St. Catharines 2004
Technical Program


SS20:  Ontario Oil and Gas, the joining of two basins 
Organizers / Organisateurs:  Ian Colquhoun, Steven Fletcher
Room / Salle:  TH 245

Date:  05/12/2004
Time:  2:40 PM
Presenter:  Langhorne B. Smith


Fault-controlled hydrothermal alteration of carbonate reservoirs

Smith, L.B. Reservoir Characterization Group, New York State Museum, Room 3140 CEC, Albany, NY, USA, 12230, lsmith@mail.nysed.gov

Hydrothermal dolomite reservoirs have received much attention in North America recently with the discovery of Ladyfern (~750 BCF) in Western Canada and continued exploration success in the Trenton Black River Play in the Appalachian Basin.  Analysis of the geology of these fields leads to the conclusion that many other carbonate reservoirs may have formed or been significantly modified by hydrothermal processes.  Hydrothermally altered reservoirs may represent one of the largest untapped hydrocarbon resources in the world because they commonly occur in subtle diagenetic traps that are easily overlooked.

Hydrothermal alteration of carbonate reservoirs is not restricted to local dolomitization around faults in a few localities, but occurs in a wide variety of structural and stratigraphic settings all over the world.  Hydrothermal alteration products may include, but are not restricted to, matrix and fracture-filling dolomite, recrystallized limestone (including development of microporosity), bedded and fracture-filling chert, induced breccias and fractures, pore- and fracture filling anhydrite, calcite, ferroan calcite, ankerite, quartz, fluorite, barite, bitumen, authigenic clay minerals, sulfides and more.  Furthermore, leaching of limestone, dolomite, and other minerals is a common occurrence in hydrothermally altered reservoirs and can be a primary control on reservoir quality.

Hydrothermal alteration occurs when high-pressure, high-temperature fluids flow up active faults and into formations that underlie sealing shales or evaporites.  Alteration occurs due to the influx of fluid from the fault, the mixing of that fluid with the fluid in the formation and during the subsequent equilibration with the formation conditions.  Solubility of carbonate (and other minerals) is directly affected by changes in temperature, pressure, pCO2, pH, and salinity and all of these are fluctuating on short time scales in hydrothermal systems.

First order-controls on hydrothermal alteration products include: composition, thickness, 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.

Structural settings where hydrothermally altered carbonates are found include around margin-bounding faults, over newly rifted basement and active normal faults, at fault intersections and around wrench faults that are activated during mountain building events.  Reefs and mounds commonly form over faults and are then altered by fluids flowing up the same faults.  A better understanding of the processes and products of hydrothermal alteration will lead to further exploration and development success.