Subsurface Corrosion of Calcite and Dolomite by Fault-Sourced Hydrothermal Fluids
Smith, Langhorne B.
Porosity in both limestone and dolomite reservoirs can be created and enhanced by fault-sourced hydrothermal fluids. Examples from the Pinda Formation of offshore Angola and the Cogollo Limestone of Venezuela will be used to show a subsurface origin for the dissolution. In the Pinda Formation of offshore Angola, both limestone and dolomite were dissolved in the subsurface. Dolomite (including some saddle dolomite) that formed at temperatures ranging from 65 to 160°C occurs as a cement in some grainstones where the grains are found as intact calcite, partially dissolved and completely dissolved. This dissolution of the calcite is therefore of a subsurface origin. Most of the porosity in some cores and thin sections is of a leached dolomite origin. The fact that the leaching occurred after this high temperature dolomitization confirms a subsurface origin. Hydrothermal fluids rich in H2S are interpreted to have flowed up active faults and into the formation where they were oxidized to create sulfuric acid which did the dissolution. Pyrite and anhydrite precipitation closely follow the dolomite dissolution. This all occurred during early burial prior to oil migration.
In the Cogollo Limestone of Venezuela, there is burial corrosion of limestone reservoirs interpreted to have occurred along strike-slip faults known to have been active during early burial of the formation. The corrosion clearly postdates early near-surface diagnesis which is largely cementation and is followed but relatively high temperature dolomite, calcite and kaolinite precipitation. There would not be a reservoir without this burial corrosion and as a result it is important to drill wells near the faults that were the source for the diagenetic fluids. At least three factors make fault-sourced hydrothermal fluids capable of corrosion of limestone: progressive cooling of the fluids, elevated salinity and increased CO2 in solution. Calcite and CO2 both have retrograde solubility so as fluids cool they become progressively undersaturated and progressively more acidic. pH generally decreases as salinity increases so hypersaline brines coming up faults should be capable of leaching limestone.
Burial corrosion of limestone and dolomite also occur in many other carbonate reservoirs. It can commonly be linked to faults (typically strike slip or transtensional faults) and might therefore be predictable in cases where good seismic data is integrated with good core and petrographic data.
AAPG Search and Discovery Article #90163©2013AAPG 2013 Annual Convention and Exhibition, Pittsburgh, Pennsylvania, May 19-22, 2013