Isotopic Analyses

Isotopic Analyses

Radiogenic and stable isotope analyses of rock samples enable us to:

  1. constrain the chronostratigraphic age of poorly dated and even palaeontologically-barren successions

  2. improve / refine stratigraphic correlations

  3. characterise diagenetic environments and trace their evolution through time

  4. determine the absolute age of diagenetic phases

While analysis of bulk rock samples is possible, analysis of separated rock components such as skeletal grains and specific authigenic mineral phases usually provides more targeted results.  Our in-house micro drill system has a minimum drill bit size of 0.1 mm, allowing the sub-sampling of thin crystal zones and mineralogically layered skeletal grains.  Laser ablation ion probe analysis may be appropriate where micro-drilling is not possible.

Radiogenic isotope analyses provide an absolute ‘radiometric’ age for rock samples based on the measurement of the ratio between the parent isotope and radioactive-decay daughter products. Historically, radiogenic isotopes have mostly been used to date metamorphic, igneous and volcanic rocks, but there are also numerous applications to sedimentary rocks. Of particular use are:

  • The Renium – Osmium isotope geochronometer (187Re – 187Os) – we have recently used this state-of-the-art technique for dating palaeontologically-barren Neoproterozoic black shale source-rock successions in East Africa and Arabia. The analysis can be performed on both core and ditch cuttings samples.

  • Potassium-Argon analysis (40K – 40Ar) can be used to determine the age of authigenic illite, and to constrain the timing of hydrocarbon charging (which normally arrests illite precipitation). The analysis is usually performed on the <2μm clay fraction.

  • Laser ablation Argon-Argon analysis (40Ar – 39Ar) can be used to determine the age of single glauconite grains which occur commonly in condensed facies and may thus provide a numerical age for the condensed section; this technique can also be used to date K-feldspar overgrowth cement rims, thus constraining the timing of cement precipitation and providing an absolute ‘not-younger-than’ age for palaeontologically-barren sandstone intervals.

These pore-bridging authigenic illite laths in a Jurassic reservoir sandstone were radiometrically dated by the K-Ar method to constrain the timing of oil charging. Scale bar is 2 microns.

Stable isotope analyses are employed to investigate secular variation in isotope ratios. Some of the more common stable isotope techniques we offer are as follows:

  • Carbon (δ13C) and Oxygen (δ18O) isotope analysis – these isotopes record global climatic variations as well as changes in oceanographic circulation, and represent a powerful correlation and age-dating technique. Carbon-isotope chemostratigraphy is most reliably deployed in deeper water sequences, as Carbon-isotope signatures of shallow-water limestones are commonly altered by marine and/or meteoric diagenesis. The modification of C- and O-isotopes beneath palaeoexposure surfaces may help to identify otherwise cryptic stratigraphic surfaces and can improve well correlation (particularly in shallow-water sequences where palaeoexposure is more prevalent)

  • Strontium isotope analysis (δ 87Sr/86Sr) is usually performed on low-Magnesium calcite bioclasts (e.g. belemnites, oysters, brachiopods) which are less-prone to diagenetic modification and best preserve the primary marine isotopic signal; this technique allows precise absolute age determinations to be made based on comparison with high-resolution seawater strontium isotope calibration curves

  • Sulphur isotope analysis (δ 34S/32S) can provide absolute age determinations of palaeontologically-barren evaporite successions, and can be performed on barite, gypsum or anhydrite from core, ditch cuttings or outcrop samples.

SEM-CL image of a Jurassic sandstone sample showing well-rounded detrital grains enclosed by dull luminescent overgrowth cements.  The numbered points are areas of overgrowth cement selected for ion probe stable isotope analysis.

Same field of view as above imaged in SEM showing ~10 micron diameter sampling pits, illustrating the precision achievable with the laser ablation technique. A Carbon-Oxygen isotope determination was made at each sampling point.

The integration of stable isotope data with cathodoluminescence petrography, fluid inclusion microthermometry, and elemental analyses, can provide major insights into the subsurface palaeo-hydrology and the provenance / nature of diagenetic fluids, leading to more robust prediction of the distribution of porosity and cement phases within reservoir units. Oxygen and carbon isotopes are also particularly sensitive to platform exposure and soil development and help in the identification of platform exposure and karstification events.