CEMENTATION OF HIGH PRESSURE RESERVOIRS: MICRO- TO REGIONAL SCALE PROCESSES

Fluid-rock interaction may result in cementation of pores and/or fractures, which significantly changes the transport properties of rocks. The study of cementation is of great interest because it seals fluids pathways, carries minerals/ ores and thus covers multiple disciplines such as crystallography, geochemistry and structural geology. Based on case studies from sedimentary environments, microstructural implications will be addressed and an outlook on future developments will be given.

Multiple fracturing and sealing events can be deduced from deformed rocks, often resulting in cementations on fracture planes, generally called veins. The clastic Triassic rocks from NW Germany were cemented with fibrous calcite in a weakly consolidated rock and later inflated by high-pressure fluids precipitating blocky anhydrite (Nollet et al. 2005, Hilgers et al. 2006). Mesozoic limestones exposed in a large anticline in Oman also show multiple fracture and sealing events during extension and compression phases (Hilgers et al 2006, Holland et al 2009). Early fibrous and blocky calcite veins are host rock buffered, and the system opens up at later stages with blocky calcite veins as precipitates from the fluid phase. In both case study areas, fibrous microstructures grow at an early stage. Isotope analyses allow inferences on transport mechanisms associated with the cements. This results in a detailed understanding of the fluid history during deformation if linked with the observed vein microstructures and the relative timing of vein sequences.

Using the microstructures observed in rocks and the transport mechanisms deduced from case study areas, experimental and numerical simulations can better quantify the conditions during cementation. It is shown that the permeability during fibrous growth must have been low, formed during small opening-sealing increments. Blocky veins grow in open or partly open fractures and thus can grow at much higher permeability. Such veins can also be used to infer the transport direction.

A better, quantitative understanding of cementation processes on the micro-scale will improve the understanding of fracture and sealing processes. Further developments will result in more quantitative tools and 3d simulations, which can be upscaled to a sedimentary environments.