Green Clays: Uses and Pitfalls

Date: Thursday, 18th April 2013

Venue: Royal Cambrian Academy headquarters in Crown Lane, Conwy @ 6:30pm.  Refreshments @ 6:00pm.

Abstract:

There was a time when field geologists were content to call green granules in sedimentary rocks glauconite and have done with it, in fact the practice continues up to a point and two of the localities I shall discuss have been so described in the past, somewhat erroneously.

The seminal work of Odin and others in the 1980s showed that, in recent sediments, green granules of the chlorite family of clays (odinite-rich verdine) were only found in recent, shallow marine, low latitude sediments, while glauconitic granules occur on the outer shelf. I shall show that, in this instance the present is not necessarily a key to the past. In ancient sediments both chloritic clays and glauconite can be demonstrated to have formed in non-marine sediments, while glauconite in situ has been recorded in shallow (<50m?) marine sediments. The picture is further “muddied” by the fact that glauconite may be reworked into fluvial/estuarine sediments.

As glauconite is more clearly associated with very low rates of sediment deposition than are the chloritic clays, it may be that glauconitised faecal pellets are more frequently at an advanced stage of maturation, and hence induration, when buried, than are pellets replaced by chlorite. In terms of reservoir quality this means that glauconite is less likely to result in poor reservoir quality than is odinite (or chlorite as it becomes with diagenesis). The fact that odinite-rich verdine deposits are found nearshore and seaward of deltas and estuaries suggests higher rates of sedimentation, and hence less chance of the pellets becoming sufficiently mineralised to be firm enough to resist compaction. If, at some stage in the transformation, clay is dissolved, this may provide the solutes necessary for chlorite rim precipitation, but at present this remains speculative.

Speaker Biography:

Dr Jenny Huggett is the proprietor of Petroclays, a consultancy specialising in clay mineral, and general clastic sediment analysis and petrography.

Petroclays was formed in 1990, and now in addition to working for the petroleum industry, works part time as a researcher and supervisor at the Natural History Museum, and the University of Oxford, Earth Sciences Department.  From 1982 until 1990 Jenny was employed as a sedimentologist by BP.  She obtained both her BSc and PhD from Imperial College, London.

Diagenetic modification of carbonate platforms from extensional basins: a step towards predictive models?

Date: Thursday, 21st March 2013

Venue: Cambrian Academy headquarters in Crown Lane, Conwy @ 6:30pm.  Refreshments @ 6:00pm.

Abstract:

It is widely acknowledged that diagenetic processes play a fundamental role in determining the volume, shape, size and distribution of porosity on carbonate platforms.  However, our ability to predict the shape, size and distribution of diagenetic products is limited.  In part this reflects the paucity of integrated, multi-disciplinary, multi-scale outcrop studies that reconstruct the temporal and spatial controls on the processes that controlled diagenetic fluid flux, and hence porosity modification.  This presentation will focus upon an ongoing research programme at University of Manchester into the diagenetic modification of carbonate platforms in extensional basins.  The studies combine quantitative, multi-scale descriptions of sedimentological, diagenetic, structural and petrophysical features.  These data are supplemented by geochemical analyses and interpreted in the context of the tectonostratigraphy and burial history of the basin.  Key datasets are taken from the Lower Carboniferous of North Wales and Northern England, and Eocene of the Sinai rift.  The diagenetic overprint of these areas will be compared, focusing in particular on dolomitisation, mineralisation and calcite cementation, and the potential to extract rules that better predict porosity distribution will be assessed.

Speaker Biography:

Dr Cathy Hollis is Senior Lecturer in Petrophysics and Petroleum Geocience at University of Manchester, where she leads a team of postgraduate and postdoctoral researchers working on industrially sponsored and research council funded projects in the UK, Europe, North Africa and the Middle East.  Her research focuses primarily on the diagenetic modification of carbonate platforms during burial  Cathy received her BSc from University of Birmingham in 1990 and PhD from University of Aberdeen in 1995.  She then worked as a consultant carbonate sedimentologist for Badley Ashton and Associates Ltd., during which time she established their Middle East office in the UAE (1999-2001).  In 2001, Cathy joined Shell International Exploration and Production in The Netherlands, where she worked for six years conducting both technical service support and research.   Between 2005 and 2007, she led the Carbonate Research Team for Shell and was Subject Matter Expert in Carbonate Diagenesis.

Figure 1: Asbian and Brigantian (Dinantian) Limestone, Great Orme, Llandudno, which underwent multiple phases of post-depositional porosity modification, including karstification and meteoric diagenesis, dolomitisation, fault controlled carbonate cementation and mineralisation

Directions to the venue:

RCA, Crown Lane, Conwy, LL32 8AN

Exploration Field Geology in the Middle East

Date: Thursday, 28th February 2013

Venue: YHA Conwy, Larkhill, Sychnant Pass Road, Conwy, LL32 8AJ @ 6:30pm.  Bar open from 6:00pm.

Abstract:  Reservoir characterization of Middle Eastern carbonate reservoirs is challenging, primarily due to the occurrence within carbonates of a wide range of heterogeneities occurring at a wide range of scales. Sources of heterogeneity in carbonate reservoirs can be divided into 3 main categories: i) Primary (depositional) heterogeneity; ii) Secondary (diagenetic) heterogeneity; iii) Structural (fracture) heterogeneity.  In the case study presented here we focus on the prolific Mid Cretaceous Sarvak and Mishrif formations of the Mesopotamian Foreland Basin and Zagros Fold Belt of Iran and Iraq. In the subsurface these reservoirs are buried to depths of 1000-5000m, and datasets available for prospect development or reservoir characterisation typically comprise 2D and sparse 3D seismic and well/core data of variable vintage and quality. However, rapid Tertiary uplift associated with river incision has resulted in spectacular pseudo-3D outcrops of the same reservoir units in the Zagros Mountains. The outcrops are often located between 10-40km from the subsurface exploration and production targets, and expose an almost identical stratigraphic and depositional succession. In addition, the outcrop diagenetic and structural template is often comparable to the subsurface. An integrated approach utilizing traditional geological field work with remote sensing and VGR (Virtual Geological Reality) techniques has been employed.

The main drivers for the VGR work has been to:-  1) Develop efficient exploration workflows in onshore frontier basins; 2) Allow rapid quantification of large outcrop geological datasets; 3) Access and utilize the inaccessible (many of the outcrop sections are km high vertical cliffs); 4) Time – Allow the field geologist to focus on observation and interpretation as opposed to traditional repetitive data collection (e.g. systematic line sampling of fracture systems); 5) To allow better bridging of the outcrop to subsurface gap.

The resulting dataset and predictive framework has formed the cornerstone for all further work in the reservoir interval of interest, including Exploration (farm ins, block bidding rounds), Field Development (reservoir modelling template), Forward Stratigraphic Modelling (e.g. Dionisos), Fracture modelling (e.g. FRED, Fracperm), Diagenetic modelling (fracture controlled dolomites), Seismic Modelling and Basin Modelling.

Speaker Biography:

Ian Sharp is a senior specialist at Statoil working a range of basins and a Professor at Bergen University. He gained his BSc at the University of Hull (1988) and PhD at the University of Edinburgh (1994). Ian has worked in industry for the last 15 years (Norsk Hydro1998-2007, Statoil 2007-2013), primarily in the field of applied industrial research and technology development/implementation. He has worked on exploration and production projects globally, but with a focus on carbonate reservoir systems of the Middle East and the South Atlantic and clastic reservoirs of NW Europe. Prior to joining Norsk Hydro Ian worked as an industry sponsored post doctoral research associate at the Universities of Manchester and Edinburgh (1994-1998), and as a mapping geologist with the British Geological Survey (1988-1989). Ian has published over 30 scientific articles and presented numerous papers at international conferences. He is an active member of AAPG, GSL, SEPM, PESGB, NPF and is an IUGS reviewer for UNESCO’s program for the conservation of globally outstanding geological heritage.

Directions to the venue: