8.6 RUDACEOUS ROCKS
377
chemical effects of diagenesis. This is in contrast to the second phase of sandstone diagenesis. With increasing burial the newly cemented sandstone moves down from the
shallow zone of reducing and alkali conditions into the catagenic zone. Here the sands
are flushed by connate waters expelled from compacting clays. Where the clays are rich
in organic matter the emitted fluids may contain carbonic acid. These may leach out
early carbonate cements and unstable grains, thus generating secondary porosity. The
minor importance of secondary solution porosity by decarboxylation during deep burial is clearly shown by the straight lines of the sandstone porosity gradients shown earlier in Fig. 8.17.
Once the carbonic acids have flushed through the sands, the pore fluid returns to an
alkaline state, and a second phase of carbonate cementation may take place. If burial
continues further the sandstone may enter the metagenic zone. Carbonate cements are
leached out again as the sandstone becomes a tightly cemented quartzite, clays recrystallize to mica, and the metamorphic zone has been reached. Evidence suggests that major sandstone diagenesis takes place in a relatively narrow thermal zone between 80 and
120~ For an average geothermal gradient of 2.6~
this is between 2500 and 4200 m.
Two events may interrupt this regular sequence. Petroleum invasion of pore spaces
expels connate water and preserves the rock from further cementation or solution (e.g.,
De Souza and De Assis, Silva, 1998). Thus porosity is often higher within petroleum
reservoirs than in adjacent water bearing zones (Fig. 8.35). Geophysical well logging
is beyond the scope of this text, but it should be noted that geophysical well logs can
identify cemented envelopes, unconformity motifs, and the change in porosity at a petroleum:water contact (Selley, 1992a) Diagenetic phenomena may also be imaged seismically. The seismic imaging of the zone of enhanced porosity beneath the Cimmerian
unconformity of the North Sea has been described by Ashcroft and Ridgway (1996). It
is also possible to identify petroleum'water contacts on seismic data. These are horizontal reflectors that cross-cut stratigraphic reflecting horizons. They are variously termed
"flat spots," "bright spots," and "DHIs" (direct hydrocarbon indicators). Not all seismically defined fiat spots actually occur beneath petroleum accumulations. It is believed
that these "phantom" flat spots mark paleo-petroleum:water contacts, where cementation occured beneath a petroleum reservoir. The trap leaked, allowing the petroleum
to escape, leaving the residual cemented zone to trap unwary petroleum explorationists. Francis et al. (1997) describe an excellent example of a phantom flat spot from the
Irish Sea (Fig. 8.36). Figure 8.37 summarizes the porosity and seismic responses of some
of these diagenetic phenomena.
With increasing burial temperature and pressure increase, petroleum degrades to
carbon. Clays recrystallize, quartz cement becomes pervasive and porosity is almost
completely destroyed. At any stage during burial, however, the sandstone may be uplifted, experience epidiagenetic porosity enhancement, undergo renewed burial, and be
recycled through the above diagenetic sequence (Fig. 8.38).
8.6 RUDACEOUS ROCKS
The rudaceous rocks are sediments at least a quarter of whose volume is made of particles larger than 2 mm in diameter. They grade down through the granulestones into
377
chemical effects of diagenesis. This is in contrast to the second phase of sandstone diagenesis. With increasing burial the newly cemented sandstone moves down from the
shallow zone of reducing and alkali conditions into the catagenic zone. Here the sands
are flushed by connate waters expelled from compacting clays. Where the clays are rich
in organic matter the emitted fluids may contain carbonic acid. These may leach out
early carbonate cements and unstable grains, thus generating secondary porosity. The
minor importance of secondary solution porosity by decarboxylation during deep burial is clearly shown by the straight lines of the sandstone porosity gradients shown earlier in Fig. 8.17.
Once the carbonic acids have flushed through the sands, the pore fluid returns to an
alkaline state, and a second phase of carbonate cementation may take place. If burial
continues further the sandstone may enter the metagenic zone. Carbonate cements are
leached out again as the sandstone becomes a tightly cemented quartzite, clays recrystallize to mica, and the metamorphic zone has been reached. Evidence suggests that major sandstone diagenesis takes place in a relatively narrow thermal zone between 80 and
120~ For an average geothermal gradient of 2.6~
this is between 2500 and 4200 m.
Two events may interrupt this regular sequence. Petroleum invasion of pore spaces
expels connate water and preserves the rock from further cementation or solution (e.g.,
De Souza and De Assis, Silva, 1998). Thus porosity is often higher within petroleum
reservoirs than in adjacent water bearing zones (Fig. 8.35). Geophysical well logging
is beyond the scope of this text, but it should be noted that geophysical well logs can
identify cemented envelopes, unconformity motifs, and the change in porosity at a petroleum:water contact (Selley, 1992a) Diagenetic phenomena may also be imaged seismically. The seismic imaging of the zone of enhanced porosity beneath the Cimmerian
unconformity of the North Sea has been described by Ashcroft and Ridgway (1996). It
is also possible to identify petroleum'water contacts on seismic data. These are horizontal reflectors that cross-cut stratigraphic reflecting horizons. They are variously termed
"flat spots," "bright spots," and "DHIs" (direct hydrocarbon indicators). Not all seismically defined fiat spots actually occur beneath petroleum accumulations. It is believed
that these "phantom" flat spots mark paleo-petroleum:water contacts, where cementation occured beneath a petroleum reservoir. The trap leaked, allowing the petroleum
to escape, leaving the residual cemented zone to trap unwary petroleum explorationists. Francis et al. (1997) describe an excellent example of a phantom flat spot from the
Irish Sea (Fig. 8.36). Figure 8.37 summarizes the porosity and seismic responses of some
of these diagenetic phenomena.
With increasing burial temperature and pressure increase, petroleum degrades to
carbon. Clays recrystallize, quartz cement becomes pervasive and porosity is almost
completely destroyed. At any stage during burial, however, the sandstone may be uplifted, experience epidiagenetic porosity enhancement, undergo renewed burial, and be
recycled through the above diagenetic sequence (Fig. 8.38).
8.6 RUDACEOUS ROCKS
The rudaceous rocks are sediments at least a quarter of whose volume is made of particles larger than 2 mm in diameter. They grade down through the granulestones into