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Diagenetic Trends and Porosity Evolution Associated With Unconformities in the Mid-Cretaceous High Relief Carbonate Platform, the El Abra Formation, Mexico


Porosity evolution of many giant carbonate petroleum reservoirs is directly related to unconformities e.g. Kirkuk, Iraq, and Golden Lane, Mexico. The mid-Cretaceous El Abra Limestone, Sierra El Abra, Mexico, an exhumed giant reservoir, provides quantitative data on porosity evolution associated with multiple exposure surfaces capped by a pre-Turonian unconformity with as much as 17 Ma duration. Pertinent variables include grain vs. mud content, mineralogy (calcite vs. aragonite layers in rudists), distance from the platform margin, and duration of exposure. Four detailed measured sections, each ~60 m thick, near the platform margin and previous studies toward the platform interior document facies relationships and repeated (1–3m) subaerial exposure surfaces marked by solution-collapse breccia, channel and vuggy porosity, pitted truncation surfaces, paleocavities, blackened clasts, and cut-and-fill structures. Grainy marginal facies are cut by platform-parallel fractures filled with pelagic biota and probable marine cements (Neptunian dikes). Transmitted light and cathodoluminescence petrography, fluid-inclusion analysis and stable-isotope analysis reveal multiple generations of cement and other diagenetic features related to surfaces of discontinuity; typical are vadose cements in dissolution pores overlain by marine cements and internal sediment with pelagic biota. Late-stage equant calcite cement precipitated from a meteoric aquifer at temperatures less than ~50°C associated with subaerial exposure, perhaps beneath the pre-Turonian unconformity. Perm plugs provide a quantitative database for extant porosity and permeability trends related to unconformities. Collectively, these results provide a reservoir analogue that has implications useful for understanding porosity permeability relationships associated with unconformities in shallow-water platform-margin carbonates, e.g. the Golden Lane, Isthmus, and Campeche reservoirs of the Gulf of Mexico, and similar reservoirs of any age.