Hydrocarbons in crystalline rocks

Burlington House, February 2001

This joint meeting of the Tectonic Studies, Petroleum and Volcanic and Magmatic Studies Groups of the Geological Society, convened by Ken McCaffrey (University of Durham), Titus Murray (Midland Valley Exploration Ltd) and Nick Petford (Kingston University), brought together academic and industry researchers interested in the occurrence of hydrocarbons in crystalline and ???basement??? rocks. Of the eighty-odd registered delegates, about one-third came from oil and associated companies and a slightly smaller proportion from academia, the rest representing a range of industrial groups and consultancies. There were 18 oral presentations (20 mins each) and four half-hour ???keynote??? talks over two days.

The occurrence of petroleum fluids in crystalline rocks has in the past been explained with recourse to unconventional theories ??? for example that the hydrocarbons are abiogenic and/or mantle-derived; or that the fractured reservoirs are associated with impact structures. However, one of the themes of this Conference was that although basement-hosted petroleum systems may in some respects be exotic, they nevertheless obey all the conventional rules of sourcing, migration and entrapment applicable to hydrocarbon accumulations in the sedimentary cover. Another theme concerned the nature and origin of complex fracture systems in crystalline rocks; fracture connectivity appears to be the key control on reservoir performance.

In an introductory talk, J. Gutmanis (Geoscience Ltd) focussed minds by showing a slide of oil seeping from granite in a Cornish tin mine. From a brief worldwide survey (related information is available at www.geoscience.co.uk), he concluded that fractured basement reservoirs tend to be accidental discoveries with very heterogeneous reservoirs characterized by high initial flow rates but rapid depletion. These characteristics typically apply, for example, to the Beruk field (Central Sumatra Basin), which was described in a later talk by T. Koning (Texaco).

The origin of porosity and permeability in igneous rocks was discussed by N. Petford (Kingston Univ.). Recent studies (e.g. McCaffrey and Petford, Journ. Geol. Soc. 154,1-4, 1997) have proposed that granitic plutons are essentially tabular not steep-sided (or ???diapiric???) as was previously thought. Dr Petford suggested that an intrusion with a sheet-like geometry will influence the orientation and location of fractures in the surrounding country rock which may, of course, itself be of igneous origin ??? the heterogeneity of so-called ???crystalline basement rocks??? (hitherto used as something of a ???bucket term??? by the oil industry) was emphasized.

The presence of fractures influences a rock???s permeability, strength, seismic velocity and anisotropy. K. McCaffrey (Durham University) showed how fracture systems in exposed crystalline rocks can be analyzed quantitatively by sampling in 1D (e.g. along outcrop traverses) or 2D (on photos and maps). It has long been recognized that fractures are scale-invariant, and that their frequency-size distribution has a power-law relationship which can be extrapolated to large and small scales for purposes of prediction. J. M. Sleight (Durham University) reported on fractures associated with the More-Trondelag fault system (central Norway) at scales between thin-section and Landsat image. Although minor ???massaging??? was required, she demonstrated the scale-invariance of spatial and connectivity parameters over eight orders of magnitude (10^-2m to 10⁵m). Later, A. Gibbs (Midland Valley) emphasized the intrinsic complexity of natural fracture systems which makes them difficult to model, and which makes flow patterns difficult to predict even at the outcrop scale. He noted that our understanding of basement fracture systems has not increased at the same rate as the volume of data which we have acquired about them.

Six presentations reported on the fractured Cretaceous granites which host commercial volumes of oil in the Tertiary Cuu Long Basin, offshore southern Vietnam. C. Sanders (Midland Valley) investigated the basin???s tectonic history from a backstripping study of the sedimentary cover. Interestingly, he showed that the youngest tectonically-induced fault activity in the basement is Oligocene, more recent fault movement being due to differential sediment compaction. Following a review of the origin of acid igneous rocks in SE Asia, S. Bergman (University of Texas) concentrated on the fractured granites at Bach Ho/White Tiger field which hold 1B brls oil in-place, sourced by prolific Tertiary lacustrine shales. These granites were intensively degraded by high-temperature high-salinity fluids, resulting in matrix porosity of up to 20%. T. W. Doe (Golder Assocs.) described modelling of the fracture network and reservoir plumbing at the nearby Rang Dong field. R. H. Maddock (Baker Atlas) showed how micro-resistivity image facies, in combination with other open-hole logs, could be used to characterize the reservoir at this field. P. M. Lloyd (Schlumberger) described the assessment of fracture character and dimension in the granites at Ruby field from combined array-acoustic and electrical image log data.

Three talks were concerned with hydrocarbon reservoirs composed of volcanic or volcaniclastic rocks. S. Bergman discussed the Miocene Kora andesitic complex (a submarine stratovolcano) in the Taranaki Basin, New Zealand. Sub-commercial volumes of oil, sourced by Early Tertiary shales, are present in reworked pyroclastic agglomerates and tuffs. Similar reservoir rocks of Neogene age host numerous small oil accumulations offshore NW Japan (K. Magara, Shimane University). L. Thomson (Midland Valley) described a recently-completed study characterizing a fractured volcanic reservoir in the San Jorge Basin (Argentina).

Of the remaining talks, three were of particular petroleum-geological interest. J. Potter (Kingston University) described laboratory analyses of abiogenic hydrocarbons present in fluid inclusions in Devonian nepheline-syenites in the Kola Peninsula, NW Russia. The most frequent type of inclusion is methane-dominated, small in size, (5-15?m), and typically aligned in a trail following a cleavage plane. Dr Potter proposed a low P-T, crustal origin for the methane arising from a Fischer-Tropsch type reaction between CO₂ and H catalyzed by Fe-oxides and Fe-silicates (and inhibited by sulphur). C. Sanders modelled secondary fractures in an extensional basement terrain, concluding that flexural isostatic adjustment contributes little to fracturing in the footwall but that fracture intensity and interaction are likely to be higher in the hangingwall block (which therefore has greater reservoir potential). Finally, C. Cornford (IGI Ltd) described a remarkable Late Archean petroleum system in the Witwatersrand Basin (South Africa), where algal/bacterial-derived pyrobitumen with uraninite inclusions occurs in pebble conglomerates in association with gold (Gray et al., Journ. Geol. Soc. 155, 39-59, 1998).

Other papers included a study of high-resolution (?/span>m-scale) fluid flow through rough synthetic fractures cast from moulds of mechanically-fractured granite (S. R. Ogilvie, University of Aberdeen); and several studies of basement rocks as potential repositories for nuclear waste materials.

The convenors plan to collate selected presentations as a Special Publication of the Geological Society which, if tightly focussed, should make interesting reading for petroleum and structural geologists and reservoir engineers.

C.G.H.T.