The .

'Vmerican Association i:»f Peirolcum Geologists Bulletin

V. 66, No. 9(SEPTEMBER 1982). P. 1271-1288,23 Figs., 1 Tabic

Seismic Detection and Evaluation of Delta and Turbidite Sequences:

Their Application to Exploration for the Subtle Trap^
O. R. BER&

ABSTRACT or impossible to find in the past now have a better chance

of being detected.
Energy conditions at the seaward edge of deltas allow The stratigraphic interpretation of seismic data has
their division into fluvial-dominated, wave-dominated, become possible because of improvements in data acquisi-
and tide-dominated deltas. Each kind of delta has a dis- tion, processing, and understanding of seismic wave
tinct framework orientation and depositional pattern response. The success of seismic stratigraphy also owes a
which results in a characteristic seismic reflection pattern. great deal to the numerous investigations of modern and
Fluvial-dominated deltas are characterized by clinoform ancient depositional systems which have resulted in a com-
seismic reflection patterns which include: oblique (tangen- prehension of their depositional evolution and frame-
tial), complex oblique (tangential), sigmoid, and complex work. Therefore, to use seismic stratigraphy successfully,
sigmoid-oblique. Seismic facies analysis can be used to the explorationist must be well grounded in the basics of
define those facies which should contain sand. geophysics and he must understand the sedimentary evo-
Wave-dominated deltas are characterized by shingled lution and stratigraphic framework of depositional
seismic reflection patterns. Seismic facies analysis of this sequences.
delta is not effective in identifying those facies which The purpose of this paper is to discuss and to interpret
should be sand-prone. Shingled reflections may be used in the sedimentary evolution, stratigraphic framework, and
determining the possible location and depositional atti- seismic response of delta and turbidite depositional
tude of strandline sands. sequences. Explorationists are usually restricted in their
Tide-dominated deltas have not yet been identified using interpretations by the absence or limited resolution of
seismic stratigraphic methods and therefore are not cov- data. Therefore, an objective of this report is to present
ered in this paper. the contents within the same restrictions. Seismic criteria
Turbidite fans are sequences of sands and shales depos- that can be used to detect deltas and turbidites are out-
ited in conjunction with and basinward of deltas or sub- lined. Deltas can be divided into end-members, each of
marine canyons. Turbidite sands can be classified which have characteristic seismic reflection patterns. Tur-
generally into channel and suprafan sands. bidites can be associated with deltas and this relation is
Certain seismic events and reflection patterns occurring described and interpreted. Specific examples of subsur-
in various combinations may suggest the presence of turbi- face delta and turbidite sequences are described and evalu-
dites. These indicators include troughs, submarine can- ated.
yons, mounds, prograded fluvial-dominated delta
reflection patterns which vary in thickness, and onlap- IDENTIFICATION AND ANALYSIS OF SEISMIC
offlap patterns on depositional slopes. SEQUENCE AND DEPOSITIONAL FACIES
Regional studies provide the best means of identifying
and mapping depositional sequences. Examples from the "Seismic stratigraphy is the study of stratigraphy and
North Sea, Gulf Coast, and Sacramento Valley illustrate depositional facies as interpreted from seismic data" (Mit-
the geologic and geophysical expression of delta and turbi- chum et al, 1977a, p. 117). To make this interpretation, it is
dite sequences and their interrelations. important to understand that seismic reflections are
chronostratigraphic in nature, following bedding or time
INTRODUCTION planes and not lithology. Therefore, the geometry of
reflections will parallel depositional geometry, and geneti-
The recent development of the discipline of seismic stra- cally related reflection patterns can be used to identify and
tigraphy has given exploration a powerful tool. By use of to map seismic sequences.
seismic stratigraphy, sedimentary basins can be analyzed "A seismic sequence is a depositional sequence identified
in systematic detail. Hydrocarbon traps that were difficult on a seismic section" (Mitchum, 1977, p. 210). "Adeposi-
tional sequence is a stratigraphic unit composed of a rela-
tively conformable succession of genetically related strata
* 1962. The American Association of Petrolejm Geologists. All rights and bounded at its top and base by unconformities or their
reserved.
'Reprinted from AAPG Memoir 32, The Deliberate Search for the Subtle correlative conformities" (Mitchum et al, 1977b, p. 53).
Trap (1982), with minor Bulletin styling. Read before the AAPG annual conven- The identification of a seismic sequence requires that its
tion, San Francisco, California, June 2,1981. Accepted for publication, July 1, boundaries be defined and correlated on seismic data.
1981.
^Andover Oil Co., Tulsa, Oklahoma 74172. Determining the boundaries of a seismic sequence can be
The writer thanks Phillips Petroleum Co. for permission to publish this paper. accomplished by locating reflection terminations on the

1271
1272 Delta and Turbldite Sequences

seismic data. These reflection terminations represent Galloway (1975) divided deltas into three end-members,
unconformities, and once identified they can be used to based on the energy source which dominates the seaward
correlate the boundaries of a seismic sequence through edge of the deUa: (1) fluvial-dominated, (2) wave domi-
areas of conformable seismic reflections. nated, and (3) tide-dominated deltas (Fig. 1). Fluvicd flow,
Mitchum et al (1977b) have discussed the concept of the wave action, and tidal currents are the three energy sources
sequence and its identification in detail, and Ramsayer that may be dominant at the leading edge of the delta.
(1979) has summarized the same subject. For a more thor- These energy sources control the deposition of elastics,
ough discussion of the sequence and its identification, which results in a distinctive sandy facies for each delta
these articles should be consulted.
A seismic sequence can be analyzed jmd its component
seismic facies defined. This process of analysis is called
"seismic facies analysis" (Sangree and Widmier, 1977).
Parameters which may be used to delineate and interpret
seismic facies include reflection continuity, amplitude, fre-
quency and interval velocities as well as reflection geome-
try and external form. Seismic facies that may be
sand-prone then may be traced and mapped. Sangree and
Widmier (1977) have discussed in detail the concept of
seismic facies analysis and the reader should refer to their
article for a more complete discussion of this subject.

DELTA SEQUENCES

Galloway (1975, p. 87) defined a delta as "a contiguous

mass of sediment, partly subaerial, deposited around the
point where a stream enters a standing body of water." A
delta will develop on the basin shelf in at least its earliest FIG. 1—Triangular classification of deltaic depositional
stages. sequences (modified from Galloway, 1975).
TABLE 1. Characteristics of Deltaic Depositional Sequences (after Galloway, 197S).

FLUVIAL WAVE TIDE

DOMINATED DOMINATED DOMINATED

GEOMETRY ELONGATE TO LOBATE ARCUATE ESTUARINE TO IRREGULAR

CHANNEL TYPE STRAIGHT TO SINUOUS MEANDERING FLARING STRAIGHT TO

DISTRIBUTARIES DISTRIBUTARIES SINUOUS DISTRIBUTARIES

BULK
COMPOSITION MUDDY TO MIXED SANDY VARIABLE

FRAMEWORK DISTRIBUTARY MOUTH COASTAL BARRIER ESTUARY FILL AND TIDAL

FACIES BAR AND CHANNEL FILL AND BEACH RIDGE SAND RIDGES
SANDS, DELTA MARGIN SANDS
SAND SHEET

FRAMEWORK PARALLELS PARALLELS PARALLELS

ORIENTATION DEPOSITIONAL SLOPE DEPOSITIONAL STRIKE DEPOSITIONAL SLOPE
 O. R. Berg 1273

MISSISSIPPI RIVER DELTA

LOBATE FLUVIAL DOMINATED ELONGATE

FLY RIVER
' DELTA
RHONE RIVER
DELTA
WAVE
DOMINATED

TIDE-
DOMINATED

FIG. 2—Modern examples of the basic delta types defined in Figure 1 (modified from Fisher et al, 1969).

type. Dehas are rarely a single, pure delta-type, but usually

are mixtures in varying degrees of two or all of these end-
members.
Sand may be trapped in distributary-mouth bars, chan- |BASINWARD )
nel sands, strandline sands, tidal sand ridges and estuary DELTA PLAIN
" DELTA FRONT
fill (Table 1). These sand bodies are the stratigraphic
framework of the delta and their framework orientation is PR OD E L T V'--..,.^^^^^^^-^^ ^^^g i^;;j^5;DIP SECTION

distinct for each delta type. The distinctive framework ori- SEA FLOOR
entation and depositional pattern of each delta type result STRIKE
in seismic reflection patterns that are also characteristic of • ^ ^ — = — .

^ - ^ ^ SECTION
that delta system. —rT^-^^--—^^
Mitchum et al (1977a) recognized that the clinoform
(Rich, 1951) seismic reflection patterns, which commonly
appear on seismic data, were usually associated with delta
systems. These chnoform patterns were caused by deltas
that had prograded seaward. Differences in the clinoform
FIG. 3—Model of a fluvial-dominated delta lobe illustrating its
reflection patterns allowed their division into geometric internal framework.
groups of reflections with similar characteristics. Each
group of clinoform seismic reflection patterns discussed is ment supply, water depth, and coastal energy conditions.
diagnostic of one of the delta end-members, with the The fluvial-dominated delta lobe model in Figure 3 illus-
exception of the tide-dominated delta. trates the vertical relation of the delta-front, delta-plain,
and prodelta facies.
Fluvial-Dominated Deltas Fluvial-dominated deltas prograde as distributary lobes,
and the sands of the delta front become difficult to sepa-
Modern deltas provide a basis for an understanding of rate from those of the delta plain. These sand bodies con-
ancient deltas. A modern example of a fluvial-dominated stitute the framework facies and in the fluvial-dominated
delta is the Mississippi delta. As Figure 2 shows, the delta delta the dominant framework orientation will parallel
may be elongate or lobate in shape, depending on sedi- depositional slope. Fine elastics of the prodelta facies lie in
1274 Delta and Turbidite Sequences

a. OBLIQUE b. COMPLEX OBLIQUE

(TANGENTIAL)

c. SIGMOID d. COMPLEX
SIGMOID OBLIQUE

e. OBLIQUE (PARALLEL) f. SHINGLED

FIG. 4—Model reflection patterns of fluvial-dominated and wave-dominated deltas (modified from Mitchum et al, 1977a).

front of and underneath the delta-plain and delta-front aries of the deltaic seismic sequence. The oblique reflec-
facies. tion pattern represents a high-energy delta in which the
The delta-plain, deUa-front, and prodelta facies com- sand-prone delta plain is coincident with the upper hori-
prise the fluvial-dominated deka. The limits of seismic res- zontal reflection event. The clinoform pattern is the seis-
olution will allow division of a fluvial-dominated mic equivalent of the prodelta facies where sands are
reflection pattern into a deha-plain seismic facies, which generally absent. The absence of stacking of horizontal
includes the delta front and plain, and a prodelta seismic reflections in the delta plain suggests the shelf was stable.
facies. In the delta-plain seismic facies, the reflections are A variation of the oblique (tangential) seismic reflection
characterized by high amplitude and good continuity, and pattern is the complex oblique (tangential) seismic reflec-
are subparallel and horizontal in attitude. This reflection tion pattern (Figs. 4b, 5b). The complex oblique pattern is
character is probably caused by interbedded sands, shales the complex sigmoid-oblique reflection pattern of Mit-
and coals. The prodeha seismic facies is characterized by chum et al (1977a). The name of the pattern is modified in
multiple clinoform reflections and the reflection character this paper to reserve the term "complex sigmoid-oblique"
of low amplitude and low to moderate continuity is proba- for another seismic reflection pattern which will be
bly due to the dominant shale content of the prodelta described in a following discussion. Although similar to
facies. the oblique (tangential) progradation pattern, clinoform
Seismic reflection patterns of a fluvial-dominated reflections of the complex oblique pattern are progres-
delta.—The oblique (tangential) sigmoid and complex sively terminated lateraJly by increasingly higher horizon-
sigmoid-oblique progradational reflection patterns as tal seismic events. This reflection pattern represents a
defined by Mitchum et al (1977a) are representative seis- high-energy environment and the delta-plain seismic facies
mic reflection patterns of a fluvial-dominated delta. is associated with the upper horizontal reflections. The
The oblique (tangential) progradational pattern (Figs. complex oblique progradation pattern represents a delta
4a, 5a) is characterized by clinoforms which terminate lobe that prograded onto an actively subsiding shelf result-
updip by toplap and downdip by downlap forming bound- ing in aggradation of the delta plain.
 0 . R. Berg 1275

a. OBLIQUE b. COMPLEX OBLIQUE

C. S I G M O I D

d. COMPLEX SIGMOID OBLIQUE e. SHINGLED

FIG. 5—Examples of seismic reflection patterns of fluvial-dominated and wave-dominated deltas (a, b, after Sangree et al, 1979;
d, origin unknown).
The sigmoid seismic reflection pattern has clinoform show oblique progradational patterns of a delta lobe
reflections which are continuous and S-shaped (Figs. 4c, blending into the sigmoid clinoforms of an interlobe area
5c). Individual clinoforms begin as essentially horizontal as is illustrated by Figure 5d.
reflectors which dip downward and then flatten out,
downlapping against the lower boundary of the seismic Wave-Dominated Deltas
sequence. This pattern represents a low-energy regime in
which little or no sand is present. The sigmoid reflection The Rhone River delta in southern France is an example
pattern is commonly adjacent to oblique and complex of a high-energy wave-dominated delta (Fig. 2). Strandline
oblique progradation patterns. The sigmoid pattern sands are widely distributed over the delta plain, with local
appears to represent delta interlobe areas that received occurrences of distributary-channel sands. The strandline
mostly clay as sediments. Where the sigmoid clinoforms sands are elongate, paralleling the position of the coastline
are stacked, then subsidence of the shelf in some degree at the time of their deposition as coastal barriers and beach
must have occurred. ridges. Where depositional conditions remain stable,
The complex sigmoid-oblique seismic reflection pattern accretionary clusters of strandline sands may develop.
consists of randomly alternating sigmoid and oblique Progradation of a wave-dominated delta tends to occur
reflection patterns within the same seismic sequence (Figs. along the overall delta front rather than being concen-
4d, 5d). The depositional environment of this kind of pro- trated as happens in the distributary depositional lobes of
gradational pattern shifts from high to low energy. The the fluvial dominated delta. Sediments derived from the
sand-prone delta-plain seismic facies is coincident with the fluvial system are reworked and redistributed by wave
upper horizontal reflections of the high-energy oblique action and longshore currents along the delta front. Depo-
progradation pattern, whereas the low-energy sigmoid sition of these sediments on the shoreline results in a sea-
pattern will be largely shale. ward progradation of the delta. Strandline sands are
An evaluation of the complex sigmoid-oblique pattern deposited on the upper leading edge of the delta, as elon-
suggests that it may be due to delta lobe switching which gate bodies whose orientation parallels depositional
may occur as a lobe changes its direction of progradation. strike. In the lower part of the delta, clay is the dominant
Lobe switching may also occur at the end of a delta lobe as sediment and is probably equivalent to the prodelta facies
the lobe loses its ability to prograde because of a dimin- of the fluvial-dominated delta (Fig. 6).
ished sediment supply, a more intense marine environ- Seismic reflection patterns of wave-dominated deltas. —
ment, or both. A seismic section across a delta should The seismic progradational patterns that appear to be
1276 Delta and Turbidite Sequences

indicative of a wave-dominated delta are the shingled and a present-day example of a tide-dominated delta (Fig. 2).
obhque (parallel) seismic reflection patterns (Fig. 4e, f). Actually, there is no delta, as the delta is represented by a
Most data presently available suggest that the shingled river bay Tide-dominated deltas are difficult to recognize
reflection pattern as shown in Figure 5e is a better indica- in the subsurface and have not been identified through
tion of a wave-dominated delta than is the oblique (paral- seismic stratigraphic methods. Therefore, the tide-
lel) reflection pattern. Development of a wave-dominated dominated delta is not discussed in this report.
delta seems to require a stable shallow depositional shelf.
Mitchum et al (1977a) interpreted the shingled seismic con- TURBIDITE SEQUENCES
figuration to be depositional units that prograded into
shallow water. Turbidites consist of a group of sedimentary rocks of
In each of these seismic reflection patterns, inclined alternating beds of sandstone and shale deposited below
reflections are terminated at their top and bottom by con- wave base in a deep water or basinal environment. They
tinuous, flat reflection events. Separation of the wave- occur in conjunction with, and basinward of, a delta or
dominated delta into a delta-plain and prodelta seismic submarine canyon (Fig. 7). Turbidites associated with a
facies does not seem to be possible (Fig. 6). The inclined canyon usually occur as a single depositional fan, whereas
reflections cross from the delta-front and plain seismic turbidites deposited seaward of a delta usually form a tur-
facies into the underlying prodelta facies, the inclined pat- bidite fan complex. A turbidite fan associated with a delta
tern being caused by the seaward progradation of the usually has a channel or delta-front trough (Moore and
delta. Although not directly indicating strandline sands, Fullam, 1975) at its apex. These troughs, cut into the pro-
the inclined reflections can be used to determine the depo- deha clays of the delta and possibly underlying older
sitional attitude and possible location of these sands. The rocks, act as sediment chutes. Sands derived from the
strandline sands will be present in the upper part of the delta-front flow down the delta-front trough as turbidity
interval and may be coincident with the occurrence of currents and are deposited on the fan. The head of a sub-
shingled seismic reflections and have their same deposi- marine canyon which reaches the wave-agitated zone of a
tional attitude. coastline can intercept sand moved by longshore currents.
These elastics then move down the canyon as turbidite
Tide-Dominated Deltas flows and are deposited near its mouth as part of a turbi-
dite fan. The sands will be of the same general grain size
The Fly River delta in the Gulf of Papua, New Guinea, is and mineralogy as their source.
The principal difference between the delta-front trough
and submarine canyon is size, the submarine canyon being
many orders of magnitude larger. The submarine canyon
is also a long-term feature existing over many thousands,
perhaps millions of years. Therefore, a turbidite fan which
is associated with a submarine canyon can be very large.
STRANDLINE The deha-front trough is short lived, existing only as long
SANDS as the delta lobe with which it is associated, remains active.
The constant shifting nature of deltaic distributaries
results in the development of a turbidite-fan complex
which is composed of smaller turbidite fans. The extent of
FIG. 6—Model of a wave-dominated delta. the turbidite complex depends on the width of activity of

Jlfll I I ' l '

TTJTV^X^^

FIG. 7—Illustration of (1) relationship of delta and its associated turbidites, and (2) submarine canyon and its associated turbidite
fan (modified from Sangree et al, 1978; based on Moore, 1966).
 0. R. Berg 1277

SUBMARINE CANYON'
SHELF SLOPE

VNNEL
SAND

SUPRAFAN
SAND

SHELF SLOPE

DIP SECTION

FIG. 8—Model of prograded turbidite fan, illustrating its internal framework (modified from Brown and Fisher, 1977).
the delta and the availability of sediments. than the suprafan sands. The suprafan sands should have
As previously mentioned, submarine canyons and delta- a wider lateral distribution than the channel sands.
fronttroughs are sediment chutes and are usually filled by In the upper fan, channel sands are the most common. In
clay when turbidite deposition ceases. As a result, canyons the middle fan, both channel sands and suprafan sands
and troughs are not good exploration targets unless it can will be present. Sands in the lower fan are usually of the
be shown that turbidite deposition may have occurred suprafan variety, though some channel sands may also be
within the channel. present. Beyond the turbidite fan are basinal sediments
which are mostly shale with some thin beds of sandstone
Turbidite-Fan Model and siltstone.
A characteristic of turbidites is the vertical stacking of
The turbidite-fan model in Figure 8 is modified from one sand bodies which form bundles of sands (R. G. Walker,
by Brown and Fisher (1977) and illustrates the strati- personal commun.). Each bundle appears to be related to
graphic framework of a turbidite-fan sequence. Turbidite the development of a suprafan lobe on the turbidite fan.
sands are distributed throughout the fan and as a family Another characteristic of turbidite sequences is the thick-
they can be broken into many members (Walker, 1978). ening and coarsening upward (Walker, 1979) of sand
However, limits on the resolution of seismic data and the bodies both in local bundles of sand and throughout the
absence of detailed well data have resulted in limiting divi- overall fan (Fig. 9). The thickening and coarsening
sion of the turbidite sands into channel and suprafan upward are the results of the progradation of suprafans
sands in this paper. Channel sands, as the name implies, and of the whole turbidite-fan complex as sand bodies of
are confined to channels beginning near the apex of the the lower fan are progressively overlapped by thicker and
fan and possibly continuing to the lower fan. These sands coarser sands of the middle and then the upper fan.
may be thick, isolated, and surrounded by claystones. Thinning- and fining-upward sequences of sandstones
Suprafan sands occur at the lower end of channels where may also occur. Walker (1978 interpreted the thinning- and
the channels braid and meander. The suprafan sand is a fining-upward sequence as the result of plugging of a tur-
lenticular body that develops as part of a suprafan lobe on bidite channel by a large, coarse flow. The plugging causes
the middle to lower portion of a turbidite fan. The dimen- the channel to be gradually abandoned and filled by thin-
sions of turbidite sand bodies will vary from basin to ner and finer grained beds.
basin, but in general, the channel sands are more massive The thick shales which separate bundles of sandstones
 1278 Delta and Turbidite Sequences

SP AND GAMMA RAY RESPONSE

UPPER
FAN

MIDDLE
FAN BASIN ^
PLAIN

C :CHANNEL SAND

FIG. 9—Model stratigraphic column of prograded turbidite sequence. Basin-plain deposits have been progressively overlapped by
sediments of lower, middle, and upper fan. Arrows indicate direction of thickening (and presumably coarsening) of a bundle of sand-
stones (modified from Walker, 1978).

are the subsurface representative of the mud blankets methods to predict the presence of sand reservoirs exist
described by Walker (1978). Suprafan lobes tend to switch (e.g., interval velocities), the best method is still direct
position periodically (Normark, 1978), causing one lobe detection by a well bore. The synthetic seismogram can be
to be abandoned and another to start. The old lobe is then used to integrate this well information into the seismic
buried by fine sediments resulting in a blanket of mud data.
which may be quite thick (e.g., 2(X) ft; 60 m). The presence of sandstone reservoirs in a depositional
The pattern of prograded, bundled turbidite sands is sequence can be predicted if the patterns of sediment
detectable on electric logs (Fig. 9) and should be useful in transport are understood. Clastics are moved from their
exploration and development. Once it is established that source by a fluvial system to a delta. From the delta, sands
turbidites are involved, the electric log can be used to may be transported as a turbidity flow directly to a turbi-
locate the well within a model turbidite sequence. Further dite fan, or moved along the coastline and then to a fan
analysis should locate other potentially productive areas through a submarine canyon. Therefore, a delta sequence
and intervals. It appears that the prograded and bundled known to have sandstone reservoirs present can be
pattern of turbidites may vary from area to area. There- expected to have sand-prone turbidites associated with it.
fore, the turbidite model may require some modification Conversely, a delta which is the source of sediments for a
to adjust to those changes. sand-deficient turbidite sequence may not have many
sandstone reservoirs.
Reservoir Rocks An examination of the mechanics of turbidity flow sug-
gests that the process does not do much to improve the res-
The reservoir rocks of delta and turbidite sequences are ervoir quality of turbidite sands. The composition and
sandstones. The discussion of deltas and turbidites in this grain-size of the clastic sediment when finally deposited
paper has had a tacit impUcation that sandstone reservoirs are generally that of the source material ahhough some
were always present. Mitchum et al (1977a) pointed out finer sediments are lost and graded bedding may occur.
that many theoretically sand-prone, seismic facies turned Therefore, the source material must be of reservoir quality
out to be largely shale and siltstone because a source of if good turbidite sands are to be deposited. Well-sorted
sand-size sediment did not exist. Though many indirect sands from distributary mouth-bars and strandline sands
 0 . R. Berg 1279

DELTA FRONT

TURBIDITES<

FIG. 10—Model of a prograded fluvial-dominated delta and associated turbidite sequence.

DELTA-PLAIN 1 s'JS
FACES < '"""^'

FIG. 11—Seismic section showing a prograded fluvial-dominated delta and turbidite sequence.

of the seaward edge of the delta and adjacent beach envi- sands through suprafan lobe switching, or the abandon-
ronments are probably the major source of good turbidite ment of channels. Mounds may develop within a turbidite
sand reservoirs. system (see section on "Examples of Seismic Stratigraphy
Studies, North Sea," in this paper) and if deposition is
Hydrocarbon Potential of 'I\irbidite Sequences rapid, rollover structures may develop on down-to-the-
coast faults (see section on "Examples of Seismic Stratig-
TUrbidite systems represent an unusual environment for raphy Studies, Tuscaloosa Formation," in this paper).
the generation and entrapment of hydrocarbons. Tlirbi- Normark (1978), Walker (1978), and Wilde et al (1978)
dite shales tend to be rich source rocks and sandstones may have discussed the stratigraphic framework and hydrocar-
be widely distributed throughout a turbidite fan as illus- bon potential of turbidite sequences; the reader should
trated by Figures 8 and 9. The proximity and mixture of refer to these articles for a detailed discussion of these sub-
source rocks, which are also excellent seals, and sandstone jects.
reservoirs create a set of conditions in which the quantity As turbidites appear to be more widely distributed than
of hydrocarbons generated and trapped can be unusually previously recognized, they may represent one of the
high. major exploration objectives of the future. Recent major
Many hydrocarbon traps may develop within a turbidite discoveries in Tertiary turbidites in the North Sea and in
sequence. These traps may be stratigraphic as a result of Upper Cretaceous rocks of the United States Gulf Coast
the updip termination of channels by the prograding of the have accentuated their importance as hydrocarbon reser-
turbidite sequence, the cutoff of channel and suprafan voirs.
1280 Delta and Turbidite Sequences

Prograded Delia and Turbidite Sequences has actively prograded, implying a good sediment supply,
would be more likely to have turbidites associated with it.
Earlier in this paper the observation was made that A prograded fluvial-dominated delta is the kind that is
many deltas have turbidites associated with them. The most likely to have an associated turbidite system.
size and extend of the turbidites depend on the availabil- Because the source has prograded, the turbidites should
ity of sediments from the delta. Therefore, a delta which have also prograded, resulting in a situation in which tur-
bidites may be under and in front of the delta (Figs. 10,
11). Detection of a turbidite sequence under these condi-
tions may be difficult.
A. TROUGH OR CANYON On a seismic dip section an indication that turbidites may
be present is the strongly prograded clinoform reflection
pattern of a fluvial-dominated delta. The turbidites if
B. MOUNDING present, will be at the toe of the clinoforms. The turbidite
1. DIP-ORIENTED SECTION seismic facies will be represented by low angle reflections
dipping into the basin, which are toplapped by clinoform
reflections of the delta (Figs. 10,11). If the depth of burial
2. STRIKE-ORIENTED SECTION
is not so great as to reduce seismic resolution, the reflec-
tion character of this seismic facies should be one of high
amplitude and good continuity of subparallel events. This
C. CLINOFORM REFLECTION PATTERNS reflection character may be due to prograded and inter-
1. THINNING CLINOFORM REFLECTION INTERVAL bedded turbidite sands and shales.
^^^:::v:$:$^^;^^
Seismic Indications of Ikirbidite Sequences
2 UNDERLYING SEISMIC FACIES INTERVAL ONLAPPING AND OFFLAPPING
A OEPOSITIONAL SLOPE

Sangree and Widmier (1977) and Sangree et al (1978)

have published detailed discussions on the recognition and
interpretation of turbidite seismic facies. The following
FIG. 12—Seismic indicators of turbidite sequences. comments are a compilation of their material integrated

EROSIONAL ZERO EDGE

OF THE PALEOCENE

50
MILES

400— SAND ISOPACHS IN FT.

(UNCERTAIN )
58°N-

SCOTLAND

4W„
\4^ \0 2°W
FIG. 13—Isopach of Paleocene sands and depositional environments, central North Sea (modified from Parker, 1975).
 0. R. Berg 1281

with the results of this study. sequence especially if overlain by a deltaic clinoform
Certain seismic events and reflection patterns, when reflection pattern (2, of Fig. 12C).
occurring in various combinations, may suggest the pres- Separately these seismic indicators do not conclusively
ence of turbidites. These seismic indicators include the fol- prove the existence of a turbidite sequence. Where they
lowing. occur together and are integrated in a regional analysis,
1. Where troughs or canyons are present on deposi- these indicators can be evidence suggesting the presence of
tional strike-oriented seismic sections, turbidites may be turbidites.
present basinward of these features (Fig. 12A).
2. Mounds, which may have an internal hummocky or EXAMPLES OF SEISMIC STRATIGRAPHY STUDIES
chaotic seismic reflection pattern, may occur on either
strike or dip seismic lines. In many places, they are Regional investigations appear to be the best method for
onlapped or toplapped by the overlying interval. Mounds identifying and analyzing depositional sequences. The
are commonly present in sea-floor lows or channels, sug- interrelation of the sequences can be determined and the
gesting that sea-floor topography controls turbidite flow geologic sequence of events established. Once these are
movement. Mounding may be the most direct indication done the interpretation of specific fades can be more eas-
of turbidite accumulations (Fig. 12B). ily accomplished and areas with hydrocarbon potential
3. Clinoform seismic reflection patterns present on a defined.
depositional dip-oriented seismic section which are indica- The following investigations are presented as examples
tive of a prograded fluvial-dominated delta. The turbidite of regional studies using the seismic stratigraphy
sequence may be present beneath or basinward of the approach. They may be used as guides in studies under-
clinoform reflections (Fig. 12C). taken by the reader.
4. A clinoform seismic reflection interval of a fluvial-
dominated delta (dip section) that remains constant in North Sea
thickness or thins into a deepening basin suggests the pos-
sibility of underlying turbidites (I, of Fig. 12C). A paper by Parker (1975) on lower Tertiary sand devel-
5. A seismic fades interval which onlaps and then possi- opment in the North Sea provides a good discussion of the
bly offlaps a depositional slope may suggest a turbidite interrelation of delta and turbidite depositional sequences.

DELTA
NW COMPLEX TURBIDITE BASIN

TOP
PALEOCENE

TOP CHALK

GAP OF 8 Ml

r BETWEEN SECTIONS
TURBIDITE BASIN

TOP
PALEOCENE
^=-1= ^ TOP CHALK

FIG. 14—Seismic sections illustrating seismic reflection patterns of Paleocene seismic facies in central North Sea. Seismic lines run
from northwest to southeast across the map in Figure 13 (modified from Parker, 1975).
1282 Delta and Turbidite Sequences

Figure 13 is an isopach of Paleocene sands in the central The mounds are unlike the rest of the turbidite seismic
North Sea. The aspect of this map to be emphasized is the facies in which they occur, suggesting a special set of con-
division into delta, slope (prodelta), and basin (turbidite) ditions for mound development. Moore and Fullam
deposits. The source of sediments for these systems was on (1975) showed that an essentially continuous transporta-
the northwest. tion network of deep sea fan channels and deep ocean
The seismic line in Figure 14 extends across these deposi- channels is available to transport elastics from their source
tional sequences from northwest to southeast over a total to as far out as the abyssal plain. This transportation sys-
distance of 40 mi (64 km), with a gap of 8 mi (12.8 km) tem also provides the means to concentrate sand-size elas-
between the pieces of the section. Unfortunately, the seis- tics into large accumulations. The assumption is that this
mic section did not reproduce well, and definition of the accumulation would be in the form of a fan. A sufficiently
seismic fades is difficult. large fan should resuh in a seismically detectable mound.
The delta plain and prodelta seismic facies are present in
their predictable order. Prograded clinoforms of the pro- United States Gulf Coast
delta facies have flattened out and continue far into the
basin at a low rate of dip. Because of the poor quality of Studies of the Upper Cretaceous Woodbine-Tuscaloosa
reproduction, the precise point of change from prodelta to Formations in the Gulf Coast of the United States have
turbidite seismic facies could not be determined and its shown that separate and different depositional sequences
position on this line is speculative. deposited these rocks (Fig. 15). The Woodbine Formation,
Subtle mounding is present within and toplapped by the present in the East Texas basin is the stratigraphic equiva-
turbidite seismic facies interval on the right side of Figure lent of the T\iscaloosa Formation in Louisiana, Missis-
14. This mounding is a considerable distance from its most sippi, Alabama, and Florida. Separated by the Sabine
probable source, the delta complex to the northwest. Simi- uplift on the Texas-Louisiana line these formations
lar mounds are present in Tertiary rocks elsewhere in the included fluvial systems that end in the south at the Lower
North Sea. These mounds contain sandstone reservoirs Cretaceous Edwards reef trend. Deltas developed where
and major fields are associated with some of them (e.g., these fluvial systems crossed the Edwards reef, and the
Forties oil field and Frigg gas field). data indicate that each delta is different (Fig. 16).

TURBIDITE
REEF TREND
..^ SHELF EDGE

FIG. 15—United States Gulf Coast showing distribution of Upper Cretaceous rocks. Sabine uplift separates Woodbine and Tbsca-
loosa Formations. Interval between Lower Cretaceous Edwards reef and shelf edge is outer carbonate shelf. Distribution of units
is exaggerated for clarity.
 O. R. Berg 1283

In east Texas, the Woodbine fluvial system crossed the these Woodbine sandstones, suggests a deep water origin
Edwards reef onto a Lower Cretaceous outer carbonate for the lower and middle Woodbine sandstones and a shelf
shelf and developed a wave-dominated delta. In central margin origin for the upper sandstones. The results of this
Louisiana, where there is no outer shelf, the Tuscaloosa investigation generally agree with the Foss interpretation,
delta developed on the Edwards reef trend and dumped but the upper sandstones are interpreted to be strandline in
turbidites into the fore-reef trough. When the turbidite origin.
section became thick enough, the fluvial-dominated delta Tuscaloosa Formation.—In central Louisiana, the seis-
prograded seaward over the turbidites. mic data (Fig. 19) shows a steep fore-reef slope south of
Woodbine Formation.—In east Texas, a seismic dip sec- the Edwards reef trend. Because of this steep slope, the
tion (Fig. 17) across the Lower Cretaceous outer carbon- Tbscaloosa delta was unable to migrate southward and
ate platform shows a shingled progradational pattern in stalled on the reef trend. The delta was abundantly sup-
the Woodbine interval. A stratigraphic cross section (Fig. plied with sediments and a massive turbidite-fan complex
18) in the same area shows that south-dipping strandline was deposited in the fore-reef trough.
sands are present in the upper part of the Woodbine sec- Within the Tuscaloosa interval in front of the reef, upper
tion. These sandstones form stratigraphic traps which inclined reflections terminate against a seismic event (A,
produce gas in several fields (e.g., Hortense and Seven on Fig. 19) by downlap; the underlying south-dipping
Oaks gas fields). Vail et al (1977) demonstrated that the reflections are toplapped by this same event. This pattern
Woodbine sandstones can be correlated between wells of seismic reflections is interpreted to be the result of the
using the reflection pattern present in the Woodbine inter- Tuscaloosa delta prograding over the underlying turbidite
val as a guide. However, the resolution of the seismic data sequence. This reflection pattern is not present on all simi-
is not good enough to permit direct detection of individual larly located seismic sections that were examined. Its
sandstones. absence may be the result of poor resolution of the seismic
The relation of turbidites to this delta is not clear. Sand- data, no progradation of the delta, or both. The subsur-
stones are present in the middle and lower part as well as face examination of well data in the area supports the
the upper part of the Woodbine section. The middle and interpretation of a delta prograding over turbidites (Fig.
lower sandstones appear to be turbidites which have been 20). The turbidites first onlapped the Lower Cretaceous
derived from strandline sands on the margin of the delta. fore-reef slope and, as the delta prograded, the turbidite
Foss (1979), in describing the depositional environment of sequence also migrated basinward, resulting in an offlap-

c SOUTH

EAST TEXAS-WOODBINE (WAVE DOMINATED)

LOUISIANA-TUSCALOOSA (FLUVIAL DOMINATED)

PROGRADED DELTA SEQUENCE

FIG. 16—Comparison of Upper Cretaceous Woodbine and Tuscaloosa deltas and turbidites.
1284 Delta and Turbidite Sequences

WOODBINE PROGRADED DELTA

EDWARDS
REEF
1 L. CRETACEOUS
h- SHELF EDGE
Sc SECS

-*:T0P
L.CRET,

FIG. 17—Shingled seismic reflections in Woodbine sequence indicates wave-dominated delta. Seismic section courtesy Seiscom
Delta, Inc.

<>
HASSIE HUNT TRUST
<>
SHELL
<> o <i i>
DIAMOND SHAMROCK DIAMOND SHAMROCK MITCHELL ENERGY GEORGE MITCHELL
<>
UNION CARBIDE
WIRT DAVIES • ! SOUTHLAND CORP CORP CO AND ASSOC PETROLEUM CO
PAPER MILLS • ! FERGUSON » 1 WBT DAVIS ET AL »1 -pnuuHciT^l^i '-"UIS BAHKLEV • 1 CHAMPION PAPER »

TOP
CRETACEOUS

— AUSTIN CHALK

WOODBINE

Ml 2 1

FIG. 18—Subsurface section showing depositional attitude of strandline sands in upper Woodbine section.
 0 . R. Berg 1285

LOCATION^ AVOYELLES
OF LINE PARISH
:+« TUSCALOOSA
L. GRET.
CARBONATES

DELTA

TURBIDITES:^:

IKSmifSECS

I TUSCALOOSA
r L. GRET.

FIG. 19—Seismic section in central Louisiana showing seismic response of Tuscaloosa delta and turbidites and their relation to
Lower Cretaceous carbonates.

NORTH,
<> ^ <> <i
s o LOUISIANA CHEVRON, HUNT PETROLEUH CORP SO LOUISIANA AMOCO PROD CO EXCHANGE OIL AND GAS CORP UNITED PROD CORP
PROD CO U S A , INC TRANS-MATCH » 1 PROD CO GEORGIA PACIFIC WARREN T PRICE » 1 BAY SCOUT OF AMERICA .
HL LAWS CO ASHLAND KIZER*1 CORP *\
»1 PLANTATION «1

AUSTIN
CHALK

TUSCALOOSA

TUSCALOOSA
FM,

FEET

•500

FIG. 20—Subsurface section illustrating stratigraphic interpretation of Tuscaloosa deltaic and turbidite sections.
1286 Delta and Turbidite Sequences

; _tt
9N - -- rf.
'-; :.:::^'
-^•s •-"• 1"--
••• "Si _t —
PUTAH

t
N
• 8N

7N
•••
SINK>' SACRAK ENTO ( # GAS FIELD

;••.
KM
t 0 5 10
6N I-
0 5 10
m Ml

RIVER
5N ISLANP.
K"'-'
A

RIO
4N ./, VISTA...-

I _ ;
•J

•'•*A
f - 4

2N ,-._ STOC <TON

^CONC DRD ^ • . : - .

f1
s
• • i _

1W IE ••::=2E 3E 4E 5E 6E .:::

FIG. 21—Southern Sacramento Valley, California, showing location of gas fields. AA' is approximate location of Figures 22 and
23 (modified from Drummond et al, 1976).

ping of the turbidite sands. within it. The Winters seismic facies is at the toe of, and
Major discoveries (e.g., False River and Irene gas fields) below, these clinoforms and has subparallel reflection
have been made in deltaic and turbidite reservoirs in large, events with fair to good continuity. The upper Winters
deep, rollover structures on down-to-the coast faults (Fig. interval in this area is characterized by high-amplitude
19). The potential for stratigraphic traps, especially in seismic events.
pinch-outs of turbidite sands, seems to be particularly The stratigraphic cross section (Fig. 23) parallels and is
good. close to the seismic section. The cross section shows that
Sacramento Valley.—In the Sacramento Valley of CaU- the Starkey sandstones are thick and laterally continuous
fornia, exploration for structural and stratigraphic traps whereas the Winters sandstones pinch out eastward, off-
in Upper Cretaceous sandstones has been under way for lap to the west, and are toplapped by the Delta shale inter-
severed years. A paper by Drummond et al (1976) devel- val.
oped a lithofacies model of some of these Upper Creta- The Starkey sandstones and Delta shale considered as a
ceous rocks placing them in shelf and basinal unit resulted in an interpretation of these Upper Creta-
environments. ceous rocks as a delta sequence. The Starkey interval rep-
Figure 21 is a map of the southern Sacramento Valley resents the delta-plain facies, and the Delta shale is the
showing the location of gas fields. A seismic section (Fig. prodeha facies. The Winters interval is an underlying tur-
22) and stratigraphic section (Fig. 23) across the Putah bidite sequence which prograded as the delta migrated sea-
Sink field illustrate the depositional patterns of the Upper ward.
Cretaceous Winters sandstone, Delta shale, and Starkey Drummond et al (1976) pointed out that trapping in
sandstone. Winters sandstones is commonly stratigraphic because of
The seismic section in Figure 22 shows the Starkey seis- updip pinch-outs of these sandstones. In the Starkey
mic fades as characterized by reflections that are parallel sequence, most traps are structural because the continuous
and horizontal, and have fair continuity. The DeUa shale nature of deltaic sand limits stratigraphic trapping possi-
seismic interval has prograded clinoform reflections bilities. The Putah Sink gas field (Fig. 23) is an example of
 O. R. Berg 1287

w
STARKEY SSl
DELTA
SHALE
WINTER SS -—"-2
>rNECK
POINT
PPCo SHELL
BELTRAMI A-1 COWELL 1-28 * 1 SHOSHONE COWELL

A SECS

0 1 KM
t-
1MI

DELTA
SHALE

FIG. 22—Seismic section in southern Sacramento Valley, California, showing seismic character and interpretation of seismic
sequences of Upper Cretaceous Starkey sandstone. Delta shale, and Winters sandstone. See Figure 21 for approximate location of
section.
PUTAH SINK GAS FIELD EAST
—
SHELL OCCIDENTAL PEARSON
PPCo
BELTRAMI-1 COWELL-1 GILDE-1 SIEBERT-1

STARKEY SS

WINTER SS

NECK POINT

FIG. 23—Subsurface section illustrating stratigraphy of Starkey sandstone, Delta shale, and Winters sandstone and their relation
to each other. Section also shows stratigraphic trap in turbidite sands of Putah Sink gas field (modified from Drummond et al,
1976). See Figure 21 for approximate location of section.
1288 Delta and Turbidite Sequences

stratigraphic trapping in a Winters turbidite sandstone. carbon exploration: AAPG Mem. 26, p. 213-248.
The trap is the result of an updip termination of a turbidite Drummond, K, F., E. W. Christensen, and K. D, Berry, 1976, Upper
sandstone. Cretaceous lithofacies model, Sacramento Valley, California, in
Tomorrow's oil from today's provinces: AAPG Pacific Sec. Misc.
Pub. 24, p. 76-88.
CONCLUSIONS Fisher, W. L., and J. H. McGowen, 1967, Depositional systems in the
Wilcox Group of Texas and their relationship to occurrences of oil
Seismic stratigraphy is the interpretation of subsurface and gas: Gulf Coast Assoc. Geol. Socs. Trans., v. 17, p. 105-125.
et al, 1969, Delta systems in the exploration for oil and gas—a
stratigraphy from seismic data. The identification of seis- research colloquium: Austin, Texas Univ. Bur. Econ. Geology.
mic sequences and the analysis of seismic facies are proce- Foss, D. C , 1979, Depositional environment of Woodbine sandstones,
dures used in the interpretation of seismic data. A regional Polk County, Texas:Gulf Coast Assoc. Geol. Socs. Trans., v. 29, p.
approach is the best method to identify depositional 83-94.
sequences. The spatial relationship of sequences can be Galloway, W. E., 1975, Evolution of deltaic systems, in Deltas, models
for exploration: Houston Geol. Soc, p. 87-89.
determined and the order of geologic events developed. and L. F. Brown, Jr., 1972, Depositional systems and shelf -
Those areas with hydrocarbon potential may be then slope relationships in Upper Pennsylvanian rocks, north-central
delineated and evaluated. Texas: Texas Univ. Bur. Econ. Geology Rept. Inv. 75,62 p.
Models of delta and turbidite sequences are useful in Mitchum, R. M., Jr., 1977, Seismic stratigraphy and global changes of
sea level, part 11: glossary of terms used in seismic stratigraphy, in
seismic stratigraphy interpretations. Models can provide a Seismic stratigraphy—applications to hydrocarbon exploration:
method to predict the location of potential reservoir rocks. AAPG Mem. 26, p. 205-212.
Deltas are classified into different groups by distinctive P. R. Vail, and J. B. Sangree, 1977a, Seismic stratigraphy and
seismic reflection patterns. An understanding of the depo- global changes of sea level, part 6: stratigraphic interpretation of
sitional framework of prograded deltas permits the analy- seismic reflection patterns in depositional sequences, in Seismic
stratigraphy—applications to hydrocarbon exploration: AAPG
sis of seismic facies to delineate those facies which may Mem. 26, p. 117-133.
contain sandstone reservoirs. Although sandstones may and S. Thompson, III, 1977b, Seismic stratigraphy and
not be directly identified by seismic methods, it is possible global changes of sea level, part 2: the depositional sequence as a
from seismic reflection geometry to predict their deposi- basic unit for stratigraphic analysis, in Seismic stratigraphy—
appHcations to hydrocarbon exploration: AAPG Mem. 26, p. 53-
tional attitude. 62.
Seismic events observed on seismic lines that might indi- Moore, D. G., 1966, Structure, htho-orogenic units, and postorogenic
cate the presence of turbidites include troughs, canyons, basin fill by reflection profiling—California continental border-
mounds, and fluvial-dominated deltaic seismic sequences. land: San Diego, U.S. Navy Electronic Lab., 151 p.
Turbidite systems are usually associated with deltas or sub- Moore, G. T, and T J. Fullam, 1975, Submarine channel systems and
their potential for petroleum localization, in Deltas, models for
marine canyons. The size and distribution of the turbidites exploration: Houston Geol. Soc, p. 165-189.
depend on the available sediment supply. Prograded Normark, W. R., 1978, Fan valleys, channels, and depositional lobes
fluvial-dominated deltas are usually abundantly supphed on modern submarine fans: characters for recognition of sandy tur-
with sediments and may be expected to have massive pro- bidite environments: AAPG Bull., v. 62, p. 912-931.
Parker, J. R., 1975, Lower Tertiary sand development in the central
graded turbidites in conjunction with them. A deltaic North Sea, in A. W. Woodland, ed.. Petroleum and the continental
clinoform reflection pattern which thins or is constant in shelf of north-west Europe, v. 1, geology: New York, John Wiley
thickness into a sedimentary basin may be an indication of and Sons, p. 447-453.
underlying turbidites. Delta-front troughs and submarine Ramsayer, G. R., 1979, Seismic stratigraphy, a fundamental explora-
canyons act as chutes for turbidity flows and are usually tion tool: Offshore Technology Conf. Proc, v. 3, paper 3568, p.
1859-1867.
filled with clay when the flows cease. The turbidite fans Rich, J. L. , 1951, Three critical environments of deposition and crite-
generally develop near the mouth of the trough or canyon. ria for recognition of rocks deposited in each of them: Geol. Soc.
Mounds may develop at some distance from their source America Bull., v. 62, p. 1-20.
and their occurrence may be largely controlled by sea- Sangree, J. B., and J. M. Widmier, 1977, Seismic stratigraphy and
floor topography. global changes of sea level, part 9: seismic interpretation of clastic
depositional facies, in Seismic stratigraphy— applications to hydro-
The stratigraphic framework of turbidite-fan sequences carbon exploration: AAPG Mem. 26, p. 165-184.
demonstrates that they are a unique hydrocarbon system. 1979, Interpretation of depositional facies from seismic
The turbidite sequence may contain rich source rocks data: Geophysics v. 44, p. 131-160.
intermixed with good reservoir sandstones. The deposi- etal, 1978, Recognitionofcontinental-slope seismic facies, off-
shore Texas-Louisiana, in Framework, facies, and oil-trapping
tional patterns and distribution of the sands are conducive characteristics of the upper continental margin: AAPG Studies
to stratigraphic and structural trapping of hydrocarbons. Geology 7, p. 87-116.
The identification of a delta or turbidite sequence does Vail, R R., R. G. Todd, and J. B. Sangree, 1977, Seismic stratigraphy
not mean that sandstone reservoirs will be present. If and global changes of sea level, part 5: chronostratigraphic signifi-
cance of seismic reflections, in Seismic stratigraphy—applications
sand-sized material was not available to the system, then to hydrocarbon exploration: AAPG Mem. 26, p. 99-116.
concentrations of sand will not be present. Walker, R. G., 1978, Deep-water sandstone facies and submarine fans:
models for exploration for stratigraphic traps: AAPG Bull., v. 62,
p. 932-966.
REFERENCES CITED 1979, Turbidites and associated coarse clastic deposits, in
Facies models: Reprint Ser. 1, Geol. Assoc. Canada, Geoscience
Canada, p. 91-103.
Brown, L. E, Jr., and W. L. Fisher, 1977, Seismic-stratigraphic inter- Wilde, R, W. R. Normark, and T. E. Chase, 1978, Channel sands and
pretation of depositional systems: examples from Brazilian rift and petroleum potential of Monterey deep-sea fan, California: AAPG
pull-apart basins, in Seismic stratigraphy—applications to hydro- Bull., v. 62, p. 967-983.