Sr isotope residual salt analysis (SrRSA)

Application

SrRSA is performed on extracted salts that have been precipitated in pore space as result of evaporation of formation water during storage of core, or on irreducible water in hydrocarbon saturated reservoir, or formation water in the water leg (Barnaby et al. 2004).

The water, and hence mixture of salts it contains, is generally homogenous unless barriers to mixing exist within the reservoir. The strontium stable isotope ratio, 87Sr/86Sr, offers a particularly sensitive measure of compositional variations (Mearns & McBride 1999).

Sample requirements

For aqueous samples, at least 1 mL is required. Where core plugs are to be used, aqueous lubricants are to be avoided, as should proximity to other plugs sites (e.g. for poroperm determination) and the periphery of cores and areas containing fractures.

Approximately 2–3 cm3 pieces of pristine, consolidated core section is required; larger pieces can be sub-sampled at APT.

It is also a good idea to analyze a mud cake sample from each well to check the implications of possible contamination of the plug samples by the mud.

Analytical procedure

Aqueous samples (0.5 mL) are evaporated to dryness and the residue dissolved in 3M HNO3 (200 µl). The Sr present is isolated by ion exchange. A Finnigan MAT 261 solid source is used for the mass spectrometry analyses.

Background levels of Sr are normally 20–30 pg, so do not have a significant influence on analytical results for different types of geological samples, including water. The minimum sample amount for the mass spectrometric analysis is 100 ng Sr.

Potential problems

  • insufficient sample/very low Sr concentrations
  • interference from Sr in mud

References

Barnaby R.J., Oetting G.C., Gao G. (2004) Strontium isotopic signatures of oil-field waters: Applications for reservoir characterization. AAPG Bulletin 88, 1677–1704.

Mearns E.W., McBride J.J. (1999) Hydrocarbon filling history and reservoir continuity of oil fields evaluated using 87Sr/86Sr isotope ratio variations in formation water, with examples from the North Sea. Petroleum Geoscience 5, 17–27.