1
MINISTRY OF EDUCATION AND TRAINING
THE HANOI UNIVERSITY OF MINING AND GEOLOGY
---------------------------------------------------------HOÀNG NGỌC ĐÔNG
GEOLOGICAL AND TECTONIC CHARACTERIZATION IN
THE NORTHERN PART OF CUU LONG BASIN DURING
EOCENE-OLIGOCENE
Specialty: Geo-Tectonics
Code: 62.44.55.05
SUMMARY OF DOCTORATE THESIS
Hanoi – 2011
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The thesis has been completed at the Department of Geology,
Geological Faculty
Hanoi University of Mining and Geology
Scientific Supervisors:
1. Assoc. Prof. Dr. Tran Thanh Hai, The Hanoi University of
Mining and Geology
2. Dr. Hoang Ngoc Dang, PetroVietnam
Examiner 1:
Examiner 2:
Examiner 3:
The thesis will be defended at the University examination Council
at the Hanoi University of Mining and Geology
At….. h, ………, 2011
This thesis can be referenced at the National Library or at the
library of the Hanoi University of Mining and Geology
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INTRODUCTION
1. The necessary of the project
In Cuu Long basin, beside two important hydrocarbon plays: PreCenozoic
fractured
basement
reservoir
and
Lower
Miocene
Sandstones, Eocene-Oligocene sandstones of Ca Coi, Tra Cu and Tra
Tân Formation are Exploration objectives need be taken into account.
However, Eocene-Oligocene stratigraphy and structural characteristics
and geological evolution as well as hydrocarbon potential have not
been studied deeply and properly. To support the exploration task,
prospectivity evaluation and risk/uncertainty analysis for these
objectives in the Northeastern part of Cuu Long Basin, the stratigraphy,
structural characteristics and geological evolution in the EoceneOligocene period are considered as essential tasks to be further studied
and clarified.
This is basic for author to chose the thesis namely “ Geological and
tectonic characterization in the Northern part of Cuu Long Basin
During Eocene-Oligocene”
2. Thesis objectives
To clarify Eocene-Oligocene stratigraphy and tectonic-structural
characteristics in the northeastern part of Cuu Long basin in order to
restore regionally geological evolution for hydrocarbon potential
evaluation features of these Formation in the study area.
3. Study Objectives and Scope
Objectives are Eocene-Oligocene Formation in the northeastern part of
Cuu Long Basin with latitude from 00’ 09” N đến 11o 00’ 01” N and
Longitude from 107o 41’ 22” E to 109o 19’ 46” E.
4. Thesis task
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1) Study in detail the stratigraphy characteristics for Eocene-Oligocene
Formations.
2) Study structural features, divide main tectonic stage and restore
geological evolution in Eocene-Oligocene.
3) Evaluate the roles of geological elements in Eocene-Oligocene for
Hydrocarbon system in the Northeastern part of Cuu Long Basin.
5. Contribution of study in Science and Practice
Science Contribution: in the basics of stratigraphy and deformation
interpretation and Evaluation in the study area as well as restoration of
geological evolution in Eocene-Oligocene, more understanding on
geological evolution of Southeastern Vietnam continental shelf in
Cenozoic will be achieved.
Practical Contribution: the outcomes of study contribute the
understanding on relationship between Eocene-Oligocene Formation
with hydrocarbon system for basics of prospectivity evaluation and
planning on exploration activity of the study area.
6. Themes of thesis
Theme 1: Eocene-Oligocene Formations in the Northeastern part of
Cuu Long Basin includes Tra Cú Formation aged Eocene?-Early
Oligocene and Tra Tan Formation aged of Late Oligocene. These
Formations are very complicated in sediment facies and lithology
composition. Tra Cu Formation is composed of clastics and alkaline
basalt with very high variation of lithology composition, much different
from stratotyp section.
Theme 2: In Eocene-Oligocene, the study area underwent 4 tectonic
phases: phase 1 experienced in Eocene-Early Oligocene relating to
continental crust extension and formed Northeast-southwest trending
graben. Phase 2 developed in late Early Oligocene and formed tectonic
and structural inversion from phase 1 and unconformity surface of Top
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Tra Cu Formation. Phase 3 was re-extensional one within Late
Oligocene to form subsidence and to merge grabens together into
common lake basin. Phase 4 was characterized by couples of strike-slip
faults; dextral lateral strike-slip faults and signitral-lateral strike-slip
faults and inversion regime to form unconformity surface of Top Tra
Tan Formation. Tectoniccondition of the study area in EoceneOligocene was relating closely to back-arch extension event and
movement of micro-plate in Southeast Asia in Cenozoic.
Theme 3: Hydrocarbon system in Eocene-Oligocene Formation in the
Northeastern part of Cuu Long Basin were controlled by geological
structures formed by tectonic phases in Eocene-Oligocene. High
organic content sediments in grabens and lake formed very high
potential source rocks. Medium to coarse clastics with high thickness
are very good reservoirs while fine grain formations of Tra Cu and Tra
Tan Formations developing widely within basin are important seal rock
to trap hydrocarbon. Interaction among NE-SW, sub-longitude, NW-SE
and sub-latitude trending structures formed closures as anticlines and
structural high developing widely within the study area.
7. New results
1) To clarfy stratigraphy characteristics and composition of EoceneOligocene Formation in the Northeastern part of Cuu Long Basin.
Especially, to clarify material composition, form and origin of Tra
Cu Formation and prove that this Formation has variation in
geological composition including clastics and volcanics formed in
complicated geological history, in early extension period of
continental crust in Southeast Vietnam continental shelf.
2) To define origin of structures basically relating to the formation and
transformation of Eocene-Oligocene Formation by interaction of 4
different tectonic phases. These tectonic phases reflect that tectonic
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regime in the Southeast Vietnam continental shelf in Early Cenozoic
was very complicated.
3) To contribute clarification of relationship between Eocene-Oligocene
geological Formation in the Northeastern part of Cuu Long Basin
with hydrocarbon system and also to identify several potential
prospects including stratigraphic trap within Tra Tan Formation and
wedge shape traps in margin areas which are recommended to be
further studied and future exploration strategy.
8. Structure of thesis
Thesis is presented in 170 pages, 03 tables, 63 Figures, 01 picture
and 121 documents of references. Besides Introduction and
Conclusion, thesis consists of number sections as following:
Chapter 1 - Study Area Overviews
Chapter 2 - Study methodology
Chapter 3 - Characteristics of Eocene-Oligocene Stratigraphy in the
Northeastern part of Cuu Long Basin.
Chapter 4 – Characteristics of Geology-Tectonic and Geological
Evolution during the Eocene-Oligocene in the Northeastern part of Cuu
Long Basin.
Chapter 5 - Relationship between Tectonic-Structures with
hydrocarbon system
9. Database
Thesis was studied based on database collected, processed,
analyzed in terms of stratigraphy, tectonics, structures in the
Northeastern part of Cuu Long Basin from 2006 upto date. Author have
interpreted and studied on over 570 2D seismic lines and some 1350
sqkm of 3D seismic data for Blocks of : 15-2/01, 15-1, 15-1/05, 01, 02,
01/97 and 02/97 and analyzed well data of wells HSN-1X, HSD-4X,
SN-1X, SN-2X, SN-3X, ST-1X, ST-2X, DM-1X, DM-2X, TL-1X và
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TL-2X. Besides that, author have collected all related reports, thesis ,
papers published in technical magazines, Forums and conferences (see
References section).
10. Place for study to be conducted
The thesis was conducted at Geology Faculty of The Hanoi
University of Mining and Geology and Thang Long Joint Operating
Company.
Chapter 1 - STUDY AREA OVERVIEWS
1.1 Tectonic location and Geological Characteristics of the Study
Area
1.1.1 Tectonic Location
The study area covers the Northeastern part of Early
Cenozoic Cuu Long Basin. Several geologists (Nguyen Xuan
Bao, Pham Huy Long, Ngo Thuong San) consider that in the
period of Eocene-Oligocene, Cuu Long was rift basin developing
on the Indochina micro-plate. Due to the affect from Eastern sea
extension and the collision of Indian plate into Eurasian Plate,
Indochina micro-plate was pushed into Southeastern direction,
Eocene-Oligocene Cuu Long basin therefore was formed. From
Late Cenozoic up to date, Cuu Long Basin is the part of Southern
Vietnam continental shelf in stable tectonic regime.
1.1.2. Geological Characteristics
1.1.2.1 Stratigraphy: Geological Formations Contributing on
geological structure in the Northeastern part of Cuu Long Basin
include Formation of Biển Đông (N2 - Qbd)- sequence A, Đồng
Nai (N13đn)-sequence BIII, Côn Sơn (N12cs)-sequence BII, Bạch
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Hổ (N11 bh)-sequence BI, Trà Tân (E32tt)-sequences C và D, Trà
Cú (E31tc)-sequence E, Cà Cối(E2 cc)-sequence F.
1.1.2.2 Hydrocarbon Potential : Up to date, there have been several
discoveries and fields in producing in Pre-Cenozoic fractured basement
reservoir, Lower Miocene sandstones and some discoveries aged of
Eocene-Oligocene, and minor discoveries in Middle Miocene
sandstones.
1.2 The history of Petroleum geological studies and existing
problem
- Stage 1- before 1975: some magnetic, gravity and seismic programs
were acquired
to evaluate hydrocarbon potential in the Southern
Vietnam continental shelf including the study area. The outcome of
study in this stage includes establishment of seismic sequences A, BIII,
BII, BI, C, D and F in Cuu Long Basin.
- Stage 2 (1975-1979): in 1976, 1.210,9 km of 2D seismic lines were
acquired by CGG; key reflector horizons namely CL20 to CL80 were
interpreted to ensure the existence of the Cenozoic Cuu Long Basin.
3.221,7 km of 2D seismic lines (3,5 x 3,5 km) were acquired and 4
exploration wells namely 15-A-1X, 15-B-1X, 15C-1X and 15-G-1X
were drilled by Deminex in Block 15.
- Stage 3 (1980-1988): Ngo Thuong San and Le Van Cu studied in
detail stratigraphy of above-mentioned 4 wells to establish Tra Tan
Formation (at 15-A-1X), Bien Dong and Dong Nai Formation (at 15-G1X). E Sequence was established on seismic data and corresponding to
Tra Cu Formation at CL-1 well.
- Stage 4 (1989 to present): series of PSC to explore hydrocarbon
were signed and carried out in Block 01&02 , 15-1/01, 15-2, 15-2/01
and several wells were drilled to capture Tra Tan Formation and Pre-
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Tertiary fracture granite basement, some to capture Tra Cu Formation
(E sequence).
1.3 Existing problem
From overviews of geological study history mentioned above, author
note that existing problem need be further studied are stratigraphy,
geological evolution history and hydrocarbon system especially for
specific area such as the Northeastern part of Cuu Long Basin.
Therefore, author have chosen study objective of Eocene-Oligocene
Geological Formation in the Northeastern part of Cuu Long Basin for
the thesis.
Chapter 2- STUDY METHODS
2.1. Approach
2.1.1. Traditional together with modern approach
2.1.2. General approach
2.1.3. Systematic approach
2.2. Study methods
2.2.1. Stratigraphy study
2.2.1.1. Seismic stratigraphy
In order to identify the regional lithological units and to interpret
depositional environments, the author applied seismic stratigraphy
method
based
on
seismic
reflection
configurations
and
on
characteristics of seismic facies. Several lithological units have been
classified based on their own physical fashions. The faulting systems
are also defined on seismic profiles based on the discontinuity and
displacement of seismic reflections across the fault surfaces. Details of
the seismic interpretation are presented in chapter 4.
2.2.1.2. Lithostratigraphy
The Lithostratigraphy was applied in order to classify
sedimentary formations into more detail units based on their physical
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properties. In this study, the principles of stratigraphy issued in 1994
with the basic Formation were applied. The Eocene-Oligocene
sediments in the region have been subdivided into 4 structural blocks:
Phan Thiet Block, Hai Su Den-Ho Den Block, Hai Su Nau-Agate
Block, and Phuong Dong-Jade-Thang Long Block. One representative
well of each block was selected for detail description, interpretation and
classification of each Eocene-Oligocene lithostratigraphic unit to
Members.
2.2.2. Well log Interpretation
In this study, the logging curves derived from wells HSĐ-4X,
HSN-1X, HST-1X were used together with lithological information
derived from chip logging, side-wall samples, cores (where is possible)
in order for lithological classification.
2.2.3. Analytical methods
2.2.3.1. Lithological/petrographic analysis
The rock samples collected from chips, cores and sidewall
materials of the well HSĐ-4X, HSN-1X, HST-1X, Emaral, Diamond,
Ruby, STĐ, STV, LĐV-1X, LĐN-1X, RĐ were used for this purpose.
The samples were described under naked eyes/hand lens first and then
they were subjected to thin section preparation prior to analysis under
microscope and electronic scanning microscope (ESM).
2.2.3.1. Petrogeochemical analysis
Four samples of volcanic rocks collected from the Well HSD-4X
have been sent for petrogeochemical analysis. The sample preparation
and analysis were done in accordance with sampling protocols proposed
by the Southern Geological Mapping Division.
2.2.4. Structural Interpretation
2.2.4.1. Structural morphology
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Structural interpretation: The structure elements such as faults,
folds, unconformities were identified on seismic profiles, well-log, and
isodepth-structure map of the top of each formation based on the
morphology/fashion of each structure type as well as based on the
penetration contacts of the rocks.
Unconformity and sediment hiatus analysis: The author has
analyzed the overlain/underlain contacts of the Cenozoic formations on
the pre-Cenozoic basement, between the Tra Tan/Tra Cu Formations
(Sequence D/Sequence E), between the Bach Ho/Tra Tan Formations
(Sequence B1/Sequence C) observed on seismic profiles based on the
change in seismic facies and/or reflection configurations. Therefore, the
unconformities and non-depositional surfaces were determined.
Sediment thickness analysis: This method was applied as follow:
-
The thickness of each seismic sequence was measured
(Sequences E+F?, D, and C) in order to compare the thickness
variation of the footwall from the hanging wall across a fault
surface.
-
Restoring the original thickness (the erosion was taken into
account). If the real thickness is known, subsidence
rate/amplitude is estimated. In addition, the subsiding gradient
of the basin could be calculated by dividing the sediment
thickness by the basin area.
-
Constructing isopach maps of the Tra Cu and Tra Tan
Formations in the region.
2.2.4.2. Kinematic analysis
The kinematic analysis was applied in order to determine the
nature of the faults, their displacements and reactivation through time.
2.2.4.3. Dynamic analysis
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This method was used to study the relation between deformation
and stresses, which resulted in the formation of faults, folds based on
the stress-strain the principles proposed by Anderson and Riedel and
based on the statistical analysis together with stereonet projections.
2.2.4.4. Cross-section Balancing
The main content of this work is restoring sedimentary formations
back to their original state before they were deformed. This is shown on
the cross-sections and back-strip seismic profiles. The length of each
formation across its top surface was measured once it was restored at
the eroded locations and then it was straightened to have the same
depth. The horizontal and thrust displacements were also taken into
accounts for the normal and reverse faults respectively.
2.2.5. Geological modeling
This method combines the data and results from the above studies
in order to build a geological model that describes the spatial
distribution pattern, tectonic evolution of different geological
formations through the Eocene-Oligocene.
Chapter 3 – EOCENE-OLIGOCENE STRATIGRAPHY
FEATURES IN THE NORTHEASTERN PART OF CUU LONG
BASIN
3.1. Overviews
3.2 Structural Block Overviews
Five structural Blocks divided by main NE-SW trending growth
faults F1, F2, F3, F4 and F5 include Phan Thiet Block(I), Hai Su DenHo Den Block (II), Hai Su Nau-Agate Block(III), Phuong Dong-JadeThang Long Block (IV) and NE Con Son Block.
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3.3. Eocene-Oligocene Stratigraphy Features in the Northeastern
part of Cuu Long Basin
3.3.1. Introduction
Eocene-Oligocene Formation was divided into Ca Coi, Tra Cu and
Tra Tan Formations by Le Van Cu, Nguyen Giao and Ngo Thuong San.
Ca Coi Formation with aged Eocene is corresponding to F Sequence;
Tra Cu Formation agded Early Oligocene is corresponding to E
Sequence; Tra Tan Formation aged Late Oligocene is corresponding to
D and C Sequences.
3.3.2. Eocene-Oligocene Formation in Hai Su Den-Ho Den
Structural Block
Eocene-Oligocene Formation encountered at HSD-4X, LDN-1X,
LDV-1X, STN-1X, DM-1X wells were selected for describing,
correlating and evaluating stratigraphic characteristics in this structural
block.
Tra Cu Formation (E31-tc)
Within this structural block, Tra Cu Formation were encountered at
HSĐ-4X, LĐN-1X, LĐV-1X, STN-1X, DM-4X wells. This Formation
is characterized by sandstones, siltstones and claystones interbedded
with extrusive rocks. These extrusive rocks were encountered at Hai Su
Den and Diamond structures by HSD-4X and DM-1X, DM-2X and
DM-3X wells. Geochemical analysis for Extrusive rock at HSD-4X
well indicated that this rock have low content of SiO2 (less than 50%),
high richness of Tio2 and MgO and therefore it was classified ad
alkaline basalt.
Thickness of this Formation were highly varied in this structural
block: from Southwest to Northeast of block thickness of Formation has
tendancy increases (at HSD-4X: 340m, SN-1X: 845m, SN-3X: 745m).
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Fossil and palyno encountered in this section determined the age of
Early Oligocene.
Tra Tan Formation (E32-tc)
This Formation distributes widely in whole Cuu Long Basin.
Depositional environments, lithology composition of this Formation
gradually change within area. In the Hai Su Den-Ho Den Block, Tra
Tan Formation is mainly composed of shale content (40-70%)
interbedded with siltstones and minor sandstones. Sand-Shale ration of
Tra Tan Formation is quite low in generally, however it have tendency
to be high in the Northeastern part of this block. Thickness of
Formation encountered at wells varying from 450m to 1000m. Age was
determined at Late Oligocene based on fossil of Cicatricosis-porites,
Verrucatosporites pachydermus và Florschuetzia Trilobeta.
3.3.3. Eocene-Oligocene Formation in Hai Su Nau- Agate structural
Block
Tra Cu Formation (E31tc)
Tra Cu Formation in this structural block have lithology contents
varying strongly in space. In the SW part of block, the thickness of this
Formation is very high with content of sandstones interbedded with
siltstones and shales. In Ruby and Emerald, this Formation is thinner
and composed of sandstones, siltstones and claysones interbedded
several extrusive layers. Sanstones here are almost fine grain size.
Extrusive rocks encountered Ruby and Emerald wells have high
thickness, 55m average, thicker than those in Hai Su Den-Ho Den
Block. At HSN-1X well, fossil and palyno encountered to determine the
age of this is Early Oligocene. The thickness of Tra Cu Formation
encountered at HSN-1X is 408m, at Ruby and Emerald around 250m.
However, based on seismic data, the thickness of Tra Cu Formation in
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Hai Su Nau- Agate varies from 200 to 1000m, and may reach to 2000m
(in the Southwestern part of Block).
Tra Tan Formation (E32-tt)
At HSN-1X well section, Tra Tan Formation were characterized by
light grey, fine grained, carbonaceous cement sandstones graded into
siltstones and dark brown shale with intercalation of thin coal beds,
occasionally glauconit.
General section of this Formation is composed of shale content of
50-80% interbedded with siltstones and sandstones. Tra Tan Formation
commonly is thin in basement high and thick in graben. The thickness
may reach to 2000m or higher in the central of grabens (Southwestern
part of this block).
3.3.4. Eocene-Oligocene Formation in Phuong Dong-Jade-Thang
Long structural block
Tra Cu Formation (E31-tc)
On this block, the characteristics of Tra Cu Formation look quite
similar to Tra Cu Formation section at the stratotyp at Cuu Long-1 well
which was composed of mainly thick bedded sandstones intercalation of
siltstones and claysones. Sandstones are fine to moderate and coarse
grain size. Occasionally gravels were encountered. Shale is light grey,
medium grey with common mica associated. Thickness of Formation
varies from 200 to 600-800m, and may reach 1000-1600 m thick.
Tra Tan Formation(E32tt)
Tra Tan Formation sediments distribute widely with high
thickness, quite thin in Thang Long area, quite thick in Phuong Dong
and Jade area. This Formation in this structural block is composed of
mainly shale with high richness of TOC interbedded with siltstones and
sandstones. Fossil and palyno encountered determined the age of Late
Oligocene for this Formation.
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3.3.5. Eocene-Oligocene Stratigraphy Correlation within the
Northeastern part of Cuu Long Basin.
3.3.5.1 Tra Cu Formation
Tra Cu Formation are characterized by fine-to-coarse grained
composition and high variation of thickness from location to location
between structural blocks and half-grabens. This Formation dominates
finest grained sediment with highest thickness in Hai Su Nau-Agate
structural Block and coarsest grained sediment with lowest thickness in
the Phuong Dong-Jade-Thang Long structural Block and moderate in
grain size of sediment and thickness in the Hai Su Den-Ho Den
structural block. Further, alkaline basalt layers intercalate between
clastics in Hai Su Den-Ho Den and Hai Su Nau-Agate Block: Diamond,
Ruby, Diamond area. .
3.3.5.2.Tra Tan Formation
Tra Tan Formation distributes widely in whole Cuu Long Basin, not
much differentiation between structural blocks. Tra Tan Formation is
characterized by mainly shale intercalated with siltstones and
sandstones and gradually variation of lithology, depositional facies
within basin-wide.
3.3.6. Formation Condition of Eocene-Oligocene sediments and
Extrusives in the Northeastern part of Cuu Long Basin
3.3.6.1 Tra Cu formation were deposited in high energy condition with
sediment source very close to location of deposition. These sediment
sources were commonly weathered basement high. Extrusive, mainly
alkaline basalts, were formed in extension of continental crust in the
study area corresponding to the early stage of rifting. Magma were
extruded out of surface through normal and growth fault plane in Hai
Su Den, Diamond, Ruby and Emerald which were formed in
extensional tectonic phase in the study area.
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3.3.6.2.Tra Tan Formation was deposited in condition of high water
level of lake, and sediment supply coming from less localized source
than that of Tra Cu deposits. Sediment supplies were transported like
from continent into the margin of the lake by rivers and channels so the
ways of sediment to be deposited are similar to that of marine condition.
Whole Cuu Long basin water in this period were merged in common
lake without separation by basement highs as in Tra Cu Formation
period. Depositional environment of Tra Tan Formation were
dominated by lacustrine, swam, estuarine condition to form sediment
with high content of fine grains and high organic matters.
Chapter 4 – CHARACTERISTICS OF STRUCTURE-TECTONIC
AND GEOLOGICAL EVOLUTION IN THE NORTHEASTERN
PART OF CUULONG BASIN DURING EOCENE-OLIGOCENE
4.1. General
4.2. Tectonic phases: the author divided tectonic evolution of the study
area into four (04) tectonic phases as bellow:
4.2.1. Tectonic phase I – Eocene-Early Oligocene extension
This phase is NW-SE extension phase in Eocene-Earlys Oligocene.
The relic of this phase are sediments interbedded with several alkali
basalt layers of Tra Cu Formation aged Eocene-Early Oligocene and
NE-SW striking syn-deposition folds and faults. Two unconformity
surfaces limited top and base of Eocene-Early Oligocene Tra Cu
Formation includes: 1-the unconformity called top basement is the
boundary surface between Pre-Cenozoic basement and Cenozoic
sediments, and 2- the unconformity called top of Tra Cu Formation is
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the boundary surface between sediments of Tra Cu Formation and Tra
Tan Formation.
Structural relics are NE-SW striking syn-deposition faults
generatiing and developing during the process of depositing volcanicclastic of Tra Cu and Ca Coi Formations (E and F? sequences). These
faults re-activated due to the affection of tectonic activities in late
phases. There are two types of faults: major faults F1.1, F1.2, F2.1,
F2.2, F2.3, F3.1, F3.2, F4, F5 and accompanied faults are minor ones
including Ft1 to Ft17. Major faults (F1 to F5) play a role to form
basement uplifted blocks and half grabens. Minor ones (Ft1 to Ft17) are
distributed inside uplifted blocks and are played a role to form the shape
and frature of basement structure.
4.2.2. Tectonic phase II – Late Early Oligocene compression
This phase is compression phase in the Late Early Oligocene
impacted on Eocene-Early Oligocene volcanic-clastic of Tra Cu
Formation. Relics of this phase are unconformity surface between
sediments of Tra Cu Formation and Tra Tan Formation. About the
faults and folds, they are very difficult to identify because they interfere
with structures which formed in other phases or this phase is not enough
strong so that the author divided temporary into phase II.
4.2.3. Tectonic phase III – Late Oligocene extension
This phase is NW-SE extension phase in Late Oligocene. The
lithologic relic of this phase are high fine grain content sediments of Tra
Tan Formation aged Late Oligocene. The structural relic are NE-SW
striking syn-deposition normal faults including F1.1, F1.2, F1.3, F2.1,
F2.2, F3.1, F4 and F5. They were formed syn-deposition in EoceneEarly Oligocene and re-activated in phase III (Late Oligocene) like as
normal faults and syn-deposition with Tra Tan Formation.
4.2.4. Tectonic phase IV – End of Late Oligocene compression
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This phase is NW-SE compression phase in the end of Late
Oligocene and identified by unconformity on top of Tra Tan Formation
and E-W striking post-deposition dextral lateral strike-slip faults (f5, f6,
f7, f8, f9, f10, f11 và f12) and N-S striking post-deposition signitrallateral strike-slip faults (f1, f2, f3 và f4). All of these faults perforated
through Eocene - Oligocene sediments of Tra Tan and Tra Cu
Formations. In additional, several NE-SW striking faults which were
initially formed in privious phases re-activated like as reverse. In this
phase, several post-deposition folds (N1, N2, N3 and N4) are formed.
4.3. Geological evolution in Eocene-Oligocene period
The geological evolution in Eocene-Oligocene period is divided into
two stages: Eocene-Early Oligocene stage (T1) and Late Oligocene
stage (T2). In each stage, it was started by extensional phase and
finished by compressional phase.
4.3.1.
Eocene-Early Oligocene stage (T1):
This stage happenned during Eocene to the end of Early Oligocene.
Specific characteristics are rift-type extension which related to
spreading to form Young East Sea. In the study area, sediments of Ca
Coi and Tra Cu Formations (called F? and E sequences) were deposited
in NE-SW direction half grabens. Tectonic activities of this phase were
created syn-deposition normal faults trending NE-SW like as F1, F2,
F3, F4 and F5 faults. Based on that, half grabens and half horsts are
same NE-SW striking with normal faults. This was the first period of
rifting process. In the end of stage, the study area occurred tectonic
inversion, which caused folds and inversion tectonic structures,
uplifting and eroded and or interrupted deposition of sediment to create
unconformity surface.
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4.3.2 . Late Oligocene stage (T2):
This stage occurs in the end of Early Oligocene to end of Late
Oligocene and includes two tectonic phases: Late Oligocene extension
phase and end of Late Oligocene compression phase.
The study area was affected by extension stress in phase III with
NW-SE extensional trending. Due to this extension activity, NE-SW
striking faults were re-activated like normal faults to extend NE-SW
striking half grabens that were created in previous time.
Beyond subsidence due to extension stress, the study area is
subsided due to thermal, so that all of half grabens are united together
by high water-level in the lake. Hence, sediments of Tra Tan Formation
are deposited in large area in Cuu Long basin, it means they are
distributing continuously in whole basin. Material-sediment supply is
stable; fine-grain rate is high.
The extension and subsidence process were completed by
compression process taking place in the end of Late Oligocene. On the
compression phase, Tra Tan Formation and older ones were deformed
as folding and faulting formation taking place gradually and formed
unconformity between Tra Tan and Bach Ho Formation.
Chapter 5- RELATIONSHIP BETWEEN TECTONICSTRUCTURES WITH HYDROCARBON SYSTEM
5.1. Overview of petroleum system in the study area
Petroleum systems of study area are plentiful. The source rocks
are the shale sequence of Tra Tan and Tra Cu formations that enriched
with high organic matter. The reservoirs include fractured granite
basement, lower Miocene, upper Oligocene sandstone, and a few of the
21
sandstone; fractured extrusive rock aged Eocene- Early Oligocene. The
seal rock are the regional top seal of the shale rock of Bach Ho
formation, the local shale rock under Bach Ho Formation and the local
shale layers of Tra Tan and Tra Cu formations. Types of traps include
structure 3 way- and 4 way-dip closure traps including pinchout traps.
5.2. The relationship between the tectonic structure and the
petroleum system
The structures of the haft grabens which were derived from
activities of tectonic phases were basic to form Eocene-Oligocene
sediments and extrusive Formation including source, reservoirs and seal
components.
In the study area, source rocks mainly include the shale layers of the
Tra Tan and Tra Cu formations which mostly were buried in the haft
garben structures. Oil generated from source rocks migrated through
coarse-grained sediments and through the faults zones to fill up into
fractured basements, Oligocene and Lower Miocene sandstones. When
all elements of structural traps, the reservoirs, the source rocks,
generated oil migration and charge were available together,
hydrocarbone accumulation were formed.
Geological evolution history in the Eocene-Oligocene controlled
and determined material composition of Formations in the study area
related to source, reservoir and also controlled fold and fault structures
which were related to morphology of structural trap of oil accumulation
in the study area.
5.3. The trap types in the area and their relationship with tectonic
structures
5.3.1 The trap types in the basement
The oil and gas discoveries in the basement reservoirs of the
study area including Su Tu Den, Su Tu Vang, Su Tu Nau, Hai Su Den,
22
Lac Da Nau, Lac Da Vang, Ruby, Diamond, Thang Long, Dong Do,
Topaz, North Topaz and South Ho Xam structures are 4 way-dip
closures of basement high which were buried and trapped by EoceneOligocene fine-grained sediments.
The integrity of traps in the basement depend on the following
factors: top seal Formation on the basement (D sequence or E
sequence), the depth to top of basement, the fault systems intersecting
inside basement structure, Structural vertical relief structure, relative
position of structures within the basin and lithological composition of
rocks.
5.3.2 The trap types in the reservoir of Tra Cu Formation
Up to date, hydrocarbon in this Formation has been discovered in
Su Tu Trang, Su Tu Nau, Diamond, Thang Long, Emerald, Hai Su Den
and Jade structures. They are mostly accumulated in the form of a 3
way- or 4 way-dip closures. The 3 way-dip closure with one way onlap
against basement such as the Hai Su Den, or one way against fault plan
such as Su Tu Trang structure. The 4 way-dip closures are almost
basement
controlled
structures
(structures
were
controlled
by
underneath basement high).
5.3.3 The trap types of the Lower Tra Tan formation
Up to date, no commercial discoveries were made within Lower
Tra
Tan
Formation
sandstones
(D
sequence). However,
the
accumulations of sandstone layers have been discovered frequently in
the study area with different types of traps. Basically these are the types
of 3 way- and 4 way-dip closures, sometime associated with fault seal.
Beside structural traps, in the study area also has high potential
stratigraphic traps, especially deep lake sand bodies.
5.3.4 The trap types upper the Tra Tan formation
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Sedimentary of upper Tra Tan Formation were formed mainly in
compression phase of late Oligocene, therefore they have closing
structure as follows: four-way dip closure as the form of post sediment
anticlines, 4 way-dip closure inherited from basement high (such as the
Su Tu Den, Hai Su Den and Pearl) and three-way dip closure and
wedge structures, the other way onlap against older Formation.
CONCLUSION AND RECOMMENDATION
The results obtained from this study bring new understandings in
sedimentology, geological evolution of a part of southeastern Vietnam
continental shelf. It allows me to draw some conclusions as follow:
1. The Eocene-Oligocene sedimentary formations in northeastern
Cuu Long Basin consist of the Tra Cu Formation in the Eocene?-Early
Oligocene and the Tra Tan Formation in the Late Oligocene. The Tra
Cu Formation unconformably overlies the pre-Cenozoic basement
within the grabens and are characterized by terrigenous clastic
sediments intercalated with alkaline basaltic rocks resulted from the
continental rifting. The lithological composition and sedimentary
characteristics of this formation in the region differs from the stratotyp
section, which was established previously. The Tra Cu Formation is
overlain by the Tra Tan Formation and is widely observed. It is
characterized by relatively uniform composition with slight facies
differentiation. This Formation consists of lacustrine and marsh
sediments deposited within graben structures.
The
Eocene-Oligocene
geology
and
tectonics
are
quite
complicated, characterized by the presence of faulting systems and
folds, which were formed during many reactivated tectonic phases. The
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earliest tectonic event occurred in the Eocene-Early Oligocene, and is
characterized by normal, syn-depositional faults, which were resulted
from the early continental rifting process to form NW-SE oriented subgraben structures within the research area. The second phase is a
compressional event occurred in the end of the Early Oligocene and is
characterized by the non-depositional/inversion phase and hence the
erosional surface was formed on top of the Tra Cu Formation. The third
tectonic event is a reactivated rifting process in the Late Oligocene. It is
characterized by the formation of syn-depositional faults and/or
reactivated faults of the phase 1. This event widely triggered subsidence
of the whole region and resulted in the formation of lacustine and marsh
environments. Phase 4 occurred in the end of the Late Oligocene. This
phase demonstrates the sub-meridian and sub-parallel strike-slip motion
as well as the NE-SW oriented thrust faulting. It is a compressional
phase oriented in NW-SE direction that resulted in the formation of
unconformable surface on top of the Tra Tan Formation.
3. The geological evolution in the Eocene-Oligocene of
northeastern Cuu Long Basin is subdivided into two periods: The early
period (T1) was linked to the back-arc rifting as a result of the
development of a magmatic arc along southeastern margin of the SE
Asia. This activity has lead to continental break and rifting followed by
the sedimentary deposition and volcanic eruption to form the Tra Cu
Formation within the intra-continental grabens. This period was ended
by an inversion event to form an unconformity on top of the Tra Cu
Formation. The second period (T2) was started by a back-arc enhanced
rifting and strong subsidence processes, which have lead to the
reactivation, the opening and the connecting of the pre-existing grabens
to form a closed lacustrine environment for the Tra Tan Formation. This
period was ended by the inversion and extrusion of the Indochina Block
25
towards the SE and hence the pre-formed sediments were deformed,
displaced, faulted and eroded to create an unconformity on top of the
Oligocene Formation.
4. The Eocene-Oligocene petroleum system in northeastern Cuu
Long Basin was controlled by all tectonic activities during this period.
The rifting process (in the Eocene-Early Oligocene and in the Late
Oligocene) resulted in the formation of sedimentary basins, which are
premises for the formation of the sources rocks, reservoir rocks, and the
cap rock in this area. The inversion events have lead to the formation of
traps, in which the 4D traps are the most typical structures within the
Eocene-Oligocene sediments and the pre-Cenozoic basement rocks. In
addition, the rocks were faulted, folded, broken, and fractured as they
were enhanced by the compressional events. As a result the porosity of
rock was increased and hence the potential reservoir rocks were
improved.
5. Unsolved problems and recommendations
Apart from the above successes, some problems are still opened
to uncertainty, which need further studies in the future. These problems
are as follow:
1. The existence of the Ca Coi Formation in the region has not
been confirmed due to the lack of clear evidences. Sedimentary
characteristics in the region demonstrate that the sediments were
continuously transited from the contact with the underlying
basement to the top of the Tra Cu Formation with no clear evidence
of the Ca Coi Formation. In order to confirm the presence of this
Formation in the region and in the Cuu Long Basin, further studies
are required but these additional studies are strongly dependent on
deep drillings, which can penetrate through the basement in the
deepest areas.
26
2. The age of the Tra Cu Formation has not been dated
precisely due to the lack of indicative fossil. The age of the
lowermost section of the Tra Cu Formation was estimated by
isotope analysis of the alkaline basalt, which is relatively common,
especially in the lowest part of this Formation. Besides, the dating
of the basaltic rocks is also important in determining the timing of
continental rifting within the Cuu Long Basin as well as in
southeastern Vietnam continental shelf.
3. In the circumstance that the reservoir rocks within the preCenozoic fractured rocks and the Lower Miocene sandstones
become more difficult to be discovered, the potential objects in the
Eocene-Oligocene formations need to be investigated further not
only in the research area but also in the whole Cuu Long Basin.
Furthermore, the potential stratigraphic traps in the deeper part of
the Tra Tan Formation, where likely to have thick sandstone
formation need to be studied in more detail.
27
LIST OF PUBLICATIONS RELATED TO THE THESIS
1.
Hoàng Ngọc Đông, 2011. Các thành tạo trầm tích - Phun trào tuổi
Oligocen sớm tại giếng khoan HSD-4X thuộc phần đông bắc bể
Cửu Long. Tạp Chí Địa Chất, loạt A số 323, Trang 36-47.
2.
Hoàng Ngọc Đông, 2011. Lịch sử phát triển địa chất trong giai
đoạn Eocen-Oligocen Đông Bắc bồn trũng Cửu Long. Tạp Chí
Dầu Khí, số 7, Trang 29-32.
3.
Hoàng Ngọc Đông, 2011. Bàn về các phân vị địa tầng của các
thành tạo Eoxen-Oligoxen phần Đông Bắc bồn trũng Cửu Long.
Tuyển tập báo cáo hội nghị khoa học công nghệ Biển toàn quốclần thứ II, Quảng Ninh.
4.
Bingjian Li, Nguyen Do Ngoc Nhi, Nguyen Quoc Quan, Hoang
Ngoc Dong and others, 2011. The Natural Fracture Evaluation in
the Unconventional Tight Oligocene Reservoirs- Case Studies
from Cuu Long Basin Southern Offshore Vietnam. Poster Section,
SPE Asia Pacific Oil & Gas Conference and Exhibition,
Indonesia.
5.
Nguyen Huy Ngoc, Nguyen Quoc Quan, Hoang Ngoc Dong, Pham
Huy Long and Tran Nhu Huy, 2010. Application of “From Seismic
Interpretation to Tectonic Reconstruction” Methodology to Study
Pre-Tertiary Fractured Granite Basement Reservoir in Cuu Long
Basin,
Southeast
Vietnam
Offshore.
AAPG
International
Conference and Exhibition, Rio de Janeiro, Brazil.
6.
Nguyen Huy Ngoc, Nguyen Quoc Quan, Hoang Ngoc Dong,
Nguyen Do Ngoc Nhi, 2010. Role of 3D Seismic Data in
prediction of High Potential Area within Pre-Tertiary Fracture
28
Granite Basement Reservoir in Cuu Long basin, Vietnam offshore.
AAPG International Conference and Exhibition, Calgary, Alberta.
7.
Nguyễn Huy Ngọc, Nguyễn Quốc Quân, Hoàng Ngọc Đông,
Nguyễn Đỗ Ngọc Nhị, 2010. Ứng dụng giải ngược địa chấn để
nghiên cức đá chứa móng Granit nứt nẻ trước Đệ Tam tại mỏ Hải
Sư Đen. Tuyển tập báo cáo hội nghị KHCN quốc tế “ Dầu khí Việt
Nam 2010: Tăng tốc phát triển ”. Tr. 495-509.