School of GeoSciences

School of GeoSciences

Diagenesis at Edinburgh

Current and Past Projects

Our group consists of PhD students and Postdoctoral workers. Current themes include:- predicting porosity anomalies, quartz cementation, illite clay growth, calibrating basin models, processes enhancing porosity during burial.

Many projects involve collaboration with researchers in the Scottish Universiites Environmental Research Centre (Tony Fallick, Rob Ellam, Fin Stuart), Universities of: Heriot-Watt (Patrick Corbett, Gary Couples, Helen Lewis, David Potter), Newcastle (Andy Aplin, Steve Larter), Leeds (Bruce Yardley, Quentin Fisher), Barcelona (CSIC - Conxita Taberner), Inst Francais de Petrole (Etienne Brosse), Leuven (Rudy Swennen). Oil companies are frequent collborators, such as : Shell, BP, Conoco, Total, Phillips, & Marathon.

Current projects include:

Illite clay growth

Illite, a fibrous clay, severely reduces reservoir permeability, the control on its growth appears to link to pore-fluid movement or hydrocarbon charging. This project is working in several sedimentary basins worldwide, examining feldspar dissolution and rates and mechanisms of illite clay growth. The timing of illite growth can be dated by K-Ar, and linked to calibration of basin modelling software. (Dr Mark Wilkinson).

Using isotopes to track past hydrocarbons

We have accumulated analyses which suggest that hydrogen isotopes can transfer from organic matter into silicates when clays grow during diagenesis. This project is attempting to establish examples of such interaction, the size sale over which the isotopic transfer occurs, and the mechanism of transfer. The isotopic record of past oil remains, even when the remigration or gas charge may have subsequently occurred (Dr Calum Macaulay).

Origins of exceptional porosity.

Many empirical curves exist to predict porosity decrease with depth, and modelling softwares are now available. However these do not predict anomalously high porosities encountered in some oil-fields, nor do they encompass processes by which porosity may increase during burial. This project will examine fields with current and past overpressure, to determine processes by which minerals have dissolved during burial, and processes by which the resulting porosity has been kept open. This may produce models to reduce uncertainty exploring for deeply buried satellite structures. (Stuart Barclay)

Quartz cementation in deepwater sands.

Porosity is reduced by quartz cement, however some oil-fields show a large variation of cement and hence a variation of porosity in the same reservoir within on structure. This project is examining several oil-fields where we can determine a systematic change of quartz cement vertically down from the structure crest in the oil zone to the aquifer. palaeo-fluids and temperatures are being constrained by oxygen isotope and fluid inclusion analyses. Timing of oil charge will be determined by basin modelling. We will improve models of porosity prediction in deep structures and small satellites. (Ann Marchand).

Cementation of oilfield sandstones

During burial of an oilfield reservoir sandstone, new minerals crystallise in the porosity between the sand grains. These minerals adversely affect the quality of sandstone as an oil reservoir, but also hold a fossil record of the ancient water conditions in which they grew. Micro-analysis of the individual crystals surrounding oilfield sand grains can be undertaken using mass-spectrometers linked to electron beams, ion beams, or laser light. The record of ancient water in these minerals is held as subtle changes of chemical composition, or as isotope ratios. This enables interpretation of the palaeo-hydrogeology of the deep subsurface during burial of sandstone. The mineral record shows that deep below ground there was a race between oil movement and crystal growth. If oil arrived in the sand before crystal growth had filled the pore-space, then a good oilfield results.

Illite growth and oil charge.

Illite clay reduces permeability, but also seems to hold a record of when oil charged a structure. We have compiled all regional age data from the Brent province and Magnus basin. New analyses have been made on the Penguin field. Basin modelling and analyses of regional seismic lines have led to new views on the timing and sources of oils. (Andrew Cavanagh).

Past oils and palaeopressure

During growth of diagenetic cements, small samples of fluid are trapped as fluid inclusions. These hold a record of different oil types pre-dating the present hydrocarbon, and can also be used to reconstruct palaeo-pressure. This project examines lateral and vertical variation of hydrocarbon in Brent-type fields, and can identfy sites of past overpressure leak-off. (Mark Parfitt)

Past postdoctoral work has included:

Development of new world-class laser analytical systems for acquiring stable isotope data.

Examination of the regional cementation and porosity patterns of the deepest reservoirs in the North Sea. These are extremely overpressured and difficult to develop commercially. This project interacted closely with sequence stratigraphic and sedimentological work, and especially with computer simulation of basin subsidence and overpressure generation. We have developed important new concepts of mass-transfer within sedimentary basins and mechanisms of porosity preservation, and porosity enhancement, during very deep burial.

Recent PhD research has published on:-

  • Carbonate cements in the Tertiary formed by oxidation of vertically migrating oil
  • Computer modelling of present day fluid flows across the Central Graben
  • The processes controlling overpressure generation in the deep graben
  • Processes of trapping hydrocarbons in fluid inclusions
  • Computer modelling of water flows and chemical transport of radionuclides around the proposed Sellafield radioactive waste site.

Earlier PhD work has tackled:

  • Carboniferous sandstones from onshore England
  • Timing and processes of cementation in North Sea fields including Leman, Brae, Brent, Thistle, Magnus, Emerald, Forties, and Andrew.