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Abstract: Role of Secondary Porosity in Sandstone Diagenesis

Volkmar Schmidt, David A. McDonald

Secondary porosity has an important role in the diagenesis of sandstones. The volume of secondary porosity equals or exceeds that of primary porosity in the sandstones of many sedimentary basins worldwide. A significant percentage of the world's reserves of natural gas and crude oil are contained in sandstone with secondary porosity. Prudhoe Bay field and the Jurassic fields of the North Sea are examples of the many giant hydrocarbon accumulations in sandstones with secondary porosity.

Chemical, physical, physicochemical, biochemical, and biophysical processes result in secondary porosity through leaching and shrinkage of rock constituents or through the opening of fractures and porous burrows and borings. Secondary porosity in sandstones can originate anywhere in the sedimentary crust: (1) before effective burial in the environment of deposition (eogenetic), (2) at any depth of burial above the zone of metamorphism (mesogenetic), and (3) during exposure following a period of burial (telogenetic). Secondary porosity occurs in sandstones of any mineralogic or textural composition and of any age. It is most common in sandstones which have undergone relatively long-lasting and deep burial and which have lost their primary porosity.

Most of the secondary porosity in ancient sandstones originated as a result of mesogenetic leaching of the carbonate minerals calcite, dolomite, and siderite. This decarbonatization removes depositional carbonate constituents and diagenetic carbonate materials such as cements or replacements. Most of the mesogenetic decarbonatization results from the decarboxylation of organic matter in strata adjacent to the sandstones during organic maturation. The process of decarboxylation leads to the production of carbon dioxide which, in the presence of water, produces carbonic acid. This, in turn, attacks the carbonate matter.

In most instances it is possible to differentiate microscopically between primary and secondary porosity; thus, it is possible to trace the loss of primary porosity during burial. In the absence of geopressures or hydrocarbons, primary porosity of sandstone cannot exist beyond specific limits of temperature-time exposure except for a small volume of irreducible lamellar porosity between grains. The limits of temperature-time exposure increase with increasing mineralogic stability of the sandstones and, subordinately, with increasing grain size.

The mesodiagenesis of sandstones can be divided into two main stages: (1) immature mesodiagenesis, when effective primary porosity can be present, and (2) mature mesodiagenesis, when effective primary porosity no longer can exist.

Decarbonatization may create significant quantities of secondary porosity during immature mesodiagenesis, particularly in mineralogically stable sandstones. However, the average addition of carbonate material to the sandstones in this diagenetic stage vastly exceeds the average amount removed.

Decarbonatization culminates during mature mesodiagenesis and it greatly surpasses carbonatization. Most secondary porosity of sandstones, therefore, originates after effective primary porosity has been lost. Fractures and irreducible lamellar porosity apparently provide sufficient access for decarbonatizing fluids to start the leaching process even in nonporous and nonpermeable sandstones. Enormous volumes of carbonate move upward in solution from diagenetically mature sandstones, and are, at least in part, reprecipitated in diagenetically immature sandstones. Within a subsiding prism of clastic sediments much of the carbonate content is recycled in this fashion as sandstones at intermediate depths are enriched in carbonate.

In the maturation of organic matter the main phase of hydrocarbon generation follows the culmination of decarboxylation. For this reason primary migration of hydrocarbons commonly follows closely after formation of secondary porosity. This close association of source and reservoir in time and space favors the accumulation of hydrocarbons in secondary porosity.

In the presence of water, secondary porosity gradually is reduced during deep burial, although at a much slower rate compared to primary porosity.

The main geologic and economic significance of secondary porosity in sandstone is that it extends the depth range for permeable sandstone porosity far below the depth limit for effective primary porosity.

AAPG Search and Discovery Article #90969©1977 AAPG-SEPM Rocky Mountain Sections Meeting, Denver, Colorado