Depositional Units in Fluvio - Lacustrine Reservoirs and Their Use as Equilibrium Regions. A Case Study in San Jorge Basin, Argentina

Lopez, Ramiro G.¹ and Fernandez Righi, Esteban
¹[email protected]

Geologic characterization and static modeling of an oil reservoir set in Upper Cretaceous fluvio-lacustrine deposits were performed to analyze the behavior of different depositional units as equilibrium regions. Using SP, resistivity and conductivity logs from more than 500 wells and porosity logs, most important shale levels with lateral continuity were correlated resulting in 30-50 m thick cycles characterized by the stacking of sand beds with normal, reverse or aggradational patterns. The variation in gradational patterns and lithofacies, vertical and lateral changes respectively, are interpreted as caused by local changes in a fluvial channel belt environment, e.g. transition between main channel to levee and floodplain deposits. Each of these stacking patterns was limited at its top and bottom by a shaly interval acting as a fluid barrier.

Fluid distribution within each unit at the beginning of field development was evaluated using completion well tests and petrophysics, leading to the recognition of capillary-gravity equilibrium with the presence of a primary gas cap in the highest parts of the structure, an oil zone in the middle and water in the lowest sections, always within faulted compartimentalization blocks of the same unit. This fluid distribution behavior is an indication of the degree of connectivity between the different reservoir units within each depositional unit or cycle.

Based on published studies, a correlation between reservoir connectivity and net-to-gross ratio (NTG) was established. However, evidence from dynamic parameters is needed to assess the degree to which this connectivity contributes to improve recovery and waterflood sweep efficiency. Along these lines, some conceptual, mechanistic dynamic simulation results are also presented showing that differences in static pressure between connected sand bodies (measured by RFT for example) can be quite large depending on the effective transmissibility of the connection and the production-injection history. Therefore, a general conclusion of the present study is that static connectivity leads to equilibrated initial fluid distributions because this is achieved in geologic time scales, whereas connectivity that is effective for reservoir depletion and sweep (at development time scales) needs to be evaluated using more complex tools such as pressure tests, tracers, and dynamic simulation.

AAPG Search and Discovery Article #90166©2013 AAPG International Conference & Exhibition, Cartagena, Colombia, 8-11 September 2013