Fault Seal Prediction and Risk Evaluation of Exploratory Prospects: Examples of Brazilian Marginal Basins

Pelosi, Ana Paula M.¹

¹E&P-Exploration, Petrobras, Rio de Janeiro, Brazil.

Fault seal methodology has been applied to analyse the fluid flow behavior of faults, especially for reservoir characterization. In a production context, geologists are able to evaluate data sets important to fault seal prediction, including fluid pressures and contacts, as well as rock properties and geometries. In exploration areas, these data are often not available. The aim of this work is to describe a workflow of structural modeling and fault seal analysis which has been used to evaluate uncertainties related to exploratory prospects at Petrobras. Examples of prospects will be shown in order to illustrate the importance of this study.

Methodologies of fault seal analysis are widely found in bibliography. The workflow shown here includes the essential steps of these methods. The first step is structural modeling, which comprises the fault geometry edition and fault throw mapping, using 3D interpretation software and seismic volumes (amplitudes and coherence). The second step comprises well logs analysis and V-Shale calculation. The third step is the projection of stratigraphy on fault surface and the modeling of fault seal parameters, including juxtaposition diagrams and the fault-rock estimate (Shale Gouge Ratio and Smear Factor).

Structural modeling can be used to evaluate risk of two important aspects: trap closures and migration pathways. This methodology was applied in one Brazilian marginal basin to estimate different spill points in siliciclastic and carbonate prospects. In both cases, fault seal prediction was able to increase the exploratory success. In carbonate sequences, fault seal was estimated by juxtaposition diagram and porosity logs. In siliciclastic sequences, threshold values of SGR were used to delimitate trap closure, lateral seals and spill points. In another case, fault seal studies evaluated migration risk, where juxtaposition of reservoirs rocks and low gouge ratio showed the most likely oil migration pathway.

Fault seal prediction is becoming frequent in prospect analysis. Further results will improve the calibration of threshold values for Brazilian basins in an exploratory context. Diagrams of reservoir juxtaposition are very reliable in exploratory areas where well data are available near the prospect. Despite not being an exact science, empirical correlation between fault-rock type and supportable across-fault pressures has demonstrated suitable results.

 

AAPG Search and Discover Article #90100©2009 AAPG International Conference and Exhibition 15-18 November 2009, Rio de Janeiro, Brazil