Introduction - 3-D Seismic Interpretation - page 10

At this stage even a small number of 2-D seismic profiles across the basin, perhaps
tens of kilometres apart, will be very helpful in defining the general thickness of
sediments and the overall structural style.
(2) Perhaps after some initial wells have been drilled in the basin with encouraging
results, exploration moves on to a more detailed study, where the aim is to de-
fine and drill valid traps. More seismic data are needed at this stage, although the
amount depends on the complexity of the structures. Simple anticlines may be ad-
equately defined from a small number of 2-D profiles, but imaging of complex
fault architectures will often be too poor on 2-D data for confident interpretation.
If wells are fairly cheap and seismic data are expensive to acquire (as is often the
case on land) it may be best to drill on the basis of a grid of 2-D lines. If wells
are very expensive compared with seismic acquisition (the typical marine case),
then it will already be worthwhile at this stage to use 3-D seismic to make sure
that wells are correctly located within the defined traps. This might, for example,
be a matter of drilling on the upthrown side of a fault, or in the correct location on
a salt flank to intersect the pinchout of a prospective horizon. An example where
3-D seismic completely changed the structural map of a field is shown in fig. 1.6(a)
(redrawn after Greenlee et al ., 1994 ) . This is the Alabaster Field, located on a salt
flank in the Gulf of Mexico. The first exploration well was drilled on the basis
of the 2-D map and was abandoned as a dry hole, encountering salt at the antic-
ipated pay horizon. The 3-D survey shows that this well was drilled just updip
of the pinchout of the main pay interval. This is a case where seismic amplitudes
are indicative of hydrocarbon presence and are much easier to map out on 3-D
seismic.
(3) After a discovery has been made, the next step is to understand how big it is.
This is the key to deciding whether development will be profitable. At this stage,
appraisal wells are needed to verify hydrocarbon presence and investigate reservoir
quality across the accumulation. Detailed seismic mapping may reduce the number
of appraisal wells needed, which will have an important impact on the overall
economics of the small developments typical of a mature hydrocarbon province.
The next step will be to plan the development. An example of the impact of 3-D on
development planning is shown in fig. 1.6(b) (redrawn after Demyttenaere et al .,
1993 ). This shows part of the Cormorant Field of the UK North Sea, where oil
is trapped in Middle Jurassic sandstones in four separate westerly dipping fault
blocks. The left-hand side of the figure shows the initial map of one of these fault
blocks based on 2-D seismic data; the absence of internal structural complexity
led to a development concept based on a row of crestal oil producers supported by
downflank water injectors. The right-hand side of the figure shows the map of the
fault block based on 3-D seismic; the compartmentalisation of the fault block led
to a revised development plan with the aim of placing producer-injector pairs in
each major compartment.