eichrom

Application Note: 503

Expertise in Sample Preparation

SrRSA: Strontium Isotope
Residual Salt Analysis

Introduction:
The isotopic geochemistry of carbon and hydrogen in gases has been extensively used in oil &gas exploration for several decades. It provides information on the origin of the gaseoushydrocarbons, on reservoir compartmentalization and on the efficiency of the cap rock. The
isotopic composition of strontium in residual salts offers a means of determining vertical andlateral connectivity in reservoir units and the chronology of hydrocarbon filling of the structures.
The Total SA, Fluids and Organic Geochemistry laboratory has developed a set of sampling andanalysis techniques for increasing the vertical resolution and precision of the measurements
while reducing the time and cost of analyses. It has thus become possible to conduct precise
isotopic screening of the zones to be characterized and thereby provide new keys tointerpretation.
One component of this characterization approach is Strontium isotope Residual Salt Analysis(SrRSA). The residual salts are extracted
from the cores by washing in an ultrasonicbath with distilled water. Another possibility
is to collect formation and/or reservoirwaters to determine their origin (paleoaquifer or present-day aquifer).
Historically, very accurate measurements ofthe strontium isotope ratios by ICP-MS orTIMS have been hindered by isobaric
interferences between 87Sr and 87Rb. To
overcome this issue, the Total approach
utilizes a sample preparation system topurify the strontium isotopes frominterfering rubidium.
Figure 1. Uptake of Various Metal Ions by Sr Resin

Sample Preparation:
The technique uses Eichrom’s Sr Resin, an extraction
chromatographic material which shows very strong andspecific affinity for strontium over rubidium, calcium andbarium, among others elements. (See Figure 1 above.)
The strontium is separated out on a solid phase extraction
system (ASPEC-XL) into which Sr Resin cartridges areincorporated. Beside the selectivity of this resin for Srover Rb, another advantage is that it can operate underpressure, which considerably reduces elution time. This
automatic system can process 200 samples a week.
Figure 2. ASPEC-XL automatedsample preparation system

01/05/2006

Application Note: 503

Isotopic Measurement:
The isotopic analysis of the strontium is undertaken using
an ICP-MS-MC (AXIOM). Over 200 samples can beanalyzed per week, with a precision better than 10ppm.
This methodology can be applied to samples with ppb
levels of strontium content.

Results:

Figure 3. Axiom ICP-MS-MC System

Figure 4 is an example of the typeof results that this technique canachieve. The isotopic ratio of87
Sr/86Sr can be plotted vs. sample
depth. Overall the ratio changes
slightly with depth. Deviations
from this line indicate geologicalformations which may besignificant in the characterizationof the petroleum field.

Figure 4. 87Sr/86Sr ratio vs. sample depth

Reference:
Isotope geochemistry in the oil & gas exploration context: progress towards a high verticalresolution screening tool; Y. Poirier, et al., Total SA, Fluids and Organic Geochemistry, Pau,
France; Poster presented at: Applied Isotope Geochemistry-5, Heron Island,. Australia May 2630 2003

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01/05/2006

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