AAPG ANNUAL CONFERENCE AND EXHIBITION
Making the Next Giant Leap in Geosciences
April 10-13, 2011, Houston, Texas, USA
Petrophysical Characterization of Bioturbated Facies from the Upper Jurassic Ula Formation, Norwegian North Sea, Europe
(1) University of Alberta, Edmonton, AB, Canada.
(2) ConocoPhillips Norway, Stavanger, Norway.
The Upper Jurassic Ula Formation of the Norwegian Central Graben provides the main producing interval in a number of offshore oil fields, including the Ula, Gyda, and Tambar. In the studied intervals, the Ula Formation largely comprises a highly bioturbated siliciclastic fabric interpreted to represent a moderate- to high-energy shallow-marine shoreface. These intervals provide an example of a reservoir within which the flow characteristics are largely dictated by the morphology and distribution of trace fossils.
The recognition of biogenically influenced flow media within the Ula Formation is significant as permeability enhancement occurs within two interrelated scenarios. These scenarios, representing both a cryptic textural heterogeneity and weakly defined textural heterogeneity, are a function of bioturbation present. Intervals influenced through cryptic bioturbation are a result of micro-organisms (such as amphipods), and represent non-discrete burrows. Likewise, reservoir intervals that contain discrete trace fossils that are considerably more permeable than the matrix, such as Ophiomorpha in this study, represent weakly defined textural heterogeneities. The abundance of these two heterogeneities within the reservoir interval (middle/upper shoreface sandstones) suggests that biogenic structures influence reservoir properties such as porosity, permeability, tortuosity, and dispersivity. Consequently, a proper treatment of the Ula Formation reservoir units, along with subsequent reservoir completion and reserve estimates, hinges on characterizing the biogenic permeability.
In this study, microtomography (Micro-CT) is coupled with spot-permeametry to provide a detailed and accurate characterization of the bioturbated media. Micro-CT models indicate that the X-Ray attenuation of the burrows is different from the matrix for weakly defined heterogeneity examples. This indicates that the sands within the burrows differ from those in the matrix. Spot-permeametry of these samples indicates that the burrows are preferentially higher in permeability relative to the matrix. Conversely, spot-permeametry illustrates that cryptobioturbation—due to the complete eradication of the primary fabric—results in a more uniform distribution of permeabilities (i.e. kv = kh). Consequently, it can be recognized that cryptic bioturbation results in a dual porosity system, whereas weakly defined textural fabrics result in a dual permeability system.