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Stratigraphic Architecture of Isolated Cretaceous Carbonate Platforms: A Case Study From the El Doctor Platform, Central Mexico

Abstract

The mid-Cretaceous (Albian-Cenomanian) El Doctor platform of central Mexico is one of a series of isolated platforms that record the final phase of shallow water carbonate deposition in the Western Gulf of Mexico. Vertical exposures of >400m provide insight into the complex facies relationships between shallow water shelfal carbonates and their marginal slope deposits. The distribution of shelf (El Abra Fm.) to basin (Cuesta del Cura Fm.) facies of the El Doctor platform suggest development of a progradational steep-walled platform that supported a rudist-dominated reef margin and associated grainy slope (Tamabra Fm.). This field-based study explores an outcrop analog for reservoirs associated with isolated carbonate platforms and their slope deposits (such as the Poza Rica field) to provide detailed lithologic characterization within platform environments. We interpret three dominant facies associations (FA) in the 730m of measured section and >300 samples collected: Platform Interior (FA1) deposits extend the length of the platform (~45km long and ~15km wide) and include miliolid wackestones, algal boundstones, and burrowed-skeletal packstones. Upward shallowing tidal cycles are characteristic of FA1. Platform Margin (FA2) deposits extend from a dominantly high energy shelf crest to the edge of the platform at the reef wall. FA2 is characterized by oo-pisolitic packstones-grainstones with fenestral porosity. Further offshore intertidal to subtidal deposits are dominated by coated grains consisting of skeletal debris (requiniids, caprinids, corals, chondrodontid clams). Skeletal content increases towards the shelf margin in the reef flat forming skeletal rudstones. A bound reef wall has not yet been identified in the field and could be attributed to the highly erosive tectonically modified nature of the platform margin, evident in the grainy nature of the fore-reef and slope deposits. Platform Slope (FA3) deposits consist of thin-bedded mudstones and packstones cut by megachannels up to 50m thick by 300m. The vertical transition from lower slope to fore-reef facies suggest a dominantly progradational system during the Albian-Cenomanian, a time when shallow water platforms reached their maximum extent around the GOM. The volume of grainy material on the slope attests to the amount of shedding from the shelfal deposits during a highstand period. The shelf to basin profile investigated here provides an important analog for reservoir scale characterization of platform margin and slope deposits that comprise significant oil and gas fields in Mexico (eg. Tuxpan, Cordoba, Campeche-Yu­catan platforms). The scale of vertical exposure at El Doctor provides a unique opportunity to study the characteristics and facies relationships of a Cretaceous shallow water carbonate platform and associated slope and basinal deposits.