AAPG Middle East Region GTW, Regional Variations in Charge Systems and the Impact on Hydrocarbon Fluid Properties in Exploration

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Geochemical Characterization of Hydrocarbon and Non-Hydrocarbon Fluids from Reservoir Rock Samples of the Upper Arab Formation, Abu Dhabi


The International Energy Agency estimates the natural gas reserves of the United Arab Emirates at over 6.2 trillion m3. Compared to these vast reserves, the production volumes of the UAE are relatively small, although gas production has been growing rapidly in recent years. The main reason for the relatively low production is the high H2S and CO2 contents of most gas accumulations which makes development both technically and economically challenging. Many gas accumulations in the UAE contain high, and highly variable, concentrations of H2S and CO2 in multiple, disconnected reservoir units. This complication is particularly prominent in the Upper Arab Formation, which consists of multiple stacked carbonate units, separated by anhydrite layers. The reservoir units are not connected and H2S concentrations range between 5 and 27%, without a clear vertical or lateral trend. With more than 10 reservoir units in the Upper Arab, it is impossible to test or to take down-hole fluid samples in each individual reservoir unit. The exact gas compositions, and thus the exact sales gas reserves, therefore remain unknown for large parts of the Upper Arab Formation. This presentation discusses our efforts to derive H2S concentrations from the geochemical analysis of molecular proxies in core and cuttings samples. Rock samples from gas reservoirs contain small amounts of adsorbed gas which were analysed by a custom-build crusher system interfaced with a isotope ratio mass spectrometer. Although the composition of the adsorbed gas is not representative for the reservoir fluids, the carbon isotope ratios of the individual gas molecules are a good proxy for the extent of the Thermochemical Sulfate Reduction (TSR), the chemical process that generates the H2S and CO2. The carbon isotope ratios can thus be used to reconstruct the composition of the gas that was originally trapped in the pore space. The extracted condensates contain a wide spectrum of organo-sulfur compounds (OSC’s). The composition of these OSC’s also reflect the progress of the TSR reaction and provide an independent validation of the gas compositions. Using drill cuttings from all wells, reservoir fluid compositions can thus be mapped away from the MDT sampling points, also in reservoir units from which no fluid samples are available. Based on the analysis of the carbon isotope ratios of the adsorbed gas and the composition of the OSC’s in ‘residual’ condensate in rock samples, we were able to reconstruct the filling history of the Upper Arab reservoirs. New data suggest that the TSR reactions that generates the H2S and CO2, are not ‘in-situ’ processes. Instead, the H2S and CO2 migrate into the reservoirs, with gradually increasing H2S concentrations, following the progress of the TSR reactions. Results indicate that H2S concentrations, which initially appeared to be totally random, reflect the reservoir filling process in a very structured way.