Structural Inversion and Hydrocarbon Prospectivity along ... · Shelf Edge South of D-18 Area,...

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.

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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.

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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

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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.

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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.

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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.

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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.

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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.

Structural Inversion and Hydrocarbon Prospectivity along ... · Shelf Edge South of D-18 Area,...

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