ABSTRACT: Pressure Seals, Abnormal Pressures, and Hydrofracture in Sedimentary Basins

WANG CHI-YUEN and LIANG GUOPING, University of California at Berkeley, Berkeley, CA

A fully coupled finite element code is used to model the thermal-hydrological evolution of the Cascadia Basin off the Oregon-Washington coast. Sediments in the basin consist of two major units: a lower unit of hemipelagites, of Late Miocene to Pliocene-Pleistocene age, and an upper unit of sandy turbidites, deposited from Pliocene-Pleistocene to the present. Sedimentation rates of the two units are estimated from the available drilling results and from interpretation of seismic reflection profiles. Data for sediment properties are available from experimental measurements on drill cores and on samples collected during ALVIN dives. Calculated results show that a hydrologic barrier develops at the top of the lower unit, owing to compaction and loss of porosity near the boundary of the tw sedimentary units. This barrier impedes further upward flow of fluids from the pressurized chamber beneath. On the other hand, episodic hydrofracturing of the pressurized chamber and the barrier is triggered when the local pore pressure exceeds the local minimum principal stress, at which the pressurized fluids are expelled rapidly upward. The repetition of these episodic events depends upon the permeability of the fractured sediments, among other factors, becoming more frequent with small fracture permeability. The rates of diffusive flow between the episodic events are relatively small; thus, diffusive flow may not contribute significantly to the total volume of fluid fluxes. Thus, the inclusion of the process of hydrofracturing in basin models may be important for predicting the timi g of hydrocarbon expulsion. Most of the compaction-induced loss in porosity also occurs during the episodic events; the loss at intervals between these events is relatively insignificant. Calculated porosity in the lower hemipelagic stratum shows subtle oscillatory patterns, which is a result of the complex interactions between pressuring, depressuring, and the nonlinear and hysteretic sediment compaction properties. Calculated temperatures in the basin are significantly lower in models with hydrofracturing than those in models without hydrofracturing. Thus, the inclusion of the processes of hydrofracturing and the associated episodic fluid expulsion in coupled thermal-hydrologic models may also be important in predicting hydrocarbon maturation in basins.

AAPG Search and Discovery Article #91012©1992 AAPG Annual Meeting, Calgary, Alberta, Canada, June 22-25, 1992 (2009)