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The AAPG/Datapages Combined Publications Database

CSPG Bulletin

Abstract


Bulletin of Canadian Petroleum Geology
Vol. 12 (1964), No. 2. (June), Pages 226-245

An Appraisal of Fracture Porosity

James M. Drummond

ABSTRACT

Fracture porosity occurs in joints and faults. It is occasionally developed in beds showing little or no folding, but more commonly occurs in folds, where it is clearly of tectonic origin. For convenience, porosity developed in folds is termed "tectonic fracture porosity" and the rarer type is termed "cryptic fracture porosity." Fractured reservoirs are defined as those in which fractures either are responsible for, or appreciably increase, hydrocarbon production. Fields in South America, Canada, the United States and other parts of the world show that fracture porosity is widely distributed. More than ten percent of world reserves occur in fractured fields in the Middle East. The world total is probably still greater, but is difficult to evaluate.

In theory, very high bulk fracture porosities can develop in folds, but even five percent development will probably be exceptional. Most known fields have porosities of three percent or less. Bit samples and electric logs may not always show potentially productive fracture porosity. Mud losses are best for identifying it and drill stem tests for evaluating it, but neither is foolproof, and economic reservoirs may be drilled through without being detected.

Fracture development and porosity development are both related to bed competency, which varies with physical conditions. There appears to be an inverse relationship between the competency of beds as measured by compositional strength and thickness, and the intensity of jointing. Fracture intensity may bear no relationship to porosity development, which mainly depends upon the fractures being open. Comparatively incompetent beds seem to form reservoirs with cryptic fracture porosity. Competent beds make the best reservoirs with tectonic fracture porosity, probably because other beds tend to adjust by flow under the stress conditions involved in folding. It is impossible to predict the occurrence of reservoirs with cryptic fracture porosity. In reservoirs with tectonic fracture porosity, most porosity is formed by jointing developed during the folding of competent beds. There is a fair correspondence at Masjid-i-Sulaiman in Iran between the bulk fracture porosity calculated on the basis of fold structure and the stratigraphy, and that derived from reservoir data. All fractures in folds tend to be tensionally open, but fractures on the periphery of a fold, in the areas of tightest folding, have optimum prospects for porosity development. Well productivities at Mara-La Paz in Venezuela and in other fractured fields tend to support this conclusion.


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