20 | Halliburton Landmark
Exploration Insights | 21
While outcrops tend to be at the margins of the
basins being explored, and often some distance
from a prospective exploration well location,
they still provide insight into the likelihood of
the presence and quality of various petroleum
systems elements. This is because outcrops
form an essential part of palaeogeographic
mapping, which provides context for what is
being observed. For example, by correlating the
outcrop of the Jurassic Blue Lias Formation,
shown in Figure 2, with nearby onshore and
offshore wells and other outcrops, and by
integrating this information with seismic facies, it
can be shown that the character of the Blue Lias
Formation at the outcrop is very similar to that
which will be encountered in exploration acreage
in the same basin. The geoscientist can make
relevant adjustments for the tectonic history of
the outcrop belt versus the subsurface — in this
case, considering different thermal histories that
could have affected source rock maturity.
Figure 1> A group of geoscientists discussing the reservoir geology of the Triassic Sherwood Sandstone, the main reservoir of the Wytch
Farm oilfield in southern England.
Figure 3> Channelized deltaic reservoir facies of the Neogene
Productive Series at an outcrop in Azerbaijan.
a conventional or unconventional reservoir that
require consideration when planning production
well placement and the use of production
enhancement techniques?
OUTCROPS AS ANALOGUES AND
DATA POINTS
The ultimate arbiter of the validity of any subsurface
model is the drill bit. Nevertheless, to risk stating
the obvious, drilling wells and taking core samples
is expensive, as is the gathering and processing
of high-quality seismic data. Therefore, petroleum
geoscientists look to outcrops as cost-effective
analogues and data points, in order to reduce
uncertainty in their subsurface models. This can be
true at exploration scale and at production scale.
Figure 2> Organic-rich Early Jurassic mudstones of the Blue Lias Formation exposed at Lyme Regis in the Wessex Basin of southern
England. Examination of their source rock quality is useful for assessing the petroleum potential of the local sedimentary basin, provided
local tectonic effects are taken into account
At exploration scale, outcrops tend to be most
useful as data points, providing, for example,
information on source rock quality (Figure 2),
possible reservoir facies and quality, and sediment
provenance. A detailed biostratigraphic study of
outcrops can create biozonation schemes, effective
for use in the correlation of equivalent rocks in the
subsurface. Tectonic events and the structural style
of a basin can also be determined from outcrops.
At the production or reservoir scale, outcrops are
invaluable. Selection of the correct analogue can
reveal much about the likely internal architecture
of a reservoir, including the likely net to gross,
and the presence of barriers and baffles, and
zones of high porosity or high permeability. This
enables better static and dynamic reservoir
models to be built.
For example, the Triassic Sherwood Sandstone
reservoir of the Wytch Farm oilfield in southern
England can be effectively modeled by studying
outcrops of the same rock unit exposed on the
coastline some 50 km+ away (Figure 1) (Newell
and Shariatipour, 2016). Models of the Neogene
Productive Series reservoir in the South Caspian
have greatly benefited from outcrop studies
(Reynolds et al., 1996; Hinds et al., 2006) (Figure
3). These studies revealed the complex reservoir
heterogeneity that could be expected to be
encountered in the subsurface and helped plan a
suitable exploitation strategy. Once well log and
core data were gathered from the subsurface,
their interpretation was greatly aided by reference
to the outcrop observations and interpretations.
Although still useful, a hammer, field notebook,
sedimentary logging sheet, tape measure,
grainsize card, hand lens, and compass
clinometer are not the only tools that can be used