Reservoir souring is a well-documented problem that appears normally when seawater is injected into sweet reservoirs during secondary recovery programs In these cases, microbiologic souring by the growth of sulphate-reducing bacteria (SRB) is the most accepted mechanism.

In some cases, H2S production is reported from the earliest stages of reservoir development. This phenomenon has been named "geologic souring"(2) because the H2S source is generated in the geological past and it is related to some element within the petroleum basin, but not-related to modern microbiological or geochemical processes.

Recently, H2S production has been recorded in El Alba-1 and Grimbeek-1 reservoirs (Manantiales Behr area, San Jorge Basin) where there are no previous reports of sources of H2S. Furthermore, water flooding has not been implemented before the H2S measurement campaing in the field. Thus, the H2S generation mechanism is the main target of this study.

Correlations of isotopic chemical analyses and isotopic measurements from gas, waters and oil samples were developed and integrated into the geological and enginnering information. After the interpretation stage, a monitoring plan and some mitigation options were suggested.

The H2S measurements, geological data and isotopic signatures do not allow locating the H2S source by thermo-chemical decomposition of sedimentary rocks (e.g. anhydrite), kerogen cracking or petroleum decomposition. Chemical data indicate that the sulphate, soluble in connate water, is the most plausible source of sulphur. Moreover microbiological activity is most likely the mechanism of biochemical reduction, even the reservoir show stressing conditions to bacterial development. According to the results, some biocides combined with H2S-scavenging are recommended to mitigate this production inconvient.

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