Universidade Fernando Pessoa

Porto, Portugal

Salt Tectonics Short Course

13-Traps associated with Salt

Contents:

13.1- Traps Associated with Autochthonous Salt
13.2- Traps Associated with Allochthonous Salt
13.3- Traps Above and Backward of Salt Nappes
13.4- Traps Above Salt Nappes
13.5- Traps Below salt Nappes
13.6- Gulf Coast Examples

The geological cross-sections of offshore Angola (fig. 283, 106, etc.) and Gulf of Mexico (fig. 105, 307, 308, etc.) clearly indicate the potential traps associated with autochthonous and allochthonous salt layers are quite different. In the offshore Angola, the old Seffel lines (1968) illustrated on fig. 319, and particularly the upper one, show huge structural traps (four way dip closure). Taking into account their horizontal scale, it is predictable that at smaller scale, a large number of normal-faults affect the apexes of the structures. They are extensional and not compressional features.

Fig. 319- The majority of the traps associated with the autochthonous salt were created by extensional tectonic regimes. Morphological and stratigraphic traps are combined with all different stages of diapirism. The traps associated with the allochthonous salt are mainly morphological by juxtaposition (potential reservoirs sealed against the salt), and structural (four way dip). A fold belt is often developed above the distal salt sheets.

Similarly, on the cross-sections of the Gulf of Mexico, the principal traps associated with the autochthonous salt are morphological by juxtaposition. All them were directly, or indirectly, created by combination of halokinesis and extensional tectonic regime. In other words, one can say that in such a traps there are always normal-faults associated with the closed surface. Therefore, due to the fault displacement, a seal rock can be put in juxtaposition with potential reservoirs. In association with the allochthonous salt, and independently of the different geological settings associated with (fig. 320), morphological traps are also frequent, particularly in deep-water settings, in which turbiditic deposits are paramount. Stratigraphic and structural traps are sometimes present. Admittedly, in the fold belts, structural traps are preponderant.

Fig. 320- The main geological settings associated with allochthonous salt sheets and the location of the principal generation petroleum subsystem of the Gulf of Mexico are illustrated in this sketch. Due to the fact that the source rocks are located between the autochthonous and allochthonous salt layer, it is quite important to indicate the more likely migration paths whenever looking for traps in mini-basins (1), sub-salt (4) or supra-salt settings. On the other hand, as the salt is a very good heat conductor, the maturation of the organic matter of the potential source rocks must be studied carefully. Very often, under the allochthonous salt, the organic matter is immature. Therefore a correct picking of the source rock interval as well as of the top and bottom of the salt layers is fundamental. On this subject, the picking of the bottom of the autochthonous salt is not relevant.

Shortly,

- With autochthonous salt, the more frequent traps are:

(i) Morphological by juxtaposition induced by normal faulting;

(ii) Stratigraphic (pinchout, facies changes, etc.).

- With allochthonous salt, the more frequent traps are:

(i) Morphological by juxtaposition against allochthonous salt;

(ii) Morphological (turdidite depositional systems);

(iii) Stratigraphic (pinchouts);

(iv) Structural (fold belts).

13.1- Traps Associated with Autochthonous Salt

This family of traps was consistently tested in Angola and Gulf Coast. It can be illustrated by two typical cross-sections (fig. 321 and 322).

Fig. 321- The majority of the traps occurring in the Kwanza basin are depicted on this composite geological cross-section (see also fig. 286). The major field was Quenguela, in which around 40 Mb were produced. However, as said previously, the petroleum system in Quenguela was very particular. The generating petroleum subsystem was composed by Paleocene-Eocene organic rich clays located in the core of the turtle back (see fig. 175 and 176).

In the offshore, the majority of the wells drilled until the discovery of Girassol (block 17) tested large antiforms associated with Cretaceous and Tertiary depocenters between rafts, and stratigraphic traps within the Pinda formation (Albian limestones). After the calibration and the understanding of the Girassol #1, explorationists started a hydrocarbon exploration based mainly in seismic amplitudes, assuming that an amplitude anomaly underlies often a hydrocarbon accumulation. After several exploratory wells, most of the explorationists became skeptical:

- The recoverable reserves do not seem so big as predicted.

- The profitability still is questionable, since the majority of the reservoirs are related to slope fan turbidite depositional systems.

Indeed, in deep water, an accumulation, in stand alone, must satisfy two major conditions:

(i) The recoverable reserves must be near 400 Mb.
(ii) The productivity per well should not lower than 10 kb/d.

The profitability is completely different from the GOM, where accumulation of 50-100 Mb recoverable can readily be developed. The cross-section from the East Texas, illustrated below (fig. 322), indicates the majority of the traps associated with the autochthonous salt are by juxtaposition induced by the relative displacement of the faulted blocks of the normal-faults.

Fig. 322- The peculiarity of this area is that several oil fields have been found in stratigraphic traps mainly associated with Jurassic organic build-ups. In onshore Alabama, similar and coeval stratigraphic traps exist in the Smackover formation. The same type of stratigraphic traps exists in offshore Angola. The only difference is their age. They are mainly Lower Cretaceous (Albian).

To sum up, one can say that in association with autochthonous salt, at least, in the Atlantic margins and Gulf Coast, the majority of the traps (fig. 323) are related with: -(i) Depocenters / Rafts; -(ii) Growth faults / Rollovers; -(iii) Salt Ridges; - (iv) Salt domes; - (v) Overhangs; - (vi) Turtle backs, etc.

Fig. 323- These block-diagrams illustrate the more likely trapping structures created in association with autochthonous salt layer. Seismic examples are shown in the next figures.

Fig. 324- Tertiary depocenters as the one illustrated on this close-up, as well as turtle-back structures, were drilled in conventional offshore and onshore Angola with some positive results.

Fig. 325- In this Tertiary depocenter located between two Mesozoic rafts, the core of the structure is mainly composed of a sand reservoir interval. The normal faulting near the apex clearly shows the structure is not an anticline, but an antiform.

Fig. 326- Typical raft structure drilled by Total in the 60’s, in the conventional offshore. Rafts being extensional structures are affected by normal faulting. Therefore, they cannot be taken as structural traps. Indeed, they are composite morphological traps by juxtaposition.

Fig. 327- In 70’s, rafts between Tertiary depocenters were the main targets in conventional offshore. Unfortunately the results were very disappointing.

Fig. 328- Morphological traps by juxtaposition associated with the normal faults bounding salt ridges are quite frequents in offshore Angola, as illustrated on this seismic line.

Fig. 329- Morphological traps by juxtaposition on the flanks of the salt domes, as the one illustrated above were also tested on the conventional onshore. The economical result of these type of traps is controversial (heavy oil).

Fig. 330- In deep water, many prospects associated with salt domes exist in the portfolios of the oil companies. This salt diapir is above a fracture zone. The translation of the overburden induced the contraction of the diapir and the development of an apparent downlap surface.

Fig. 331- This line illustrates an overhang of a disconnected salt stock. Under the salt stock there is a vertical salt weld (Tertiary salt weld).

Fig. 332- Oil fields associated with morphological by juxtaposition traps, in which the reservoirs are sealed against the salt have been known for long time. Halbouty and Arbenz published some of them.

Fig. 333- Back-thrust diapirs creating huge basinward tilted mini-basins in associated with the autochthonous salt layer are frequent in deep-water Angola. The hydrocarbon potential of the traps associated with this type of structures is controversial. Migration timing and retention seem to be key parameters.

Fig. 334- The noteworthy of the salt domes illustrated on this lines is that they were shortened during the Upper Tertiary due to the reactivation of complex fracture zone (two branches are recognized).

Fig. 335- Asymmetric turtle-back structures, between salt domes, are frequent in offshore Angola. In deep-water, salt domes were often reactivated by compression. The overlying depocenters, created during a previous collapse of the domes (extensional phase), are inverted.

Fig. 336- The so called false turtle-backs that in fact correspond to back-raft structures have been tested in onshore Angola since the end 60’s. Some of these structures show a to four way dip closure. This kind of trap is quite frequent in deep offshore Brazil as illustrated in next figures.

Fig. 337- This seismic line, from deep offshore Campos, illustrates the importance of the apparent turtle-back structures. The geometry of such structures is better seen on the close-up illustrated below (fig. 338).

Fig. 338- The internal configuration of this apparent turtle-back structure shows a strong divergence of the chronostratigraphic lines toward the growth fault plane. A relic salt roller is recognized in the up-thrown faulted block. The geometry of the sub-aerial lava flows, which form the infrastructure (sub-salt strata), is quite evident.

13.2- Traps Associated with Allochthonous Salt

The cross-sections of the deep offshore Angola and Gulf of Mexico (fig. 339 and 340) show the kind of trapping associated with allochthonous salt. On the cross section of offshore Angola (see fig. 106), one can say:

(i) In the proximal area of the cross-section, the substratum of the salt is composed of continental crust (rift-type basin or basement), while in the distal part it is probably composed of volcanic crust (SDRs or oceanic crust).

(ii) In the distal part, the salt is allochthonous and thickened by thrusting.

Fig. 339- Allochthonous salt and the more likely associated potential traps can be recognized on this geological interpretation of a regional seismic line located on the southern offshore Angola.

(iii) A fold belt is developed on the overlying sediments.

(iv) In the central part, the allochthonous salt is clearly separated from of the autochthonous salt.

(v) Within the allochthonous salt nappe, a small salt expulsion basin was developed.

(vi) Potential traps associated with the allochthonous salt are easily discernible from the geometry of the seismic reflectors of the overburden.

Similarly, on the time cross section of the Gulf Coast illustrated in fig. 340, the traps associated with the allochthonous salt are easy to distinguish using:

a) The geometry of the overburden reflectors.
b) The limits of the allochthonous salt.
c) The cartography of the fault planes.

Fig.340- The majority of the traps associated with the allochthonous salt in Gulf of Mexico can be recognized on this geological interpretation of a regional seismic line. They are described and depicted on the next figures (fig. 341 to 343).

13.3- Traps Above and Backward of Salt Nappes

Fig. 341- On this geological setting the traps 1 and 5 are structural (they have a four way dip). The trap 2 is stratigraphic, while 3 and 4 are morphological by juxtaposition.

13.4- Traps Above Salt Nappes

Fig. 342- Above of the salt nappes, one can distinguish: (a) structural traps: (1) antiform (top diapir), (2) antiform (top mound), (b) non structural traps: (b.1) morphological traps by juxtaposition: (3) Hanging wall (normal faulting), (4) Footwall (reverse faulting), (5) Footwall (normal faulting), (6) and (9) Footwall (growth faulting), and (b.2) stratigraphic traps: (7) and (8) Haling out.

13.5- Traps Below Salt Nappes

Fig. 343- Below canopies there are a large variety of traps: (a) structural - (1) Antiform, (2) Turtle Backs, (b) Non Structural traps, (b.1) Morphological by juxtaposition: (3) Hanging wall (normal faulting), (4) Hanging wall (reverse fault), (5) Footwall (normal fault), (6), Footwall (grabens), (7) Hanging wall (horst), (8) Juxtaposition (salt), b.2) Morphological (unconformity): (11) Below unconformity, (2) and (3) Below allochthonous salt.

13.6- Gulf Coast Examples

To finish this chapter, we will illustrate the principal effective traps found recently in the deep water province of the Gulf of Mexico, where one can consider five major geological provinces (fig. 344) with characteristic hydrocarbon parameters.

Fig. 344- The delimited geological provinces have particular hydrocarbon parameters, which can be summarized as indicated below.

In area 1 (fig. 345), where the more characteristic effective traps are illustrated in fig. 345, the hydrocarbon parameters can be summarized as follows:

A) Reservoir, Good. Miocene to Pliocene turbidite depositional systems. Mainly associated with slope fans and debris flows.

B) Migration, Good.

C) Seal, Good (salt).

D) Maturation, Can be a problem. Migration is influenced by the distribution of allochthonous salt.

E) Trap, Good.

Fig. 345- Examples of hydrocarbon accumulations in area 1. Morphological traps by juxtaposition (against salt) associated with 1st generation salt nappes, with salt stocks and bottom allochthonous salt.

In area 2 (fig. 346), the hydrocarbon parameter can be summarized as:

A) Reservoir, Good. Plio-Pleistocene shingled turbidites, mainly within salt expulsion basins.

Fig. 346- Area 2. Juxtaposition traps associated with salt expulsion basins developed above 1st generation sal nappes, salt welds or the bottom of allochthonous salt.

B) Migration,Good, through salt welds and fault planes.

C) Seal, Good. However, when the juxtaposition is not against salt, sealing problems can occur.

D) Maturation, Good.

E) Trap, Good, but difficult to map.

In area 3 (fig. 347), the hydrocarbon parameters can be summarized as follows:

A) Reservoir, Good. However, it is difficult to identify and map the reservoirs, but if they are underline by amplitude anomalies.

B) Migration, Good.

Fig. 347- Area 3. Stratigraphic Traps. Onlapping of Miocene turbidite against halokinetic structures.

C) Seal, Good.

D) Maturation, Good.

E) Trap, Difficult to map without amplitude anomalies associated.

In area 4 (fig. 348), the hydrocarbons parameters can be roughly evaluated as follows:

A) Reservoir, Plio-Pleistocene turbidites, mainly shingled turbidites and basin floor fans. The extension and geometry of the salt expulsion basin is necessary.

B) Migration, Generally bad. The presence of a salt weld is necessary.

Fig. 348- Area 4. Traps by juxtaposition (against salt) associated with salt nappes of last generation with salt stocks or bottom of 2nd generation salt nappes.

C) Seal, Good.

D) Maturation, Generally good. However, it can be insufficient near the fold belts

E) Trap, Good, if potential reservoirs are well located.

In area 5 (fig. 349), the parameters are:

A) Reservoir, Good, mainly Middle Miocene turbidites.

B) Migration, Good.

Fig. 349- Area 5. Structural traps in front or under 1st generation allochthonous salt nappe.

C) Seal, Good.

D) Maturation, Bad. So far, there is no evidence of source rocks.

E) Trap, Good.

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