Universidade Fernando Pessoa
Porto, Portugal
X- Megasutures & Sedimentary Basins
The global result of the geological evolution described previously can be summarized by the following condensed tectonic synthesis:
1) Oceans are products of Paleozoic and Mesozoic-Cenozoic ocean spreading and extension.
2) Paleozoic and Ceno-Mesozoic megasutures of the world are the compressional contemporaneous equivalent of the ocean spreading.
Fig. 53- Ceno-Mesozoic megasuture, post-Pangea oceanic crust and associated geologic features
3) The combined Paleozoic foldbelts which, except for Circum-Pacific areas, represent the Paleozoic megasutures now bound on both sides by continental crustal and by A subduction zones.
Paleozoic “B” subductions can only be surmised, because Paleozoic oceanic crust has not been preserved in its pristine undeformed shape and only minor amounts of Paleozoic oceanic crust occur in form of obducted ophiolites.
Two basic alternatives may be suggested:
(a) the B subduction process has been so effective that most Paleozoic oceanic crust was disposed of, or
(b) Paleozoic geosynclines did not have substantial oceanic areas.
4) The Precambrian fold belts of the world represent several Precambrian megasutures. Again, no pristine Precambrian oceanic crust is believed to occur.
A megasuture can be described using the worldwide Ceno-Mesozoic mobile fold belts as an example.
The boundaries of this megasuture are defined to include all products of Ceno-Mesozoic orogenic and associated igneous activities. Emphasis is laid on the dominant compressional mode of deformation, but we are mindful of the observation that in these belts, extensional deformation and basin formation are widespread.
The processes responsible for the extensional deformation are deemed to be subordinate and a consequence of the complex subduction processes.
Megasutures show a record of folding, thrust faulting and igneous activity. They are often associated with ductile flow of crustal rocks. Intrusive and metamorphic structures are extensive. Thus, megasutures, when viewed over time spans in order of 10 M to 100 My, are very mobile realms. It is unwise to tract them as part of rigid plates, when in fact, they are pervasively plastically deformed. Because of the great difficulties in palinspastically reconstructing megasutures, it is most hazardous to extrapolate paleomagnetic and paleogeographic control data from onto adjacent, more rigid cratons.
Fig. 54- Classification of sedimentary basins of Bally and Snelson slightly modified.
Using the megasutures, and particularly the Ceno-Mesozoic megasuture, A. Bally1 proposed, in 1980, a classification of the sedimentary basins in three large families (see figs. 54 & 55):
a) Episutural
Ex: Back-arc, Pannonian, Mediterranean, Foldbelts, etc,
b) Perisutural
Fore-arc, Fore-deep, etc., and
c) Basins not associated with the formation of megasutures
Type-Rift , Cratonic and Divergent margin.
This basin classification (figs. 54 & 55) combining realms of subsidence and tectonics regimes, is, in my opinion, the most appropriated for hydrocarbon exploration. It will be used during this short course.
Fig. 55- The basic principles of Bally & Snelson classification of sedimentary basins are schematised illustrated in this figure. This basin classification combining realms of subsidence and tectonic regimes is, in our opinion, the most appropriated for hydrocarbon exploration.
XI- Seismic Interpretation Background
Contrary to popular textbook myths, it is rare for a geologist to make very many observations without already having a tentative , or working hypothesis in mind to test. In fact, in seismic interpretation there is a constant feedback among observation and hypotheses. Moreover, some hydrocarbon exploration breakthroughs have resulted from intuitive flashes based upon skimpy evidence1.
Thus, before ending the first part of this short course, I would like to illustrate here below what is often called the mental setting preparation, or the making ready for performing interpretation of seismic data. Actually, first of all, it is strongly recommended to review the geological setting of the basin where the lines were shot. Such a knowledge allows to be acquainted with the major geological events which took place in the basin and to understand their seismic signature. The knowledge of the geological setting of the area under study gives crucial informations to evaluate correctly the majority of hydrocarbon parameters, such as:
- potential source rocks,
- migration time & migration paths,
- potential traps and their age,
- the potential reservoirs and
- the retention.
The understanding of the interconnections between the geological settings and the hydrocarbon parameters, is expressed in Bally's classification where the plate tectonics theory and the genesis of subsidence play a major role.
Thus, it is utterly useful to class, in space and time, the different stacked basins which compose the area of interest and take into account, at least in the begriming of the interpretation, the basic a priori geological hypotheses.
Let's illustrate the preparation for interpretation with two seismic lines taken from classic hydrocarbon basins: Kwanza basin, in Angola, and Saigon basin, in offshore Vietnam.
Offshore Angola:
The interpretation of seismic lines from offshore Angola, such as that illustrated on plate 59, requires an a priori knowledge of the geological set- ting of the Western African basins. Otherwise, the interpreters will spend months, even years, to recognize the major geological events of these basins and the associated seismic patterns.
Fig. 56- Seismic line of conventional Angola offshore (Block 2)
Using systems thinking approach the interpreter will progress as follows:
a) The offshore Angola is associated with the break-up of supercontinent Pangea. Meso-Cenozoic sediments overlain a Pre-Pangea substratum (Paleozoic or Precambrian).
b) The break-up of Pangea took place in two distinct phases:
(i) rifting phase and (ii) thermal phase.
c) The rifting phase characterizes the stretching of the continental crust (lithosphere above Moho), created basins type-Rift (grabens and or half-grabens) bounded by normal faults.
d) The thermal phase is characterized by a thermal subsidence induced by cooling of (i) subaerial volcanism (seaward dipping reflectors) and (ii) oceanic volcanism. The thermal subsidence and sedimentary loading created the divergent margin.
e) An angular unconformity is often recognized between the basins type-Rift and the divergent margin. The age of this unconformity is posterior to the break-up. It is associated with readjustment of lithospheric plates in the onset of oceanization.
f) In distal parts of the continental crustal the divergent margin overlies the seaward dipping reflectors and basins type-Rift.
g) The divergent margin represents the post-Pangea continental encroachment stratigraphic cycle which is associated with the Meso-Cenozoic eustatic cycle.
h) The divergent margin is composed by a transgressive phase and a regressive phase. The transgressive phase is associated with the 1st order eustatic sea level rise and the regressive phase with the 1st order eustatic sea level fall. The maximum of transgression took place in Cenomanian-Turonian.
i) On the seismic lines the transgressive phase is characterized by a retrogradational or backstepping geometry, while the regressive phase is characterized by a progradational or forestepping geometry. A major downlap surface underlines the limit between these phases.
k) At the break-up, lavas flowed from the spreading centers toward the continent across the distal basins type-Rift. Where the lavas locally flowed into lakes or epicontinental seas, they frozen abruptly, preserving steep inward dips seismically resemble deltaic fore- set beds.
l) When the underlying lithosphere cooled, thermal subsidence lowered the lavas flows below sea level. Environment became marine, and the spreading center submerged to become an oceanic ridge. Lavas flows no longer flooded laterally because they frozen rapidly under the sea.
m) At an early stage of submergence, marine circulation in shallow, irregular basins was restricted by tectonic and volcanic barriers. That favored formation of local evaporite basins that gradually coalesced as they deepened at about 115 Ma. In this scenario, the opposing Aptian salt basins of the South Atlantic were never contiguous but were separated by a spreading center.
n) Aptian evaporites or sandstones mark the onset of the transgressive phase that continued until the rend of Cenomanian- Turonian.
o) Eustatic fall of sea level during the Tertiary induced the regressive phase which was enhanced by epeirogenic uplift of continental crust.
With these a priori hypotheses the seismic line can be easily be interpreted as illustrate in fig. 56. If some of these hypotheses are not corroborated by the interpretation of several lines, the interpreter must advance new hypotheses which in turn must be tested by a new interpretation and so on.
Offshore Vietnam:
a) L'offshore Vietnam is located within the Meso-Cenozoic Megasuture.
b) A back-arc basin and a divergent margin non-Atlantic form this offshore.
c) Within the back-arc, a basins type-Rift and a cratonic basin were individualized.
d) In the basins type-Rift lacustrine shales rich in organic matter were deposited whenever the rate of subsidence was not balanced by terrigeneous influx.
e) The cratonic basin is characterized by sandprone stratigraphic interval within good potential reservoirs are found.
f) Normal faulting induced by thinning of the lithosphere are well recognized in both basins (type-Rift and cratonic).
g) The extension in the back-arc basin resulted of a seaward displacement of the trench, i.e. the trench moved away from the overlying lithospheric plate.
h) The divergent margin was associated with the oceanization of a marginal sea. This oceanization took place in the Late Tertiary. The geometry of sediments composing the divergence margin is, globally, progradational. The ratio progradation versus aggradation was a direct function of the rate of sea level rise. When the sea level rose slowly the outbuilding became predominant (upbuilding is very low), (ii) when the rate of sea level rise was rapid the upbuilding became significant and, locally, more important than the progradation.
Fig. 57- Seismic line of SE Vietnam offshore located on SW flank of Dai Hung field.
i) The tectonic context of this offshore is compressional. Actually, it is located inside of the Meso-Cenozoic Megasuture. Thus, in the Late Tertiary the old normal faults, created during the type-Rift and cratonic basins, were reactivated as reverse.
j) The compressional tectonic regime and the reactivation of the previous normal faults, created null points along the fault planes. When the tectonic inversion is not total, in spite of the last fault movement be reverse, normal and reverse fault geometries are found. Normal fault geometries are found below the null points and reverse geometries above the null points.
k) The majority of the traps for hydrocarbons found in this offshore are structural. They are associated with the tectonic inversions.
However, one should not forget that these traps are very young and they are often posterior to the migration time of the hydrocarbons. In fact, the potential source rocks reached maturation during de- position of cratonic sediments.
Take into account all these hypotheses the seismic line illustrated on plate 61 can easily be interpreted as proposed on plate 62, where the basin type-Rift, the cratonic basin and the divergent margin no-Atlantic have been picked.
On the other hand, the inverted structures associated with the reactivation of the faults created during the stretching of the lithosphere are clearly recognized. The traps associated with these structures are generally dry. However, as illustrated by the location of the DH#1 discovery well, the traps filled with hydrocarbons are older, i.e. anterior to the migration time of the hydrocarbons.
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Notes (see Bibliography):
Megatures & Sedimentary basins
1- see Bally (1985), pp. 17
Interpretation approach
1- see Dott, (1994).
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