José Manuel Grajales-Nishimura1,
(1) Instituto Mexicano del Petróleo, Mexico, Mexico
(2) Instituto Mexicano del Petróleo, México, D.F, Mexico
Abstract: Burial dolomitization of the KT carbonate breccia, subsurface Campeche, Southeastern Mexico
The oil producing carbonate breccia of subsurface Campeche is a ~ 300 m thick unit of KT age that has been a major hydrocarbon exploration target for more than 20 years. The deposition of the breccia took place at the base-of-slope of ancient Campeche –Yucatan platform as a consequence of the Chicxulub impact. The breccia is composed at the base by reefal and hemipelagic limestone fragments and evaporitic and shallow and lagoonal limestone fragments toward the top. Study of the geologic and sedimentologic framework, petrography, cathodoluminesence, fluid inclusions, trace element and radiogenic isotopes indicates the following dolomitization processes. Due to its high porosity and permeability, later enhanced by fracturing, the breccia unit was affected by circulation of huge amounts of fluids which produced dissolution and dolomitization. The porosity was totally or partially cemented by dolomite, calcite and calcium sulfate. The fluid inclusions, trace element and radiogenic isotopes suggest that the dolomitization took place at burial setting due to circulation of hot diagenetic fluids. The fluid inclusion homogenization temperatures of 80-110 oC and salinities of 2-8 % wt NaCl, suggest a hot saline water or modified seawater as the dolomitizing fluids. The low concentration of Mn and Fe (20-300 ppm) in the dolomite seems to indicate a “clean” fluid with little or no interaction with siliciclastic rocks. In contrast, the strontium isotopes values of 0.709 200 obtained from the dolomites suggest a Strontium-87 rich fluid. These high values of Strontium isotopes ratios can also be explained if we considered Neogene seawater as the dolomitizing fluids of the KT breccia.
AAPG Search and Discovery Article #90914©2000 AAPG Annual Convention, New Orleans, Louisiana