Hydrocarbon Retention and the Case for Vertical Migration

Complex hydrocarbon charging and distribution in which reservoirs are filled by oil and gas phases with different densities and genetic types inter-fingering within the basin, are common phenomena, and often attributed to vertical migration. This paper discusses the factors that control vertical hydrocarbon migration and presents modelling of the hydrocarbon charging and entrapment history in a tertiary basin in Southeast Asia as a case study. According to the Young-Laplace flow theory of the secondary hydrocarbon migration mechanics, migration occurs in a state of capillary equilibrium in a flow regime dominated by buoyancy and capillary forces. In this study, the invasion percolation simulation algorithm, based on the Young-Laplace flow, was used. During the simulation, three-dimensional (3D) seismic data were used as the high-resolution base grid for migration to capture the effect of both structure and facies heterogeneities on fluid flow. A model of an unfaulted system was presented to make the case. In the study area there is inter-fingering between oil and gas across different formations; most oils are trapped in the deeper formation, oil and gas inter-fingering occurs in the middle formation, and the upper formation contains mostly gas. This arrangement is possible because of the interplay between the expelled fluid buoyancy and relatively weak intra-formational seals within the basin. The modeling results were then calibrated to known accumulations or fluid presence in wells. In a basin dominated by a vertical migration regime, hydrocarbons are prevented from travelling far from the kitchen, thus decreasing prospectivity away from the kitchen. Through a case study, this paper helps to understand the factors that influence hydrocarbon retention and migration that control fluid distribution within a basin. Eventually the study helps geologists to understand prospectivity risking related to hydrocarbon charging, which is one of the main risks in exploration especially in mature basins.