Carbon and oxygen stable isotopes in the Toa Baja Well, Puerto Rico: implications for burial diagenesis and hydrocarbon generation

Abstract

The Toa Baja Well was drilled on the coastal plains of northern Puerto Rico with a total depth of 2705 m [Larue, 1990]. Interstratified limestone, quartz-bearing calcareous sandstones, and shales dominate the uppermost 580 m and are separated from underlying rocks by an unconformity. Below this unconformity continuing to total depth, lithologies encountered consist of volcaniclastic sandstones/siltstones, pelagic carbonates, volcanic flows and either plutonic rocks or coarse-grained immature sandstones derived from plutonic bodies. Stable isotopic data of carbonates suggest diagenetic modification under a meteoric and meteoric-marine mixing environments for sediments above 915 m. A fracture zone near 915 m coincides with an abrupt shift in δ18O compositions indicating the presence of hotter fluids during alteration of these sediments. The depleted δ13C signatures occurring above a fault at 1220 m are suggestive of isotopically light gaseous hydrocarbons migrating through the fault and being oxidized as they dispersed through the sediment column. The abrupt shift in δ18O compositions and its coincidence with the fracture zone at 915 m suggest geopressuring and thermal buildup due to accumulation of hotter, upward migrating formational fluids. Estimated burial temperatures for the interval above 915 m do not exceed 40°C and are possibly lower due to circulating colder meteoric fluids. Maximum estimated burial temperatures for sediments below 915 m range from 80° to a a maximum of 150°C at 2400 m. The data suggests that the bulk of the sediment pile has not been exposed to temperatures above the oil window and possibly hydrocarbons have been generated deeper in the basin.