Isotopes
Over the years, Chemostrat has expanded the services on offer to now include more advanced isotopes techniques that so far have been used only in academia. The information presented below briefly outlines how we are utilising these techniques, to assist the petroleum industry in the goals of oil and gas exploration.
Rhenium - Osmium Isotopes Dating Technique
Establishing when organic shales were deposited
Rhenium and Osmium become concentrated in the organic, hydrogenous fraction of organic rich shales, thus the time when the shale was deposited can be dated. The technique can be applied to any organic rich shale older than ~10 Ma and can include lacustrine and Proterozoic shales for which biostratigraphical information is lacking. In general, between six and eight samples from the same stratigraphic interval are analysed to produce an isochron that provides a Re - Os age for the deposition of the shale with a typical uncertainty of between 1% and 5%. The 187Os/188Os value calculated at the time of deposition (Osi) can also provide information on whether deposition occurred in a lacustrine or an open marine environment.
Establishing dates for the generation of oils
When oil is formed from a source rock, a proportion of the Re and Os from the source rock is transferred to the oil, thus one can date the formation process using the Re - Os geochronometer. Furthermore, as Re and Os have been inherited from the source rocks, Os isotopes can be employed to correlate an oil with its source. Re - Os dating and oil - source rock correlation is actually helped if the oils are highly biodegraded, e.g., oil sands, tar mats and bitumen-stained sandstones (where traditional techniques such as biomarker identification are disrupted), as this concentrates Re and Os within an oil making analysis easier.
Establishing the formation date of sulphide minerals
The Re - Os technique can also be utilised to date the formation of sulphide minerals. Despite this application mainly being associated with the economic extraction of minerals, it can be employed by the oil and gas industry to date the formation of disseminated sulphide minerals within basement rocks and so the basement itself. Moreover, when these minerals are found in fractures it can be used to provide a minimum date for when the fracture network was open.
Carbon 13 for Shale Plays Services
The proportion of 13C to 12C in organic matter, or δ13Corg, can be used to a) provide chronostratigraphic correlations for basins (or indeed globally), by comparison with well understood and published δ13Corg curves. Chemostrat are able to obtain data from both core and cuttings and from shales as lean as TOC=0.3%. More local fluctuations in this value can be integrated with other variables such as Sulphur34, TOC, Total N, Total S and elemental data to provide key data on productivity and preservation of OC in sediments.
Sulphur 34 (δS34pyr) for Shale Plays Services
δ34Spyr data from early pyrite provide information regarding euxinia and depth of water. When integrated with Carbon 13, TOC, Total N, Total S and elemental data δ34Spyr is a powerful tool in understanding palaeoredox conditions.
