LINKS BETWEEN THRUST EMPLACEMENT AND QUARTZ CEMENTATION IN SANDSTONE RESERVOIRS OF THE EASTERN VENEZUELA BASIN AND THE EASTERN CORDILLERA OF COLOMBIA

Quartz cemented sandstone reservoirs of various ages were studied in six structures in the Eastern Venezuela Basin and the Colombian Llanos. We have combined detailed petrography, fluid inclusions, and thermal modelling to assess the relative timing of quartz cementation and tectonic events. Both ages and temperatures of silicification vary from a structure to another. Nevertheless, in each of these structures, onset of quartz precipitation was contemporaneous with the first compressive phase in the hinterland and with the synflexural series deposition in the foreland. Quartz cementation started when the reservoir still was in the foreland and went on after thrust emplacement. It appears that there has been a temporal control on the onset of quartz cementation, certainly related to the available silica sources. In situ pressure solution of quartz grains and diagenetic transformation in neighbouring shaley levels have been identified as the main local sources of silica. Rapid burial resulting from the syntectonic deposits sedimentation increased pressure solution of quartz grains. Layer parallel shortening induced by Andean compression accentuated this process. Increasing burial also induced clay mineral transformation in shaley formations. An important quantity of silica has thus been released at the beginning of the compressive events, and later on during deformation episodes. By reducing porosity, rapid burial and horizontal compression could also have induced circulation of fluids trapped in the sediments. In addition, topography creation during the orogenic phase may have driven meteoric water circulation forelandward. Therefore, diagenetic reactive elements have possibly been conveyed from the deformed zones in the foothills to the non-deformed areas in the foreland. Quartz cementation probably stopped in response to the combination of several factors resulting in the drying up of the silica source such as hydrocarbon trapping (reservoir oil saturation and precipitation inhibition), tectonic accretion (system closure), erosion (decreasing burial).