P-435
Anisotropic Depth Imaging of Heera Panna Bassein
– An interpretive case study
Deepak Sareen, Parul Pandit*, A.Kavitha, C.M. Varadrajan, S.K. Das, ONGC
Summary
A significant variation within the Miocene sequences was observed in the producing wells in the southern Heera - Panna-
Bassein block of Mumbai Offshore Basin. An area of approximately 890 sq. km was identified for velocity modelling and
depth imaging. The area is having maximum occurrences of patchy carbonates, amplitude anomalies, velocity anomalies and
pseudo structures. Migration algorithms dealing with imaging are not able to handle the depth conversion issues accurately.
Imaging becomes more challenging when high density and high velocity formations are present.
It has been shown that anisotropic PSDM (APSDM) outputs are better positioned than conventional depth imaging outputs.
APSDM incorporates accurate imaging as well as depth positioning. The methodology to incorporate anisotropy (Thomson,
1986) has been illustrated by taking examples from Mumbai offshore area. Integrated analysis of 3D seismic data, well logs,
well markers and VSP data including geological inputs has been carried out to understand the depositional framework.
Keywords: Thomson’s Parameters (Epsilon and delta)
Introduction Objective of the study
The area of study is located in the southern Heera-Panna- This study was aimed to map actual velocity variation
Bassein (HPB) block of Mumbai Offshore Basin (Fig. associated with relatively thin and patchy Miocene
1). This block, positioned in the east of Mumbai carbonates (Fig. 2).
High/Platform and south of Surat Depression, has three
distinct N-S to NW-SE trending tectonic units which lose Miocene Carbonates, occurring in patches affect the
their identity in Miocene. The western block is a composite structural disposition and reflection responses of deeper
high block dissected by a number of small grabens. The levels. It has been found that these sporadic depositions lead
Central graben is a syn sedimentary sink during Paleogene to the incorrect depth values. The Miocene carbonates are
and Early Neogene. The eastern block is a gentle eastward having higher velocities than enclosing shale. Because of
rising homocline. lateral and temporal variation in velocities at shallower
levels, pseudo structures are created at deeper levels. As a
The facies distribution and the depositional setting of the result the success ratio decreases for the placement of
Miocene carbonates in the HPB area indicates development production wells within the reservoir.
of carbonate facies in structurally higher area and
alternations of carbonate/shale in the lows and graben area
(Mishra et al, 2011).
GEOPIC, ONGC, Dehradun. India.
parul1611pandit@gmail.com
Anisotropic Depth Imaging of Heera Panna Bassein
- An interpretive case study
Fig. 2: RMS amplitude map where dark blue patches indicate
sporadic depositions of high density high velocity Miocene
carbonates.
Analytical Framework
Fig. 1: Tectonic map of Western Mumbai Offshore Basin (after
Pandey et al., 1998). Most of the crustal rocks are anisotropic in nature. This is
mainly due to fracturing, layering and complex crystal
The study area is characterized by massive shale (Backus, structure of rocks Imaging addresses the proper focusing
1962) around 800-1000m underlain by thick limestone - and lateral positioning of reflectors, but does not result in a
shale alternations. The anisotropic behaviour of shale is true depth data set, even if depth migration is used (Al-
well established. Thus, an anisotropic approach was utilized Chalabi,1994; Schultz,1999).Even in totally isotropic
to evolve a reliable depth image, high resolution media, therefore, unless well data are incorporated into the
velocity volume and to address depth conversion issues velocity model (Alkhalifah & Tsvankin, 1995), there will
in the southern HPB block. probably be misties - especially due to the tendency of
picking towards higher side to discriminate against
multiples.
Fig. 3: Definition of Anisotropic Parameters
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Anisotropic Depth Imaging of Heera Panna Bassein
- An interpretive case study
Estimation of Anisotropic Parameters (δ, ε, V0a)
The anisotropic parameters include anisotropic interval
velocity, Epsilon and Delta which are Thomson’s
Parameters (Thomson, 1986). For the estimation of delta,
picked time migrated (TM) Maps along dominant reflectors
are scaled to depth maps.
The delta (δ) is calculated according to the following
relationship (Tsvankin, 2001):
δ = ½ [{∆Z I / ∆Z A} 2 -1]
I
where ∆Z is isotropy layer thickness (measured from the
structural model or from the top horizon map to bottom)
Fig. 4: Isotropic interval velocity volume v/s anisotropic interval
which is
velocity volume.
∆Z I (Formation1) = Formation1 bottom -Formation1 top
After incorporation of anisotropic parameters a
and ∆Z A is anisotropy layer thickness (measured from the better interval velocity volume was obtained (Fig 4).The
well time - depth pairs log between T1 and T2 or top velocity variation due to presence of Miocene carbonates
horizon marker to bottom).(Fig. 3) is clearly visible in the volume. APSDM was executed
taking the estimated anisotropic parameters as input.
Epsilon can be derived from Eta analysis for far offsets but Subsequently the depth section shows much improved
small inaccuracy in Eta may lead to a big inaccuracy match with the well tops (Fig 5, Fig.6, Fig 7).
in Epsilon. Thus, as a guess value we can begin with
setting, δ(delta) equivalent to ε (Epsilon).
δ =ε
The isotropic Interval Velocity (V0) is scaled with delta
volume to create Anisotropic Interval Velocity (V0a) based
on following relation ((Tsvankin, 2001)):
V0a = V0 /(1+2δ) 1/2
Application
The APSDM method described above has been applied to
3D seismic reflection offshore data. The target zone under
study is a reservoir located in Heera-Panna-Bassein block of
Mumbai Offshore Basin.
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Anisotropic Depth Imaging of Heera Panna Bassein
- An interpretive case study
Fig. 8: The gather flatness displaying the QC of isotropic
interval velocity volume v/s anisotropic interval velocity
volume.
Fig. 7: Isotropic depth section v/s anisotropic depth section passing
through Well A, B and C.
The QC of interval velocity has been provided in Fig 8.The
flat gathers and improved image validates the accuracy of
anisotropic interval velocity volume over isotropic interval
velocity volume.
Moreover the compatibility of Sonic Log with the
anisotropic interval velocity model validates the same
(Fig9).Fig. 10 shows the flowchart to estimate the values Fig. 9: Compatibility of Sonic Log (Pink color) with the
of anisotropic parameters using Anisotropic Pre stack updated interval velocity (Blue color)
depth migration.
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Anisotropic Depth Imaging of Heera Panna Bassein
- An interpretive case study
Conclusion
The utility of true depth imaging using Anisotropic
Approach has been demonstrated in the present study. It
has been found that data from most of the wells are in
agreement with well markers at major horizons. The ratio of
success in well matching is quite high. Thus the
deliverables including anisotropic velocity volume and
PSDM gathers are highly amiable for structural modeling,
inversion and attribute analysis.
References
Al-khalifah, T. and Tsvankin, I., 1995, Velocity analysis for
transversely isotropic media; Geophysics, 60, 150-1556.
Backus, G.E.,1962, Long-wave elastic anisotropy produced by
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Mishra, J.,Singh, G., Paul, R.,Rath, B. K.,Phukan,
R.K.,2011.Facies Analysis of Early Miocene Bombay
Formation in Panna-Bassein-Heera area, Mumbai Offshore
Basin. The 2nd South Asain Geoscience Conference and
Exhibition, GEOIndia2011, New Delhi, India
Pandey, J., and A. Dave, 1998, Stratigraphy of the Indian
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Fig. 10: A flow chart showing the workflow of APSDM.
Tsvankin, I., 2001, Seismic signatures and analysis of
reflection data in anisotropic media: Elsevier Science, Inc.
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Anisotropic Depth Imaging of Heera Panna Bassein
- An interpretive case study
Acknowledgment
The authors place on record their sincere thanks to Director
(Exploration), ONGC, for his kind permission to publish
this work. Valuable interactions with Sh. Vidyasagar
from IRS, Ahemdabad are duly acknowledged. Thanks
are due to Shri C. B. Yadava, CG (S), GEOPIC,
ONGC for valuable discussions. They also acknowledge
Mrs. Sunita Kashyap and Mr. M. M. Pandey for the system
support. The views expressed in this work are solely of the
authors and do not necessarily reflect the views of ONGC.
