A carbonate and marly stratigraphic sequence accumulated on a broad, shallow to deep marine platform during the Late Jurassic to Late Cretaceous in southeast Mexico. The Reforma-Jalpa area was studied in subsurface with the purpose of reconstructing the tectonic evolution, depositional setting, and diagenesis.
During Late Jurassic, the sedimentary province was a broad and shallow marine carbonate ramp tilted initially from north to south and later northward. This tectonic movement caused the break up of the ramp into two local platforms that were separated by a small basin.
Sedimentation began progressively deeper from south to north with the following sequence: (a) Supratidal environment: nodular anhydrite and interlayers of early microdolomite and clayey and silty micrite; (b) Tidal-flat and shallow subtidal environments: early dolomitized micrite with scattered gypsum crystals, algal stromatolites with mud cracks, and burrowed fossiliferous pelmicrite; (c) Shallow lagoon environment: bioturbated biomicrite, pelmicrite with Favreina salevensis and oncolite-maollusk biomicrudite; (d) Shallow inner platform environment: sandy and silty oosparite, mollusk biomicrudite, calcareous banks of red corallinaceous algae, rudist fragments, echinoids and other calcareous macrofossils; (e) Outer platform environment: scattered mounds of rudist, red coralline algae and other encrusting organisms interbedded with layers rich in tintinnids, radiolaria, and other planktonic microfossils; (f) Slope environment: wedges and lenses of breccias and debris flows of limestone fragments, rudist, and other broken and unsorted calcareous constituents with interlayers of pelagic sediments; (g) Basin environment: wavy beds of poorly washed biosparite and even beds of clayey and silty biomicrite with tintinnids and radiolaria.
Through Cretaceous, the sedimentary province was subjected to continuous subsidence and deposition of marine facies. During Late Cretaceous and Early Tertiary, the stratigraphic sequence was intensively deformed by Laramide stresses, resulting in subsiding faulted and fractured carbonate platform facies. The associated clayey and silty slope and basinal facies were uplifted during this tectonic event.
The distribution of the sedimentary facies and the tectonic evolution of the area played an important role in the origin, migration, and accumulation of oil. Oil is heavier and immature in the slope and basinal environments, whereas it is lighter and more mature in the carbonate platforms where the sedimentary facies are more permeable. Moreover, the primary porosity of the carbonate mounds was enhanced by tectonic fracturing, stylolitization, and late dolomitization. Thus, the mounds are the most important zones for petroleum exploration.
