Basement Geology and Structural Modeling of the Santos Basin, Brazil: Petroleum System Implications

L. A. Jensen¹ and J. P. Teasdale^2
1Shell International Exploration & Production, Inc., Houston, TX, USA
²Shell International Exploration & Production BV, Rijswijk, The Netherlands

The regional pre-salt geology of the South Atlantic margin basins is challenging to decipher using conventional, “top-down” seismic-based interpretation, largely due to poor seismic imaging beneath the salt. We present an alternative “bottom-up” workflow, where:

(i) Analysis of the basement terranes beneath and adjacent to the South Atlantic margins and plate reconstructions are used to predict pre-salt structural style
(ii) Innovative, integrated use of gravity, magnetic, and 2D PSDM seismic data are used to map top-basement topography and structure
(iii) Regional, crustal-scale cross sections and horizon interpretations are interpreted using (i) and (ii) above, and subsequently restored in 2D and 3D, to validate structural geometries, illustrate basin kinematic evolution, and the timing of key petroleum system elements

Jurassic-Early Cretaceous, south-to-north propagation of rifting and breakup in the South Atlantic exploited pre-existing basement structures; principally a series of Neoproterozoic-Early Paleozoic mobile belts that formed during the amalgamation of Gondwana. The contrasting crustal-scale structural framework of each mobile belt is reflected in the related geometry of each overlying basin compartment. Understanding basement geology, inherited structures, and rifting kinematics can be used to predict the asymmetry, width, and structural style of each basin.

The Santos-Namibe Basin Compartment overlies the Cabo Frio Terrane, an early Paleozoic accretionary complex with a pervasive, shallowly east-dipping structural grain. Both model and natural analogs show that when extended, shallowly-dipping, pre-existing basement structures are often reactivated via simple shear to form highly asymmetric conjugate margins; often termed “upper” plate (narrow) and “lower” plate (wide) margins. In the wide, lower-plate Santos margin, analogs that predict high levels of upper crustal extension with sub-horizontal detachments and highly rotated fault blocks are at odds with the conventional notion that the Santos pre-salt is dominated by a simple, upright “horst-and-graben” rift geometry.

Independent tectonic plate reconstructions that produce a “tight-fit” model, which restores the Santos Basin portion of the South Atlantic to a pre-rift configuration, require a beta factor of between 2.7 and 4.6. This value cannot be reconciled with domino-style, high-angle faulting alone. Our current model accounts for the required high values of extension with a major, low-angle, detachment-style fault that soles out into an inherited, low-angle suture zone in the inboard portion of the basin. A large component of stretching is also accommodated in a ductile fashion in the lower crust. In totality, however, neither simple shear nor pure shear end-member models can explain the observed structuration and style of extension in the Santos. A hybrid model, with faulting controlled by pre-existing basement terrane and overlying cover rheology and structure, is more realistic and suggests that these inherited fabrics play a large role in future basin evolution. Other mechanisms, which are not readily visible on the available data, such as rotated normal fault blocks that were continually re-faulted once a critical angle is reached in the tensional stress field, are also likely to contribute to accommodating extension.

Integrated imaging, modeling, and interpretation of gravity, magnetic and 2D PSDM seismic data verifies this view of the pre-salt basin geometry in the Santos Basin. A set of 2D crustal cross sections were constructed to capture the pre- and syn-rift structure of the basin. These cross sections were validated and improved using gravity modeling and structural restoration, including decompaction and flexural isostatic adjustment. Enhancement processing and imaging of magnetic and gravity data were undertaken to reveal map-view basement structures and igneous feature, which were combined with the 2D cross sections to interpret the map-view structural geometry of the basin. Shell’s proprietary BathogramÔ technology was used to model depth to basement from magnetics. Finally, integrating all of these outputs generated a sculpted, top-basement surface. 3D structural restorations, again including backstripping and flexural isostatic effects, of the pre- and syn-rift sequences yielded an enhanced understanding of accommodation space development, gross-depositional environment, timing of trap formation, and potential fracture intensity. The 2D and 3D restoration results were utilized in subsequent basin modeling efforts, effectively “closing the loop” and taking advantage of new structural analysis and potential field techniques.

This new “bottom-up” view of the Santos Basin can be used to understand and predict pre-salt paleogeography and petroleum system distribution and timing, and to generate “sweet spots” for hydrocarbon exploration. The analysis shows that:

• Early syn-rift sediments are likely to be irregularly distributed and structurally rotated (with implications for source distribution and maturity)
• A significant proportion of the basin is floored by low-angle detachment fault planes
• The steepest basement topography is likely to occur at the top of fault blocks
• Fault block tips may be eroded, with implications for syn-rift clastic reservoir distribution
• Base-salt structural highs (i.e. main hydrocarbon traps) form above basement and/or igneous highs due to differential compaction

AAPG Search and Discovery Article #90091©2009 AAPG Hedberg Research Conference, May 3-7, 2009 - Napa, California, U.S.A.