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The AAPG/Datapages Combined Publications Database

CSPG Bulletin


Bulletin of Canadian Petroleum Geology
Vol. 39 (1991), No. 3. (September), Pages 260-269

Assessment of Porosity from Outcrops of Vuggy Carbonate and Application to Cores

L.B. McNamara, N.C. Wardlaw, M. McKellar


Porosity in vuggy carbonate rocks, which form important hydrocarbon reservoirs in western Canada, is commonly estimated from analysis of full diameter cores and interpretation of geophysical well logs. An outcrop of the Cairn Formation at Grassi Lakes, near Canmore, Alberta, contains large vugs and fossil molds in a dense, dolomite matrix and the outcrop is an analog, with respect to characteristics of the pore system, of the producing zones of some Devonian reservoirs in the subsurface of western Canada.

The outcrop was systematically photographed and sampled in order to determine whether full diameter cores from vuggy carbonate rocks provide representative samples for estimation of porosity at larger scales and to examine the relationship of sample size to estimated porosity and porosity variability. Vug areas were digitized by manually tracing their boundaries on the photographs and the ratio of vug area to photo area is the porosity within visible pores in the outcrop.

Total porosity in the outcrop sample is 16.9%, with 12.5% being visible porosity contributed by vugs and 4.4% in the dense matrix. In the photo data sets that provided representative porosity estimates, variability of porosity increases with decreasing sample size. Since the efficiency of hydrocarbon recovery is related to local variability of petrophysical properties or heterogeneity, estimates of recovery efficiency will also be a function of the sample size or the scale at which the measurement is made.

Full diameter cores were simulated at random through the outcrop photos. In cases where a megapore crossed 9 cm or more of the core diameter, the core was considered broken and the pore volume contained within the megapore that caused the break was not included in the porosity calculation. The simulated cores averaged 9.55% porosity in visible vugs, 2.95% lower than the outcrop value. It is concluded that a subsurface formation, with a similar pore size distribution to that recorded in the Cairn Formation outcrop, would have porosity underestimated if measured from full diameter cores.

Porosity measured from cores of the Leduc Formation, Westerose Field, is compared with porosity calculated from well logs over the cored interval. The similarity between core and density log porosity, averaged over the same interval, indicates that there is little porosity truncation at the core scale for full diameter core samples from the Westerose Field.


La porosite des roches carbonatees vacuolaires, qui forment d'importants reservoirs d'hydrocarbures dans l'ouest du Canada, est communement estimee a partir d'analyses de carottes de plein-diametre et d'interpretations de diagraphies. Un affleurement de la formation Cairn a Grassi Lakes, pres de Canmore, Alberta, contient de larges cavites et des moules de fossiles dans une matrice dense de dolomie, et l'affleurement est un analogue, en ce qui concerne les caracteristiques du systeme poreux, des zones productrices de certains reservoirs Devoniens dans le sous-sol de l'ouest canadien.

L'affleurement fut systematiquement photographie et echantillonne dans le but de determiner si des carottes de plein-diametre dans des roches carbonatees vacuolaires fournissent des echantillons representatifs pour l'estimation de la porosite a plus grande echelle, et dans le but d'examiner la relation entre la taille de l'echantillon et la porosite estimee et la variabilite de la porosite. La surface des cavites fut digitisee en tracant manuellement leurs contours sur les photos, et le rapport entre la surface des cavites et la surface des photos represente la porosite visible en affleurement.

La porosite totale de l'echantillon d'affleurement est de 16.9%, dont 12.5% representent la porosite visible contribuee par les cavites et 4.4% representent la porosite dans la matrice dense. Dans les series de photos qui fournirent des estimations representatives de la porosite, la variabilite de la porosite, augmente lorsque la taille de l'echantillon

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diminue. Puisque l'efficacite de la recuperation d'hydrocarbures est liee a la variabilite des proprietes petrophysiques ou heterogeneite, les estimations d'efficacite de recuperation vont aussi etre fonction de la taille de l'echantillon ou de l'echelle a laquelle la mesure est effectuee.

Des carottes de plein-diametre furent simulees au hasard sur les photos d'affleurement. Dans les cas ou un megapore traversa 9 cm ou plus du diametre de la carotte, celle-ci fut consideree brisee et le volume poreux contenu dans ce megapore ne fut pas inclu dans le calcul de la porosite. Les carottes simulees ont une porosite visible moyenne de 9.55%, soit 2.95% de moins que la valeur d'affleurement. Il est conclu qu'une formation enfouie, ayant une distribution de taille de pores similaire a celle relevee dans la formation Cairn en d'affleurement, aurait sa porosite sousestimee si celle-ci etait mesuree a partir de carottes de plein-diametre.

La porosite mesuree a partir de carottes de la formation Leduc, champ de Westerose, est comparee a la porosite calculee sur base de diagraphies correspondant a l'intervalle carotte. La similitude entre la porosite moyenne de la carotte et celle derivee des diagraphies de densite pour le meme intervalle, indique que la porosite est peu tronquee a l'echelle de la carotte pour les echantillons de carotte de plein-diametre venant du champ de Westerose.

Traduit par Patrice de Caritat

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