Understanding the diagenetic evolution of potential reservoirs in fold/thrust belts: an example from eastern Venezuela

Abstract

Major oil discoveries in foothill areas have recently focused the interest of exploration companies sub-Andean basins. The SUBTRAP consortium was initiated in order to unravel the petroleum system associated with sub-thrust carbonate and sandstone reservoirs in various foreland fold-and-thrust belts around the world. As an illustration of the methodology defined by the consortium, this paper summarizes the main results of the integrated study using surface and subsurface geological data and basin modelling tools along a regional transect from eastern Venezuela. The focus of the study was to understand the porosity reduction and, therefore, the origin of the palaeofluids in the Upper Cretaceous–Oligocene sandstone reservoirs of the El Furrial structure (Venezuela). Basin modelling was performed using the Thrustpack , Locace and Ceres tools. The temperature and nature of the fluids obtained by this modelling were compared to fluid inclusions and oxygen isotope data on quartz overgrowths. Four observations were made. From 65 Ma to 20 Ma fluids were at thermal equilibrium with the sediments. They were continuously expelled vertically toward the surface during compaction-driven de-watering processes. From 20 Ma to 12 Ma, as a result of the regional tilting and the deposition of the synflexural Naricual Formation, the Cretaceous and Oligocene sandstones of the El Furrial structure became efficient conduits for fluids circulating from the north. These thrust-driven fluids (‘squeegee 1’) were at thermal equilibrium with the Cretaceous and Oligocene sandstones and seem to be correlated with the first generation of quartz overgrowths. This episode is characterized by an increase in the overpressure in the Oligocene and Upper Cretaceous sandstones correlated with a hydraulic fracturing of the sealing Carapita black shales. From 12 Ma to 8 Ma fluids were expelled laterally from the Cretaceous sediments of the Pirital hanging-wall unit located immediately north of the El Furrial structure (‘squeegee 2’). These fluids were probably in chemical disequilibrium and their temperature was higher than the temperature of adjacent sediments. This probably resulted in additional, but minor, episodes of quartz precipitation. This hypothesis is consistent with the information obtained from oxygen isotope analyses, which suggest that subsequent generations of quartz cement probably formed from evolved basinal fluids. At around 8 Ma a reduction in the intensity of the flow and then an inversion of this flow marked the sealing of the southern structural closure. Closure of the northern flank occurred at around 5Ma, as indicated by the present velocity of the fluids close to zero in the El Furrial reservoirs and the filling of the structure by hydrocarbons.