Oklahoma’s induced seismicity: did we revive intraplate earthquakes?

Abstract:

Paleozoic platform carbonates on the midcontinent of North America commonly contain saline, high-temperature fluid-inclusions suggesting episodic invasion of the sedimentary column by basinal fluids originating from sub-basement depths. Such fluids frequently are also associated with basement involved fault systems. Why fluids should occasionally be forced upwards from the deeper subsurface remains unclear.

We propose a model of mineralization-driven episodic pore pressure build-up and release in basement channels to explain the apparent episodic injection of fluids from the basement. We envision these channels as inter-fingering, permeable features within negligibly permeable host rock. Physically they may be fault gouges or highly weathered slivers of host rock. Pivotal to our model is the existence of a mineralized seal between two aquifers creating a step increase in pore pressures from the overlying to the underlying formations. An example is the Arbuckle formation in Oklahoma and Kansas, which is sub-hydrostatic and overlies a basement that is expected to have a hydrostatic pore-pressure gradient below a certain (currently unknown) depth. The pore-pressure barrier can be created by sealing the basement channels with minerals that typically precipitate from the basinal brines upon their mixing with less saline sedimentary pore fluids. The seal can be ruptured by re-dissolving the precipitated minerals, i.e., by changing the fluid chemistry of either aquifer.

In our model, the overlying aquifer is an unconfined system; it has higher permeability and more fluids. The underlying aquifer is confined and the fluids are only present in the channels. Seal rupture leads to an instantaneous lowering of the underlying aquifer’s pore pressure while leaving the overlying aquifer’s pore pressure rather unchanged. Mathematically, this corresponds to a negative pressure pulse propagating downwards in the channels through time. Under appropriate conditions, the propagating pressure pulse can destabilize a naturally hyperstable zone such as a decollement or deep overpressured reservoir, which, in turn, can reactivate pre-existing fault systems and inject basement fluids into the overlying sedimentary column. It is possible that the wastewater disposal in the Arbuckle Group might be rupturing such a seal.