Development Challenges in a Fracture-Enhanced Carbonate Grainstone Reservoir, Polvo Field, Brazil - from Reservoir Characterization to Dynamic Model

Schwans, Peter ¹; Lukas, Theodore C.³; Cohen, Martin ²; Zauderer, Kim ¹; Bird, Scott D.¹; Hindlet, Francois ¹; Gross, Michael R.^1
1 International Exploitation, Devon Energy, Houston, TX.
² Department of Earth Science, Florida International University, Miami, FL.
³ Consultant, Houston, TX.

A combined fracture and matrix static and dynamic model was built for carbonate shoal sequences of Polvo Field, offshore Campos Basin, Brazil. The Albian-age carbonates of the Quissama Member (Macaé Formation) were deposited in shallow marine to intertidal, shoaling-upward sequences on a partially emergent shelf. Shoal complexes are retrogradationally stacked, probably in response to onset of drowning of the shelf in the Late Albian. They are unconformably overlain by the transgressive shales and marls of the Outeiro Member. Individual shoal sequences comprise burrowed subtidal packstones grading upward to oncolitic-oolitic grainstones often capped by hardgrounds. The carbonate sequences are often only partially preserved and stack vertically and offlap laterally in complex fashion to form three shoal complexes. Reservoir heterogeneity occurs at the facies and sequence-stratigraphic level; later diagenesis and subsequent fracturing and faulting added additional complexity.

A matrix and a mechanical stratigraphy model with discrete fracture networks were built to model the dual porosity-permeability systems. In the matrix model oil saturated grainstones of the younger shoal complexes form a high permeability veneer overlying older and cemented shoal complexes with patchy matrix permeability; the latter exhibit abundant fracturing, however. Detailed core descriptions were linked to logs to define probability petrofacies and associated permeability distributions. The fracture systems are identified in cores and via FMI logs. Their distribution and character was used to define a mechanical stratigraphy and associated DFN’s. Together with seismically mapped faults, the fractures play a significant role in maintaining reservoir energy and vertical connectivity in this highly compartmentalized system where neighboring wells do not communicate. Instead, wells exhibit a high amount of reservoir energy that exceeds the amount accounted for by the mapped hydrocarbon and aquifer zones.

AAPG Search and Discovery Article #90090©2009 AAPG Annual Convention and Exhibition, Denver, Colorado, June 7-10, 2009