Geology Reference
In-Depth Information
The search for hydrocarbons, onshore and offshore,
nevertheless remains by far the largest single application
of reflection surveying. This reflects the particular
strength of the method in producing well-resolved
images of sedimentary sequences down to a depth of
several kilometres.The method is used at all stages of an
exploration programme for hydrocarbons, from the
early reconnaissance stage through to the detailed map-
ping of specific structural targets in preparation for ex-
ploration drilling, and on into the field development
stage when the overall reservoir geometry requires fur-
ther detailing.
Because of its relatively high cost, three-dimensional
seismic surveying still does not find routine application
in hydrocarbon exploration programmes. However,
whereas it was originally used only at the field develop-
ment stage, it now finds widespread application also at
the exploration stage in some oilfields. Vertical seismic
profiling is another important technique that is being
applied increasingly at the stage of oilfield development
because of its ability to reveal subsurface detail that is
generally unobtainable from surface seismic data alone.
In the quest for ever more detailed subsurface informa-
tion, three component (3C) surveys are becoming more
common. The value of repeated surveys during oilfield
production is now established and 'time lapse' or 4D
surveys are also increasing in usage.
The initial round of seismic exploration for hydrocar-
bons normally involves speculative surveys along
widely-spaced profile lines covering large areas. In this
way the major structural or stratigraphic elements of the
regional geology are delineated, so enabling the plan-
ning of detailed, follow-up reflection surveys in more
restricted areas containing the main prospective targets.
Where good geological mapping of known sedimentary
sequences exists, the need for expenditure on initial
speculative seismic surveys is often much reduced and
effort can be concentrated from an early stage on the
seismic investigation of areas of particular interest.
Detailed reflection surveys involve closely-spaced
profile lines and a high density of profile intersection
points in order that reflection events can be traced reli-
ably from profile to profile and used to define the pre-
vailing structure. Initial seismic interpretation is likely
to in-volve structural mapping, using time-structure
and/or isochron maps (Section 4.14.1) in the search for
the structural closures that may contain oil or gas. Any
closures that are identified may need further delineation
by a second round of detailed seismic surveying before
the geophysicist is sufficiently confident to select the lo-
cation of an exploration borehole from a time-structure
map. Three-dimensional seismics may need to be em-
ployed when critical structural details are unresolved by
interpretation of the two-dimensional survey data.
Exploration boreholes are normally sited on seismic
profile lines so that the borehole logs can be correlated
directly with the local seismic section. This facilitates
precise geological identification of specific seismic re-
flectors, especially if vertical seismic profiling surveys
(Section 4.13) are carried out at the site of the borehole.
Particularly in offshore areas, where the best quality
seismic data are generally obtained, the methods of
seismic stratigraphy (Section 4.14.2) are increasingly
employed on sections displaying seismic sequences to
obtain insight into the associated sedimentary lithologies
and depositional environments. Such stratigraphic in-
formation, derived from seismic facies analysis of
the individual sequences, is often of great value to an
exploration programme in highlighting the location
of potential source rocks (e.g. organic-rich mudstones)
and potential reservoir rocks (e.g. a deltaic or reef facies).
The contribution of reflection surveying to the devel-
opment of hydrocarbon reserves does not end with the
discovery of an oil or gas field. Refinement of the seismic
interpretation using information from, variously, addi-
tional seismic profiles, three-dimensional seismics and
vertical seismic profiling data will assist in optimizing the
location of production boreholes. In addition, seismic
modelling (Section 4.14.3) of amplitude variations and
other aspects of reflection character displayed on seismic
sections across the producing zone can be used to obtain
detailed information on the geometry of the reservoir
and on internal lithological variations that may affect
the hydrocarbon yield. 4D surveying of producing fields
(Section 4.12) has demonstrated that the detection of
unexploited areas in a producing field is feasible and
adequately repays the cost of the geophysical survey.
Examples of seismic sections from hydrocarbon fields
in the North Sea area are shown in Figs 4.61 and 4.62.
Figure 4.61 represents a seismic section across the North
Viking gas field in the southern North Sea. The gas is
trapped in the core of a NW-SE trending anticlinal
structure that is extensively faulted at the level of the
Lower Permian. A typical combined structural/strati-
graphic trap in the northern North Sea is represented by
the Brent oilfield structure, and Fig. 4.62 illustrates a
seismic section across the field. A tilted fault block con-
taining Upper Palaeozoic, Triassic and Jurassic strata is
overlain unconformably by Upper Jurassic, Cretaceous
and Tertiary sediments. Two Jurassic sands in the tilted
