Fluid Property and Geochemical Evaluation of from Exploration Well in the South Rub' Al Khali Basin: Implications for the Regional Subsurface Model

Abstract

The sampling and analysis of sour gas and its associated condensate is a difficult task: H2S is highly reactive and the route from reservoir to laboratory is covered with potential ‘sulphur sinks’. The downhole sampling program is further complicated by the fact that gases with high H2S concentrations tend to have low condensate gas ratio’s (CGR’s) and the slightest drilling mud contamination can potentially effect the fluid composition and lead to erroneous concentrations of H2S. Yet, accurate fluid property data like H2S content and condensate gas ratio (CGR) are critical for reserve estimates and economic evaluation, for field development planning and for facility design. The South Rub Al-Khali Company Limited (SRAK) recently completed a deep exploration well which targeted objectives at several stratigraphic levels. The well found sour gas in the Jurassic Arab Formation and an extensive down-hole sampling program was initiated to acquire representative fluid samples from all potential Arab pay-zones. In addition, a comprehensive geochemical analysis program was carried out with the objective to update the regional hydrocarbon habitat and basin models. This paper discusses the integration of all fluid property, geochemical and basin modeling data, which together provide a rigorous quality check of the sampling and analysis program. H2S concentration and CGR, for instance, are not independent parameters, as both are governed by Thermochemical Sulphate Reduction (TSR), the in-reservoir process that leads to the formation of H2S and CO2. Also the molecular and isotopic composition of the gases are dependent on the TSR process and provide an important consistency check. Subsurface temperatures and source rock quality and maturity data were used to calibrate the regional basin model and to update the understanding of the hydrocarbon habitat of the Southern Gulf region. Finally, the newly acquired subsurface temperatures provided further insight into the hydro-dynamic flow regime of the South Rub al Khali Basin, which were described earlier after the first SRAK exploration well.