Petrographical and petrophysical characterization of the late cretaceous Naparima Hill Formation, Central Range, Trinidad, West Indies

The late Cretaceous Naparima Hill Formation in the South Trinidad Basin (southeast Caribbean) is a prolific source rock with a potential for unconventional hydrocarbon exploration. It is exposed across the Central Range and extends to the city of San Fernando. This study characterized the Naparima Hill source rock using petrographic analysis techniques (X-ray diffraction and thin section analysis) and petrophysical techniques. These techniques were applied to outcrop samples that were collected from 11 locations. Firstly, petrographic analysis was used to determine the mineral composition and lithofacies. Secondly, plugs were taken perpendicular to bedding and the bulk density was determined in dry conditions. Thirdly, porosity and grain density were determined using a porosimeter at ambient conditions. Finally, permeability measurements were obtained at 10 MPa to 130 MPa effective pressures using the transient pulse decay technique. The results show that the source rock is dominated by silica with carbonate as secondary minerals. Four lithofacies were identified: (a) siliceous-calcareous mudstone (b) calcareous mudstones interbedded with black chert, (c) carbonate-rich mudstone with nodular chert and (d) siliceous mudstone. Ranges of 5.9 to 30.8%, 2362 to 2668 kg/m³ and 1804 to 2350 kg/m³ were determined for porosity, grain density and bulk density, respectively. The permeability ranges from 3.96 × 10⁻²⁰ to 5.94 × 10⁻¹⁸ m² and shows very little dependence on the effective pressure. Permeability and porosity are moderately correlated (R² = 0.6). There is no relationship between permeability and mineralogical composition. The observed microfractures, diagenetic processes and grain sorting, are factors that have affected the permeability-porosity relationship to varying degrees. However, none of these factors can be considered as the main intrinsic factor controlling the permeability of the samples. Overall, it is difficult to predict the permeability of the Naparima Hill Formation with any of the proxies (density, mineralogy and porosity) determined in this study.