--> Abstract: Relationship of Tectonic Style to the Occurrence and Distribution of Fracture Porosity, by K. J. Weber and U. O. Freyer; #91012 (1992).

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ABSTRACT: Relationship of Tectonic Style to the Occurrence and Distribution of Fracture Porosity

WEBER, K. J., and U. O. FREYER, Shell Internationale Petroleum Maatschappij B.V., The Hague, Netherlands

Fracture porosity is still the most elusive of reservoir parameters. Modern borehole wall imaging tools and cores can provide fracture orientation, spacing and rough estimates of fracture widths. Fracture distribution models in combination with well test analysis can lead to semiquantitative estimates of the permeability, porosity, and connectivity of the fracture system.

Studies of outcrops of fractured formations are mainly of interest to establish the fracture distribution and to reconstruct the development and relative age of the various fracture systems. This information can be applied to model the fracture distribution in analogous field structures in combination with the well data. Some of the most reliable fracture porosity estimates are derived from long-term production data and careful monitoring of hydrocarbon-water contacts.

Literature on fractured reservoirs contains little reliable data on fracture porosity. A survey of worldwide literature has provided a limited number of well-documented cases from which realistic ranges of fracture porosity in various tectonic regimes have been derived. Thrust structures and structures formed by strike-slip faulting dominate the scene. Even in strongly folded reservoirs the fracture porosity rarely surpasses 0.2% of rock bulk volume unless leaching has enhanced fracture width.

Fracture porosity is often overestimated by a factor of 10 or more. This is mainly caused by the often spectacular well productivities in fractured reservoirs. Fracture systems with an overall porosity of only 0.01% of rock bulk volume can yield sustained well productivities of several thousands of barrels. Minor fracture porosity related to extension faulting is often responsible for the economic production of tight gas fields.

 

AAPG Search and Discovery Article #91012©1992 AAPG Annual Meeting, Calgary, Alberta, Canada, June 22-25, 1992 (2009)