Understanding Fluid Variations in the Arab Formation in Abu Dhabi: New Technologies for Detailed Reservoir Fluid Characterisation

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Summary The reservoir fluids in the Arab Formation in Abu Dhabi are highly variable. In the offshore, the fluid phase in Arab reservoirs can vary in a single well from a low GOR black oil to an ultra-sour, dry gas. Also Arab accumulations onshore show large variations in reservoir fluid composition, with H2S concentrations ranging from 5 to 38 %. Understanding these variations on a field scale is a requirement for designing development strategies and formulating economic forecasts. Detailed studies of Arab reservoir fluids remain challenging, however, as they are frustrated by a systematic shortage of (down-hole) fluid samples and by the fact that fluid gradients cannot be established accurately in isolated, thin reservoir units. Extrapolating fluid compositions away from the sparse MDT sampling points further adds to the uncertainties, as many of the Arab reservoir contain fluid gradients. This paper describes the results of a research study into gas released from core samples with the objective to derive information about fluid phase and fluid composition. Isotopic and molecular compositions of gas released from core samples were measured from one offshore well and four onshore wells. A purpose-built rock-crusher coupled to a gas chromatograph (GC) and a gas chromatograph isotope ratio mass spectrometer (GCIRMS) was used to release and analyze traces of residual gas on core samples. As observed in the reservoir fluids, the acquired analytical data show very large variations, both in isotopic and molecular composition. Remarkably, the isomer and isotope ratios show the same overall depth trend in all wells that were analyzed. Based on the generated data, isotope and isomer logs were constructed and interpreted in terms of charge history and fluid composition. Analysis results suggest complex charge histories with an initial charge of black oil, followed by at least two episodes of gas charge. The resulting variations in fluid phase are the outcome of (incomplete) mixing of fluids from these three different charge events, with present-day compositions reflecting the filling sequence and filling direction of the reservoirs. The charge history of the studied Arab reservoir units inferred from the new data is as follows: An initial charge of a black oil, most likely derived from the source rocks of the Diyab Group in the deep synclines surrounding the Arab structures.A second charge of sour gas. This gas is interpreted as a ‘secondary gas’, derived from oil-to-gas and condensate-to-gas cracking under H2S catalysis. Oil-to-Gas cracking is a known side-reaction of the Thermochemical Sulfate Reduction (TSR) that generates the H2S and CO2. As no lithological evidence for TSR is found in the Arab anhydrites, it is concluded that the TSR and oil-to-gas cracking takes place in a deeper accumulation.A third charge of dry, low H2S gas. This gas is interpreted as early mature methane, originating from source rock intervals immediately below the Arab accumulations. The gas is very similar to the ‘shale’ gas identified in the Hanifa/Tuwaiq Mountain source rocks in the Jafurah Basin in Saudi Arabia. The existence of a third gas charge into the Arab reservoirs has not been recognized before and the gas has not been sampled in its pure state. The gas is extremely dry, contains no H2S and its concentration increases with depth, both onshore and offshore. The light carbon isotope ratios of methane suggest that it was generated in the initial stages of thermal maturity and that it may therefore be less volumetrically significant than the gas generated during the main phase of oil generation.