Physical Modeling of Fluid Overpressure and Hydraulic Fracturing in Source Rocks in Various Tectonic Contexts

Zanella, Alain; Cobbold, Peter R.

As a result of an increasing emphasis on unconventional resources, new data are becoming available for source rocks. Abnormally high values of pore fluid pressure are especially common within mature source rock, probably as a result of chemical compaction and increases in volume during hydrocarbon generation. Geological evidence for overpressure is the widespread presence of « beef » (bedding-parallel veins of fibrous calcite) in or around source rocks. We have studied such features in two basins, the Neuquén Basin of Argentina and the Wessex Basin of southern England. To investigate processes of chemical compaction and hydraulic fracturing, we have also developed new techniques of physical modeling. Mixtures of silica powder and beeswax microspheres (50% by volume) represented source rock, whereas the overburden consisted of pure silica powder. By submerging the materials in water, we avoided the high surface tensions, which arise within pores containing both air and liquids. Also we were able to measure pore fluid pressure in a model well. We built and deformed the models within a rectangular box, which rested on an electric flatbed heater. During heating, the basal temperature of the model surpassed the melting point of beeswax (62°C), reaching a maximum of 90°C. To investigate three different tectonic contexts, we applied horizontal displacements, using a piston. Thus the experimental variables were (1) rate of heating, (2) amount and sense of piston displacement, and (3) piston velocity. When the piston was static, rapid melting led to compaction of the source layer and to development of high overpressure (lithostatic or above). The molten wax migrated through pore space and into open hydraulic fractures. Most of these fractures were horizontal and in apparently random positions. In experiments, for which the piston moved outward, allowing horizontal extension of the model, some of the intrusive bodies (especially those close to the piston) were dykes, whereas others were sills. In experiments, for which the piston moved inward, causing compression of the model, all of the developing structures were sills. However, these were thicker than in static models and some of them were subject to folding or faulting. Thus in our experiments, transformation from solid to liquid wax led to chemical compaction and hydraulic fracturing, but the details varied according to the tectonic context.

AAPG Search and Discovery Article #90163©2013AAPG 2013 Annual Convention and Exhibition, Pittsburgh, Pennsylvania, May 19-22, 2013