It is important to realize that most secondary sandstone porosity closely resembles or exactly mimics primary porosity. This is the reason why secondary sandstone porosity has largely remained unnoticed until recently. It follows that, in the study of secondary sandstone porosity, it is necessary to develop readily observable diagnostic criteria for recognizing or suspecting its presence. Additionally, a suitable analytical approach must be chosen for verifying suspected cases and for recognizing the quantity, texture, origin, and textural modification of secondary porosity.
Petrographic Criteria. — In many instances, the secondary nature of sandstone porosity can be identified by using petrographic criteria that are easily observable in thin sections under the petrographic microscope. Some of these can also be observed, on occasion, in hand specimens. The following eight petrographic criteria appear to be the most useful (Figure 18): (1) partial dissolution; (2) molds; (3) inhomogeneity of packing; (4) oversized pores; (5) elongate pores; (6) corroded grain margins; (7) intra-constituent pores; and, (8) fractured grains.
Partial dissolution of soluble constituents is by far the most conclusive criterion and it is also very common. Dissolution of soluble sedimentary and authigenic constituents is, in many instances, incomplete and patches of remnant material occur adjacent to pores (Figure 21). Frequently the remnants have a corroded appearance. Care must be taken to determine the partial dissolution of cements because incomplete cementation of porosity may produce very similar textures. Indicators for the partial dissolution of authigenic cements are: (1) corroded surfaces; (2) uniform extinction of isolated remnants of
Figures & Tables
Secondary Reservoir Porosity in the Course of Sandstone Diagenesis
Secondary porosity n sandstones can be classified according to origin and pore texture. Five significant genetic classes of secondary porosity are defined by processes of origin: Fracturing; shrinkage; dissolution of sedimentary grains and matrix; dissolution of authigenic pore filling cement; and dissolution of authigenic replacive material. This publication provides information on the genetic-textural classes of secondary sandstone porosity; the textural spectrum of secondary sandstone porosity; the recognition of secondary sandstone porosity; the geological occurrence and diagenetic origin of secondary sandstone porosity; the textural stages of sandstone mesodiagenesis; the diagenesis of quartz arenites; the diagenesis of sandstones of intermediate and low mineralogical maturity; examples of porosity distribution, and reservoir aspects.