Diagenesis of Hydrocarbon-bearing Concretions in South-central Puerto-rico: Tectonism and Early hydrocarbon Generation

Carbonate concretions within tuffaceous mudstones in the Upper Cretaceous Cariblanco Formation of south-central Puerto Rico that contain solid and liquid hydrocarbons were affected by: 1) Three distinct events of vein/fracture formation accompanied or followed by sediment infilling; 2) pyrite formation throughout the concretion matrix, in foraminiferal chambers, and sediment vein fills; 3) four events of larger vein and fracture formation infilled by distinct calcite cements that postdate sediment infilled veins; 4) a late quartz void filling cement; and 5) formation of calcite-filled veinlets that crosscut all components. Petrographic and isotopic data suggest early concretion formation and septarian vein fills, close to the sediment-water interface, prior to any significant dewatering of infilling sediments. The δ13C values of the matrix and sediment infills (-15 to -30‰ PDB), their brightly luminescent character, and the sequestering of Fe into pyrite indicate formation in a sulfate-reducing environment with influx of diffusing methane. Fluid inclusion data, isotopic composition of carbonate cements (13C enrichments from − 18 to -8‰ and 18O depletion from − 4 to -12‰), and organic matter maturation suggest maximum burial temperatures of 150 to 200°C. Calcite cements and microspars were formed by the circulation of progressively warmer fluids, with warming induced by the gradual emplacement of the nearby Los Panes intrusion. The intrusion probably caused intense normal faulting, induced extensive warm fluid circulation, and resulted in a high geothermal gradient responsible for early hydrocarbon generation.