The radiocarbon of the local reservoir effect (RE) was observed in many sectors along the Argentinean Patagonic coast. Results show variations in the 14C offsets and differences between marine and continental species growing within the same locality, ranging from about 80–1100 yr BP. It is postulated that such variations are mainly due to local factors, including the coast morphology and the contribution of continental waters. The relevance of these kinds of studies for the interpretation of age in archaeological samples is highlighted in this paper.

We present here the results of dating of 80 archaeological and paleoenvironmental samples from Argentina and Uruguay, processed between 1986 and 1988 by M A Albero and M A Gonzalez. Series of samples and single samples are grouped by province and then by locality or archeological site, from north to south. See sample location maps for details.Procedures for sample pretreatment, counting, statistical analysis, and age calculation were essentially the same as previously described by Albero and Angiolini (1985). Results are reported as conventional 14C dates in years before AD 1950. They are corrected for isotopic fractionation. 14C contents of some paleoenvironmental samples are expressed in percent modern carbon (pMC).

A case study is presented which addressed the origin of Hydrogen Sulphide (H2S) in produced gas associated with the oil in Las Heras - Cerro Grande oilfield in Gulf of San Jorge Basin, Argentina. The reservoir was initially sweet. However, after waterflooding an increase in H2S concentration was detected in several wells. The H2S amount varies across the field with the highest values found in wells where water injection was initiated and the wells produce with a high water cut. Knowledge of H2S origin contributes to effective control and therefore, to reduce environmental risk, minimize emissions, improve quality of produced hydrocarbons and reduce corrosion rate. The main stages followed include, H2S measurements, H2S source identification and mitigation treatments. The possible mechanisms of reservoir souring have been analyzed by sulphur isotope ratio analysis. These results have been integrated with geological factors and other parameters related to waterflooding. This case is unusual because of water composition. The formation and injection waters have low salinity and are not rich in sulphate ion concentration, very different from sea water composition. Nutrients such as fatty acids are present in both formation and injection waters. The increase in H2S production is attributed to sulphate reducing bacteria (SRB) living in the reservoir. The proposed model to explain H2S origin suggests a closed system with reactive-limited reaction where H2S production is controlled by low sulphate concentration present in waters. A strong correlation was demonstrated between produced H2S, injection water parameters and subsequent souring. A decrease in sulphate concentration present in water production appears to correspond with an increase in H2S concentration. At present, the produced gas is being treated with scavengers. A nitrate injection project is being analyzed to control reservoir souring. The development of a biofilter to oxidize H2S in plant treatment is in laboratory phase. Introduction Las Heras -Cerro Grande oilfield belongs to the main area called Las Heras field, in Gulf of San Jorge Basin, Santa Cruz Province -Argentina (Figure 1). The zone of the field to study covers an area of around 75 Km2 with 250 wells. The field is characterized by its complex structure which is crossed by two main faults NNO-SSE with NE and SO dips that create a central fossa. It is limited by the two faults. The hydrocarbon accumulations are in sandstone, lithic sandstone and tobaceous sandstone of Bajo Barreal Formation - Chubut Group, in Middle / Upper Cretaceous Age, being the origin of the sediments aluvional, ephemeral fluvial and brief lacustrine (Figure 2).Most of the reservoirs of the block comprise the lower section of Bajo Barreal Formation, not reaching the Castillo Formation. On the basis of the paleographic interpretations the coexistence of three groups of geometries at different depths was determined: canalized layers, layers poorly canalized and lobulate layers. The geometry of the channels agrees with the direction of the main faults. Three waterflooding projects were implemented successively to increase oil recovery. Water injection began in 1997, in a pilot well with, an average injection rate of 140 m3/day that was obtained from a water producer. After approximately 6 months, the conversion of remaining injection wells of the first project was carried out and it was followed with wells of two remaining projects, arriving at the total injection in the middle of 2000 with approximately 6000 m3/day distributed among 59 injection wells, with 175 associated producing wells (Figure 3). The initial reservoir conditions prior to waterflooding were 42°C and 85 bar pressure. Produced fluids are separated in surface facilities and the produced water is reinjected into the reservoir after being mixed with produced water from other areas and with a small percentage of Rio Senguer water in Las Heras 3 Plant. The injection water is pumped from LH3 Plant to injection wells.

By mid-2000, the H2S content in gas from the Chihuido Lomitas field separator started to increase significantly up to an average of 2,000 ppm. A multidisciplinary team was formed to address the problem, which was deemed to be very likely due to bacterial activity in the reservoir. Therefore, the group focused on the issue of concern, which is generally known as reservoir souring. It was concluded that using sulphur isotope measurement techniques was one of the few tools available to effectively determine the cause of the souring. A group of specialists in isotope assessments from a local Research Institute was called in to join the team. Based on the team's findings, supported by a geochemical model, it was shown that H2S generation in the Troncoso and Avile formations was due to the activity of sulphate reducing bacteria living in the reservoir. The model developed relates increasing bleeding water injection to the phenomenon studied. A thermo-chemical reduction was proposed to explain the H2S content in El Filon Reservoir. Innovative H2S removal techniques are currently being applied. Introduction The Chihuido Lomitas light oil field is located in the Neuquen Basin, in Argentina. The main producer reservoirs (Agrio Formation and Avile Member) are in the Mendoza Group (Late Jurassic - Early Cretaceous). One non-conventional reservoir produces from an igneous rock (see Figure 1). The field started to produce in the early 1980s, and is currently Repsol YPF's main oil field in Latin America. Water injection started in 1996 and, by the end of 2001, nearly 100,000 m3/day were injected through 480 injection wells. Initially, just fresh water was injected, but it was gradually replaced by production water. When the H2S content in the separator gas started to increase, a team was formed including field staff from Engineering and researchers from Repsol-YPF's Applied Technology Centre (CTA) in order to address the problem by looking at different approaches. Measurements were thoroughly checked, and a sampling survey at key points was designed. Different gas treating alternatives were discussed. It was concluded that sulphur isotope measurement techniques were one of the few tools available to effectively determine the origin of the problem. Sampling H2S collection was accomplished with a sampling device developed at INGEIS, including a flow meter and two H2S traps, with two online flasks containing AgNO3 or NaOH. Sixty samples were taken from all batteries, including formation, production, and injection water, oxygen scavengers, and crude oil. Isotope analyses were performed at INGEIS using VG 602 and Finnigan Delta-S mass spectrometers. CTA performed a standard geochemical analysis, and determined the level of fatty acids in the formation waters. Results and Discussion The main data are shown in Tables 1 and 2. We carefully analysed the different possibilities for H2S generation in the reservoir, on the basis of the isotopes results, geology, temperature range, presence of sulphides in reservoir formations, and injection/production water characteristics. The analysis also took into account the exploitation history, and the absence of H2S in the early years.