Distribution and Continuity of Carbonate Reservoirs
Authors: Jardine D. , Andrews D.P. , Wishart J.W. , Young J.W. 
Published: December 2007

Carbonate reservoirs are characterized by extreme heterogeneity of porosity, and permeability. This is related to the complexities of the porosity, and permeability. This is related to the complexities of the original depositional environment and the diagenetic influences that can modify the original textures. Four pools are discussed to illustrate the relationship between internal reservoir geometry and pool performance.

Introduction

Production experience in Canada includes carbonates Production experience in Canada includes carbonates ranging in age from Ordovician to Triassic. This experience encompasses the entire domain of carbonate reservoir types, from the small pinnacle reefs of Silurian and Middle Devonian age in Ontario and the Rainbow basin of northern Alberta, through the large Leduc and Slave Point reef complexes of central Alberta, to widespread Point reef complexes of central Alberta, to widespread shoals and carbonate banks of Devonian, Mississippian, and Triassic age extending over much of the Western Canada basin.

Carbonate reservoirs are characterized by extreme heterogeneity of porosity and permeability, often within a single pool. They range from massive, vuggy, and fractured reservoir types in the organic-reef facies to highly stratified, often vertically discontinuous reservoirs in the back-reef and shoal facies.

Depletion plans for these pools, most of which are on enhanced recovery; operation, require detailed, integrated geological-engineering studies. Initially. reservoir description, consisting of lithofacies, correlation crosssections, and fluid saturation studies, is used to develop reservoir engineering numerical models. Continued monitoring of operating performance is essential to assure that the geological model is valid.

The first part of this paper describes, in general terms, the geometry and internal porosity distribution of various types of carbonate bodies and the diagenetic influences that can modify the original textures. To relate this to real cases, four typical carbonate pools in Western Canada are discussed in the second part. Each pool has unique reservoir characteristics that require detailed integration of the geological-engineering disciplines for optimum development. In the third part, one of these pools, the Judy Creek Devonian reef reservoir, is described in detail to illustrate how integrated geological-engineering studies can change the operating practices within a pool.

Porosity of Carbonates Porosity of Carbonates Primary Porosity Primary Porosity Carbonate rock types shown schematically in Fig. 1 and in the core photographs in Fig. 2 are found in a wide variety of depositional settings. These settings range from reefs covering less than 1 sq mile to carbonate banks that often extend over thousands of square miles. In Fig. 1, the types are arranged in order of decreasing energy of the depositional environment. There is a parallel decrease in average grain size, pore size, and permeability.

Biohermal reefs are characterized by their relatively small size, high degree of relief above the sea floor, steeply sloping sides, and a high percentage of skeletal material. An interior lagoonal facies of finer, more bedded carbonates is usually present. Porosity and permeability are highest in the skeletal-rich rim sediments (Fig. permeability are highest in the skeletal-rich rim sediments (Fig. 2a). The interior is less porous and often has poor vertical permeability because of interbedding of porous and permeability because of interbedding of porous and nonporous sediments. Biohermal reefs tend to develop when basin subsidence is relatively rapid.

Biostromal reefs are more sheet-like deposits that are associated with slowly subsiding basins. They can extend over hundreds of square miles.

JPT

P. 873