Table Of ContentIntroduction to deepwater
sedimentary environments
Rev: 1.1 10/01/01
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Table of contents
INTRODUCTION TO DEEPWATER SEDIMENTARY ENVIRONMENTS........................1
Table of contents...........................................................................................................................2
Weathering and erosion.................................................................................................................3
Deepwater marine environments...................................................................................................4
Deepwater Depositional processes................................................................................................5
Transportation agents....................................................................................................................6
Turbidity currents..........................................................................................................................8
Debrites.........................................................................................................................................9
INFLUENCING CHARACTERISTICS.....................................................................................10
Geological characteristics of deep marine environments............................................................10
Formation characteristics............................................................................................................13
Formation Characteristics: Rules of thumb.................................................................................16
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Weathering and erosion
Mechanisms of weathering and erosion cause sediments to formed where they are
eventually transported and deposited in differing environments as can be seen from
figure 1.
Figure 1: Classic Sedimentary environments
When considering deepwater sediments it is also
important to note that sediment grains are
modified the further the distance they are
transported. This is illustrated in figure 2.
Figure 2: Sediment transporation
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Deepwater marine environments.
Rotary drilling in deepwater marine environments begins with the drilled cuttings returned
to the seabed using sea water and viscous mud sweeps pumped through the drill string
and out through the wellbore. Here the Sedimentary Environments drilled are broadly
described as “Shallow to Deep marine.”
Figure 3: Typical Atlantic passive continental margin.
Shallow marine
“Shallow” marine extends from the shore to the edges of the continental shelves. Lime,
mud’s and sands are the principle solids deposited.
Deep marine
“Deep” marine characterises the deep oceans beyond the continental slopes and
include deep sea fans and abyssal plains. Sands and mud’s are the principle sediments
deposited
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Deepwater Depositional processes.
Fig. 3 presents that specific factors are contributory to sediments transportation and
deposition to result in the deepwater sedimentary rock and facies produced. On a gross
scale, overriding factors are;
1. Depositional processes
2. Depositional environment
3. Tectonic content
4. The climate.
In deepwater specific geological factors therefore combine to form a unique identity to
the sediments deposited that ultimately have then to be drilled to find and develop
commercial hydrocarbons. A principle fundamental for driller’s to appreciate.
The environment by which sediments are transported however change as sub sea
topography will vary from region to region. E.g. West of Shetland, Gulf of Mexico, West
Africa, Brazil deepwater environment do have individual characteristics. However in most
deep open water environments, tectonic content in the first three to four thousand feet
below the mudline is limited, although effects of storms, earthquakes etc. (slumping, mass
shifting of sediments) can play their part in sediment deposition.
Finally climate, notably
water temperature and
overburden of the water
itself does however play a
significant part as to how
sediments are deposited
and to the diagenetic
effects that take place. I.e.
Once deposited, sediments
are subjected to processes of
diagenesis, that is physical,
chemical, and biological
processes, that result in
compaction, cementation, re-
crystallisation and other
modifications to the original
Figure 4: Sedimentary processes sediment. In deepwater
these processes result in
sediments of differing characteristics in relation to other shallower water drilling
environments.
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Transportation agents.
The agent for transportation of deep open water sediments is through a sequence of
repeating sliding, slumping, sediment gravity flows, and in particular ocean (turbidity)
currents, debris flow, and less important grain flows and fluidised sediment flows.
In addition ocean bottom currents produced through thermo line density difference can
transport and re-work sediments. Apart from these re-sedimentation processes deeper
water environments are sites of pelagic and hemi pelagic deposition.
Slides and slumps
Slides and slumps (see fig 5) involve small to large masses of sediment, with more
internal deformation, occurring in slumps. Slumps may develop into sediment-gravity
flows. Slides and slumps are typical for slope environments and give rise to scars and
discontinuities in generally evenly bedded fine-grained sediments.
Slope failure
Slope failure, generating slumps and sediment gravity flows can be induced by
earthquake shocks, but also by storm wave loading. Over steepening of slopes by rapid
sedimentation is also important.
Sedimentary gravity flows
Five types of sedimentary gravity flows are recognised based on their rheology (liquid vs.
plastic behaviour) and particle support mechanism. (table 1; Lowe 1982.)
Table 1: Classification of laminar sediment gravity flows based on flow rheology and particle
suport mechanisms.
Flow behaviour Flow type Sediment support mechanism
Fluid Turbidity current Fluid turbulence
Fluidised flow Escaping pore fluid
Liquefied flow Escaping pore fluid
Plastic Grain flow Dispersive pressure
Matrix density and strength
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- Turbidity currents, sediment is supported buy the fluid turbulence and low-high
density flows can be distinguished.
- Fluidised flows, sediment is supported by upward –moving pore fluid.
- liquefied flows, sediment is not fully supported; the grains settle through the fluid
which is displaced upwards.
- grain flows sediment is supported by the dispersive pressure arising from grain
collisions.
- Debris flows also called mud or cohesive flows, are ones where the sediment is
supported by a cohesive matrix. Sediment is deposited from decelerating gravity
flows by two different mechanisms. In fluid flows grains are deposited individually,
either from the bed load (traction sedimentation) or from suspension, so that
deposition takes place from the base of the bed upwards. With debris flows , the
flow freezes as the shear stress falls below the yield strength of the moving
material , so that deposition takes place en masse or from the outside inwards.
Figure 5: How turbidity currents develop
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Turbidity currents.
Turbidity currents are the most important of the sedimentary gravity flows and they
deposit sediments with characteristic internal structure that change along the
transportation path.
High density turbidity
High density turbidity currents can carry
gravel and coarse sands, mostly in the form
of a traction carpet at the base of the flow
and in suspension just above. Fluid
turbulence, dispersive pressure from grain
collisions, and finer sediment exerting a
matrix buoyancy life, keep the gravel and
sands moving until the flow decelerates
through increasing slope or dilution.
Low density turbidity
Low density turbidity currents transport
sediment up to medium sands, kept in
suspension by the fluid turbulence. As the
flow decelerates sediment is moved as bed
load in a traction carpet. The deposits of
these currents are commonly “classic”
turbidites. Note: Typical bed thickness for each
type is in the range 0.3m (1ft) – 1m (3ft). (Based
on Lowe, 1982) Figures 5 & 6 illustrate how
turbidity currents typically form in the ocean.
Currents can erode, and transport large
quantities of sand, mud, deposits etc. that
Figure 6: Turbidity sequence
are transported and deposited down the
continental slopes and into the Abyssal
plains.
Turbidite sequences can be less than one metre to several metres thick and can be
traced for many miles (fig 6.). Not all sub units may be present and exact patterns vary
with differing environments.
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Debrites
Finally, of the other sediment gravity-flow
deposits, debris-flow deposits (debrites) are
the more significant.
The strength of the muddy matrix in some
debris flows is sufficient to transport block
many metres across. Debrites are typically
mud-dominated with scattered clasts, in a
matrix-support fabric.
Due to the relative geological time period
involved and depositional age of the
sediments they are deposited in nearly
horizontal layers. Typical sequences are
illustrated in figure 7 and are typical of
“open water” formation sequences that may
be encountered in the drilling process.
Figure 7: Debrite flow sequences
Thus understanding the depositional region
can be appreciated to be fundamental even
for drillers.
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Influencing characteristics
Geological characteristics of deep marine
environments.
In the deep open water marine environments, formations drilled are composed of
sedimentary rocks, i.e. Sands, Muds silts and limestone, and as previously illustrated are
deposited mainly from suspension, turbidity currents and debrites to deposit massive,
laminated and graded beds.
All other factors being equal, Sandstone are classed as medium strength, Shale, high
and limestone very high. Note: Sandstone exhibits the lowest compressive and tensile rock
strength. This is a key fundamental in deepwater drilling when considering pore and
fracture pressure understanding, and evaluating both in-situ stresses, rock and stability
mechanics.
Other key influencing characteristics presented when drilling such sedimentary
stratigraphy in the open water environments are the depositional process, depositional
environment, porosity, permeability and the stresses of the formation(s). The main
process after the depositional sequence occurs in relation to the formations ultimately to
be drilled in the open water below the seabed, is burial and then compaction. This is
especially important in the mud rocks where compaction expels water, reducing the
thickness of the sediment by a factor up to ten.
When initially deposited, mud rocks may contain 70-90% water by volume compaction
through overburden, reducing to approximately 30% at a burial depth of 1Km below the
seabed. Much of the water contained within the rocks is not then free pore water but is
contained in the lattice of the clay minerals and absorbed by the clays. As water depth
increase however the overburden of the water itself results in much more water being
contained in the lattice of the clay minerals and absorbed by the clays that would occur
at shallow depths. Thus a 50m interval of clay minerals will tend to have much > porosity
and permeability that would be exhibited if it was deposited in shallower waters.
Sedimentary rocks drilled in open water environments can therefore be concluded to
have much higher porosity and permeability’s and are probably not as fully compacted or
cemented, than formations buried and compacted more deeply. The overall formation
integrity of sedimentary rocks can therefore be concluded to be poor.
While drilling, every measure should therefore be taken to ensure that the well bore
remains intact to maintain both hole quality and to prevent well instability, hole collapse
or enlargement. In that, preventative measures while drilling and tripping must ensure
that formations are not eroded, enlarged, mechanically or hydraulically agitated
sufficiently to cause formations to become unstable or collapse. backreamer’s take note!
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