IV.6 Depositional Environment, Diagenesis and
Reservoir Properties of Oncolitic Packstones, Macae
Formation (Albian-Cenomanian), Campos Basin,
Offshore Rio de Janeiro, Brazil
Albert V. Carozzi 1, Frank U. H. Falkenhein 2, and Milton R. Franke 2
1 Introduction and Geologic Framework
The stratigraphy of the Campos Basin (Fig. 1 A, B) was controlled by the following sequence of tectonic stages characteristic of rifted continental margins (Ponte
et al. 1980); pre-rift arching: denudation of Paleozoic cover (Permian-Jurassic);
intracratonic rift valley: N-S collapse trough enlarged to rift valley system (late
Jurassic - early Cretaceous) filled by Lagoa Feia fluvio-deltaic to lacustrine
clastics; proto-oceanic gulf: restricted embayment (late Aptian) with deposition
of Lagoa Feia evaporites; open marine continental margin: subsidence and
seaward tilting, generation of carbonate platform consisting of offshore bars of
oncolitic packs tones separated by mudstones (Lower Macae, Albian); further
tilting, subsidence of platform, transgression of argillaceous pelagic mudstones
and intercalated turbidites (Upper Macae, Cenomanian) followed by basinal
shales and turbidites (Carapebus Member, Campos Formation, SenonianEocene); renewed tilting of continental margin and rejuvenation of source areas
with intense deltaic progradation (Ubatuba Member, Campos Formation (Oligocene - Pliocene).
The structural framework (Fig. 1 C, D) is characterized by basement tectonics
restricted to Aptian rifting and overlain by numerous dome-like structures separated by curved growth faults. The latter display a down-to-basin arrangement
and disrupt only the Macae Formation and the lower part of the Campos
Formation. During the Albian-Cenomanian basinward tilting, growth faults
developed with related antithetic rotation of the down-thrown blocks by gravity
gliding of the limestone section over the underlying Lagoa Feia evaporites acting
as a lubricant. A major result was the formation of a large SW-NE trending
positive area (Fig. 2) that strongly influenced carbonate depositional patterns.
I Department of Geology, University of Illinois at Urbana-Champaign, 254 N.H. Building,
Urbana, IL, 61801, USA
2 Petr6leo Brasileiro S. A. PETROBRAs, Exploration Department (DEPEX), Rio de Janeiro,
R. J., 20035, Brazil
Coated Grains (ed. by T.M. Peryt)
© Springer-Verlag Berlin Heidelberg 1983
Depositional Environment, Diagenesis and Reservoir Properties of Oncolitic Packstones
331
Onshore Geology
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Fig. 1. Campos Basin: location, geologic column and structural framework
2 Recent Models of Formation of Oncoids
Marine oncoids are forming today in tropical shallow subtidal settings by disruption of blue-green algal mats in Bermuda (Gebelein 1969), the Bahamas (Neumann et al. 1970, Scoffin 1970, Gebelein 1976), and Bonaire (Pratt 1979).
In all these cases, algal mats occur adjacent to areas of extensive growth of
marine grasses (Tha/assia) and seaward of sandy bottoms. The seagrass beds support a dense epiphytic growth of calcareous red algae, bryozoans and foramini-
A. V. Carozzi et al.
332
Slice 1
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Depositional Environment, Diagenesis and Reservoir Properties of Oncolitic Packstones
333
fers as well as numerous benthic organisms such as pelecypods and gastropods.
Algal mats do not produce laminites because of environmental turbulence which
continuously destroys them. Their chips, together with bioclasts from the adjacent seagrass community, become the cores of oncoids growing by means of
further algal coatings and sediment binding. Final accumulation of oncoids occurs on sandy bottoms at depths of less than 1 m and accumulations of 10 to 100
oncoids m - 2 are common.
Seagrass beds adjacent to algal mats are the critical ecologic factor. They
have three main functions: (1) they provide a baffle effect which lowers the turbulence of the environment so as to protect the growth of algal mats but still allow enough periodical energy to generate oncoids from them; (2) they provide
habitat and ecologic niches for the establishment of a stable and rich community
(Brasier 1975, Taylor and Lewis 1970) which includes grazers and burrowers
which otherwise would destroy the adjacent algal mats; (3) they generate large
quantities of micrite mud from the disintegration of epibionts (Land 1970, Patriquin 1972).
3 Oncoids of the Macae Formation
They have an irregular spheroidal to ellipsoidal shape, often deformed by compaction (Figs. 3A and 6D), and consist of pisooncoids (2 to 10 mm, in size) and
microoncoids (less than 2 mm in size). Under the petrographic microscope a
vague concentric layering is visible around cores of various types of bioclasts,
chips of algal mats and grains of detrital quartz. Even under SEM pisooncoids no
longer show filaments of blue-green algae, but only a typical internal structure of
alternating porous and dense layers (Fig. 3 B). The former assumed to have been
originally rich in filaments represent periods of growth of the pisooncoids whereas the latter apparently devoid of filaments represent periods of interruption of
growth. There are numerous instances of major interruptions of growth superimposed on the finer couplets (Fig. 3 E). Natural broken surface of porous layers
under SEM consists of a mosaic of micrite-size (0.5 to 4.0 J.lm) subhedral calcite
crystals that does not give any indication of its origin (Fig. 3 C), in fact it is identical to that of micrite mud. Pore casts under SEM show a spongy network of
microporosity with pores smaller than 2 J.lm (Fig. 3 D). A common feature of
pisooncoids is the presence of filaments of endolithic boring algae appearing as
empty flattened tubes (Fig. 3 F).
4 Application of Recent Models to Macae Oncolitic Carbonates
Open marine shelf, shallow subtidal turbulent waters and baffle systems appear
to be the required conditions to generate oncolitic carbonates. They would ideally result in the juxtaposition of three distinct microfacies: mudstones, wackestones (baffle system) and oncolitic packstones, but in reality the first two cannot
be easily distinguished. Thalassia baffle systems can only be traced back as far as
the late Cretaceous, therefore it is assumed that seagrass-like communities such
Fig. 3A - F. SEM pictures of pisooncoids. A Pisooncoids extracted from a friable pisooncolitic packstone. Arrow points to deformation by compaction. B Pore cast of internal structure of pisooncoid.
Couplets of porous and dense concentric layers around nucleus at left. C Broken natural surface of
porous layer. Micrite size (0.5 to 4.0 ~m) subhedral calcite crystals with no indication of origin.
D Pore cast of spongy microporous network. E Internal structure of pisooncoid showing thick
nonporous concentric layers corresponding to major interruptions of growth. F Filament of
endolithic boring alga appearing as noncalcified empty tube of organic matter inside pisooncoid
Depositional Environment, Diagenesis and Reservoir Properties of Oncolitic Packstones
335
as green algae with subordinate stalked pelmatozoans and red algae played a similar role in the Lower Macae carbonates. The effects of baffle systems is revealed
by the following lithologic associations: the juxtaposition of mudstones with
relatively abundant and diversified benthic bioclasts and packstones consisting
only of microoncoids indicates a poorly-developed seagrass community whose
ineffective baffle action allowed dispersal of bioclasts in all directions and afforded little protection to algal mats. The juxtaposition of mudstones with few
bioclasts among which planktics predominate and packstones consisting of
pisooncoids indicates a stable and rich seagrass community with an effective baffle action releasing bioclasts only in the direction of algal mats and thus adequately protecting them.
In order to differentiate among oncolitic packstones, an index of biotic diversity of benthics (IBDB) is used for bioclasts forming the nuclei of oncoids and for
those scattered in the matrix. The index ranges from 1 to 8 by cumulative increments of one for each of the following constituents: echinoids, gastropods,
molluscs, foraminifers, annelids, red algae, rudistids and hexacorals. The percentage of planktics (ostracods, foraminifers and calcispheres) relative to the
total amount of bioclasts is used to differentiate among mudstones.
5 Paleoenvironmental Maps
They illustrate the depositional settings of four parachronostratigraphic slices
representing basinwide shallowing tendencies (Falkenhein 1981). The datum line
is the base of the Upper Macae, a reliable chronostratigraphic surface.
Slice 1 (1000 -700 m below datum, Fig. 2). Shallow subtidal to high intertidal
shoreface shoals developed in the central part of the basin. They consisted of
grain-supported arenaceous pisooncolitic to oolitic packstones with micrite
matrix and sparite cement: IBDB of bioclasts in matrix: 8, of bioclasts as cores of
pisooncoids and ooids: 6. The highest areas of the shoals were exposed and
beachrock reservoirs generated. Adjacent mudstones: biogenic content: 7ft/o,
IBDB: 3, planktics: 18%. The baffle system of stable seagrass community is efficient.
Slice 2 (700 - 400 m below datum, Fig. 2). Migration of the shoreface shoals of
slice 1 to southwest part of basin. Development in east-central area of subtidal
offshore shoals. They consisted of grain-supported microoncolitic packstones
with micrite matrix and rare sparite cement. The microoncoids lacked internal
structure. IBDB of bioclasts in matrix: 2, of bioclasts as cores of microoncoids:
1.5. Adjacent mudstones: biogenic content: 10%, IBDB: 5, planktics: 450,10. The
baffle system of immature or unstable seagrass community is inefficient.
Slice 3 (400 - 200 m below datum, Fig. 2). The shoreface shoals of slice 1 persist
in southwest part of basin, but without development of reservoir conditions.
Southern lobe of system changed into an offshore subtidal microoncolitic shoal
with adjacent relatively fossiliferous mudstones. In east-central area, the
offshore subtidal microoncolitic shoals of slice 2 changed into shallow subtidal to
336
A. V. Carozzi et al.
intertidal pisooncolitic shoals. The pisooncolitic packstones with micrite matrix
and sparite cement have: IBDB of bioclasts in matrix: 6, of bioclasts as cores of
pisooncoids: 4 - 5. Beachrock reservoirs are occluded by meteoric phreatic
cementation. Adjacent mudstones: biogenic content: 5OJo, IBDB: 1, plantics:
86%. The baffle system of stable seagrass community is efficient.
Slice 4 (200 - 0 below datum, Fig. 2). Final stage of conditions of slice 3. It is
characterized by extensive shallowing controlled by synsedimentary tectonism.
Related development of reservoirs in the offshore pisooncolitic shoals of eastcentral area (Garoupa field), at northern tip of shoreface pisooncolitic-oolitic
shoals and in offshore microoncolitic shoals of southern part of basin (Pampo
field).
6 Comparison with Other Mesozoic Oncolitic Environments
Oncolitic and oncolitic-oolitic packstones of Mesozoic carbonate shelves have
been described on a worldwide basis: Dachstein of Austria (Fischer 1964), Jurassic of Paris Basin (Purser 1978) and of the Jura Mountains of Switzerland (Bolliger and Burri 1970), Jurassic Smackover of the Gulf Coast (Bishop 1968,
Becher and Moore 1979), Albian Edwards Limestone of Texas (Bebout et al.
1977), and Albian-Cenomanian Regencia Formation of Espirito Santo Basin,
Brazil (Tibana and Alves 1973). The Albian-Cenomanian Madiela and Catumbella Formations of Gabon and Angola basins also contain undescribed oncolitic
carbonates (de Klasz 1978).
The above-mentioned examples are similar to those described here in the following critical aspects: occurrence as subtidal to intertidal offshore shoals with
subordinate shoreface shoals and longshore belts, associated subtidal mudstones
with very low fossil content, absence of other types of packstones and general
paucity of fossils of carbonate platforms bearing large oncolite deposits.
7 Depositional-Diagenetic Sequence
The oil traps in the upper part of the Lower Macae are mixed structural-stratigraphic resulting from closure by domes and growth faults combined with
favorable depositional-diagenetic conditions. The latter are part of a sequence of
ten stages of early to burial diagenesis distinguished on petrographic and cathode
luminescence textures, and confirmed by stable isotope data (Franke 1981).
Low Intertidal Environment. Stage 1 (Fig. 4): deposition of initial unconsolidated sediment consisting of a framework of abraded pisooncoids with interstitial
sand-size bioclasts and intraclasts set in a finer matrix interpreted as "oncoid
flour" derived from the abrasion of the larger pisooncoids. Stage 2 (Figs. 4A and
5A, B): diagenetic, precipitation of thin isopachous rim cement of fibrous calcite
(high magnesium and/or aragonite?).
Depositional Environment, Diagenesis and Reservoir Properties of Oncolitic Packs tones
Stage 1- Depositional
Stage 2 - Diagenet ic: cementat ion
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337
338
A. V. Carozzi et al.
High Intertidal Environment. Stage 3 (Figs. 4 and SA - C): depositional, re-
moval and reorganization of interstitial constituents into an internal sediment
with geopetal features and horizontal surfaces. Energy high, predominant downward circulation of seawater, probably adjacent to a beachrock.
Beachrock Environment. Stage 4 (Figs. 4 and SD): diagenetic, lithification of in-
ternal sediment by interparticle sparite cement followed by solution which generated secondary vuggy to channel porosity cutting across pisooncoids and internal
sediment. Stage S (Figs. 4 and SE): diagenetic, precipitation of rim cement of
bladed calcite (high magnesium and/ or aragonite?) along boundaries of all previously generated open spaces. If preserved after burial, this stage is a reservoir.
Beachrock Vadose Environment. Stage 6 (Figs. 4 and SF - H): diagenetic, its re-
sults whenever preserved after burial correspond to the best reservoir rocks. Intense vadose dissolution after exposure of beachrock to subaerial conditions but
without generation of a freshwater lens: removal of large amounts of bladed rim
cement, corrosion of margins of pisooncoids, deep etching of upper surface of
cemented internal sediment and local differential solution of nuclei and concentric layers of pisoooncoids. These mesopores provide a measured porosity of 2S
to 300;0 and 200 to 400 md permeability.
Freshwater Meteoric Phreatic Environment. Stage 7 (Figs. 4 and 6A, B): diagene-
tic, with cementation in a freshwater lens generated upon extensive subaerial exposure. Reservoirs formed during the previous beachrock stage, including any
enhancement by subsequent meteoric vadose conditions were occluded by calcite
precipitation when transferred by subsidence into the meteoric phreatic zone
prior to permanent burial. The phreatic calcite cement occurs as single crystals
and coarse crystalline irregular to highly interlocked equant mosaic. Frequent oc-
currence of intense and distorted twinning indicates that cementation preceded
major compaction.
Burial Environment. Stage 8 (Figs. 4 and 6C - E): compaction with grain break-
age by radial microfractures, reciprocal grain interpenetration by pressure solution and stylolitization, spalling of bladed rim cement with penetration in pisooncoids, pisooncoid spalling, non-fabric selective fracturation at least in two gener-
Fig. 5 A-H. Submarine to beachrock vadose diagenesis. Plane polarized pictures of thin sections impregnated with blue plastic, porosity appears in various shades of gray. A Submarine isopachous rim
cement of fibrous calcite with traces of dissolution (arrow) and surrounding graded internal sediment. B Isopachous rim of fibrous calcite cement overlain by graded internal sediment with geopetal
attitude. Note vadose dissolution effects preceding phreatic sparite cementation. C Horizontal
surface of geopetal internal sediment. D Vuggy to channel secondary porosity cutting across pisooncoid (right side) and internal sediment. Pores rimmed by bladed calcite cement (arrow). E Rim
cement of bladed calcite (upper right), elsewhere same calcite is cavity-filling. F Relic of bladed
calcite rim cement on pisooncoid surface (arrow) from vadose dissolution before phreatic sparite
cementation. G Reservoir rock showing vadose dissolution effects on bladed calcite rim cement
(arrow). H Reservoir rock showing vadose dissolution of bladed calcite rim cement and of cortical
layers of pisooncoid
Depositional Environment, Diagenesis and Reservoir Properties of Oncolitic Packstones
339
340
A. V. Carozzi et al.
Depositional Environment, Diagenesis and Reservoir Properties of Oncolitic Packstones
341
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References
~
Milliman, 1974;
James and Ginsburg, 1979.
- Keith and Weber, 1964.
A Prezbindowski, 1977.
Fig. 7. Stable isotope measurements
..
Fig. 6A - H. Meteoric phreatic to burial diagenesis. Plane polarized pictures of thin sections impregnated with blue plastic, porosity appears in various shades of gray. A Reservoir occlusion by
phreatic coarsely crystalline equant mosaic of calcite. Earlier rim cement still visible. B Almost complete occlusion of pores by single crystal of calcite overgrowth on echinoid spine. Earlier bladed rim
cement still visible. C Compaction effects: spalling of bladed calcite rim cement, grain breakage by
microfractures, bladed rim cement penetrating pisooncoid margin (arrow). D Compaction effects:
deformed pisooncoids, reciprocally penetrating pisooncoids and stylolitized contacts. E Compaction
effects: first generation fracture cemented and second generation fracture (trending vertical) displacing the first and open. F Late dissolution pore (arrow) by differential solution of concentric layers of
pisooncoid and showing thin rim of late calcite cement. G Thin rim of late calcite cement filling
fractures of pisooncoid deformed by compaction. H Late cement as perfect rhombohedral crystals
with oil staining growing on relics of earlier bladed calcite rim cement
342
A. V. Carozzi et al.
ations. Early fractures cemented, other enlarged by late dissolution. In general,
pores generated during compaction remain uncemented or show a very thin rim
of late calcite cement. Stage 9 (Figs. 4 and 6F): late dissolution with enlargement
of all previous compaction features and generation of intraparticle porosity by
differential solution of some concentric layers of pisooncoids. Stage 10 (Figs. 4
and 6F - H): late cementation is widespread but not an important porosity
reducer. It occurs as a thin rim of calcite crystals and as perfect rhombohedral
crystals with frequent growth lines and inclusions (oil?). This cement may be
coeval with oil migration.
Other minor aspects of burial diagenesis are: aggrading neomorphism appearing as relatively large cloudy calcite crystals with diffused boundaries and
abundant inclusions of unreplaced material; rare dolomite rhombs replacing
matrix and cement; extremely rare silicification as overgrowth on detrital quartz
grains and post-dating dolomitization; anhydritization selectively replacing
pisooncoid layers and sparite cement.
8 Geochemistry of Diagenesis
The beachrock rim cement of bladed calcite and the pisooncoids gave values of
8C13 and 80 18 typical of marine cements, while the cavity-filling sparite cement
gave very light values of 8C13 corresponding to freshwater phreatic conditions
(Fig. 7). The latter are further confirmed by a lack of cathode luminescence
indicating an iron-rich, manganese-poor calcite (Meyers 1978).
9 Summary of Events and Oil Migration
The growth fault activity was responsible for the successive phases of vertical
evolution displayed by the oncolitic shoals, the most evident being in the eastcentral part of the basin where the change of offshore shoals from microoncolitic
to pisooncolitic coincided with a general high of the structural map on top of the
Lower Macae Formation. The same synsedimentary tectonic activity which localized the shoals also uplifted them into beachrock conditions, and higher into islands with extensive subaerial exposure and freshwater lenses. Subsidence of
beachrocks preserved their reservoirs because no freshwater lenses had been related to them, whereas islands had their reservoirs occluded when subsidence
transferred them through the phreatic environment. Reactivation of growth
faults provided the necessary physical connection for the lateral migration of oil
between source-beds (radioactive shale, Carapebus Member, Campos Formation) and the underlying oncolitic reservoirs.
Acknowledgements. The authors are very grateful to the Board of Directors of Petroleo Brasileiro
S. A. Petro bras for permission to publish this paper.
Depositional Environment, Diagenesis and Reservoir Properties of Oncolitic Packstones
343
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