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GEOLOGICAL JOURNAL, VOL. 21, 225-255 (1986)
Comparative Tertiary petroleum geology of the Gulf Coast, Niger, and Beaufort-Mackenzie delta areas
Doris M. Curtis Curtis and Echols, Consultants, Houston, Texas, U.S.A.
Oil and gas are produced from Tertiary sandstone reservoirs in deltaic and related depositional systems in the Gulf Coast (U.S.), Niger (Africa), and Beaufort-Mackenzie (Canada-Alaska) basins. In each area there is an orderly. predictable interrelationship of sedimentation, stratigraphy, depositional environment, and structure, with the characteristics, ages, and distribution of producing trends.
In comparing and contrasting the three areas, it is apparent that they have many essential aspects in common, resulting from the fact that they are relatively young, subsiding paralic basins on ‘Atlantic type’ or passive margins. They contain thick accumulations of deltaic terrigenous sediments that have prograded in regressive basin-filling sequences as the basins subsided. Therefore each has a vertical gross lithologic sequence with shale at the base, overlain by interbedded sandstones and shales, overlain by massive sandstones. The vertical sequence is repeated laterally from the basin landward. In each basin the stratigraphic units of the sequences thicken basinward across a series of normal, listric, down-to-the-basin syndepositional faults, with which are associated ‘rollover’ anticlines which form traps. Trapping associated with diapiric structures is also characteristic of all three areas.
Although similarities among these areas is striking, significant differences are related to their different geologic settings and geologic histories. For example, the pksence of salt in the Gulf Coast basin has resulted in a wide variety of salt dome-related trapping mechanisms in addition to the shale diapirs and roll-over anticlines common to all three areas. Pre-Tertiary tectonic settings, different in each case, control basin configurations and affect structural trends. Vertical and lateral differences in depositional systems and sequences, as well as variations in delta morphology and sandstone geome- tries, result from variations in ratios of rates of deposition to rates of subsidence.
The framework for the occurrence of oil and gas is well understood in the maturely-explored and intensively-studied Gulf Coast Tertiary basin. Concepts developed there can be applied to developing the less-explored Niger basin and to exploring the frontier Beaufort-Mackenzie basin. An extensive bibliography is included.
KEY WORDS Deltas Tertiary Gulf Coast Niger delta Beaufort-Mackenzie delta Stratigraphy Sedi- mentation Contemporaneous faulting Growth faults Gravity tectonics Petroleum habitat
1. Introduction
A large portion of the world’s oil and gas reserves are in Tertiary terrigenous fill on passive continental margins. The Gulf Coast (U.S. Gulf of Mexico), the Niger
OO72-1050/86/03022~3 1$15.50 0 1986 by John Wiley & Sons, Ltd. Accepted 30 October 1985
226 D. M. CURTIS
delta, and Canadian Beaufort-Mackenzie delta (Figure 1) represent three of the most significant hydrocarbon accumulations of this type. These areas are characterized by similar geological features related to the fact that they are sites of Tertiary deltaic depositional systems dominated by gravity tectonics. They also have interesting, but less significant, differences related to their geographic setting, tectonic setting, and geological history. The purpose of this paper is to compare the three areas and to emphasize the basic similarities.
Each area represents a different stage of exploration and development. The U.S. Gulf Coast onshore Tertiary province is a maturely explored and developed portion of the Gulf of Mexico basin; the Niger delta Tertiary province is less maturely explored and developed; and the Beaufort-Mackenzie delta is still a frontier area. The framework for the occurrence of oil and gas is well understood in the maturely-explored and intensively studied Gulf Coast Tertiary basin. Concepts developed there can be applied to developing the less-explored Niger delta and to exploring the frontier Mackenzie-Beaufort basin.
The unifying concept for understanding basins of this type is the orderly, predictable interrelationship in time and space of stratigraphy, tectonics, and hydrocarbon distribution. Since the concepts are well documented for the Gulf Coast and the Niger delta no attempt at redocumentation is made in this paper. Instead, an extensive bibliography has been included. The relevant concepts are incorporated in a broader treatment of terrigenous sedimentation by Galloway and Hobday (1983).
Figure 1 . Index maps: 1-Gulf Coast (U.S.) , 2-Niger (Nigeria, Africa), 3-Beaufort-Mackenzie (Canada-Alaska).
PETROLEUM GEOLOGY TERTIARY DELTAS 227
The paper is organized in four sections: Geologic Setting, Regional Geology, Petroleum Geology and Hydrocarbon Habitat, and Conclusions. The regional geology of the Gulf Coast Tertiary province (stratigraphy and structure) forms the major portion of this paper.
2. Geologic Setting
2a. Gulf Coast Tertiary Province The Tertiary sediments of the U.S. Gulf Coast (Figure 2) are part of a broad
and extensive Mesozoic and Cenozoic coastal plain that rims the Gulf of Mexico and is surrounded by Palaeozoic and Precambrian ‘basement’ rocks. The Tertiary sediments partly fill a bowl-shaped basin (Gulf of Mexico) defined by a Lower Cretaceous carbonate shelf edge. Several prominent older structural features surround the present coastal plain and continental shelf (Figure 3).
The coastal plain was built by progradation of huge volumes of terrigenous sediment derived from the extensive continental interior drainage basin that began
LEGEND
Paleocene - Eocene
@ Piio-Pieistocene
Mio-Pliocene Cretaceous
Oligocene .Basement.
Figure 2. Generalized stratigraphic map, Gulf Coast
228 D. M. CURTIS
Figure 3. Generalized tectonic map, Gulf Coast, showing major structural features, salt basins, Lower Cretaceous shelf edge, and major growth-fault trends.
to develop in early Tertiary time during the Laramide orogeny. Construction of the coastal plain proceeded generally from southwest to northeast as deposition in successively younger depocentres prograded into the Gulf of Mexico over continental crust to build the present continental shelf.
2b. Niger Delta Tertiary Province The Tertiary Niger delta is partly confined in a structural trough that contains
Upper Cretaceous and Tertiary sediments and is bounded by positive elements containing older Cretaceous rocks and a pre-Mesozoic ‘basement’ (Figure 4). Beginning in late Cretaceous time, an extensive interior drainage system has funneled huge volumes of terrigenous sediment into this structural depression, so that Niger delta deposits have filled the trough and prograded across the narrow pre-existing continental shelf forming a narrow Tertiary-Quaternary coastal plain (Figure 5 ) and prograding beyond the continental margin over oceanic crust.
2c. Beaufort-Mackenzie Tertiary Province The Beaufort-Mackenzie Tertiary province consists of the Mackenzie delta and
the Beaufort basin, in a structural depression developed at the junction of three structural elements: a fold belt on the southwest, a fault zone on the southeast, and an ocean basin on the north (Figure 6). Since Cretaceous time, huge volumes of terrigenous sediment derived from an extensive drainage basin in the Yukon and Northwest Territories built a narrow coastal plain and prograded over the Beaufort shelf into the Canada Basin (Figure 7) underlain by oceanic crust.
PETROLEUM GEOLOGY TERTIARY DELTAS 229
Figure 4. Generalized geologic map, Niger delta (after Weber and Daukoru 1975)
3. Regional Geology
3a. Gulf Coast Tertiary Province Comprehensive treatment of the details of Gulf Coast Tertiary petroleum
geology does not exist in the public domain. Framework for the Occurrence of Oil and Gas in the Gulf Coast Tertiary (Robertson Research with Curtis and Echols 1980) is a summary of the geology; Jackson and Galloway (1984) develop and apply concepts. The highly generalized stratigraphic map of the Gulf Coast (Figure 2) shows the coastal plain deposits becoming progressively younger gulfward, forming a gently dipping monocline. Tertiary progradation across the old Lower Cretaceous shelf edge began in the southwest, where thick sequences of Palaeo- cene and Eocene sands and shales derived from the Laramide orogeny were concentrated in depocentres. As Tertiary sedimentation began to fill the basin margin area, depocentres of deltaic sedimentation (Figure 8) shifted eastward and gulfward, first being supplied by relatively small rivers draining from the west and northwest, and later, beginning in Miocene time, by the more-integrated ancestral Mississippi River drainage. The locus of major Palaeogene depocentres was gen- erally in the western Gulf between the Rio Grande and the Texas-Louisiana border. Major Neogene depocentres, related to an integrated Mississippi River drainage system, were concentrated in the northcentral, or Louisiana, portion of the Gulf.
Stratigraphy. The Gulf Coast Tertiary stratigraphy represents a typical, gen- erally regressive, offlapping, prograding, basin-filling sequence in a late stage of
230 D. M. CURTIS
100km I
LEGEND
Plio-Pleistocene Paleocene -
Mio-Pliocene Cretaceous
Eocene
I Oligocene 'Basement' I Figure 5 . Generalized stratigraphic map, Niger delta (after Weber and Daukoru 1975).
history of a passive Atlantic-type continental margin. The entire thick Tertiary prism (more than 15 km of sandstone and shale) has prograded over older sandstones, shales, limestones, and evaporites on continental crust, building the continental shelf out into the Gulf basin in a series of thick progradational wedges in successively younger depocentres. Oceanic crust underlies the centre of the basin.
The vertical and lateral stratigraphic sequences that develop in the offlapping passive margin filling are distinctive. In each depocentre the sequence represents numerous regressive-transgressive cycles. In general, each regressive sequence represents an 'offlap', or seaward progradation beyond an earlier sequence, estab- lishing a new depositional shelf edge. Each transgressive sequence represents an advance or onlap of the sea landward beyond an older shoreline.
The transgressive phase of each cycle is a relatively thin marine shale with a basal transgressive sand. The regressive phase, or sequence of regressive cycles, consists laterally of four generalized facies (Figure 9): (1) a landward non-marine and near-shore massive sand facies representing alluvial and upper delta plain deposits; (2) a mixed near-shore deltaic and shallow marine sand-shale facies representing lower delta plain, delta fringe, and delta front deposits; (3) a marine
PETROLEUM GEOLOGY TERTIARY DELTAS 23 1
LEGEND / Coastal plain Basement
Figure 6. Generalized geologic map, Beaufort-Mackenzie delta area (after Hawkings and Hatfield 1975).
Figure 7. Generalized stratigraphic map, Beaufort-Mackenzie area (after Hawkings and Hatfield, 1975).
232 D. M. CURTIS
LEGEND
Paleocene - Eocene
Plio-Pleistocene
Mio-Pliocene Cretaceous
Oligocene 'Basement'
Figure 8. Generalized map of depocentres, Gulf Coast, showing eastward and gulfward progression with time.
LEGEND
Pro-delta
Slope
River and
Lower delta and
DOWNDIP upper delta
m delta fringe
UPDIP
Figure 9. Diagram showing lateral and vertical facies relationships in prograding delta sedimentation.
PETROLEUM GEOLOGY TERTIARY DELTAS 233
shaly facies representing pro-delta and open marine deposits; (4) bathyal marine shales. As each regressive package progrades, a younger more-landward facies comes to overlie older more seaward facies, so that the resulting vertical-upward sequence (shale to sand-shale to sand) is the same as the lateral seaward to landward sequence (shale to sand-shale to sand).
The vertical sequence that results from repeated deposition of offlapping regress- ive sequences that prograde into the Gulf with time combines similar facies of different ages into lithologic units or formations that are diachronous (Figure 10). Each formation can range in age from early Tertiary through Quaternary. The marine shale formation forms the bottom of the sequence. It is an undercompacted, overpressured, low density mobile shale. The sandstone-shale formation forms the middle of the vertical sequence, and the sandstone formation the top. For a given time interval the facies tracts can be segregated biostratigraphically using the fossil assemblages in the transgressive marine shales between regressive packages. Figure 11, illustrating this concept, is drawn from data from wells penetrating Miocene sections in the Gulf Coast.
The vertical and lateral geometry of facies tracts in delta stratigraphy is primarily related to two factors in the geographic and geologic setting: (1) dominance of rivers over waves and tides in the delta system; (2) interrelation of variable rates of sediment supply and subsidence. Many variations in delta morphology lie between two end-members representing river dominance and wave dominance (Figure 12). The birdsfoot delta of the Mississippi River is river dominated (I, Figure 12). Dip-oriented sand bodies deposited in distributary channel systems are characteristic of river-dominated deltas. In the Gulf Coast, river-dominated delta deposits have been identified in major depocentres throughout the Tertiary. Wave- and longshore current-dominance produce an arcuate-shaped delta like the Niger delta (VI, Figire 12), with prominent strike-oriented sand bodies including beaches, bars, and delta fringe deposits. Deltaic deposits of this type can also be recognized in many parts of the Gulf Coast Tertiary.
Development of three-dimensional delta geometry through time (Figure 13) is largely a function of sedimentationfsubsidence dynamics in the basin. The Gulf Coast continental crust has subsided relatively slowly, as compared with the high rate of sedimentation. Normally, in the Gulf Coast Tertiary offlapping sequences, the rate of sediment supply exceeds the rate of subsidence, river-dominated deposits are emphasized, and seaward progradation dominates. The resulting typical lithostratigraphy (Figure 14) is as described. However, when a rate of sediment supply comparable with rate of subsidence is maintained through time, sediments tend to accumulate vertically or aggrade rather than prograding (Figure 15). The lithofacies units thus tend to stack, so that the three major facies or formation boundaries can be almost vertical. When rate of sediment supply remains less than rate of subsidence, neither progradation nor continued vertical aggradation takes place. Instead the ‘regressive’ sediment sequences tend to be retrogradational (Figure 16) so that the net effect is lateral accretion. The sand facies forms the base of the sequence, with the marine shaly facies at the top. In effect, this sequence appears to be transgressive because the rate of sedimentation is less than rate of basin subsidence.
Throughout the Gulf Coast Tertiary each of these situations has occurred, but the dominant mode has been progradation so that the entire broad margin of the Gulf basin (coastal plain and continental shelf) has been built from the Lower Cretaceous rim out to the present shelf edge. Extensive progradation has taken place over the slowly subsiding continental crust. Several significant effects, result-
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Figure 12. Examples of delta morphology and sand-body orientation in modern deltas. Arrows indicate regional depositional strike (after Coleman and Wright 1974).
ing from density inversions in the lithologic sequence, are typical of areas of deltaic deposition:
1. Growth faults associated with each depocentre originate where regressive sands prograde beyond older marine shales across a former shelf edge (Figure 17), or where slope failure of the low-density undercompacted shale is initiated. Regional growth fault belts are a direct record of how the delta has prograded with time. Movement on these faults continues through several cycles so that for each depocentre there is a family or system of growth faults (Figures Ma, b) associated with each of the many regressive cycles that make up a given depocentre fill. Major growth on each fault is progressively younger seaward. Slump faults at depositional shelf edges outside of major depocentres are a similar feature.
2. The marine shale facies tends to be undercompacted and abnormally over- pressured or geopressured (Figure 19); that is, fluid pressures exceed hydro- static pressure. Sands within the geopressured shale section tend to be
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PETROLEUM GEOLOGY TERTIARY DELTAS DIST4i EDGE OF U N I T I DELTb
241
1 1 1
UNIT I S L O P E :I: UNIT I S H E L F
O I A G R A M D E F I N I N G T L A M S
__L_ U N I T 2 D E L T A
U N l T I DELTA
/ - L__1
C O L U M N A R S E C T I O N
/ I
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RELATION OF DEtTAlC SEDIMENTATION A N D C O N T E M P O R A N E O U S STRUCTURE
CONCEPTUAL DIAGRAMS l S C 4 L E N O N E I
PALEOSTRUCTURAL DIP SECTIONS DELTAIC SEDIMENTATION A N D STRUCTURE
CONCEPTUAL DIAGRAMS
( S C A L E N O N E I
Figure 17. Conceptual diagrams showing interrelation of contemporaneous sedimentation and structure (Curtis 1970).
242 D. M. CURTIS
a
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P ! t <
I N D E X MAP
NW SE
Q- MIOCENE SECTION (SCHEMATIC) 0 10 20 Y) 0 50 17.10’
VfR I ICAL EXAGGERATION I 1 0
Figure 18. Map and cross-section showing progradation of Miocene deltas (18a) and growth faults in southeast Louisiana, Gulf Coast. Individual deltas (A-N) have prograded from northwest
A, B, C . . .) to southeast ( J , K , L . . .).
PETROLEUM GEOLOGY TERTIARY DELTAS 243
‘sh
Figure 19. Diagram showing relation of depositional environment and lithofacies, geopressure, and position of principal hydrocarbon accumulation. Left to right: Vertical distribution of depositional environments; vertical pore pressure gradient from hydrostatic or normal (0.465 psuft.) to abnormal or overpressured; electric log curves and hydrocarbon symbols; trend of shale resistivity with pore pressure.
abnormally-pressured as well. Such sands are detached from deltaic depos- itional systems; they originate in turbidite or debris flow deposits, and in submarine channels and fans. Because such sand bodies are generally enclosed in shales, formation waters cannot be easily expelled during burial and abnormal pressures result.
3. The undercompacted shales are highly mobile. Shale domes, which may become diapiric, develop in the marine shale facies at the toe of major listric fault systems (Figure 20), or wherever sufficient density contrast exists.
Structure. Very few Gulf Coast structural elements have been inherited from older tectonic settings. The prevailing and dominant tectonic style results from gravity tectonics, reflecting the intimate interrelation of stratigraphy and structure. Systems of normal listric faults, arcuate and subparallel with the depositional strike, are contemporaneous with deposition and are intimately related in time and space with the depocentres. These faults offset surfaces of active deposition. They are known by various names such as syndepositional faults, growth faults, synsedimentary faults, contemporaneous faults. Fewer than ten per cent of these regional growth faults are shown on Figure 3. The majority of the faults are down- to-the-coast. As these faults continue to be active during deposition, a considerably greater thickness of sediments accumulates on the downthrown side of each fault. Two kinds of structures are directly related to this type of sedimentation-structure unit. One is a ‘rollover’ into the fault (i.e. dip reversal) in the downthrown block, resulting from ‘reverse drag’ that accompanies the synsedimentary faulting. The other is a broad uplift or upwarp at the toe of the fault. As these structures are
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PETROLEUM GEOLOGY TERTIARY DELTAS 245
buried they are preserved as anomalies that interrupt the gentle regional gulfward dip. The rollover is preserved as a rollover anticline. Antithetic (counter-regional) faults are commonly associated with the rollover structures. The broad uplift becomes a deep-seated, low-relief domal structure which may become a shale diapir. Both of these structures are typical results of gravity tectonics in delta settings. Families of growth faults and related structural anomalies associated with the principal progradational wedges have been termed ‘megatectonic’ units.
The highly generalized cross-section (Figure 21) shows the influence of Jurassic salt which intrudes the Tertiary sediments in diapiric structures: salt domes and salt ridges. Salt domes and salt ridges are prominent and distinctive features of Gulf Coast geology. These diapiric structures are caused by the gravitational instability resulting from the contrast between the underlying low-density Jurassic salt and the overlying terrigenous sediments.
3b. Niger Delta Tertiary Province A detailed and comprehensive summary of the petroleum geology of the Niger
delta is found in Whiteman (1982). The very generalized geologic map of the Niger delta (Figure 4) shows the coastal plain deposits becoming progressively younger seaward as they fill the pre-Tertiary structural depression and prograde across the narrow continental shelf. The direction of progradation of deltaic fill is controlled by the pre-Tertiary tectonic framework. Depocentres shifted seaward as filling progressed. Tertiary progradation (Figure 5) has built the sedimentary prism across the narrow continental shelf out over oceanic crust.
Stratigraphy. The vertical and lateral stratigraphic sequences are the product of a typical offlapping delta sequence or progradational fill, but because a balance has been maintained between sediment supply and subsidence, progradation has been less extensive and vertical building has been more prominent than in the Gulf Coast. The delta has prograded over a relatively rapidly subsiding oceanic crust, so that the high sedimentation rate has filled the subsiding basin with aggradational, vertically stacked sequences. Further, since the Niger delta has
A A’
LEGEND Salt Oceanic crust
Basement
Figure 21. Schematic dip section, Gulf Coast. Line of section A-A’ is shown on Figure 3
246 D. M. CURTIS
developed in a typically wave- and current-dominated setting, strike-oriented, wave- and current-dominated sand bodies are prevalent (VI, Figure 12).
As in the Gulf Coast each lateral sequence, of whatever age, grades from a sandstone facies landward or updip to a marine shale seaward or downdip. Each vertical sequence grades from marine shale at the base up through fluvial and alluvial sands at the top. Three typical diachronous lithofacies (given formation names in Nigeria) closely resemble the regional stratigraphy in the Gulf Coast. Because of the lower sedimentation-subsidence ratio, however, the facies bound- aries tend to have a stronger vertical component. The marine shale facies is an undercompacted, overpressured, low-density mobile shale in which listric growth faults and diapiric shale structures originate. In Figures 22a and b the relation of progradation (22a) and aggradation (22b) to the development of growth faults is compared. Antithetic faults tend to develop in association with aggradational sequences, and are characteristic of Niger delta tectonics.
Structure. Pre-Tertiary megatectonic structural elements appear to have no influence on the complex Tertiary growth fault trends, only a few of which are shown in Figure 4. The generalized diagrammatic cross-section (Figure 23) differs from that for the Gulf Coast because of the absence of the underlying Mesozoic salt layer. This area was not an evaporite basin in the early rifting stage which begaii the opening of the Atlantic, so the salt domes and ridges characteristic of the Gulf basin are absent. Besides the complex system of subparallel arcuate listric normal growth faults and associated rollover structures, diapiric shale domes and ridges are the most common structural features of Niger delta geology. Antithetic faults are common. The growth faults and related structural features associated with each major aggradational sequence become progressively younger seaward.
3c. Beaufort-Mackenzie Tertiary Province The Beaufort-Mackenzie basin is a frontier area about which much less is
known. A generalized summary of the area can be found in Lane and Jackson (1980), but newer seismic data have become available (Hubbard, Pape, and Roberts 1985). The generalized geologic map (Figure 6) shows Cretaceous and Tertiary Mackenzie delta deposits prograding out onto the Beaufort shelf in a complex setting bounded on the southeast and southwest by Mesozoic fold and fault trends that influence Tertiary depositional and structural trends. Progradation of Tertiary deltas (Figure 7) has advanced from southwest to northeast with depositional strike roughly parallel with older northwest-trending structural elements.
Stratigraphy. As in the Niger and the U.S. Gulf Coast, the vertical and lateral stratigraphic sequences in the Tertiary deltas represent repeated offlapping regressive cycles. A core from an offshore well in the Canadian Beaufort Sea shows aggradational and progradational delta cycles with grain-size sequences and sedimentary structures that are virtually indistinguishable from those in other Tertiary delta sequences. Marine shales are overlain by pro-delta clays; these in turn are overlain by delta-front sands, distributary mouth bar and distributary channel deposits of the lower delta plain. Upper delta plain fluvial-alluvial portions of the cycle are not represented in downdip locations beyond the present shelf edge. The Beaufort Sea portion of the delta has prograded into the Canada Basin which is underlain by oceanic crust, so that the distal delta sequences offshore are
/-
PETROLEUM GEOLOGY TERTIARY DELTAS
RD > RS
3
2
LEGEND
Sandy sequences
[ Shaley sequences
fZJ Sea
RD Rs
247
a
1
b
LEGEND
Sandy sequences
Shaley sequences
Sea
1
Figure 22. Conce tual diagrams showing development of basinward-dipping growth faults during progradation (22af, and antithetic growth faults during aggradation (22b).
aggradational. (The high sedimentation rate is comparable with the high sub- sidence rate.)
The diachronous lithofacies units that result from the repetition of regressive offlapping cycles are virtually indistinguishable from those in other similar basins: a mobile shale facies at the base, grading upward to a massive sand facies at the top.
Structure. Growth faults and other features related to gravitational instability and gravity inversion originate in the overpressured, undercompacted, low-density
248 D. M. CURTIS
S W NE
km
LEGEND
Cenozoic Basement
Cretaceous Oceanlc crust
Figure 23 Schematic dip-section, Niger delta (after Huff 1980)
shales. As sedimentation and contemporaneous faulting continue, reverse drag into the growth faults forms broad rollover anticlines in downthrown blocks in the sand-shale facies. The systems of Tertiary delta-related syndepositional faults, a few of which are shown in Figure 6, are related in time and space to the northward growth of the Tertiary deltas over the Beaufort shelf into the Arctic Canada Basin. These faults cross a series of Mesozoic and younger faults that are subparallel with an older northeastward structural trend (Figure 24). In the off- shore Beaufort Sea, shale diapirs have developed in the mobile shale facies (Figure 25).
3d. Comparison of regional geology
Summary of similarities. These three Tertiary delta areas have a number of similar characteristics. These include: 1. Vertical and lateral deltaic depositional sequences 2. Pattern of lithofacies units 3. Growth faults 4. Rollover structures 5. Diapiric structures 6. Abnormal pressures
Summary of differences. The significant differences in these three areas are related to: 1. Pre-Tertiary geologic history 2. Tectonic setting 3. Ratio of rate of deposition to rate of subsidence 4. Palaeoenvironmental aspects (e.g. wave-tide-river dominance , climate)
4. Petroleum geology and hydrocarbon habitat
Tertiary deltaic deposits on passive continental margins are ideal hydrocarbon
PETROLEUM GEOLOGY TERTIARY DELTAS 249
habitats because of their orderly, predictable, and intimate interrelation of stra- tigraphy and structure. More than 112 billion barrels of oil (and equivalent) have been found in deltas, but giant fields (more than 1 billion barrels) are rare (Huff 1980).
4a. Source rocks The identity of source rocks for Tertiary-reservoired oils is controversial. Few
thermally mature organic-rich shales have been identified in beds younger than
LEGEND
Cenozoic Basement
0 Cretaceous
N W krn SE
km 0
5
10
Figure 24. Schematic dip section, Beaufort-Mackenzie area (after Huff 1980).
N S
LEGEND
Cenozolc tJ Cretaceous Basemem
Figure 25. Schematic dip section, Beaufort-Mackenzie area.
250 D. M. CURTIS
Cretaceous. The problem of reservoired hydrocarbons associated with immature organic-rich shales seems to be common to the three provinces discussed. It is probable, but not demonstrated, that local source rocks were formed in separate closed anoxic basins in a slope environment, within the bathyal marine shale facies, in successively younger trends related to the locus of successive depocentres. The depth to the oil and gas window depends on time and temperature and the type of kerogen in the organic material. For sediments of the same age the burial depths needed for maturation are thus dependent on the geothermal gradient. Therefore, where the geothermal gradient is lower, source rocks of a given age must be buried deeper. Bv the time traps formed in a much younger offlapping stratigraphic-structural unit, source rocks had been buried to sufficient depths to expel hydrocarbons. The timing of migration in relation to the timing of trap formation has been an area of controversy among geologists and geochemists. The geological evidence that demands migration soon after traps were formed in a given stratigraphic unit (‘early migration’ concept) is not in conflict with the geochemical evidence that demands thermal maturation and late migration out of the source rock (‘late migration’ concept) (Curtis 1979). There is no evidence that the neritic shales interbedded with reservoir sandstones are source rocks for oils in these reservoirs.
4b. Reservoirs Reservoirs in deltaic settings are mostly in the mixed sand-shale facies where
sands and shales are interbedded and the percentage of sand is less than 50 per cent. The most common depositional environments for reservoir sands are distributary channels, distributary mouth bars, delta fringe, barrier bar, and off- shore bars. Sands in deltaic depositional systems are generally dip-oriented in river-dominated deltas, and tend to be more strike-oriented in wave-dominated deltas. Strike-oriented beach, shoreline, and bar reservoirs are associated with strand-plain depositional systems. In any given producing trend of the same age, multiple reservoirs are generally present. Turbidites and slump deposits may also contain reservoir sands. Submarine channel and submarine fan deposits are probably associated with lowered sea levels, with progradation over oceanic crust (Burke 1972), or with areas of high sedimentation supply and slump faulting. Such deposits are becoming better understood and more predictable (Bouma et al. 1985).
4c. Traps Most traps in delta settings are structural, or combination struc-
tural-stratigraphic traps. In all three basins the trapping is associated with rollover anticlines, and in upthrown and downthrown fault traps in beds with regional dip. Also common are deep-seated domal uplift and diapiric shale domes and ridges. In the Gulf Coast, salt domes and salt ridges provide trapping mechanisms in the complex fault systems associated with the diapirism, as well as in stratigraphic pinchouts on flanks of structures. Stratigraphic traps are less common and more difficult to,find. There are examples, however, of pinchouts on counter-regional dip. The important traps in all three basins are related to gravity tectonics.
Timing of structural growth and development is intimately related with stra- tigraphy. That is, structure and stratigraphy are closely related in time and space. The most productive segment of any given Tertiary stratigraphic-structural unit in the Gulf Coast is in a depocentre, in a mixed sand-shale sequence deposited in a lower delta plain environment, overlain by middle neritic transgressive shales,
PETROLEUM GEOLOGY TERTIARY DELTAS 25 1
downthrown on a major growth fault, and normally-pressured to slightly over- pressured. Multiple reservoirs in similar age rocks are present in each fault block. Because Niger and Beaufort deltas are predominantly aggradational, vertically- stacked reservoirs of a greater age range are present in each fault block.
5. Conclusions
Generalizations derived from U.S. Gulf Coast can probably be applied, with some variation, to all three basins because of their basic geological similarities. Such generalizations include information about field sizes and reserve volumes, as well as about the habitat of hydrocarbons. Some of the common characteristics can be summarized as follows:
1. Individual field sizes are relatively small, by world standards, and there are few giant fields, but total basin reserves are quite large.
2 . Surprisingly enough, petroleum reserves in Tertiary deltas comprise less than 10 per cent of the total world reserves, possibly because the only maturely- explored Tertiary delta area is the onshore Gulf Coast, and large Tertiary delta reserves remain to be found in offshore provinces.
3. Tertiary deltas tend to be gas prone, and the liquids are normally intermediate to high-gravity oils and condensates, low in sulphur.
4. In reservoirs of equivalent age, gas is more common than oil in areas with higher geothermal gradients.
5. Oil is more common updip; gas more common downdip. The deeper the reservoir the more likely it is to be gas bearing.
6. Oil is more likely to be found in normally-pressured reservoirs; a geo- pressured reservoir is more likely to contain gas.
7. The traps are characterized by syndepositional diapiric structures, growth faults, and rollover structures.
8. The reservoirs are mostly sands deposited in a near-shore or lower delta plain setting.
9. Multiple reservoirs are present in the mixed sand-shale facies, generally above the geopressured shale section or in the slightly geopressured section.
The unifying concept is the systematic, orderly, predictable relationship of stratigraphy, structure, and hydrocarbon accumulation. The intimate interrelation of all the elements in the synsedimentary delta tectonics is not random, but is an integral part of the self-contained geologic history repeated many times in each producing trend. The framework for the occurrence of oil and gas is well under- stood in the maturely-explored and intensively studied Gulf Coast Tertiary basin. Concepts developed there can be applied to exploring and developing other Tertiary delta complexes.
Acknowledgments. I thank Dr. Carol Williams and Lori Davidson for reviewing the manuscript and offering constructive suggestions. The paper evolved to its present form as a result of stimulating discussions with audiences in many parts of North America during my participation as a speaker in the Distinguished Lecture Program of the American Association of Petroleum Geologists in 1983.
252 D. M. CURTJS
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