Benzocarbazole and methylxanthone molecular migration tracers unravel migration routes and filling history of Veslefrikk, Oseberg-East and Brage reservoirs in the Norwegian North Sea

Abstract

Identification of petroleum migration routes and reservoir filling processes is feasible by applying carbazoles or xanthones as molecular migration tracers. To verify their utility, a well-constrained migration route was chosen for this study: the petroleum migration along the Veslefrikk and Oseberg East fields until reaching the Statfjord Formation compartment within the Brage field in the Norwegian North Sea. This migration route comprises an ideal test case because the oil is sourced from the same kitchen area, yielding very similar oil families in all three fields, and because the maturity remains within the main oil window, below 0.9 Ro % vitrinite reflectance equivalence.Different methodologies applied by two laboratories, from sample preparation to analytical methods, yielded identical results of migration direction and route using the benzocarbazole ratio ([a]/([a]+[c]), thus underlining practicability and robustness of this molecular migration parameter. The benzocarbazole ratio (BCR) in this study was neither affected by source rock facies, and/or biodegradation. A thermal maturity effect on BCR is considered here to be of only subordinate impact whereas it substantially affects benzocarbazole concentrations. The BCR declined from values of 0.68 at onset of migration to 0.42 at end of migration, whereby the absolute concentrations of benzocarbazoles in parallel declined by a factor of 10, from 5000 to 500 ng/g oil. The distribution and concentration of methylxanthones evolved comparable to that of benzocarbazoles. The methylxanthones ratios (MXR) from onset of migration until reaching the Brage field declined from a value of 0.85 to 0.42. The benzocarbazoles and methylxanthones, like other geo/biomarkers, preferentially reflect the first charge of the most polar and least mature oil delivered upon petroleum migration and could help to understand the reservoir filling history.Migrating oils derived from the Sogn Graben kitchen entered the Veslefrikk oil field first and continued until the structure was filled to spill point. Thereafter, migration continued to the Oseberg East oil field and after reaching spill point there it continued to the Brage field and its Statfjord formation compartment. The complex migration route represents a classical fill to spill scenario and benzocarbazole fractionation upon migration delineates the initial filling trends. The maturity values of crude oils are in the mid-oil window and show trends of covariance with the molecular migration indicators, thus proving to be supporting tracers in a classical fill to spill scenario, where maturity values are lowest when migration distances are greatest. This is due to the first expelled oil being of lowest thermal maturity entering the nearest reservoir first and then being displaced into farther reservoirs along the migration route upon arrival of later oil (more mature) charges.