Sedimentological control on the reservoir and caprock properties of a bleached palaeoreservoir in the Entrada Formation at Salt Wash Graben, Green River, Utah Environmental Modelling Programme Open Report OR/15/005 BRITISH GEOLOGICAL SURVEY ENVIRONMENTAL MODELLING PROGRAMME OPEN REPORT OR/15/005 Sedimentological control on the reservoir and caprock properties of a bleached palaeoreservoir in the Entrada Formation at Salt Wash Graben, Green River, Utah Andrew J. Newell & Andrew S. Butcher Keywords Contributor/editor Entrada Formation, bleached palaeoreservoir, aeolian, Antoni Milodowksi reservoir, caprock. Front cover Bleached palaeoreservoir in the Entrada Formation, Utah Bibliographical reference NEWELL, ANDREW J & BUTCHER, ANDREW S. 2015. Sedimentological control on the reservoir and caprock properties of a bleached palaeoreservoir in the Entrada Formation at Salt Wash Graben, Green River, Utah. British Geological Survey Internal Report, OR/15/005. 45pp. Copyright in materials derived from the British Geological Survey’s work is owned by the Natural Environment Research Council (NERC) and/or the authority that commissioned the work. You may not copy or adapt this publication without first obtaining permission. Contact the BGS Intellectual Property Rights Section, British Geological Survey, Keyworth, e-mail [email protected]. You may quote extracts of a reasonable length without prior permission, provided a full acknowledgement is given of the source of the extract. © NERC 2015. All rights reserved Keyworth, Nottingham British Geological Survey 2015 BRITISH GEOLOGICAL SURVEY The full range of our publications is available from BGS shops at British Geological Survey offices Nottingham, Edinburgh, London and Cardiff (Welsh publications only) see contact details below or shop online at www.geologyshop.com BGS Central Enquiries Desk Tel 0115 936 3143 Fax 0115 936 3276 The London Information Office also maintains a reference collection of BGS publications, including maps, for consultation. email [email protected] We publish an annual catalogue of our maps and other publications; this catalogue is available online or from any of the Environmental Science Centre, Keyworth, Nottingham BGS shops. NG12 5GG Tel 0115 936 3241 Fax 0115 936 3488 The British Geological Survey carries out the geological survey of email [email protected] Great Britain and Northern Ireland (the latter as an agency service for the government of Northern Ireland), and of the surrounding continental shelf, as well as basic research projects. Murchison House, West Mains Road, Edinburgh EH9 3LA It also undertakes programmes of technical aid in geology in Tel 0131 667 1000 Fax 0131 668 2683 developing countries. email [email protected] The British Geological Survey is a component body of the Natural Environment Research Council. Natural History Museum, Cromwell Road, London SW7 5BD Tel 020 7589 4090 Fax 020 7584 8270 Tel 020 7942 5344/45 email [email protected] Columbus House, Greenmeadow Springs, Tongwynlais, Cardiff CF15 7NE Tel 029 2052 1962 Fax 029 2052 1963 Maclean Building, Crowmarsh Gifford, Wallingford OX10 8BB Tel 01491 838800 Fax 01491 692345 Geological Survey of Northern Ireland, Colby House, Stranmillis Court, Belfast BT9 5BF Tel 028 9038 8462 Fax 028 9038 8461 www.bgs.ac.uk/gsni/ Parent Body Natural Environment Research Council, Polaris House, North Star Avenue, Swindon SN2 1EU Tel 01793 411500 Fax 01793 411501 www.nerc.ac.uk Website www.bgs.ac.uk Shop online at www.geologyshop.com OR/15/005 Contents Contents ........................................................................................................................................... i Summary ....................................................................................................................................... iv 1 Introduction ............................................................................................................................ 1 1.1 Background ..................................................................................................................... 1 1.2 The Salt wash graben palaeoreservoir ............................................................................ 2 1.3 Methods .......................................................................................................................... 3 2 Geological background .......................................................................................................... 5 2.1 The Paradox basin .......................................................................................................... 5 2.2 Stuctural setting of the salt wash graben palaeoreservoir ............................................... 6 2.3 Entrada Formation Stratigraphy ..................................................................................... 8 2.4 Depositional system ........................................................................................................ 9 3 Palaeoreservoir sedimentology ........................................................................................... 10 3.1 Logged sections ............................................................................................................ 10 3.2 Lithofacies of the Slick Rock Member – Reservoir sandstone .................................... 13 3.3 Lithofacies of the Earthy Member – the topseal ........................................................... 17 4 Evidence for fluid flow ......................................................................................................... 18 4.1 Distribution of the bleaching ........................................................................................ 18 4.2 Nature of the reducing fluids ........................................................................................ 22 4.3 Density of the reducing fluids ...................................................................................... 23 4.4 Regional Flow Pathways .............................................................................................. 24 4.5 Depth, temperature and age of bleaching ..................................................................... 26 4.6 CO2 Phase ..................................................................................................................... 26 4.7 Reservoir fracturing and the escape of reducing fluids ................................................ 26 5 Conclusions ........................................................................................................................... 28 References .................................................................................................................................... 36 FIGURES Figure 1 Bleaching of red sandstones leaves evidence for the movement of reducing fluids along this fracture zone in the Entrada Formation of south central Utah. .................... 1 Figure 2 Site location (black rectangle) adjacent to the Salt Wash Graben (major faults are shown in red). The town of Green River is just to the north of the map and the Green River itself meanders from north to south. The site is located on Middle Jurassic Entrada Formation, which crops as a small sliver on the northern footwall of the Salt Wash Graben. To the northeast of the site, the Entrada Formation passes upwards through a stepped series of scarps and dip slopes into the younger Morrison, Cedar Mountain and Mancos Shale formations. The southwest deflection of the outcrop i OR/15/005 trace in the western half of the map results from the development of the Green River Anticline which plunges to the north and is truncated by the Little Grand Wash Fault. Geology and fault patterns are from the digital version of the Utah Geological Survey Map 180 (Doelling 2001). ............................................................................................. 2 Figure 3 Map showing the extent of data capture and some of the key geological features. Thick white arrows indicate the general dip direction, note the four way structural closure around the bleached (pale grey) palaeoreservoir. White stars indicate the location of the west (left) and east (right) logged sections. .......................................... 4 Figure 4 Burial and thermal history of the Paradox Basin since the end Jurassic at Green River, Utah (Nuccio and Condon 1996). Red dashed line indicates the approximate burial path of the Entrada Formation. Note the rapid exhumation of the formation from 37Ma onwards associated with uplift and erosion of the Colorado Plateau. Green dashed line indicates the temperature of the fluids thought to have caused the bleaching of the Entrada Formation. ............................................................................. 5 Figure 5 Map showing the location of major fold (blue) and fault (red) structures. Note the coalignment of the majority of faults and folds, with the Salt Wash Graben and Little Grand Wash Faults being notable exceptions. The Salt Wash Graben forms a northwestern extension of the Moab Fault Zone. Geological structures are from Utah Geological Survey Map 180 (Doelling 2001). .............................................................. 6 Figure 6 Two views of the northern bounding fault of the Salt Wash Graben Fault along the southern perimeter of the palaeoreservoir. The fault juxtaposes south-dipping Cretaceous Morrison/Cedar Mountain Formation in the hanging wall against north- dipping, partially bleached, Jurassic Entrada Formation on the footwall. ...................