High-resolution sequence stratigraphy of complex gas reservoirs: Patao and Dragon fields, offshore Venezuela

Abstract

The stratigraphic complexity of near-shore depositional systems, together with the occurrence of submarine mass-failure deposits in the area of the Patao and Dragon fields, represents a challenge for devising a development plan for the gas reservoirs identified in the area. The high drilling costs in offshore areas and the sparse knowledge in this frontier basin require an integration of all available data to optimize the development of production projects. This work aims to refine the geological model, particularly with respect to understanding the complexity of the sedimentary deposits in this area and its impact on the exploitation of the associated reservoirs. The Patao and Dragon fields belong to the Carupano Basin, offshore northeastern Venezuela, and contain hydrocarbon accumulations mostly trapped in late Miocene and early Pliocene clastic reservoirs. Existing well data in the area are still relatively scarce, but provide valuable calibration of the available seismic data. This study involves the construction of a sequence stratigraphic framework, based on the integration of sedimentological and petrophysical studies, vertical proportion curves of lithofacies (VPC), seismic-stratigraphic interpretation, petro-acoustic characterization, and reservoir analysis, which enable the identification of stratigraphic units and the correlation between wells. Seven systems tracts have been identified, including the FSST3 (falling-stage systems tract 3) and TST3 (transgressive systems tract 3) which host the most prolific gas reservoirs in the area, comprised of coalescent sands with distinctive rocks properties. There is evidence that the FSST3 is affected by syn-depositional submarine mass movement that distorted the sedimentary architecture and the internal structure of the deposits. These considerations, as well as the identification of potential permeability barriers associated with marine flooding surfaces, allowed the establishment of different scenarios of hydraulic unit configuration, key for the evaluation of these gas reservoirs in terms of their storage capacity and subsequent production.