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Exploration Insights | 19
The Ongoing Value of Outcrop Geology in
Exploration and Production Studies
by: Mike Simmons
A Triassic outcrop at Landram Bay, southern United Kingdom, an excellent reservoir analogue.
For most geoscientists, it is the examination of
rocks in the field that forms the turning point
in their understanding. Theories and concepts
learned in the classroom come to life when the
geoscientist is confronted with an outcrop. This
is true for all branches of geoscience, not least,
petroleum geology. Petroleum geoscientists are
engaged in building models of the subsurface,
yet they often turn to surface outcrops to develop
insight. During times of cost saving and concerns
regarding health and safety, the value and
necessity of field trips and outcrop studies can be
challenged. This article argues that there is still
much value to be gained from outcrop geology.
In particular, two key themes will be explored:
the deepening and reinforcement of skills; and
outcrops as analogues and data points to reduce
uncertainty in building models of the subsurface,
at any scale.
OUTCROPS FOR SKILLS
DEVELOPMENT
Fieldwork forms a key component of the training
of a geoscientist. It would be impossible to
become a geoscientist without spending
time mapping and describing outcrops, and
sampling them for subsequent detailed study.
Geoscientists learn geology in the field through
the process of deduction. They observe, and
then interpret these observations within the
framework of the theory of geological processes,
progressively developing an understanding
of the interpretation of an outcrop, or series
of outcrops. Attainment of this logical ability
(observe — describe — interpret — integrate) is
extremely important in the career progression
of a geoscientist, and is the skill set used at the
office workstation, as well as in the field.
Geology is a diverse subject and a university-
based training is only the beginning of a lifetime
of learning. For the petroleum geoscientist, there
are countless variations of sedimentary facies
and tectonic structures to understand and relate
to subsurface data, such as wireline logs and
seismic. There is a famous maxim, “The best
geologist is, other things being equal, one who
has seen most rocks” (Read, 1940). This implies
that geoscientists can never rest on their laurels
— they need to go out into the field and see more
rocks. By doing so, they reinforce their existing
knowledge and deepen their understanding.
Typically, this is achieved through discussion with
colleagues. to flow when the reservoir is in production.
In this example, the rock unit was deposited
in a braided stream environment on an arid
continental flood plain. The outcrop mostly
represents a series of cross-cutting channels that
can have permeability barriers at their bases.
Geology is a visual subject, so there is no better
place to discuss and understand, for example,
the internal architecture of a reservoir, than by
discussion in front of a relevant outcrop analogue
(Figure 1). Ideas and knowledge are shared
and new insights are developed. Figure 1 is an
outcrop of the Triassic Sherwood Sandstone, the
main reservoir in the giant Wytch Farm oilfield in
southern England. By studying this outcrop and
understanding the depositional environment it
represents, geoscientists and reservoir engineers
are immediately able to visualize the types of
sedimentary architecture that a realistic reservoir
model should contain. Especially important are
the lateral and vertical distribution of porous
and permeable units, versus those of the non-
permeable units that will be barriers and baffles It is not only petroleum geoscientists who
can benefit from visiting outcrops. Reservoir
engineers and drillers can also develop a deeper
understanding of the rocks they are trying to
produce from, or drill through, by visiting suitable
analogue outcrops. In fact, everyone involved in
the exploration and production process can gain
value from developing a ‘geological mindset.’
The subsurface is seldom layer-cake in its
organization. This becomes apparent in the field.
Put simply, the complexities and uncertainties
of subsurface geology at both exploration and
production scale become clear through the
study of outcrops. For example, how certain
are predictions of the occurrence of reservoir or
source rocks away from data points? What are
the complexities in the internal organization of