10
1 INTRODUCTION
The initial main objective of this phase is to predict the lateral extent and thickness of
porous formations adjacent to potential source rocks and within the optimum thermal
envelope. Regional sedimentological studies may thus define productivity fairways such
as a line of reefs, a delta front, or a broad belt of shoal sands. The location of individual
traps may be structurally controlled within such fairways. Sedimentology becomes more
important still when the structural traps have all been seismically located and drilled.
The large body of data now available can be used to locate subtle stratigraphic oil fields.
This is the major application of sedimentology in the oil industry and is discussed at
length in the next section.
Concluding this review of the role of sedimentology in the history of the exploitation
of a basin, it is worth noting the contribution it can make not just in finding fields, but
in their development and production. Few reservoir formations are petrophysically isotropic. Most oil fields show some internal variation not only in reservoir thickness, but
also in its porosity and permeability (see Fig. 3.14). These differences can be both vertical or horizontal and, in the case of permeability, there is often a preferred azimuth of
optimum flow (see Fig. 3.28). These variations are due either to primary depositional
features or to secondary diagenetic changes.
Primary factors are common in sandstone reservoirs. Gross variations in porosity
within a reservoir formation may relate to the location of discrete clean sand bodies
such as channels or-bars, within an overall muddy sand. Variations in the direction of
maximum flow potential (i.e., permeability) may relate to a gross sand body trend or to
sand-grain orientation. In carbonate reservoirs, on the other hand, depositional variations in porosity and permeability tend to be masked by subsequent diagenetic changes.
Hence the interest shown by oil companies in carbonate diagenesis. It is important to
understand the petrophysical variations within a reservoir. This assists in the development drilling of a field by predicting well locations that will produce the maximum
amount of petroleum and the minimum amount of water. Subsequently, secondary recovery techniques can also utilize this knowledge. Selection of wells for water or gas
injection should take into account the direction of optimum permeability within the
reservoir formation.
Concluding this review of the applications of sedimentology through the evolution of
a productive oil basin, it is important to note how close integration is necessary with
geophysics. First, to elucidate gross structure and stratigraphy of a basin. Subsequent
seismic surveys establish drillable prospects and may image petroleum accumulations.
The final phase of development drilling and production necessitates close liaison between geophysicists, geologists, and engineers. The relationship between petrography
and petrophysics is most important at this time. The preceding account of the applications of sedimentology in the search for petroleum needs to be put in perspective. A lot
of the stuff was found before most oil men could even spell "sedimentology."
1.3.2 Sedimentary Ores: General Aspects
Sedimentology has never been used by the mining industry to the extent that it has been
employed in the search for hydrocarbons (Parnell et al., 1990; Evans, 1995). There are
two good reasons for this. First, many metallic ores occur within, or juxtaposed to, igneous and metamorphic rocks. In such situations sedimentology can neither determine
1 INTRODUCTION
The initial main objective of this phase is to predict the lateral extent and thickness of
porous formations adjacent to potential source rocks and within the optimum thermal
envelope. Regional sedimentological studies may thus define productivity fairways such
as a line of reefs, a delta front, or a broad belt of shoal sands. The location of individual
traps may be structurally controlled within such fairways. Sedimentology becomes more
important still when the structural traps have all been seismically located and drilled.
The large body of data now available can be used to locate subtle stratigraphic oil fields.
This is the major application of sedimentology in the oil industry and is discussed at
length in the next section.
Concluding this review of the role of sedimentology in the history of the exploitation
of a basin, it is worth noting the contribution it can make not just in finding fields, but
in their development and production. Few reservoir formations are petrophysically isotropic. Most oil fields show some internal variation not only in reservoir thickness, but
also in its porosity and permeability (see Fig. 3.14). These differences can be both vertical or horizontal and, in the case of permeability, there is often a preferred azimuth of
optimum flow (see Fig. 3.28). These variations are due either to primary depositional
features or to secondary diagenetic changes.
Primary factors are common in sandstone reservoirs. Gross variations in porosity
within a reservoir formation may relate to the location of discrete clean sand bodies
such as channels or-bars, within an overall muddy sand. Variations in the direction of
maximum flow potential (i.e., permeability) may relate to a gross sand body trend or to
sand-grain orientation. In carbonate reservoirs, on the other hand, depositional variations in porosity and permeability tend to be masked by subsequent diagenetic changes.
Hence the interest shown by oil companies in carbonate diagenesis. It is important to
understand the petrophysical variations within a reservoir. This assists in the development drilling of a field by predicting well locations that will produce the maximum
amount of petroleum and the minimum amount of water. Subsequently, secondary recovery techniques can also utilize this knowledge. Selection of wells for water or gas
injection should take into account the direction of optimum permeability within the
reservoir formation.
Concluding this review of the applications of sedimentology through the evolution of
a productive oil basin, it is important to note how close integration is necessary with
geophysics. First, to elucidate gross structure and stratigraphy of a basin. Subsequent
seismic surveys establish drillable prospects and may image petroleum accumulations.
The final phase of development drilling and production necessitates close liaison between geophysicists, geologists, and engineers. The relationship between petrography
and petrophysics is most important at this time. The preceding account of the applications of sedimentology in the search for petroleum needs to be put in perspective. A lot
of the stuff was found before most oil men could even spell "sedimentology."
1.3.2 Sedimentary Ores: General Aspects
Sedimentology has never been used by the mining industry to the extent that it has been
employed in the search for hydrocarbons (Parnell et al., 1990; Evans, 1995). There are
two good reasons for this. First, many metallic ores occur within, or juxtaposed to, igneous and metamorphic rocks. In such situations sedimentology can neither determine