--> Fundamental Pressure Controls in Unconventional Petroleum Systems

AAPG ACE 2018

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Fundamental Pressure Controls in Unconventional Petroleum Systems

Abstract

Petroleum systems of unconventional plays share similar risk elements with conventional plays including source, trap, reservoir, seal and timing, but they take different forms and exhibit dependencies that do not exist in conventional petroleum systems. In an unconventional hydrocarbon accumulation an updip waterline separates a hydrocarbon saturated reservoir from a wet reservoir but a detectable seal, a permeability contrast, is not present. In the reservoir downdip from the waterline, pore pressures are less than hydrostatic pressure but at depths significantly below the waterline pressures increase and gradients may approach the lithostatic gradient. Unconventional reservoirs occur proximally to mature source intervals of variable quality and contain little or no mobile water. These attributes are distinct from conventional reservoirs where buoyancy and trap style are the main controls on accumulations.

Reservoir pressures less than hydrostatic immediately below the waterline occur because low density hydrocarbon fluids carry the fluid load. Pressure gradients deeper into the reservoir are greater than hydrostatic and are more difficult to explain. Initial reservoir pressure data suggests that the fluids are at saturation pressure and temperature, however, oil and gas contacts are not observed. Uplift and erosion cooled the reservoirs, which ended hydrocarbon generation, and lowered pressure which placed the fluids at saturated conditions.

The equilibrium condition for saturated hydrocarbon fluids is expressed through the Clausius-Clapeyron equation which shows that the initial reservoir pressure gradient is dependent on the fluid composition and the temperature gradient. Fluid coupling between pressure, temperature and composition means that zones of higher heat flow are zones with higher pressure gradients and enhanced production.

Lack of mobile water and a mature source proximal to the reservoir implies an additional dependency in unconventional reservoirs that does not occur in conventional reservoirs. Price and Helgeson developed a thermodynamic model that shows approximately 60 moles of water are consumed for every mole of hydrocarbon produced from kerogen. Water consumption during generation leads to a system where buoyancy between water and hydrocarbons no longer occurs and the system becomes unconditionally stable.