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INTRODUCTION
The necessity of thesis
Carbonate Miocene formations were formed and developed in different
mechanisms, geographic conditions and became crucial oil and gas
reservoirs in many parts of the world. The model of sediment
depositing, secondary variations of rock are dominated by paleoclimatology and regional tectonics and affect quality of carbonate rock.
The South East Asia is located in the humid tropical zone, with the
warming up climate which is favorable conditions to strongly develop
coral platform or other calcite creatures which form carbonate sediment
zone in Miocene period to now.
Identifying gas in carbonate traps of early and middle Miocene age, in
drilling hole complexes namely: Well 115A-1X, 117-STB-1X, 118CVX-1X, 119-CH-1X in the South of Red River gave orientation to oil
and gas geologists to research in platform carbonate objects, build-up,
reef of other potential basins such as Phu Khanh, Eastern Nam Con
Son, Tu Chinh – Vung May, Hoang Sa, Truong Sa basin groups on
Vietnamese continental shelf.
Originating from factual requirement and scientific significance of
carbonate objects, in order to clarify matter composition, sediment
condition, oil and gas potential of carbonate object to orientate the oil
and gas exploration in the South area of Red River sediment, research
student selects topic “Characteristics and model of carbonate sediment
of Miocene age in the South of Red River sediment basin” to prepare
Doctor of Philosophy thesis in order to respond requirement of
practice.
Purpose of research
The purpose of this thesis is to clarify the geological features, evolution
process of carbonate rock of Miocene age in the South of Red river
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sediment basin to facilitate the construction of sediment model,
positively contribute to improve efficiency in oil and gas exploration –
research for these objects.
Research task
- Research and clarify the exist, classify and identify carbonate rock
of Miocene age in the South of Red river sediment basin on the
basis of analyzing and totaling up the geology – geophysics and
exploring drill,
- Identify geological, geophysical features and interpret environment
facies; analyze sediment composition in two carbonate formations
of early and middle Miocene age on Tri Ton high/ horst,
- Build up geological evolution process for Tri Ton high/ horst
structure and sediment model as well as relationship between oil &
gas and carbonate rock composition of Miocene age in the South of
Red river basin.
Research content
- Study, integrate the ecological characteristics in the region, and
zone the structure, geological development history in the South of
Red River basin on the basis of ecological, geophysics materials
and drilling result,
- Sum up the theory of forming origin, affective factors, environment
of composition and carbonate rock classification,
- Total up knowledge on methodology of lithologic, paleobiology,
seismic sequence stratigraphy, geophysics of drilling hole;
principles, actual application of those methodologies,
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Interpret/ Explain clearly the seismic materials to figure out the
diversity boundary, identify seismic facies, in combination with
geophysics result of drilling holes and paleobiology, lithology
analysis to identify sediment facies complex. Pursuant to the result,
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we can initially determine the distribution scope as well as
environment of composition, thereby interpret the formation and
development process, properties and nature of carbonate,
Build up evolution model of carbonate Miocene sediment,
primarily assess relationship between rock and ability of forming,
storing for oil and gas potential in the region.
Research methodology
To exploit the approved materials to maximum capacity and in an
effective way and to meet the research target of the thesis, we use result
of research methodologies complex such as:
- Geological methods: sediment lithology, paleo-biology and
stratum,
- Geophysics methods: seismic stratigraphy, sequence stratigraphy,
drilling geophysics analysis (well logs).
Object, scope and data base
In the South of Red River basin, the carbonate layers are distributed
evenly on Tri Ton high structure, basin bed thickness sharply varies
without following the rule, on the other hands, storing composition
suffers from secondary change, forms various kinds of pore in both
horizontal and vertical direction etc… Researched object of this thesis
is potential carbonate sediment which contains oil and gas of early,
middle Miocene age in this high structure.
The thesis mainly employs 2D seismic acquisition materials BP, BHP
from 1989 to 1991; the 04 wells drilled by BP, BHP in the stage of
1990-1995 in the scope of Tri Ton high structure along with geological
sample documents (lithology, geochemistry) and other geophysics
materials. Moreover, geological research result from the published
work in continental shelf in Central of Vietnam in blocks 117, 118, 119
and 120 in the South of Red river sediment basin is also used as
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reference.
Defense theoretical points
1. Carbonate sediment composition of chemical-biological origin is
created, developed and degraded mainly in early and middle
Miocene stage in independent high structure. Sedimentary
composition is mainly controlled by composition structure,
affected by gradual transgression in late syn-rift and closely relates
to environment factor. Carbonate Miocene component consists of
different carbonate composition as platform, build up, reef.
2. The composition and development of carbonate Miocene
experiences four main stages: a) Start up within early carbonate
Miocene; b) Build up and build out during early, middle Miocene;
c) Back stepping to decrease its distribution scope in near top of
middle Miocene; d) The platform carbonate stage transfers to
downgrade stage and to drowning in the top of middle Miocene
and covered then by younger sediment deposited from late
Miocene to present.
New points of thesis
1. This is the first research which studies the feature, classification
and identification of carbonate rock of Miocene sequences in
accordance with geology – geophysics materials in the South of
Red river sediment basin.
2. Based on new structural maps (both in time and depth) in the scope
of four blocks namely 117, 118, 119 and 120, we identify and
clarify composition factor, environment facies, and impact on the
forming, development and downing of this rock.
3. We restored the general geological development history, as well as
the evolution of carbonate sediment of Miocene age and referred to
the oil and gas potential of basin. Moreover, we built geological
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facies to forecast oil and gas exploration model in a more effective
way.
4. The outcome of this thesis had confirmed the application of general
method in studying the carbonate layers in Cenozoic on
Vietnamese continental shelf. Concurrently, the research and
prediction results had been verified and shown conformable to two
new drilling holes in block 118 and 119 in the year 2011.
Scientific and practical significance of thesis
Scientific significance
Sediment basin of Red river in Vietnamese continental shelf is the first
Cenozoic sediment basin which is studied rather specifically and fully
for carbonate sediment of Miocene age.
- Research outcome of thesis contributes new understanding on
geological development process, builds carbonate sediment facies
and the relation with oil and gas potential in the region,
- Through deciphering position of strata and analyzing
environmental factors, thesis outcome had confirmed the ability of
applying research methodology complex for carbonate formations
in the South of Red river basin and showed that this methodology
will accord other areas in Vietnamese continental shelf.
Practical significance
Practical significance of this thesis is that we can thoroughly exploit
information from seismic materials and other geologic, geophysics to
identify space distribution as well as features of carbonate sediment of
early, middle Miocene age.
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Clarify features of carbonate sediment of Miocene age in the South
of Red river basin and predict them in other Cenozoic sediment
basin in Vietnamese continental platform,
Build up face model and sediment development stages to clarify oil
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and gas potential for carbonate Miocene objects in the South of
Red river sediment basin is an issue of great attention of Petro
Vietnam (PVN) as well as oil and gas companies in South East
Asia,
- Prove the relationship of carbonate rock with oil and gas system in
the region; contribute practical effectiveness to selection and
location of drilling hole for oil and gas exploration.
Structure of thesis
This thesis includes 134 pages, in which there are 8 tables, 65 figures
and diagrams, 3 attached appendices, 10 published work items relating
to the thesis and 43 references (27 in Vietnamese, 16 in English) shown
in 4 chapters excluding introduction and conclusion.
Chapter 1.
GEOLOGICAL FEATURE AND PETROLEUM
SYSTEM IN SOUTHERN OF SONG HONG BASIN
1.1.
Study area
1.1.1. Study area location
The study area covers southern Song Hong basin including the
blocks 117, 118, 119 and 120 with the area of approximately 40,000
sq.km.
1.1.2. Exploration history
Pre-1975: has acquired regional seismic lines and identified regional
structures roughly.
1975-2000: wildcat well results have proven that the presence of PreTertiary carbonate basement as well as Miocene carbonates existed.
2000-present: some researches conducted on carbonates of the shelf
area and got better understanding on their nature. From 2009 to now,
Exxon Mobil drilled 2 exploration wells in area of blocks 117-119 and
one of them has shown surprising results in carbonate target.
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1.2.
Geological overview
1.2.1. Database
Seismic data used in the study include a selected part of 2D seismic
surveys with 6200km of 2.5x10km (BP, 1989), 4000km of 2.5x5km
(BP, 1991), 3000km of 4x4km, 4x8km (BHP, 1991). Well data include
of 4 wells 117-STB-1X, 118-CVX-1X, 119-CH-1X, 120-CS-1X, app.
50 thin sections and lithological samples and 12 paleontological samples.
1.2.2. Seismic interpretation and well correlation
Five key horizons were identified, interpreted and correlated are (a)
Top basement, (b) Top Oligocene, (c) Top Lower Miocene, (d) Top
Middle Miocene, (e) Top Upper Miocene. Within these, there are 3 key
seismic sequences were divided into:
- A sequence is characterized by amplitude variance with reflection
discontinuity and low frequency. It is interpreted to be a
sedimentary succession developed in grabens and half-graben as
consequence of extension during Paleogene and is composed of
coarse grained and poor sorted sediments discharged from nearby
basement highs.
- B sequence corresponds with Miocene sediments with stronger
amplitudes and more continuous reflections than sequence A. In
the study area, sequence B can be further divided into B1 and B2
ones. B1 corresponds with Early-Middle Miocene sediments
characterized by anomalous reflections and velocities associated
with carbonate development. B2 is characterized by weak and
parallel reflections and corresponds with late Miocene sediments.
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C sequence corresponds with Pliocene-Quaternary sediments
characterized by parallel, divergent reflections with moderate to
strong amplitudes and good continuity. These features are typical
of successive sedimentary depositions in response to cycles of
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marine transgression and regression on the regional subsidence
background.
1.2.3. Stratigraphy
a. Pre-Tertiary basement
To present, only well 115-A-1X encountered metamorphic Pre-Teriary
basement.
b. Cenozoic sedimentary column
Oligocene sediments - Bach Tri formation
Terrestrial sediments directly overlying the basement were deposited in
alluvial, lacustrine and deltaic environments and consist of claystone,
siltstones interbedded with sandstones, occasionally thin coal beds.
Lower Miocene - Song Huong formation
Consists of hard grey, light grey claystones containing carbonates,
lignites, occasionally foram interbedded with thin light grey thin
sandstones intercalated with carbonates. Depositional environments are
typical of coastal plain and continental shelf.
Middle Miocene - Triton formation
In well 119-CH-1X, Tri Ton formation is present from 1454-2165m
and consists of white, light grey carbonates conformably overlying the
Song Huong formation.
Upper Miocene - Quang Ngai formation consists of claystones
intercalated with siltstones, sandstones and occasionally containing
carbonates, coal materials, pyrites, glauconites and forams. Their
depositional environments vary from coastal plain, continental shelf,
shallow marine to deep marine.
Pliocene-Quaternary - Bien Dong formation
Is present in well 114-KT-1X from 750-1084m and consists of
unconsolidated light grey, red grey claystones, siltsone and finegrained to medium-grained, light gey, dark grey sandstones
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1.2.4. Structural elements
Southern Song Hong basin can be divided into following main
structural elements: Danang shelf, Quang Ngai graben, Tri Ton horst,
differentiated eastern Tri Ton syncline.
1.2.5. Fault systems
Three main fault systems were identified as NE-SW, sub-longitude and
sub-latitude.
1.3.
Geological evolution
Four geological evolution phases in the area of blocks 117-120 were
restored as:
- Eocene? – Early to Middle Oligocene syn-rift (35-26my.a)
- Late Oligocene strike-slip (26-24my.a)
- Early-Middle Miocene stable sag (20-10my.a)
- Late Miocene-present rapid sag (10my.a- now)
1.4.
Petroleum systems
1.4.1. Discoveries
Several of oil and gas discoveries were encountered in early-middle
Miocene carbonates. Analysis results show hydrocarbon of both
terrestrial and marine source.
1.4.2. Source rocks
Oligocene organic-rich shales, coaly shales and early-middle Miocene
marine shales.
1.4.3. Reservoirs
Pre-Tertiary basement, Oligocene terrestrial sediments, Miocene
carbonates.
1.4.4. Seals
Oligocene and Miocene interbedded shales
1.4.5. Traps
Three main trap styles are basement high, structural closures (including
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four ways closured and fault tittled block), Early- Middle Miocene
reefs, buid ups.
Chapter 2- THEORETICAL BASIS AND RESEARCH
METHODOLOGY OF CARBONATE SEDIMENT
2.1.
Theoretical background
2.1.1. General
Marine carbonate development is controlled by various factors
including water depth, temperature, light, nutrient source, depositional
environment, time.
2.1.2. Carbonate development
Carbonate system started to develop if suitable and favorable
environment conditions are consistent. In favour condition,
development of platform carbonate will be coincided with rising
velocity of sea level. This development is normally followed by 3 stage
according to the period of sea level change. In the stat-up stage,
carbonate begin to develop, sedimentary supply source is still low, in
general it is not catched up with high rising velocity of sea level. In
next stage, growth of carbonate is changed to catch-up and keep-up
stage with enough material source after high rising velocity of sea
level. However, when sea level is quick rising and occurs in a short
time, it can result in death and degradation of coral carbonate system.
Besides of sea level change, there are many different independence
factors that results in the degradation and death of carbonate growth
system such as supply source; sedimentary amount; salty change;
dissolution of oxygen in water; coral eating of different species.
Expose over water surface is one of the reasons that make corals die.
2.1.3. Carbonate classifications
a. By Embry and Klovan, 1971: combined with Dunham (1962)
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studied and based on the rock compositions and also structured,
sorted carbonate.
b. By Wilson, 1975: carbonate environments are divided into three
main zones as: (i) basinal (pelagic); (ii) slope; (iii) platform.
c. By Ginsburg and James, 1974: there are two carbonate groups such
as : (i) rimmed shelves; (ii) opened shelves.
2.2.
The study methodology
2.2.1. Carbonate sequence stratigraphy
Due to different originals, environment for carbonate development,
variation of carbonate with relatively sea level change have been also
occurred quickly and multiform that resulted in basical difference
between stratigraphy sequence modeling, carbonate sedimentary
system area and clastics deposits.
The area of Lowstand System Tract (LST)
In the carbonate environment, Sea level change has strongly effected to
the formation and the amount of sedimentary material. When sea level
decreases under shelf margin, carbonate platform is exposed on sea
surface and it is unable to grow. The area of carbonate growth is only
limited on slope, the shelf margin forward to the sea where it is under
sea level. The process of karstation is strongly occurred in the place
where it is eroded and exposed.
The area of Transgression System Tract (TST)
In the carbonate environment, the increase of relative sea level can be
lack of deposits and subducted, destroyed carbonate platform. In
almost cases, the development of carbonate platform is often occurred
following 3 phases after the high increase period of sea level. Start-up
phase, catch-up phase and keep-up phase are carbonate materials
enough under the increase of sea level.
The area of Highstand System Tract (HST)
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Carbonate deposits are always grown the strongest in the high increase
period of sea level. The coastal line is trended forward to the sea. The
slope and bottom floor is received deposits supplied from the shelf and
shelf margin to create turbidities facies carbonates and clastics flows.
2.2.2. Borehole geophysical method
The complex of borehole data including logging curves as GR, PS, RT,
DT, RHOB have been used to accurate stratigraphy, lithology and
depositional environment, correlate with seismic sections and then
correlate seismic facies with depositional facies.
2.2.3. Litho-stratigraphy method
Thin section analysis methods to determine lithological and mineral
composition, texture as sortness, roundness, cementation, secondary
variation, pore styles have been used in thesis.
2.2.4. Bio-stratigraphy
Application of biostratigraphy data to study carbonates was used to
analyse, interprete creatures spices as coral, forams etc. that can be
concluded that formations bearing sub-creature spices are developed in
shallow tidal environment.
Chapter 3- CHARACTERISTICS OF MIOCENE CARBONATE
SEDIMENTS IN THE SOUTH OF RED RIVER BASIN
3.1.
Carbonate sedimentary characteristics
3.1.1. Seismic facies characteristic
a. Chaotic form: This facies is characterized by downlap form at two
ends of seismic termination. Reflections are parallel with the
boundary of above sequence, meanwhile reflection of below
sequence are convex-up and typical of chaotic configuration. The
characteristic of this seismic form is releted to rather high porosity
and permeability as their composition is grainstone, boundstone
created from marine algae and corals similar to core samples.
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b. Progradation facies: This facies is determined by onlap form at the
top of sequence boundary and downlap at the maximun flooding
surfaca or related to the bottom of above sequence. Within this
sequence, reflections has sigma and clinoform configuration. On
seismic section this facies is characterized by carbonate of
boundstone form in company with shelf margin facies, reef facies
or carbonates that are developed from shelf margin to local coastal
line.
c. Chaotic facies: This facies is characterized by reflections that are
distributed in chaotic form. On seismic section this facies is
occurred at the special location of linear type at the platform
carbonate, but it can also be ellipsoid form highlighted on shelf
carbonate.
d. Parallel seismic facies: It is characterized by parallel seismic
configuration or concordant with reflection surface between small
sequences. This facies is characterized by continuos reflections,
semi-continuos, they are parallel with sequence boundary or create
sine form with sequence boundary.
On seismic section, this facies has parallel form that often occurred in
inner shelf and characterized for wide variation of carbonates with
composition from coarse to fine grain.
e. Incline/slope facies: These reflections have two configurations of
Incline/slope facies that are normally characterized for locations in
which shelf is forward to the central basin in theorical or to coastal
line (continental) in the slope cape. The main characteristic of this
facies is that reflections are regularly inclined, but decrease into the
slope cape and still keep parallel with the sequence boundary.
3.1.2. Geological and well logging characteristics
Based on the complex of biostratigraphy, lithostratigraphy analysis
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results, the biological composition that created carbonate and related to
the rocks can be determined as groups bellowed:
- Corals, rhodolite of boundstone,
- Red marine algae, skeletal, equinoderm of packstone- grainstone,
- Red marine algae, mollusk, bryozoa of grainstone - packstone,
- Flooding ostracode, red marine algae of grainstone - wackestone.
Therefore, it can be concluded that carbonates in Tri Ton horst are
original from creatures and plant fossils mentioned above that were
formed in Early and Middle Miocene time. The lower part of the
section was varied by chemical process due to dolomization.
The results of thin section analysis have proven the existence of
carbonate according to core data (1469÷1486m of the Well 119-CH1X), well logging and mechanical physical data it has porosity varying
from high value of 30÷20% to low 10÷5% .
Based on core and thin section analysis data it can be determined
sedimentary cycle of Miocene carbonate deposits in block 117-120.
The most typical evidence of this sequence was started by development
of boundstone coral layers and then move upward into coarser grained
rthodolite facies with poor sortness, then finer grains and finally was
very fine grain with good sortness and created red corals and forams.
The statistic results of lithology have identified the relation of grain
size and general porosity when the grain size is smaller than 0,6mm,
then porosity is about 4÷13% and porosity is bigger than 0,6mm,
porosity varies in the interval of 9÷27%. With the above mentioned
results, based on the grain size and porosity identified by lithology and
mechanical physical analysis in combination with well logging curve
analysis, the sedimentary cycle can be determined by this complex
comparison.
Comparison with mechanical physical, thin section and well data
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analysis of three well 118-CVX-1X, 119-CH-1X, 120-CS-1X, it can be
seen that tendency of porosity distribution is well coincided with
analysis results of geological samples and well data. High porosity
zone can be related to coarse grain rthodolite that created in shallow
water with poor roundness and poor sortness. In the other side, the
lower porosity zone is clearly shown by good sortness and good
roundness related to coral, forams, marine algaes created in deeper
water.
3.2.
The factors effected to the development of Tri Ton
carbonates
The structure is the most important factor for beginning and
development of carbonates in Tri Ton horst. In the highest location of
Tri Ton, there are all of live conditions for platform carbonates to
develop. The stable activities of normal faults controlling Tri Ton horst
in Early Miocene period are also good condition for progradation
development and then extension of Tri Ton platform carbonates.
Besides of structural factor, Tri Ton platform carbonates are also
effected by relative sea level chance and environment variation as well
as windward or leeward direction.
Chapter 4- MODEL OF CARBONATE SEDIMENT OF
MIOCENE AGE AND ITS RELATION TO OIL AND GAS
4.1.
Geological Model
Based on structural configuration analysis, time relation analysis of
each structural unit and other factor such as tectonics, sedimentary
environment that effected to the development and drown of Tri Ton
platform carbonate, geological models can be done.
4.1.1. Building the sections of development history and drowning of
carbonates
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a. Stage 1 - Start up: Platform carbonates of Song Huong formation
of early Miocene (c.24÷16my.a) was well started up, the strongest
is at 20my.a (million years ago)on Tri Ton high zone inherited the
basement uplift zone locally or extended by marine troughs with
depth varying from some tens meters to hundred meters. The most
typical point in this stage is that the tendency of vertical
development is dominated. The western and eastern marginal parts
are developed by mound, chaotic, complex seismic facies with the
interpretation that marginal reef is gentle in rimmed area. The
common development of Song Huong formation carbonate is
dominated by platform carbonates in large area and by the process
of carbonates creation increasing in comparison with the increase
of sea level.
b. Stage 2 - Extension development: Tri Ton carbonates was well
catched up, keeped up and built out in the period of 24÷13my.a. In
this period the area is the larggest as 7500 sq. km. The typical
seismic facies is parallel. High amplitude made up almost western
margin of carbonates. The unification of carbonates is shown by
the strong development into the Tri Ton central high zone in
16÷13my.a. In the marginal parts, the seismic interpretation results
have shown that composition of coarse grain is dominated and
carbonates are gradually developed into the East and the West in
comparison with central Tri Ton horst. Therefore, the development
of lower and middle parts is mainly platform one dominated in
horizon, but carbonate materials increase corresponding with the
increase of sea level.
c. Stage 3 - Reduction area or backstepping: Platform carbonate is
developing gradually and reduction of the development area in
Middle Miocene (12÷10my.a) is in the direction from the North
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(block 117) to the South, Southeast (block 120). Creating build up
is mainly in Tri Ton horst and is the margin of structure in eastern
block 119 and northeastern block 120. At that time the area of
carbonate remain about 1000 sq. km at the late stage. Therefore, at
the end of Middle Miocene, the area of platform carbonates were
lessened and only existed on the area of locally high relief that was
inherited from the stage 2. With the above mentioned analysis it
can be seen that in the study are, platform carbonates of block 115
is the earliest destruction, then is block 117 and continuously
toward to the southeast and final is in block 120. However the
backstepping process is dominated in the whole stage but it is
continuous to develop further some types of build-up that extended
further tens of kilometers toward to southeast, south of block 120
and northern part of block 121. Lithological facies of carbonate is
more multicipal cycle by shallow biological creatures with larger
size compared with the previous stage.
d. Stage 4 - Drowning: Carbonates of Tri Ton formation were
drowned and led to completely disappeared at the end of middle
Miocene when sea level increases fast in a large area (10my.a) due
to strongly tectonic subduction. At the same time with this process,
the small adjacent grabens were filled up by clastic deposits
interbedded with thick carbonate layers that were destroyed from
previous formations. Finally, the whole carbonates on Tri Ton
horst were covered by terregenous deposits in plain delta and
coastal marine, deep marine of Quang Ngai formation (6my.a) and
Bien Dong formation until now. Carbonate deposits were slowly
developed or not developed in that stage that led to the end of the
carbonate formation. It was related to sea level to increase strongly
and rather continuity on the whole area of Tri Ton horst.
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Figure 4.1. Geological development and drowning for carbonate
formations in Tri Ton high
4.1.2. Building porosity distribution (model based on seismic facies)
In seismic facies analysis, porosity distribution model of carbonate in
Tri Ton horst is reduced in the following order of seismic facies:
- Mound, progradation and chaotic facies with highest porosity,
- Parallel in isolated places of platform carbonates with high
porosity and Incline/ slope facies with lowest porosity.
In comparison with logging data at the wells drilled as 118-CVX-1X,
119-CH-1X, 120-CS-1X and mechanical physical analysis, the level of
common porosity application in the study area scientifically included
three types suggested as: very high porosity (>25%), high porosity (525%) and low porosity (<5%) as in figure 4.2. Corresponding to these
porosity types, proposed model for carbonate reservoirs can be build in
the blocks 117-120 and adjacent area.
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119-CH-1X
Progradational
Progradation direction
118-CVX-1X
Chaotic
High pore (>25%)
Inclined
(slope)
Mounded
Parallel
(platf orm hight)
SB2
Parallel
(platf orm
undif f erentiated)
Parallel
(platf orm low)
SB1
Parallel
(trough)
300m
30km
NORTH
Low pore(<5%)
Medium pore (5-25%)
Fig 4.2. Model for carbonate porosity distribution in Tri Ton horst
after seismic facies data
4.1.3. Model of lithological facies section related to depositional
environment
Building lithological - depositional model for platform carbonates in
Tri Ton horst plays an important roll in the thesis as the name of the
rock as well as the biological creature composition of rock creation in
vertical and horizontal orientation were determined by core samples.
The results by theoritical study are relatively concided with well data
from recently drilled wells. Morever, the result of biostratigraphical
analysis are also allowed to identify creatures that carbonates were
created on Tri Ton horst.
Based on paleo-environmental condition it can be predicted in the best
concide with shallow marine (horizontal section including two zones as
shallow tidal and deep tidal ones) as facies and environmental section
model detailed in figure 4.3.
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Figure 4.3. Model for carbonate lithology facies in Tri Ton high
4.2.
Relation of hydrocarbon to carbonate sediments
4.2.1. Source rock
Generation ability of carbonate is not fully evaluated as TOC in rock is
rather poor, fine grained composition near top of Tri Ton formation is
not reached source rock standard with TOC potential that is average
level and not reachs to maturity threshold.
4.2.2. Reservoir
Dolomite limestone - Song Huong formation- Early Miocene
The results of the studies have shown that the ability of dolomization
can make general volume of rock decreased, it means that it created
better porosity without filling up by cementation. In that case, each
dolomization sequence are still considered as potential reservoirs with
gas or sealing with heavy oil with high liquids in the area of Nam Con
Son basin.
Limestone - Tri Ton formation aged Middle Miocene
The carbonates fractured and karstered to create vugs and cases are
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good reservoir targets that have been proven by well data. The good
porosity of fractured and vuggy carbonates was also proven by well
data. Their distribution is rather complicated depending on fault zones
and erosion surface as submarine canyon proven by 2D seismic
interpretation.
4.2.3. Sealing
In the area of study area, carbonate is widely and thickly distributed at
the depth of 1.473÷1.503m at well 120-CS-1X with small porosity of
3÷5% that can seal heavy oil if it is located at deeper depth. After
seismic data with confidential quality it is able to identify local seal
sequence in Tri Ton platform carbonate area.
4.3.
Practical application of geological facies modelling in
hydrocarbon exploration
The above studies have proven that carbonates were formed in Tri Ton
high zone independently and area of coral reefs developed shallow
water and around area is deeper water. The carbonates of Tri Ton
formation with typical reservoir is hydrocarbon trap in the south of
Song Hong basin and in the area covered by sealing rocks aged late
Miocene and Pliocene. The traps with good porosity can be identified
by seismic data as bioclastic carbonates were formed by lowstand
system tract and under effect of wave and coastal flows. This reason
was confirmed by core sample analysis compared with well logging
data at the wells as in figures 4.2 and 4.3.
It can be seen on the seismic sections that the most important traps in
the studied area are reef created in trangresive system tract with the
nature being volume framework with amount of big porosity and
platform carbonate with high porosity related to fractures, vugs, moldic
belonging to Middle Miocene Tri Ton formation. The other traps are
the ones created by favourable activities of build up or isolated reef in
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the area. Besides that, when analysing seismic facies configuration to
compare with the reservoir quality to identify carbonate reservoir
targets at windward direction or leeward are both for good reservoir
qualities.
CONCLUSION AND PROPOSAL
The outcomes of thesis issue the following conclusion:
1. Continental shelf of Central of Vietnam (in the South of Red river
basin) consists of main structural elements namely: Da Nang
monocline, Quang Ngai graben, Tri Ton horst and Dong Tri Ton
diferenciated zone. The KZ sediment section in the studied area
experiences four stages of tectonics, structures development: main
syn-rift, strike slip phase, stable and rapid phase. These geological
variations dominate and pose various affect on the process of
forming, developing, and degrading and relate to the distribution of
structure model of carbonate rock in the South of Red river
sediment basin.
2. The carbonate strata in Tri Ton high structure has chemicalbiological composition on independent up-thrown block which is
separated from Vietnam central block at the end of Oligocene time.
Specific features of carbonate tectonics are shown in seismic,
geophysics material of drilling hole and especially sample analysis
results: the lower section in Song Huong formation is early Miocene
carbonate in platform which is dolomited with poorer pore, formed
in the shallow water environment; the upper section in Tri Ton
formation is middle Miocene platform carbonate, build up and reef
in shallower water environment.
3. The composition and development of carbonate Miocene
experiences four main stages: a) Start up within early carbonate
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Miocene; b) Build up and build out during early, middle Miocene;
c) Back stepping to decrease its distribution scope in top of middle
Miocene; d) The platform carbonate stage transfers to downgrade
stage and to drowning in the top of middle Miocene and covered by
younger sediment from late Miocene to present (fig. 4.1).
4. The carbonate pore distribution model based on seismic faces in Tri
Ton high structure includes three zones: high, medium and low
porosity (fig. 4.2). In terms of sediment facies, Tri Ton carbonate is
formed in two environmental zones: shallower tide (including the
top or the section exposed from water surface/ subsurface) and
shallow tide (fig. 4.3).
5. The petroleum potential in rock containing carbonate in the South of
Red river basin highly relates to hydrocarbon gas, however, due to
distribution model and carbonate creating mechanisms with
lithologic elements which suffer from secondary variation and then
create large and complex porosity, affect gas containing potential of
the area in both vertical and horizontal direction (cross section and
structural contour map)
From the research outcomes mentioned above, author issues the
following proposals:
1. Because there has not been any parameter drilling hole which is
drilled to the thickest carbonate cross section of Miocene age to
present in Red river basin, age of youngest carbonate strata on Tri
Ton high structure pursuant to official document is only middle
Miocene. Seismic materials of some separate wells in the Western
East (to Hoang Sa archipelago) show the development of reef
carbonate, build up exists to late Miocene age- Pliocene. On the
other hands, the above linkage still reveals some limit, and on site
materials of neighbouring regions has not observed the presence of
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carbonate and requires at least one drilling hole to survey the entire
cross section. Thus, PVN need to drill an wildcat well at least.
2. The seismic materials BP and BHP with grid of 4km x 8km are
major materials used in methodology of the thesis. However, due to
the strong seismic reflectional energy absorption on top of carbonate
boundary and component of carbonate rock changes rapidly on Tri
Ton horst in accordance with horizon direction, we should use the
more specific seismic grid (in-filled or the materials must be reanalyzed with specific specification) then meet the requirement of
region with distribution of platform carbonate rock.
3. The researches on assessing petroleum potential of Red river basin
have not been proportional to storing ability of carbonate strata
system, especially this is the largest Cenozoic sediment basin
nationwide and the converging area of three typical kitchen areas
with different tectonics. Therefore, the gas generated source for
researched area is from three different basin sources such as Red
river, East Tri Ton and Phu Khanh will relate to the carbonate hydro
thermal and may release CO2. On the other hands, the outcome of
this thesis on study of carbonate sediment and its relation to
petroleum with other sediment basins on Vietnamese continental
platform is just the first study results. Therefore, PVN should
encourage scientists in deeper research on carbonate characteristic
in Vietnam, then specify geological model to set up plan on drilling
for exploration or assess petroleum reserves for this object.