Fault-Related
Hydrothermal Alteration: An Important Model for Carbonate Reservoir Diagenesis
Smith, Langhorne1, Graham R.
Davies2 (1)
Most carbonate diagenesis
is interpreted to have occurred very near the surface (meteoric, mixing zone,
reflux models) or at depths of more than a kilometer (deep burial dolomitization, etc). Upon closer analysis, it appears that
a significant amount of diagenesis previously
attributed to those near-surface or deep models may have occurred due to
high-temperature, high-pressure fluid flow up faults and into formations at
relatively shallow depths of less than a kilometer (commonly less than 500 m).
These hydrothermal fluids are interpreted to flow up faults at very high rates
(up to meters/second) when the faults were active. Carbonate solubility is
affected by variations in composition, temperature, pressure, PCO2, pH, and
salinity and all of these may fluctuate on very short time scales in
fault-controlled hydrothermal systems. Diagenesis may
be done solely by the fault-sourced fluids or by mixing with in situ fluids.
Reservoir-enhancing processes include dolomitization,
leaching of limestone and dolomite, development of microporosity,
brecciation and propping of open fractures with
minerals. Reservoir-destroying processes include precipitation of pore and
fracture filling dolomite, anhydrite, calcite, quartz, fluorite, barite,
bitumen, authigenic clay minerals, sulfides and more.
Common structural settings for hydrothermal alteration include strike-slip,
extensional and especially transtensional faults.
Subtle faults with little vertical offset may be the best conduits.
Fault-controlled hydrothermal diagenesis is most
easily interpreted using a holistic approach involving field relations,
structural geology, seismic interpretation, stratigraphy,
geochemistry, fluid inclusions, petrography, hydrology, rock mechanics and
more. A focus on only one of those disciplines may lead to erroneous
interpretations.
AAPG Search and Discover Article #90063©2007 AAPG Annual Convention, Long Beach, California