Molecular Density Segregation and Subtle Multiple Charging in a Cool Nonbiodegraded, Undersaturated Oil Reservoir

Abstract

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.