Upload
others
View
1
Download
0
Embed Size (px)
Citation preview
Oil and Natural Gas Corporation Ltd., VRC(Panvel), WOB, ONGC, Mumbai.
1 GEOPIC, Oil and Natural Gas Corporation Ltd., Dehradun
2 Oil and Natural Gas Corporation Ltd., WOB, Mumbai
E-mail: [email protected]
P-401
Structural Inversion and Hydrocarbon Prospectivity along the Shelf Edge
South of D-18 Area, Mumbai Offshore Basin
Sucheta Dotiwala*, Ravi Kant1, P.S. Basak, K. Yasodha, T. Mukherjee,
P. Prabhakar, P.H.Rao, V. Vairavan2, ONGC
Summary
The D18 and D1 fields of the Deep Continental Shelf (DCS) area of the Mumbai offshore Bassein are close to the Oligo -
Miocene Shelf edge. Both these fields are oil producers from the Mukta and Panvel formations of the Oligo- Miocene age
and have also given gas shows in the Early-Eocene – Paleocene Devgarh/ Panna formations. The South Mumbai Low, which
has accumulated a very large thickness of the syn-rift Early-Eocene– Paleocene Panna clastics, is taken as the main kitchen
of the Mumbai High – DCS area. WO-15/16 fields to the NNE of this low and D33 field to the NW of it are oil producers
from Panna Clastics from wedge-out and pinch- out traps at different intervals. The D18 field to the immediate west of the
South Mumbai Low, has given oil and gas from Mukta and lower Panvel (LVI) in 5 wells and a gas show from Panna
siltstone in one well.
A very similar geological setup, as in D33 and WO15, is seen to the south and SW of D-18 main field, which is thus,
interesting from hydrocarbon exploration point of view for the Early-Eocene – Paleocene Panna clastics wedge-outs.
This area shows the presence of sudden increase in the shelf slope, during Early Miocene age, over which have been
deposited pro-grading carbonate sequences. These pro-grading carbonate sequences have as yet to prove their hydrocarbon
prospectivity as very little study has been done on them. The area shows evidence of structural inversion after the deposition
of the Devgarh and Panna formations during the Middle to Late Eocene times. The deposition of the prograding Oligo -
Miocene carbonates seems to be after this structural inversion.
Paleo-tectonic analysis, 3D visualization and seismic attribute studies were done on PSTM seismic data and promising
hydrocarbon area has been brought out southwest of D18 field, for both Oligocene-Miocene carbonates as well as the Early
Eocene- Paleocene silici-clastic sequence, over a prominent basement high. This is expected to open an area for exploration
for Panna clastics.
Keywords: Mumbai offshore Basin, Deep Continental Shelf (DCS), Attribute analysis, 3D visualization
Introduction
The study area lies in the southern part of Western
Offshore Basin of India (Fig. 1) and encompassing 700 sq.
km. of southwestern part, in which D18 field lies. It has
been studied in details using 3D PSTM data. The analysis
is focused to explore the prospectivity of mainly Oligo-
Miocene and Early Eocene- Paleocene formations.
Hydrocarbon accumulations have been established from
three different levels of Oligocene Limestone reservoir in
5 wells in strati-structural entrapment closer to Mukta
level. Adjoining this strati-structure feature, towards the
south there appears to be a prominent shelf slope system
associated with the pro-grading carbonate sequences of
Oligo-Miocene age and the Early Eocene- Paleocene
syn-depositional clastics.
2
Structural Inversion and Hydrocarbon Prospectivity along the
Shelf Edge South of D-18 Area, Mumbai Offshore Basin
This is the main objective of the study using seismic
attribute analysis and paleo-tectonic analysis from the
3D PSTM seismic data, to prove their hydrocarbon
prospectivity and identify promising reservoir locales.
Stratigraphy
The Basement is unconformably overlain by Paleocene -
Early Eocene Panna-Devgarh equivalent sediments. The
Panna litho-facies vary from trap wash, sandstone,
siltstone, siltyclaystone, carbonaceous shale, coal to
mudstone deposited in transitional to shallow marine
environment. Bassein Formation of Middle to Upper
Eocene age unconformably overlies Devgarh Formation.
The Formation is dominantly limestone with very thin
shale layers. A regional unconformity separates Bassein
Formation from the overlying Early Oligocene Mukta
Formation, which was deposited in a very shallow open
marine environment and comprises of thick limestone
with occasional thin shale bands. The limestone shows
primary inter-granular and also secondary porosity in the
form of vertical channels and pressure solution features.
The Panvel Formation of late Oligocene age
unconformably overlies the Mukta Formation. This
sequence consists of limestone with thin shale
intercalations.
Present Study
An attempt has been made to decipher the structural style,
hydrocarbon accumulations, porosity pods etc. of the
payloads of the D-18 area. The systematic mapping of
several levels, their paleo- tectonics and cross correlation
of porosity/ saturation/ seismic impedance, to bring out
possible areas of better reservoir has been attempted.
Structural Mapping
Seismic interpretation has aided in mapping the horizons
equivalent to LVI (lower part of Panvel formation), Mukta
top (H3A), Bassein top (H3B), Panna top, some
prominent reflectors within Panna and the Basement
Top. Time maps at all these horizons were prepared
(Figure 2).
The well D-E and D-G are the only wells within this 3D
seismic volume which have penetrated Panna top. In D-G
the zone within Panna has indicated gas. Thus, there is
very little well control for mapping horizons below Panna
top. 3D PSTM seismic data interpretation has aided in
mapping the horizons at deeper levels where there is no
well control i.e. below Panna top (figure 3).
The reflectors within Panna are seen pinching out /
wedging out towards the east, northeast and the north of
the main D18 field on the rising flank of the basement
towards the WO-15-16 and D33 areas respectively. The
lowest reflector, below which the seismic signature is
patchy, has been taken as Basement top. Panna
sequence is deposited over the paleo-high, well sorted
sands are expected to be encountered here and thus, make
a good exploratory locale.
3
Structural Inversion and Hydrocarbon Prospectivity along the
Shelf Edge South of D-18 Area, Mumbai Offshore Basin
At the upper levels (H3A and LVI) a distinct hinge can be
marked at the shelf edge towards the east and south of the
D18 wells. This hinge is marked by a thick prograding
sequence which can be interpreted above the Mukta top
and below LVI top. A top of one such sequence has been
mapped as HingeHrz. (figures 2 & 3). This horizon brings
out a structural high over the Basement as discussed
earlier. The seismic signature seen for the upper part of
Mukta formation and the lower part of Panvel formation
(LVI layer) in this area are very similar to those of the
producing layers of D18 wells. The thick prograding
sequences which are seen over the basement high are
present only as much thinner extensions of the same, very
near to the southern bounding fault of the D18 main field
also, the successive Panna wedge-outs present over the
basement high are not seen here.
All Panna wedge-out limits are of exploratory interest.
Towards the west well developed Panna is observed over
a basement paleo-high. As the Panna sequence is
deposited over the paleo-high, well sorted sands are
expected to be encountered here and thus, make a good
exploratory locale.
The fault pattern has been marked accurately towards the
south of D18 using time slices from StructureCube
volume. One such slice at 2500 ms is shown in figure 3
along with the time structure map at Mukta top which
clearly shows the increase in the shelf slope angle to the
south of D18 field.
Paleo-tectonic Analysis
Paleotectonic analysis was carried out on a line passing
across the basement high towards the south of D18.
The study brought out clearly that the inversion of the
structure was at the end of Early Oligocene i.e. deposition
of Mukta formation and the on-set of progradations within
the lower part of Panvel formation took place thereafter.
The analysis indicate that the present day low towards SW
at basement level was a part of the paleo high and got
inverted at the end of early Oligocene (Mukta) due to the
reactivation of older faults. This is supported by
isochronopach maps between different formations with the
basement as indicated in figure 5.
Figure 2: Time Maps at Different Levels
4
Structural Inversion and Hydrocarbon Prospectivity along the
Shelf Edge South of D-18 Area, Mumbai Offshore Basin
5
Structural Inversion and Hydrocarbon Prospectivity along the
Shelf Edge South of D-18 Area, Mumbai Offshore Basin
The various sequences within the Panna formation are
pinching out against the paleo-high. The D18 field is
immediately to the west and WNW of the South Mumbai
Low, which is the kitchen for a large part of Mumbai High
and the adjacent fields.D18 is a prolific producer of
hydrocarbons from Mukta and Panvel formations. Thus, it
is in an established hydrocarbon migration pathway.
Towards the northeast and the northwest of the South
Mumbai Low in the WO-15/16 (figure 3) and the D33 fields
respectively, these pinch-outs have produced hydrocarbons.
The same conditions of deposition and spatial distribution
can be envisaged for the Panna clastics against the paleo-
high to the south of D18 and thus, it can be expected that
the pinch-outs/ wedge- outs of the Panna clastics here will
also form good stratigraphic reservoirs. Thick sedimentary
sequence is observed in the west and east of D-18 but the
Panna is thin on the paleo-high, as seen from the
isochronopach map between LVI and basement (figure 5B).
D-F, the deepest well in this area, was drilled up to 4350
mts encountered silts at 4152 mts and on further drilling
upto 4350 continued in carbonates which could be of
Devgarh formation, indicating that the Panna clastic section
is not penetrated in any of the wells. The seismic
section indicates that Panna clastic section is much below
than expected and Panna on this paleo high may have good
reservoir characteristics.
Figure 5:A- Seismic section passing through the southern Paleo high successively flattened at younger horizons showing
the formation of the inversion structure after deposition of Mukta formation in Late Oligocene (blue horizon). B- Present
day configuration of the Basement surface (lower surface) and the configuration of the basement at flattened successive
younger surfaces. At LVI flattening the formation of the inversion structure at Basement level is apparent. C- Isohronopachs
of sediments between different horizons and Basement. Highest thickness of sediments is seen in the South Mumbai Low
(purple) and lowest over the southwestern corner of the area i.e. over the basement high (red- orange) – except in the
isochronopach between LVI (Lower Panvel formation) and Basement, in which a relative increase in sediment thickness is
seen in the southwestern corner corroborating the structural inversion. D- Present day structural configuration at the paleo-
high. E- Time map at the Hinge horizon (part of Oligocene-Miocene carbonate progradations) showing the presence of a
four-way closure over the paleo-high. F- Same structural closure is also present at the Basement level as seen on the Time
Map near the Basement Top.
6
Structural Inversion and Hydrocarbon Prospectivity along the
Shelf Edge South of D-18 Area, Mumbai Offshore Basin
Reservoir Facies
Panna / Devgarh Formation:Well data reveals very little
details about the Panna of this area as only D-E and D-G
have penetrated the uppermost part of Panna formation
only. The nearest producing wells for Panna clastics are in
the D33 field. The coarser clastics were deposited
immediately near shorelines whereas fines were dispersed
as plumes. The younger part of Panna sequence has more
carbonate influence and can thus be differentiated as
Devgarh formation as one goes away from the provenance.
A number of hydrocarbon shows have been observed
while drilling through the Devgarh limestone in this area,
viz., D-A, D-E, D-F & D33-B. This is attributed to low
porosity development (in general 0-3% in D-33 wells & 2-
5% in D-18 wells). The older Panna stratigraphy which has
more clastics (not penetrated by D18 wells but the seismic
horizon could be traced from the D33 area) has a sand
geometry which suggests redistribution by low tidal
influence that might have been enhanced along the
embayment. The Panna sediments have been deposited in
an overall transgressive system. The envisaged overall
Panna / Devgarh depositional model is depicted in the
seismo-geological section on an arbitrary line from D33
area through D18 and the southern basement high
(figure 6).
Mukta Formation: The reservoir character in the D18
field, of the carbonate sequence of Early Oligocene age
deposited in a shallow open marine environment, is well
developed (~ 12-20% porosity). Similar characters are
expected to the south and SW as well.
Panvel Formation: Late Oligocene Panvel Formation
consists of limestone interlaminated with thin shale streaks.
The lower part of the Panvel Formation (LVI) exhibits
better porosity development (9-11% in the form of micro
and macro vugs, fracture and molds) and has proved to be
hydrocarbon bearing in five wells of D18 field. This
sequence will exhibit similar porosity trend and
hydrocarbon presence even in the south and SW of the D18
field.
During the interpretation of the reprocessed PSTM data a
prograding sequence within LVI - towards the WSW of
the D-18 field, has been observed. This has been
interpreted as the prograding carbonates on the shelf margin
(Oligo-Miocene) deposited after the structural inversion and
during a transgressive phase. The transparent seismic
signature patches within the prograding sequence could be
pods of higher porosity.
Figure 6: Arbitrary line through D-33 & D18 wells and the Paleo-High to the south of D18 field showing probable
development of sand equivalent of D33-C & Awells.
7
Structural Inversion and Hydrocarbon Prospectivity along the
Shelf Edge South of D-18 Area, Mumbai Offshore Basin
Seismic Attribute Studies
To understand the facies variation within Panvel, Mukta and
Panna formations, Hampson- Russel’s model based
constraint inversion has been attempted. This, along with
other attribute studies, depicts a more or less similar trend
of D18-A, B, C, E and H as well as south and SW of D18
field.
The window based impedance analysis of the Devgarh /
Panna section show the presence of certain stratigraphic
intervals with lowering of values indicating increase in
porosity (figure 8A). Even though, the older Panna
section has not been penetrated in any of the D18 wells,
two wells have been drilled i.e. D-E and D-H, just within
the upper part of Panna formation and both have given
gas shows. These results have been considered during the
analysis of Panna prospectivity. The producing zone of D33
wells also gives similar AAA as well as impedance
values when compared with the window attributes at the
basement high for lower part of Devgarh and upper part of
Panna formation. This is demonstrated in figure 10 thus
proving the promise of Panna prospect south of D18.
A conspicuous low amplitude transparent feature has been
observed within Devgarh Formation to the south of D18
field which may be attributed due to the possible
development of reefal limestone in this area. Several
attribute analysis have been attempted to establish the
character of the reefal body. Reflection strength and
Average Absolute Amplitude (AAA) (figure 8B) between
Devgarh & Panna indicates lowering of amplitude analysis
at the reefal body over the Basement high. Interval velocity
between Devgarh & Panna depicts lowering of velocity
over the Basement high which further gives support to the
theory that this area is a good locale for hydrocarbon
exploration.
Figure 7: Impedance studies for Lower Panvel & Mukta formations.
8
Structural Inversion and Hydrocarbon Prospectivity along the
Shelf Edge South of D-18 Area, Mumbai Offshore Basin
9
Structural Inversion and Hydrocarbon Prospectivity along the
Shelf Edge South of D-18 Area, Mumbai Offshore Basin
below the LVI top indicating the development of porosity in
the upper part of Hinge Zone. The isochronopach map for
Hinge1 horizon indicates considerable thickness of the
prograding carbonates over the southern Basement high.
The lowering of impedance values at most of the producing
wells of D18 field is taken as the bench mark which is then
compared with the impedance values to the south of the
main field. The stratigraphic levels corresponding to the
interval from LVI to upper Mukta, formation over the
southern Basement high, show similar impedance trend as
those for the producing zones of D18 wells (figure 7).
An attempt was made by the same team to study the
combined response of impedance and Gamma Ray
(Uranium free GR values used) data in the main D18 field
(Oligo-Miocene carbonate reservoirs) (figures 9A, 9B,
9C, & 9D) with the specific intention of distinguishing
the fluid filled and non-fluid filled porosity zones using
enhanced frequency data by Spectral Bluing technique of
the OPENDTECT software.
The philosophy of this technique is that impedance
variations give only the porosity distribution picture,
while, on combining this analysis with the GR and fluid
saturation data the presence or absence of fluid within the
strata can be differentiated and further used to map
porous-fluid-filled and porous- dry areas.
Cross plots between impedance, gamma ray and porosity
(PIGN)/ water saturation (SUWI) values were made for
all the wells. Low GR with higher impedance values with
low porosity indicates tight carbonates, whereas, low GR
with high porosity has lowered the impedance values. The
same range of impedance values for SWUI are interpreted
as that the fluid distribution within the limestones is not
affecting the impedance range but indicates only the
porosity variation. The cross-plot of GR/(1-phi) vs
Impedance vs. PIGN indicate lowering of GR per unit grain
volume decreased with increasing porosity and GR
increases with decrease in porosity, this indicates that
primary porosity has remained unaltered. The cross plots
also indicate that the GR per unit grain volume decreasing
with increasing porosity and vice-versa. This in turn
indicates the coherence between the grain size and porosity
which is an indicator of presence of inter-granular porosity.
It is also brought out that primary porosity also plays a
role here and in turn holds a promise that connectivity
of secondary porosity is expected to be seen in the area of
preserved primary porosity. Based on this phenomenon
seismic data inversion was carried out to identify low
impedance areas within the pay to attribute to probable
porosity development.
The study could bring out only the distribution of the
porous zones within the Mukta pays of D18 field but the
fluid distribution pattern could not be discerned accurately.
Conclusions
The paleo-tectonic studies demonstrate the existence of
a paleo-high at D18 and southwest of it, up to the deposition
of the Mukta formation and after that there is structural
inversion. The area has a good potential hydrocarbon traps
at various stratigraphic intervals due to structural inversion.
The wedge-outs and pinch-outs in the Panna clastics on
the flanks of the South Mumbai Low and especially over
the paleo-highs highs appear to be promising.
Window based seismic attributes like Reflection Strength,
Average Absolute Amplitude and Interval Velocities have
been very useful along with impedance studies to zero in on
the lower porosity locale to the south of D18 field in the
Devgarh / Panna section.
The seismic attribute studies especially, inversion studies
have helped to bring out the locales of lowered acoustic
impedance indicating the presence of good porosity almost
at par to the D18 porosity percentages for lower Panvel and
Mukta formations.
New techniques like use of GR and Impedance cross-plots
and frequency enhancement by Spectral Bluing from
OpenDtect which have been tested in the main D18 area
and been proved successful can also be tried over the
southern part of the seismic volume.
The area to the south and southwest of the producing
D18 field, near the shelf margin, shows a good promise
as a locale for further hydrocarbon exploration.
10
Structural Inversion and Hydrocarbon Prospectivity along the
Shelf Edge South of D-18 Area, Mumbai Offshore Basin
Acknowledgements
The authors are grateful to the Management of Oil and
Natural Gas Corporation Ltd. for assigning this
project, providing an opportunity to complete the project
and giving permission to present this paper.