Origin of diagenetic fluids in the SE Mexican oil fields

Several world-class oil fields are located inland and offshore the Gulf of Mexico, SE Mexico. The host carbonates had a complex diagenetic history, including pervasive dolomitization, fragile and hydraulic fracturing and breccia formation, several generations of cement, anhydrite injection and late hydrocarbon impregnation. Dolomitization and related cements are controlled by major structures and primary sedimentary textures. Mesozoic tectonics were characterized by passive margin development and early salt mobilization; the Tertiary, post-Chixchulub tectonic history is marked by complex tectonics, due to an ocean-continent oblique collision, the formation of a Tertiary foreland basin and very active salt and clay diapirism. Several researchers link the salt diapirism to the mobilization and upflow of dolomitizing fluids, and a heat increase causing organic matter maturation and oil expulsion. A fluid inclusions study of diagenetic cements (dolomite and calcite) samples taken from oil wells throughout the mesozoic sedimentary sequence was undertaken. Fluid inclusions were classified as two-phase brine-bearing (LAQ-V), two-phase oil-bearing (LHC-V); and poly-phase (LAQ-LHC-VS). The petroleum inclusions belong to the “black oil” family, while the majority of aqueous inclusions are methane-depleted with salinities between 8 and 18 wt% eq. NaCl and no evidence of hypersaline brines. Halogen data obtained from both dolomite cement and late-calcite plot very close and parallel to the seawater evaporation trend in a Na/Br vs Cl/Br diagram, past the point of halite precipitation, clustering around the point of epsomite-sylvite precipitation. This excludes the involvement of secondary brines from the dissolution of salt diapirs. The salinities of the fluids are too low for this degree of seawater evaporation, indicating the evaporite-related fluids probably mixed with dilute, seawaterlike fluids during diagenesis.