Journal of Petroleum Geology, Vol.10,
No.3, pp. 245-266, 1987
©Copyright 2000 Scientific Press,
Ltd.
A WORLDWIDE LOOK AT THE
CAP-ROCK PROBLEM
Hans R. Grunau*
* Petroconsultants S.A., 8-10 Rue
Muzy, Case postale 228, 1211 Geneva 6, Switzerland.
Abstract
Geological aspects of the cap-rock problem
were investigated on a worldwide scale in order to provide
guide-lines for assessing the sealing and retention risk in
exploration ventures. The most common cap-rock lithologies are
shales and evaporites, which have typical thicknesses of tens to
hundreds of meters. Regional seals are often provided by
evaporitic sabkha environments and transgressive and regressive
shales. Cap-rocks must be placed in the frame of a hydrocarbon
habitat which evolves with time to judge properly the retention
risk. Important parameters are rate of diffusion of hydrocarbons
through cap-rocks, and fracture development as a function of the
intensity of geomechanical deformation. When losses due to seal
failure and diffusion occur, replenishment from an active
generation-migration system may counter-balance the loss.
"Giant" oil- and gas fields serve as a yardstick for
measuring shale-evaporite ratios and seal depths. Surface
seepages can provide a measure of seal failure. As the cap-rock
problem is in an initial stage of research, many uncertainties
affect the reliability of semi-quantitative statements on risks
of sealing and retention.
