NONLINEAR DYNAMICAL ASPECTS OF DEEP BASIN HYDROLOGY - FLUID COMPARTMENT FORMATION AND EPISODIC FLUID RELEASE

The notion of abnormally pressured fluid or ''pressure'' compartments is a new concept associated with sedimentary basins and has implications for petroleum formation and transport, mineralization accompanying fluid explusion from great depths, and overall basin hydrology. In this paper, the coupling between fluid mass conservation, time-dependent compaction, water-rock reaction, and natural hydrofracturing is explored in the context of fluid compartment genesis and preservation during sediment burial. Focus is placed upon examining the complexity associated with this coupling between hydrological, geochemical, and structural factors associated with sedimentary basin development. A natural consequence of at least the early stages of compartment evolution is found to be the episodic (and possibly chaotic) release of fluid to shallow portions of the basin. A simple topological analysis of nonlinearly coupled differential equations for fluid pressure production and dissipation (through a top seal bounding a fluid compartment) and seal hydrofracturing suggests a variety of dynamical phenomena including steady unfractured-states, steady fractured-states, and oscillatory fluid expulsion through a seal accompanying cyclic fracture growth and healing. A quantitative reaction-transport-mechanical model is developed which co-evolves in time and space the variables of fluid pressure and chemical composition, cement content, porosity, and fracture radius, density and aperture. An often-ignored facet of basin evolution is the role of time-dependent processes, such as solution-transfer or pressure solution, in altering porosity and permeability. Although operating on longer time scales than poro-elastic compression accompanying sediment loading, time-dependent compaction offers a larger potential for generating fluid overpressures and plays an integral role in the development of the variety of dynamical states discussed herein. One-dimensional numerical solutions of the model equations simulate generation of abnormal fluid pressures and show consequences of hydrofracturing if overpressure exceeds a critical level. Fluid pressure within overpressured domains may oscillate accompanying hydrofracture-related fluid expulsion and slow build-up with compactional pore volume loss and thermal expansion of pore fluid. One example basinal scenario revealed by simulations involves, upon the initiation of hydrofracturing, a transient phase of oscillatory fluid release, followed by a steady fractured-state reflecting a balance between rates of fracture growth and healing and fluid pressure production and dissipation. The upper boundary of the hydrofractured zone (a low permeability zone or seal), upon achieving this steady state, undergoes dynamic readjustment with subsidence, occupying a fixed position in depth and forming a moving ''front'' in the rock-fixed reference frame. Tops of geopressured zones buried several kilometers in actively subsiding basins may follow a similar dynamic, remaining at a constant elevation. Changes in character of fluid explusion from overpressured domains depend on variations in burial rate, the functional relationship between rock texture and permeability, and magnitude of lateral stress.