4 Geoscientific Workflow Process in Drilling a Deep-Water Well,
Offshore Morocco
3
Van Dyke, Staffan*
Vanco Energy Co.
Papers
Houston, Texas
* Current address: Aera Energy LLC
Bakersfield, California
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Abstract
Author
Analysis for placement of a well in deep water but is relatively insignificant and therefore not consid-
ered to be a potential geohazard.
typically begins with a thorough study of the seafloor.
This is followed by a shallow geohazard report, which Gullies and canyons are the most prominent fea-
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tures in the study area. They include active modern
is used to identify zones of instability in the shallow
sediment pathways, which may be subject to slumps
subsurface (faulting, over-pressured zones, etc.).
and slides and therefore may negatively impact nearby
Help
An example from a 415.55 km2 (160.44 mi2) 3D seabed structures. Older groups of buried channels that
seismic survey, offshore Morocco, is presented. Ampli- may be sand prone and/or associated with pore pressure
tude extraction and stratal slice maps were generated anomalies were also mapped.
Print
within the focus area of the Ras Tafelney 3D seismic Sand-rich facies in the near seafloor sediment
8.5 x 11
data-set volume. Three horizons have been mapped in column are not in themselves hazards but should be
the subsurface to track reflection events that showed characterized because of the potential for problems
bright positive amplitudes. In the survey area, the main related to setting casing points. Sandy facies are also
potential hazards appear to be active sediment path- host to shallow water-flow conditions, shallow gas res-
ways (gullies) and shallow sands, both of which can be ervoirs, and hydrates.
the site for shallow water-flow conditions. Minor fault- Improvements in estimating drilling risk and
ing is present through different stratigraphic intervals costs that could be carried out include the analysis of
Reservoir Characterization: Integrating Technology and Business Practices
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4 offset logs, velocity data, sediment properties, and pres- tions and other displays that may reduce drilling risk
sure data. In concert with the existing seismic data, and costs.
these data can be used to create pore pressure cross sec-
3
Introduction
7 Emerging technologies, such as 3D modeling and ity and content. Thus, the subsequent interpretations
visualization, can be used in conjunction with funda- play a major role in determining potential timing and
mental geoscience principles to locate potential zones migration of the hydrocarbons, as well as determining
of interest for deep- water reservoir targets. Potential
the locations for acceptable source and reservoir rocks.
Papers drilling locations are determined after regional geologi-
Despite the lack of hard data in these frontier areas,
cal studies, sedimentological and stratigraphic studies,
when sound geological principles are applied, the final
possibly field work, basin modeling, and all interpreted
Start interpretations can make a target very attractive for
geophysical data. Typically in frontier areas, the work
is highly interpretative due to the lack of data availabil- drilling.
Author
Methodology
The 3D model that was developed for this study possible reservoir targets. This comparison is important
Search compared the character of the images in a shallow geo- because shallow seismic records contain a larger com-
hazards seismic dataset to that of deeper seismic ponent of high-frequency data than do the underlying
reflections records, to form a more complete under- exploration seismic records, thus providing improved
standing of the underlying strata and to identify interpretation of the deeper data.
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Study area
The first deep-water well drilled offshore drilled in May, 2004, and was operated by Vanco
Energy Company in its Ras Tafelney block (Fig. 1).
Morocco, the Shark B-1 was located at latitude
31.00846º N and longitude 11.18078ºW, 5n a water Analysis for placement of the Shark B-1 well
depth of 2,120 meters (based on 1,493 m/sec water commenced with a thorough study of the seafloor. This
velocity), and had a TD of 4,162 meters. The well was was accomplished through a shallow geohazard 3D
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Geoscientific Workflow Process in Drilling a Deep-Water Well, Offshore Morocco
4 seismic survey, which was used to identify zones of located offshore northwest Africa in waters adminis-
instability in the shallow subsurface (faulting, over- tered by the country of Morocco. The live data area is
pressured zones, etc.). A pore-pressure analysis was
approximately 3,000 km2 or about 1,160 mi2. Ampli-
3 done at the same time to complement interpretations
tude extraction and stratal slice maps were generated
from the shallow hazard study. These studies were then
within the focus area of the Ras Tafelney 3D seismic
used in tandem to construct a robust 3D geological/geo-
7 physical model to help determine the character and dataset, delineated within the southern region of the full
quality of the potential underlying reservoir strata. seismic volume, bounded to the east by inline 100, to
The following figures and content contained in
the west by inline 1000, to the south by crossline 1000,
this paper are based on the shallow geohazard survey
Papers to the north by crossline 2600, and covers an area of
and the pore pressure study from the Shark B-1 well
415.55 km2 or 160.44 mi2 (Fig. 2).
area. The area of interest surrounding Shark B-1 is
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Pore pressure analysis and shallow geohazard survey
The pore pressure analysis was carried out on a Jointly, the pore pressure analysis and the 3D
Author
smaller data set, having an area of 25 km2, and centered seismic volume analysis summarized slope instability
issues related to bathymetry, shallow sediment strength
over the well location. The initial seismic velocity vol-
ume contained 25 migration velocity functions over the and coherency, and shallow sediment stratigraphy,
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25 km2 target area. These functions were corrected for including identification of sand prone intervals. In addi-
water velocity variations, interpolated and smoothed to tion, the study provided evidence for shallow water
create an interval depth velocity volume. Ultimately,
Help flow conditions and issues related to sand control. The
40,401 velocity functions were analyzed for pore pres- shallow geohazard study focused primarily on visualiz-
sure anomalies. Each function in the volume was
ing seismic anomalies and mapping sedimentary
analyzed independently for overburden gradient, nor-
features related to one or more of the issues listed above
mal compaction trend, and pore pressure gradient. The
(Fig. 3). Specifically, the geohazards study presented
velocity function at the proposed location was extracted
maps and slices over the first several hundred meters of
from the volume and was analyzed for fracture gradi-
the sedimentary column as well as interpretation and
ent, kick tolerance, and maximum sustainable column
height. commentary.
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4 Geologic setting
Figure 4 is a composite seismic line of 2D and 3D region of the continental margin. The angle of the slope
data transecting part of the slope environment. The of the water bottom in this area averages about 2
3 composite line shows substantial vertical exaggeration. degrees. Updip, 95 km from the study area, is the shelf/
The 3D seismic dataset rests in the lower slope/rise slope break.
7 Environments of deposition and facies description
Figure 5 shows an arbitrary northwest-southeast groups of buried channels that may be sand prone and/
seismic line. The top picture represents the uninter- or associated with pore pressure anomalies also have
Papers
preted line, while the lower picture shows the same been mapped. These buried channels are grouped
seismic line with interpreted seismic-depositional within three major channel-belt systems. Small-scale
facies overlain. Five main channel-belt facies are recog- fans and overbank features are associated with some of
Start nized in the dataset; they are channel-fill facies, the gullies. These regions can be the site of rapid depo-
proximal levee facies, distal levee facies, sheet fan sition of coarse clastics. Rapid deposition of the sand
facies, and a debrite facies. The interpretation shows and burial of an impermeable shale is a condition asso-
Author aggradationally stacked channel-fills encased within a ciated with enhanced risk of shallow water flow.
relatively dim section interpreted to represent shale
The numerous modern and buried channel-fill
deposited by hemipelagic settling.
Search deposits also are the site of strong negative amplitude
The channel-fill facies is the most prominent in
values that appear to be caused by sand-rich fans, chan-
the study area (Fig. 6). Channels may be active modern
nel axes, and overbank deposits. This paper documents
seafloor sediment pathways and may contain channel-
Help the facies distribution and depositional environment of
fill subfacies, such as slumps and slides, and therefore
may negatively impact nearby seabed structures. Older the entire study area.
Seismic interpretation
Figure 7 shows a close-up view of the water bot- location, another lies south-southeast, and the third lies
tom amplitude extraction; the location of the proposed directly northeast of the well location. Sediment waves
Shark B well is indicated. Three slumps are clearly vis- can be clearly seen on the seafloor. An amplitude
ible on the surface; one lies northwest of the well extraction map is useful for revealing hazards at that
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Geoscientific Workflow Process in Drilling a Deep-Water Well, Offshore Morocco
4 interface, but it is designed also to see beneath the sur- horizons are: H100 horizon (blue); H160 horizon
face, at least for a few hundred meters. Clearly, a time (green); H180 horizon (pink); and H200 horizon (yel-
slice at 1.0 seconds would not follow a stratal surface. low) (Fig. 6). Also, it should be noted that the faint sub-
3 Therefore, the four horizons mapped for the shallow
vertical lines seen on most amplitude extraction maps
geohazard survey have been used as datums to flatten
and stratal slices are the acquisition footprint from the
the cube volume at these horizons in order to compare
7 with deeper horizons (discussed below). These four 3D seismic survey.
Depositional history of shallow subsurface strata via stratal slices and amplitude
extractions
Papers
All of the amplitude extractions in this paper denoted by green arrows, while the proposed well loca-
show sedimentological features that help to describe the tion is marked by the yellow dot. At this depth,
depositional history of the area. Color bars have been unconsolidated and unstable conditions should be
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chosen that are meant to heighten the contrast between expected while drilling.
anomalous amplitudes and the background reflectivity. Figure 9 is an amplitude extraction map gener-
Strong negative reflection values indicate coarser mate- ated on the H100 horizon. The slump feature in the
Author
rial. The area around the channels is typically northwest now encroaches onto the wellsite location.
quiescent, therefore positive reflection values are This region may once again cause problems for drilling
shown in white (for monochrome extractions) and pur- as the sediments may still be unconsolidated and unsta-
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ple (for full colored extractions) and strong positive ble at such shallow subsurface depths. All three channel
amplitudes are in blue (for monochrome extractions) systems are present; stronger amplitudes within these
and red (for full colored extractions). regions infer they are filled with coarse clastics. Sys-
Help
tems 1 and 2 (Fig. 6) are abruptly cut in the northern
After flattening, time slices reveal much about
regions of the study area due to an interpreted slump
the structure and properties of the shallow sediments.
block trending from the northwest to the southeast,
Figure 8 shows the result at 52 milliseconds below the
moving in a southerly direction.
flattened water bottom. Here, channels cut the seafloor,
and sediment transport direction is shown by red Figure 10 is an amplitude extraction map
arrows. The three slumps previously described are still between the seafloor and H100 horizon. It shows a pro-
present, however, one has completely encroached the nounced slump feature in the northwest region of the
well location. Their direction of mass transport is study area. System 3 shows bifurcation of its channel
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4 directly south of the well location. The region immedi- potential hazard for drilling because the sediments may
ately surrounding the well location shows a debris-flow still be unconsolidated and unstable at these shallow
character in the seismic response. Sediment waves also subsea depths.
3 appear to be present. This region is once again inter-
Figure 13 is an amplitude extraction map
preted to contain a relatively thick package of debrites
between the H180 and H200 horizons. Channel Sys-
that may cause instability while drilling.
7 tems 1 and 2 are not present within this time interval,
Figure 11 shows a time slice from the flattened but Channel 3 is present. The dominant feature, how-
cube volume generated from the H160 horizon. Figure ever, is the paleo-Agadir Canyon located in the
11 is located 52 milliseconds below the flattened hori- southern region of the study area. The region immedi-
Papers
zon and shows the prominent northwest-southeast ately surrounding the well location shows very weak
trending slump feature terminating directly west of the reflection events, interpreted to represent a thick,
Start wellsite. A large expression of a sheet fan is also inter- argillaceous sequence deposited during this time
preted to be fed from the mouth of System 3 into the interval.
southwestern part of the study area.
Figure 14 is a stratal slice located 100 millisec-
Author
Figure 12 is an amplitude extraction map onds above the H200 horizon that was generated from
between the H160 and H180 horizons. Channel Sys-
the flattened cube volume from that horizon. The main
tems 1 and 2 can no longer be seen because the conduit
Search feature is the east-west trending paleo-Agadir Canyon,
that fed these systems is not present at this time. Chan-
which exhibits extremely strong negative reflections
nel System 3 dominates the map by its strong positive
within the canyon boundaries. Another large debris-
Help reflection character; it has a broader geometry than
flow deposit occurs in the northern channel margin.
before, particularly in the southwest regions where faint
Figure 15 shows a gated window of 40 millisec-
fan-like geometry is visible. A northwest to southeast
onds at about the H200 horizon (20 ms above and 20
trending debris flow deposit dominates the relatively
ms below). The main feature manifested on the map is a
quiescent area outside of System 3’s environment. The
series of extremely strong negative reflections confined
region immediately surrounding the well location
within the paleo-Agadir Canyon, but the region around
shows stronger reflectors than previous maps and is
thus interpreted as a debris-flow. This interval may be a the wellsite remains quiescent.
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Geoscientific Workflow Process in Drilling a Deep-Water Well, Offshore Morocco
4 Economics of shallow hazard survey
Most shallow hazard surveys are accompanied by Typically, a branch on the diagram is representative of
a pore-pressure analysis. Pore pressure imaging is a particular horizon (geologic time or event) and its
3 based on mapping seismic velocity to pore pressure associated hazard (e.g., an over-pressured zone).
using a petrophysical model. Any uncertainty in the The best way to quantify a numerical value for
velocity analysis will impact the pore pressure predic- the economic exposure to each potential geohazard is to
7 tion, as well as the calibration of the pore pressure multiply the likelihood of the risk’s existence versus the
model. Thus, pore pressure prediction is heavily reliant total cost of alleviating the problem (e.g., 20% risk of
on a well developed, and accurate, velocity model. Typ- encountering an over-pressured zone in the Oligocene/
Papers ically this is not exact, but it can be close to exact if a Miocene boundary at a 200m depth below the mud line
high-quality seismic dataset is available and the proper at location X,Y). Historically, no oil blowout has
time is spent developing the velocity model. Particu- occurred in deep water, but the industry has spent hun-
larly in frontier regions, where little, if any, true data dreds of millions of dollars actively preventing shallow
Start
such as well logs are available, a pore pressure analysis water flows and underground blowouts. Thus far, how-
will lead to a better understanding of the shallow sedi- ever, three broached mud line gas flows and one BOP
ments and their associated geohazard risk potential. failure resulting in a gas blow-out have occurred in the
Author
deep water. The estimated costs of these four events
In order to grasp the economic impact that
totaled $40MM. During a gas or oil blowout, the loss of
accompanies a potential hazard disclosed by the shal-
the ~$1MM tool assembly and the additional 10 days to
low hazard survey or pore pressure prediction analysis,
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sidetrack and recover the lost hole, can cost the operator
a dendrogram can be drawn to gather a more complete
a significant amount of money to get back to normal
understanding of each hazard. This diagram can be
drilling operations; e.g., a minor underground blowout
accompanied by a spreadsheet that relates a quantitative
Help
can result in as much as an $8MM claim. It should be
cost to each hazard represented in the diagram; an over-
noted that many of these well control problems are
simplified dendrogram can be seen in Figure 16. Each
unique to deep-water operations.
branch, or pathway, on the dendrogram represents a
potential hazard and associated cost based on its risk Many other potential risks exist, such as encoun-
percentage; therefore there can be numerous branches tering a shallow gas pocket that might be breached, thus
on each diagram. Depending on the size and detail of releasing the gas upward, engulfing the drill ship or
the shallow hazard survey and the pore pressure predic- semi-submersible, and in a worst case scenario, sinking
tion analysis, nearly infinite branches can exist. the vessel. The costs involved with this type of disaster
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4 can be many hundred of millions of dollars, not includ- hazard reports in any region of the world where a deep-
ing the potential for loss in human life. It should be water well is to be drilled. In the example for this paper,
noted, that nowadays, total costs for some deepwater the shallow Moroccan sediments studied are considered
3 wells can be as high as $95MM, and daily rig costs can
to be relatively tight and there is little risk for over-
be >$500,000/day.
pressured zones or other anomalies that are generally
Evaluating the economic impact that these poten-
7 encountered in other deep-water regions of the world,
tial disasters can cause is a helpful and debatably
necessary exercise that should accompany all shallow such as those commonly found in the Gulf of Mexico.
Conclusions
Papers
In summation, these studies suffered from a lack could perhaps be verified. However, all of the maps and
of well control, shallow geo-boring data, and drilling stratal slices that were generated from these studies
Start and wireline data from deeper boreholes. If such data help to show the basic geometry of the channel systems
could have been obtained, then many of the assump- present, which can be inferred to deeper reservoir tar-
tions as to sediment and fluid characteristics of the area gets for their improved characterization.
Author
Acknowledgements
The author would like to thank Dr. Roger Slatt, providing the opportunity to present the material within
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Vanco Energy Company, and Aera Energy LLC for this paper.
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Figure 1. Ras Tafelney block, offshore Morocco.
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Figure 2. Ras Tafelney 3D seismic dataset with area of interest highlighted. Proposed well location is shown by yellow
circle.
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Figure 3. Example of amplitude extractions performed within shallow subsurface between H100 and H160 (see text for
explanation of symbols).
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Figure 4. West-east composite seismic transect showing shelf-slope break. Vertical exaggeration is 3X.
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Figure 5. Uninterpreted and interpreted seismic line showing channel subenvironments.
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Figure 6. Seismic line showing the 3 major channel-belt systems. Crossline 1830.
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Geoscientific Workflow Process in Drilling a Deep-Water Well, Offshore Morocco
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Help Figure 7. Maximum peak amplitude extraction of seafloor showing slumps and slides (outlined in yellow); green
arrows point in the direction of down-slope movement, sediment waves, and channels; and red arrows point in the
direction of presumed sediment transport.
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Figure 8. Flattened time slice 52 ms below the seafloor showing the features illustrated in Figure 7.
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Figure 9. Maximum peak amplitude extraction for H100 Horizon showing slumps/slides and sediment transport direc-
tions.
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Figure 10. RMS amplitude extraction between the seafloor and H100 horizon.
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Figure 11. Flattened time slice 52 ms below the H160 horizon showing slumps, channels, and sheet fan.
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Figure 12. RMS amplitude extraction between H160 and H180 horizons.
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Figure 13. RMS amplitude extraction between H180 and H200 horizons showing the paleo Agidar Canyon.
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Figure 14. Flattened time slice 100ms above the H200 horizon showing paleo Agadar Canyon and slump scar.
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Figure 15. H200 horizon amplitude extraction with 40ms gated window (20ms above and below) showing the paleo
Agadir Canyon.
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4 Geologic
Potential Projected
Boundary Rock Type Probability
Geohazard
3 Cost
Lower
7 $5 MM
Pliocene 0%
Possible
Oil
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SS
Blowout
Plio-
$8 MM
10%
Miocene
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Boundary
Sh
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Possible
$10 MM
Upper
Gas 25%
Miocene
Blowout
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LS
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Lower $15 MM
50%
Miocene
Figure 16. Simplified representation of a typical dendrogram.
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