ing from 150 to 3,000 m with the slope dips ..... and is considered as the onset of post-rift ac- tivities. .... Phu Khanh basin can reach up to 3,000 me- ters.
Chapter
8
Phu Khanh Sedimentary Basin and Petroleum Potential
Chapter 8. Phu Khanh Sedimentary Basin and Petroleum Potential
1. Introduction The Phu Khanh basin is located along
where the basement is high and in zone of
the continental margin off central Vietnam.
subsidence there are small depocenters. Due
It extends from 11oN to 14oN latitude and
to fault system activities, the shelf basement
109o20’E to 111oE longitude and possibly
rocks are block faulted, forming a step-like
extends to the East (see Fig. 5.1 in Chapter
and eastward-dipping topography. The shelf
5). The Phu Khanh basin is bordered by the
slope adjacent to the Da Nang and Phan Rang
Cuu Long basin to the South, the Nam Con
shelves is an area with the water depth varying from 150 to 3,000 m with the slope dips
Son basin to the Southeast, the Song Hong
varying from two to tens of degrees with a
basin to the North, the Hoang Sa basin to the
width of about 20 to 200 km. Here the seg-
Northeast, and the Da Nang and Phan Rang
mentation of the slope is more intensive rela-
shelves to the West. To the East, where data
tive to the shelf, with subterrane mountain and
are missing, it is possible that another basin
canyon systems. In the North, corresponding
exists between the Phu Khanh basin and the
to the Quang Nam and Binh Dinh provinces,
deepest part of the East Sea.
and to the South, corresponding to the Binh
The seabed topography of this region
Thuan and Ninh Thuan provinces, the shelf
is very complex. The marginal sea setting
slope is relatively wide. Conversely, the shelf
is comprised of a continental shelf, a shelf
slope is very narrow in the middle part corre-
slope and a shelf margin slope (base of
sponding to the South Binh Dinh and Khanh
slope) system which connect to an oceanic
Hoa provinces (in some place only 18 km).
plain with local grabens and uplifts. The wa-
Here it takes the horseshoe form that marks
ter depth varies from 0 to 3,000 m. The geo-
the sea spreading area that intrudes furthest
logical units here are situated on continental
near the Kontum block. Pelagic plains with
crust and a transiting zone lying between the
a relatively flat topography lie off the base
continental and oceanic crusts. It is possible
of the continental slope. Although seismic
parts of these units are situated on a suboce-
data is lacking, based upon the gravimetric
anic crust to the East. The continental shelf,
data interpretation results, it is possible that
where the water depth varies from 0 to 200
under these plains lie the grabens of various
m, is narrow and lies above the differentiated
dimensions that are depocenters where the
granite basement. There are buried horsts
sediment thickness can be of 3 - 4 km. 249
The Petroleum Geology and Resources of Vietnam
Lying mainly in the deep water, Phu
the Vietnam Oil and Gas General Directorate
Khanh basin has not been studied extensively
and Petrovietnam. Up to now, several
with almost no exploration drilling program.
seismic surveys offshore of central Vietnam
Therefore, the main geological features of
have been completed, i.e. GSI (US, 1974),
the basin were predicted from regional seis-
Malugin (former USSR, 1984), GECO-
mic data as well as extrapolated data from
PRAKLA (1993), and NOPEC (1993) with
adjacent basins, which were studied in more
a total volume of up to 17,000 km of seismic
detail and in which oil and gas reservoirs
lines.
have been discovered and produced.
Onshore studies have been continued
2. Geological Studies & Oil and Gas
since 1977 by many researchers, such as Le
Exploration History
Nhu Lai, Nguyen Quang Hinh (1977), Phan
Although the Phu Khanh basin is poor-
Huy Quynh (1980), Sladen, Nguyen Quang Bo (1991), Tran Tinh (1988-1997), and Le
ly explored, the onshore geological studies
Thanh (1998). They analyzed samples from
West of Phu Khanh basin had been done very
Thi Nai area and discovered that the oil seeps
early by French geologists during geologi-
had similar characteristics to those coming
cal mapping (at a scale of 1:500,000) of Da
from Miocene carbonates retrieved from
Nang (1935), Nha Trang (1937), and Quy
well 119-CH-1X. Therefore, it was specu-
Nhon (1942).
lated that the seeped oil may originated from
The first oil seep was discovered between
the deep part of the Phu Khanh basin that
1920 and 1923 at Thi Nai embayement (Quy
migrated there via the faulting systems. In
Nhon city), on land adjacent to the Phu Khanh
2000, Vietnam Petroleum Institute (VPI) co-
basin. In 1944, the French carried out some
operated with Japan Geology Institute (JGI)
exploration drilling but now the data has been
to study outcrops from Nong Son to Kontum,
lost. From 1944 to 1964, Saurin researched
Ba River, and Thi Nai embayment. Pham
the oil seepages at Thi Nai and came to the
Quang Trung et al. (2002, 2003) of VPI also
conclusion that the oil origin did not link to
had analyzed samples from Thi Nai. Those
the Neogene sediments but possibly to the
aforementioned studies have come to con-
algae-rich sapropels from Quy Nhon bay [27,
flicting conclusions. Therefore, the origin
28]. After 1960, several surveys were carried
of the seeped oil and source rock quality are
out with the East Sea research programs by
problems that have yet to solved and warrant
various French, American, German, Japanese,
further investigations.
and Chinese geologists and geophysicists. In the early 1970s, some foreign oil companies
From 2001 to 2004, VPI was in charge
performed geological and geophysical studies
of a national level research program coded
with the former South Vietnam government
KC-09-06 to research the geodynamics
in an effort to locate preliminary oil and gas
and oil and gas potentials of Vietnam deep
prospects. Since 1979, these studies have
water regions, including Phu Khanh basin.
been continued under the administration of
Also within this time period, VPI and 250
Chapter 8. Phu Khanh Sedimentary Basin and Petroleum Potential
GEUS (Geological Survey of Denmark and Greenland) had collaborated in the ENRECA project for a complete study of the geology and oil and gas potential of Phu Khanh basin, including an extensive sedimentological and geochemical study of Thi Nai embayment and Ba River lowland, which are adjacent to the Phu Khanh basin. Results of those studies mentioned in the references of this chapter have been reported in several seminars, workshops, conferences, journals as well as archived at Petroleum Information Center (PIC) and VPI (subsidiaries of Petrovietnam) and the Geology Museum Information Center (subsidiary of Vietnam Figure 8.1. The Phu Khanh basin and adjacent regions’ structural units
Department of Geology and Minerals). Most of the material of this chapter comes from these references. It is important to note that
• Main grabens of Phu Khanh basin or Phu
all the results provided here are based on seis-
Khanh trench;
mic analyses, simply because drilling activi-
• Tuy Hoa shear zone.
ties have not yet been implemented. Analysis
The Da Nang shelf lies to the Northwest
results were correlated with well data from
of Phu Khanh basin and is elongated in the
nearby sedimentary basins (Song Hong, Cuu
North - South direction. Water depth there is
Long, Nam Con Son); these basins are sepa-
less than 100 m. It has thin Cenozoic sedi-
rated by complicated fault and uplift systems
mentary formation whose thickness increas-
with different geological structures, therefore
es to the East (Fig. 8.2).
there are limitations for these correlations.
The Phan Rang shelf lies to the South-
3. Tectono-Structural Characteristics
west of Phu Khanh basin. The two shelves
3.1. Main geological structures
are the eastern boundary of the Kontum geo-
Based on Pre-Cenozoic bedrock map-
block and have been relatively stable during
ping, Phu Khanh basin and its vicinity can
the formation and development of Phu Khanh
be divided into the following main geologic
basin. During the rifting process, most parts
structures (Fig. 8.1):
of the two shelves remained unchanged, ex-
• Da Nang shelf;
cept for some weak spots (mostly concentrated in the Da Nang shelf) influenced by
• Phan Rang shelf;
faulted blocks where small grabens, half-
• Tri Ton horst;
grabens, and horsts were formed (Fig. 8.2). 251
The Petroleum Geology and Resources of Vietnam
Figure 8.2. Seismic profile along the VOR-93-101 line, crossing Da Nang shelf and Da Nang fault zone
The tectonic activity on the shelf appears to
The Tri Ton horst lies to the East of
be very minor (Fig. 8.3). Similar to the Da
the Quang Ngai trench, North of the Da
Nang shelf, the Phan Rang shelf has layers
Nang fault zone of the Phu Khanh trench.
of Tertiary sedimentary rocks, whose thick-
Some authors also referred to this zone as
ness varies from tens of meters to more than
the “Quy Nhon uplift zone” [23]. Seismic
a thousand meters in the East. The main
data indicate that the region underwent
sedimentary rocks are clastic, although some
compressional movement, folding, and uplift
Miocene carbonate formations can be found
during the Miocene period, to such a degree
along the eastern edge.
that it was above sea level, and got eroded
Figure 8.3. Seismic profile along the VOR-93-110 line, crossing Phu Khanh deep depression 252
Chapter 8. Phu Khanh Sedimentary Basin and Petroleum Potential
Figure 8.4. Seismic profile along the VOR-301 line, situated longitudinally in Phu Khanh basin from 11th to 14th parallels
and weathered for a considerable period of
to the North of Vietnam mainland. Accord-
time. These activities stopped at the early
ing to Tapponnier (1982), this shear zone
Late Miocene and were followed by a period
may be related to the transform megashear
of subsidence, allowing for the deposits of
zone developed as a result of Indochina’s re-
Upper Miocene and Pliocene - Quaternary
moval from Eurasia. The Tuy Hoa shear zone
periods to stay relatively flat and stable.
depression was formed prior to the majority of the grabens that were created during the
The Phu Khanh trench lies in the deep
main rifting phase and may contain depos-
water region, bounded to the West by a steep-
its of Lower Oligocene and probably Eocene
ly dipping continental slope. This is a region
sediments as well.
of greatest sedimentary thickness in the Phu Khanh basin (Figs 8.3, 8.4). The gravimetric
Recently some authors from VPI (Le Dinh
anomaly map shows that the eastern part of
Thang, Le Van Dung, 2003) proposed yet an-
Phu Khanh basin is an anomalously negative
other main structure called the Da Nang fault
area and appears as a sub-isometric form.
zone. This fault zone lies to the South of the
The deepest area lies at the intersection of
Tri Ton uplift around latitude 13o30’N and is
110o20’E longitude and 13oN latitude. Its
the northern limit of the Phu Khanh basin.
eastern boundary lies near longitude 1120E,
This fault zone is characterized by a series of
which transitions to the deepest area of the
strike-slip faults oriented along the NE - SW
East Sea.
and stepped, deepened towards the SE direction. Faults and steps were formed primarily
The Tuy Hoa shear zone is located to
in the first rifting phase and Eocene(?) and
the Southwest of the Phu Khanh basin and
Oligocene sediments were deposited in adja-
is expressed as an area with high amplitude
cent grabens (Fig. 8.2).
faults, some of them originating from within the basement (Fig. 8.5). The shear zone is
Combined with the Tuy Hoa shear zone
oriented in a NW - SE direction, similar to
in the south, the Da Nang faulting area forms
the Song Hong fault system, which extends
a horse-shoe shaped opened to the east of 253
The Petroleum Geology and Resources of Vietnam
Figure 8.5. Seismic profile along the PK-03-084 line crossing Tuy Hoa shear zone
Phu Khanh basin. Apart from those structural
the following main tectonic phases according
elements mentioned above, some studies ef-
to the view point of Petrovietnam geologists
fectued in this region introduce the NE Cuu [37]: Long basin and NW Nam Con Son basin as
Pre-rift stage (Late Cretaceous - Eocene)
a part of the Phu Khanh basin [37]. Readers can refer to Chapters 9 and 10 of this book for further discussion.
In the Late Cretaceous period, the extrusion of the SW East Sea area was controlled mainly by strike-slip activities in the Song
3.2. Phu Khanh basin evolution history
Hong, Tuy Hoa and Three Pagoda fault sys-
The Phu Khanh basin is a passive tems. The Late Cretaceous - Paleocene pecontinental margin basin or may be considered riod is characterized by a widespread acidic as a continental margin basin linked to the eruption activity and extensive erosion imcollision of the Indian and Eurasian plates and mediately following extrusion event. In the the spreading of the East Sea with a multi- Late Miocene. This tectonic activity resulted phased development history. The thicknesses in the creation of the proto-East Sea. The exof the sedimentary successions vary from 500
tension process that began at this time caused
Based on the results of tectonic studies carried
to the Kontum geoblock, creating the basis for
m in the western margin to more than 10,000 the shortening in the Pre-Tertiary basement, m in the main depocenters in the eastern part. which was consolidated and closely attached out by Vietnamese and foreign authors, the the formation of the Phu Khanh basin as a retectonic evolution of Phu Khanh basin shares sult of the moving and rotation of the Kontum common characteristics with other Tertiary geoblock and the extension and following rosedimentary basins surrounding the East Sea tation and spreading of the East Sea. area. The tectonic evolution can be divided into 254
Syn-rift stage (Late Eocene ?-Oligocene)
Chapter 8. Phu Khanh Sedimentary Basin and Petroleum Potential
The subduction of the proto-East Sea
dominantly extentional and transtensional
along the North Borneo trough continued,
stress fields. However, transpressional defor-
creating extentional tensions in the subducted
mation was produced along a few strike-slip
plate which intensified the Indochina margin
faults combined with some compression in
extentional forces; the peak of this activity
the dip direction. The extension and subsid-
marked the beginning of the seafloor spread-
ence were synchronous with the rifting of the
ing in the East Sea deepwater in the Middle
Phu Khanh basin and were accompanied by
Oligocene. This was the strongest spreading
the deposition of coarse and minor effusive
phase, which occurred relatively simultane-
material. The uplift period terminated with a
ously in all Tertiary basins in the SE East Sea
regional unconformity surface at the Oligo-
area. In the Phu Khanh basin, this phase re-
cene/Miocene boundary, which marked the
sulted in the formation of grabens parallel to
differential tectonic movements in the area
the opening direction of the East Sea and cre-
(Fig. 8.7). However, many authors admit that
ated the subcontinental sedimentation envi-
the syn-rift phase in the Phu Khanh basin
ronment (Fig. 8.6). The crustal extension and
may have continued into the Early Miocene
subsidence in this area reached its peak in
(late rift). This issue needs to be addressed
the Oligocene. All major structural elements,
and clarified in the future.
positive or negative, in the Phu Khanh basin
Regional post-rift subsidence
were formed in the main syn-rift phase with
Figure 8.7. Isodepth map of the top of Oligocene, Phu Khanh basin
Figure 8.6. Isodepth map of acoustic basement, Phu Khanh basin 255
The Petroleum Geology and Resources of Vietnam
Thermal subsidence started in the Early Miocene and gradually extended eastwards and is considered as the onset of post-rift activities. The reduction in the rate of uplift of the SE Indochina block at this time caused reduced sinistral movements along the Song Hong faults and also marked the end of the rotation of blocks at a large scale. During this time, the East Sea spreading direction changed from N-S to NW-SE and the polarity of the East Sea subduction reversed from eastwards to westwards. In the middle and late Middle Miocene, two significant tectonic events occurred: intra-basin setting inversion, probably caused by the Asian – Australian plate collision, followed by the most pronounced period of regional erosion and/or non-deposition that is clearly seen as uncon-
Figure 8.8. Isodepth map of the top of Miocene, Phu Khanh basin
formities on seismic sections. In the Middle Miocene, the main stress field was transpres-
basin, the Late Miocene erosional surface
sional, which resulted in tectonic inversion
was unconformably covered by younger
and the formation of flower structures in the
fine-grained sediments of the Pliocene -
sedimentary section. Along the major faults
Quaternary formations. However, because of
crossing the slope of the Phu Khanh basin,
the short duration and weak intensity of the
the effects of transtensional deformation are
unconformity, the Late Miocene - Pliocene
also clearly observed.
boundary is very difficult to be recognized on
In the Late Miocene, all the East Sea re-
seismic sections.
gion experienced compressional forces; these
Apart from these view points, geologists
forces combined with the dextral faults on the
from the Ha Noi National University (Phan
Vietnam continental shelf were probably the
Van Quynh and Ta Trong Thang) believe that
dynamic factors that produced the temporal
the Phu Khanh basin was formed in an orogenic
uplift and partial inversion of the Phu Khanh
trough at the beginning of the Paleogene with
basin at the end of the Late Miocene and cre-
the filling of continental molasse formations
ated locally observed erosional uncomformi-
(orogenic phase at 50 million years ago) and
ties (Fig. 8.8).
the basin architecture developed mainly by
In the Pliocene, the transgression signifi-
the pull-apart mechanism along deformation
cantly influenced the East Sea area. Similar to
zones at the boundaries of the Indochina
other basins in the region, in the Phu Khanh
extrusion wedge. 256
Chapter 8. Phu Khanh Sedimentary Basin and Petroleum Potential
3.3 Fault characteristics
pretation results, the thickness of Tertiary sediments varies from 500 m along the west-
The Phu Khanh basin was formed and
ern margin to 7,000 - 8,000 m in the deep de-
controlled by 3 major fault systems: a NE-
pressions at the eastern part of the basin. The
SW trending system in the north, a N-S trend-
predicted stratigraphy of Phu Khanh basin is
ing system along the outer margin of the Da Nang shelf (longitude 109o30’) and a NW-
as follow (Fig. 8.9).
SE trending system in the south. The N-S
4.1. Pre-Tertiary basement
fault system is divided by some authors into
The Pre-Tertiary basement in the Phu
3 parallel faults, along the 109o30’, 110o10’,
Khanh basin is comprised of igneous and
and 110o20’ longitudes. These fault systems
metamorphic rocks with different composi-
follow deep depressions in the basement and
tions and ages, including granitic weathered/
thus control the main depocenters. Because
fissured formations extending from the adja-
the Phu Khanh basin formation was mainly
cent Phan Rang shelf, Tuy Hoa shear, and Da
governed by transtensional stress fields and
Nang shelf.
developed along the megashear zone at the
4.2. Paleocene - Eocene (?)
boundary between relatively rigid continental blocks and the more mobile spreading
The Paleocene – Eocene(?) deposits were
zone of the East Sea, the tectonic style of all
accumulated in grabens and half-grabens, and
fault systems are pronouncedly different and
are comprised mainly of coarse clastic sedi-
are conditioned by the change in stress along
ments and bedded conglomerates in the basal
the fault segments with varying directions of
part. On seismic sections, they are character-
strike within the megashear zone.
ized by moderate to high amplitude and poor continuity reflections.
Normal tensional faults developed along the shelf margins and older fault system
4.3. Oligocene
trends were reactivated in the following tec-
The Oligocene sediments unconformably
tonic phase.
overlie Eocene(?) deposits and are comprised
4. Stratigraphy
of fine-grained sediments such as sands,
As above mentioned, in the Phu Khanh
clays, and some intercalated coal beds. On
basin, no exploration drilling has been car-
seismic sections, these formations appear
ried out to date. Therefore, the stratigraphic
as moderate amplitude, low frequency, and
age of unconformities etc. are based only
poor continuity reflection packages. In some
on the geophysical interpretation correlated
areas, they appear as chaotic and high ampli-
with well data from the Song Hong, Cuu
tude reflections. The thickness of Oligocene
Long, and Nam Con Son basins. Due to this
sediments varies from hundreds of meters at
circumstance, the interpretation of the basin
the basin margins to thousands of meters in
stratigraphy differs from author to author. To
the central part of the basin. The organic rich
resolve this issue, it is necessary to have well
lacustrine and lagoonal black shales are the
data in the area. According to seismic inter-
potential source rocks in the Phu Khanh ba257
The Petroleum Geology and Resources of Vietnam
Figure 8.9. General lithostratigraphic column of the Phu Khanh basin 258
Chapter 8. Phu Khanh Sedimentary Basin and Petroleum Potential
sin.
as parallel or sub-parallel reflectors with low
4.4. Miocene
to high amplitudes and moderate frequency and continuity. The coastal area is character-
The Miocene succession is thought to
ized by onlap reflectors, while the deepwater
be made up of mainly detrital clastic deltaic
region is dominated with downlap reflectors.
sediments with intercalated marine and shal-
In the western and northern shelves of
low marine sediments. In the east part of the
Phu Khanh basin, the Middle Miocene sedi-
Da Nang and Phan Rang shelves, shelf car-
ments are generally dominated by clastic for-
bonates and reef carbonates are developed.
mations due to the close proximity to the ter-
Based on the seismic reflection characteris-
rigenous sources of adjacent land masses. In
tics, the Miocene succession can be divided
the south, the Oligocene and Lower Miocene
into 3 groups: Lower, Middle, and Upper
lagoonal sediments are overlain by Middle
Miocene formations.
Miocene clay, sand, and carbonate platform
The Lower Miocene section unconform-
deposits.
ably overlies the Oligocene formations. On
Miocene turbiditic formations are wide-
seismic sections, this package is identified
spread. Together with submarine alluvial fans
by sub-parallel to parallel, low to high am-
that formed along the dip slope of Phu Khanh
plitude, moderate continuity and moderate to
basin, they are probable reservoirs with good
steeply dipping reflections. In the northern
petroleum accumulation capacity. Further-
part of the basin, the wedge-shape reflection
more, the carbonate platform unit is well de-
packages suggest the presence of alluvial
veloped along the eastern shelf margin. The
fans or deltaic fans; while the high amplitude
carbonate buildup blocks that jutted out from
reflections indicate the existence of platform
the ancient sea level are found sporadically
carbonate formations.
on seismic sections and they are usually el-
The Lower Miocene formations are
evated fault blocks. Reef breccias at the reef
mainly terrigenous sediments with occasion-
front associated with carbonate reefs and
al carbonate deposits. The depositional envi-
buildups accumulated and developed along
ronments are deltaic and lacustrine in which
the reefs themselves are also the reservoirs
coals, shallow marine and paralic sediments
worth of attention although they are not as
are deposited. The Lower Miocene organic
abundant.
rich black shales and coal shale beds belong
The Upper Miocene sections are iden-
to the lacustrine and lagoonal facies and are
tified by parallel to slightly divergent reflec-
the source rocks in the Phu Khanh basin. The
tions with low to moderate amplitudes and
thickness of lower Miocene formations in the
good continuity in the western basin; these
deep part of the basin can reach more than
sediments have a sigmoidal and wedge-
2,000 m.
shaped form on the shelf slope towards the
The Middle Miocene successions are
open sea. Some grabens and half-grabens
identified and correlated on seismic sections
are filled with wedge-shape sediment bodies, 259
The Petroleum Geology and Resources of Vietnam
5. Hydrocarbon System
which have steeply dipping reflectors dipping away from the bounding faults. The sediment
Due to the lack of well data in the study
bodies are thick near the bounding faults and
area, estimates of hydrocarbon system of the
thin or wedge out away from the faults. The
Phu Khanh is based on the knowledge of geo-
wedge-shape bodies are interpreted as a se-
logical well data from southern Song Hong,
ries of alluvial fans or fan deltas that prograde
northeastern Cuu Long, and southern Nam
into graben and half-graben lake systems.
Con Son basins, which have commercial dis-
The subsidence of the Phu Khanh basin
coveries and producing fields. The main ele-
following the interruption of sedimentation
ments of Phu Khanh hydrocarbon system are
in the late Middle Miocene and the abundant
described in the following section.
terrigenous sediment supply, especially from
5.1. Source rocks
the Song Hong estuary, caused a regional
Geochemical analysis of oil samples col-
marine transgression of the shelf margin
lected from well 120-CS-1X in the southern
with decreasing intensity southwards. The
Song Hong basin indicates the presence of
Upper Miocene sediment thickness of the
non-degraded oil associated with biodegrad-
Phu Khanh basin can reach up to 3,000 me-
ed oil; oil was generated from terrestrial veg-
ters. The uppermost part of this succession
etal sources that accumulated in deltaic or
is comprised of marine deltaic, beach, or
swamp environments. Another geochemical
coastal sediments while to the west there are
analysis of oil samples extracted from car-
fluvial and non-marine delta formations. In
bonaceous mudstones in well 121-CM-1X
the lowermost part of the section, slope tur-
suggests that oil was generated from other
bidites are laterally replaced by more distal
secondary source rocks that accumulated in
submarine fans, which may form valuable
an anoxic marine environment.
stratigraphic traps.
The study of oil seeps in the Thi Nai
The Pliocene - Quaternary sedimentary
lagoon (Quy Nhon, Binh Dinh) conducted by
succession consists of unconsolidated sands,
VPI and GEUS specialists shows that samples
muds, and platform and deep water clays
collected from weathered/fractured granites
related to the creation of the East Sea. On
have a mixed carbonaceous and terrestrial
seismic sections, these sediments are easily
oil origin with the presence of oleanane
identified and correlated with other Pliocene
and 30-norhopane. Therefore, oil may have
– Quaternary sediments of the East Sea for-
migrated from Tertiary source rocks in the
mation in Vietnam. In the eastern part, thick
Phu Khanh basin (Fig. 8.10). Oil/bitumen is
wedges are usually found, characterized
also found in fractures of several samples of
by slope sediments and in the western part,
Cretaceous granites collected at the eastern
these rocks appear as parallel reflectors with
Thi Nai lagoon. Results of geochemical
good continuity, moderate amplitudes, and
analysis demonstrate that it is biodegraded
low frequency, related to inner to outer shelf
oil with biomarkers indicating both terrestrial
facies in the region.
and marine origins. Samples collected 260
Chapter 8. Phu Khanh Sedimentary Basin and Petroleum Potential
Figure 8.10. Analysis results of oil samples collected from weathered granites of Thi Nai lagoon (after VPI-GEUS, 2004)
from shrimp farming in the intertidal zone
identical to those of Cenozoic lacustrine
(northern Thi Nai lagoon) show the presence
oils found in well B10-STB-1X in the Song
of rather heavily biodegraded oil with some
Hong basin lying further North from the Phu
remnant biological markers. However, the
Khanh basin. Their similarity indicates that
tricyclic triterpane distribution is similar to
lacustrine source rocks may be present in
that of oil samples extracted from fractured
the Phu Khanh basin situated East of the Thi
granites mentioned above. Therefore, we can
Nai lagoon. The presence of Oligocene and
confirm that biodegraded oil has a common
Lower Miocene oil source rocks is proven
origin with oil at the site. The samples
in both the Cuu Long and Nam Con Son
collected from southern Thi Nai lagoonal
basins, i.e. in the adjacent areas to the SW
beach contain high wax and low biodegraded
and South of the Phu Khanh basin. Within
oil, with biomarkers that are simple and a
these formations, Oligocene source rocks
very high estimated hopane/sterane ratio.
in the Cuu Long basin are highly oil-prone
The characteristics of samples are largely
lacustrine, organic-rich shales. 261
The Petroleum Geology and Resources of Vietnam
Therefore, in the Phu Khanh basin, there
depth that corresponds to Ro = 1. 2% is 3,100
are two potential source rocks namely the
m measured from the sea floor. Thus, source
Oligocene and Early Miocene lacustrine
rocks in the Phu Khanh basin lie within the
clays/shales, coal and deltaic coal shale. Both
oil generation window. In the Cuu Long and
Type II and III kerogen have oil and gas gen-
Nam Con Son basins, the geothermal gradi-
eration capacity. Besides these formations,
ent varies from 2. 26 - 3.35oC/100m.
carbonaceous mudstones could be present as
Heat flow history and subsidence rate in
secondary source rocks. The high amplitude
this area are quantified using the minimal er-
reflections in syn-rift intervals commonly
ror method between computed (theoretical)
represent lacustrine sediments with good
and measured temperatures and vitrinite val-
source rock quality. In the Phu Khanh basin,
ues of real wells (15-G-1X and 04-A-1X).
the moderate to high amplitude reflections in
The computed paleo-temperature history
the syn-rift unit, which is confined to the gra-
was then compared with pyrolisis and chro-
bens and half-grabens, may represent lacus-
matography results to validate the certainty
trine sediments with source rock quality. In
of geothermal data obtained from organic
the post-rift section, the ramp trangressive
matter burial process. Study results prove
interval and the starved basin-ward portion
that oil was generated at well 15-G-1X as
of the trangressive and early highstand sys-
shallow as 1,810 m for Type II kerogene and
tem tracts within the regressive interval may
2,087 m depth for Type III kerogene. Peak of
have source rock potential. The lower sedi-
oil generation occurred at a depth of 2,737
ment accumulation rate in the Phu Khanh
m for Type II kerogene and at 2,825 m for
basin may have favored the deposition of
Type III kerogene. This information indicates
lacustrine sediments with source rock qual-
that Oligocene source rocks generated oil as
ity. However, the Lower Miocene and part of
evidenced by the abovementioned well. The
Paleogene source, however, may lie below
geochemical analysis results indicate that the
the oil window and is probably gas-prone.
maturity depth of organic matter in the north-
The geothermal data from adjacent ba-
ern part of Phu Khanh basin is shallower than
sins are used to predict the Phu Khanh basin
that in the southern part because of a higher
heat flow. In the southern Song Hong basin
geothermal gradient. The present-day organ-
(North of the Phu Khanh basin), the geother-
ic matter maturity maps are shown in figures
mal gradient varies from 3.65oC/100m (well
8.11 and 8.12.
120 CS-1X) to 3.81oC/100m (well 121-CM1X). Study results indicate that the vitrinite
In NOPEC evaluation (1993), pseudo-
reflectance values attain only 0.45% at 1,650
wells are constructed on all seismic lines with
m buried depth, a temperature not sufficient
input data obtained from wells 15-G-1X and
to generate hydrocarbons. Based on observed
04-A-1X. The highest computed temperature
and modeled vitrinite values, it is possible that
value at the base of the Upper Oligocene is
the peak oil generation window (Ro = 0.6%)
361oC in block 123. The vitrinite reflectance
is at 1,900 m buried depth. The oil window
values computed at the base of the two main 262
Chapter 8. Phu Khanh Sedimentary Basin and Petroleum Potential
Figure 8.12. Maturity map at present day of Miocene source rocks (after Petrovietnam, 2004)
Figure 8.11. Maturity map at present day of Oligocene source rocks (after Petrovietnam, 2004)
source rock formations in the lowest and the
clastics, and carbonate complexes.
highest part of the Upper Oligocene vary
Weathered/fractured reservoir rocks
from 0.2% Ro at pseudo-well 53 (block 125)
In Vietnam, weathered/fractured base-
to 5.47% Ro at pseudo-well 67 (block 123).
ment (granite, granodiorite) is widely known
In the southern half of the Phu Khanh basin,
as the major reservoir rock. Its thickness can
the average computed vitrinite reflectance is
vary from tens to over thousands of meters.
3.87% Ro. Thus, for Oligocene hydrocarbon
In the Cuu Long basin, weathered/fractured
source rocks, the majority of this area is prob-
granitic basement is the most important res-
ably gas-prone and is in the gas window.
ervoir which makes up 80% of the Bach Ho
5.2. Reservoirs
(White Tiger), Rong (Dragon), Rang Dong
Based on the geological analogy between known
adjacent
basins,
(Aurora), Hong Ngoc (Ruby), and Su Tu Den
Petrovietnam
(Black Lion) field reserves. This play type is
geologists suggest that the Phu Khanh basin
also found to contain oil in Dai Hung (Big
contains 3 potential reservoirs, including
Bear) field of the Nam Con Son basin. Pre-
the weathered/fractured Pre-Tertiary rocks,
Tertiary carbonate reservoir rocks are en263
The Petroleum Geology and Resources of Vietnam
countered in well B-10 in the Hanoi trough
In the Miocene, deltas retrograded north-
and in the Bach Tri structure in Song Hong
westwards, and the depositional environ-
basin.
ment most likely to changed to marine setting. Sandstone formations became the com-
Basement highs disrupted by faults are
mon reservoirs in the west part of the basin
widely distributed in Phu Khanh basin, while
and were deposited in submarine slope fan
the overlying Oligocene sedimentary cover
systems. In the Lower Pliocene, turbidites
acts as good seal (Fig. 8.13).
were developed on the slope flanks, creating
Clastic reservoir rocks
important clastic reservoirs for lithological -
Sandstones, the most common reservoirs
stratigraphic traps.
in Tertiary sedimentary basins in Vietnam,
Carbonate reservoirs
are distributed from the Oligocene to the Lower Pliocene. In the Oligocene and the
In sedimentary basins adjacent to the
lower part of the Lower Miocene sections,
Phu Khanh basin, carbonate reservoirs are
sandstones are deposited in non-marine del-
of the Middle Miocene to Late Miocene in
taic, fluvial, and channel environments. They
age (Figs. 8.13, 8.14, 8.15). Gas-bearing
become major reservoir formations of aver-
carbonates have been discovered in many
age to good quality, depending upon their
wells drilled in the southern part of Song
depth. Sandstones in fluvial - alluvial fans
Hong basin (118-CVX-1X, 121CM-1X) and
may also act as good reservoirs.
in Nam Con Son basin (04 A-1X, Dai Hung,
Figure 8.13. Geological section along the VOR-93-209 seimic line showing traps - tilted and rotated fault blocks in the basement, reservoirs (fractured basement, sandstones and carbonate) and seals (Upper Miocene - Pliocene siltstones and shale/clay), (NOPEC, 1993) 264
Chapter 8. Phu Khanh Sedimentary Basin and Petroleum Potential
Figure 8.14. Geological section along the seimic line VOR-93-101 showing flower structure trap in Oligocene - Middle Miocene formations Reservoirs: Lower - Middle Miocene sandstones and carbonate (NOPEC, 1993)
Lan Tay, Lan Do fields). These reservoirs
of excellent reservoir quality. In Phu Khanh
have both primary and secondary porosities,
basin, Miocene carbonate reservoirs are
fracture and vuggy porosities and thus are
mainly distributed along the East Da Nang
Figure 8.15. Geological section along the VOR-93-103 seismic line showing traps - Middle Miocene carbonate build-ups/reefs and seal rocks - Pliocene siltstones and shale/clay), (NOPEC, 1993) 265
The Petroleum Geology and Resources of Vietnam
and Phan Rang shelves.
part of the Phu Khanh basin could be the
5.3. Seals
migration of hydrocarbons along deep faults. Moreover, in several places, unconformities
The regional seal in the Phu Khanh basin
could also act as the conduits for lateral
is the Pliocene - Pleistocene marine shale/
migration.
clay. The thickness of this sequence attains
5.5. Petroleum plays and traps
its maximum value in the depocenters and minimum value in the adjacent uplifted
On the basis of proven petroleum plays in
zones. The 200 - 300 m thick shale sequence
the Vietnam continental shelf, the Phu Khanh
in the upper section of the Lower Miocene
basin is predicted to contain the following
corresponding to the Rotalia shale in the Cuu
plays: fractured/weathered Pre-Tertiary base-
Long basin also acts as a good regional seal.
ment, Oligocene clastics, Miocene clastics,
Besides these regional seals, shale and silt-
and Miocene carbonate plays. Besides, NO-
stone interbeds within Oligocene and Mio-
PEC specialists [21] also introduce another
cene reservoirs and along the fault surface
play type named the near basement basaltic
are also expected to act as good local seals.
play.
5.4.
Migration and charge
Fractured/weathered play or Pre-Tertiary play (play 1)
In Phu Khanh basin, hydrocarbons generated from mature kitchens can migrate
Pre-Tertiary play in the Phu Khanh basin
upwards into the overlying traps through
consists of the Cretaceous fractured granites,
sandstone bodies and along fault surfaces
possibly similar to the Pre-Tertiary play in the
under the capillary mechanism. The most
Cuu Long basin, where oil and gas have been
important migration mechanism in the deep
discovered and are currently in production.
Figure 8.16. Hydrocarbon migration from source rocks to traps (after VPI-GEUS, 2004) 266
Chapter 8. Phu Khanh Sedimentary Basin and Petroleum Potential
Source rocks may be Oligocene lacustrine
The abovementioned structures are com-
shales rich in organic matter either onlap-
monly encountered in those regions where
ping and/or burying basement high blocks.
strike-slip faults are developed.
These rocks also act as both the lateral and
Miocene clastic play (play 3)
vertical seal for the basement high traps. In
Source rocks that supply hydrocarbons
Phu Khanh basin, the fractured/weathered
for the Miocene clastic play include the Oli-
basement play is thought to distribute along
gocene lacustrine shale and siltstone, Early
the 1090 fault zone (along the eastern por-
Miocene coals, and calcareous mudstones.
tion of the Da Nang shelf and the Phan Rang
Coals in swamps and delta plains as well as
shelves), and the Tuy Hoa shear zone.
calcareous mudstones are the gas generating
Oligocene clastic play (play 2)
source rocks in the Phu Khanh basin.
In this play type, source rocks are organic
Reservoirs of this play consist of delta-
rich shales and coals within Oligocene and
ic, fluvial, and shallow marine sand bodies.
Eocene (?) formations. These source rocks
Sedimentary sequences from the Lower Mio-
should generate both oil and gas. However,
cene to Middle Miocene are probably depos-
in the deep Phu Khanh troughs, these rocks
ited in littoral and inner shelf setting. Further
are deeply buried to an extent that they are
to the East, they are deposited in outer shelf,
mainly in the gas window.
slope, and deep marine environments. Early
Reservoir rocks consist of sandstones ac-
Miocene sandstones are usually of littoral
cumulated in lacustrine, fluvial, deltaic, and
and shelf facies with good reservoir qual-
alluvial to shallow littoral environments. The
ity. Slope and seafloor sandstones are only
Early Oligocene to Late Oligocene fluvial
seen in the Middle Miocene section. The Up-
sandstones of average to good porosity are
per Miocene section is dominated with tur-
the potential objectives in the area.
bidites, which are widely distributed in the deeper parts of the Phu Khanh basin.
Seals are shale and siltstone layers intercalated within the Middle to Upper Oligo-
Seals (local) of this play are claystone
cene and Lower Miocene formations.
and siltstone interbeds within Miocene formations. Trangressive shale sequences are
The dominant Oligocene trap types are
the Top Miocene seal.
elongated anticlines and faulted anticlines. These traps are mostly sealed by fault sur-
In the Miocene, dipping fault blocks are
faces. Tilted and rotated fault blocks are the
the common trap types, which are usually
common trap types in Oligocene formations
three-way closure against a fault or two-way
of the Phu Khanh basin. These trap types are
closure with two sealing faults. Stratigraphic
usually two way or three way closures against
traps such as alluvial and/or fluvial fans, sub-
faults. Flower structures are also identified on
marine slope fans, pinch-outs or truncated
some seismic sections, and are usually sealed
beds are also present in the Miocene clastic
by faults as well.
play. 267
The Petroleum Geology and Resources of Vietnam
Miocene carbonate play (play 4)
are normally reefs, buildups, and reef flank
The Miocene gas-bearing carbonate play
deposits. Secondary porosity, which plays an important role in carbonates, varies signifi-
is very common in the Nam Con Son and
cantly with the level of dolomitization, frac-
Song Hong basins, which are adjacent to the
ture intensity, and karstification.
Phu Khanh basin.
Seal of the carbonate play is the Mio-
Source rocks of this play consist of Oli-
cene to Pliocene calcareous shale and marine
gocene lacustrine shale and Early Miocene
mudstone.
marine/deltaic calcareous shales.
The major traps are carbonate buildups
Carbonate reservoirs that formed from
of four-way dip closure. Non-structural traps
the end of Early Miocene to the Middle Miocene can be identified on many seismic sec-
related to lateral lithological change in plat-
tions. Reefs and carbonate buildups are usu-
form carbonates might also exist. Buildup-
ally mixed with muds and silts; thus, increas-
like reef bodies formed in the platform mar-
ing their cementing capacity while reducing
gins with good porosity and permeability are
their porosity and permeability. Besides reefs
also the potential traps.
and carbonate buildups, other carbonate fa-
Basalt play (play 5)
cies are also considered potential reservoirs.
According to NOPEC specialists, the Phu
Reef flank facies are usually limestone con-
Khanh basin may also have another play type
glomerates and coarse-grained lime sands
called the near basement basalt play. This
originated from reef talus. Inter-reef facies
play type was found to contain oil in Rong
or the shallow marine subtidal limestones are
field of Cuu Long basin.
not related to reefs or fine-grained siliciclastic sediments. However, the best reservoirs
Source rocks of this play are also the
Figure 8.17. Model of hydrocarbon system for the Phu Khanh basin (after Petrovietnam, 2004) 268
Chapter 8. Phu Khanh Sedimentary Basin and Petroleum Potential
Figure 8.18. Prospective Plays in the Phu Khanh basin (after VITRA, 2004)
Oligocene organic rich lacustrine shale.
favorable condition for hydrocarbons to be
Reservoirs are primarily made up of
charged into the reservoirs during migration.
Paleogene basaltic intrusive or extrusive units
The preservation of hydrocarbon is of
of good primary porosity and good secondary
relatively low risk thanks to the presence
porosity formed by fractruing. Traps consist
of regional and local impermeable beds
of lithological - stratigraphic traps of limited
intercalated within the reservoirs in the
aerial extent. Source rock sequences also act
same hydrocarbon system. Although faulting
as good local seals for these traps.
activities are common, gas chimneys along
5.6. Trap formation, hydrocarbon migration
faults are not seismically observed on the
and preservation
seafloor thus, these faults are thought to act
Seismic interpretation and geochemical
as seals. Biodegradation of hydrocarbons
modeling results suggest that trap formation
might also occur due to the relatively high
predates hydrocarbon migration. This is a
temperature in the basin. 269
The Petroleum Geology and Resources of Vietnam
6. Oil and gas potential
tons oil equivalent, mainly in plays 2, 3, and 4 (Fig. 8.19). These figures confirm that the
Within the past few years, the Phu Khanh
Phu Khanh basin has significant hydrocarbon
basin has been chosen as the research objec-
potential and should be extensively studied
tive by Danish, Japanese, and Vietnamese
and explored.
geologists in many projects, such as in Enreca project (Denmark) or the VPI/JGI (Ja-
However, one needs to be aware that
pan) joint study [36]. Results of hydrocarbon
these conclusions are drawn primarily from
potential, determined based upon the data
indirect information. Therefore, hydrocarbon
and information from a series of newly dis-
potential will be better assessed in the future
covered oil and gas fields in the surrounding
with the availability of new direct and quan-
area in 2003 - 2004, are generally optimis-
titative data.
tic and encouraging. Hydrocarbon shows are
7. Conclusions
recognized from oil seeps (?) in the western
Phu Khanh basin lies offshore along
area (onshore) and/or indirectly from seismic
the South central Vietnam coastline; part
sections, such as abnormal amplitude, bright
of this area belongs to the continental shelf
spot, flat spot, etc. All
petroleum
system
criteria
and the other to the continental slope and
are
the base of the continental slope. The basin
identified based upon onshore geological
was formed during the Oligocene period due
data as well as geophysical data and models
to rifting and the southwestward opening
constructed from input parameters from
of the East Sea. Gravimetric analysis and
adjacent basins combined with those from
seismic interpretation indicates that the basin
the Phu Khanh basin. Hydrocarbon potential
depocenter lies on the base of the continental
assessment results of the VITRA II project
slope and extends in the North – South
[39] suggest that the Phu Khanh basin
direction, which is approximately parallel to
occupies approximately 10% of the total
the coastline. The western continental shelf
Vietnam oil and gas resource or 400 million
is long and narrow, and is compartmentalized and complicated by the 109oE longitudinal fault system and the NW – SE trending Tuy Hoa shear zone. Sediments are primarily clastic formations with scattered coal deposits. Carbonate formations are developed in the Early and Middle Miocene sections along the eastern part of Da Nang shelf and along the Phan Rang continental shelf. The thickness of these sedimentary sections varies greatly, from hundreds of meters in the west to a
Figure 8.19. Distribution of petroleum potential based on play types (after VITRA)
maximum of approximately 8,000 m in the 270
Chapter 8. Phu Khanh Sedimentary Basin and Petroleum Potential
Phu Khanh trench.
In summary, although only being pre-
Data from adjacent basins indicates that
liminarily studied, Phu Khanh basin is con-
source rocks in the Phu Khanh basin are pri-
sidered to be of great potential and should be
marily made up of Oligocene and Lower Mio-
extensively invested and/or explored in the
cene lacustrine shales and coal shales, which
near future. Primary exploration objectives
are capable of generating both oil and gas.
might include the structural traps on local
In Phu Khanh trench, Oligocene and Lower
highs located in shallow water (less than 200
Miocene source rocks are in the gas window.
m water depth). Structural and non-structural
Given a favorable petroleum system with the presence of important potential plays such as
traps in deeper water should be subsequently
the Oligocene and Miocene sandstones, Mio-
explored when technological advancement
cene carbonates, the weathered and fractured
becomes available so that the oil and gas po-
Pre-Cenozoic basement, and the near base-
tential of the basin can be precisely assessed.
ment basalt plays, Phu Khanh basin is con-
With a favorable geographical location (close
sidered a high potential hydrocarbon-bearing
to shore and near the construction sites of
basin.
many refineries), the Phu Khanh basin enjoys
Moreover, the occurrence of gas chimneys
great advantages once oil and gas are discov-
observed on seismic sections in the eastern
ered, as it can supply hydrocarbons to the
edge of Phu Khanh basin, where water
enormous energy markets in the central, the
depth exceeds 1,000 m, reveals the possible existence of methane hydrate, which is
North, as well as the eastern South Vietnam,
considered the second most valuable source of
where the economy is currently developing
energy besides conventional hydrocarbons.
at a very fast rate.
271
The Petroleum Geology and Resources of Vietnam
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