Assessment of Geological Uncertainties and Geological Modeling of a Giant Turbidite Field, Deepwater Campos Basin, Brazil

Aranha, Denise V.¹; Blauth, Marcelo ¹; Neto, Ricardo R.¹
(1)Petrobras, Macaé, Brazil.

This paper is focused on the assessment of the main geological uncertainties and on the building of 3D geological models of a giant, Upper Cretaceous turbidite oilfield from deep water Campos Basin, Brazil.

There have been identified four main critical uncertainties for the characterization and modeling of the reservoirs: 1- time vs. depth conversion: there are two seismic volumes- a depth-volume generated by a geostatistical methodology, and a newer calibrated PSDM; 2- occurrence of basalts interbedded with reservoirs, the real thickness can not be precisely estimated by the resolution of the available seismic data (basalts as thick as 100 m have been interpreted, despite the drilled wells have found only 10 to 20 m so far); 3- difficulties in the seismic mapping the reservoir base: the reservoir base was not well adjusted in the calibration process and, on PSDM volume, the seismic horizon interpreted as the reservoir base is shallower than shown by the drilled wells; and 4- the presence of an undrilled area in the field, where the occurrence and/or thickness of the reservoirs are uncertain.

Three different geocellular models were built with the purpose of describing the geological uncertainties mentioned above: 1- Upper Case Model: structural model using the seismic with the higher depths; basalt thickness controlled by well data limited to 25 m; and reservoir base mapped on seismic, adjusted in the entire field, but honoring well data; 2- Base Case Model: structural model using the seismic with the lower depths (PSDM); basalt thickness guided by wells and seismic horizons limited to 45 m; and reservoir base mapped on seismic using local adjustment to honor the wells; 3- Lower Case Model: structural model using the seismic with the lower depths; basalt thickness mapped on seismic using local adjustment to honor the wells; and reservoir base mapped on seismic using local adjustment to honor the wells; and no oil in the undrilled area.

The last stage was to evaluate the uncertainties of reservoir properties within each of the scenarios previously established. Two hundred simulations were performed by combining the following variables: reservoir tops, average porosity, average water saturation and depth of oil/water contact in the undrilled area.

The described uncertainty analysis allowed to access the risks involved in the project, considering an STOOIP variation between 1.1 and 2.9 billion bbl supporting the development of a risk mitigation strategy.

AAPG Search and Discovery Article #90135©2011 AAPG International Conference and Exhibition, Milan, Italy, 23-26 October 2011.