ABSTRACT: Diagenesis of Hydrocarbon-Bearing Concretions in South-Central Puerto Rico: Tectonism and Early Hydrocarbon Generation

GONZALEZ, LUIS A., University of Iowa, Iowa City, IA

Hydrocarbon-bearing calcareous concretions occur within calcareous/tuffaceous mudstones in the Upper Cretaceous (Santonian-Campanian) Cariblanco Formation of south-central Puerto Rico. Concretions commonly contain septarian veins, infilled by four, and possibly five, distinct sediment generations. The first two consist of brownish pelleted muds, the third (and possibly a fourth) consist of grayish nonpelleted sediment, and final infills consist of a brownish-gray mud with intraclasts and structures suggesting soft sediment deformation. Pyrite is abundant throughout the concretion matrix, within the sediment vein fills, and infills most foraminiferal chambers. Some concretion sediment infills have been recrystallized to micrite and microspar. Four distinct calcite cements, postdating a l sediment fills, fill new as well as reactivated septarian veins: (1) brownish bladed cement, (2) ferroan brownish cement, (3) inclusion-rich brownish bladed cement, and (4) late clear spar. Numerous calcite veinlets crosscut all components in many of these concretions.

Concretion matrix, sediment fills, and all but the ferroan cement and isolated patches of sediment fills are brightly luminescent. The late clear spar and the inclusion-rich cement exhibit constructive luminescent zonation. The matrix and sediment infills have Carbon 13 compositions ranging from -15 to -33o/oo PDB, and Oxygen 18 compositions between -2 and -4o/oo PDB. Calcite cements microspars exhibit progressive Carbon 13 enrichments from -18 to -8o/oo and Oxygen 18 depletion from -4 to -12o/oo.

Petrographic and isotopic characteristics suggest early concretion formation and septarian vein fills, close to sediment-water interface, certainly prior to any significant dehydration of overlying sediments. The brightly luminescent character and sequestering of Fe into pyrite indicate an anoxic, possibly sulfate-reducing environment. The extreme Carbon 13 values (-33o/oo) suggest the influx of oxidized methane (sourced by early hydrocarbon generation). Calcite cements and microspars were formed by the circulation of fluids undergoing progressive warming. Warming was most likely 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 gradien responsible for early hydrocarbon generation.

AAPG Search and Discovery Article #91012©1992 AAPG Annual Meeting, Calgary, Alberta, Canada, June 22-25, 1992 (2009)