Smith, Langhorne B.¹ 
(1) New York State Museum, Albany, NY

ABSTRACT: Hydrothermal Alteration of Carbonate Reservoirs: How Common Is It?

Fault-related hydrothermal alteration is a common process that has enhanced and/or destroyed porosity in carbonate reservoirs in a variety of settings around the world. Reservoir-enhancing processes include dolomitization, leaching of limestone and dolomite, development of microporosity 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. 
Carbonate solubility is affected by variations in temperature, pressure, PCO₂, pH, and salinity and all of these fluctuate on very short time scales in fault-controlled hydrothermal systems. Minerals precipitate in fractures from fluids flowing up faults and minerals precipitate in the matrix from a mixture of fluids flowing up faults and fluids residing in the formation. Repeated changes in pressure, temperature and composition promotes replacement of matrix limestones with dolomite, stable forms of calcite and to a lesser extent chert. 
First-order controls on the type of hydrothermal alteration product include the burial depth, composition, porosity and permeability of the host rock, the pressure, temperature and chemistry of the hydrothermal fluid, the effectiveness of the seal, the efficiency of the fluid recharge, distance to the basement and basal sandstone aquifer, and the type and timing of faulting. 
Common structural settings for hydrothermal alteration include strike-slip faults with associated Reidel Shears, platform-margin bounding faults and extensional faults. Subtle faults with little vertical offset may be the best conduits for hydrothermal alteration because they are less likely to breach overlying seals.

AAPG Search and Discovery Article #90026©2004 AAPG Annual Meeting, Dallas, Texas, April 18-21, 2004.