Phu Khanh Sedimentary Basin and Petroleum ...

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


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


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


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


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)


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


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


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


Figure 8.9. General lithostratigraphic column of the Phu Khanh basin 258


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


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


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


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


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


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


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


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


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


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


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


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


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


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