Simulation of Mineral Diagenesis in Reservoirs. Application to Illite Formation in Feldspathic Sandstones
Etienne Brosse, Brigitte Bazin, Yann Le Gallo, and Olivier
Petroleum geologists and production engineers are faced with reservoirs where porosities and permeabilities (poroperm) have been reduced by mineral phases precipitated during the geological evolution. Diagenesis of sandstones is influenced by many factors: initial composition of the sediment, burial history, composition of infiltrated waters. An appraisal of poroperm decline due to mineral diagenesis only can result from an integration of these factors. A quantitative evaluation of diagenetic phenomena is possible using numerical modelling. A first approach of the mineral transformations can be made using a new geochemical modelling software (NEWKIN) applied to ^Ltclosed cells^Gt, where aqueous solution and minerals are not in equilibrium initially. The routine c lculates, as a function of time, the dissolved and precipitated quantities of mineral phases, occurring when the system evolves toward equilibrium.
Cements of illite and quartz frequently occur in sandstones bearing feldspar, such as Middle Jurassic reservoirs of the Brent Group (East Shetland Basin, North Sea) which today lie between 3500 and 4500 m depth. Results of ^Ltclosed cells^Gt simulations are presented, which explore the conditions of illite and silica authigenesis in this Province, particularly in terms of temperature, water composition, and kinetics (oversaturation of the waters with respect to quartz, low pH).
Another key of non-equilibrium, in pervious rocks, is the flow of interstitial water. Its role must be appraised by a ^Ltreaction-transport^Gt code. A new software is presented (DIAPHORE) able to solve, at the reservoir scale, in a coupled way: 1) advection of water and chemical elements in the porous volume; 2) mass balance of the considered chemical elements in the rock volume; 3) dissolution-precipitation phenomena occurring locally (using the geochemical code precedently described); 4) a feedback of the mineral transformations on permeability and reactive surface areas through a ^Lttextural^Gt model at the grain scale.
Simulations are presented, which show the possible role of water infiltration in the illite and quartz cementation of the Brent sandstones. The effect of water velocity on the scale of transfer of chemical elements such as Si, Al, Na, K is discussed.
AAPG Search and Discover Article #91019©1996 AAPG Convention and Exhibition 19-22 May 1996, San Diego, California