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Figure 1. Location map of Blocks A-1 and A-3, offshore western
Myanmar (Green color represents the original block boundary before
relinquishment)
SEAPEX Exploration Conference 2007
Orchard Hotel, Singapore
24th 26th April 2007
A Success of the New Play Concept Offshore Northwestern
Myanmar:
Pliocene Deepmarine Turbidites of the Eastern Bengal Fan
Su-Yeong Yang, Michelle Kim Daewoo International Corporation
INTRODUCTION
Daewoo International Corporation acquired
Block A-1 and Block A-3 in 2000 and 2004
respectively offshore northwestern
Myanmar (offshore Rakhine) in the Bay of
Bengal in the Indian Ocean (Figure 1).
There had been exploration activities in this
region in 1970s by several international oil companies,
resulting in only minor
hydrocarbon shows in poor reservoirs. Since
then, the region had been barren for more
than twenty years until Daewoo started
exploration in Block A-1 in 2000. A new
concept of deepmarine turbidites play was
introduced by Daewoo and resulted in three
gas discoveries in this area, Shwe and Shwe
Phyu in Block A-1 and Mya in Block A-3.
REGIONCAL GEOLOGY
The offshore Rakhine is located in the
eastern part of the Bengal basin surrounded
by the Indian craton to the west, the
Shillong Plateau to the north, and the Indo-
Burman ranges to the east, but opens to the south, extending
into the Bay of Bengal
containing the Bengal deep-sea fan.
The structural architecture of the offshore Rakhine has been
controlled by oblique subduction
of the Indian plate beneath the Burman plate (Gani and Alam,
1999). As the Indian plate
subducts beneath the Burman plate, the NW-SE or N-S trending
Chittagong-Tripura Fold Belt
(CTFB) and the westward-migrating Neogene accretionary prism
complex are developed in
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the east of offshore Rakhine (Figure 2) (Curray, 1991). Highly
faulted and folded structure,
sometimes related with mud diapirs, are developed nearshore
Rakhine. The intensity of
tectonic deformation is relatively severe in the east and
gradually diminishes to the west
developing two anticlinal structures in Blocks A-1 and A-3, Ngwe
and Shwe in the NNW-SSE
direction. Strike-slip faults with flower structures are
observed in the Ngwe anticline and in
the south of the Shwe anticline.
PREVIOUS EXPLORATION
The previous exploration activities were concentrated in
structural highs in the shallow-water
areas. A total of seven wells were drilled offshore western
Myanmar by international oil
companies in 1970s. The main reservoir objectives were the
Miocene and Oligocene sediments transported from inland Myanmar and
deposited near the coast of western
Myanmar. The drilling results revealed that these sediments are
shale-rich deposits with minor
oil or gas shows. It was concluded that the lack of reservoir
was the main reason for failure in
the previous exploration, and there were no exploration
activities for more than twenty years
offshore western Myanmar since then.
DEVELOPMENT OF A NEW PLAY CONCEPT
2D Seismic Interpretation
After Daewoo had acquired Block A-1 in 2000, the existing
1,800km of 2D seismic data
acquired in 1970s were reprocessed. The seismic interpretation
of the 2D reprocessed data during the Study Period indicated the
existence of deepmarine turbidites in the submarine fan
system transported from the northwest to the southeast. New 2D
seismic data of 3,800 L-km
Figure 2. Tectonic setting, offshore northwest Myanmar (D.A. et.
al., 1988)
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were acquired subsequently after entering into the Exploration
Period in 2001. The seismic
sequence stratigraphic analysis using the new 2D seismic data
suggested that there are two
types of deposition in the late Pliocene, one from the northwest
as part of the Bengal fan
(lower G-series sediments) and the other from the northeast
inland Myanmar (the D-series
and upper G-series sediments) (Figure 3).
Figure 3. Two types of sediment source in the late Pliocene
deposited in Blocks A-1 and A-3
The lower G-series sediments show an elongated mounded shape
trending NW-SE direction
(Figure 4a). These sediments are characterized by high-amplitude
continuous reflections
which onlap onto a late Pliocene sequence boundary, PLSB2, near
the base of the slope and
spread out widely basinward to the southeast (Figure 4b). The
seismic facies and the external
form of this deposition represent a basin-floor-fan system
containing sand-rich deepmarine
turbidite deposits transported from the northwest. The upper
G-series sediments show a
prograding wedge shape in the eastern area of Block A-1, of
which sediments were
transported from northeast inland Myanmar and deposited in the
slope. This prograding
wedge thins out basinward to the west or southwest.
Figure 4a. Isopach of the G- series sediments showing two types
of sedimentation: a submarine fan from the northwest lower
G-series) and a prograding wedge from the northeast (upper
G-series)
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Figure 4b. Seismic section showing the PLSB2 sequence boundary
and the lower G-series submarine fan (basin floor fan)
The D-series sediments which overlie the G-series sediments were
all transported from the
northeast inland Myanmar and deposited in the slope as
prograding wedges (Figure 5a and
Figure 5b). The prograding wedges of the D-series and upper
G-series sediments show
generally low-amplitude reflections. Some high-amplitude
reflections at the D- and upper G-
series slope sediments were interpreted as shingled turbidites
with isolated sand bodies.
Three prospects were generated based on the 2D seismic
interpretation in Block A-1: Shwe,
Shwe Phyu and Ngwe which mean Gold, Platinum and Silver in
Myanmar respectively. The
lower G-series sediments showing high-amplitude continuous
reflections in stratigraphic traps
were the primary exploration objectives, whereas the overlying
upper G-series and D-series
Figure 5a. Isopach of the D-series sediments showing prograding
wedges from the northeast
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Figure 6a. Seismic section correlated with the Shwe-1 and -1A
log interpretation of total porosity (left) and gas saturation
(right), showing the D1 and D2 formations and the G3.2 and G5.2 gas
reservoirs.
sediments showing high-amplitude reflections in structural traps
were the secondary
exploration objectives.
Figure 5b. Seismic section flattened to the PLSB2 sequence
boundary to show the D-series lowstand prograding wedge
Drilling Result of the First Exploratory Well, Shwe-1/1A
The first exploratory well, Shwe-1, was drilled vertically to
penetrate the two objectives in the
D-series sediments (D1 and D2) and an objective in the upper
G-series sediments (G1)
located at the structural crest. And then the sidetrack well,
Shwe-1A, was drilled to test the
offset G5 objective which could not be reached by the vertical
well. Significant gas kicks
were observed in the mud-log data at the D-series objectives
where reflections show high
amplitude. However, there are no sand reservoirs in this
interval. The high amplitude at the D-
series objectives is due to the minor gas trapped in the shale
or siltstone at the crest of the
structural high (Figure 6a).
After the disappointing result with
the poor reservoirs at the first three
objectives from the vertical well,
the sidetrack well, Shwe-1A,
encountered gas-charged sand
reservoirs at the G3.2 and G5
formations (Figure 6a). A massive
turbidite sand unit was discovered
at the high-amplitude continuous
reflection of the G5 objective
which pinchouts toward the
structural high to the east (Figure
6b). The G5 formation was
subdivided later into the G5.1 and
G5.2 formations by detailed 3D
seismic interpretation and log
interpretation. The exploratory
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Figure 6b. Perspective view of Amplitude at Top G5 based on 2D
seismic interpretation (subdivided into G5.1 and G5.2 later). The
Shwe-1A well penetrated the eastern edge of the G5 which pinchouts
toward the structural high to the east.
well, Shwe-1A, penetrated the G5.1 sand and the following
appraisal wells penetrated the
G5.2 sand. The log interpretation of the Shwe-1A shows that the
G5.1 sand has the average
porosity of 24%, the average water saturation of 28% and the net
thickness of 20m. A series
of relatively thin sand units of the G3.2 formation, which was
subdivided from the G3
formation, also contains gas although there is no high amplitude
observed at the top of the
G.3.2 formation and this formation was not an objective. The low
amplitude at the gas-bearing
G3.2 formation is caused by tuning effect of thin sand
units.
3D Seismic Interpretation
After the Shwe-1/1A was drilled, the 3D seismic data of 1,200
square kilometers were
acquired immediately over the Shwe discovery and the Shwe Phyu
and Ngwe prospects to
locate the appraisal wells and additional exploratory wells. The
3D seismic interpretation
confirmed that the lower G-series sediments are a submarine fan
system transported from the
northwest. The horizon slice of the 3D seismic data shows a
feeder channel coming from the
northwest to develop a submarine fan of the G5.2 reservoir
(Figure 7).
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Figure 7. Seismic amplitude at Top G5.2 showing the sediments
transported from the northwest through a feeder channel and
deposited as a fan lobe
Figure 8. Gas discoveries in Blocks A-1 and A-3
Success of the New Play Concept
The drilling result of the exploratory well and the 3D seismic
data proved the new play
concept of deepmarine turbidites deposited in the submarine fan
system offshore northwestern
Myanmar as part of the eastern Bengal fan.
After the discovery of Shwe, Daewoo have made two more
additional discoveries in Blocks
A-1 and A-3 by applying this concept of the deepmarine turbidite
play: the Shwe Phyu
discovery in Block A-1 in 2005 and the Mya (which means Emerald
in Myanmar) discovery
in Block A-3 in 2006 (Figure 8).
The Daewoos discoveries have awaked interest of international
oil companies in exploration in this region. As a result, all of
the blocks offshore western Myanmar and some deepwater
blocks in the north have been already awarded to several
international oil companies and joint
ventures in last few years.
PETROLUEM SYSTEM
Reservoirs
The lower G-series sediments (G5.1, G5.2 and G6.1) of late
Pliocene time, transported from
the northwest and deposited in the submarine fan systems as part
of the eastern Bengal fan,
provide excellent reservoirs offshore northwestern Myanmar. The
G6.1 gas sand of the Mya
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discovery shows continuous reflections which extend to the
southeast with an elongated shape.
The seismic facies and the Mya-1 well data indicate that the
G6.1 sand was deposited in the
channelized fan system. The overlying G5.2 sand in the Shwe
discovery is the most
prominent gas reservoirs in this region. A widespread fan lobe,
represented by high-amplitude
continuous reflections extending a few tens of kilometers, is
developed by a long feeder
channel passing through Block A-1 from the northwest. The G5.2
fan lobe contains massive
turbidite sands in the proximal area, and the turbidites become
less sandy in the distal part of
the fan lobe (Figure 9). The G5.1 sand was deposited in a splay
lobe and is also an excellent
reservoir in the Shwe discovery.
The upper G-series sediments (G3.2 and G2.2) are mainly
channel-levee deposits transported
both from the northwest as part of the Bengal fan and from the
northeast inland Myanmar.
The G3.2 sands in the Shwe discovery and in the western part of
the Shwe Phyu discovery are
channel-levee deposits or splay lobes trending in the NW-SE
direction. Thick, porous sand
units of the G3.2 formation are developed in or near the
channels or at the splay lobe, whereas
thin strings of levee sand are deposited in the overbank/levee
environment in the Shwe
discovery (Figure 10). The G3.2 formation in the eastern part of
the Shwe Phyu discovery is
submarine fan deposits transported from the northeast (Figure
11). The size of the fan lobe
from the northeast is relatively small and reservoir quality of
the fan-lobe deposits is not as
good as the one transported from the northwest, indicating that
the depositional energy was
relatively low or the sediment source was less sandy. The
overlying G2.2 formation in the
Shwe Phyu discovery is channel-levee deposits transported both
from northeast and northwest.
Figure 9. Well to seismic correlation in the Shwe discovery: (a)
Amplitude map of Top G5.2, (b) Seismic section correlated with log
interpretation of total porosity (left) and gas saturation
(right)
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Figure 10. Well to seismic correlation in the Shwe discovery:
(a) Amplitude map of Top G3.2, (b) Seismic section correlated with
log interpretation of total porosity (left) and gas saturation
(right)
Figure 11. Well to seismic correlation in the Shwe Phyu
discovery: (a) Amplitude map of Top G3.2, (b) Seismic section
correlated with log interpretation of total porosity (left) and gas
saturation (right)
The D-series sediments transported from the northeast and
deposited in the slope as
prograding wedges above the G-series sediments do not show good
reservoirs in this region.
However, the D-series sediments or overlying sediments may
provide good reservoirs when
they are deposited in the basin as submarine fan deposits.
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Source
Although no wells have penetrated any source rock offshore
Rakhine and source rock is still
unknown, the accumulation of gas in this region already has
proved the existence of source
rock. A thick accumulation of the Miocene and Pliocene
deepmarine shale in the Bengal fan
which thickens to the west may play as source rock of the gas
accumulated offshore Rakhine.
Seals
The turbidite reservoirs are overlain by a thick section of
Pliocene deep-marine shale which
makes impermeable top seals.
Traps
All of the discoveries in Blocks A-1 and A-3 are made in the
stratigraphic traps developed in
the western flank of the Shwe anticlinal structure. The
channel-fill shale in the updip location
is the main trapping mechanism in the Shwe discovery.
Strike-slip faults or stratigraphic
pinchouts also act as trapping mechanism in the region.
CONCLUSION
Deepmarine turbidites of the late Pliocene, deposited in the
submarine fan or channel-levee
complex systems, were proved to be excellent gas reservoirs
offshore northwestern Myanmar.
The Daewoos discoveries with the new play concept have upgraded
the potential of the whole offshore western Myanmar. The future
exploration in this region needs to be targeted
for deepmarine turbidites in the basinal area.
REFERENCES
Curray, J. R., 1991, Possible greenschist metamorphism at the
base of a ~ 22-km sedimentary
section, Bay of Bengal. Geology, v. 19, p. 1097-1100.
D.A. Pivnik et al., 1988, Polyphase deformation in a
fore-arc/back-arc basin, Salin subbasin,
Myanmar (Burma). AAPG Bulletin 82, No. 10, 1837~1856
Gani, M. R. and Alam, M. M., 1999, Trench-slope controlled
deep-sea clastics in the exposed
lower Surma Group in south-eastern Fold Belt of the Bengal
Basin, Bangladesh. Sedimentary
Geology, v. 127, p. 221-236.