Physical Modeling of Fluid Overpressure and Compaction During Hydrocarbon Generation in Source Rock of Low Permeability

Lemrabott, Ahmed¹
 Cobbold, Peter R.¹

Although many authors have suggested that hydrocarbon generation can lead to fluid overpressure, there is no consensus as to the mechanism. Is the overpressure due to a volume increase during transformation of solid kerogen to oil or gas, or is it due to chemical compaction? We have investigated these problems by using scaled physical models. Our model materials were mixtures. The basic material was a silica powder (Millisil C4). Its grain size was less than 0.15 mm and its intrinsic permeability was about 1.6 darcy. To simulate source rock, we mixed the silica powder (50% by volume) with micro-spheres of beeswax, 1 mm in diameter. The melting point of the beeswax was 61.5 °C. The housing for the models was a rectangular box, 28 cm long, 18 cm wide, and 15 cm deep. It had a thermally conducting basal plate of aluminum and transparent thermally insulating sidewalls. Each model consisted of several layers. We poured them successively into the box, scraping their upper surfaces flat. Then we saturated the model with water. The final water level was 1 cm higher than the upper surface of the model. To measure fluid pressure within a layer, we inserted a thin glass tube vertically down to the required depth. A fine sieve prevented solid grains from entering the tube, while allowing water to do so. The water in the tube rose to the level of the water outside. Then we heated the box from below with an electric flatbed heater. This had fine controls on temperature and power. The rate of heating was about 1°C per minute. When the temperature in the source layer reached the melting point of the wax, the water level in the glass tube started to rise. It soon reached a steady head, which was equivalent to the effective weight of the overburden (allowing for buoyancy). During partial melting, the source layer compacted. Liquid wax migrated upward through pore space, displacing water. In some experiments, the wax also filled flat-lying hydraulic fractures. We attribute these fractures to seepage forces, resulting from Darcy flow through the porous medium. Locally, the overpressure exceeded the weight of overburden. In some experiments, liquid wax breached an uppermost sealing layer of silica powder and extruded at the surface, forming structures like lava tubes or lava flows. The experiments illustrate the power of chemical compaction as a mechanism for generating high overpressure in maturing source rock.