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Abstract: Fault Seal as the Main Hydrocarbon Trap Mechanism in the Apodi Graben - Onshore Potiguar Basin - Brazil

Borges, Wellington R. Evelim - Petrobras/E&P

Faulting sealing capability is a key issue when characterizing Petroleum System in the Apodi Graben. This graben is the southernmost rifted compartment of the onshore Potiguar basin (Northeast Brazil). After no more than two decades of exploration, the basin reached the status of the most profitable and successfully onshore basin of Brazil, mainly due to its fast exploratory results.

Important accumulations in the region occur within the sin-rift Cretaceous clastic sequence of the Pendência Formation. Geochemical analyses define the lacustrine fresh-water shales of the basal rift sequences as the main source rock in the region.

Even though facies distribution and reservoir quality are other important factors in the Apodi Graben, the key point is the structural component defined by the sealing/ non-sealing behavior of a complex set of NE and NW trending faults (Fig. 01). This is a direct consequence of a unique rift architecture dominated by low-angle southeast dipping normal faults as regional counter faults of majors intracrustral detachments, defining an array of NW tilted faulted blocks (Fig. 02). These faults acted either as seals or as the main petroleum pathways, linking deep source and shallow clastic reservoirs.

Almost all hydrocarbon accumulations in the Pendência Formation are related to fault trapping mechanisms and are located either in hangingwall or in the footwall of rift related faults. Hangingwall accumulations occur preferentially near the main rift borders and are related to a series of fault splays, responsible for the juxtaposition of younger hangingwall reservoirs to older and more compacted shaley sequences. In general, these accumulations have semi-circular shape in map view. The diagnostic factors of its fault sealing control are net-pays greater than the structural closure of the traps, and anomalous fluid pressure data. Footwall accumulations are the most common type, having broader distribution within the rift and occurring at depths varying between 800 and 2600 m. They have an elongated shape following the closure mechanism, the sealing fault. These traps constitute an uneasy exploratory target due to their small width. The use of 3D seismic has been fundamental to improve the exploration and exploitation of these traps.

A minor number of rift-generated hydrocarbon accumulations in post-rift formations is related to later migration. These oil fields have more restricted geologic settings and are located in internal basement highs or faulted rift borders, where the major faults provided the main oil pathways from the source to the host formations as demonstrated by small amount of hydrocarbons in basement faults and fractures of these internal highs.

AAPG Search and Discovery Article #90933©1998 ABGP/AAPG International Conference and Exhibition, Rio de Janeiro, Brazil