8.5 SANDSTONES
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to form the dominant cementing mineral. These minerals include barytes, celestite, anhydrite (gypsum at outcrop), halite, hematite, and feldspar.
The net effect of all these mineral cements is to diminish or completely destroy the
primary intergranular porosity and the permeability of the sandstone. Because of their
abundance and significance, the origin of carbonate, silica, and clay cements is described next.
8.5.3.3~1 Carbonate cements
Carbonate cements in sandstones consist of calcite, dolomite, and occasionally siderite.
Modern sediments have been found with cements of both aragonite and calcite (e.g.,
Allen et al., 1969; Garrison et al., 1969, respectively). These reports indicate that carbonate cements can form at surface temperatures and pressures. Their genesis does not
require high temperatures or pressures. These cements are precipitated from solutions
which gained their calcium carbonate both from connate water expelled by compaction
and from the dissolution of shells.
In ancient sandstones aragonite is largely unknown as a cement due to reversion to
calcite, the stable form of calcium carbonate. Carbonate cements range from fringes of
small crystals rimming detrital grains through sparite-filled pores, to single crystals, centimeters across, which completely envelop the sand fabric. This latter type of texture is
termed "poikilitic" or "poikiloblastic" (Plate 4A). It is easily identified in hand specimens because the sandstone tends to break along cleavage fractures which twinkle in
the sunlight. This is known as "lustre mottling." Calcite cements can, therefore, be present in a sandstone in sufficient quantities to infill all primary intergranular porosity.
Dolomite is the second common type of carbonate cement found in sandstones. It occurs typically in rhomb-shaped crystals which, by themselves, seldom completely destroy porosity. In argillaceous sandstones microcrystalline calcite, dolomite, and siderite are often present within the clay matrix. As previously discussed, the presence of a
carbonate cement indicates that the sand has been bathed in alkaline pore fluids.
Carbonate cements will form in alkaline pore fluids, irrespective of Eh, as shown in
Figs. 8.21 and 8.22, respectively. These conditions are commonplace in the subsurface,
but are especially found over breached petroleum accumulations. It is believed that bacterial oxidation of petroleum liberates CO2, causing the precipitation of carbonate mineral cements. Experienced drillers commonly report a slowdown in rate of penetration
of the bit when drilling through the cemented shale cap rock of a petroleum accumulation. This type of carbonate cemented capping, commonly referred to as an HRDZ (hydrocarbon-related diagenetic zone) can sometimes be imaged seismically (O'Brian and
Woods, 1995). When the escaping fluids reach the sea floor, carbonate mud mounds,
termed "cold seeps," form from methanogenic bacteria and higher carbonate-secreting
life forms. These have been particularly well documented from the North Sea and the
Gulf of Mexico (see Hovland et al., 1987, and Sassen et al., 1993, respectively).
The spatial distribution of carbonate cements is much more varied than that of silica (Morad, 1998). Cement distribution may be uniform, layered (Fig. 8.23A), concretionary (colloquially referred to as "doggers") (Fig. 8.23B), or occuring as envelopes at sand:shale contacts (Fig. 8.23C). Carbonate-cemented envelopes at sand:shale
363
to form the dominant cementing mineral. These minerals include barytes, celestite, anhydrite (gypsum at outcrop), halite, hematite, and feldspar.
The net effect of all these mineral cements is to diminish or completely destroy the
primary intergranular porosity and the permeability of the sandstone. Because of their
abundance and significance, the origin of carbonate, silica, and clay cements is described next.
8.5.3.3~1 Carbonate cements
Carbonate cements in sandstones consist of calcite, dolomite, and occasionally siderite.
Modern sediments have been found with cements of both aragonite and calcite (e.g.,
Allen et al., 1969; Garrison et al., 1969, respectively). These reports indicate that carbonate cements can form at surface temperatures and pressures. Their genesis does not
require high temperatures or pressures. These cements are precipitated from solutions
which gained their calcium carbonate both from connate water expelled by compaction
and from the dissolution of shells.
In ancient sandstones aragonite is largely unknown as a cement due to reversion to
calcite, the stable form of calcium carbonate. Carbonate cements range from fringes of
small crystals rimming detrital grains through sparite-filled pores, to single crystals, centimeters across, which completely envelop the sand fabric. This latter type of texture is
termed "poikilitic" or "poikiloblastic" (Plate 4A). It is easily identified in hand specimens because the sandstone tends to break along cleavage fractures which twinkle in
the sunlight. This is known as "lustre mottling." Calcite cements can, therefore, be present in a sandstone in sufficient quantities to infill all primary intergranular porosity.
Dolomite is the second common type of carbonate cement found in sandstones. It occurs typically in rhomb-shaped crystals which, by themselves, seldom completely destroy porosity. In argillaceous sandstones microcrystalline calcite, dolomite, and siderite are often present within the clay matrix. As previously discussed, the presence of a
carbonate cement indicates that the sand has been bathed in alkaline pore fluids.
Carbonate cements will form in alkaline pore fluids, irrespective of Eh, as shown in
Figs. 8.21 and 8.22, respectively. These conditions are commonplace in the subsurface,
but are especially found over breached petroleum accumulations. It is believed that bacterial oxidation of petroleum liberates CO2, causing the precipitation of carbonate mineral cements. Experienced drillers commonly report a slowdown in rate of penetration
of the bit when drilling through the cemented shale cap rock of a petroleum accumulation. This type of carbonate cemented capping, commonly referred to as an HRDZ (hydrocarbon-related diagenetic zone) can sometimes be imaged seismically (O'Brian and
Woods, 1995). When the escaping fluids reach the sea floor, carbonate mud mounds,
termed "cold seeps," form from methanogenic bacteria and higher carbonate-secreting
life forms. These have been particularly well documented from the North Sea and the
Gulf of Mexico (see Hovland et al., 1987, and Sassen et al., 1993, respectively).
The spatial distribution of carbonate cements is much more varied than that of silica (Morad, 1998). Cement distribution may be uniform, layered (Fig. 8.23A), concretionary (colloquially referred to as "doggers") (Fig. 8.23B), or occuring as envelopes at sand:shale contacts (Fig. 8.23C). Carbonate-cemented envelopes at sand:shale