Tracking gas migration using reservoir PVT data as reality check for migration models

Tracking gas migration geochemically is more challenging than tracking oil migration due to the deficiency in biomarkers and migration tracers, traditionally restricting gas tracking to isotope fingerprinting. This highlights the need for developing new fluid interpretation workflows that maximize the value of reservoir fluid data to constrain gas migration models. Systematic analysis of reservoir PVT fluid properties from 49 wells in a complex carbonate-evaporite sequence across four adjacent Paleozoic gas fields on the eastern part of the Arabian Plate, supported by multivariate statistical analysis, enabled the tracking of gas migration. Results were used to test current basin models, differentiate fault migration from carrier migration, and evaluate the impact of migration style and dynamics on fluid properties and distributions. The first basin model invokes a very long-range migration in excess of 300 km, accumulating first in the northernmost field before spilling over to the south. The second model entails filling from the east via multiple parallel pathways. Regional northward increase in gas-oil ratio and gas maturity, concomitant with enrichment of nitrogen and carbon-13, seemingly supports the north-charging model, but on closer inspection of reservoir fluid properties, the east-charging model is favored. Multiple parallel charging along the eastern flank is reflected in parallel but overlapping fluid maturity and density segregation trends, supported by ratios of reservoir pressure to dewpoint pressure that almost double towards the drier gas in the deeper north due to increased pressurization from more-mature charges coupled with increased hydrocarbon consumption by thermochemical sulfate reduction. The observed patterns in fluid properties are contrary to what would be expected in a single southward fill-spill setting or a single accumulation in equilibrium encompassing the four fields. Migration style played a major control on gas properties and distributions, with carrier migration yielding maturity and density profiles that follow reservoir depth, supplemented in certain areas by fault migration that increased gas dryness and column height. Findings have significant implications for targeting drier gas in stratigraphic and diagenetic traps along the deeper east, particularly where downdip faults meet migration pathways. Similar workflows can be used to constrain reservoir models.