◄ Carnets Geol. 19 (4) ►
OUTLINE:
[1. Introduction]
[2. Petrographical background]
[3. Material, terminology, and method]
[4. Early cementation in the Pinda Group]
[5. Essentials of cement stratigraphy (using cathodoluminescence)]
[6. Essentials of compaction]
[7. Essentials of carbonate leaching]
[8. More petrographic analyses of some Pinda thin sections]
[9. Conclusions]
[Editorial notes]
[Bibliographic references] and ... [Unpublished reports]
"Cátedra
Franco-Brasileira no Estado de São Paulo 2015", UNESP - Universidade
Estadual Paulista, Center for Geosciences Applied to Petroleum (UNESPetro),
Caixa Postal 178, Av. 24 A, no. 1515, Bela Vista, CEP13506-900 - Rio Claro - SP
(Brazil);
Dépt. STU, Fac. Sci. Tech., UBO, 6 avenue Le Gorgeu, CS 93837, F-29238 Brest (France)
Published online in final form (pdf) on February 15, 2019
DOI 10.4267/2042/69773
[Editor: Michel Moullade; language editor:
Robert W. Scott; technical editor: Bruno Granier]
This is the second contribution to the stratigraphic and genetic model of the Pinda Group. The mid-Cretaceous Pinda Group of the Congo basin in the northern Angolan offshore is a transgressive supersequence. Its lower part is made of Bufalo and Pacassa facies that correspond respectively to mixed and calcareous-dolomitic grain-dominated fabrics. Both facies were strongly diagenetically altered, mostly by dolomitic replacement and leaching. Contrary to opinions expressed by other authors that both styles of alteration are early diagenetic phenomena, there is substantial evidence for a burial origin with dolomitic replacement and leaching of calcite having started at depths not less than 150 m and 400 m, respectively.
• Angola;
•
Pinda;
• Albian;
• Cenomanian;
• reservoirs;
• dolomite;
• replacement;
• cementation;
• mechanical compaction;
• chemical compaction;
• leaching
Granier B. (2019).- New stratigraphic and genetic model for the dolomitic Cretaceous Pinda reservoirs in Angola. Part II - Compelling arguments against early dolomitization and early leaching.- Carnets Geol., Madrid, vol. 19, no. 4, p. 47-70.
Nouveau modèle stratigraphique et génétique pour les réservoirs dolomitiques du Pinda (Crétacé, Angola). IIe Partie – Preuves irréfutables contre une dolomitization et une dissolution précoces.- Il s'agit ici de la deuxième contribution au modèle génétique et stratigraphique du Pinda. Le Groupe Pinda du Crétacé moyen constitue une mégaquence transgressive au large de la partie septentrionale de l'Angola dans le bassin du Congo. Sa partie inférieure est constituée par les faciès Búfalo et Pacassa qui correspondent respectivement à des roches sédimentaires mixtes et calcaires-dolomitiques à dominante grenue. Ces deux types de sédiments ont subi de fortes altérations diagénétiques, essentiellement par du remplacement de la calcite par de la dolomite et par dissolution de la calcite restante. Contrairement aux opinions exprimées par d'autres auteurs selon lesquelles ces processus sont attribuables à une diagénèse précoce, il existe un faisceau d'arguments solides en faveur d'une origine liée au processus d'enfouissement avec notamment un remplacement par de la dolomite et de la dissolution ayant débuté respectivement à des profondeurs d'enfouissement d'au moins 150m et 400m.
• Angola ;
• Pinda ;
• Albien ;
• Cénomanien ;
• réservoirs ;
• dolomie ;
• remplacement ;
• cimentation ;
• compaction mécanique ;
• compaction chimique ;
• dissolution
Novo modelo estratigráfico e genético para os reservatórios dolomíticos Pinda (Cretáceo, Angola). Parte II – Provas irrefutáveis contra a dolomitização e a lixiviação precoces.- Esta é a segunda contribuição ao modelo genético e estratigráfico do Pinda. O Grupo Pinda, Cretáceo médio da Bacia do Congo em região costa afora do norte angolano, constitui uma supersequência transgressiva. Sua parte inferior é composta pelas fácies Búfalo e Pacassa que correspondem, respectivamente, a tramas mistas e calcário-dolomíticas dominadas por grãos. Tais fácies sofreram fortes alterações diagenéticas, sobretudo por processos de dolomitização (substituição dolomítica da carbonato de cálcio) e dissolução da cálcita restante. Contrariamente a opiniões de outros autores que interpretaram tais processos como tendo sido ligados à diagênese precoce, há indícios concretos de que eles tiveram uma origem associada ao soterramento, tendo a substituição dolomítica e a dissolução sido iniciadas em profundidades não menores que 150m e 400m, respectivamente.
• Angola;
• Pinda;
• Albiano;
• Cenomaniano;
• reservatórios;
• dolomito;
• substituição;
• cimentação;
• compactação mecânica;
• compactação química;
• dissolução
In
Angola's offshore licence blocks 2 and 3 (Fig. 1 
), carbonate rocks of the Bufalo, Pacassa and
even Punja facies of the Pinda Group (Fig. 2 )
are commonly dolomitic, resulting
mostly from dolomitic replacement of calcium carbonate (both aragonite and
calcite) and partly from dolomitic cementation of porosity. Dolomitized Bufalo
and Pacassa facies represent the best reservoir rocks in terms of porosity and
permeability. Where such rocks were not fully dolomitized, later dissolution
phases eventually affected the non-dolomitic parts, i.e., mostly residual
calcite, and further enhanced porosity and permeability. In contrast, later
cementation, including anhydrite and high-temperature saddle dolomite, has
significantly reduced both porosity and permeability. Understanding these
processes and their timing relative to the oil and gas charge of the reservoirs
is a requisite for a better assessment of exploration risk and production
capacity of Angola's Pinda structures.
Contrary to opinions expressed by other authors (Stark, 1991; Eichenseer et al., 1999) that dolomitic replacement and leaching are early diagenetic phenomena, this second contribution to the stratigraphic and genetic model of the Pinda Group provides substantial evidence for a burial origin of both styles of diagenetic alteration.
|
Figure 1:
Location of offshore licence blocks 2 (blue) and 3 (red) in Angola's
concessions (excerpt from GAD201501-DMC0001-I-A), excerpt from Granier,
2017. |
|
Figure 2:
Pinda stratigraphic nomenclature (formations, members, facies, and key
surfaces), excerpt from Granier, 2017. |
According to Stark (1991, p. I-39), "Reservoir qualities in the Albian A and B are the result of polyphasic dissolution of the calcareous elements, followed by strong dolomitization (dissolution of the allochems and of the calcareous cements). The dissolution mechanism was closely related to early meteoric water infiltration within the carbonate granular network. Porosity is mainly vuggy and/or intercrystalline and is locally increased by fracture porosity. Dissolution was followed by a number of cementation phases which led to dolomitization, anhydritization and calcitization. This cementation, however, did not totally obliterate the earlier dissolution porosity."
According to Eichenseer et al. (1999), "The combined stratigraphic and geochemical approach allowed us to determine the Pinda Group dolomites as early diagenetic in origin, whereas fluid inclusion and oxygen isotope analysis alone suggested a late burial dolomite formation. Dolomite neomorphism at higher burial temperatures modified most of the original geochemical signatures of early, near-surface dolomitization. Cathodoluminescence data and trace element analysis suggest that dolomitization was associated with gradually decreasing pore water salinity. Accordingly, barrier oolites were completely dolomitized and developed intercrystalline porosity during the earliest stages of diagenesis. At a later phase of early diagenesis, as waters became less saline and undersaturated with respect to calcium carbonate, fabric-selective dissolution of the remaining calcareous components occurred. The decrease in pore water salinity and associated porosity generation are genetically linked with high-frequency events of relative sea level lowering."
Since the early 1990s the present author (Granier, 2009, and earlier unpublished reports) has always taken the exact opposite stance to both Stark (1991) and Eichenseer et al. (1999), based on observations such as the fact that "fabric-selective dissolution" in the Pinda Group reservoirs always postdates stratiform stylolites (see § 8.). The present study provides alternative evidence to the effect that most, if not all, the dolomitization stages, as well as calcite leaching, occurred in fact during the rather late stages of diagenesis rather than "during the earliest stages of diagenesis" as stated by Eichenseer et al. (1999). Neither are they related to "early meteoric water infiltration" (op. cit.). "The most likely source for the Mg of the Pinda dolomites was" neither meteoric water, marine water nor marine-derived brines but probably the "brine from the polyhalites of the underlying Loeme Salt" (Granier, 2009). It will be demonstrated that Stark's statements (1991) and some arguments used by Eichenseer et al. (1999) to support their hypotheses are erroneous.
The
original material (thin sections, cores, well logs) is the property of Sociedade
Nacional de Combustíveis de Angola Empresa Pública – Sonangol E.P. It was
not available for re-study but a special agreement granted permission to re-use
a set of older photomicrographs (Granier, 2017:
Pls. 1-7) and other data
(op. cit.). These traditional silver emulsion microphotographs of thin
sections with transmitted light (TL) and polarized light (PL analyzed) were part
of the standard petrographic analyses. They are a primary technical or
scientific source material. Nowadays, electronic imagery makes it easy to
capture more images. In addition to TL and PL silver emulsion microphotographs
of thin sections, the present author used some microphotographs with reflected
light (RL), which were usually poorly contrasted. Nowadays, electronic imagery
and associated tools permit to "optimize" such RL photos, either scans of
silver emulsion photos or recently captured electronic photos, that will be
referred to as "photos with enhanced reflected light" (ERL); they also permit various combinations
(Figs. 3 ,
4.A-D ).
Carbonate rocks are made of a combination of grains (mostly calcareous allochems, some quartz and feldspar), organic framework (building, binding, or entrapping as baffles), mud (micrite), pore spaces (occupied either by gas, brine or oil/bitumen), insoluble residues (e.g., iron oxide, clay minerals, and organic matter), cements (crystals), and "replacement" (crystals). "Dolomitization" is commonly seen as replacement, although it may include some cementation:
Primary dolomite (also referred to as "depositional" dolomite, early dolomite, synsedimentary dolomite, ...), which is either bacterially induced, i.e., biocements interspersed with bacterial EPS (extracellular polymeric substances) and which is considered as a rather marginal phenomenon, or the result of cementation (i.e., crystals growing centripetally in pores);
Secondary dolomite (also referred to as late dolomite, post-depositional dolomite, ...), which is the result of either replacement (i.e., a mosaic of crystals with associated intercrystalline porosity) or cementation (i.e., crystal overgrowths or crystals growing centripetally in pores and open fractures). During the replacement of a calcareous allochem, the intragranular microporosity is filled with dolomite; such a dolomite "microcement" cannot be distinguished from the coeval replacing dolomite.
This terminology is not precise and can be advantageously replaced by either "dolomitic replacement" or "dolomitic cementation".
As already stated in the first part of this report (Granier, 2017), "The combination of petrographic observations using a standard polarizing microscope with transmitted light, crossed polars, reflected light with a "white card" (...), eventually supplemented by cathodoluminescence analyses (...), allows discrimination of dolomitic replacement (reddish brown, locally with inclusions) leading to a porosity increase from later dolomite (dull), calcite (orange and yellow) and anhydrite (non-luminescent) cementations leading to a porosity decrease".
Recognition
of microfacies, both depositional and diagenetic (grains, matrix, porosity,
cement, replacement), entails more or less complex approaches. Where
dolomite rhombs are few and scattered in the thin section, the microfacies was
defined using the Dunham (1962) classification or the expanded version by
Embry and Klovan (1971).
Where dolomitization is more extensive the Dunham
texture label comes with the prefix "dolo-"; if the primary texture
cannot be recognized it is referred to as a dolostone (or dolocrystalstone).
Very often, not only in the case of the Pinda
Group, the
fine crystals of the dolomicrite mosaic in dolomudstones or dolowackestones
resulted from the replacement of micritic mud whereas the coarser crystals of
the dolosparitic mosaic in the dolograinstones resulted from the replacement of
allochems and cement. The mosaic pattern results from the initial
random/scattered arrangement of the crystallization nuclei/seeds. Considering
the dolograinstones, ghosts of grains are often visible in transmitted light
within the replacing dolomite crystals (Figs. 3.A ,
4.A , 5.F ) , but it is not
always clear (Fig. 4.E-F ) whether intergranular space is filled with a dolomitic
cement occluding an original intergranular pore or with a dolomitic replacement
after an original calcitic cement.
|
Figure 3: Pinda
Group. A-F)
Dolograinstone : A) TL transmitted light; B) PL polarized light
(analyzed); C) RL reflected light; D-E) ERL enhanced reflected light; F) a
combination of TL and RL. Black arrows: sutured contacts; red arrows: grain
break; yellow arrows: fracture. For confidentiality purposes the sample depths
and the name of wells are not provided. |
|
|
Figure 4:
Pinda
Group. A-D)
Dolograinstone with some leaching vugs: A) TL transmitted light; B) PL
polarized light (analyzed); C) RL reflected light; D) ERL enhanced reflected
light. E-F) Partly dolomitized grainstone. The micritic ooid cortexes and the
echinodermic remains are not dolomitized, nor leached: E) TL transmitted
light; F) PL polarized light (analyzed). For confidentiality purposes the
sample depths and the name of wells are not provided. |
The next step is the proper identification of the paragenetic sequence, i.e., the sequence of discrete diagenetic events, which is usually displayed either along a scale of relative time (commonly opposing "early" —i.e., near-surface diagenesis— and "late" —i.e., burial diagenesis—) or, in the best cases only, along a chronostratigraphic scale (e.g., Granier, 1994; Granier & Staffelbach, 2009). At this stage of the workflow cathodoluminescence (CL) microscopy may reveal some hidden details (see § 5.) as, for instance, zoning that is the signature of changes in the composition of the brines. Such magnified and coloured CL views are the cornerstones of cement stratigraphy.
In the Pinda Group case, there are very few identifiable features to build on. Early cementation (see § 4.), compaction (first mechanical and later chemical compaction, see § 6.), replacement, leaching (see § 7.), late cementation, fracturing, and the hydrocarbon reservoir charge are either diachronous from one structure to the next or poorly time-constrained events. Therefore the time scale used herein is relative time at the scale of individual structures.
The "pervasive" dolomitization has wiped out almost all evidences of early cementation. Locally, very few are still preserved:
Few pebble-sized intraclasts in a partly dolomitized, oolitic grainstone.
They are encrusted (by oysters: Fig. 5.C ; Granier,
2017,
Pl.
1 , fig. 9)
and bored, both features are the mark of early lithification. The mesoborings (Fig.
5.D-E ; Granier,
2014)
are
ascribed to Trypanites ichnosp. (Granier, 2017, Pl. 7 , figs. 12, 15);
Macroborings
in a fully dolomitized facies. Where the borer
shell is still in situ, cf. Lithophaga spp. (Fig.
5.A-B ; Granier,
2017,
Pl. 5 ,
figs. 4-5), these macroborings are ascribed
to Gastrochaenolites ichnosp.
|
Figure 5: Pinda
Group. A)
Macroboring with borer shell. B)
Macroboring with borer shell in a dolowackestone. C) Oyster shell encrusting a pebble. Grains are dolomitized at
their edges. D) Mesoborings (yellow arrows) in a pebble. E) Mesoboring (yellow
arrow) in an encrusted pebble. F) Replacement dolomite after an aragonitic
gastropod shell. All photos in transmitted light (TL). For confidentiality
purposes the sample depths and the name of wells are not provided. |
Such
features are rarely observed, even in the field. The author did not get the
opportunity to do field work in Angola, but Brazil offers some outcrop analogs.
For instance, in a quarry (10°46'42.8"S 37°15'38.2"W) near Areia
Branca, some 30 km NW of Aracaju in the Sergipe basin, together with Dimas Dias-Brito
and Mitsuru Arai, we identified a bored hardground capping a slightly
dolomitized, oolitic limestone unit within the Riachuelo Formation (Fig. 6 ).
|
Figure 6: A-B) Quarry section near Areia
Branca. Arrows point to the hardground at
top of the oolitic unit. C) Hardground (arrow). D-F) Borings in transmitted
light (TL): D) macroboring and encruster (arrow); E) mesoboring (arrow) and
macroboring; F) mesoborings. |
Cement stratigraphy, which is one essential component of the paragenetic sequence, requires a search for the larger crystals. A skilled petrographer will first look for echinoderm remains and then focus on their syntaxial cements because such rims are usually "much wider than" any crystal "formed on the surface of other kinds of grain" (Granier & Staffelbach, 2009).
The
skeletal elements of echinoderms have a monocrystalline optical behaviour (e.g.,
Lucia, 1962; Evamy & Shearman,
1965, 1969; Towe,
1967), although they were made of polycrystalline aggregates that promptly
recrystallized in their original organic framework (Towe,
1967). As
pointed out by Granier and Staffelbach
(2009), owing to this
monocrystalline behaviour, "they are capable of acting as "germs"
for calcite cementation. (...) The syntaxial cements developed
around these echinodermic bioclasts register in detail all the phases of
cementation that affected" the rock. These details are made visible by
cathodoluminescence (CL) microscopy, which reveals the occurrence of color
zoning (Fig. 7.B, E-F ) related to trace elements such as Mg, Fe and Mn.
It is
assumed that the larger zoned crystals (Fig. 7.B, F ) should be syntaxial
overgrowths on echinoderm remains. Because most calcitic cementation stopped
when dolomitic replacement started (due to huge changes in the composition of
the brines), calcitic syntaxial overgrowths predate dolomitic replacement. Thus,
the boundary of replacement dolomite and calcite crystals is locally jagged (Fig.
7.F ), or some dolomite rhombs crosscut or look like they are superimposed
on calcite crystals (Fig. 7.F ), i.e., they make it clear that dolomitic
replacement postdates calcitic cementation.
Incidentally, on the basis of crystal growth and morphology, one should question the dolomitic nature of the zoned "limpid epitaxic cement" illustrated by Eichenseer et al. (1999: Fig. 16). It could well prove to be another example of calcitic syntaxial overgrowth on echinoderm remains. Unfortunately the author cannot get access to this proprietary material to validate this statement.
|
Figure
7:
A) Dissolution zoning of calcite cement on a polished, then slightly etched
surface, SEM (Brazilian material). B-F) Pinda
Group. B) Slightly
dolomitic oolitic grainstone. Here a large dolomite crystal in red color (at
the top) is located in an intergranular cement position. The large calcite
crystal (in the center) is zoned with a first thick zone dark brown in color,
then light brown and orange luminescent zones. This single crystal probably
grew on an echinoderm bioclast. This calcite cementation preceded
dolomite cementation. C-D) Slightly dolomitized oolitic grainstone with an
echinoid spine (e) and its calcite rim cement (c). It is not clear whether all
dolomite is replacement only, or replacement and cement (d): C) TL transmitted
light; D) CL cathodoluminescence. E-F) cathodoluminescent views of slightly
dolomitic oolitic grainstones: E) one zoned calcite crystal is corroded by red
dolomite (arrow); F) the large zoned calcite crystal that probably grew on an
echinoderm bioclast is partly replaced by dolomite as evidenced here by two
rhombic shapes (arrows). For confidentiality purposes the sample depths and
the name of wells are not provided. |
The behaviour of a calcareous sediment, more specifically the grain-dominated fabrics (i.e., in the reservoir units), with respect to compaction should significantly vary depending on whether the sediment remains unconsolidated, whether it is cemented early (see § 4.), or whether it suffers more porosity-reducing cementation.
In a case of the loose calcareous grains documented by Nicolaides and Wallace (1997), when the sedimentary pile reaches some 150 m, compaction, which is first mechanical, consists of grain rearrangements, eventually grain breakage, and subordinate intergranular porosity reduction. When the sedimentary pile exceeds 400 m, then compaction becomes chemical. Dissolution is first located at "sutured" grain contacts and a limited subordinate calcitic cementation may occur, also contributing to porosity reduction. With increasing depth "fitted-fabrics and microstylolites" are found. According to Nicolaides and Wallace (1997), these features are "well-developed (...) between 430 and 550 m" in the material they studied. On the basis of rather limited set of information, they reported that genuine stratiform stylolites visible to the naked eye as opposed to microstylolites that require a microscope to be seen, "are totally absent to depths of approximately 670 m". As a matter of fact, pressure-solution features may occur at lesser depths in early-cemented grain-dominated fabrics than in unconsolidated ones. The amount of early cementation, with dominant precipitation of low-Mg calcite in unconfined (phreatic) continental groundwaters and of high-Mg calcite or aragonite in unconfined (phreatic) marine groundwaters, could play a key role in the relative timing of chemical compaction.
In contrast, in mud-dominated fabrics (i.e., in the non-reservoir, overburden units), pressure-solution features occur at lesser depths. For example, still according to Nicolaides and Wallace (1997), "dissolution seams are observed at depths as shallow as 190 m", but "they appear to be better developed in the clay-rich portions of samples buried at depths of 340 m or deeper". Note that because the rates of sedimentation and of mechanical compaction differ significantly in grain-supported and mud-supported fabrics the same depth of their respective sedimentary piles should represent significantly different durations.
To the author's knowledge, there are few localities with fully-cored drilling in shallow-water carbonates on passive margins. The present author had once the opportunity to study a full set of thin sections from DSDP site 392A (Benson et al., 1978) drilled off the southeastern United States coast at a water depth of 2600 m. This core cuts through almost 100 m of nannofossil ooze and clays overlying more than 200 m of uppermost Jurassic (sensu Oppel, i.e., including the Berriasian) shallow-water carbonates (Fourcade & Granier, 1989). In this more than 300 m thick section, the absence of any pressure solution feature, either in grain-dominated (commonly partly cemented) or mud-dominated fabrics, is striking. This fact is broadly corroborated by the observations of Nicolaides and Wallace (1997).
Last but
not least, because dolomite has a lower pressure solubility than calcite, rhombs
are commonly found concentrated at the boundaries of stylolites or as part of
the insoluble material (Railsback, 2002, Fig. 3.8). For instance, in
the sample of Figure 12.G , which is a slightly dolomitized wackestone with
planktonic foraminifers, few dissolution seams are observed with some dolomite
rhombs concentrated at these specific levels. Accordingly in this mud-dominated,
non-reservoir facies the limited dolomitic replacement predates the seams.
However, in former grain-dominated reservoir facies, it looks like dolomitic
replacement wiped out preexisting solution seams (Fig. 8.A-D ). That could
explain the scarcity of stylolites in dolomitic intervals; alternatively the
latter may be also due to the relative inherent resistance to solubility of
dolomitized intervals.
|
Figure
8:
Pinda
Group. A-F) Yellow
arrows: "sutured" grain contacts; white arrows: outer zoning in dolomite
crystals, i.e., dolomite overgrowths (? replacement, then cement). A,
C, E) TL transmitted light; B, D, F) CL cathodoluminescence. A-D) Porous
dolograinstone, with porosity resulting from the late leaching of the
undolomitized core of the former calcareous grains. In B), a feldspar grain
appears in blue color. E-F) Dolograinstone with leaching vugs cemented by late
anhydrite. For confidentiality purposes the sample depths and the name of
wells are not provided. |
To summarize, although there is no absolute depth scale for compaction, either mechanical or chemical, one can always deal with the few available data and a relative depth scale to calibrate any paragenetic sequence.
Note:
Stratiform stylolites are not restricted to limestones and dolomites. Very good examples
are in quartzites of the Devonian "Grès armoricain" at
Pointe de Pen Hir (48°16'03.1"N 4°36'57.1"W), near Camaret in
Brittany, France (Fig. 9 ). There, stylolites definitely required much deeper
burial to develop than in limestones. Gratier et al.
(2005) who
produced "experimental microstylolites in quartz" pointed out the effects
not only of obvious determining factors such as pressure and temperature, but
also those of the nature of the brine and the duration. These authors
successfully produced microstylolites in quartz grains under a vertical
uniaxial stress of 200 MPa, with a silica solution pressure of 150 MPa, i.e.,
with a differential stress of 50 MPa, and at a temperature of 350°C.
Experiments by Zhang et al.
(2010) on "wet granular calcite" required
lesser stresses (20 to 47 MPa) and temperatures (28 to 150°C). In
conclusion, these experiments suggest that the burial of the "Grès armoricain"
in western Brittany should not be estimated at hundreds of meters as for the
Pinda limestones and dolomites, but in
kilometres.
|
Figure
9:
A-C) Stylolites in quartzites of the Devonian "Grès armoricain" at
Pointe de Pen Hir, near Camaret in Brittany (France): B) detail of a
macrostylolite; C) mesostylolites highlighting the low-angle oblique
lamination. Scale: the 2 cents coin is 1.8 cm in diameter. |
Artificial
leaching provides information on natural leaching. For instance, preparation of
SEM samples requires first polishing of the rock surface and then etching it to
visualize some features. A polished limestone sample from the Riachuelo
Formation collected near Areia Branca,
some 30 km NW of Aracaju, Brazil, was lightly etched in a less than 10% aqueous
solution of hydrochloric acid for not more than 20 seconds. Under the SEM the
larger calcite crystals display zoned scours and grooves (Fig.
7.A ). This
dissolution zoning, which is similar to the color zoning observed in
cathodoluminescence views, probably results from slight variations in the mol%
of trace elements, notably in the magnesium content. Accordingly, the scours
possibly correspond to high-Mg calcite (HMC), i.e., calcite with
a magnesium content greater than 8-12 mol%, and the groves to low-Mg
calcite (LMC), i.e., calcite with a magnesium content less than 8 mol%.
However, whereas LMC is the most stable polymorph of calcium carbonate and HMC is even more soluble than aragonite, stating that LMC is more stable than aragonite that in turn is more stable than HMC may prove to be incorrect. As a matter of fact, there are several limitations to the leaching of some former HMC, namely the biogenic HMC as opposed to the non-biogenic HMC (chemical cements):
For instance, modern rhodophyta with 10-30 mol% of MgCO3 and echinoderms with 10-15 mol% are rarely leached. A plausible explanation for this phenomenon is the occurrence of organic matter originally embedded in these biogenic calcite crystals that prevented or restricted dissolution (Granier, 2014; Granier et al., 2014);
Biogenic HMC often loses some Mg and is converted / "promptly recrystallised" / into the most stable LMC (Towe, 1967), a phenomenon which is still "not well understood" (Flügel, 2004) but that would prevent later dissolution;
Skeletal elements of echinoderms look like they are strongly resistant to leaching because calcite crystals resulting from the combination of an echinoderm nucleus and its overgrowth are commonly the largest crystals in a thin section. Crystal size may be another limiting factor;
Further on the subject of echinoderm remains, according to Granier and Staffelbach (2009), "Syntaxial cements developed at their peripheries are much wider than those formed on the surface of other kinds of grain". Owing to their thickness and continuity these LMC overgrowths may contribute to the preservation of the former HMC echinoderm cores.
Dissolution may also affect some dolomite crystals. Similar
to calcite with LMC versus HMC, two dolomite types can be
differentiated on the basis of their calcium content: low-Ca dolomite (LCD), i.e.,
dolomite with
a calcium content lower than 55 mol%, versus high-Ca
dolomite (HCD). As reported by Jones (2007), LCD is
more stable than HCD. Hollow (Fig. 10.A-B, D ) or "Russian-doll"
(Fig. 10.E ) rhombic crystals are regarded as the result of differential
dissolution of HCD compared to LCD. As documented here (Fig.
10.C-D ), such
small crystals commonly occur as replacement of the inner part of micritic
allochems next to
larger crystals replacing their outer part.
Leaching
of hollow dolomite rhombic crystals (Fig. 10.A-B, D ) and that of some calcitic
remains (Fig. 10.C ) are probably coeval (note that
Fig. 10.D
is a detail of Fig. 10.C ). In some other cases (e.g.,
Fig. 10.F ), cathodoluminescence
analyses eventually reveal inside-out dolomite sensu Jones
(2007) that may suggest a story more complex than originally thought.
|
Figure
10:
Pinda
Group. A-B) Hollow dolomite rhombic
crystals: A) TL transmitted light; B) PL polarized light (analyzed). C-D)
Oolitic dolograinstone, TL transmitted light: C) red arrows point to a
stylolite evidenced by insoluble impurities and small quartz grains; D) detail
of the previous image with hollow dolomite rhombic crystals within the
original allochems. E) "Russian-doll" dolomite rhombic crystals, TL
transmitted light. F) Possible inside-out dolomite sensu Jones
(2007), CL cathodoluminescence. For confidentiality purposes the sample depths
and the name of wells are not provided. |
In the original data set, former grain-supported textures are better documented, probably because they correspond to cored intervals in the best reservoir zones and they captured more attention of the O&G practical petrographers.
Echinoderm
remains are definitely the most remarkable bioclasts (see § 5.). Both
echinoderm remains and their syntaxial cements are visible in many poorly
dolomitized samples (Figs. 4.E-F ,
7.C-D ) as well as in fully dolomitized samples
(Fig. 11.A-F ):
The sample in Figure 4.E-F
has a grain-supported texture. It is an oolitic grainstone, partly
dolomitized, with micritic ooids. The echinoderm remains, some of which are
coated, did not undergo much dolomitization, i.e.,
replacement which is restricted to the periphery of the grains, without
excluding possible cementation in the original intergranular pores;
The sample in Figure 4.C-D
is a slightly dolomitized oolitic grainstone. Here the calcite rim
cement of the echinoid spine was apparently not affected by "dolomitization"
and it is not clear whether all dolomite is replacement only, or replacement and
cement;
The
sample in Figure 11.A-D
has also a grain-supported texture, but this grainstone was fully dolomitized as
indicated by the
entirely red-coloured cathodoluminescence. An echinoid spine (fully dolomitized) covers one-third of
the photo. It has kept its monocrystalline behavior even after the original calcite
was replaced by dolomite. The original syntaxial rim cement
surrounding it is still visible and predates dolomitization;
The
sample in Figure 11.E-F
is also a dolograinstone. An echinoid theca covers one
third of the photo. Individual pieces constituting the theca have a
monocrystalline behavior. Here again they were dolomitized and have developed
syntaxial cement on the inner side of the empty thecal cavity. However, shapes
of these syntaxial crystals bear the morphological signature of dolomite.
Therefore, part of the echinoderm syntaxial rim cement is here related to the
dolomitization process. The remaining space of the thecal cavity was later
occluded by anhydrite.
|
Figure
11:
Pinda
Group. A-D)
Dolograinstone with a large echinoid spine : A) TL transmitted light; B)
PL polarized light (analyzed); C-D) ERL enhanced reflected light. E-F)
Dolograinstone with an echinoid theca : E) TL transmitted light; F) PL
polarized light (analyzed). For confidentiality purposes the sample depths and
the name of wells are not provided. |
The
sample in Figure 3
is a fully dolomitized grainstone. Ghost of the
grains are visible in both transmitted light (TL) and reflected light (RL);
these sub-rounded allochems are poorly sorted. Shapes of the coarse replacement
dolomite crystals are visible in both transmitted light (TL) and polarized light
(PL). In addition, in this case, the enhanced version of the reflected light
photo (ERL) reveals broken grains, sutured grain contacts, and a larger fracture
cutting through several grains. Broken grains (Fig. 3.E , red arrows) and sutured
grain contacts (Fig. 3.E , black arrows)
indicate burial compaction,
mechanical and chemical, of a loose calcareous sand. The fracture (Fig.
3.E ,
yellow arrows) indicates some burial cementation and the
conversion of the loose calcareous sand to a grainstone. Because the sutures of
some grains, the breaks of individual grains, and those of the grainstone itself
are locally embedded in large dolomite crystals, it is assumed that all these
features predate dolomitic replacement.
The
sample in Figure 4.A-D
is similar to that of Figure 3 . It is a fully dolomitized
grainstone of very poorly sorted sub-rounded allochems and some vugs resulting
from the dissolution of the undolomitized cores of the grains. In turn the
sample in Figure 4.E-F
is similar to that in Figure 4.A-D . The grains edges are
dolomitized but, in this case, there is no porosity because the
undolomitized cores of the grains were not leached. In both samples,
grain sutures predate dolomitic replacement.
As
already noted above, sutured grain contacts are commonly observed embedded in
large dolomite crystals (Figs. 3.E , black arrows ;
8.A-D , yellow arrows).
According to Nicolaides and Wallace (1997), this
phenomenon indicates the initiation of chemical compaction, which occurs
in grain-dominated fabrics at depths definitely exceeding 150 m and possibly 400
m. In the examples of the Pinda Group carbonates, sutured grain contacts predate dolomitic
replacement, suggesting that we are dealing with burial dolomitization.
Locally
some small dolomite rhombs are in solution seams in muddy non-reservoir
facies (Fig. 12.G ) or in thin stylolites in grainy reservoir facies
(Fig. 12.E ).
These small rhombs therefore partly predate these compaction features that are
supposed to occur at different depths depending on the facies (see §
6.).
However, in other cases, it could be a matter of debate to determine whether
some stratiform stylolites (Figs. 10.C ,
12.D-F ) preceded, coincided, or followed
this burial dolomitization stage. It cannot be excluded —but not definitively
proven— here that the thickest stylolites in grainy reservoir facies (Fig.
12.D ) may have started before initiation of the dolomitic replacement and that,
once started, they may have continued during the replacement and even after.
|
Figure
12:
Pinda
Group. A) Fracture
cemented by baroque dolomite and anhydrite in a porous doloscrystalstone. The
"intercrystalline" porosity results from the leaching of formerly
undolomitized parts. It is the latest event in the sequence dolomitic
replacement, fracturing, cementation with baroque dolomite, anhydritic
cementation, and leaching. B-C) Cementation of a large cavity with baroque
dolomite first and anhydrite second. D) Thick stylolitic contact with a porous
dolograinstone. The "intercrystalline" porosity results from the leaching
of formerly undolomitized parts. It postdates the stylolite. E) Thin
stylolitic contact between two porous dolograinstones. Some dolomite rhombs
(red arrows) are found at the contact and may well predate the stylolite. F)
Thicker stylolitic contact between a porous dolowackestone and a tight
doloscrystalstone, cemented by anhydrite. Porosity in the dolowackestone
results from a leaching phase postdating the stylolite. G) Slightly
dolomitized wackestone with planktonic foraminifers. Red arrows point to
dolomite rhombs at the contact of dissolution seams, predating them. A-B, D-F)
TL transmitted light; C) PL polarized light (analyzed). For confidentiality
purposes the sample depths and the name of wells are not provided. |
In some
cases, leaching happened later in the paragenetic sequence, and significantly
contributed to porosity and permeability enhancement in formerly grain-dominated
fabrics. Such facies (Fig. 3
- 4
, 8 , 11 ,
13
- 14.A-D ) can be subdivided into a range of
different subfacies on the basis of the extent of the dolomitic replacement and
the occurrence of pores:
fully
dolomitized tight samples (Figs. 3.A-F ,
11.A-D ),
"fully"
dolomitized tight samples with former pores cemented by anhydrite (Figs.
8.E-F ,
11.E-F ),
"fully"
dolomitized porous samples (Figs. 4.A-D , 5.F ,
8.A-D ,
14.E-F ),
partly
dolomitized porous samples with pores resulting from incomplete leaching (Figs.
13.F ,
14.A-D ).
The
last category is the most interesting because it represents the intermediate
stage between partly dolomitized tight samples (Fig. 13.A-E ), which have
retained some calcitic parts (i.e., remaining allochem cores), and "fully" dolomitized porous or tight samples. These partly dolomitized porous
samples (Figs. 13.F ,
14.A-D ) document how leaching proceeded with undolomitized
allochem cores. Such leaching was not centripetal but, in the case of ooid
cortices, it appears to affect discrete layers differently (Figs.
13.F ,
14.A-D ).
That could be related either to the original amount of organic matter in the
ooid cortices or to their original magnesium content.
|
Figure
13:
Pinda
Group. A-D) Tight dolograinstone, partly dolomitized. Only the outer parts of calcareous grains
are replaced by cloudy dolomite, the cores are not. It is not clear whether
the rather hyaline dolomite between the allochems resulted from replacement
only or it includes some cement. C-E) Some "intercrystalline porosity" can
be detected. F) Red arrows point to limited leaching porosity within
undolomitized grain cores. A, C-F) TL transmitted light; B) PL polarized light
(analyzed). For confidentiality purposes the sample depths and the name of
wells are not provided. |
|
Figure
14:
Pinda
Group. A-D) Slightly
porous dolograinstone, partly dolomitized. Only the outer parts of calcareous
grains are replaced by cloudy dolomite, the cores are not. It is not clear
whether the rather hyaline dolomite between the allochems results from
replacement only or it includes some cement. The allochem cores, mostly ooids,
are partly leached. E-F) "Fully" dolomitized porous dolograinstone.
Porosity probably results from the complete leaching of undolomitized allochem
cores in a former partly dolomitized tight dolograinstone. A-F) TL transmitted
light. For confidentiality purposes the sample depths and the name of wells
are not provided. |
Some
fracturing (Figs. 12.A , 15 ) postdates dolomitic replacement. High-temperature
saddle dolomite and later anhydrite, i.e., cements, filled in such fractures (Figs.
12.A , 15 ), suggesting that the fractures were the pathway to
high-temperature
brines. However, leaching of calcitic remains, which postdates stratiform
stylolites, also postdates such
fractures and their cements (Fig. 12.A ). In most cases, leaching of calcitic
relicts of grains
(Figs. 4.A-D
, 5.F ,
8.A-D ,
10.C ,
12.D-F ) is commonly the last stage preceding the
hydrocarbon reservoir charge (Fig. 16.A-C ).
In any
case, leaching of calcitic relicts of grains (e.g., micritic ooid cores, cortexes
and nuclei) postdates all stratiform stylolites (Figs. 10.C ,
12.F ). Accordingly,
leaching started at least below the first 400 m of burial and even possibly
below the first 600 m.
|
Figure
15:
Pinda
Group. The paragenetic sequence documented in the photomicrograph of Figure
12.A |
|
Figure
16:
Pinda
Group. A-C) Bitumen
in "intercrystalline" and moldic porosity resulting from leaching. TL
transmitted light. A) Porous dolograinstone. Anhydrite is located in the
largest pores. B) Vuggy dolograinstone. C) "Intercrystalline" resulting from
leaching in a dolograinstone. For confidentiality purposes the sample depths
and the name of wells are not provided. |
According to Eichenseer et al. (1999), "From our integrated stratigraphic and geochemical study we see no strong evidence for deep burial dolomitization in the Lower Congo Basin, although fluid inclusion and isotope data suggest otherwise. The pervasively dolomitized reservoir section is best explained by early shallow dolomitization and carbonate dissolution". All of that, as well as Stark's hypotheses (1991), prove to be wrong. A forthcoming publication will show that, although their diagenetic model failed to account for some relationships listed below, the depositional model of Eichenseer et al. (1999), which drew on Granier (unpublished reports), was in some degree valid.
Although our samples come from several wells, paragenetic sequences have very similar patterns:
In
mud-dominated fabrics (e.g., in the cap rock and in the remainder of the
non-reservoir overburden), chemical compaction first occurred between 150 and
400 m of burial (see § 6.). Although a limited dolomitic replacement is
observed there too and slightly predates the chemical compaction (Fig.
17.B ), it
cannot be related in any sort to "early shallow dolomitization" because this
replacement occurred in sediments typical of relatively deep-water environments
with relatively low-sedimentation rates;
In
grain-dominated fabrics (i.e., in reservoir units), first came early
cementation that was locally responsible of some hardgrounds and lithoclasts
derived from them. This stage documented by
boring or encrusting organisms (see § 4.) is rare
(Fig. 5.A-E );
Some early
fracturing (Figs. 3 , 17.C ), possibly related to the synsedimentary rafting of
the Pinda structures on top of the Loeme salt, occurred during the episode of
calcitic cementation. It postdates early phases of calcitic cementation (partly
including syntaxial overgrowth of echinoderm remains);
Still, in
grain-dominated fabrics, mechanical compaction occurred in the first 150 m
—or ? less— of burial whereas first stages of chemical compaction
occurred below the first 400 m —or ? less— of burial (see §
6.). In the studied samples of the Pinda Group dolomitic replacement partly masked the weaker dissolution features
present in calcitic precursors (Figs. 3 ,
4.A-D
, 8.A-D ), i.e., it
postdates them. This dolomitic replacement should not be interpreted as "early
shallow dolomitization"
because it definitely is a case of burial dolomitization (> 400 m);
Some late
fracturing, partly occluded by high-temperature saddle dolomite (Figs.
12.A , 17.D ), also postdates dolomitic replacement;
Leaching of
calcitic remains postdates all stratiform stylolites (Figs.
10.C ,
12.D-F ).
Accordingly, leaching started at least below the first 400 m of burial and even
possibly below the first 600 m. In most cases, leaching of calcitic grains (Figs.
4.A-D , 5.F ,
8.A-D ,
10.C ,
12.E-F )
was commonly the last stage preceding the
hydrocarbon reservoir charge (Fig. 16.A-C ). This leaching should not be interpreted as
"early shallow (...) carbonate dissolution",
because it obviously is a case of burial leaching (> 600 m).
In the paragenetic sequence described above, first
dolomitic replacement and then leaching postdate early stages ("fitted-fabrics
and microstylolites": Fig. 17.A ), and eventually later stages ("stratiform
stylolites")
of chemical compaction. The burial depth of the Pinda oil and gas reservoirs
probably exceeded 150 m when dolomitization started whereas it should have being
not less than 400 m with respect to leaching. Both processes represent burial
diagenetic features and disprove the hypotheses of Stark
(1991) and of Eichenseer
et al. (1999). In the alternative
model discussed above, the source of Mg in the dolomitizing solutions was
neither meteoric water, nor marine water, nor marine-derived brines but probably from
the underlying Aptian Loeme Salt Group (Granier, 2009, and earlier
unpublished reports). The incorporation of these new pieces of information help
to complete the "new" stratigraphic and genetic model for the dolomitic
Cretaceous Pinda reservoirs that will be presented in a forthcoming publication
(Granier, nearing completion).
|
Figure
17:
Pinda
Group. A)
Pressure-solution seam predating dolomitic replacement in a dolograinstone. B) Stratiform
stylolite (? partly) postdating a limited dolomitic replacement in a slightly
dolomitized mudstone. C)
Fracture predating dolomitic replacement in a dolograinstone. D) Fracture
predating dolomitic
(baroque) cementation in a mudstone. Figure caption: White/light grey: dolomite; blue:
porosity; green: micrite (i.e., microcrystalline calcite); black: insoluble residues (e.g., iron oxide,
clay minerals, and organic matter). |
The
author would like to thank Sonangol for having granted permission through an
agreement dated April 13th, 2017, to use some of their material in
this publication. He is very grateful to Isabel Policarpo da Silva and
Jean Colsoul who acted as facilitators in the agreement with Sociedade
Nacional de Combustíveis de Angola Empresa Pública - Sonangol E.P. Thanks go
to Trevor Burchette and L. Bruce Railsback for their
constructive comments, to Phil Salvador for his help
with an early version of this manuscript, to Robert W. Scott
for having checked the final English version, and to Dimas Dias-Brito
for having translated the abstract
into Portuguese. SEM photography in Figure 7.A
was by Hermes Dias Brito of the UNESPetro. The author is solely
responsible for the interpretations (and opinions) expressed here, which do not
necessarily reflect those of Sonangol.
Results presented hereafter question the "scientific" value of many "technical" papers published in professional journals (e.g., AAPG Bulletin, GeoArabia, and SPE publications) or book series (e.g., Schlumberger Well Evaluation Conference) because it is often not possible for external and independant researchers to duplicate observations or experiments based on proprietary data. The author is very grateful to Sociedade Nacional de Combustíveis de Angola Empresa Pública - Sonangol E.P. for permission to re-use some material and to re-evaluate previous results.
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