Geology Reference
In-Depth Information
Plate
149 ReservoirRelated Outcrop Analog Study: Late Tertiary of Sulawesi, Indonesia
Tertiary shallow-marine tropical carbonates are major hydrocarbon reservoirs in various parts of the world,
including Egypt, Iran, Iraq, southeastern Asia (South China Sea, Indonesia, Philippines) as well as Central and
South America. The depositional environments of these predominantly Oligocene to Miocene deposits include
platforms and ramps, shelf edge and platform reefs, and pinnacle reefs surrounded by deeper-water facies. Many
Miocene carbonates, forming stratigraphic traps, exhibit well-preserved primary and secondary porosities. Fresh-
water leaching of aragonitic bioclasts during stages of emersion, dolomitization and fracturing of partly lithified
sediment (enhancing permeability) are common controls on the reservoir rock qualities in the Philippines and
various parts of Indonesia (Epting 1980; Longman 1985; Ascaria et al. 1997; Grötsch and Mercadier 1999;
Wilson et al. 2000). Important reefbuilders are corals, red algae, bryozoans and encrusting foraminifera. Larger
benthic foraminifera, miliolid foraminifera, green algae (e.g.
Halimeda
) and gastropods prevailed in lagoonal
and ramp as well as in peri-reef environments.
Microfacies, facies-controlled diagenesis and the distributional patterns of Larger foraminifera and algae offer
a possibility for differentiating the various parts of reef complexes in cores and outcrops (Hallock and Glenn
1985; Sect. 14.2, Fig. 14.7).
The plate shows the preserved primary and secondary porosity types and biota common in Miocene reef
limestones. The impregnation of the samples with blue-dye stained resins reveals open pores. Open biomoldic
pores, linked by fracture pores, may form some permeability. Note that particles originally consisting of arago-
nite (e.g.
Halimeda
and gastropods) exhibit stronger dissolution and moldic porosity (-> 3, 5) than primary
calcitic constitutents (e.g. Larger foraminifera, -> 8). Although visible porosity appears high, effective porosity
is low, because most pores are disconnected.
The samples are from the Selayar Limestone of the Miocene Walanae Formation in the southern tip of South
Sulawesi, Indonesia. Common facies types of the reefal deposits studied are coral limestones (-> 1, 2), foramin-
iferal limestones (-> 7, 8) and
Halimeda
limestones (-> 3, 4). Because of volcanism active during the formation
of the carbonates, thickness and width of these limestones are limited to a few tens of meters (Personal commu-
nication A. Imram, Makassar, 2000).
1
Coral limestone.
Corals (C) are encrusted by coralline red algae (RA) and attached foraminifera (F). Matrix is a bioclastic
wackestone with algal debris. Porosity: Open macroborings (MB) and solution vugs (SV).
2 The
reef framework
consists of corals (C) and coralline red algae (RA). In addition, encrusting bryozoans (B) and gastro-
pods (G) occur.
3
Halimeda
floatstone.
Note the strong selective dissolution of the primary aragonitic algal blades, resulting in high biomoldic
porosity (BP). Chambers in foraminiferal (F) and gastropods shells (G) are filled with blocky meteoric cement.
4
Halimeda
rudstone.
In contrast to -> 3, algal blades are well preserved. Interparticle pores are partly occluded by several
generations of submarine cement, consisting of isopachous rim cement (arrows) followed by white fibrous calcite ce-
ment. Similar to -> 1 and 2, there is only poor intraparticle porosity within the coral (C) due to the protection by red algal
crusts (RA) and rapid submarine cementation. H:
Halimeda
.
5
Gastropod shell
exhibiting intraskeletal and moldic porosity (MP) due to leaching. Note the still preserved cross-lamellar
shell structure (arrow). Dissolution vug at right.
6
Oncoid (acervulinid-corallinacean macroid)
composed of a coral (C), overgrown by coralline red algae (RA), competing
with encrusting foraminifera (
Solenomeris
, SO).
Solenomeris
is a reef-building acervulinid foraminifer that formed mono-
specific reefs in the Early Tertiary, particularly in the Eocene (Plaziat and Perrin 1992). The genus, however, is also
known from Miocene non-reef environments contributing to the formation of oncoids. Open porosity is restricted to few
dissolution vugs (top right).
7
Foraminiferal grainstone/rudstone
with
distinct solution voids (SV) that cross the depositional fabric. Larger foramin-
ifera include
Amphistegina
(A),
Myogypsina
(MY) and
Heterostegina
(HET). In Indo-Pacific Miocene carbonates the
association of these foraminifera is diagnostic of shallow high-energy conditions (see Fig. 14.7) and perireef and forereef
environments.
8
Packed foraminiferal rudstone
consisting predominantly of shells of
Heterostegina
. The primary calcitic foraminifera
show no dissolution. Porosity is restricted to solution voids partly cutting the skeletal grains (arrows). Low energy, pro-
tected inner shelf facies.
Heterostegina
requires calm water in sheltered reef parts or in deeper low-energy environments,
attached to hard substrates or plants.
-> 1-8: Courtesy of A. Imram (Makassar, Sulawesi, Indonesia).
