Hydrocarbons in crystalline rocks: an introduction

N I C K P E T F O R D 1 & K E N M c C A F F R E Y 2

1Centre for Earth and Environmental Science Research, Kingston University, Kingston, KT1 2EE, UK

2Department o f Geological Sciences, University of Durham, Durham DH1 3LE, UK

Commercial oil deposits in basement rocks are not geological 'accidents' but are oil accumu- lations which obey all the rules ofoil sourcing migration and entrapment; therefore in areas of not too deep basement, oil deposits within basement rocks should be explored with the same professional skill and zeal as accumulations in the overlying sediments. Landes et aL (1960), American Association of Petroleum Geologists Bulletin

Oil and gas fields in crystalline basement are discovered mostly by accident, usually when the welt operator notices hydrocarbon shows and tests the welt. However, as shown in this book, such reservoirs can be very prolific, especially if the basement rock is highly faulted or fractured (the Bach-Ho fractured granite reservoir, Viet- nam, produced some 130,000BOPD). The standard definition of crystalline basement by petroleum geologists is any metamorphic or igneous rock unconformably overJain by a sedi- mentary sequence. However, crystalline rocks need not be metamorphosed, nor significantly older than their sedimentary cover. Perhaps for a more appropriate definition of crystalline base- merit, we must again look to Lartdes et al. (t 960): 'the only major difference between basement rock and the overlying sedimentary rock oil deposits is that in the former case the original oil-yielding formation (source rock) cannot underlie the reservoir'. As such, further expMra- tion involving geological, geochemical and geo- physical studies may lead to a sign~cant revision of the defirfifion and nataare of basement rocks in a particular area, with the possibility of discovering hydrocarbon source rocks located stratigraphically within rocks previously regarded as basement. Examples of where hydrocarbons have migrated into older porous metamorphic or igneous rocks to form a base- ment reservoir include the volcanic reservoirs of Japan, the oil fields of Mexico and the Mara- caibo Basin of Venezuela (see Sehutter 2003). Although still often dismissed as exotic curios, this may be a mistake. A case m point (discussed in Koning 2003) is the Suban field, southern Sumatra. Prior to its discovery, the search for oil was confined to structural highs in Tertiary sediments. While a number of wells were drilled into basement in order to tie the top of basement into seismic data, it was presumably not thought worthwhile to investigate the basement itself for hydrocarbons, it was not until 1999 that Gulf

penetrated sufficiently deeply to discover the giant Suban gas field where hydrocarbons were found in the basement rocks. Transient heat from igneous rocks can also contribute to the maturation process in sediments that have been heated rapidly by magmatic intrusion (e.g. Saxby & Stephenson 1987; Stagpoole & FunnelI 200I; Schutter 2003), making excellent cap rocks (Chert et at. 1999).

The moral here must be that the explora- tionisfs definition of basement rock needs to be less narrow and more responsive to new geo- logical ideas and data (e.g. Lamb I997). Indeed, under the right conditions, igneous rocks, either as volcanic extrusive, or high-level intrusions, come as a package of heat source and reservoir rock combined. It is for these reasons that we believe crystalline basement comprising igneous rocks, and their potential for hydrocarbon reservoirs, is deserving of in- depth study. The purpose of this book is to encourage further work in this ,4Jrection.

Crystall ine basement and inorganic hydrocarbons

While the majority of natural hydrocarbons form through thermal decomposition of orgamc material and associated microbial processes, some authors have argued that their presence in crystalline rocks is proof that all hydrocarbons are non-biogenic in origin (e.g. Gold 1998). AdmittedIy, the idea that abiogenic hydro- carbons contribute significantly to global hydro. carbon reservoirs has proved hard to challenge, due to uncertainties in carbon isotopic signatures between both groups. However, a recent study by Sherwood Lollar et al. (2002) has shown conclusively that (abiogenic) hydrocarbons in crystalline rocks from the Canadian Shield differ significantly in isotopic composition from therrnogenic hydrocarbons, effectively ruling

From: PEa'I~RD, N. & MCCAF~.EY, K. J. W. (eds) 2003. Hydrocarbons in Crystalline Rocks. Geological Society, London, Specia~ Publications, 214, t-5. 0305-8719/03/$15 �9 The Geological Society of London.

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2 N. PETFORD & K. McCAFFREY

out abiogenic hydrocarbons as a major source of oil and gas. Hydrocarbons do form inorganically via Fischer-Tropsch reactions (Anderson 1984), but only in relatively small amounts. However, the geological conditions required to promote such reactions (cooling of magma and hydrother- mal systems) can result in significant alteration (e.g. serpentinization) of the host rocks, leading to the formation of a secondary porosity that may provide important migration pathways. Further detailed study of these processes may also help improve our understanding of the rela- tionship between metals (notably U, Pb-Zn, Au, Hg and Mo) and hydrocarbons.

Thermogenic/organic hydrocarbons in igneous rocks

Hydrocarbons have been discovered in associa- tion with many different types of igneous rocks (e.g. Powers 1932). Figure 1 shows a breakdown of lithologies in which hydrocarbon deposits have been described from around the world, based on the compilation provided by Schutter (2003). While not all are of economic value, the

data reveal that volcanic rocks (basalts, andesites and rhyolites) appear most closely associated with hydrocarbons, despite the fact that most large scale production is currently from granitic and associated plutonic rocks. Unfortunately, there are still insufficient data to be able to conclude whether hydrocarbons occur in some igneous rocks simply because of post-emplace- ment migration, or if there is something inherent in magma composition that results in preferential accumulation.

Since most hydrocarbon systems begin outside crystalline rock, this requires hydrocarbons in the adjacent sediments. Any distinction between hydrocarbons around, as well as within, igneous rocks is thus arbitrary, and exploration for hydrocarbons in igneous rocks may well create opportunities in the adjacent sediments. A case in point is the Athabasca tar sands, Canada, where the operator Uranium Power Corporation plans to re-enter a c. 1,770 m well on the western outskirts of Fort McMurray. Drilled originally in 1994 and considered the first North American well to target Precambrian granite as a potential hydrocarbon reservoir, the original effort stalled due to lack of funds. Oil is currently believed to

Fig. 1. The distribution of hydrocarbons in and around igneous rocks according to lithology (from Schutter 2003, Table 1). The highest reported occurrences are in basalts, followed by andesite and rhyolite tufts and lavas. Although volcanic rocks in this survey constitute close to three-quarters of all hydrocarbon-bearing lithotypes, the majority of production and global reserves appears to be confined predominantly to fractured and weathered granitic rocks. A compilation of hydrocarbon production from fractured basement reservoirs can also be found at http://www.geoscience.co.uk/.

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INTRODUCTION 3

be trapped in fractures in the granite (Oil & Gas Journal Online 2002). More exotically, impact structures in basement (and sedimentary) cover may hold giant field potential. Of the 17 con- firmed impact structures occurring in petro- liferous areas of North America, nine are being exploited for commercial hydrocarbons. Produc- tion comes from impact-affected granites, as well as carbonate rocks and sandstones, yielding between 30 b/d to over 2 million b/d of oil and over 1.4 bcfd of gas. In some basins, the hydro- carbon systems occur beneath volcanic cover, and as well as acting as reservoirs, the igneous rocks may also provide the principal seals. For example, in the Paran/t Basin of Brazil, one of the principal potential trap systems are the lacco- liths and sills beneath the flood basalts. Although sub-basalt seismic imaging currently poses a technical problem, fractured sills here have pro- duced gas, and igneous activity played an impor- tant role in the maturation process. In another example (the Phetchebun Basin, Thailand), thermal maturation of lacustrine sediments has resulted in a good sized (10 to c. 30 million barrel) oil field, reservoired in dolerite and sealed by lacustrine sediments, which were preferentially intruded by the rising magma. The laccolithic structure of the intrusion pro- vides 'closure'. This is an excellent example of ways in which crystalline rocks can contribute significantly to hydrocarbon formation and accumulation.

This volume

The 12 papers in this volume cover a diverse range of topics related broadly to the theme of hydrocarbons in crystalline rocks.

The first set of papers are reviews that help to set the scene for some of the more process- oriented studies that follow. Schutter provides two timely and extremely thorough contribu- tions on hydrocarbons in igneous rocks. His primary objective is to show that hydrocarbons in and around igneous rocks are not isolated anomalies, but rather are sufficiently common and orderly that exploration can be done system- atically, and included in a regional exploration plan. The problem often is trying to convince those who control the finances to be less risk- adverse. A companion paper provides a broad data base identifying many of the known occur- rences of hydrocarbons in and around igneous rocks. There may be more than you think! in a short contribution, Magara reviews the main Japanese oil producing areas that lie on the Japan Sea side of Honshu island. Although the

total reserve here is small and production sup- plies only three-tenths of a percent of total Japanese oil consumption, the main reservoir rocks are volcanic and primary oil and gas migration seems to have taken place downward from the overlying source rocks. Marine volcanic activity since 15 Ma formed the main reservoir sections along with significant secondary poros- ity development. Thick and continuous deposi- tion of organic-rich shales and mudstones followed and lower parts of these fine-grained rocks became the main source rocks. Koning con- tinues in a similar vein, showing that basement rocks are important oil and gas reservoirs in various areas around the world. Such reservoirs include fractured or weathered granites, quart- zites and other metamorphic rocks. In the USA, basement-derived oil production occurs in a number of areas, including California (Wil- mington and Edison fields), Kansas (El Dorado and Orth fields) and Texas (Apco field). In SE Asia, basement reservoirs are the main contri- butor of oil production in Vietnam. Although in Indonesia, hydrocarbon production from basement rocks to date has been minimal, the recent large gas discovery in pre-Tertiary frac- tured granites in southern Sumatra has led to a focusing of exploration in basement reservoirs. Major oil production has also been obtained from basement reservoirs in the La Paz and Mara oil fields in Venezuela. He ends by sum- marizing some of the lessons learnt by companies operating in crystalline basement.

Petford reviews some of the processes contri- buting to the development of primary porosity in igneous rocks due to the cooling and crystalli- zation of magma. A distinction is made between volcanic and plutonic rocks, and crystalline and granular volcanic material. The porosity in each rock type is classified according to a proposed effective length scale and geometry into diffusive (Class D) and macroscopic flow (Class F) fea- tures. Some types of primary poromty m igneous rocks are strongly time- and scale-dependent due to thermal effects associated with the emplace- ment of magmas. Tectonic reworking of the primary petrophysical properties of basement- forming igneous rocks may be significant in the development of regions of anisotropy and enhanced permeability. McCaffrey et al. provide a quantitative description of fracture attributes from one-dimensional samples across exposures of typical crystalline rocks. Vein thickness and fracture aperture data show predominantly power-law distributions, while vein and fracture spacing data are best described by exponential distributions with negative slopes, and appear to vary with composition in intrusive rocks. The

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4 N. PETFORD & K. McCAFFREY

fracture systems exhibit a range of anfi-e|ustered to clustered patterns and densities are an order of magnitude higher for joints compared to veins. They show that thermal stress-related joint pat- terns are distinguishable from tectonic-related fractures in platonic rocks, and that fracture density and clustering increases towards a major reactivated basement fault. Ogilvie ~ a/, provide a charaetefi~tion of the rough surfaces of frarc- tures and their resulting apertures as an impor- tant step toward an improved understanding of the factors controlling fluid flow in crystalline rocks. Significantly, their tests have allowed the standard deviation of surface asperity heights, the fractal dimension and the matching param- eters to be related to the resulting aperture of the fractures.

Kaenders & Petford present the results of an analytical study of the mechanical effects asso- ciated with the emplacement and cooling of a magma body in the continental crust. The temperature and subsequent strain fields as a function of both position and time are calculated, with the latter providing information on the primary (cooling-related) fracture formation pattern and direction within and immediately surrounding the intrusion. Large strain jumps across the intrusion-country rock contact suggest that fracture formation will be maximized at the edges and comers of the intrusion. Low predicted ,strains and assumed low fracture connectivity in the centre of the intrusion imply that deformation associated with emplacement, or later tectonic motions, may be important in improving reser- voir quality by providing enhanced fracture connectivity within the rock mass.

Potter & Kommeru~Madsen discuss the pres- ence of hydrocarbons in igneous rocks, showing that while most occurrences are due to the incor- poration of organic material into the magmatic system, hydrocarbons formed by inorganic pro- cesses may not be as rare as previously thought and may have implications for natural gas resources in the future. This paper reviews these occurrences and the models proposed for the generation of these hydrocarbons, concluding that the Fischer-Tropsch synthesis of hydro- carbons in igneous rocks seems to be a more applicable model for a wide variety of igneous rock types. While not dealing explicitly with hydrocarbons in crystalline rocks, the paper by PsyrHios et al, explores the important relation- ship between fluid flow, regional tectonics and hydrothermat alteration in granitic rocks, and complements similar studies of hydrocarbon migration in granitoid basement. They propose a new genetic model for the formation of the St Austell kaolin deposits in southwestern England,

showing from fluid inclusion evidence that the kaotinizafion is a low-temperature hydrothermal event (50-100 ~ coincident with the oil genera- tion window. The kaolinization appears con- temporary with a major period of uplift that affected the Cornubian Massif as a consequence of offshore rifting. The most plausible fluid types for the kaolinization are either basinal brines expelled from Permo-Triassic sediments, or highly evolved meteoric waters that circulated through the sediments enclosing the pluton. The kaolinization process converted large volumes of fractured granite to a porous quartz-kaolin rock matrix. Degtma e~ aL provide an important crossover into the hydrogeology of low- permeability, fractured rocks. For over 20 years, intensive efforts have been underway in a number of countries to find suitable locations for underground repositories for the disposal of radioactive wastes. Such investigations have concentrated on characterizing fluid flow in low-permeability rocks, and the potential for developing and applying a breakcross-industry understanding is clear. The article summarizes the results of an eight-year study by Nirex on the detailed groundwater flow properties of a rock volume near Sellafield, northwestern Eng- land, as part of a site characterization programme to determine whether the site was suitab|e as a deep repository for radioactive wastes, The investigations showed that groundwater flow occurred predominantly through a limited subset of fractures, parts of which formed net- works of connected channels referred to collec- tively as Potential Flowing Features 0PFEs). These authors show how the detailed information about the geometrical and hydrogeological properties of the PFFs was used to calculate the upscaled effective parameters that are required for regional-scale flow calculations and to deter- mine the uncertainties associated with the upscated parameters.

Finally, Saaders et aL use observations :from an extensional basin in Vietnam to simulate and analyse fracture systems typical of crystalline basement in such structural settings. Information from field observations, seismic surveys and three-dimensional structural modelling were integrated and used to build geologically realistic three-dimensional fracture networks. Their results suggest that during flexural uplift, the hanging wall is deformed significantly, contain- ing fracture populations related to kinematic hanging wall deformation, flexural isostatic uplift and primary (cooling-related) fractures. In contrast, the footwatl blocks will probably only host primary fractures. Their study brings together many important aspects set out in

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INTRODUCTION 5

previous chapters (fracture density studies, sur- face roughness~ fluid flow and knowledge o f pri- mary joint setsL and highlights the impor tance of a multidisciplinary approach where a proper characterization of f rac tured basement is needed.

We would like to thank S. Sehutter, T. Koning, S. Bergman and P. Degnan fNIREX) for helpful correspondence and guidance regarding the industry perspective on hydrocarbon exploration in crystalline rocks. R. Swat'brick and J. Turner are thanked for a careful reading of the manuscript.

R e f e r e n c e s

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GOLD, T. ~998. The Deep Hot Biosphere. Copermens, New York.

LANt~, K. K., A~ORUSO, J. J., Cr~a_~woR~t, L. J., HEA~, F. & LWS~NCE, P. J. 1960. Petroleum resources in basement rocks. American A~eciation of Petroleum Geologists, Bulletin, 44, i682-i691.

LAMB, C. F. 1997. Basement reservoirs--an overlooked opportunity. Canadian Society of Petroleum

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Komr~c, T. 2003. Oil and gas production from base- m.ent reservoirs: examples trom Indonesia, USA and Vietnam. In:: I~TFOnD, N. & McCa~m~Y, K. J. W. (eels) Hydrocarbons in Crystalline Rocks. Geological Society, London, Special Publications, 214, 83--92.

Oil & Gas Journal Online 21~12. Oil in gramte concept due tests under Canada's Athabasea area. http// o~.c~n~.

PowEw% S. 1932. Notes on minor occurrences of oil, gas and bitumen with igneous and metamorphic rocks. America~ A~ciat ion of Petrolemn Geolo- gists, Bulletin, 16, 837-858.

SAX~Y, J. D. & S ~ s o N , L. C. 1987. Effect of an igneons intrnsion on oil shale at Rundle (Au~tra- ha). Chemical Geology, 6:~, t-16.

SnrorTr~ S. R. 2603. Hydrocarbon occurrence and exploration in and around igneous rocks. In: Pwr- FORD, N. & MCCAF~'REu K. J. W. (eds) Hydro- carbons in Crystalline .Rocks: Creoto~eat Society, London, Speeml Publications, 214, 7-33.

Sma~WOOD LotJ_~R, B.,. ~,Vt~a'GA~ T. D., WhgD, J. A., SLATE~, G. F. & Lac~a~w~-CouLo~ O. 2062. Abiogenic formation of a[kanes in the Earth's crust as a minor somv, e for global hydrocarbon reservoirs. Nature, 146, 522-524.

Sraor.oo~, V. & Fure~m, R. 2001. Arc magmafism and hydrocarbon ggncration m northern Taranaki Basin, New Zealand. Petroleum Geo~ciences, 7, 255-267.

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