Interlaboratory study of a method for determining nonvolatile organic carbon in aquifer materials

The organic carbon fraction in aquifer materials exerts a major influence on the subsurface mobilities of organic and organic-associated contaminants. The spatial distribution of total organic carbon (TOC) in aquifer materials must be determined before the transport of hydrophobic organic pollutants in aquifers can be modeled accurately. Previous interlaboratory studies showed that it is difficult to measure TOC concentrations 1%. We have tested a new analytical method designed to improve the accuracy and precision of nonvolatile TOC quantitation in geologic materials that also contain carbonate minerals. Four authentic aquifer materials and one NIST standard reference material were selected as test materials for a blind collaborative study. Nonvolatile TOC in these materials ranged from 0.05 to 1.4%, while TIC ranged from 0.46 to 12.6%. Sample replicates were digested with sulfurous acid, dried at 40°C, and then combusted at 950°C using LECO or UIC instruments. For the three test materials that contained >2% TIC, incomplete acidification resulted in a systematic positive bias of TOC values reported by five of the six laboratories that used the test method. Participants did not have enough time to become proficient with the new method before they analyzed the test materials. A seventh laboratory successfully used an alternative method that analyzed separate liquid and solid fractions of the acidified sample residues.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/46768/1/254_2004_Article_BF00770471.pd

Dolomitization and burial history of lower triassic carbonate reservoir-rocks in the Persian Gulf (Salman offshore field)

International audienceShallow marine carbonate sedimentation prevailed during the Early Triassic in the northern part of the Persian Gulf (Kangan Formation; offshore Iran). Associated near surface and (to a lesser extent) burial diagenesis had significant impacts on the evolution of reservoir properties of the Kangan Formation. In some Iranian offshore fields such as Salman, pervasive, early dolomitization of the Kangan carbonate rocks is believed to have played an important role in improving the reservoir characteristics of this unit. Detailed petrographic and geochemical analyses indicate that such dolomitization could be explained by means of the sabkha/seepage-reflux model. Furthermore, three types of dolomites, found in Kangan rocks in the Salman Field, have been described: (1) fabric-retentive, (2) fabric-destructive, and (3) cement. Fabric-retentive dolomites are the most frequent commonly found in the studied field, with respect to the other types. Oxygen and carbon-stable isotopic analyses confirm that the fabric-retentive dolomites and associated anhydrites formed at surface and near-surface conditions, whereas the fabric-destructive dolomite formed at temperatures above 50 °C. The burial history of the Salman Field is divided into three major phases (tectonic quiescence and low subsidence rates, and two active phases with high subsidence rates). The fabric-retentive dolomites are mostly related to the first phase, while the fabric-destructive and dolomite cements are related to the second and third phases including shallow and deep burial, respectively