The Permeabilities of Overpressure Shale Seals and Source Rock Reservoirs are the Same

The permeability of rocks in the subsurface varies in magnitude from too high to be a useful concept to too low to be measurable. The division between conventional petroleum systems and continuous accumulations is approximately 0.1 mD. At that point, relative permeability and capillary pressures create the trapping seal. Weak barostratigraphic seals become common in the µDrange. Good overpressure seals are modeled to be in the 10 to 100 nD range. The flow of water is slow enough at these permeabilities that the interstitial water bears a portion of the overburden load and is overpressured (undercompaction disequilibrium).

Source-rock reservoirs (SRR) are present in ‘shales’ with permeabilities that are also in the 10 to 100 nD range and are capable of producing gas at commercial flow rates. This paradox is addressed by examining the geologic history of the SRR. Generation, maturation (including the cracking of oil to gas), and the expulsion of hydrocarbons creates high internal overpressures sufficient to fracture the host rock so that the hydrocarbons can be expelled through a microfracture network. The generation of hydrocarbons also creates pore space within the kerogen grains themselves. After expulsion ceases, cementation and diagenesis occludes the larger fractures and primary migration routes in the SRR, thereby isolating the kerogen and microfracture system. Hydraulic fracturing reopens the natural fractures and connects to the oil-wet, gas-filled porosity in the SRR kerogens. The remaining unexpelled free and adsorbed gas is then available to be produced.

Owing to the expulsion of hydrocarbons and associated water, SRRs may not be water-wet, but may be hydrophobic. Furthermore, the laminated nature of many source-rock shales, and the presence of oil and gas in the pore space creates a relative permeability reduction to the flow of water and also facilitates the formation of capillary seals. SRRs may be effective pressure seals. The separate gas-filled microporosity system is isolated within the matrix of the SRR and can be accessed through artificial fracturing. The conventional interstitial and interparticle porosity is water-wet and may be gas-filled and produces by Darcy flow. The kerogen and microporosity system is oil-wet and gas-filled with an adsorbed gas component. It produces by diffusion flow. The combination of the two systems is what is seen at the wellbore.

AAPG Search and Discovery Article #90142 © 2012 AAPG Annual Convention and Exhibition, April 22-25, 2012, Long Beach, California