PORE GEOMETRY AND RESERVOIR ASPECTS OF SECONDARY POROSITY IN SANDSTONES

Abstract

ABSTRACT Much secondary porosity has been created in sandstones after their burial by leaching of carbonate and sulphate minerals. Secondary porosity constitutes the predominant or exclusive form of effective porosity in many Canadian oil and gas fields where the reservoir sandstones have undergone a relatively long-lasting and deep burial. Secondary porosity can be found in sandstones of any mineralogical and textural composition, in all major sedimentary basins of Canada, and may occur in sandstones ranging in age from Cambrian to Tertiary. The carbonate and evaporite minerals which are the precursors of the secondary porosity occur as three textural components: 1) primary depositional constituents, 2) authigenic pore-filling cement, and 3) authigenic replacement of other minerals including quartz and feldspars. Excepting fractures, secondary porosity consists essentially of molds of only one of the three textural components or any combination of them. Secondary porosity resembles primary porosity only in those rare cases where the nonsoluble framework of the sandstone remained unchanged after cementation and where exclusively pore-filling cement was dissolved. In most instances, however, secondary porosity differs from primary porosity in comparable sandstones as follows: 1) the range of pore sizes and pore shapes is wider; 2) the spectra of sizes and shapes of pore throats are radically different; 3) the distribution of porosity is more variable. As a result of their pore geometry, most sandstones with secondary porosity show strikingly different reservoir characteristics and physical properties if compared with sandstones of the same original lithology showing an equal percentage of primary porosity. The affected parameters include 1) permeability, 2) irreducible water saturation, 3) mechanical competence, 4) sonic travel time. The presence of secondary porosity modifies the response to several well-logging methods. Relatively low water saturations are characteristic for many hydrocarbon reservoirs with secondary porosity. The reservoir aspects of fields producing from secondary sandstone porosity are often more like those of carbonate reservoirs rather than those of fields with primary sandstone porosity. The distribution of secondary porosity may not necessarily show a direct relationship with depositional lithofacies or burial history and can be difficult or even impossible to predict. However, detailed geological and petrological analysis of the factors that control the occurrence of secondary porosity often greatly enhance the understanding and prediction of its distribution. Sandstone reservoirs with secondary porosity have special problems and offer special opportunities for the exploitationist. It is necessary to recognize the secondary nature of the porosity of these reservoirs and to survey their pore geometry in order to optimize recovery.