a drilling rig’s rate of penetration (ROP) and the potential hazards of moving from fast to slow and back in repeated succession of lava flows. Additionally, fractured and vesicle rich zones that contain very high porosity/perme- ability are also concerns, as drilling fluids may be lost which adds further hazards to the planned well. All this, combined with an uncertain depth to the base of the volcanics, makes for a challenging set of drilling issues that are present when attempting to pen- etrate moderate to thick volcanic successions (see summary Figure 6)10, 13, 14, 5. Advances in drill bit technology and improvements in understanding the hazards through thick volcanics can help to mitigate some of these issues13. In many cases, the rock units drilled in volcanic successions are rarely cored. Only partial information is available on what exactly you are drilling through, such as remote data from the ROP and geophysical tools, as well as small rock cuttings that are brought up to the surface while drilling15. Such data needs to be assessed as effectively as possible. Utilizing information and understanding from other sources, such as outcrop analogues, helps to build an under- standing of the behaviours of volcanic facies at depth.

7 Figure 6. Drilling through volcanics. Oil and gas companies require good images in order to see ‘through’ volca- nics and generate a best estimate of where the volcanics end. Drilling teams can encounter a number of problems from high to low ROP and fluid loss to the drill bit sticking. A number of new technologies can help with drilling, as well as with post-well analysis of what volcanics one has10, 13, 14, 5.

SUCCESS AND FAILURE; WHAT LESSONS HAVE BEEN LEARNED Failure comes at an excessive cost where little to no hydrocarbons are found, or other technical difficulties lead to the early abandonment of wells. Problems with the inter- pretation of the base basalt position and the target depth of the reservoir have often meant that far more volcanic material has been encountered whilst drilling than pre- dicted. Also, the complex rock properties associated with lava flows and volcaniclastic rocks can result in drilling problems during penetration. In truth the targets drilled where the volcanic stratigraphy is thick have had very limited success (e.g., Lagavulin and Brugden wells in the North Atlantic)16, with perhaps the technical successes in terms of safe drilling and operation in challenging deepwater being the best results to take away. However, examples where the volcanic units are somewhat thinner have yielded successes, such as the Rosebank discovery in the North Atlantic, which is currently under appraisal/development17, 18.

8 Discoveries and failures involving volcanic rocks in the South Atlantic (e.g., Brazil, Angola and Namibia), including some cases with complex and poorly understood work- ing petroleum systems, amplify the need to better understand volcanic margins. Significant gas discoveries have also been made in China where various volcanic reser- voirs are implicated in the discoveries. However, limited exposure of this information to the wider scientific community until recently19 has somewhat limited the poten- tial exchange of information concerning their exploration. Further afield, the recent discovery of continental crust beneath Iceland20 could extend the areas where potential exploration possibilities exist in volcanic rifted margins beyond our normal models. Somewhat surprisingly, petroleum reservoirs have also been found beneath volcanoes themselves. Actually drilling into a volcano may seem undoable. Normally, one would avoid any major volcanic rocks or active volcano systems. Yet in the volcanoes and vol- canic plumbing in the Neuquén Basin, Argentina, a tale of hot rocks and oil exists that has been repeatedly successful for oil and gas production21. In this example, the volcanic plumbing systems often cut into organic rich shale horizons which are the main source rock in the region. The hot magma can locally mature this horizon where it has not been buried deep enough. It may also fracture and contract when they cool, providing their own fracture networks that acts as a reservoir. In parts of the Neuquén Basin, one may see small drilling hole pockmarks riddled over some of the volcanoes, as they have tapped into this volcanically-hosted reserve. The hydrocarbons themselves have been mobilised in places to form spectacular bitumen dykes (see Figure 7).

Figure 7. Spectacular bitumen dykes associated with volcanic oil fields in Argentina; a) GoogleEarth image of the area (2014 Inav/Geosystemas SRL, 2014 DigitalGlobe, 2014 CNES/Astium) of Fortuna IV and Toribia bitumen mines. Note the proximity of the mines with hydrocarbon-producing wells. Note also that the bitumen dykes are parallel to very recent volcanic eruptive fissure (right). b & c) field photographs of the spectacular Toribia bitumen mine. The bitumen dyke is up to 8 m in thickness (Photos courtesy of S. Planke)5.

9 IS THE FUTURE VOLCANIC? What is the future for hot rocks and their use in the exploration and successful discovery of new hydrocarbon resources? Volcanic rocks have actually always been useful in sedimentary successions as they may be used to date sequences due to radioactive elements hidden within them. These rock ages that result from erupted episodes provide valuable marker horizons with which one can better understand the conditions within sedimentary rocks. However, these dateable examples are usually of low volume, such as thin ash beds that once blanketed the basins. But, it is the volcanic margins that potentially hold big prizes if the industry can successfully explore in the sub-basalt domain. If the industry can unlock the subsurface with better imaging techniques and improved acquisition of seismic data, O&G companies will be more confident to drill targets within the volcanic domain. Continued research into the onshore analogues where sediments and volcanics interact will undoubtedly find new insights into the potential petroleum prospectivity, as disciplines cross from sedimentology through and geophysics. In turn, this will help better risk these sub-surface targets. Our best chance of properly identifying the types of volcanic rocks and systems is through taking an integrated approach. This includes looking at all the available data from the rate of penetration at the drill bit, through the cuttings that arrive at the surface, and down to borehole geophysics and imaging with a volcanics perspective. This, along with systematic post well analysis of the available sampled volcanic material (cuttings, side wall cores, etc.), can help with additional temperature, density and, where possible, age constraints of the system in question. These insights, when applied with other research findings, are the ways to improve our understanding of exploration plays, reduce the risks and lead to better exploration outcomes, as the industry ventures further into the volcanic basins and margins of the world.

ACKNOWLEDGMENTS I would like to thank the many colleagues and friends that I have worked with on various aspects of modern and ancient volcanic systems, and volcanic margins over the years. My recent collaborations with Sverre Planke, Henrik Svensen and the ‘Earth Crisis’ team at the Centre for Earth Evolution and Dynamics, University of Oslo (Centres of Excellence funding project number 223272, CEED), has greatly inspired my continued interests in large volcanic events. John Millett is particularly thanked for drafting help with figures and for detailed discussions on earlier versions of this contribution. TGS are thanked for providing seismic images. Brendan O’Keefe and Claudine Berti are thanked for initial project discussions and editorial guidance.

10 REFERENCES 1. Dougal Jerram, 2011. Introducing Volcanology: A Guide to Hot exploration. AAPG Bulletin, V. 98, No. 3 (March 2014), P. Rocks, Dunedin Academic Press, ISBN 978-1906716226. 449–463. 2. Iceland Met Office news reports 13th March, 2015 13. Planke, S. 1994. Geophysical response of flood basalts from http://en.vedur.is/about-imo/news/nr/3097 analysis of wire line logs: Ocean Drilling Program Site 642, 3. Jerram, D.A., Single, R.T, Hobbs, R.W. and Nelson, C.E. 2009. Vøring volcanic margin. JGR Volume 99, Issue B5, Pages Understanding the offshore flood basalt sequence using 9279–9296. onshore volcanic facies analogues: an example from the Faroe– Shetland basin Geol. Mag. 146 (3), pp. 353–367. 14. Rickard, W., Bailey, A., Pahler, M., and Cory, S.,2014. Kymera TM Hybrid Bit Technology Reduces Drilling Cost, in Thirty- 4. Bryan, S.E., and Ferrari, L., 2013. Large igneous provinces Ninth Workshop on Geothermal Reservoir Engineering. and silicic large igneous provinces: Progress in our Stanford, California, 1–12. un­derstanding over the last 25 years. GSA Bulletin, 125 (7- 8), 1376. 15. Millett, J.M., Hole, M.J., and Jolley, D.W. 2014. A fresh approach to ditch cutting analysis as an aid to exploration 5. Millett, J.M., Jerram, D.A. and Planke, S. 2015. in areas affected by large igneous province (LIP) volcanism. Volcanic Margin Petroleum Prospectivity, Multi-client Geological Society, London, Special Publications, 397, Core Module reports http://www.vbpr.no/index.php/ 193-207. products/17-products-category/special-studies/75-vmapp 16. BBC News article on Lagavulin well 13 6. Rateau, R., Schofield, N. and Smith, M., 2013. The potential June 2011 http://www.bbc.co.uk/news/ role of igneous intrusions on hydrocarbon migration, West uk-scotland-scotland-business-13747638 of Shetland. Petroleum Geoscience, Vol. 19, pp 259-272. 17. DEVEX European Production and Development Conference 7. Archer, S.G., Bergman, S.C., Iliffe, J., Murphy, C.M., and presentation 2A1605, Aberdeen 2009 http://www.devex- Thornton, M. 2005. Palaeogene igneous rocks reveal new conference.org/pdf/Presentations_2009/2A1605%20 insights into the geodynamic evolution and petroleum Chevron%20The%20Rosebank%20Discovery%20-%20 potential of the Rockall Trough, NE Atlantic Margin. Basin new%20play%20type%20in%20intra%20basalt%20 Research, 17, 171–201. reservoirs%20of%20the%20North%20Atlantic%20 8. Jerram, D.A. and Widdowson, M. 2005. The anatomy of volcanic%20province.pdf Continental Flood Basalt Provinces: Geological constraints 18. Schofield, N., and Jolley, D.W., 2013. Development of on the processes and products of flood volcanism. Lithos 79, intrabasaltic lava-field drainage systems within the Faroe- 385-405. Shetland Basin. Petroleum Geoscience, 19(3), 273–288. 9. Gallagher J.W., Dromgoole D. 2007. Exploring below the 19. Volcanic Gas Reservoir Characterization. By Qiquan Ran, basalt, offshore Faroes: a case history of sub-basalt imaging. Yongjun Wang, Yuanhui Sun, Lin Yan, Min Tong. 15 May Petroleum Geoscience 13:213–225. 2014. Gulf Professional Publishing (Elsevier) 10. Woodburn, N., Hardwick, A., Masoomzadeh, H. and Pages: 604. Travis, T. 2014. Improved signal processing for sub-basalt 20. Torsvik, T.H. et al. (2015) Continental crust beneath imaging Geological Society, London, Special Publications, v. southeast Iceland. Proceedings of the national academy of 397, p. 163-171. sciences. Published online March 30, 2015, doi: 10.1073/ 11. Nelson, C.E., Jerram, D.A., and Hobbs, R.W., 2009, pnas.1423099112 Flood basalt facies from borehole data: implications for 21. Rodriguez Monreal, F.R., Villar, H.J., Baudino, R., Delpino, prospectivity and volcanology in volcanic rifted margins: D., and Zencich, S., 2009. Modeling an atypical petroleum Petroleum Geoscience, v. 15, p. 313-324. system: A case study of hydrocarbon generation, migration 12. Watton, T.J., Wright, K. A., Jerram, D.A. and Brown, R.J. and accumulation related to igneous intrusions in the 2014. The petrophysical and petrographical properties Neuquen Basin, Argentina. Marine and Petroleum Geology, of hyaloclastite deposits: Implications for petroleum 26(4), 590–605.

11 ABOUT THE AUTHOR Dougal Jerram is an award winning Earth Scientist (Murchison Fund, Geological

on key subjects, and undertaken eldwork in an array of locations around the world. He has helped foster links between academia and industry setting up consortiums and research projects around petroleum prospectivity in volcanic basins and margins. Media work includes on screen, live TV and Radio, public speaking and popular

worldwide, Hungton post blogs). He holds a Prof II position at the Centre for Earth Evolution and Dynamics (CEED) at the University of Oslo, and runs his own company DougalEARTH LTD. Email: [email protected] Personal webpage: www.dougalearth.com Centre for Earth Evolution and Dynamics (CEED), University of Oslo, Norway. DougalEARTH Ltd. Solihull, UK (www.dougalearth.com).

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