Integrating 3D Structural Modeling, Surface Restoration and Fracture Characterization in a Deep-Water Gravity Gliding Context (Campos Basin, Brazil)

Mário Araújo¹, Rodrigo Dias Lima¹, Fernanda Guilardi¹, Isabelle Moretti², and Luca Micarelli^3
1Petróleo Brasileiro S.A. - Petrobrás, Brazil, 21941
²Institut Français du Pétrole, Av. de Bois Préau, 92852, Rueil Malmaison, France
³Beicip Franlab, Av. Napoléon Bonaparte, 92502, Rueil Malmaison, France

This paper presents the results of the integration among 3D structural modeling; surface restorations and fracture characterization in a thrusted and fractured fault block in deep-water Albian carbonate rocks of Campos Basin, southeast Brazil. The construction of a mathematical and conceptual model will support the future development of this fractured carbonate reservoir. The Campos Basin is the most prolific Brazilian oil-producing Basin, which presents nowadays these kinds of carbonate fractured reservoirs as the ultimate exploration frontier. The study area is situated 200 km from SE of the Brazilian Coast below 3000m of water depth. The target is about 200 m thick layer of Albian carbonates (e.g., plantonic mudstones) covering an area of 528427 km². They are structured by compressional folds and thrusts formed by downslope sliding on a salt detachment, which is connected to the upslope portion with roll-over anticlines and listric normal faults, formed during late Albian. Latter this carbonates folds and thrust belt was affected by Upper Cretaceous salt pillow deformation. The late Albian downslope gliding of the carbonate platform is responsible for the formation of N-S trending east-directed thrust faults, backthrusts and gentle to open east verging folds. The sequential surface restoration revealed that during this time the carbonates were deformed above a gently west-dipping salt layer. The East-west shortening has been at this time of about 15%. From the end of Albian up to Late Turonian sediment supply was calm as suggested by isopach maps. After Late Turonian a massive sediment supply induced the deformation of the overburden above the salt layer, favoring the extensive development of salt diapirs and forming a regional NE trending turtle back anticline. This structure reworked the Lower-Albian thrust-related ones mainly with relative differential vertical displacements without new significant compressive phase.
During the restoration steps, the cumulative dilatational strain was calculated to be subsequently used as probability maps for the next step of fracture network simulations. The well has drilled a flank of one of the fold and crossed one of the thrust.
Fracture measurements from FMI logs in this well show that in the carbonate rocks most fractures are nearly perpendicular to the layer boundaries and show low dip-angles. However, by rotating those fractures to make the bedding horizontal, they acquire high dip-angles and define clear N-S preferential strikes. This is in agreement with the main thrust faults orientation. The degree of tilting is too much high and it doesn’t spread over a quite large area to be related to the Upper Cretaceous doming (strata at 60-70° and consequently low-angle dipping fractures). Additionally, the thrust faults themselves being not tilted by the latter phase, the strong variation in dip of the strata along the well, and consequently the dip variation of the fractures, can be attributed to local flexures in the proximity of the fault planes. Therefore, most fractures developed during Albian thrusting were progressively tilted during the east-direct transport of the thrust sheet. This analyze of the structural position of the fractures detected in the well combined with the probability map deduced from the precise fault network geometry and the restoration allows us to quantify accurately the fractures density and to model the expected fracture porosity.

AAPG Search and Discover Article #90066©2007 AAPG Hedberg Conference, The Hague, The Netherlands

AAPG Search and Discover Article #90066©2007 AAPG Hedberg Conference, The Hague, The Netherlands