MARQUEZ, XIOMARA, McGill University, Department of Earth and Planetary Sciences, Montreal

ABSTRACT: Sedimentological and Diagenetic Controls on Pore Systems in a Dolomitized Reservoir, A Case Study: The Ricinus West Gas Field, Central Alberta

Any attempt to study reservoir quality or predict reservoir performance can not be accomplished without establishing the depositional framework and diagenetic history of the reservoir. The Ricinus West gas field, in the deep part of the Alberta basin (>4000 m) illustrates the control of depositional facies on the pore types and their distribution in a reservoir despite a strong diagenetic overprint by replacement dolomites.

Core and well log data indicate that the pay zone of the Ricinus West reservoir (upper 196 m) is completely replaced by dolomites. Despite this complete dolomitization, two different pore systems are recognized on the basis of the vertical and lateral arrangement of vuggy, moldic, fenestral-like and intercrystalline pores that resemble meter-scale shallowing upward depositional cycles, described elsewhere from the Leduc Formation (and outcrop equivalents).

The lower part of the pay zone is characterized by: (1) large (2 to 5 cm) and irregular vugs that appear to be related to the dissolution of bulbous stromatoporoids. (2) medium (0.5 to 2 cm) cylindrical moldic pores are associated with Amphipora facies, and (3) tabular and irregular (elongated) fenestral-like pores associated with algal laminites. Thicknesses of the sesequences varies from 2 to 8 meters. Present porosities ranges from 4 to 12% (avg. 8%) and permeabilities vary from 4 to 171 md (K<h>) and 1 to 20 md (K<v>).

The upper 60 m of the pay zone consists of thick (up to 10 m) intervals with predominantly intercrystalline porosity that suggest dolomitization of shallow water grainstones. Porosities in this section are similar to the lower part of the pay zone (5 to 20%, avg. 8.8%) but permeabilities are considerably higher (eg. K<h> from 1 to 947, avg. 570 md).

The main diagenetic processes influencing present porosity and permeability values are dolomitization, dissolution, fracturing and cementation by late, dolomite, calcite, anhydrite and minor elemental sulphur. Any enhanced recovery model should consider that the pore types and shapes and their distribution within the buildup have been largely inherited from the original limestone facies.

AAPG Search and Discovery Article #90987©1993 AAPG Annual Convention, New Orleans, Louisiana, April 25-28, 1993.