High Frequency Cycles to Supersequences: Stratigraphic Controls on Early Dolomite Distributions within Peritidal and Subtidal Carbonates, the Cupido Formation, Sierra Madre Oriental, Northeast Mexico

Younis K. Altobi
The University of Texas at Austin, Austin, TX

Lithofacies and stacking patterns of high frequency cycles define 11 depositional sequences within the Cupido carbonates. The Cupido Formation (700-1200 m thick) is a Barremian to Aptian low-angle carbonate bank that rimmed parts of the ancestral Gulf of Mexico. It comprises of shallow water carbonates and was deposited during the second order HST of the 120 my supersequence and the early TST of the Albian supersequence. Cupido cyclic carbonates were significantly dolomitized at early stages of deposition by hypersaline fluids analogous to modern sabkha models. A depositional model and a sequence stratigraphic framework for the Cupido carbonates were developed in this study through interpreted measured stratigraphic sections, petrographic analysis, and two-dimensional correlation of depositional sequences. Correlated high-frequency cycles show that dolomitization is not facies specific and is laterally continuous between closely spaced sections. Statistical and geochemical analysis of dolomitized Cupido cycles shows a link between sequence stratigraphy and dolomite distribution. Eustacy controls stratigraphic packaging which, in turn, controls dolomite distribution. At the high frequency scale, dolomitization is dominant on cycle caps and the transgressive (lowest) part of the overlying cycle. At the depositional sequence scale, cyclic TST deposits are extensively dolomitized compared to the HST cycles. The overall dolomite/limestone ratio within depositional sequences decreases towards the 120 Ma sequence boundary. This relationship is valid for the HST deposits of the 120 Ma supersequence. The TST deposits of the Albian supersequence are mostly limestone. The proposed model allows dolomite distribution prediction within shallow carbonate reservoirs as well as mechanical bed continuity modeling for reservoir characterization.