This Special Publication was created following a conference at the Geological Society, London in early 2006 on ‘Structurally Complex Reservoirs’. The four editors are widely recognized as being at the top of their game – structural geology – and I expected them to deliver an outstanding book. I was not disappointed. Four of the papers were of direct relevance to the work in which I am currently involved. However, leaving aside my own particular interests, let us first examine the scope and balance of the book. The volume contains 25 papers, a large portion of which are research-orientated, being either analysis of structural problems or modelling approaches to structural ‘solutions’. Seven case studies are presented from the North Sea, USA and New Zealand. A large part of the volume is given to description of the static – geological – picture, albeit in some instances using dynamic data. However, and very importantly, the final two papers in the volume address the dynamic behaviour of complex reservoirs, fractures and faults to waterflooding and flow modelling. The volume contains some practical and useful tools that will help both petroleum explorers and producers. For example, Welbon et al. provide a robust approach to exploration/ development of landslide reservoirs, which are such a common feature of rift basins and passive margins and, as the authors point out, such reservoirs are often the last to be exploited because of their complexity. Similarly, the methodology used by Manzocchi et al. to understand connectivity as a function of net: gross in turbidite reservoirs will be of use to those trying to produce petroleum, calculate sweep efficiency or even establish reserves in low net:gross systems. One of the editors, Barr, explores the impact of both conductive and sealing fractures on the performance of a gas reservoir in the UK Southern North Sea. Of particular note in this paper is the quantity and quality of information extracted from an aging field. The time-series data are particularly fascinating, enabling the identification of sealed compartments, partially depleted compartments etc., both in the field and more recently exploited satellites. As with many of the other papers, this gives the reader the opportunity to understand how their fields might perform. I thoroughly recommended this Special Publication to geoscientists and reservoir engineers alike; indeed, to anyone trying to understand complex reservoirs.

The Cretaceous chalk exposed on the coast around the Isle of Thanet in Kent, southeast England, provides an analogue for many producing North Sea reservoirs. Mode I dilatant fractures are an important component of these reservoirs, increasing the permeabi

Open fractures were described in core and Formation Micro Image (FMI) image logs in the Jurassic sandstones of the Tubåen, Nordmela and Stø formations in the Snøhvit Field, and 3D fracture network properties analysed using Computer Tomography (CT)scanning in selected core samples. The most frequent open fracture type is short stylolite-related fractures (F1), but longer open fractures are also present, with no obvious relationship to stylolites (F2). The F1 fracture densities are related generally to the clay content of the host rock, which controls the occurrence and spacing of the stylolites. The fractures are steep, with a N–S-dominant strike azimuth and significant spread. Although, generally, the F1 fractures are short, a percolating and 3D connected open fracture network across the core was found in most of the CT-scan samples. Open fractures were also found in the damage zone of a lately reactivated fault. The formation of the open fractures in the Snøhvit Field is related most likely to thermoelastic processes during removal of overburden in late Tertiary time. The presence of open fractures may influence reservoir flow, particularly in intervals containing a high frequency of stylolites and in the damage zones of reactivated faults.

Little is known about the nature of faults that cut the Triassic Argilo-Greseux Inferieur reservoir of the Berkine Basin, Algeria. To rectify this, an analysis was performed of faults in the Ourhoud Field in terms of their geometry, connectivity and fault sealing potential. Different oil-water contacts had been identified across the field, suggesting a degree of fault compartmentalization. Fault-rock material from cored wells in the Ourhoud Field was analysed to determine the type, permeability and capillary threshold pressure. The suite of fault rock present is highly varied, ranging from disaggregation faults and cataclasites in clean sands to clay smears. All the fault rocks have lower permeabilities and higher capillary threshold pressures than the undeformed host rock of the Triassic Argilo-Greseux Inferieur. The observed oil water contact and pressure differences can all be supported by the dominant fault rocks if their clay content exceeds 20%. Fault-rock data were also used to generate transmissibility multipliers for incorporation into the Ourhoud geological and simulation models. Significant reductions in transmissibility multiplier are generated by more clay-rich fault rocks separating reservoir units with permeabilities ranging from 10 mD to 1000mD.

Three-dimensional seismic interpretation and well-core data were analysed to characterize the fault populations in two offshore U.K. hydrocarbon fields. Comparison of multiline fault-throw samples from seismic-scale faults with displacement samples, measured on well-core fractures, shows that the displacement population is described by a power law. Faults seen in core and on seismic data are consistent with one another and appear to belong to the same continuous population; therefore the number of intermediate-scale faults can be predicted. Fault lengths are much more difficult to measure due to resolution effects and the size of the sampling area. Fault linkage also introduces some problems, as it may prove difficult to determine which splay should be included in the length measurement when faults branch. Well-core fault and fracture sets have been analysed in terms of their spacing, which also appears to be described by a power law and is clearly distinguishable from regular spacing. Comparison of core-scale fracture orientations with seismically-mapped faults shows that the two have similar azimuth distributions. Subsurface data, either well core or seismic, is particularly amenable to analysis by one-dimensional sampling.

In favourable circumstances, seismic reflection data can give an unrivalled view of faulted rocks in the sub-surface, imaging features down to the seismic resolution (typically 20-30 m). The lack of finer detail can, in part, be addressed by analysing well cores through the same rock volume. Samples of fault populations from such data often exhibit power-law size distributions where 'fault size' can be trace-length or fault-displacement. Analysis of a synthetic fractal model (the 'fragmentation model') demonstrates that changing the dimension of the sampling domain (e.g. volume to plane, plane to line) changes the power-law exponent of the sample's size distribution. The synthetic model also suggests how best to treat faults that extend out of the sample area, and illustrates potential problems in comparing samples from very different scales (e.g. regional and detailed mapping).Analysis of a variety of interpreted seismic-reflection data sets has provided a range of power-law exponents for different sample types:(i) fault-trace lengths (two-dimensional samples): -1.1 to -2.0; (ii) fault-trace maximum displacements (two-dimensional sample): -1.0 to -1.5;(iii) 'arbitrary' displacements (one-dimensional sample): -0.5 to -1.0.Fault-trace lengths are very sensitive to truncation (resolution) effects, and rip regions should be re-assessed using displacement gradients. Maximum displacements, and displacements obtained by line-sampling, are much more robust attributes. Well data are useful in constraining the extrapolation of populations to smaller scales. Fault populations scale differently than earthquake populations, because the latter represent only the instantaneous deformation, whereas fault populations represent the deformation accrued over geological time. A valuable dataset to clarify these relationships would be a true three-dimensional sample of faults in an actively-deforming area.

The term ‘inversion’ to describe an inverted basin was first used by Glennie & Boegner (1981) although inverted basins had been recognized many years before e.g. Lamplugh (1920) and Stille (1924). During this meeting it became apparent that the application of the term had broadened to such an extent that the understanding of ‘inversion’ in the petroleum industry was incompatible with much of the current usage. The discussion that follows illustrates many of the points of disagreement, perhaps the most contentious of which is the use of the term ‘negative inversion’ although this was also introduced by Glennie & Boegner (1981). Most of the discussion was presented verbally at the meeting and was recorded, transcribed and returned to speakers for their corrections. In addition, a number of written contributions were received. All contributions to the discussion have been edited as gently as possible so as to retain the exact meaning intended by the contributor. All contributors are included as co-authors in this discussion article but clearly this does not mean that individuals necessarily accept all the points made by other contributors. The editors have identified individual contributions. The discussion commenced with some proposals by the editors which are briefly reproduced here. This discussion article concludes with a considered revision of the proposals on nomenclature which aims to satisfy some of the shortcomings identified in the discussions. Our initial definition of inversion relied on the concept of regional elevation. The regional elevation of a marker horizon is the structural elevation of the horizon

"Venous" ulceration is usually ascribed to deep venous insufficiency. We record the cases of 20 patients with 23 ulcers without a history suggestive of deep vein disease who were found to have a normal deep venous system when evaluated by Doppler ultrasound, ambulatory venous pressures, and photoplethysmography. All had gross varicose veins present for many years (mean 24 years; range 10 to 35 years), and only 14 limbs had incompetent calf perforating veins. Effective treatment is facilitated by recognition of the relationship of varicose ulcer to superficial venous disease, usually incompetence of the saphenofemoral junction, with or without the presence of incompetent calf perforating veins.

The effect of local controlled cooling on the digital systolic blood pressure in the hand was studied in 25 healthy volunteers and 25 patients with Raynaud's syndrome. Arterial systolic pressure was indirectly measured using a digital cuff over the proximal phalanx and a photoplethysmographic probe over the digital pulp to detect pulsation. Pressures were measured after occlusion of the circulation (cuff at 180 mmHg) for 5 minutes with water circulating initially at 30 degrees C and then at 10 degrees C. An air filled cuff was used simultaneously on a control finger. The digital systolic pressure at 10 degrees C in the patients was significantly lower (P less than 0.001 Mann-Whitney U test) than in the normal volunteers whilst calculation of the percentage change in pressure induced by cooling the digit to 10 degrees C allowed complete separation between both groups. The range in the controls was +2 per cent to -26 per cent and in the patients, -33 per cent to -100 per cent. The test gives reproducible results (coefficient of variation 5-11 per cent) and offers an objective method of identifying patients with excessive vasospasm due to a cold stimulus. It may prove useful in studying natural history and the effect of drug therapy.

Fifty-one patients (55 limbs) who had had deep venous thrombosis (DVT) extending into the femoral or iliofemoral segment three to five years earlier and ten limbs of ten healthy volunteers were studied. The ambulatory venous pressure (AVP) was measured by inserting a needle in a vein on the foot; the presence of reflux in the popliteal vein was determined by a directional Doppler ultrasonic blood velocity detector. All patients had ascending venography. The results suggest that the most important factor in determining the AVP and ulceration in postthrombotic limbs is the condition of the popliteal valves. Ulceration does not occur even in the presence of occlusion if the popliteal valves are competent. The extent of DVT and recanalization or the failure of recanalization is of secondary importance.

Ambulatory calf volume plethysmography has been used to study venous insufficiency in 50 lower limbs. The results demonstrate that it is possible to determine the presence of venous insufficiency and differentiate between normal limbs, limbs with superficial venous insufficiency only, limbs with deep venous insufficiency and limbs with deep venous insufficiency and occlusion by determining the ambulatory volume change and maximum venous outflow or venous volume.

Simultaneous clearance of 99Tcm from the gastrocnemius and quadriceps muscles has been studied in 74 limbs of 62 patients with claudication and in 20 normal limbs of 15 volunteers. The local decay curve for 10 min at rest and for 20 min after a 3-min treadmill walk at 4.5 km/h was recorded. The changes in blood flow which occurred after exercise were characteristic of the arteriographic lesions and they explain the haemodynamics of claudication.