(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.

Current understanding of porosity preservation in deeply buried sandstone reservoirs tends to be focused on how diagenetic grain coatings of clay minerals and microquartz can inhibit macroquartz cementation. However, the importance of overpressure developed during initial (shallow) burial in maintaining high primary porosity during subsequent burial has generally not been appreciated. Where pore fluid pressures are high, and the vertical effective stress is low, the shallow arrest of compaction can allow preservation of high porosity and permeability at depths normally considered uneconomic. The deeply buried fluvial sandstone reservoirs of the Triassic Skagerrak Formation in the Central Graben, North Sea, show anomalously high porosities at depths greater than 3500 metres below sea floor (mbsf). Pore pressures can exceed 80 MPa in the upper part of the Skagerrak Formation at depths of 4000-5000 mbsf, where temperatures are above 1408C. The Skagerrak reservoirs commonly have high primary porosities of up to 35%, little macroquartz cement and variable amounts of diagenetic chlorite grain coats. This research sheds light on the complex controls on reservoir quality in the fluvial sandstones of the Skagerrak Formation by identifying the role of shallow overpressure in arresting mechanical compaction and the importance of chlorite detrital grain coatings in inhibiting macroquartz cement overgrowth as temperature increases during progressive burial.Gold Open Access: This article is published under the terms of the CC-BY 3.0 license.

Additional information: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-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. 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.

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.

The Triassic fluvial sandstones of the Skagerrak Formation were deposited in a series of salt-walled mini-basins and act as important hydrocarbon reservoirs for several high-pressure, high-temperature (HPHT) fields in the Central Graben, North Sea. The HPHT reservoirs exhibit excellent reservoir quality considering their depth of burial and hence have been of high interest for hydrocarbon exploration. This research uses a multidisciplinary approach to assess the Skagerrak Formation fluvial reservoir quality from the Seagull field incorporating core analysis, petrography, electron microscopy, XRD analysis, fluid inclusion appraisal and burial history modelling. Halokinesis and salt withdrawal at the margin of the saltwalled mini-basin induced early disaggregation bands and fractures at shallow burial and led to increased influx of meteoric water and clay mineral infiltration from overlying sedimentation. The density of disaggregation bands correlates with the occurrence and magnitude of pore-filling authigenic clay minerals, concentrated along the margin of the saltwalled mini-basin. The fluvial channel sandstones of the Skagerrak Formation are subject to strong intra-basinal spatial reservoir quality variations despite diagenesis and low vertical effective stress having played a favourable role in arresting porosity loss.