Mass transfer and problems of secondary porosity creation in deeply buried hydrocarbon reservoirs

Abstract

Secondary porosity created by the post-depositional leaching of detrital grains and cements can be of great economic importance. The term secondary porosity is only descriptive and does not give any information on whether the porosity of the rock has been enhanced. In reality, there are two end member volumetric outcomes to the leaching process. Firstly, the mineral can be completely dissolved and its components transported out of the system. In such an instance the porosity of the rock will be enhanced. Secondly, it is possible that a mineral will decompose and by-product minerals will be precipitated which will occupy approximately the same volume as the original mineral. The latter case is more likely where the pore waters are close to equilibrium with the majority of the minerals of the rock. This will occur at elevated temperatures and/or where the pore waters have been in contact with the rock for a long time. These conditions are commonly met during intermediate to deep burial. Many metastable minerals such as high temperature feldspars, probably decompose in this way. A consequence of the decomposition of such metastable minerals is that primary porosity will be swapped for secondary and that older and more deeply buried rocks will contain a higher proportion of secondary porosity. The mass-transport equation may be used to examine the physico-chemical environment where enhanced porosities most likely occur during leaching. By using a simple non-equilibrium mass-transport model based on the silica system it is possible to demonstrate the relative importance of flow rate, reaction rates, specific surfaces and temperature in controlling the areal extent of leaching. Wide leaching zones are related to high flow rates, which, in turn, promote a high throughput of undersaturated fluid and dispersion. Leaching over a wide area is also favoured by low reaction rates, which will occur at low temperatures. Although it is true to say that less of a given silicate mineral can be dissolved by an undersaturated pore water at low temperature than at high temperature, it is probable that pore fluids from deeply buried sediments are more likely to be close to equilibrium with a given mineral than, for instance, meteoric water. Thus the most likely environment for the production of widespread enhanced porosity is in the shallow subsurface. The lower flow rates and higher temperatures that are encountered during deep burial diagenesis will result in far narrower, more intense leaching zones. As a result, providing sufficient reactive mineral is present any undersaturated pore water will approach equilibrium in a short distance. In order to transport undersaturated pore waters appreciable distances in the deep subsurface, it is necessary to minimize their interaction with the host rock. This could occur if the pore fluid were to move through a fracture system where it would interact with a far smaller surface area of reactive mineral than if it moved through the pore network.