ABSTRACT A migration geochemical study was conducted over an area of approximately 15,000 km 2 (5792 mi 2 ) in northeast Saudi Arabia to define regional migration patterns and de-risk oil charge away from the Late Jurassic source kitchen of the Gotnia Basin to prospects further to the south and west on the shelf margin and Summan Platform. Discovered accumulations range in depth from more than 10,000 ft (3048 m) subsea on the shelf margin in the north to around 5000 ft (1524 m) on the Summan Platform in the south. Shallowing south/southwestward is associated with a wide API gravity variation (15–35°) and gentler molecular and isotopic maturity trends. Defining migration patterns based on bulk, isotopic, and saturated and aromatic hydrocarbon distributions alone proved ineffective, especially given that all oils examined from the target Arab-A reservoir have a narrow peak-oil window maturity and a largely common source, that is sulfur-rich (up to 6.1 wt.%) anoxic marine carbonate, presumably within the Hanifa/Tuwaiq Mountain formations or their equivalent Najmah/Sargelu formations in the Gotnia Basin. Molecular geotracers, based primarily on compositional correlation coefficients of aromatic nitrogen compounds, identified several southwest-trending migration pathways that have charged traps in the Late Jurassic Arab Formation. Prospects falling along inferred migration pathways or entry/spill routes are therefore high-graded.

ABSTRACT Determination of petroleum charge history in a field is a critical segment to resolve within regional migration analysis. This, however, is often complicated by a number of alteration processes, such as density segregation. Later charges can disturb chemical equilibrium and compositional grading of oil columns. Controls over fluid compositional variations of oil field H in the northern part of Saudi Arabia were investigated to help predict fluid properties of future delineation and development wells and help constrain regional migration scenarios. Combined effects of charge history, density segregation, and compartmentalization appear to have been key controls on fluid compositional variations in this field, with density segregation manifested at bulk and molecular levels in this single-phase, undersaturated fluid system. The molecular and isotopic maturity profiles mirror the API gravity–asphaltene–depth profiles, which indicate the combination of multiple charging and density segregation. Density segregation within the reservoir has apparently expanded the API gravity range originally entered the trap, resulting in a mismatch between the wide API gravity range (15–26°) and the narrow maturity range (0.78–0.81% calculated vitrinite reflectance). Due to possible interference from molecular segregation, maturity parameters that rely on compounds of differing molecular densities and chemical reactivities should be used with caution. Sterilization of the oil column at temperatures greater than 80°C (176°F) during its deepest burial events in the Eocene and Miocene times may explain the nonbiodegraded nature of this shallow (<5400 ft [1646 m] depth) and relatively cool (54–63°C [129–145°F]) accumulation.