Chapter 7 Carbonate Oil Reservoir Rocks

Abstract

Summary Half or more of the world's petroleum is produced from carbonate reservoir rocks. The oil-reservoir characteristics of carbonate rocks are largely functions of porosity and relative permeability, which, in turn, have been affected by initial composition of the rocks and their subsequent history. Porosity of carbonate rocks may be arbitrarily divided into (1) primary porosity (formed during deposition), (2) secondary porosity (formed by solution, fracturing or other changes after deposition), and (3) sucrose dolomite porosity (resulting from replacement of calcite by dolomite). Primary porosity may in turn be subdivided into (a) framework porosity resulting from pores that remained as a result of the “sheltering” effect of rigid or loosely-aggregated frameworks, (b) mud porosity, consisting mostly of minute pores that remained in partly compacted carbonate mud that was subsequently lithified, and (c) sand porosity consisting of voids between sorted sand and gravel-sized carbonate particles. Most primary pores have been modified by solution (and cementation). Consequently, there is no sharp dividing line between primary porosity and secondary porosity resulting from solution. Sucrose dolomite porosity is important in many oil reservoirs. In these rocks, porosity and permeability have been strongly influenced by composition of the original carbonate sediment and the degree to which the rock has been replaced by sucrose dolomite. For example, in certain Devonian rocks in west Texas, which originally consisted of varying proportions of lime mud and of crinoid-stem fragments, the greatest porosity occurs in rocks that have been most highly dolomitized. Here, the percentage of dolomite tends to be greatest in rocks which originally contained about 45% lime mud and 55 % crinoid-stem fragments. The performance of carbonate reservoirs depends to a substantial degree on shapes and dimensions of pores and their geometric arrangement with respect to each other. Under oil-reservoir conditions, pores in rocks are generally occupied by either water or oil. Ordinarily, the reservoir rock is water wet, that is, each rock grain is surrounded by a thin film of water, and oil is generally the non-wetting phase. Isolated oil globules ordinarily will not migrate through the rock because the interfacial tension between water and oil is so high that the globules will not pass through the throats of pore interconnections. Before the oil can move as a separate phase, the displacement pressure between the oil-water interface must exceed the entry pressure of the pore interconnections. The displacement pressure is influenced principally by buoyancy, whereas entry pressure depends on the interfacial tension between water and oil, and on pore geometry. The minimum height of an oil column necessary for buoyant rise through a water-wet carbonate rock thus partly depends on the diameters of throats of pores and diameters of the interiors of pores. The reservoir performance of carbonate rocks may be predicted by injecting mercury into cores from reservoirs. Mercury, a non-wetting fluid, is forced into the core sample under increasing pressure. A graph of the data, showing injection pressure, versus cumulative volume of mercury injected, is an effective guide to the conditions required for oil to move in the rock. Ancient depositional environments exert strong influence on carbonate deposits formed in them, and, in turn, have subsequent effect on oil-reservoir conditions in carbonate rocks. Many examples could be cited. In Mississippian carbonate reservoirs in southeastern Saskatchewan, oolitic and pseudo-oolitic limestones interpreted to have been formed through chemical precipitation in a barrier bank environment, serve Iocally as excellent, highly permeable oil reservoirs. In west Texas, the Pennsylvanian-Permian Horseshoe atoll is a horseshoe-shaped mass of limestone about 90miles across in an east-west direction and 70 miles from north to south. It is interpreted to be analogous to modern reef atolls of the East Indies. In the Paradox Basin of southeastern Utah, limestone lenses composed largely of leaflike calcareous Algae serve as oil reservoirs. In Alberta, much oil is produced from Devonian rocks in which favorable reservoir conditions are closely associated with stromatoporoids and calcareous Algae. The geographic outlines of certain oil fields in Alberta, such as Red-water field, are essentially parallel to the trends of ancient organism communities. Thus, there is strong incentive to interpret ancient carbonate environments and organism communities, and to understand their effects on oil-reservoir properties.