Temperature, Compositional and Geological Controls on the Formation of Pervasively Saturated Hydrocarbon Accumulations

Pervasively-saturated hydrocarbon reservoirs is a term used to describe non-conventionally trapped hydrocarbon systems that have greater hydrocarbon saturations than conventionally trapped reservoirs and are typically more aerially extensive. In this paper, we develop a physical model of the geological controls that lead to the formation of this hydrocarbon reservoir system based on pressure, temperature and compositional data. We first focus on the attributes of pervasively-saturated reservoirs (e.g. the association of the updip regional waterline with early to mid-maturity source rocks and lack of oil, gas contacts). Next, we discuss in detail the geological settings of the Leduc, Montney and Viking formations to illustrate the differences between conventionally trapped reservoirs and pervasively-saturated reservoirs in terms of pressure, temperature and fluid composition. Finally, we use pressure, temperature and composition data to understand the phase behavior of the reservoir to build a working model that describes the formation of pervasively-saturated reservoir systems. We show that: capillary pressures cannot lead to the observed high hydrocarbon saturations in low porosity reservoirs; pervasively-saturated Viking reservoirs were, at one time, migration pathways for bubble point oils and dewpoint gases buoyantly migrating from the deep basin; maturation of organic matter during burial leads to hydrolytic disproportionation of organic matter which desiccates the reservoir system and when the water saturation becomes low enough, leads to a loss of hydrocarbon buoyancy and stationary hydrocarbon fluids. Uplift and erosion places the saturated hydrocarbon fluids at greater pressures for their temperature than conventionally trapped reservoir systems, because pressure is controlled by the temperature and the composition of the hydrocarbon fluids. We close with two applications involving the sequestration of CO2 that take advantage of the high reservoir pressures to show that the injected CO2 has little to no buoyancy with respect to the original fluids in place. These results suggest these fluids are unconditionally stable which contrasts with saline aquifer CO2 storage applications where the fluids are unconditionally unstable and will always buoyantly migrate away from the point of injection.