Diagenetic evolution of the Cretaceous-Paleogene carbonate breccia, Cantarell, Mexico: Evidence from geochemical proxies

Abstract

The Cretaceous-Paleogene (K-Pg) carbonate breccia in the Cantarell Field, Gulf of Mexico, consists of a fining-upward succession of breccias capped by an ejecta-rich seal layer. Petrographic analyses (cathodoluminescence, SEM, fluid inclusion microthermometry) and geochemical data (stable isotopes, elemental, trace elements) reveal a complex sequence of diagenetic processes that significantly impacted reservoir quality.

The breccia shows the highest porosity (∼9.4%) at the base of succession, decreasing upward. Six carbonate phases were identified including lime mudstone (C1), late blocky calcite (C2, 350 μm‒15 mm, concentric CL zoning), early replacement dolomite (D1, <50 μm, bright to dull CL), medium to coarse replacement dolomite (D2, 50–155 μm, cloudy cores with bright CL and clear rims with dull CL), planar-e dolomite cement (D3a, 80–180 μm), and saddle dolomite (D3b, 450–950 μm) with undulose extinction and CL zoning.

Trace element data provide clues about the fluid chemistry evolution. The low Sr content of D1 (94 ± 18 ppm), combined with its near-micritic grain size and the estimated δ¹⁸O values of the parent dolomitizing fluid (−4.9‰ to 0.3‰ SMOW), suggest an early-stage dolomitization at low temperatures of near-surface conditions that was driven by a mixture of meteoric and seawater. The Sr contents decrease from D1 to D2 (78 ± 19 ppm), suggesting progressive burial, which is supported by the relative decrease in δ¹⁸O values (D1 = −0.3 ± 1.8‰ VPDB; D2 = −4.3 ± 2.7‰ VPDB). The increase of [Sr] in D3a (265 ± 238 ppm) followed by a decrease in D3b (108 ± 40 ppm) correlates with tectonic uplift and subsequent burial. The δ¹³C values of D3b and δ¹⁸O values reflect higher fluid-rock interaction during deeper burial, supported by high T_h values (105.7 ± 7.7 °C).

The shale-normalized REE patterns show that those of D1 and D2 mimic that of modern seawater and almost coincide with the C1 pattern, which suggests that C1 was likely the precursor. In contrast, the later phases (C2, D3a, D3b) show very low HREE concentrations, attributed to high fluid-rock interaction during tectonic uplift. This study of the K-Pg carbonate breccia provides a better understanding of its dynamic diagenetic history, shaped by fluid flow and tectonics, which critically influenced porosity and reservoir quality.