Abstract

Palaeogeographical reconstruction of the Oxfordian-age (Jurassic) depositional systems in the southern Gulf of Mexico is supported by detailed sedimentological analyses, detrital zircon geothermochronology and plate tectonic restorations. This integrated approach departs from prior local studies by placing the Bacab Sandstone, a major reservoir in offshore Mexico, in a larger basin- to regional-scale context.

Sedimentary characteristics derived from detailed core description show how remarkably similar the Bacab Sandstone is to coeval sandstones of the Norphlet Formation of the northern Gulf of Mexico. A comparison of lithofacies associations suggests similar depositional processes and palaeoclimate regimes, with aeolian dunes forming near base level with adequate wind, sediment supply and arid climatic conditions promoting development of dunes and adjacent sabhkas. Construction of prominent ergs (aeolian sand seas) with lateral transitions to updip fluvial–wadi systems is envisioned in Mexico, comparable to the Norphlet of the northern Gulf of Mexico.

Reservoir quality over the southern Mexican offshore area is variable, as numerous well penetrations over several decades have demonstrated. However, in the core area of the Ek-Balam Field, the best reservoirs have surprisingly good porosity and permeability for their age (Jurassic) and present-day depth of burial (>4000 m). However, published information is not definitive on the factors mitigating subsurface reservoir quality destruction. It is possible that similar processes preserving or enhancing porosity under deep burial conditions for the Norphlet are likely to have operated in the Ek and Balam well locations.

Available petrographical data on framework grains are similar for Oxfordian sandstones in both the northern and southern Gulf of Mexico. Detrital zircon U–Pb geochronology, however, indicates that different source terranes supplied clastics to the Oxfordian Bacab and Norphlet sandstones. Detrital zircon geothermochronology age spectra indicate that the Mayan (Yucatán) Block was the primary terrane for the Bacab Sandstone. This is separate and distinct from Norphlet source areas that vary from Appalachian (Laurentian) to Pan-African (Gondwanan and peri-Gondwanan) terranes. While it possible that the Bacab and Norphlet sandstones were continuous and connected across the Yucatán margin that lies between the two areas, detrital zircon provenance results do not show any indication of common source terranes. Dimensional considerations, such as the contrasting ratios of Bacab and Norphlet source areas to their respective documented areas of deposition, also support the notion of separated aeolian palaeoenvironments.