Print this pageDiagenetic Model for Fault-Related Dolomitization in the Cretaceous of Northern Spain: Subsurface Reservoir Implications
Fault-controlled dolomitization has significant impact on reservoir properties in the subsurface. Origin, geometry, and internal reservoir characteristics of such dolomite bodies constrain adequate reservoir modeling and production. Fault-related dolomites in Albian platform carbonates (Basque-Cantabrian Basin, Spain) provide a detailed analog for subsurface reservoirs.
Early diagenesis includes marine cements, exposure and phreatic freshwater cements. Dolomite bodies (<900 m thick) are developed along fault-strike; km-scale branches extend from the main bodies parallel to bedding. Dolomitization is pervasive close to faults, but it follows more permeable trends at distance to the fault. Xenotopic, sucrosic and saddle dolomite occur as replacement and cement. A first generation of calcite is associated with dolomite. Dolomitization was followed by stylolites and telogenetic calcite cement.
Petrography, isotopes, and fluid inclusions indicate polyphase hydrothermal dolomitization from basinal fluids moving up deep-rooted extensional-transtensional faults. Dolomitization most likely occurred in the Latest Albian - Early Turonian at shallow depths and max. burial temperatures of ~80°C. Minimal dolomite precipitation temperatures were 75°C-240°C; salinities were up to 22wt.% NaClequiv.
Outcrops reflect spatial gradients of decreasing temperature from deeper to higher stratigraphic levels of dolomitization, and away from a fault. Dolomitization occurred from three fluid pulses, which possibly derived from a single parent fluid. Salinity differences between pulses may reflect variable water-rock ratios during leaching of Triassic evaporites. Little thermal interaction with the host rock occurred during fluid movement, consistent with rapid fluid expulsion. Between pulses, the fault conduit was cemented by dolomite and calcite. Calcite likely precipitated from the hydrothermal fluids at lower temperature than dolomite, as a result of Mg exhaustion and/or a drop in fluid temperature at constant Ca/Mg.
The diagenetic model provides some predictive rules for fault-related dolomites in subsurface reservoirs. Body geometry can be tied to its position relative to feeder faults. Hydraulic brecciation, dolomite and calcite cementation decrease reservoir properties in immediate vicinity to the faults. At some distance to the faults, dolomitization remains beneficial for reservoir properties through the creation of vuggy, biomoldic and intercrystalline pore networks.
AAPG Search and Discover Article #90100©2009 AAPG International Conference and Exhibition 15-18 November 2009, Rio de Janeiro, Brazil