(2016) 'Exceptional reservoir quality in HPHT reservoir settings : examples from the Skagerrak Formation of the Heron Cluster, UK, North Sea.', Marine and petroleum geology., 77 . pp. 198-215. Further information on publisher's website: Use policyThe full-text may be used and/or reproduced, and given to third parties in any format or medium, without prior permission or charge, for personal research or study, educational, or not-for-pro t purposes provided that:• a full bibliographic reference is made to the original source • a link is made to the metadata record in DRO • the full-text is not changed in any way The full-text must not be sold in any format or medium without the formal permission of the copyright holders.Please consult the full DRO policy for further details. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. cementation and has had little to no effect on porosity preservation. The formation of welldeveloped authigenic chlorite (>70% surface coating) and, to a lesser extent illite clay coats with burial had a positive effect on porosity preservation even though permeability was marginally reduced in the illite-rich sandstones. A schematic porosity and quartz cement evolution model has been developed which allows for pre-drill prediction of reservoir quality in the Heron Cluster and provide valuable insights for other complex high-pressure hightemperature reservoirs.

The reservoir quality (porosity and permeability) of deeply buried hydrocarbon reservoir sandstones in sedimentary basins is significantly affected by burial diagenesis. Many deep reservoirs develop anomalous fluid overpressures during burial. Previous studies on the effect of fluid overpressure on reservoir quality in these deep reservoirs have been inconclusive because of the difficulty in constraining the individual contributions of various porosity preserving factors that are simultaneously active in these reservoirs. Owing to its rapid burial and low burial temperatures, the Neogene gravity-flow sandstone reservoirs from the Magnolia Field, Gulf of Mexico, offers a unique opportunity to investigate in isolation the effect of fluid overpressure on reservoir quality. Examination of petrography, pore pressure and routine core analysis datasets showed a positive correlation between high fluid overpressure and enhanced reservoir quality. This study confirms that fluid overpressure preserves reservoir quality in deeply buried sandstone reservoirs in compaction dominated, high sedimentation basin settings.

Whether addition of oil to sandstones slows, stops, or has no influence upon pressure solution and quartz cementation has long been disputed, despite having major implications for petroleum exploration and appraisal strategies. To elucidate the effect of addition of oil to compaction, pressure solution, and cementation processes, this study utilizes an experimental approach simulating isochemical, volumetric compaction using granular halite in the presence of variable brineoil mixtures. Each experiment, at 500 kPa effective stress, lasted 48 hours and involved repeated measurements of volumetric strain. The lithified products of experiments were examined using SEM techniques. After 30 hours, approximately 4% volumetric strain occurs when the brine-oil ratio is 100:0 (pure brine). Fractures are rare in samples from experiments undertaken with pure brine so that pressure solution, and concomitant cementation, must be the compaction mechanism. When the brine-oil ratio is decreased from 100:0 to 40:60, the volumetric strain remains about 4% but there are more fractures, as quantified from SEM image analysis. These observations suggest that, although pressure solution has occurred, some of the volumetric strain is the result of fracturing, implying that pressure solution has been less effective in the presence of some oil than it is in the presence of pure brine. When the brine-oil ratio is decreased further from 40:60 to 5:95, the volumetric strain increases up to 7.3%; SEM image analysis reveals that the strain is predominantly due to grain fracturing. This change from pressure solution to grain fracturing is likely due to heterogeneous pressure solution resulting from heterogeneous distribution of stresses. Brine-coated grain contacts undergo pressure solution but oil-coated grain contacts experience increasing stress up to the point of failure, as the surrounding grain pack compacts. When the brine-oil ratio is decreased to less than 5:95, volumetric strain decreased to about 1% with few induced fractures and little pressure solution. Compaction is negligible when the brine-oil ratio is 0:100 (only oil in the pores) and fractures are negligible. Halite is more water-soluble and more hygroscopic than quartz so that water will cling to halite surfaces more tenaciously than to quartz. Since even halite has its mechanism of volumetric strain affected when the brine-oil ratio is less than 40:60, quartz pressure solution and subsequent cementation is even more likely to be affected by oil filling. This experimental study suggests that the mechanism and extent of compaction and subsequent cementation in sandstones is strongly affected by the addition of oil.

Critical stress theory states that fault transmissivity is strongly dependent upon orientation with respect to the stress tensor. This paper describes an experimental study aimed at verifying critical stress theory using a bespoke angled shear rig designed to examine the relationship between gas flows along a kaolinite-filled synthetic fault as a function of fault dip. A total of 22 gas injection experiments were conducted on faults oriented 0°, 15°, 30°, and 45°to horizontal; both with and without active shear. Gas flow was seen to be complex; repeat gas injection testing showed a consistent gas entry pressure but considerably different, nonrepeatable, gas peak or breakthrough pressure. Gas flow occurred along discrete, dilatant pathways. The physics governing the pressure at which these features formed was repeatable; however, permeability was dependent on the number, distribution, and geometry of the resultant pathways. The nonrepeatable gas response suggests that the number of pathways was dependent on very subtle variations in gouge properties. No fault orientations were seen to exhibit nonflow characteristics, although critical stress theory predicted that two of the investigated fault angles should be effective seals. However, a small variation in gas entry pressure was seen with fault angle as a result of varying normal and shear stress acting on the gouge material. Shear was seen to enhance gas movement by reducing gas entry pressure and increased permeability once gas became mobile. Therefore, in kaolinite gouge-filled faults, shear is not an effective self-sealing mechanism to gas flow. IntroductionDiscontinuities (fractures, faults, joints, interfaces, etc.) play a pivotal role in controlling the movement of water and gas around an underground Geological Disposal Facility (GDF) for radioactive waste. High Level Waste, Intermediate Level Waste, and some long-lived low-level radioactive waste and spent fuel are planned to be disposed of in a GDF within stable geological formations at depth (~50-800 m) by a number of countries. The radioactive waste is securely isolated and contained by the engineered and geological barriers at such a facility. At depth the rock mass may be a naturally fractured environment, as in the case for crystalline rocks, and the excavation of the GDF is recognized to induce additional fractures in both crystalline and clay-rich host rocks [Bossart et al., 2002;Rutqvist et al., 2009]. Therefore, most current disposal concepts will result in the formation of a multitude of discontinuities as part of the natural and engineered environment. Depending on the in situ stress conditions, preferential pathways may form along any, or all, of these discontinuities.Gas and water are expected to play a significant role in the transport of radionuclides away from the GDF. The conductivity of fluids through discontinuities is understood to be controlled by the interplay of their orientation and stress tensor direction [Barton et al., 1995;Finkbeiner et al., 1997]. Around the GDF there...

Understanding flow along fractures and faults is of importance to the performance assessment (PA) of a geological disposal facility (GDF) for radioactive waste. Flow can occur along pre-existing fractures in the host-rock or along fractures created during the construction of the GDF within the excavation damage zone (EDZ). The complex fracture network will have a range of orientations and will exist within a complex stress regime. Critical stress theory suggests that fractures close to localized shear failure are critically stressed and therefore most conductive to fluid flow. Analysis of fault geometry and stress conditions at Sellafield has revealed that no features were found to be, or even close to being, classified as critically stressed, despite some being conductive. In order to understand the underlying reasons why non-critically stressed fractures were conductive a series of laboratory experiments were performed. A bespoke angled shear rig (ASR) was built in order to study the relationship between fluid flow (water and gas) through a fracture surface as a function of normal load. Fluid flow reduced with an increase in normal load, as expected. During unloading considerable hysteresis was seen in flow and shear stress. Fracture flow was only partially recovered for water injection, whereas gas flow increased remarkably during unloading. The ratio of shear stress to normal stress seems to control the fluid flow properties during the unloading stage of the experiment demonstrating its significance in fracture flow. The exhumation of the Sellafield area during the PalaeogeneÀNeogene resulted in considerable stress relaxation and in fractures becoming non-critically stressed. The hysteresis in shear stress during uplift has resulted in faults remaining, or becoming, conductive. The field and laboratory observations illustrate that understanding the stress-history of a fractured rock mass is essential, and a mere understanding of the current stress regime is insufficient to estimate the flow characteristics of presentday fractures.

Abstract

The full-text may be used and/or reproduced, and given to third parties in any format or medium, without prior permission or charge, for personal research or study, educational, or not-for-prot purposes provided that:• a full bibliographic reference is made to the original source • a link is made to the metadata record in DRO • the full-text is not changed in any way The full-text must not be sold in any format or medium without the formal permission of the copyright holders.Please consult the full DRO policy for further details. Overpressure preventing quartz cementation? -a replyStephan Stricker, Stuart Jones and Shanvas Sathar IntroductionWe are delighted that our recent contribution on sandstone diagenesis and reservoir quality of the Heron Cluster reservoirs, Central North Sea, UK (Stricker et al., 2016b) has raised further interest and opportunity for discussion on the role played by pore fluid overpressure during burial diagenesis (chemical compaction). Maast (2016) raised several interesting points that merit a reply following the discussion of our paper on the maintenance of reservoir quality within the Heron Cluster reservoir sandstones (Stricker et al., 2016b). We used a multidisciplinary approach comprising petrographic, SEM, fluid inclusion, and burial history modelling studies to analyse the evolution of reservoir quality in HPHT environments. We highlighted several processes which contribute to the maintenance of exceptional porosity (>30%) and reservoir quality at depth. We concluded that a combination of A) well-developed chlorite and mixed chlorite/illite grain coatings, B) shallow onset of overpressure and vertical effective stress (VES) reduction, and C) continuous overpressure maintenance (low VES) maintained reservoir quality within the fluvial channel sandstones of the Skagerrak Formation. Maast (2016), however, insists in his comment that pore fluid overpressure does not play a role and that reduced VES does not benefit the maintenance of reservoir quality. We will address this comment by not providing a recount of our petrographical observations and modelling (Stricker et al., 2016b) but will refer to further observations and analyses from the Triassic Skagerrak Formation in the North Sea (see Stricker et al., 2016a;Stricker and Jones, 2016).

 Importance of stress history on fracture flow.  Stress dependency of fracture flow described by a power-law or cubic relationship.  Fracture flow dependent on fracture roughness, thickness of gouge material, saturation state, permeability of the host material, clay mineralogy, and the degree of shearing.