Geology of the pre-salt in Mexico: which salt?

Abstract

In any discussion on the ‘pre-salt’ stratigraphy of Mexico the first issue that requires discussion is the stratigraphic position of the autocthonous salt. In many areas this is not a particularly difficult exercise because clear and relatively well-dated Upper Jurassic stratigraphy (Tepexic, Santiago, San Andres, Pimienta formations, etc.) overlie salt and equivalents (e.g. Huehuetepec Formation) and clear latest Triassic-Middle Jurassic formations (Huizachal, Huayacocotla and Cahuasas formations). In the onshore and shallow offshore Sureste Basin it is also evident that there is a thick and well-drilled sedimentary succession overlying autocthonous salt. However, problems arise in the deeper offshore area where there is now significant interest in deeper Mesozoic targets. Here the CNH data and Pemex final well report information shows what can be termed the ‘post-salt’ stratigraphy as being relatively thin, perhaps an order of magnitude thinner than reported a relatively short distance to the SE. However, analysis of key well stratigraphies indicates that the uppermost evaporites and red bed clastics within this part of the basin is most likely to be of Kimmeridgian rather than Callovian age, as part of the infill of a series of intrashelf basins that developed within a widespread Upper Jurassic carbonate platform. An analogous basin system has in fact been comprehensively drilled-up in the Villahermosa area (deeper wells in the greater Cunduacan-Tres Pueblo Field areas) and contains many hundreds of metres of subaqueous-deposited anhydrites intercalated with and overlying restricted intrashelf basin carbonate facies. It is completely reasonable expected that such basin systems are present in the deeper water parts of the Gulf of Mexico and the presence of both bedded anhydrite and associated halite would be expected, given the nature of similar basin systems elsewhere (Zechstein of NW Europe, Gotnia-Hith of the Arabian Plate). This then places the likely true Callovian ‘autocthonous salt’ much deeper within the stratigraphy and means that the apparent drastic thinning of the ‘post-salt’ Upper Jurassic into the deeper water GOM is in fact a consequence of miss-picking. We suggest that caution should be applied to mapping ‘top salt’ and recommend a programme of enhanced biostratigraphic and strontium isotopic analysis of key well stratigraphies to test this model.

AAPG Datapages/Search and Discovery Article #90369 © 2020 AAPG Hedberg Conference, Geology and Hydrocarbon Potential of the Circum-Gulf of Mexico Pre-salt Section, Mexico City, Mexico, February 4-6, 2020