Limitations

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Weathering and transportation is followed by the sedimentation of
material. The
depositional environment can be defined as an area with a typical
set of physical,
chemical and biological processes which result in a specific type of
rock. The
characteristics of the resulting sediment package are dependent on
the intensity and
duration of these processes. The physical, chemical, biological and
geomorphic variables
79
show considerable differences between and within particular
environments. As a result,
we have to expect very different behaviour of such reservoirs
during hydrocarbon
production. Depositional processes control porosity, permeability,
net to gross ratio,
extent and lateral variability of reservoir properties. Hence the
production profile and
ultimate recovery of individual wells and accumulations are heavily
influenced by the
environment of deposition.
For example, the many deepwater fields located in the Gulf of
Mexico are of Tertiary age
and are comprised of complex sand bodies which were deposited
in a deepwater turbidite
sequence. The BP Prudhoe Bay sandstone reservoir in Alaska is
of Triassic/Cretaceous
age and was deposited by a large shallow water fluvial-alluvial fan
delta system. The
Saudi Arabian Ghawar limestone reservoir is of Jurassic age and
was deposited in a
warm, shallow marine sea. Although these reservoirs were
deposited in very different

depositional environments they all contain producible
accumulations of hydrocarbons,
though the fraction of recoverable oil varies. In fact, Prudhoe Bay
and Ghawar are amongst
the largest in the world, each containing over 20 billion barrels of
oil.
There exists an important relationship between the depositional
environment, reservoir
distribution and the production characteristics of a field (Figure 5.3).
Depositional Reservoir Production
Environment Distribution Characteristic
Deltaic
(distributary channel)
Isolated or stacked channels
usually with fine grained
sands. May or may not
be in communication
Good producers; permeabilities
of 500-5000mD. Insufficient
communication between
channels may require infill
wells in late stage of development
Shallow marine/
coastal (clastic)
Sand bars, tidal channels.
Generally coarsening
upwards. High subsidence
rate results in 'stacked'
reservoirs. Reservoir
distribution dependent
on wave and tide action
Prolific producers as a result
of 'clean' and continuous
sand bodies. Shale layers
may cause vertical barriers

to fluid flow
Shallow water
carbonate (reefs &
carbonate muds)
Shelf (clastics)
Reservoir quality governed
by diagenetic processes
and structural history
(fracturing)
Sheet-like sandbodies
resulting from storms or
transgression. Usually thin
but very continuous sands,
well sorted and coarse
between marine clays
Prolific production from
karstified carbonates.
High and early water production
possible. 'Dual porosity' systems
in fractured carbonates.
Dolomites may produce H2S
Very high productivity but high
quality sands may act as
'thief zones' during water or gas
injection. Action of sediment
burrowing organisms may impact
on reservoir quality
Figure 5.3 Characteristics of selected environments
80
It is important to realise that knowledge of depositional processes
and features in a
given reservoir will be vital for the correct siting of the optimum
number of appraisal and
development wells, the sizing of facilities and the definition of a
reservoir management

policy.
To derive a reservoir geological model, various methods and
techniques are employed;
mainly the analysis of core material, wireline logs, high resolution
seismic and outcrop
studies. These data gathering techniques are further discussed in
Sections 5.3 and
2.2.
The most valuable tools for a detailed environmental analysis are
cores and wireline
logs. In particular the gamma ray (GR) response is useful since it
captures the changes
in energy during deposition. Figure 5.4 links depositional
environments to GR response.
The GR response measures the level of natural gamma ray activity
in the rock formation.
Shales have a high GR response, while sands have low responses.
PLAN VIEW
deltaic &
shallow marine
~ , A'
A .~ ~-.~..:-.~ =B== =======================
_
channel
SECTION GR Log
~• •,,:,,,:1, :,1, ,:.,.:,, .: ,l i:i~i , • ~ ~ . :. :.. :. : . : ~ ~
~ ~ : ' , ' , ' ' , ' ' - - - - ~ 2 . . . . . . . . . . . . . . 2222
A I A'
0 150
"Bell" shape
":."':-"'.'" "- ~S
0eta Funne sha0e
B B'
Figure 5.4 Depositional Environments, sand distribution and GR
log response

81
A funnel shaped GR log is often indicative of a deltaic environment
whereby clastic,
increasingly coarse sedimentation follows deposition of marine
clays. Bell shaped GR
logs often represent a channel environment where a fining upwards
sequence reflects
decreasing energy across the vertical channel profile. A modern
technique for
sedimentological studies is the use of formation imaging tools
which provide a very
high quality picture of the formations forming the borehole wall.
5.1.2 Reservoir Structures
As discussed in Section 2.0 (Exploration), the earth's crust is part
of a dynamic system
and movements within the crust are accommodated partly by rock
deformation. Like
any other material, rocks may react to stress with an elastic, ductile
or brittle response,
as described in the stress-strain diagram in Figure 5.5.
03
yie_ld_ p~,o~~ ductile
e l a s t i c - ~ ,,mit / \
Strain
Figure 5.5 The stress - strain diagram for a reservoir rock
It is rare to be able to observe elastic deformations (which occur for
instance during
earthquakes) since by definition an elastic deformation does not
leave any record.
However, many subsurface or surface features are related to the
other two modes of
deformation. The composition of the material, confining pressure,
rate of deformation
and temperature determine which type of deformation will be
initiated.

If a rock is sufficiently stressed, the yield point will eventually be
reached. If a brittle
failure is initiated a plane of failure will develop which we describe
as a fault. Figure 5.6
shows the terminology used to describe normal, reverse and
wrench faults.
Since faults are zones of inherent weakness they may be
reactivated over geologic
time. Usually, faulting occurs well after the sediments have been
deposited. An exception
to this is a growth fault (also termed a syn-sedimentary fault),
shown in Figure 5.7. They
are extensional structures and can frequently be observed on
seismic sections through
deltaic sequences. The fault plane is curved and in a three
dimensional view has the
shape of a spoon. This type of plane is called listric. Growth faults
can be visualised as
submarine landslides caused by rapid deposition of large quantities
of water-saturated
82
sediments and subsequent slope failure. The process is continuous
and concurrent
with sediment supply, hence the sediment thickness on the
downthrown (continuously
downward moving) block is expanded compared to the upthrown
block.
'~
al Fault ~ _ ~
"Thrust Fault" if displaced over "~.
long distance (km range)
Wrench Fault
Figure 5.6 Types of faulting
A secondary feature is the development of rollover anticlines which
form as a result of

the downward movement close to the fault plane which decreases
with increasing
distance from the plane. Rollover anticlines may trap considerable
amounts of
hydrocarbons.
Growth faulted deltaic areas are highly prospective since they
comprise of thick sections
of good quality reservoir sands. Deltas usually overlay organic rich
marine clays which
can source the structures on maturation. Examples are the Niger,
Baram or Mississippi
Deltas. Clays, deposited within deltaic sequences may restrict the
water expulsion during
the rapid sedimentation / compaction. This can lead to the
generation of overpressures.
Fault plane Axis of rollover anticline
t.:.:.:.:.:.:.:.:.:.:.:. ~

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