MTCs of the Brazos-Trinity Slope System;
Thoughts on the Sequence Stratigraphy of
MTCs and Their Possible Roles in Shaping
Hydrocarbon Traps
R.T. Beaubouef and V. Abreu
Abstract Mass Transport Complexes (MTCs) are significant constituents of the
fill of the basins of the Brazos-Trinity Slope System. These MTCs are composite
stratigraphic bodies consisting of resedimented materials associated with slumps,
slides, debris flows, but also subordinate turbidites, and hemipelagites. Although
their composition is variable, they tend to be mud-rich. The MTCs exhibit a variety
of external geometries. They commonly have erosive bases characterized by “boxy”,
grooved scours and mounded tops. Commonly, they are internally chaotic, but
sometimes show crude stratification or fabrics related to patterns of movement and/
or deformation. Because of increased awareness of the abundance of MTCs within
deep marine settings and their association with other well known architectural elements, recent attempts have been made to incorporate the occurrence of MTCs into
predictive sequence stratigraphic models. The models emphasize the role of relative
sea level cycles in the generation of MTCs and relate this to formative processes and
positions within an idealized depositional sequence. However, while some BrazosTrinity MTCs may conform to these concepts, others do not. As such, utility of
MTCs for developing sequence stratigraphic frameworks for the Brazos-Trinity system is dubious. Based on our understanding of the Brazos-Trinity Slope System it is
proposed that MTCs play a role in shaping hyrdrocarbon traps in analogous subsurface systems. Although some portions of these MTCs contain sufficient porous and
permeable sediment to constitute hydrocarbon reservoirs, most represent seal and
“waste rock” units. They can form basal, vertical and lateral seals to reservoir complexes, as well as intra-reservoir seals and baffles. Because of their erosive nature,
the emplacement of MTC’s has often resulted in localized or widespread termination
of reservoir strata. MTC’s with clay-rich lithologies form low permeability, high
capillary entry pressure layers overlying these erosional surfaces. Thus, the geometry and transmissibility of contacts between MTCs and reservoir-bearing strata can
R.T. Beaubouef ()
Hess Corporation, Houston, TX, USA
e-mail: rbeaubouef@hess.com
V. Abreu
ExxonMobil, Houston, TX, USA
e-mail: vitor.abreu@exxonmobil.com
D.C. Mosher et al. (eds.), Submarine Mass Movements and Their Consequences,
Advances in Natural and Technological Hazards Research, Vol 28,
© Springer Science + Business Media B.V. 2010
475
476
R.T. Beaubouef and V. Abreu
play an important role in defining the stratigraphic component of hyrdrocarbon traps
and represent baffles or barriers to fluid flow during production.
Keywords Mass transport complexes • seismic stratigraphy • core and log
characteristics • sequence stratigraphy • stratigraphic traps
1
Introduction
The subject of this paper is the occurrence and characteristics of Mass Transport
Complexes (MTCs) in the continental slope of the Gulf of Mexico. The examples
discussed are from a well known system of “mini-basins” located in the Texas continental slope; the Brazos-Trinity Slope System (Fig. 1). The system contains a
variety of deposit types. Notable among these are MTCs. MTCs are here defined as
composite stratigraphic bodies dominated by deposits resulting from mass wasting
and remobilization of intra-slope sediments (slides, slumps, debrites), but also
containing subordinate deposits of turbidity currents and hemipelagic processes.
Fig. 1 Maps of study area. (a) Bathymetric map of Gulf of Mexico showing location, (b) isochron
map of Basins 1–4 (Beaubouef and Friedman (2000), (c) TWT map of seafloor in study area
showing location of seismic, log and core data (Beaubouef and Abreu 2006)
MTCs of the Brazos-Trinity Slope System
477
Although these MTCs comprise roughly half the fill of these basins they have
received less attention than their sand-rich, turbidite-dominated counterparts. In this
paper, we describe some of the MTCs within the system. The methodology employed
in this study was to use existing work on the seismic stratigraphy of these basins in
combination with recent core and down-hole log data to describe and characterize
these deposits. The objectives of this paper are to: (a) discuss the seismic core, and
log characteristics of the Brazos-Trinity MTCs, (b) briefly describe the biostratigraphic and chronostratigraphic controls used to build a preliminary age model, (c)
discuss implications for sequence stratigraphic models, and (d) set forth some basic
ideas for the role of MTCs in the stratigraphic component of hydrocarbon traps. It is
hoped that the specific insights gained from this case study have applicability in the
description and analysis of deep-water depositional systems in general.
2
Previous Work
The Brazos-Trinity Slope System has been studied by numerous workers since the
early 1990s. For a review of the previous work carried out in the area the reader is
referred to Winker (1996), Beaubouef and Friedman (2000), Badalini et al. (2000),
Beaubouef et al. (2003), Beaubouef and Abreu (2004), Beaubouef and Abreu
(2006), Mallarino et al. (2006), and Expedition Scientists (2005). The most recent
work has focused on Basin 4. Beaubouef et al. (2003) used a large, short-offset,
ultra-high resolution 3D seismic survey to study the basin. A second phase of
investigation was designed to calibrate the seismic data through coring and logging.
Mallarino et al. (2006) utilized 15 piston cores (MD-03 cores) collected by the R/V
Mario Dufresne (IPEV) in 2003 (Fig. 1). From these cores, bio-stratigraphic and
isotopic records were obtained. For the first time, a preliminary age model was
established for the filling of Basin 4 (Mallarino et al. 2006) providing critical information regarding the chronostratigraphy of the system and the relationship between
basin filling episodes and well known, late Pleistocene sea level cycles. Most
recently, Basin 4 was investigated during IODP Expedition 308 (Expedition 308
Scientists 2005). During this investigation cores and downhole logs were collected
at three sites (U1320, 1321, 1319; Fig. 1). These boreholes sampled the basin filling
succession and some of the underlying, pre-basin deposits. These long cores and
down-hole logs represent valuable data for the lithologic calibration of the available
seismic data. The initial shipboard results from this investigation are available in
the Preliminary Report from this leg of the cruise (Expedition Scientists 2005).
3
Geologic Setting and History of Brazos Trinity Slope System
The Brazos-Trinity Slope System (Fig. 1) is a chain of four oval-shaped intra-slope
basins (Basins 1–4) of Pleistocene age in the upper to middle continental slope.
These basins were directly fed from sediment delivered to the shelf edge by the
478
R.T. Beaubouef and V. Abreu
ancestral Brazos and Trinity rivers and associated deltas. Sediment delivery to the
slope portion of the systems tract was associated with the stepped sea-level fall
during the latest Pliestocene. Deposition within the system occurred during a period
of approximately 100 Ky (Beaubouef and Friedman 2000). The basins were abandoned during the Holocene transgression and the present highstand of sea level.
The four basins are connected by a southerly trending system of slope channels
present at the sea floor. The channel system heads in a tributary network within
Basin 1 and terminates in a distributary network in Basin 4. Basin 4 is the terminus
of the depositional system. These basins are filled by stratigraphic successions
ranging from less than 75 m to more than 400 m in thickness.
4
Characteristics of MTCs in Brazos Trinity Slope System
Based on seismic stratigraphy and facies, Beaubouef and Friedman (2000) interpreted the fill of these basins to exhibit vertical cyclicity reflecting alternating
deposition of mass transport complexes (MTC), Distrtibutary Channel-Lobe
Complexes (DLC), Leveed-Channel Complexes (LCC) and hemipelagic drape
complexes (DC). The MTCs are low amplitude, chaotic seismic facies units that
occur within each of the four basins. In Basin 2 there are three MTCs (Fig. 2),
each of which is overlain by DLCs. In stratigraphic order, these MTCs are here
referred to as B2_MTC1, _MTC2, and _MTC3. In proximal basin settings, their
bases are defined by prominent erosional surfaces (surfaces of reflection termination
Fig. 2 2D seismic line from Basin 2 (Beaubouef and Friedmann 2000)
MTCs of the Brazos-Trinity Slope System
479
and truncation). In more distal settings these basal surfaces are relatively conformable with underlying units. The tops of the MTCs are defined by the bases
of the overlying high amplitude DLC units. The lowermost MTC (B2_MTC1) is
restricted to the deepest portion of the basin and is the most aerially limited. The
middle (B2_MTC2) and upper MTCs (B2_MTC3) are more tabular, widespread
units that exhibit irregular tops and bases. As seen in Fig. 2, both units have
highly erosive bases that truncate underlying DLCs and, in some cases, the prebasin hemipelagic section.
In Basin 4, there are also three separate MTCs (Fig. 3) interlayered with
DLCs (Lower, Middle and Upper Fans of Beaubouef et al. 2003). In stratigraphic order, these are here referred to as B4_MTC1, _MTC2, and _MTC3. The
base of B4_MTC1 is defined by the prominent truncation surface above the
Lower Fan. The top of the unit is defined by the compound unconformity linking
the bases of Middle Fan and the younger MTCs. B4_MTC1 can subdivided into
an upper and lower division. Separating these divisions in the central and southern part of the basin is a prominent reflection with concave geometry (Fig. 3).
The lower division is the thickest and exhibits low amplitude, chaotic seismic
facies in the areas of greatest thickness (where basal erosion is greatest), and
very low amplitude, semi-continuous seismic facies in areas where it thins. The
upper division is a thin unit with moderate to low amplitude, continuous seismic
facies which appears to be “ponded” on top of the lower division. In proximal
portions of the basin this upper division is truncated at the base of overlying units.
Fig. 3 In-line from EBHR3D, Basin 4 (Beaubouef et al. 2003)
480
R.T. Beaubouef and V. Abreu
B4_MTC2 is the thinnest of the Basin 4 MTCs. It is a lozenge-shaped unit with
low amplitude, chaotic seismic facies that occurs within the lower part of the
Middle Fan. The nature of the relationship between these units is not clear; it
appears that B4_MTC2 was emplaced during deposition of the Middle Fan. In
places, the base of B4_MTC2 is defined by terminations of the basal high amplitude reflections of the Middle Fan. In other places, it is in direct contact with the
underlying B4_MTC1. The top is mounded and is onlapped and draped by
reflections within the upper third of the Middle Fan. B4_MTC3 is the largest,
most laterally extensive MTC. Its base is defined by truncation of reflections at
the top of the Middle Fan. The top is defined by the base of the Upper Fan.
B4_MTC3 is subdivided into three divisions (Fig. 3). The lower division a thick,
wedge-shaped unit characterized by transparent to low amplitude, chaotic seismic facies. This base of the lower division is highly erosive. The middle division
occurs only in the southern portion of the basin and “wedges-out” against the top
of the lower division. Seismic facies within the middle division is mainly very
low amplitude and continuous with reflection amplitude and continuity increasing upward. The upper division is a thin, continuous unit that thins from south
to north and “pinches out” beneath the Upper Fan in the proximal portion of the
basin. The base of this upper division is marked by high amplitude, continuous
reflection passing over both the lower and middle divisions. The top of the upper
division is the sharp base of the Upper Fan.
Fig. 4 Electric log cross-section from Basin 2
MTCs of the Brazos-Trinity Slope System
481
Electrical logs and cores are available from boreholes within Basin 2 and Basin 4
(Fig. 1). Figure 4 shows the log characteristics of the MTC-bound sequences in
Basin 2. B2_MTC1 was not encountered and is shown schematically. The MTCs
are characterized by high, but variable gamma ray (GR) curves consistent with
clay-rich lithologies with minor amounts of silt and sand. The IODP 308 boreholes
provide over 600 m of subsurface information from Basin 4 (Expedition Scientists
2005). Site 1320 is positioned to provide stratigraphic information about the basinfill and some of the pre-basin filling strata (Fig. 5). The information confirms the
previous interpretations that the seismic units characterized by high- to moderate
amplitude, onlapping reflection patterns are sand-bearing (Beaubouef et al. 2003).
These sandy fans are characterized by packets of very fine to lower medium sand
beds inter-bedded with mud (Expedition Scientists 2005). The low amplitude, chaotic seismic facies units contain mud-rich facies containing deformed and contorted
facies consistent with the previous interpretations of these units as Mass Transport
Complexes (Beaubouef and Friedman 2000; Badalini et al. 2000; Beaubouef et al.
2003), but contain other facies as well.
Reworked Planktonic
and Benthic Forams
Reworked Mesozoic Nannofossils
Mixed Assemblage
QAZ1
E. Huxleyi Acme Zone
1H
10
2H
20
3H
30
40
50
70
80
90
U1320B GR RAB
(gAPI)
0
20
40
60
80 100
Lith.
unit
I
* refers to new, informal
stratigraphic zones
Upper Fan
IIA
4H
R10
IIB
MTC 3 Upper*
IIC
“Suprise”
MTC
Sand *
MTC 3 Lower*
5H
6H
7H
60
Revised from
Beaubouef et al 2003
U1320A FMS GR
(gAPI)
sand
silt
mud/clay
0
Seismic
Reworked
Assemblages
Core
Hole
U1320A
Z
Y
This Study
Log and Core Stratigraphy
Recovery
Benthic
Forams
Biostratigraphy
Planktonic
Forams
Nannofossil
Zone
Expedition Scientists, 2005. IODP Prelimary Report, 308
8H
10X
S
Middle Fan
R20
11X
MTC 2
?
12X
100 13X
Middle Fan
IID
110 14X
Upper*
120 15X
B
X
Laticarinina
W
Depth (mbsf)
QAZ2
Transition Zone
A
Bokvina-Bulmina
Assembalge
130 16X
3
MTC 1
R30
140 17X
IIE
III
S
S
Lower*
Abandonment Interval*
Hole
U1320B
150 18X
IV
160 19X
20 m
Lower Fan
170 20X
R40
?
180 21X
190 22X
V
200 23X
Pre-Basin
Hemipelagics
clay/Mud
Sand
Silt
Ash
Foraminifer-bearing
clay
Shell fragments
Plant/wood fragments
Mud clasts
S Mass transport deposits
Sand packages
Fig. 5 Log, core characteristics of IODP308 site 1320, Basin 4. Modified from Expedition
Scientists (2005)
482
R.T. Beaubouef and V. Abreu
Fig. 6 Core and log characteristics of B4_MTC1, site 1320
Figure 6 shows the internal zonation of B4_MTC1. The core in the basal part of
the lower division of B4_MTC1 consists of 17 m of light gray to reddish brown clay
with; irregular bands of organic rich black clay, rounded clasts of dark gray clay, a
contorted, mud-clast bearing interval with large angular clasts and minor, finegrained sand. Core from the upper part of the lower division reveals greenish-gray,
brownish to dark grey mottled clay with; a contorted, mud-clast bearing interval
with apparent shear fabrics and rare, thin beds of silt and very fine grained sand.
Core from the upper division contains 10 m of greenish-gray, brown and black mottled clay with; alternations of thin silt and fine sand beds, abundant organic matter,
and occasional silt-filled burrows. Examples of core photographs for these divisions are shown in Fig. 7. Figure 8 shows the internal zonation of B4_MTC3.
Given previous interpretations, the cores and logs from Sites 1320 (and 1321) provided an unexpected result within MTC3. While the lower division contains a
muddy, chaotic unit, the middle division consists almost entirely of sand (hence the
informal name, “Surprise Sand”). The core from the lower division contains
8 meters of contorted, mottled, dark gray-black clay with; recumbent folds, large
clay clasts, abundant organic materials and minor, thin silts and sands. The middle
division is an approximately 20 m thick, fining upward interval of very fine sand.
The upper division of B4_MTC3 is 9 m of massive, relatively homogeneous dark
green clay with occasional organic material. This unit corresponds to the “slumpderived debrite of Beaubouef et al. (2003) and the “non-hemipelagic mud” of Unit
3 of Mallarino et al. (2006).
MTCs of the Brazos-Trinity Slope System
483
Fig. 7 Core photographs of B4_MTC_1. Locations shown in Fig. 6 (from Expedition Scientists
2005)
Fig. 8 Core and log characteristics of B4_MTC3, site 1320
484
5
R.T. Beaubouef and V. Abreu
Biostratigraphy and Chronostratigraphy
Biostratigraphic and chronostratigraphic control within Basin 4 is provided from
analysis of the MD-03 cores (Mallarino et al. 2006) and from preliminary analysis
of the IODP 308 cores (Expedition Scientists 2005). The biostratigraphic zonation
for site 1320 is shown in Fig. 5. All of the MTCs in Basin 4 belong to the QAZ1
nannofossil zone, and the Y foraminifera zone of the Upper Pleistocene. No further biostratigraphic zonation of site 1320 is available at this time. The presence
of reworked microfossils complicates the biostratigraphic zonation within the
interval, but sheds some light on the depositional processes. The onset of high
abundances of reworked Mesozoic nannofossils coincides with the base of B4_
MTC1 marking a phase of high energy sediment transport following a period of
sediment starvation within the basin marked by the abandonment interval (Fig. 6).
Reworked benthic and planktonic forams are observed from the base of the Middle
Fan to the middle portions of the Upper Fan. The abundance of these reworked
forams reaches a maximum near the base of B4_MTC3 suggesting that erosion
and mixing of sediment from different stratigraphic intervals was associated with
the deposition of this unit. Integration of this information with results of the other
IODP 308 boreholes and the work of Mallarino et al. (2006) allows a preliminary
age model to be constructed, as discussed by Beaubouef and Abreu (2006), and
shown in Fig. 9. The MTCs in Basin 4 were deposited in a relatively short period
Fig. 9 Preliminary age model for Basin 4 (Beaubouef and Abreu 2006)
MTCs of the Brazos-Trinity Slope System
485
of time between MIS-3 and MIS-4. No additional detail about the precise ages of
the MTCs and their relationship to the sea-level curve are provided by the biostratigraphic or isotopic data.
6
Stratigraphic Architectures of MTCs in Brazos Trinity
Slope System
Geologic cross-sections through Basin 2 and Basin 4 are shown in Figs. 4 and 10,
respectively. The bases of the MTCs correspond to prominent erosion surfaces and
thickness variations of the intervening sand packages (DLCs) is largely due to this
erosion. The erosion is greatest in the proximal, northern regions of the basins and
diminishes in a southerly direction where the bases of the MTCs become progressively more conformable with the underlying units. Due to additional core and log
information and HR 3D seismic data, we have much greater control on the architectures of MTCs within Basin 4. The lower division of B4_MTC1 is clearly
derived from collapse of the eastern margin of the basin (Fig. 11). Wavy and arcuate amplitude lineations observed in datumed time slices are interpreted as indicators of flow patterns (Fig. 11b). The flow patterns indicate mass transport was
directed west to southwest. Erosion and/or mass wasting at the base completely
removed the northwestern portion of the Lower Fan. The upper division is interpreted as a thin-bedded, fine-grained turbidite succession that may have been
deposited from dilute turbidity currents generated from the slumping and mass
wasting events associated with deposition of the lower division. B4 _MTC2 is
“inter-fingered” with the Middle Fan (Fig. 3). This MTC was derived from the
western portion of the basin (Fig. 12) and may have been associated with the initial formation of the Western Feeder (Fig. 1). The erosional contact with the lower
part of the Middle Fan is characterized by “boxy”, grooved scours with abrupt
terminations (Fig. 12).
Where the lower portions of the Middle Fan are removed, the two MTCs are
in direct contact with one another. Of the Basin 4 MTCs, B4_MTC2 is the smallest. B4_MTC3 is the uppermost, and largest of the MTCs within Basin 4. Based
on map patterns it is interpreted that his unit was mainly derived from the
Western Feeder (Fig. 1). The lower division has a highly erosive base, in places
cutting through the Middle Fan and down to the level of B4_MTC1. Erosion is
greatest in the northern portion of the basin and decreases toward the distal
reaches. Accompanying this basal erosion trend is an overall southerly thinning.
As the lower division thins, it is replaced by the middle division (“Surprise
Sand”). This sand-rich division is restricted to the southern regions of the basin.
Based on seismic and log correlation, the middle division is continuous for over
5 km in the dip-direction. Although speculative, the middle division may have
been deposited by flows generated via the emplacement of the lower division.
The sand-rich nature of this division made be due to reworking of the sands from
the Middle Fan. The upper division is continuous across the basin except where
it is cut out by the Upper Fan in the north. Beaubouef et al. (2003) interpreted
Fig. 10 Log and core-based cross-section for Basin 4. Shown are logs from IODP 308 Sites 1319, 1320, 1321 (Expedition Scientists 2005) and MD-03
cores (Mallarino et al. 2006)
486
R.T. Beaubouef and V. Abreu
MTCs of the Brazos-Trinity Slope System
487
Fig. 11 Seismic amplitude maps illustrating characteristic of B4_MTC1. Images are derived
from “slices” through a seismic volume “flattened” at the top of the Lower Fan interval. Variations
in colors represent variations in seismic amplitude characteristics within the slices (Beaubouef and
Abreu 2004)
Fig. 12 Seismic amplitude maps illustrating characteristic of B4_MTC2. Images are derived
from “slices” through a seismic volume “flattened” at the top of the Middle Fan interval.
Variations in colors represent variations in seismic amplitude characteristics within the slices
(Beaubouef and Abreu 2004)
488
R.T. Beaubouef and V. Abreu
this unit as a slump-derived debrite associated with continued mass wasting of
the steep, eastern basin margin.
7
Origins of MTCs and Implications for Sequence
Stratigraphic Models
Beaubouef and Friedman (2000) proposed a sequence stratigraphic model in
which MTCs occur in the basal portions of lowstand systems tracts. Sequence
boundaries were placed at the prominent unconformities at the bases of these
distinctive units. It was inferred that: (a) mass wasting and mass transport of
muddy slope materials occurred in response to the earliest portion of sea level
fall and preceded the major influx of sand during the maximum sea level lowstands, and (b) these patterns repeated in response to high frequency sea level
cycles during the latest Pliestocene (Fig. 9). Analysis of more recently acquired
data requires aspects of the Beaubouef and Friedman model be reconsidered.
Specifically, not all of the Brazos-Trinity MTCs conform to such sequence
stratigraphic predictions. The MTCs are of two different types, here classified as
intra-basinal and extra-basinal. Intra-basinal MTCs are derived from within the
basins as slumps or slides from over-steepened, collapsed basin margin areas.
B4_MTC1 is a good example of an intra-basinal MTC. Intra-basinal MTCs
occur in response to local processes unique to each basin, and may occur at any
time during the basin evolution regardless of sea-level stage. Extra-basinal
MTCs are derived from areas outside and up-dip of the basins and entered the
basins via the various feeder channels (Beaubouef and Friedman 2000). The
extra-basinal MTCs tend to be the most laterally extensive and have point
sourced, fan-like map patterns. Examples of extra-basinal MTCs are B2_MTC3
and the lower divisions of B4_MTC3. Extra-basinal MTCs may have occurred
in response to allocyclic processes, and therefore, may have sequence stratigraphic significance. However, it can not be ruled out that extra-basinal MTCs
may also have more local, autocyclic origins. Furthermore, it is questionable
whether the MTCs can be reliably tied to the late Pliestocene sea level curve
using biostratigraphic or isotopic data. Hence, the actual relationships between
the MTCs and the high frequency sea level cycles, if any, are still not well understood. Finally, in some cases, the origins of the MTCs are not clear and the classification is indeterminate. For example, although B4_MTC2 clearly flowed into
the basin from the vicinity of the Western Feeder, the unit may have formed as
a local slump of the basin margin at the mouth of the channel. The observation
that this MTC is “inter-fingered” with the Middle Fan is at odds with the
Beaubouef and Friedman model. At this point, it would appear that the utility of
MTCs for developing sequence stratigraphic frameworks for the Brazos-Trinity
system is in question.
MTCs of the Brazos-Trinity Slope System
8
489
Discussion of Potential for Stratigraphic Traps Associated
with MTCs
It has been previously shown that the sand-rich, turbidite-dominated successions
within the fills of these basins constitute excellent analogs for intra-slope reservoirs. The role of the MTCs in analogous hydrocarbon traps has been less explored.
In this paper, we have shown that the Brazos-Trinity MTCs contain a variety of
lithofacies types and exhibit a variety of stratigraphic architectures. Some divisions
of the MTCs contain sufficient porous and permeable sediment to constitute hydrocarbon reservoirs, most notably the middle division of the B4_MTC3 (the “Surprise
Sand”). The other, mud-dominated divisions would represent seal and “waste rock”
units within hydrocarbon traps. This concept has also been discussed by Moscardelli
et al. (2006). Because of erosion at the basal surfaces of the Brazos-Trinity MTCs,
localized or widespread termination of reservoir strata below these surfaces has
occurred. As such, these erosion surfaces play an important role in determining the
geometry and continuity of reservoirs in these settings. Furthermore, the 3D geometry of the truncated reservoir bodies often are not controlled by a single erosion
surface, but rather two or more erosion surfaces that form a compound unconformity. The muddy portions of the MTCs represent basal, vertical and lateral seals to
the reservoir complexes. These stratigraphic arrangements would likely represent
poly-seal traps; traps involve more than one sealing surface (Milton and Bertram
1992; Corchoran; 2006). The type of lithologies directly overlying these erosion
surfaces, and hence, the transmissibility of the sealing surfaces, represents a key
factor in impeding fluid migration through the stratigraphic succession (Moscardelli
et al. 2006). In the fill of Basin 4, reservoir-prone units are represented by the
Lower, Middle and Upper Fans and the “Surprise Sand”. Each of these units is
partially or completely surrounded by mud-rich facies of the MTCs (Fig. 10). In the
case of the Lower Fan (Fig. 11), base and lateral seal is provided by the clay-rich,
pre-basin hemipelagic section. An unconformable top seal is provided by the lower
division of B4_MTC3. The case of the Middle Fan is more complicated (Figs. 3
and 12). In some areas, base seal is provided by either pre-basin hemipelagics or
the very fine-grained, distal equivalents of the Lower Fan. In other areas, B4_
MTC1 provides base seal. An unconformable top seal is provided by the lower
division of B4_MTC1. The Middle Fan may be further complicated by the intraformational seal formed by B4_MTC2. Depending on the lithology of this uncored
unit, it may partially segment or compartmentalize the trap. For the “Surprise
Sand”, base seal is provided by the lower division of B4_MTC3, lateral seal is
provided by steeply dipping hemipelagic section in the distal portion of the basin,
and top seal is provided by the homogenous clay of the upper division of B4_
MTC3. The primary risks for these traps are; (a) an incomplete join between top
and base seal, (b) incomplete separation of sand-rich units by the MTCs, and (c)
fluid migration pathways provided by connections between reservoir units via the
thinly bedded sand and silts existing in some divisions of the MTCs. In cases where
490
R.T. Beaubouef and V. Abreu
MTC’s with mud-rich, high capillary entry pressure layers overly the sealing surfaces,
the fluid transmissibility of the contacts will be very low and stratigraphicallyassisted trapping of hydrocarbons is possible. From this brief analysis, we conclude
that the distribution of mud-rich sediment is equally important as the nature of the
sand-rich intervals in determining the characteristics of stratigraphic component of
traps. MTCs are the main contributors of mud to these basins, but also contribute
reservoir-grade material. As such, further study of the role of MTCs in shaping
hydrocarbon traps is warranted.
Acknowledgments A review by Sam Algar and suggestions from Lorena Moscardelli has
enhanced this paper and are gratefully acknowledged.
References
Badalini G, Kneller B and Winker CD (2000) Architecture and process in the Late Pleistocene
Trinity-Brazos turbidite system. Gulf of Mexico. In: Weimer, P. et al. (eds.), Deep-Water
Reservoirs of the World, Gulf Coast Soc SEPM.
Beaubouef RT and Abreu V (2006) Basin 4 of the Brazos-Trinity slope system: anatomy of a
Lowstand systems tract. Gulf Coast Soc Geolog Soc Trans 56: 39–49.
Beaubouef RT and Abreu V (2004) Basin 4 of the Brazos-Trinity slope system. Europ Assoc Geol
Eng Paris, France, Paper A028.
Beaubouef RT, Abreu V and Van Wagoner JC (2003) Basin 4 of the Brazos-Trinity slope system:
the terminal portion of a late Pliestocene lowstand systems tract. In: Roberts, H. et al. (eds.),
Shelf Margin Deltas and Linked Down Slope Petroleum Systems: Global Significance and
Future Exploration Potential, Gulf Coast Soc SEPM.
Beaubouef RT and Friedman SJ (2000) High resolution seismic/sequence stratigraphic framework
for the evolution of Pleistocene intra slope basins, Western Gulf of Mexico: depositional models and reservoir analogs. In: Weimer, P. et al. (eds.), Deep-Water Reservoirs of the World,
Gulf Coast SEPM.
Corchoran J (2006) Application of a sealing surface classification for stratigraphic related traps in
the UK Central North Sea. In: Allen, MR. et al. (eds.), The Deliberate Search for the
Stratigraphic Trap, Geol Soc Lond Spec Pub 254: 207–223.
Expedition Scientists (2005) Overpressure and fluid flow processes in the deepwater Gulf of
Mexico: slope stability, seeps, and shallow-water flow. IODP Prelim Rep 308. doi:10:2204/
iodp.pr.308.2005.
Mallarino G, Beaubouef RT, Droxler AW, Abreu V and Labeyrie L (2006) Sea level influence on
the nature and timing of a minibasin sedimentary fill (northwestern slope of the Gulf of
Mexico). Am Assoc Petrol Geol Bull 90: 1089–1119.
Milton NJ and Bertram GT (1992) Trap styles, a new classification based on sealing surfaces. Am
Assoc Petrol Geol Bull 76: 983–999.
Moscardelli L, Wood L and Mann P (2006) Mass-transport complexes and associated processes in
the offshore area of Trinidad and Venezuela. Am Assoc Petrol Geol 90 (7): 1059–1088.
Winker CD (1996) High-resolution seismic stratigraphy of a late Pleistocene submarine fan ponded by salt-withdrawal minibasins on the Gulf of Mexico continental slope. In: Proc 28th
Annual Offshore Tech Conf 28 (1): 619–628.