--> Abstract: A Comparison of Dual-Permeability Systems in Highly-Heterogeneous Reservoirs: Burrowed Siliciclastics Vs. Fractured Limestones, by Polo, Camilo A.; Borrero, Mary Luz; Andrew, La Croix; Gingras, Murray; and Pemberton, George; #90166 (2013)

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A Comparison of Dual-Permeability Systems in Highly-Heterogeneous Reservoirs: Burrowed Siliciclastics Vs. Fractured Limestones

Polo, Camilo A.1; Borrero, Mary Luz; Andrew, La Croix; Gingras, Murray; and Pemberton, George
1[email protected]

Recent studies have demonstrated that reservoir permeability enhancement associated with trace fossils can be expressed as dual-permeability flow-media that bear some conceptual similarities to fracture systems. For the purposes of this paper, dual-permeability occurs when the matrix permeability and the burrow- or fracture-associated permeability differ by more than two orders of magnitude. This study compares dual-permeability flow systems developed in burrowed sandstones; from those developed in fractured limestones in order to document the role of ichnofossils in intensely burrowed reservoirs. Herein, two case studies are examined: 1) a bioturbated sandstone from the Upper Cretaceous Nise Formation (offshore Norway), constituting a primary reservoir target on the continental shelf; and, 2) a fractured limestone from the Rosablanca Formation, a Cretaceous carbonate-reservoir in the Middle Magdalena Valley Basin in Colombia.

In the Nise Formation, spot-permeametry and petrographic assessments show that trace- fossil permeability is up to two orders of magnitude higher than the matrix. This results from the sorting of grains by bioturbating organisms near the time of sediment deposition. Three dimensional 3D imaging via X-ray microtomography (Micro-CT) scanning allowed the identification of an interconnected burrow-network that is anisotropic mostly in the horizontal direction. This is the result of a diverse ichnofossil assemblage that comprises horizontally oriented deposit-feeding traces with subordinate dwellings of inferred suspension feeders that reflects the proximal through distal Cruziana Ichnofacies. Therein, these modifications resemble a fracture system in the sense that they constitute interconnected and spatially continuous fluid-flow pathways. Alternatively, the Rosablanca Formation is characterized by an interconnected fracture system especially in mudstone and packstone facies. Outcrop mapping and hand-sample analyses were used to characterize the fracture network in the Rosablanca Formation. Microfractures values were measured using conventional petrography and cathodolominiscence imaging that revealed microfractures that were otherwise not visible using a conventional transmitted-light microscope. These analyses showed a clear relationship between the aperture and fracture intensity of microfractures and mesofractures within the range scale investigated.

Despite the fact that clastic and carbonate systems differ in their origin and diagenetic history, in this example, both burrow networks (Nise Fm.) and fracture systems (Rosablanca Fm.) tend to enhance the permeability and transmissivity of an otherwise low-permeability matrix. This is primary influenced by: 1) the concentration and preferential orientation of permeable trace fossils/fractures; 2) the degree of permeability contrast between burrows/fractures and the surrounding matrix; and, 3) the three-dimensional connectivity of the burrow and/or fracture system. Compared to the Rosablanca fracture system, burrow-associated permeability in the Nise Formation case, typically occupy a greater volume of the rock. Additionally, Rosablanca fractures represent "planar" elements that can be geometrically "predictable". In contrast, due to the complex and chaotic nature of ichnofossil geometry, attempts to assess the effectivity of the bioturbated flow net¬works by numerical simulation can be more challenging when compared to fracture systems. However, ichnological models likely can be developed to suitably model ichnological permeability.

 

AAPG Search and Discovery Article #90166©2013 AAPG International Conference & Exhibition, Cartagena, Colombia, 8-11 September 2013