The South Caspian Basin hosts more than 600 onshore and offshore oil and gas fields characterized by large variations in fluid properties impacting field development and nearby exploration efforts. A total of 32 surface and downhole fluid samples collected from six distinct Pliocene sandstones in an offshore anticline field in the eastern South Caspian Basin were analyzed (gas, light- and midrange–fraction composition, biomarkers, and bulk and gas compound-specific carbon isotopes) to investigate the processes controlling the fluid properties at the field scale. Sampling and sample handling methodologies enabled preservation and recovery of representative light-end compositions.
Lateral fluid property variations within the main productive sandstone reservoir (API gravity from 36.8° to 53.4° and gas–oil ratio from 544 to 947 SCF/bbl) were found to result from the combined effects of biodegradation, water washing, and a light charge contribution, the latter having the predominant impact on the observed fluid properties variations. Ongoing migration of higher-maturity fluids generated deeper in the South Caspian Basin or evaporative fractionation, which likely affected fluids in the deeper sandstones, may be responsible for the contribution of a volatile enriched charge to the shallow reservoirs. A comparison based on source and maturity diagnostic parameters with Pliocene reservoir oils from the western South Caspian Basin is consistent with a similar reservoir filling history but more complex migration pathways for the eastern South Caspian Basin reservoirs.