Inference of Flow Rates in Fracture Systems Based on Outcrop Observations: Examples From the Miocene Monterey Formation, Coastal California.

Peter EICHHUBL and James R. BOLES
Geological Sciences, University of California, Santa Barbara, CA 93106

Fault and fracture systems exposed in coastal sections of the Monterey Formation provide structural and compositional evidence for highly variable conditions of basinal fluid expulsion. Rates of fluid flow along these faults and in connected fracture systems prior to their exhumation can be estimated based on three criteria: (1) Geopetal and sorted sedimentary deposits of rock fragments in fracture cavities suggest fragment transport in suspension indicative of minimum flow velocities of 1 m/s. (2) Bifurcation of extension fractures is interpreted as the result of fluid injection into these fractures at rates of 1 to 10 m/s. (3) Flow velocities of less than 20 m/day are inferred based on the lateral extent of a fluid temperature anomaly within and adjacent to a fault zone. Fluid temperatures are estimated based on the oxygen isotopic and fluid inclusion composition of vein and fault cement. The last estimate of flow rate is likely to represent a long-term average flow velocity whereas the first two estimates are considered peak flow rates during episodic expulsion of over-pressured pore fluid. Variable fluid flow conditions are also indicated by cyclic changes in vein cement composition and fluid inclusion characteristics. Because fluid expelled at rates of about 1 m/s has to be derived from connected fracture space within and adjacent to fault zones rather than from the low-permeability country rock, the volume of rapidly expelled fluid and thus the duration of rapid flow events is necessarily limited to short bursts. The observed variability in flow velocity is likely to be the consequence of seismic fault slip, rupturing seals and aquitards and providing temporary pathways for the expulsion of basinal fluid. The number of observed cycles of rupturing and subsequent cementation in fault zones is small compared to the number of cyclic cement bands within these faults. The relative abundance of cement bands suggests that pulses of fluid flow may be triggered by seismic slip along distant faults within the basin, affecting subsurface fluid flow on a basinal scale.