The Role of Secondary Porosity in the Course of Sandstone Diagenesis
Secondary porosity plays an important role in the diagenesis of some sandstones. The volume of secondary porosity equals or exceeds that of primary porosity in the sandstones of many sedimentary basins worldwide, and a significant percentage of the world's reserves of natural gas and crude oil are contained in secondary sandstone porosity. Prudhoe Bay Field and the Jurassic fields of the North Sea are examples of the many giant hydrocarbon accumulations in secondary sandstone porosity.
Chemical, physical, physicochemical, biochemical and biophysical processes result in secondary sandstone porosity through leaching and shrinkage of rock constituents, or through the opening of fractures and porous burrows and borings. Secondary sandstone porosity can originate anywhere in the sedimentary crust: (1) before effective burial in the environment of deposition (eogenetic)2; (2) at any depth of burial above the zone of metamorphism (mesogenetic)2; and (3) during exposure following a period of burial (telogenetic)2. Secondary porosity may occur in sandstones of any mineralogical or textural composition and of any Phanerozoic age. It is most common in sandstones that have undergone relatively long lasting, deep burial and 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 such as cements or replacements. Most of the mesogenetic decarbonatization results from the decarboxylation of organic matter in strata adjacent to the sandstone during the course of organic maturation. The process of decarboxylation leads to the generation of carbon dioxide which, in the presence of water, produces carbonic acid. This acid reacts with the carbonate minerals.
In most instances it is possible to differentiate microscopically between primary and secondary sandstone porosity. Thus it is possible to trace the loss of primary porosity during burial. In the presence of water and hydrostatic pressure, primary sandstone porosity cannot exist beyond specific limits of temperature-time exposure except for a small volume of irreducible lamellar porosity between grains. The limiting temperature-time exposure increases with increasing mineralogical stability of the sandstones and, subordinately, with increasing grain size.
The mesodiagenesis of sandstones can be divided into four stages: (1) immature—mechanical compaction; (2) semi-mature—chemical compaction of primary porosity; (3) mature—only secondary porosity present; and, (4) supermature—no effective primary or secondary porosity.
Decarbonatization may create considerable quantities of secondary porosity during semi-mature mesodiagenesis. However, the average addition of carbonate to the sandstone in this diagenetic stage usually exceeds the average carbonate removal.
Decarbonatization culminates during mature mesodiagenesis at which stage it greatly outweighs carbonatization. Much secondary sandstone porosity, 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 sandstones of low permeability.
Enormous volumes of carbonate move upwards in solution from diagenetically mature sandstones and are, at least in part, reprecipitated in immature and semi-mature sandstones. Within a subsiding prism of clastic sediments much of the carbonate content is being recycled upwards in this fashion and sandstones at shallower depths are being enriched in carbonate.
Primary migration of hydrocarbons commonly follows closely after the secondary porosity has been formed, because in the maturation of organic matter, the main phase of hydrocarbon generation follows after the culmination of decarboxylation. This close association of source and reservoir in time and space favours the accumulation of hydrocarbons in secondary porosity.
In the presence of pore water, secondary porosity is gradually reduced during deep burial although at a much slower rate compared to primary porosity.
The main geological and economic significance of secondary sandstone porosity is that it extends the depth range for effective sandstone porosity far below the generally accepted depth limit for effective primary porosity. Generation and primary migration of hydrocarbons occurs mainly below the range of effective primary porosity.
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Aspects of Diagenesis
There are a number of gaping holes in accumulated knowledge within the discipline of sedimentology. Perhaps one of the largest holes has been the general subject of diagenesis in clastic rocks. It was therefore fortuitous that two symposia covering various aspects of diagenesis (mainly in clastics) were presented a year apart in different parts of the country but with the same motivation – to contribute to the closing of that knowledge gap. Sedimentologists now have a fairly good idea of the what and the how of sediment deposition. What happens after the sediments are lithified has frequently been ignored. It was the aim of both editors of this publication to approach the subject from two different viewpoints. Schluger directed a symposium which looked mainly at clastic reservoirs, and Scholle presented a symposium which examined various aspects of paleotemperature control of diagenesis.