Frequently Asked Questions
Yes, SrRSA isotope studies have been carried out on cores of all ages from freshly cut cores that have just arrived in the core store (in which case there may still be water as well residual salt in the pores), to “old” cores that have been in storage for many years. Drying of the cores and precipitation of residual salts in the pores does not affect the 87Sr/86Sr isotope ratio of the pore water.
No. We have tried SrRSA studies on rotary side wall cores in the past where oil based mud was used, but the results have always been unsatisfactory owing to contamination by drilling mud filtrate. We recommend that core is used for SrRSA.
Yes. We have carried out successful reservoir compatmentalization studies of both carbonate and clastic strata using SrRSA. A key aspect of SrRSA from carbonate rocks is to make sure the measured 87Sr/86Sr ratio is from the pore-filling formation water or residual salt and not from carbonate minerals. The sample processing is carried out to ensure there is no contamination from the rock in the salt extraction. We have done several studies where it is clear that the formation water in carbonate strata have significantly different 87Sr/86Sr ratios from the “host” carbonate rock.
The key points to consider when considering investigating reservoir compartmentalization using SrRSA are as follows:
SrRSA requires core.
Wells drilled with oil based mud provide a better chance of obtaining reliable SrRSA data than wells drilled with water based mud. This is because the risk of contamination of the pore water by aqueous mud filtrate is greater in cores drilled with water based mud.
It may be possible to screen samples from wells drilled with KCl polymer water based mud or wells in which the drilling muds were doped with Deuterium to analyse least contaminated samples.
Mud filtrate contamination may be less in lower to medium permeability strata than very high permeability strata.
The effects of contamination on SrRSA 87Sr/86Sr ratios may be less in strata with high salinity, high strontium concentration pore waters.
Obviously it depends on the number of samples, but a normal turn around time is 4 weeks for delivery of results with a further 1-2 weeks for completion of a written report. This is for a “typical” study which may include up to 40 core samples for isotope analyses and interpretation of the results.
Strontium isotope stratigraphy is applicable to marine facies, bioclastic or chemical precipitate sedimentary rocks. The analysed mineral has to have retained the original 87Sr/86Sr isotope ratio of the global sea water at the time the sediment formed. Therefore, any post depositional alteration may result in spurious strontium stratigraphy age dates.
There are a number of mineral and chemical correlation techniques that have been developed to provide correlations of biostratigraphically barren strata. Some of these are more applicable to mudstones (e.g. chemostratigraphy), while others are applicable to sandstones (e.g. heavy mineral stratigraphy). Diagenetic overprinting of the mineralogy and chemistry may be a limiting factor for some of these methods. The Sm Nd isotope method is applicable to both sandstone and mudstone facies and the SmNd isotope compositions of sediments are generally not affected by diagenesis.