A new method for dating mineral cements containing minute fluid inclusions may ultimately aid the discovery of new oil reservoirs. Directly establishing the timing of oil migration through a sedimentary basin could help the discovery process, but only indirect dating of oil migration has been possible so far.
Scientists at the University of Aberdeen are using new analytical techniques to determine when oil accumulated in sandstone rocks deep under the sea between the Shetland Isles and the Faroes.
Not all rocks contain oil and over the years billions of pounds has been spent drilling into rocks, which very often prove to be barren. Many things need to be right to trap an economic accumulation of oil in porous sandstone rocks and one of the critical factors is the timing of oil movement.
This new research is being lead by Professor John Parnell in the School of Geosciences at the University of Aberdeen, and Dr. Simon Kelley in the Department of Earth Sciences at the Open University.
Professor Parnell explains: “If the potential trap formed before oil flowed into it, there are no problems, however, if it formed after oil was moving, then this is too late and the oil is likely to have leaked to the surface and been lost many millions of years ago.
“Therefore in oil exploration, it is extremely important to be able to predict when oil was flowing in a particular region and this is the basis of our research, in collaboration with our colleagues at The Open University.”
To date, geologists have had to try and predict the oil flow using indirect evidence or theoretical modelling along with quite a bit of ‘guesswork’. Now, a novel approach has been proved which can constrain when oil entered a subsurface reservoir. The integrated technique determines the temperature, composition, and timing of past-fluid flow. The technique uses a mineral, potassium feldspar, which occurs as a natural cement in many oil reservoirs filling pore spaces and coating sand grains.
The presence of oil during mineral precipitation is recorded in tiny bubbles of oil trapped inside the mineral cement. The age of the feldspar cement can be determined by measuring the amount of argon produced by radioactive decay of potassium. Dating the mineral also dates the occurrence of oil in the reservoir.
This combination of oil detection and mineral dating brings together collaborative expertise at the University of Aberdeen and The Open University.
The painstaking laboratory work has been undertaken by Aberdeen University graduate Darren Mark, involving high-resolution microscopy techniques in Aberdeen and measurement of argon isotopes at the Open University. Darren and the rest of the team, including Sarah Sherlock of The Open University, Martin Lee of the University of Glasgow and Andy Carr of Advanced Geochemical Systems Ltd report a successful case study in this week’s edition of the leading scientific journal, Science.
The case study used in the research, based on the UK Atlantic Margin, has constrained the date at which oil entered a deep water reservoir 50 km west of the Shetland Isles, and also when the oil was subsequently lost from the reservoir.
“This is a very powerful technique,” said Professor Parnell, “which will not only help the oil industry to understand the timing of oil movement into subsurface reservoirs worldwide, but will also help scientists to date fluid movement in the Earth’s crust.”
Funding for the research has been provided from the UK Natural Environment Research Council.