Tạp chí của tập đoàn dầu khí quốc gia Việt Nam - Petrovietnam - Số 06 - 2011 pdf

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An Official Publication of The Vietnam National Oil and Gas Group Vol 6 - 2011
PETRO
VIETNAM
Petro
ietnam
Te Giac Trang field: Geological features,
reservoirs and field development concepts
Biofacies and sequence stratigraphy,
Oligocene to Pliocene, Cuu Long
and Nam Con Son Basins, Vietnam
PETRO
VIETNAM
Petro
ietnam
Editor-in-chief
Deputy Editor-in-chief
Editorial Board Members
Secretary
Contact Address
Designed by
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Dr. Nguyen Van M inh
Dr. Phan Ngoc Trung
Dr. Vu Van Vien
Dr. Sc. Lam Quang Chien
Dr. Hoang Ngoc Dang
Dr. Nguyen Minh Da
BSc. Vu Khanh Dong
Dr. Nguyen Anh Duc
MSc. Tran Hung Hien

MSc. Dao Duy K hu
MSc. Le Ngoc Son
MSc. Nguyen Van Tuan
Dr
Dr. Nguyen Tien Vinh
Dr. Nguyen Hoang Yen
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BSc. Vu Van Huan
Mobile: 0982288671
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Contents
Contents
PETROLEUM EXPLORATION & PRODUCTION
Te Giac Trang field: Geological features, reservoirs
and field development concepts
38
60
PETROLEUM PROCESSING
NEWS
PVFCCo launches a new fertilizer brand - Phu My NPK
74
Development of a Kinetic Model for the Aromatisation of Propane and
Propene over H-ZSM-5 Catalyst under Deactivating Conditions
The characteristics of Miocene sedimentary rocks in the
Western Cuu Long Basin
3
PETROVIETNAM - JOURNAL VOL 6/2011
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1. Introduction
The Cuu Long Basin lies just o shore from the Eastern
Sea coast of Southern Vietnam. It is separated to the South
by a basement high, the Con Son Swell, from a larger re-
gion of several sub-basins comprising the Nam Con Son
Basin that straddles a large part of Vietnam’s territorial wa-

ters and partially overlaps into other basins in Malaysian
and Indonesian waters (Fig. 1).
Basin initiation has been related to phases of exten-
sion associated with the collision of India and Asia causing
‘extrusion’ in SE Asia along major lineaments, (Tapponier,
et al. 1982). Movements in the Middle Miocene caused
brief compression and basin inversion with a regionally
recognised Middle Miocene unconformity.
The sedimentary sequence of the Cuu Long and Nam
Con Son Basins largely comprise heterogeneous but
Biofacies and sequence stratigraphy, Oligocene to
Pliocene, Cuu Long and Nam Con Son Basins, Vietnam
Jim Cole
Tie-Point Geoscience
Abstract
Detailed biostratigraphical studies on wells from the Cuu Long and Nam Con Son Basins reveal five genetic 2
nd

order megacyclical tectonostratigraphical sequences bounded by regional flooding surfaces, dated as Late Oligocene,
Early Miocene, Middle - earliest Late Miocene, Late Miocene and Plio-Pleistocene respectively.
The section in both basins is non-marine to marginal marine over the Oligocene to Middle Miocene, where paly-
nology is the only viable microfossil group for age dating, though there are actually few good strict palynostrati-
graphical markers. However stratigraphical resolution is considerably enhanced by the recognition of progressive
and recurrent palynofacies to mark biofacies zones. An attempt has been made at relating these zones to transgres-
sive, highstand and lowstand system tracts, making use of the known palaeoenvironmental limits of certain palyno-
morph taxa whose analogues occur in modern environments.
In this way twenty-five regionally correlatable, possibly sea-level controlled, 3
rd
order cycles have been recog-
nised within the five 2

nd
order cycles. This scheme is in its infancy and is open to refinement as more wells are studied,
but is currently complete enough for application to any new well drilled in either basin. It has already led to some radi-
cally revised age assignments amongst legacy wells so far studied. In addition, there are some important differences
in sedimentary regime between the Cuu Long and Nam Con Son Basins, reflected in the palynofacies assemblages,
particularly within the Late Oligocene and Early Miocene.
Age constraining of sedimentary units is particularly important in petroleum exploration of the Vietnam offshore.
The section has a plethora of repetitive clastic lithologies. The characteristics of some of these intervals as potential
migration zones or stratigraphical traps (as detectable from fluid inclusion studies), need to be accurately delineated
within a workable regional sequence stratigraphical scheme.
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PETROVIETNAM - JOURNAL VOL 6/2011
PETROLEUM EXPLORATION & PRODUCTION
monotonously alternating sandstones and mudstones
that are di cult to constrain and correlate lithostrati-
graphically and by wireline logs. Major limestone units are
present in the Nam Con Son Basin (Fig. 2).
Additional di culties arise from the presence of few
age restricted microfossils for strict biostratigraphical age
determination. Much of the interval of exploration interest
(Oligocene - Middle Miocene) is non-marine or marginal
marine, yielding only long ranging palynomorphs (Fig. 3),
though more open marine sections in the later Neogene
yield good micropalaeontological and calcareous nanno-
fossil age markers. Marine in uence commenced earlier
and is more strongly evident in the biofacies of the Nam
Con Son than the Cuu Long Basin.
Though most are long ranging, palyno-
morphs are present in large numbers within
most cuttings samples available from the Late

Palaeogene - Early Neogene. Many of them have
analogues in the present day across a range of
depositional settings making them valuable pa-
laeoenvironmental markers. This permits known
palaeoecological controls to be taken back into
the Tertiary in depositional environment analy-
sis. Fluctuations in the abundance and relative
frequency of eco-speci c palynomorphs over
a stratigraphical section are therefore a proxy
of environmental change and position with re-
spect to the global sea-level cycle.
Palynofacies and microfacies analysis (Fig.
4) reveals a progressive overall trend of increas-
ing and deepening marine in uence through-
out the Tertiary in both the Cuu Long and Nam
Con Son Basins. For this reason sea-level change
must be a prime underlying mechanism in sedi-
mentation. Morley (1991) has indicated that
two of the principle controls on vegetation
distribution are orogeny and climate, factors
themselves that are part under the control of
sea-level change.
Sequence stratigraphy provides a way of “…
subdividing basin- ll into a series of time and
spatially related units, which o er a dynamic
representation of lateral facies relationships
by tracing the evolution of depositional sys-
tems in response to transgressions and regres-
sions forced by cycles of relative sea level change”
(Courel et al. 2008).

This paper investigates the opportunity of using
biofacies trends at a more re ned level in sequence
stratigraphical analysis. Identi cation of similar biofa-
cies trends across a number of intra-basin wells should
provide several additional correlation tie-points. If such
trends can be demonstrated to be sea-level controlled,
their value as time synchronous cross-facies stratal sur-
faces can be validated. In this way a number of supple-
mentary time-lines become available to add to the FAD’s
and LAD’s ( rst and last occurrence datums) from age
diagnostic biostratigraphical taxa alone. A sea-level driv-
Fig. 1. Cuu Long Basin - Con Son Swell - Nam Con Son Basin, Vietnam
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PETROVIETNAM - JOURNAL VOL 6/2011
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en sequence stratigraphical model may also provide a
means of inter-basin correlation between the Cuu Long
and Nam Con Son Basins.
Morley (1991) used quantitative palynomorph zones
in improved stratigraphical resolution in the SE Asian Ter-
tiary and mentioned the elimination of non-stratigraphi-
cal quantitative events whose correlation lines cut across
good biostratigraphical correlation datums. A recommen-
dation of a 4 well minimum database for initiation of a
quantitative local scheme within a basin was mentioned
with a further 4 wells, a total of 8 therefore, suggested
as the requirement for full elaboration and assembly of
a scheme erected on a series of quantitative events for
basin-wide predictive purposes.
2. Megasequences

Five megasequences based on gross palynofacies
characteristics have been recognised within the Tertiary
sequence of o shore Vietnam. These have been given the
su x V and are numbered stratigraphically, V0 - Late Oli-
gocene; V1 - Early Miocene; V2 - Middle Miocene to earli-
est Late Miocene; V3 - Late Miocene and V4 - Plio-Pleisto-
cene, (Fig. 2).
These megasequences encap-
sulate Geological Stages based on
the Geologic Time Scale of Grad-
stein et al. (2004), as revised from
Haq et al. (1988) and Hardenbol et
al. (1998). They are mostly placed
at Sequence Boundaries (SB) in
sequence stratigraphy parlance at
the shift from highstand systems
tract (HST) to lowstand systems
tract (LST). These are dated at:
23.03 Ma - top Oligocene
(Chattanian, Ch4/Aq1).
16.97 Ma - top Early Miocene
(Aquitanian/Burdigalian, Bur5/
Lan 1).
11.61 Ma – top Middle Mio-
cene (Langhian/Serravallian, Ser4/
Tor1).
5.33 Ma – top Late Miocene
(Tortonian/Messinian).
0.05 Ma – top Plio-Pleistocene
(Zanclean/Piacensian/Gelasian/

Pleistocene).
Each of the megasequences
V1 - V4 are marked at their base
inception by a major increase in
marine microfossils. The actual
biofacies boundary may then, be
better de ned using the scheme
of Galloway (1989) that places
Fig. 2. Sequence Stratigraphy Scheme, Cuu Long and Nam Con Son Basins
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PETROLEUM EXPLORATION & PRODUCTION
sequence boundaries at  ooding surfaces. This occurs
slightly younger in the section, with the exception of the
Miocene - Pliocene boundary (V3-V4) at 5.33 Ma that is al-
ready placed at a  ooding surface on the Geological Time
Scale of Gradstein et al. (2004).
Megasequence boundaries at the initial transgres-
sive surface of the next transgressive systems tract (TST)
or maximum  ooding surface at the base of the ensuing
high stand systems tract (HST), as used in this study are
as follows:
22.3 Ma - Late Oligocene, V0.
15.8 Ma - Early Miocene, V1.
8.0 Ma - Middle Miocene - early Late Miocene, V2.
5.33 Ma - Late Miocene, V3.
0.05 Ma - Plio-Pleistocene, V4.
Megasequence V2 is dated as Middle Miocene to early
Late Miocene (Langhian, Serravallian and Early Tortonian).
Age of the upper boundary of this megasequence is

based on microfaunal and nannoplankton evidence. The
biozones of these marine microfossils are N16 and NN9
respectively (early Late Miocene) that are recognised to
be present in the upper part of V2 megasequence.
Microfaunal and nanno oral evidence is not available
at earlier stages than the Late Miocene due to the pau-
city of marine facies in the Vietnam basins. A convenient
major  ooding surface, post N16 for the V2 - V3 boundary
occurs at 8.0Ma within the Tortonian Stage.
Megasequence V3 encapsulates the remainder of the
Late Miocene, nannofossil zones NN10-NN11, (Late Torto-
nian - Messinian) with its upper boundary, as mentioned
at a  ooding surface rather than sequence boundary, at
5.33Ma on the global scheme. Megasequence V4 encom-
passes the Pliocene and Pleistocene.
3. Biofacies in sequence recognition
This section examines some of the eco-speci c paly-
nomorphs and palynofacies of the Vietnam Tertiary section
and the way they might be expected to respond in within
transgressive, highstand and lowstand systems tracts.
3.1. Transgressive system tract (TST)
Following lowstand sedimentation an initial  ood-
ing surface marks commencement of deposition of the
transgressive systems tract, with back-stepping retro-
gradational  ner grained shelfal sediments. Ravinement
erosion of the underlying sediments may occur with the
pene-contemporaneous reworking, making the bound-
ary appear less clear cut in terms of its biofacies.
Rapid creation of accommodation space leads to a
distinctive shallow marine microfauna if underlying open

marine ingress into the basin is near and strong enough.
Mangrove pollen with marine dinocysts and other marine
palynomorphs such as chitinous microforaminiferal lin-
ings may rapidly become an increasingly important com-
ponent of each successive parasequence up through the
sequence. In more high angle beach settings, pollen of
plants such as Casuarina, Marginipollis concinnus, Pandanii-
dites sp. and Echiperiporites estelae are particularly evident.
If the basin remains in an interior continental setting
or is distal with respect to the direction of marine ingress,
raised base level will be manifest by an increase in lacus-
trine algae and pteridophyte spores of associated water-
logged terrain, such as Magnastriatites howardi.
Fig. 3. Palynofloral Zones, Cuu Long and Nam Con Son Basins
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PETROVIETNAM - JOURNAL VOL 6/2011
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3.2. Highstand systems tract (HST)
Following the R-in ection point of maximum rate of
sea-level rise, with the maximum  ooding surface, sedi-
mentation may continue over an area of plentiful accom-
modation space, but it can become more regressive, pro-
gradational in aspect. Palyno oral assemblages will be
dominated by lowland freshwater pollen types and where
waterlogging persists in a non-marine setting, pterido-
phyte spores, including Magnastriatites howardi from the
aquatic fern Ceratopteris.
Restricted basins or those distal to the open marine
realm may register continued high freshwater base level
from the earlier TST phase, recording raised lacustrine al-

gae in the biofacies as alluvial run-o tends to be trapped
and ponded within the basin.
This is indicative of the highstand systems tract. If
 ooding surfaces within the early
phase of high stand sedimenta-
tion are su ciently open marine
in the later phase of 2
nd
order ther-
mal sag, they will yield correlat-
able micro oral and microfaunal
assemblages.
3.3. Lowstand systems tract (LST)
During falling stage sediment
accommodation space is at a pre-
mium. This e ectively reduces
the area available for deltaic and
coastal plain sedimentation, lead-
ing to a reduction in mangrove
and lowland freshwater pollen
representation in the palyno-
morph assemblages.
Withdrawal of the sea from
the shelf leads to incision as
clastic detritus is conveyed to
the slope and basin o the shelf
edge. Carbonate sedimentation
on the shelf edge may be pro-
moted. Sediment by-pass of the
shelf and upper slope may lead

to non-sequences and higher
energy of  ow as the ‘nick’ point
erodes back over an increased al-
luvial gradient. If sea-level does not fall below the exist-
ing shelf sediments of the earlier highstand, then a shelf
margin wedge systems tract may be produced.
With either shelf margin wedge sedimentation or
shelf incision, coarser clastic lithologies tend to predomi-
nate with the higher energy run-o alluvial  ow rates.
The winnowing e ect of the current leads to dispersal
and destruction of organic material. Palynomorph as-
semblages tend to be quite poor, coinciding with coarser
clastic lithologies. Where palynomorphs are present they
are dominated by a lower diversity of taxa and a greater
concentration of montane gymnosperm pollen such as
Piceapollenites sp., Pinuspollenites sp. Alnipollenites sp. and
Zonalasporites sp.
Lowstand sedimentation coincides with glacial maxi-
ma, which in the tropics may lead to increased seasonality
Fig. 4. Microfacies Scheme, Cuu Long and Nam Con Son Basins
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with a more marked dry season. Lowland everwet rainfor-
est pollen types may be reduced in favour of shrub  oras
dominated by fern spores and savanna grassland with
Monoporites annulatus.
4. Biofacies of megasequences
In this section broad trends of environmentally sig-
ni cant taxa from a number of wells are examined in

terms of their value as sea-level indicators for both basins.
This has permitted compilation of summary palyno oral
biofacies (palynofacies) for the Cuu Long Basin, (Fig. 5)
and for the Nam Con Son Basin, (Fig. 6). These diagrams
provide a general summary of correlatable ‘bioevents’ for
these two basins…
4.1. V0 Megasequence (Fig. 7)
Sediments of the Palaeogene, rift- ll V0 megase-
quence, Tra Tan Formation of the Cuu Long Basin are typi-
cally characterised by the Oligocene marker taxon Ver-
rutricolporites pachydermus (an early morphotype within
the Florschuetzia trilobata complex) with abundant fresh-
water algae Bosedinia sp., Pediastrum spp. and Botryococ-
cus spp., with a very distinctive  u y brown sapropelic
organic matter (SOM).
The latter is highly organic and hydrogen rich, typi-
cal of deposition in meromictic (strati ed) water bodies of
lacustrine character. Rare marine palynomorphs and ma-
rine microfauna occur, but they are probably caved.
This is a well known and a primary regional source
rock of numerous back-arc, Pannonian and continental rift
fracture basins around the periphery of the Sunda micro-
plate of the Eastern Sea, Malaysia and Western Indonesia,
(Cole & Crittenden, 1997).
In the Nam Con Son Basin, studies to date have re-
vealed very little of the Cau Formation section. Samples
that have been examined yield poor palynomorph recov-
ery with V. pachydermus and other freshwater pollen plus
records of the lacustrine alga Pe-
diastrum. Samples yield common

inertinitic kerogen derived from
coal seams indicative of freshwa-
ter swamp deposition.
4.1.1. Sequences
The V0 sequence in the Cuu
Long Basin reveals some marked
downhole changes in abundance
of the lacustrine algae Botryococ-
cus, Bosedinia and Pediastrum in
a few wells that have penetrated
a good interval of the Late Oligo-
cene. There are also changes in
the kerogen type. It is uncertain
that these changes can be related
to sea-level e ects within these
continental rift basins. Morley
(1991) has noted that mangrove
taxa of the Florschuetzia group
and the back mangrove taxon Dis-
coidites borneensis may have had
a freshwater swamp origin in the
Palaeogene.
Sequences V0a, V0b and V0c
have been tentatively recognised
Fig. 5. Palynofacies Defined Genetic Sequences, Cuu Long Basin, Vietnam
9
PETROVIETNAM - JOURNAL VOL 6/2011
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pending further studies, but they may be more related to
local changes in water conditions, though base level ef-

fects within non-marine settings are known, (Cole & Crit-
tenden 1997; Cole et al. 2005).
Very little of this sequence has been seen in the Nam
Con Son Basin, but its depositional facies is di erent from
the Cuu Long Basin. Lacustrine facies are present repre-
senting small ephemeral ponds and lagoons associated
with peatswamps, not the deep strati ed semipermanent
lakes of the Cuu Long Basin.
4.2. V1 Megasequence (Fig. 8)
Early Miocene sediments are distinguished palyno-
logically by the occurrence of Florschuetzia semilobata
and F. trilobata below the evolutionary appearance of F.
meridionalis.
The Bach Ho Formation of the Cuulong Basin records
the  rst occurrence of in situ marine microfossils. This ma-
rine indication is only slight, in the form of rare dinocysts,
chitinous foraminiferal linings in the palynomorph assem-
blage, plus locally common occurrences of the brackish
marsh agglutinating foraminifera Jadammina cf. macre-
scens and the gastropod Littorina spp. in the microfaunal
analysis. There is an absence of open marine planktonic
foraminifera and calcareous nannofossils.
In the palynomorph assemblage freshwater lacus-
trine algae remain very prominent, particularly Botryococ-
cus spp. and Pediastrum spp., but reduced occurrences
of Bosedinia spp. There is a diversity of freshwater pollen
and spore taxa, particularly Magnastriatites howardi. Ker-
ogen assemblages yield common amorphous kerogen
(AOM) though not the SOM rich component seen in the
V0 megasequence. None-

the-less these lacustrine
phases provide an im-
portant subsidiary source
rock facies, though to a
lesser degree than in the
syn-rift section.
Dua Formation as-
semblages from the Nam
Con Son Basin yield an
abundance of the lacus-
trine alga Pediastrum spp,
but not records of Botryo-
coccus spp, or Bosedinia
sp., indicative of smaller,
less permanent, ephem-
eral lakes that may have
been subject to minor
marine inundation. Pte-
ridophyte spores and
gymnosperm pollen are
comparatively common.
Marine palynomorphs
are present in increasing
numbers up through the
megasequence and the
microfaunal analysis re-
veals J. cf. macrescens plus
Elphidium cf. tikutoensis
and rare ostracods.
Fig. 6. Palynofacies Defined Genetic Sequences, Nam Con Son Basin, Vietnam

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4.2.1. Transgressive Sequences
Transgressive sequences in the Cuu Long Basin (V1c
and V1f, Fig. 8) are marked by an increase in mangrove
taxa and marine palynomorphs such as chitinous micro-
foraminifera and marine dinocysts. A variety of lacustrine
algae (Bosedinia, Botryococcus and Pediastrum), plus acme
occurrences of the aquatic fern spore Magnastriatites
howardi occur.
Semi-permanent lakes may have been at their greatest
extent, with sedimentation occurring across a broad ac-
commodation space of expansive lower coastal plain. Ma-
rine  ooding events may have introduced marine palyno-
morphs and restricted brackish marine microfaunas to the
lower strati ed layers of the lake allowing freshwater algae
to continue to thrive in the surface photic zone. Damp ar-
eas of slow moving water courses on the lake fringes may
have favoured the aquatic fern Ceratopteris and abundanc-
es of its spore Magnastriatites howardi In ux of the brack-
ish intertidal marsh agglutinating foraminifera Jadammina
cf. macrescens is most evident in the V1f sequence.
These sequences in the Nam Con Son Basin are char-
acterised by prominent occurrences of the freshwater
algae, Pediastrum spp. only indicative of smaller less per-
manent ephemeral lakes subject to greater and more
frequent marine in uence than in the Cuu Long Basin.
Surrounding areas may have
been less choked by water-

courses and more stable for
a greater diversity of pterido-
phytes and clearer develop-
ment of mangroves, but less
favourable for lowland forest
to become established.
4.2.2. Highstand Sequences
Highstand sequences
(V1a & V1d) are character-
ised in the Cuu Long Basin by
prominent lacustrine facies,
but on a slightly reduced
scale than seen in the TST,
similarly of the lake fringing
facies of Ceratopteris and its
spore M. howardi.
Assemblages of lowland
freshwater pollen and spores tend to be richer and more
diverse. These biofacies re ect the development of stable
plant communities and diverse habitats over extensive
tracts of prograding upper and lower coastal plain, during
this phase of diminishing rate of sea-level rise.
In the Nam Con Son Basin marine in uences are
overall greater and lacustrine palynofacies, with M. how-
ardi lesser than in the Cuu Long highstand sequences
V1a and V1d.
4.2.3. Lowstand Sequences
In the Cuu Long Basin lowstands V1b and V1e are
characterised by poor palynomorph recovery and diver-
sity as shelfal accommodation space was rapidly reduced.

In some cases lacustrine algae are common (as in V1e)
probably indicating a non-sequence during lowstand, this
interval not being distinguishable from the overlying TST.
In the Nam Con Son Basin poor recovery interval V1e
corresponds to higher run-o rates and reduced accom-
modation space. Raised percentages of gymnosperms in
V1b plus freshwater pollen and spores, but reduced man-
grove and marine in uences may re ect greater exposure
of terra  rma for upper coastal plain habitats and a widen-
ing of montane substrates.
Fig. 7. Late oligocene (VO) Megasequence, Cuu Long and Nam Con Son Basins
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PETROVIETNAM - JOURNAL VOL 6/2011
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4.3. V2 Megasequence (Fig. 9)
Onset of the Middle Miocene is marked by a pro-
found biofacies change in both the Cuu Long Basin (Cuu
Long/Nam Con Son Formations) and Nam Con Son Ba-
sin (Thong/Mang Cau Formations). This has been related
to the 15.8 Ma  ooding event that brought in a marked
change in sedimentation style in both basins, an event
that occurred just above the base of the Middle Miocene
within the Langhian.
In the Nam Con Son Basin (including the Northern
Nam Con Son Basin of MIA/block 4 area where sedimen-
tation was initiated over basement highs) thick intervals
of reef carbonates, clayey limestones, coquinoidal lime-
stones, mudstones and packstones (Areshev et al. 1992)
were deposited. The Nam Con Son Basin may have be-
come linked with the East Natuna Basin of Indonesia to

the South where May & Eyles (1985) have documented
Middle Miocene reefal carbonates.
4.3.1. Lowstand Sequences
Some of the thickest limestone development in the
Nam Con Son Basin has been linked to lowstand sedimen-
tation (sequence V2b), though in some areas of the basin
clastics remained dominant.
In the Cuu Long Basin no carbonates are encoun-
tered as the basin remained landward of the shelf edge.
Sedimentation was characterised by valley incision and
high sedimentation rates of coarse clastics in depocen-
tres. Sequence V2b yields abundant gymnosperm pollen
indicative of greater upper coastal plain/montane in u-
ence in the depositional environment and it is poorly dis-
tinguishable from the ensuing TST. Sequence V2e is a
poor recovery interval in some areas of the Cuu Long
Basin, whilst sequence V2i is rarely observed, perhaps
re ecting a non-sequence during sediment by-pass at
this time in the basin.
4.3.2. Transgressive Sequences
Cuu Long Basin sediments of the V2c sequence
are characterised by common mangrove pollen and
M. howardi indicative of the creation of abundant sedi-
ment accommodation space with wide tracts of lower
coastal plain for these plants to  ourish.
Nam Con Son Basin V2c sediments display a
marked increase in marine palynomorphs but lime-
stones are not always evident probably due to ravine-
ment erosion and renewed extensive shelfal clastic
sedimentation leading to inundation of former reefs.

This erosion, plus that of the underlying LST, may lead
to di culty separating sequence V2c from the under-
lying V2b.
4.3.3. Highstand Sequences
Highstand sedimentation in the Cuu Long Basin
is well represented by the V2a sequence that registers
the earliest consistent marine palynomorphs in this
basin plus horizons of prominent mangroves, all in
response to the 15.8 Ma maximum  ooding surface.
The V2d HST is well represented in the Cuu Long
Basin by a marked increase in M. howardi, accompa-
Fig. 8. Early Miocene (V1) Megasequence, Cuu Long and Nam Con Son
Basins
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PETROLEUM EXPLORATION & PRODUCTION
nied by prominent lowland rainforest pollen, a response
to wide development of lowland waterlogged habitats
at a time of maximum accommodation. HST sequences
V2g and V2h are represented by increased numbers of Pe-
diastrum spp. plus abundant and diverse freshwater pol-
len and spores, as similar responses to prograding sedi-
mentation over an area of wide accommodation space
following the decline in rate of sea-level rise.
Sequence V2h occurs immediately following the mid-
Miocene unconformity (Tjia & Liew, 1996) and documents
a further stage of regional thermal sag with progressive
marine in uence as indicated by an uphole increase in
marine and mangrove taxa.
In the Nam Con Son Basin the V2a sequence is indi-

cated by an uphole increase in marine to ter-
restrial (M:T) palynomorph ratio, a much more
marked change than that seen in the Cuu Long
Basin. This boundary also coincides with the
 rst appearance of limestones in the Nam Con
Son Basin, lithologies completely absent in the
Cuu Long Basin. Shallow marine benthonic mi-
crofaunas (microfacies 3 - 4) indicative of an in-
ner to middle neritic palaeoenvironment occur
at the base V2 megasequence, where none are
seen in the Cuu Long Basin.
The V2d sequence (coeval with an acme of
M. howardi in Cuulong Basin) is marked by a fur-
ther uphole increase in marine palynomorphs
and mangrove taxa.
The V2h sequence is dated as middle Mio-
cene (intra NN6 nannofossil zone) throughout
the Nam Con Son Basin. Palynomorph analysis
reveals a further increase in marine taxa (M:T
ratio) within this sequence, with progressive
subsidence following the middle Miocene in-
version and unconformity. This unconformity
shows diachroneity of its base and top across
the Cuu Long Basin from the apparent absence
of some sequences in some wells, but it ap-
pears to be an intra NN6 event.
4.4. V3 Megasequence (Fig. 10)
This megasequence is marked by a further
uphole increase in marine taxa in the palyno-
morph assemblage.

An open marine, lower neritic to upper-?middle
bathyal depositional palaeoenvironment is indicated by
microfaunal evidence throughout the V3 sequences of
the Nam Con Son Basin (Nam Con Son Formation, Fig.
2). Very diverse and abundant deepwater planktonic and
benthonic foraminifera of high planktonic to benthonic
(P:B) ratio, microfacies zone 5 are observed.
A further indication of restriction/restricted marine
in uence in the Cuu Long Basin is perceived from the ab-
sence of these deep marine faunas of microfacies zone 5,
in the Dong Nai Formation.
4.5. V4 Megasequence (Fig. 10)
Re ned foraminiferal and nannofossil zonations have
Fig. 9. Middle Miocene - early Late Miocene (V2) Megasequence
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permitted recognition of this megasequence, that forms
the major part of the sedimentary pile in the northern
part of the Nam Con Son Basin (MIA/Block 4 area).
Sedimentation in both the Cuu Long and Nam Con
Son Basins became linked as the con nes of their former
basin margins were overstepped. Sediments assigned to
the regional Bien Dong Formation are recognised in both
basins.
The megasequence is dated as NN12 - NN19 and
N18 - N23 and includes a major Late Pliocene tectonic
event. Sequences may be recognised on microfaunal,
palyno oral and nanno oral evidence of water depth
changes and intermittent input of terrestrial palyno oras.

5. Conclusions
An attempt has been made to identify corre-
latable sea-level controlled biofacies events for
improved stratigraphical resolution in the non-
marine and marginal marine sections (Oligocene
to mid-Miocene) of the Cuulong and Nam Con
Son basins. This has been achieved by recognis-
ing frequency trends of some environmentally
controlled, but long ranging palynomorphs and
microfaunal microfacies.
It has been possible to demonstrate di er-
ences between coeval sequences in the 2 basins.
Such provincialism is indicative of isolation and
separate autonomous basin  lling histories. The
independent e ects of marine transgression
and forced regression have been invoked. An
overall sea-level control may permit correlation
of sequences between these unconnected inte-
rior fracture continental rift basins.
These di erences diminish into the Middle
Miocene, as progressive marine transgression
led to more regionally synchronous sedimen-
tation. It was not until the Late Miocene that
planktonic and benthonic foraminifera plus cal-
careous nannofossils become abundant, indica-
tive of widespread marine transgression of the
regional thermal sag phase.
These studies have allowed the assembly
of initial stratigraphical templates that provide
improved stratigraphical resolution through the

rapidly alternating clastics of the Cuu Long Basin
and clastics/carbonates of the Nam Con Son Basin, o -
shore Vietnam Tertiary interval.
This work has also permitted a revision of the age dat-
ing, with intervals of section previously assigned to the
Oligocene now recognised as Miocene. Intervals earlier
dated as Plio-Pleistocene are now dated older, also within
the Miocene. This has led in some cases to more than dou-
bling the previously identi ed Miocene section.
The biofacies templates provide a means of stratal
calibration for future work on chemostratigraphy and  uid
inclusion stratigraphy (FIS). These latter new methods may
also provide independent veri cation of correlatable da-
tums so far identi ed on biostratigraphical criteria alone.
Fig. 10. Late Miocene (V3) & Plio-Pleistocene (V4) Megasequences Cuu Long
& Nam Con Son Basins
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PETROLEUM EXPLORATION & PRODUCTION
6. Acknowledgements
Thanks are due to Marcus Jakeman, Phil Jones and
Claire Murphy for help with the palynology. Nannofossil
work was done by Steve Starkie. Microfaunal studies were
carried out by Graham Coles, who is especially accred-
ited for compiling and making available the microfacies
scheme.
References
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& Cenozoic chronostratigraphy and cycles of sea level. In:
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H., De Graciansky, P. C., & Vail, P. R., 1998. Pmesozoic and
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C., Hardenbol, J., Jacquin, T. H., Farley, M., & Vail, P. R. eds
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Peel, F. J., 1997. Structure, stratigraphy and petroleum geol-
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Geology of Southeast Asia, Geological Society Special
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Ann. Conv. Indonesian
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& Cobbold, P., 1982. Propogating extrusion tectonics in Asia:
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Spec. Pub. 106: 291 - 306.
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Introduction
Facies and depositional environment determination
are fundamental work to be carried out when geoscien-
tists study any clastic reservoir for the purpose of hydro-
carbon exploration and development. Such work can be
relatively straightforward when outcrop data is available.
In the subsurface, however, it can be problematic when
only very limited core or sometimes even no core is avail-
able. The idea of using wireline logs as sedimentological

tools can be traced back to the middle 1950’s when geo-
scientists and engineers studied the shape of SP and GR
curves in association with facies and environments (Serra,
2003). Since then, more studies were done in the use of
log patterns for grain size indication, vertical sequence, fa-
cies and environment determination (Visher, 1965, Serra,
1975, Galloway & Hobday, 1983). Each well log curve gives
a particular spectral picture of the rock properties. How-
ever, we have to acknowledge that the use of any single
or combination of most available conventional open-hole
logs is often insu cient to clearly de ne a facies or depo-
sitional environment due to the lack of some crucial infor-
mation such as the sedimentary structure and texture of
the rock.
With the introduction of borehole image logs in the
middle 1980’s, the accuracy for facies and environment
delineation has been improved signi cantly through the
integration of image data as part of the inputs for sedi-
mentological interpretation. It is unfortunate that the
popularity of using the image logs for studying facies and
environments is still far behind expectation until today.
Part of the reason why image logs were underutilized
might be related to the lack of awareness and su cient
training of various image interpretation techniques to
many end users. Another part of the reason can be attrib-
uted to the still limited case studies published with work-
 ow details explained.
This paper aims to present a case study from  uvio-
estuarine deposits to illustrate the log-based sedimento-
logical characterization work ow in detail. It is expected

to help geoscientists to get a clear picture of what type of
sedimentological information is available on images and
how to use them in facies and environmental analysis, and
ultimately explain how geological uncertainty can be re-
duced with full integration of acquired image data with
conventional open-hole logs in clastic reservoirs.
Electrofacies and depositional environment
interpretation for a clastic reservoir utilizing
electrical image logs
Bingjian Li
Schlumberger Oil Field Services (Vietnam)
Facies are crucial inputs for depositional environment interpretation, reservoir property prediction and reser-
voir modeling. Traditionally, fancies description for classic reservoirs has relied heavily on core data. This is a time-
consuming data acquisition process with high well cost. This paper presents an alternative technique for electrofacies
characterization of classic sands utilizing electrical image logs, together with a case study. Electrofacies are deter-
mined with integration of sedimentary structure and texture information interpreted on image logs and lithology
from conventional open-hole logs. Analysis of facies association, sequence trend and paleocurrent directions pro-
vides satisfactory data to assist the depositional environment construction. This case study along with the devel-
oped work ow promotes more complete use of the acquired borehole images and conventional open-hole logs with
cost-e ective solutions for electrofacies and environmental determination. The presented data in the paper is from a
 uvio-estuarine sequence but the approach developed can be applied in any clastic reservoir in the local Miocene or
Oligocene formations in the subsurface of o shore Vietnam.
Abstract
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PETROLEUM EXPLORATION & PRODUCTION
Fig. 2. Sedimentary structures detected on FMI images. Plate A (up-
per) - Cross-bedding along with truncation surface and mud rip-up
clasts (red arrow indicated). Plate B (lower) - burrows appear as
white curved features on FMI

Facies and electro-facies
Facies refer to stratigraphic units distinguished by
lithologic, structural, and organic characteristics detect-
able in the  eld (Boggs, 1987). Electrofacies is de ned
as “the set of log responses which characterizes an elec-
trobed and permits it to be distinguished from the others”
(Serra, 1972).
Work ow for electrofacies determination
The work ow for electrofacies used in this paper
includes determination of lithology from conventional
open hole logs, sedimentary structures and texture from
FMI images.
Lithology using conventional open hole logs
Lithology can be de ned using conventional open-
hole logs. In the studied formation in this paper, it is proved
that the GR log is a good lithology indicator based on pre-
vious  eld experience. The following GR cuto s have been
used for the major lithologies: 65 API or less for sand; 65
- 75 API for silt; 75 API or greater for shale. Other logs, i.e.
RHOZ, NPHI, PEF and FMI are also referred to for lithology.
Fig. 1 shows the de ned lithology for the studied well.
Sedimentary structure using FMI images
As commented by Selley (1970), sedimentary struc-
tures “unlike lithology and fossils are undoubtedly gener-
ated in place and can never have been brought in from
outside”. Therefore, they are one of the key elements to
be known for facies de nition. Many types of sedimentary
structures can be recognized on borehole images with
or without core calibration. They include predepositional
structures such as various scales of erosional surfaces,

primary or syndepositional such as organic structures
formed in connection with an animal or plant organic ac-
tivity (borrow, root traces, etc) and inorganic structures
resulted in physical agents and postdepositional struc-
tures such as slumping, mud-cracks, dissolution and con-
cretions. Fig. 2 shows two examples of the sedimentary
structures; one includes both predepositional, small scale
erosional surfaces, as well as syndepositional inorganic
structure, cross-bedding, (plate A), and another one is the
syndepositional organic structure, burrows, (plate B).
One needs to bear in mind that some structures might
be ambiguous on electro-images; for example small
1 m
Cross-bed sets
Truncation surface
Truncation surfaceTruncation surface
Truncation surface
Mud rip
Mud ripMud rip
Mud rip-

-up
up up
up clasts
clastsclasts
clasts
Cross
CrossCross
Cross-

-bed
bed bed
bed set
testes
set
Cross
CrossCross
Cross-

-bed
bed bed
bed set
testes
set
0.5m
1m
Burrows
A
B
Fig. 1. Lithology definition for the studied well. using GR data. Track
1 - logs curves including GR, RHOZ and PEFZ. Track 2 - depth in me-
ter. Track 3 - lithology using previously defined GR cutoffs. Track 4
- lithology defined for the entire logged zone
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PETROVIETNAM - JOURNAL VOL 6/2011
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scale dissolution features and a pyrite nodule or small
shale clast might look similar (conductive) on images. In
this case, a core calibration might be needed to ensure the
interpretation accuracy.

Rock texture using SandTex*
A relative sorting index can be estimated from the
resistivity spectral analysis (SandTex* software) using the
electrical borehole images - FMI/FMS or OBMI (Newberry
et al, 2004). All points crossing the wellbore on electro-
images are grouped into three divisions - the “matrix” frac-
tion which is the background of the rock texture, resistive
fraction which is the portions having resistivities greater
than matrix and conductive fraction which has resistivi-
ties less than the “matrix” fraction. SandTex* calculates the
actual resistivity represented by each of the fractions and
the percentages of the three fractions to derive a sorting
index. Fig. 3 gives an example showing the outputs from
SandTex*: track 1 is the fractional percentages, track 2
showing the peak resistivity as well as the upper and low-
er boundaries for a well sorted sand overlain on the vari-
able density log display and the computed sorting index
(white curve). Fig. 4 shows an example of an image de-
rived sorting index (black curve in track 2) having reason-
able agreement with the sorting index (red dot) gained
from core analysis.
Electrofacies analysis workflow
In this paper, an integrated work ow is used to de ne
the electrofacies (Fig. 5). First of all, lithologies are deter-
mined using the conventional open-hole logs mainly GR
data as described in the previous section above. Second-
ly, sedimentary structures are interpreted on FMI images.
Thirdly, a sorting index is derived on FMI images using the
SandTex* described above.
Then electrofacies is determined for any particular

zone by integrating the lithology, sedimentary structure
and sorting index in the zone. For example, a cross-bed-
ded sand is de ned if the zone has sand lithology (GR is
65 API or less in the studied well) and cross-beds observed
on FMI interpretation. A relative sorting is indicated by the
sorting index curve for sandstone facies only.
Fig. 3. Resistivity spectrum analysis display: Track 1 - percentages of
conductive fraction (blue), matrix fraction (tan) and resistive frac-
tion (red); track 2 - Variable density log display (VDL) of continuous
resistivity histogram showing peak distribution (orange) and upper
and lower bounds in yellow and sorting index in white line (from
Newberry, 2004)

Fig. 4. Sorting index derived from borehole images against the
sorting index from core analysis (red dots in track 2). Track 3 - re-
sistivity spectrum analysis displayed in VDL versus core mean grain
size (white dots) (from Newberry, 2004)
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Typical electrofacies observed in the studied well
There are seven types of electrofacies de ned in the
studied well as listed and described below.
A) Mud rip-up clast-rich sandstones (MCSt).
B) Massive sandstones (MSt).
C) Cross-bedded sandstones (CBSt).
D) Bedded sandstones (BSt).
E) Bedded shale (BSh).
F) Massive shale (MSh).
G) Bedded silt (BSIt).

Electrofacies
ElectrofaciesElectrofacies
Electrofacies Determination Flowchart
Determination FlowchartDetermination Flowchart
Determination Flowchart
X
XX
X-

-bedded SST
bedded SSTbedded SST
bedded SST
Laminated SH
Laminated SHLaminated SH
Laminated SH
Massive SILT
Massive SILTMassive SILT
Massive SILT
Open-hole Logs
(GR-Pe-RHOZ
-NPHI)
FMI
Lithology
Core
calibration
Electro
ElectroElectro
Electro-

-

f

facies
aciesacies
acies
Sedimentary
Structure
Textural
information
Laminated SH
Laminated SHLaminated SH
Laminated SH
Fig. 5. Electrofacies determination workflow chart used in this study
A B
Fig. 6. Examples showing mud rip-up clast-rich sandstones (MCSt) and massive sandstones (MSt). Track 1 for both plates A & B - GR, BS
and borehole deviation. Track 2 - depth (m), track 3 - RHOB and NPHI. Track 4 - static FMI images. Track 5 - Dip tadpole plot (0-40o scale).
Track 6 - dynamic FMI. Track 7 - Resitivity spectrum analysis VDL. Track 8 - sorting index (yellow shaded zones as good sorted sands, purple
shaded zone as poorly sorted and non-shaded zones as moderately sorted sands). Track 9 - Electrofacies and Track 10 - Lithology
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Mud rip-up clast-rich sandstones (MCSt): It usu-
ally has higher GR reading than clean sandstones due to
the mud rip-up clast content. The mud rip-up clasts have
various sizes and are usually quite angular (see Plate A in
Fig. 6). They can be recognized well on FMI images as con-
ductive clasts. The sorting is mostly poor for this type of
sandstone (see track 8 in Plate A, Fig. 6). Massive sand-
stones (MSt): It shows low GR as clean sands and hardly
any sedimentary structure. An example can be seen in

the upper section of the Plate A in Fig. 6. Sorting is usu-
ally good for the massive sands. Cross-bedded sand-
stones (CBSt): It is one of the most common facies in the
studied well. It has low GR response as clean sands and
intensive cross-beds. Fig. 8 shows an example of four dif-
ferent sets of cross-beds separated by truncation surfaces.
Some mud rip-up clasts might be observable at the base
of a truncation surface (see 471.4m of Fig. 8). The cross-
bedded sands can be either well- or medium-sorted. Bed-
ded sandstones (BSt): It has either low or slightly higher
GR than clean sands. Horizontal or low angle bedding (or
lamination) can be observed on FMI images for bedded
sandstones. An example of bedded sandstone is given in
Plate B of Fig. 6. Bedded shale (BSh): It has high GR read-
ing (>75 API) with bedding or lamination observed on FMI
images (see Plate A in Fig. 7). Massive shale (MSh): It has
high GR readings (>75 API) without any bedding structure
observed. An example can be seen in the upper section
of Plate B, Fig. 7). Bedded silt (BSIt): It has a GR reading
between 65 API and 75 API with bedding or lamination
observable on images. An example of such facies is given
in the lower section of Plate B, Fig. 7.
Depositional environment interpretation
One of the main goals for electrofacies analysis is to
reconstruct the depositional environment. However, as
Walker (1978) pointed out, “many, if not most, facies de-
 ned in the  eld have ambiguous interpretations - a cross-
bedded sandstone, for example, could be formed in a me-
andering or braided river, a tidal channel, an o shore area
dominated by alongshore currents”. Therefore, it is impor-

tant to analyze the facies association or sequence as well
as paleocurrent patterns for more accurate interpretation
on depositional environments.
Paleocurrent analysis
Some sedimentary structures provide directional
Fig. 7. Examples of bedded shale (Plate A), massive shale (upper
section of Plate B) and bedded silt (lower section of Plot B). Track
details refer Fig. 6
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data that shows the direction the ancient current  owed
at the time of deposition. For instance, the dip direction
of cross-bed foresets can be used as a good indicator for
paleocurrent  ow direction. Additionally, directional data
has also environmental signi cance. Bogges (1987) sum-
marized two basic types of paleocurrent pattern as shown
in Fig. 9 as well as the environmental signi cance of pa-
leocurrent patterns (Fig. 10). In the case study in this pa-
per, the paleocurrent pattern is dominantly unimodal (see
track 5, Fig. 11).
Sequence Analysis
An electrosequence by Serra
(1970) meant “a depth interval thick-
er than the vertical resolution of the
measuring tool, presenting a pro-
gressive and continuous evolution
between two extreme values of mea-
sured parameter, tracing a ramp”.
Environemental interpretation is

commonly hampered by the fact that
very similar facies can be produced
in di erent environmental settings
(Boggs, 1987). It is often impossible
to make a unique environmental in-
terpretation based on a single facies.
Depositional environment reconstruc-
tion can be improved if we study the
facies associations and sequences
once all individual facies have been
de ned. The sequence in which fa-
cies communally occur contributes
as much information as facies them-
selves for depositional environmental
interpretation.
An overall  ning-upward se-
quence has been de ned based on
sequence analysis utilizing the open-
hole logs and interpreted electrofacies
in the studied well as showing in Track
3, Fig. 11. The  ning-upward sequence
starts at 489m where an erosional
surface detected on FMI with mud
rip-up clasts as lag deposits above it
and ends at about 451m where shale
is deposited. The sequence indicates a
decrease in transporting power of cur-
rents during deposition.
Depositional Environment Interpretation
A particular depositional environment is de ned by a

particular set of physical, chemical and biological param-
eters that correspond to a geomorphic unit of a particu-
lar size and geometry. Traditionally environmental inter-
pretation was based on outcrops or core in which those
needed parameters might mostly be available. Environ-
Fig. 8. Example of typical cross-bedded sandstones (CBSt). Track details refer Fig. 6
Fig. 9. Paleocurrent data plotted as rose diagrams. A - Bimodal pattern. B. Unimodal
pattern (From Boggs, 1987)
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PETROVIETNAM
mental interpretation can be possible when su cient
data extracted from images and conventional open-hole
logs described above is available with limited core cali-
bration or su cient knowledge about the mostly  uvio-
estuarine settings based on previous core studies in the
 eld. The sequence is therefore interpreted in details as
following.
478.8 - 489m: The section begins with channel  oor
lag deposits (mud rip-up clasts, MCSt) on an erosional
surface at 489m, overlain by bedded sand (BSt). The lag
deposits are mostly poorly sorted but the bedded sands
seem moderately sorted. These deposits were eroded by a
possible di erent channel evidenced by another erosion-
al surface at 486.2m succeeded upward by channel  oor
mud rip-up clasts as lag deposits. The overlain deposits
are mostly cross-bedded sands (CBSt) with minor bedded
sand (BSt) and massive sand (MSt) interbeds in the top.
The cross-bedded sands are well - to moderately-sorted.
The section is interpreted as stacked  uvial channel de-

posits with the top small interval from 474.8 - 475.4m as
possible levee or lateral accretion sediments. The paleo-
current direction in this channel is dominantly due north
with a unimodal pattern.
467.8 - 474.8m: The low section is comprised of
moderately sorted cross-bedded sands (CBSt) deposited
on an erosional surface at 474.8m. The upper section is
consisted of interbeds of wellsorted bedded sands, mas-
sive sands and cross-bedded sands.
The azimuth of the cross-bed dip in-
dicates an overall NNW paleocurrent
direction (see track 5 in Fig. 11). The
section is interpreted as  uvial chan-
nel deposits.
455.6 - 467.8m: The base contact
of the section is gradual other than
an erosional one with the underlain
channel deposits. The electrofacies
include interbeds of bedded sands
(BSt), bedded silts (BSIt) and bedded
shale (BSh). The section is interpreted
as lateral accretion deposits. The azi-
muth of the lateral accretion beds are
dominantly dipping towards SW.
451- 455.6m: The section is com-
prised of mainly bedded shale (BSh)
or mudstone and interpreted as  ood-
plain deposits.
447.3 - 451m: The section is comprised of bedded
silts (BSIt) and shale and interpreted as brackish bay

shoreface deposits.
In summary, the entire studied succession is inter-
preted as stacked  uvial channel deposits overlain by
lateral accretion beds which are followed by  oodplain
and brackish bay deposits in the upper part. It is a  ning-
upward sequence deposited in currents decreasing in
transporting power as deposition progressed. The plaeo-
current  owed dominantly northward in the nearby well
location area.
Other potential applications oh the paleocurrent data
Better understanding on well location in respect to the
sandbody geometry
Plot A in Fig. 12 shows a theoretical dip azimuthal rela-
tionship between current beds and lateral accretion beds
for wells in di erent locations within a  uvial meandering
point bar. The left rose diagram in Plot C shows that the
angle between current bed azimuth and and lateral ac-
cretion azimuth is greater than 90
o
for the studied well in
this paper. This indicates that the well is likely located in
the upstream part of the point bar as showing in the right
cartoon in Plot C of Fig. 12. An o set well can be recom-
Fig. 10. Environmental significance of paleocurrent patterns (Selley, 1978, Boggs, 1987)
22
PETROVIETNAM - JOURNAL VOL 6/2011
PETROLEUM EXPLORATION & PRODUCTION
Fig. 11. Electrofacies and depositional environment interpretation results for the studied
well. Track 1- GR & PEF. Track 2 -depth (MD). Track 3 - sequence analysis result. Track 4 -
dip tadpoles (0-40o scale). Track 5 - Dip azimuth rose diagram (current beds - light blue

and lateral accretion - red). Track 6 - sorting index (yellow shaded zone - well sorted, pink
shaded zone - poor sorted and the rest zone - moderate sorted). Track 7 - lithology. Track
8 - FMI static images. Track 9 - Electrofacies. Track 10 - facies code. Track 11 - facies descrip-
tion. Track 12 - depositional environment interpretation.
mended to be placed towards the North of the current
well in order to trace the same point bar in the appraisal
or in ll drilling. Plot B is a cross section view of the channel
showing in Plot C (right) along the direction of E-W.
Optimum position for injection wells
When water injection wells are required for second-
ary recovery for this type of reservoir, it
is recommended to place the injectors
either on the upstream (South, refer
plot C in Fig. 12) and the downstream
(North, refer to plot C in Fig. 12) end
of the longitudinal sand bar for better
sweeping e ciency. This is because t
the favorable permeability direction is
at E-W due to the dominant northward
cross-beds dip. Water injection needs
to avoid going along with the favor-
able permeability direction with the
producers within the high perm anisot-
ropy reservoirs.
Conclusions
A log-based sedimentological
characterization work ow for electro-
facies and depositional environment
analysis is proposed in this paper. Elec-
trofacies is determined with integra-

tion of sedimentary structural and tex-
tural information interpreted on image
logs and lithology from conventional
open-hole logs. Analysis of facies asso-
ciation, sequence trend and paleocur-
rent directions and patterns provides
satisfactory data to assist the depo-
sitional environment reconstruction.
A case study from a  uvio-estuarine
deposit is presented to illustrate the
work ow.
This case study along with the
developed work ow promotes more
complete use of the acquired bore-
hole images and conventional open-
hole logs with cost-e ective solutions
for electrofacies and environmental
determination. It is believed that the
approach developed herein can be applied to any clastic
reservoir in the local Miocene or Oligocene formations in
the subsurface of o shore Vietnam.
Temrms and de nitions
FMI - Is a Fullbore Formation MicroImager tool pro-
viding an electrical borehole images.
23
PETROVIETNAM - JOURNAL VOL 6/2011
PETROVIETNAM
SandTex* - Is a GeoFrame Geology module that ana-
lyze resistivity spectrum for deriving a sorting index utiliz-
ing electro-image logs like FMI, FMS or OBMI.

OBMI - Is an Oil-base MicroImager tool used in non-
conductive, invert-emulsion mud environments.
References
1. Boggs, Sam Jr., 1987. Principles of sedimentology
and stratigraphy. Merrill Publishing Company, p.784.
2. Galloway, W.E. and
Hobday, D.K., 1983. Terrige-
nous clastic depositional envi-
ronmental systems. Springer,
New York.
3. Newberry, B., Han-
sen, S. and Perrett T., 2004. A
Method for Analyzing Textural
Changes within Clastic.
4. Environments Utiliz-
ing Electrical Borehole Imag-
es. Gulf Coast Association of
Geologic Societies (GCAGS).
5. Convention. San Anto-
nio Texas, USA, Oct. 2004.
6. Selley, R.C. , 1970. An-
cient sedimentary environ-
ments. 1
st
ed. Chapman &
Hall, London.
7. Serra, O., 1972. Diag-
raohies and stratigraphie. In:
Mem. B.R.G. M., 77, p. 775-
832.

8. Serra, O., and Sulpice,
L., 1975. Sedimentological
analysis of shale-sand series
from well logs.
9. Serra, O., June 17-20,
1985. Lithology determina-
tion from well logs: Case stud-
ies, SPWLA 26 Annual Logging Symposium.
10. Serra, O., et al, 2003. Well Logging and Geology,
Editions Serralog. P. 436.
11. Visher, G.S., 1965. Use of vertical pro le in environ-
mental reconstruction. Bull. Amer. Assoc. Petroleum Geol.,
49, p. 41 - 46.
Fig. 12. Dip azimuthal relationship between current beds and lateral accretion beds for wells in
di erent locations within the  uvial point bar
24
PETROVIETNAM - JOURNAL VOL 6/2011
PETROLEUM EXPLORATION & PRODUCTION
The Te Giac Trang (TGT) Field is located in the central
part of Block 16-1, Cuu Long Basin, offshore Vietnam,
approximately 100km Southeast of Vung Tau City, near the
Bach Ho Field and Rang Dong Field (Fig. 1).
Following the initial discovery by well TGT-1X in 2005,
six appraisal wells were drilled, all of which flowed oil at
commercial rates except for one, TGT-4X.
Oil has been found in the Lower Bach Ho Formation
(Early Miocene) and Upper Tra Tan (Late Oligocene)
sandstone reservoirs (Fig. 2) in 6 accumulations separated
by either dip closures or WSW-ENE faults. From North to
South, the fault blocks have been named as H1.1, H1.2, H2,

H3N, H3 and H4 (Fig. 3), and comprise stacks of numerous
individual reservoirs in thinly bedded sand layers of
lacustrine and fluvial origin.
In the TGT field, the dominant structural features are
ENE-WSW trending en-echelon faults, which are primarily
listric in character, with a strike-slip component. Most
faults die out upwards in the Lower Miocene strata but
some extend through the Bach Ho shale into the Middle
Miocene. An example is the major fault separating the H1
fault block and the Hai Su Trang field in the North of the
TGT field. Some faults extend downwards to basement,
whilst many sole out in the D Sequence shale (Fig. 4).
At different structural horizons, the reservoirs have
been subdivided into numerous compartments in fault
blocks or dip closures combined. The existence of the
compartments was supported by the PVT and RCI data
from many TGT drilled wells.
A zonation scheme was developed for reservoirs in
stacked sand systems in the fault blocks based on the log,
biostratigraphic and pressure data. As a result, these have
been subdivided into four main zones with 56 reservoirs
(Fig. 5).
The Intra Lower Bach Ho 5.1 reservoirs are very thin,
consisting of very fine- to medium-grained sandstones,
Te Giac Trang field: Geological features,
reservoirs and field development concepts
Pham Tuan Dung, Pham Thi Thuy, Nguyen Manh Tuan, Branimir Gojsic
Hoang Long JOC
Abstract
The Te Giac Trang (White Rhinoceros) Field is located in the Northern part of Block 16-1 in the Cuu Long Basin

approximately 100km Southeast of Vung Tau. The field was discovered in 2005 and seven wells have been drilled
during the exploration and appraisal period. Individual reservoir intervals have been tested at rates of over 8,000
barrels of oil per day and 4 million standard cubic feet of gas per day.
The Te Giac Trang (TGT) Field is comprised of numerous separate fault block reservoirs of Early Miocene and Late
Oligocene age. The reservoirs are confined to some small areas over the field and composed of normally pressured,
vertically stacked sand layers. The reservoir zones are made up of vertically isolated geological complexes that
preclude highly deviated or sub-horizontal development wells in the reservoir zones.
In order to optimise the field development, the reservoirs in the fault blocks were planned to be developed through
two well head platforms. One of them is located in the North of the field to develop reservoirs in the H1 and H2 fault
blocks, the other, in the South, to cover reservoirs in the H3 and H4 fault blocks.