Diagenetic evolution and origin of cements of the Cretaceous carbonates in the Akal Block, southeast Gulf of Mexico

The Cretaceous succession of deep marine facies (580 m-thick) of the Akal Block, southeast of the Gulf of Mexico, is dominated by lime mudstones that have been extensively affected by dolomitization. Petrographic examination, including cathodoluminescence (CL), and microthermometry reveal three phases of dolomitization that occurred as replacement and pore-/fracture-filling cements. These are (1) an early replacement dolomicrite (D1, <50 mu m) with dull orange CL, (2) eu-to subhedral dolomite (D2, 60 mu m-150 mu m) with distinctive cloudy cores (dull CL) and clean edges (non-luminescent to dull CL), and intercrystalline porosity (6 %), and (3) anhedral vug-/fracture-filling dolomite (D3, 200 mu m-500 mu m) exhibiting CL zonation. The mean delta O-18 values of those dolomites show a trend of slight decrease from D1 to D3 (D1 = 0.0 +/- 1.3 parts per thousand VPDB, D2 = -0.2 +/- 1.1 parts per thousand VPDB, D3 = -1.2 +/- 1.6 parts per thousand VPDB), which reflects the influence of progressive burial. The low Sr contents (101 +/- 29 ppm) of D1, along with its near-micritic grain size and estimated delta O-18(D1fluid) of parent dolomitizing fluid (-3.6 parts per thousand-1.9 parts per thousand SMOW), support early-stage dolomitization at low temperature of near-surface conditions from solutions of mixed meteoric and seawaters. The higher estimated delta O-18(fluid) values of D2 and D3 (3.0-13.9 parts per thousand SMOW) are expected for fluids in burial settings of high temperatures. The homogenization temperatures of the primary two-phase fluid inclusions of D2 (T-h = 99.1 +/- 7.9 degrees C) and D3 (T-h = 89.5 +/- 11.2 degrees C) suggest precipitation at deeper burial settings, which is also consistent with their lower Sr contents (80 +/- 14 ppm and 98 +/- 33 ppm, respectively). The lower mean T-h value of D3 than that of D2 likely reflects the influence of tectonic uplift that affected the Akal Block. The shale-normalized (REESN) patterns of the dolomites mimic that of modern seawater and those of D1 and D2, in particular, coincide with that of the lime mudstones (C1), thus suggesting that C1 was their precursor and also D3 originated almost from the same parent fluid that its geochemical composition evolved with progressive burial. The latest fracture-filling calcite cement (C2) postdated D3. The investigation of the products of burial diagenesis, such as fracturing, dissolution, and dolomitization events and their relationship with the tectonic uplift of the Akal Block allows a better understanding of the factors controlling the quality of Cretaceous reservoir rocks.