Types and Properties of Dolomite Reservoirs in Carboniferous of
East Margin of Caspian Basin
Yanfeng Wang
1
, Shuqin Wang
2, *
, Jianxin Li
2
,Wenqi Zhao
2
, Weiqiang Li
2
and Changhai Li
2
1
CNPC Aktobe Munaigas JSC;
2
PetroChina Research Institute of Petroleum Exploration and Development.
Keywords: Carbonate reservoir, dolomite type, porosity, permeability
Abstract: Taking the Carboniferous KT-I reservoirs in the NT Oilfield in the eastern margin of the Caspian Basin as a
case, this paper studies the petrology and physical properties of carbonate reservoirs using core data. The
carbonate reservoirs in the study area were deposited in the platform environment. The limestone deposited
has experienced multiple stages of diagenetic alteration, among which the dolomitization played an
important role in improving the reservoir quality. The dolomites in the study area are diverse in type and are
characterized by the development of eight types of dolomites, including foraminifer dolomite, fuzulinid
dolomite, bioclastic dolomite, micritic dolomite, micritic-powdery dolomite, powdery dolomite and lime
dolomite, and karst breccia dolomite. Different types of dolomite have different characteristics observed on
thin sections, and the physical properties vary too. Foraminifera, fuzulinid and bioclastic dolomites belong
to grain dolomite with obvious biological characteristics, relatively high permeability and average
permeability of more than 100mD. Micrite, micritic-powdery and powdery dolomites are crystalline
dolomites which are composed of dolomite crystals arranged closely, and the average permeability is less
than 35mD.
1 INTRODUCTION
The sedimentation and diagenesis of the carbonatite
reservoir is extremely complex, making it very
difficult to study the depositional environment,
genesis, properties and distribution (Colin et al.,
2004). The complexity of carbonate reservoir has
great influence on oilfield development. Taking the
Carboniferous KT-I carbonate reservoir in the NT
Oilfield in the eastern margin of the Pre-Caspian
Basin as a case, this paper evaluates the properties,
distribution, storage and seepage of the dolomites.
2 GEOLOGICAL CONDITIONS
The NT Oilfield is located on the eastern margin of
the Caspian Basin in the Republic of Kazakhstan in
Central Asia(Figure 1). Under the oilfield, the
Carboniferous to Quaternary formations were drilled
in from bottom to top, in which the Carboniferous
are carbonate reservoirs composed of KT-I and KT-
II, and between which is a clastic interlayer about
300m thick. The KT-I carbonate reservoir
experienced multiple stages of diagenesis, of which
dolomitization is one of the major diagenetic
processes, and played an important role in
improving the storage capacity of the reservoir
(López-Horgue et al., 2010). The dolomites account
for 52.7% and limestones account for 47.3%, and
dolomites are primary reservoirs in the KT-I
carbonate section.
3 TYPES AND PROPERTIES OF
DOLOMITES
The dolomites in the KT-I section in the NT Oilfield
are dominated by crystalline dolomites, accounting
for 74.26%, including micrite, micritic-powdery and
powdery dolomites. They generally contain
biological debris of different amounts, but without
Wang, Y., Wang, S., Li, J., Zhao, W., Li, W. and Li, C.
Types and Properties of Dolomite Reservoirs in Carboniferous of East Margin of Caspian Basin.
In Proceedings of the International Workshop on Environment and Geoscience (IWEG 2018), pages 319-324
ISBN: 978-989-758-342-1
Copyright © 2018 by SCITEPRESS – Science and Technology Publications, Lda. All rights reserved
319

fine, medium and coarse dolomites. The content of
grain dolomite is less, accounting for 15.44% (Table
1).
Figure 1: Location map of the study area.
Table 1: Rock type statistics of dolomite in KT-I Layer in
NT Oilfield.
Lithology
Numbe
r of
samples
Conten
t (%)
Subtota
l (%)
Grain
dolomite
Foraminifera
dolomite
3 2.21
15.44
Fuzulinid
dolomite
6 4.41
Bioclastic
dolomite
12 8.82
Crystalline
dolomite
Micritic
dolomite
45 33.09
74.26
Mic-powdery
dolomite
22 16.18
Powdery
dolomite
34 25
Other
Lime (lime-
bearing)
dolomite
10 7.35 10.29
3.1 Foraminifera Dolomite
The foraminifera dolomite is dominated by
foraminifera (35% - 50%) and fuzulinid (2 - 10%),
which all have been dissolved into residual debris.
In addition, algae lumps and debris of chlorophyta
were observed; cavities and granular cast pores were
found common; and semi-euhedral dolomite with
sparse contact is intergranular fillings (Figure 2).
3.2 Fuzulinid Dolomite
Fuzulinid dolomite contains a large amount of
fuzulinid (30%-45%). Intense dissolution almost
made the chambers empty, only leaving residual
Schwarzkopf debris being identified
Powdery residual
foraminifera dolomite .
Residual micritic
foraminifera dolomite.
Figure 2: Foraminifera dolomite .
in some ones. In addition, there found chlorella
and rhodophyta fragments and foraminifera (5% -
15%), and echinoderm and bryozoan residues. The
shells of all the creatures are composed of
dolomicrite, which were often dissolved to form
casted shell pores. The intergranular fillings are
micritic-powdery dolomites accounting for 20% - 45%
(Figure 3).
Micritic bioclastic fuzulinid
dolomite
Residual micritic fuzulinid
dolomite
Figure 3: Fuzulinid dolomite.
3.3 Bioclastic Dolomite
Bioclastic dolomite is dominated by foraminifera
(1%-30%) and fuzulinid (5%- 30%). The
paleontological content is great with common
observation of bryozoans, brachiopods,
echinodermata, and relatively large amount of non-
abrasive algal lump. More non-selective
IWEG 2018 - International Workshop on Environment and Geoscience
320

dolomitization formed coarse- powdery -level
rhombohedron. Strong dolomitization and
dissolution made the grain almost indistinguishable.
Only the traces of species of fuzulinid and
chlorophyta could be vaguely observed. Some
biological debris came into being casted pores, and
some became single and multiple crystal grains, but
most of the bioclasts are micritic dolomite. The
intergranular material is fine-powdery dolomite
which is semi-euhedral. It contains a large number
of residual bioclastic dolomite composed of micritic
dolomite, partly forming grain casted pores and shell
wall pores (Figure 4).
Micritic residual bioclastic
dolomite
Micritic-powdery residual
bioclastic dolomite
Figure 4: Bioclastic dolomite.
3.4 Micritic Dolomite
Micritic dolomite is dominated by micritic
accounting for 56%-98%. With varying amounts of
biological debris, it contains foraminifera of 2% - 25%
and fuzulinid of 1% - 18%. The organism is less in
its amount but miscellaneous in classes. There
observed shallow benthic organisms, including sea
urchin spine, sea lily stems, paleobraspora, small
brachiopods individuals, sponge fragments,
lamellibranch, and the likes. Deepwater
phytoplankton could also be observed, including
thin-shell bivalves, thin-shell brachiopods, and
fibrous thin-shell organisms. In the dissolved pores,
there found semi-filled, fine-coarse dolomite
rhombohedra and kaolinite, and occasional
occurrence of semi-filled casted pores with gypsum
crystalline. Locally, it contains a small amount of
terrigenous debris extremely fine to fine. It is
dominated by quartz and a minor of quartzite debris.
The biological shell wall consists of
cryptocrystalline dolomite. The bioclastic represents
residual structures, parts of which were completely
dissolved to form biological molds, and foraminifera,
fuzulinid individuals, ostracods, lamellibranch and
the likes can be identified. A minor of bioclastic
bodies were filled or semi-filled with fluorite,
kaolinite and calcite. Locally there is the observation
of casted molds of gypsum crystalline. Judged from
the developmental characteristics of shallow benthic
organisms such as spine and shellfish, the marlite
should be formed in a subtidal environment. It is the
marl deposited in restricted platform lagoon was
dolomitized (Lucia, 2011), and such dolomitization
was strong and complete (Figure 5).
Micritic dolomite
Siliceous spine micritic
dolomite
Figure 5: Micritic dolomite.
3.5 Micritic-Powdery Dolomite
Micritic-powdery dolomites are almost marlite-
power crystalline dolomite accounting for 52% -
92%. They contain varying amount of bioclastic,
including 2%-20% of foraminifera, 2%-35% of
fuzulinid, and a minor of echinacea, ostracods,
lamellibranch, gastropods, brachiopods, algae, and
the likes. In summary, they are less in categories and
amount, and in which intense and destructive
dissolution created many molded holes and shell
wall holes. The micrite-powdery crystalline was
formed by strong and complete dolomization of marl
deposited in the subtidal saline lagoon in a restricted
platform (Figure 6).
Residual bioclastic micritic-
powdery dolomite
Micritic-
p
owdery dolomite
Figure 6: Micritic-powdery dolomite.
Types and Properties of Dolomite Reservoirs in Carboniferous of East Margin of Caspian Basin
321

3.6 Powdery Dolomite
Powdery dolomites are fine-medium- coarse
powdery crystals, semi-euhedral and euhedral. They
contain varying amounts of foraminifers, fuzulinid,
ostracods, gastropods, lamellibranch, trilobite,
echinoderms and algae, ranging from 12% to 70%,
and mostly fragments and remnants often dissolved
into casted mold pores. Shell walls of a small
portion of gastropods,
echinoderms, ostracods are still calcite. Most
foraminifera and fuzulinid species are remnants of
debris due to dolomization and dissolution. Those
well preserved are bradyid, paleobraspora and
internal worms, and casted molds of chlorella,
ostracods and lamellibranch, and occasional
occurrence of echinacea fragments. Locally, there
observed chlorophyta lumps filled with coarse
calcite after dissolution, and large caves completely
filled with deformed dolomite and lapis lazuli. In
addition, a small amount of anhydrite was found
from two origins: the anhydrite replacing biological
debris, which shows wave absorption, and the
single-crystal anhydrite filling into biological
molded pores. Dissolved fractures semi-filled with
anhydrite can also be observed. More gypsum
molded pores were found, rectangular, cylindrical or
like bar and other shapes (Figure 7).
Coarse podwery dolomite
Residual foraminifer powdery
dolomite
Figure 7: Powdery dolomite.
From the point of more shallow water organisms,
the powdery crystalline should be subtidal deposits.
The occurrence of a small amount of gypsum salt
indicates that the sea water was salinized. That is,
the powdery crystalline was formed by the
dolomization of biological marl deposited in the
subtidal saline lagoon in a restricted platform (Ma,
1999), and it was not formed in a supratidal Sabkha
environment (Zhao et al., 2016).
3.7 Lime (lime-bearing) Dolomite
The lime (lime-bearing) dolomite is mainly of
micritic structure, with sparse distribution of
deepwater organisms, including monactines, thin-
shell organisms, echinoderm fragments and
phosphorus bioclastics. The distribution of calcite is
more uniform, but that of echinacea and fuzulinid is
not uniform, and a few echinacea have the
phenomenon of coaxial growth. A few of fuzulinid
were replaced by calcite. It shows strongly
fragmented organism, less in amount, and more in
kingdoms, with the observation of trilobite
fragments and tail thorns, and bryozoans,
brachiopods, echinoderms and the likes. What is
extremely peculiar is that there are both the
occurrence of tabular and columnar pseudo-crystals
of gypsum, and that of monactines and thin-shell
creatures. Also, there is the observation of normal
shallow marine creatures. The dolomite is divided
into two types, marlite and powdery crystalline,
which are distributed disorderly. The biological
burrow structure can be observed, representing ditch,
stream, rooted and other shapes. All micrites are
calcite, and powdery crystallines are semi-euhedral
and euhedral. The content of dolomite is equivalent
to that of lime, forming a transitional lithology. In
the micrite, there are occasional occurrences of the
remnants of echinoderm, fuzulinid and foraminifer.
The replacement of chalcedony and gypsum (with
wave absorption) can be observed locally. The
breccia with a suspected genesis of biological
burrowing or disturbance can also be observed and
the dolomitization is not uniform. In the place where
deformation is strong, found powdery dolomite and
lime dolomite. In the place where replacement is
weak, found dolomitic limestone, marlite, and
sparsely distributed brachiopoda, porous bryozoans,
echinoderms, fuzulinid, foraminifera fragments, and
occasionally phosphorus bio-shaped barbed. The
micritic structure contains coarser coral fragments,
monactines and triactines. With normal shallow
marine organisms, the lime (lime-bearing) dolomite
should be formed by incomplete dolomization of
deposits in restricted shallow sea (lagoon) (Figure 8).
IWEG 2018 - International Workshop on Environment and Geoscience
322

Powdery lime dolomite
Micritic breccia lime
dolomite
Figure 8: Lime (lime-bearing) dolomization.
3.8 Karst Breccia Dolomite
The karst breccia dolomite, which contains karst
breccia formed by rupture-dissolution, is mainly
composed of marlite and pellets, mostly in angular,
messy accumulation, but in relatively concentrated
distribution. Among the breccia is filled with semi-
euhedral powdery crystalline dolomite or coarse
rhombohedral dolomite in the first generation, or
semi-filled or fully-filled with coarse rhombohedral
dolomite in the second generation. Among the
grains is semi-filled with fine crystalline dolomite
and coarse rhombohedral dolomite. The latest
fillings are bitumen or kaolinite. Kaolinite fillings
are more commonly observed in dissolved pores.
Strong karstification created karst breccia dolomite,
indicating the existence of ancient weathering crust.
The dolomite was altered by dissolution and became
the favorable reservoir in this area (Figure 9).
Micritic-powdery breccia
dolomite
Karst breccia dolomite
Figure 9: Karst breccia dolomite.
Table 2: Statistics of physical properties of different types of dolomites.
Lithology
Number of
samples
Porosity (%) Permeability (mD)
Max. Min. Average Max. Min. Average
Foraminifera /
Fuzulinid dolomite
9 21.62 13.9 18.48 443 11.6 135.44
Bioclastic dolomite
12 16.5 5 11.09 611 0.0009 148.39
Micritic
dolomite
45 39.2 4.9 15.6 400 0.0003 33.89
Micritic-powdery
dolomite
23 30.72 7 14.93 179 0.0246 25.38
Powdery
dolomite
31 18.46 3.62 13.8 126 0.0011 20.2
Lime/Breccia dolomite
15 26.54 1.56 12.49 1431.41 0.004 104.32
Figure 10: The porosity and permeability distribution of different types of dolomite.
Types and Properties of Dolomite Reservoirs in Carboniferous of East Margin of Caspian Basin
323

4 PHYSICAL PROPERTIES OF
DOLOMITES
Statistics of thin section observations and dolomite
porosity and permeability indicates that different
types of dolomites have different physical properties
(Table 2, Figure 10). The average porosity of the
dolomite reservoirs in different types ranges from
11.09% to 18.48%. The porosity is a little different,
but the permeability is greatly different. The average
permeability of foraminifer/fuzulinid,bioclastic
dolomites and Lime/Breccia dolomite is more than
100mD, while that of micritic, micritic-powdery and
powdery dolomites is below 35mD, because the
latter three types are of grain structure with
relatively homogenous grains. Although they have
similar porosity, the distribution of the throat size is
more extensive in grain dolomite; therefore grain
dolomite shows obviously high permeability (Wang
et al., 2010).
5 CONCLUSIONS
The Carboniferous KT-I carbonate reservoir in the
NT Oilfield was deposited in a restricted platform.
Early limestone experienced multiple stages of
diagenesis, and the dolomitization played an
important role in improving the reservoir quality.
There found eight types of dolomites in the study
area, including foraminifera, fuzulinid, bioclastic
dolomite and micrite, micritic-crystalline, crystalline
dolomite, lime dolomite, and karst breccia. Different
types of dolomites have different appearances
observed on thin sections, and their physical
properties are different too. Foraminifer/fuzulinid,
bioclastic dolomites and Lime/Breccia dolomite are
granular, with obvious biological characteristics and
relatively high permeability (more than 100mD on
average). Micrite, micritic-powdery and powder
dolomites are crystalline and composed of dolomite
crystals. The crystal grains are relatively close, and
the average permeability is less than 35mD.
REFERENCES
Colin J.R.Braithwaite,Giancarlo Rizzi and Gillian Darke
2004 The geometry and petrogenesis ofdolomite
hydrocarbon reservoirs Geological society
López-Horgue M.A, Iriarte E, Schröder S, Fernández-
Mendiola P.A. 2010 Structurally controlled
hydrothermal dolomites in Albian carbonates of the
Asón valley, Basque Cantabrian Basin, Northern
Spain Marine and Petroleum Geology 27(5) 1069-
1092
Lucia F J 2011 Carbonate Reservoir Characterization [M]
Beijing: Petroleum Industry Press 16-22
Ma Yongsheng 1999 Carbonate reservoir sedimentology
Beijing: Geological Publishing House
Wang Dang, Chen Daizhao, Y Changchun 2010 Dolomite
structure type in buried environment. Acta
Sedimentologica Sinica 28(1) 17-25
Zhao Lun, Wang Shuqin, Zhao Wenqin and Luo Man
2016 Combination and distribution of reservoir space
in complex carbonate rocks.[J].Petroleum Science 13
450-462.
IWEG 2018 - International Workshop on Environment and Geoscience
324