Cairn India Limited - Cairn India Updated Shareholding Information Dec 2011

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RAVVA
Celebrating 16 Years of Technical Excellence
Innovating Development Seismic Interpretation
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Ravva Innovating Development
Seismic Interpretation
Seismic data is indispensable for exploration and
development to understand the subsurface
structures. 3D seismic data is free from off
plane reflection, increases imaging to a great
extent and provides denser sampling of the
subsurface strata structures.
Seismic interpretation provides valuable inputs
for optimal field development by precisely
mapping subsurface structures and suitably
placing producer and injector locations. Many
seismic surveys, 2D and 3D were acquired in the
Ravva block area of which, the 3D steamer 1990
was used for phase I field development. In 2000,
Ocean Bottom Cable (OBC) 3D seismic was acquired
and the data was used for the subsequent infill
development of the field.
Seismic attributes like Amplitude Verus Offset
(AVO) and impedance inversion and rock property
volumes like total porosity, clay volume and
fluid saturation, calibrated with well
information gives insights for a better placement
of infill wells and extension of field life.
The advanced seismic interpretation tools use
interactive workstations for large amounts of
seismic data by applying techniques like manual
picking, interpolation, autotracking, voxel
tracking and horizon slicing. Well-ties are
adopted to characterise the seismic signatures of
the reservoir intervals through construction of
synthetic seismograms. In Ravva, with the
availability of the well data, excellent well to
seismic ties were established. The identified
seismic events in Ravva data are correlated
blockwide in the 3D volume along with faults to
improve the structural framework and to position
the infill producers optimally. In addition to
providing excellent structural images, the
surface slices in the zone of interest provide
vital stratigraphic information for the
characterisation of the reservoir.
A Rock Physics Analysis was conducted to
understand the log and seismic responses of the
reservoir sands and shales. The ultimate goal of
the analysis was to gain insights into the
petrophysical properties of reservoir such as
lithology, porosity, and fluid content through
AVO analysis or seismic inversion.
This included depth-trend analysis, cross-plot
analysis, fluid substitution modeling, AVO
interface modeling, 2D wedge modeling and offset
synthetic modeling. AVO classification was
performed with modeled responses. In Ravva, the
Miocene oil bearing sand has been classified as
class II/III responses with good AVO gradient.
Based on the Rock Physics analysis, AVO inversion
was carried out to generate P-Impedance, SImpedanc
e,
Poisson ratio, Fluid Factor, Lambda and Mu. The
other attributes such as coherency,
spectral decomposition, stochastic rock
properties, and enhanced restricted gradient were
also used for the reservoir characterisation.
The stochastic rock properties were used for the
detailed reservoir characterisation work. It has
provided a technically definable method to
populate the properties between and away from the
well points. The method has helped to increase
the confidence level to estimate the in-place
volumes of Ravva. The effective visualisation of
3D seismic data volumes is of great value to
geoscientists, as it brings greater flexibility
and power for maximum impact on GG workflows.
The visualisation environment allows the display
of different volumes and attributes
simultaneously, which enhances the quality
of interpretation.
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Ravva Innovating Development
The power of 3D visualisation comes from volume
rendering, which uses colour and opacity to
filter seismic data attributes for selective
display in three dimensions. Opacity tool allows
the user to pick and choose which amplitudes to
display within volumes.
The 3D visualisation also includes interpretation
of seismic attributes related to rock and fluid
properties and time-lapse seismic interpretation
to trace the movement of fluids within the
reservoir during production.
This technique was used extensively in Ravva and
has helped to visualise the geobodies and
channel geometries in Pliocene and late Miocene
strata. The identified geobodies were analysed
for hydrocarbon potential in the Ravva block.
Seismic Attribute Analysis
Ravva block has many seismic volumes generated
over a period of 15 years. Amplititudes versus
Offset (AVO) attributes, saturation, effective
porosity, Continuous Wavelet Transform (CWT),
Coherence/ Variance and Enhanced Restricted
Gradient (ERG), etc. are some of the volumes,
which have been generated using state of the art
technology. The attributes are used to the best
of their potential to decipher, delineate, and
characterise the producing reservoirs as well as
the exploration targets.
Seismic attribute analysis radically changes
exploration for hydrocarbons. It facilitates
extracting the maximum amount of value from the
seismic data by providing more detail on the
subtle lithological variations of the reservoir.
They are extracted with reference to the top of
the marker or extracted window to decipher the
geological information and understand the
distribution of reservoir facies for placement
of additional development locations to recover
more oil and gas from the reservoirs or to add
more resources by suitable exploration well
locations in a virgin area. Most of the
attributes routinely run on 3D seismic data are
Root Mean Square (RMS) Amplitudes, Maximum of
Positive and Negative Amplitudes, or
Instantaneous Amplitudes extracted from the
correlated horizon.
The attached figure is an example of RMS
amplitude extraction attribute from Pre-Stack
Time Migration (PSTM) from the reservoir Miocene
section, which delineated the extent of the
reservoir sands and subsequently was proved
successful by drilling. Multi trace seismic
attributes are extracted using more than one
seismic trace as input and provide information
about lateral variations in the seismic
data. Seismic Coherence is a measure of the
trace-to-trace similarity of the seismic waveform
within an analysis window over the entire volume
of the data set. The Coherence volume/variance
cube helps in the interpretation of the
variations in the faults and sedimentary facies,
and the delineation of the sedimentary facies
zones within favourable hydrocarbon reservoirs.
The coherence slices are helpful in
the delineation and distribution of faults, and
thus, significant in the exploration and
development of oil and gas.The variance cube was
generated for the Ravva block to study the
variance among the seismic traces in the lower
late Miocene sequence. The variance cube was
flattened with reference to the mapped horizon
and horizon slices were generated. The horizon
slice corresponding to1500 msec had
clearly brought out the channel morphology with
associated faulting at this level.
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Ravva Innovating Development
Spectral Decomposition of Seismic Data
Spectral decomposition is an invaluable tool to
identify the channel geometry and associated
geological features, especially in a fluvial
environment, where morphology is the key
indicator to understand the depositional
environment. There are primarily two types of
software applications for Spectral
Decomposition-1) SWFFT and 2) CWT. Of late, CWT
has been widely used for its frequency
localisation aspects of the signal. CWT is the
analysis of the frequency of the data at local
level and does not require a window to carry out
the analysis. However, the data generated varies
with the frequency of the volumes and is blended
to highlight the anomalies associated with the
sequence to understand the morphological evidences
to arrive at the probable geometry of the
reservoir sands in order to place the
development locations. Extensive studies were
carried out in the Ravva Block to bring out the
morphology of the discontinued sands, which are
hydrocarbons bearing in lower late Miocene
sequence. This was addressed by subjecting PSTM
volume to CWT of frequencies ranging from 8 Hz to
42 Hz, thus, extracting the geological
information pertaining to the reservoir sands.
The analysis clearly brought out the channel
geometry and gave substantial insights into the
probable depositional environment of these sands
as well as the extent of the hydrocarbon bearing
sands. This helped in understanding
the opportunities available for these sands to be
of primary/secondary targets for
exploration/development.
AVO
Amplitude versus offset or AVO analysis is
perhaps the most commonly utilised direct
hydrocarbon indicator in exploration reflection
seismology. Hydrocarbon related AVO anomalies
may show increasing or decreasing amplitude
variation with offset. Conversely, brinesaturated
background rocks may show increasing or
decreasing AVO. The AVO interpretation is
facilitated by cross plotting AVO intercept
(A) and gradient (B). Under a variety of
reasonable geological circumstances, in a
well-defined background trend. AVO anomalies
are properly viewed as deviations from this
background and maybe related to hydrocarbons or
lithologic factors.
AVO anomalies have been observed prominently in
the main reservoirs of the Ravva block. The
various attribute volumes like Lambda-Dlambda,
Mu, Rho volumes have been adequately
characterised by the fluid effects. The
attributes derived from these volumes have
successfully demonstrated the efficacy of AVO and
have been used for delineation and subsequent
placement of the wells.
ERG Attribute
AVOS derived cross plotting techniques have been
invaluable in identifying hydrocarbon bearing
sands. Apart from the cross-plotting, forward
modelling studies show the response of amplitudes
with offset when substituted with different
fluids, and on calibrating the responses of
hydrocarbons and brine fluids. This phenomenon is
seen very clearly on angle stacks processed from
full stack responses of the seismic data.
Qualitative attributes of these angled stacks
will give fluid response with different angle
stacks. It has been observed that ultra far
(angles beyond 50 ) stacks indicate bright
response for hydrocarbon bearing sands whereas
near angle and mid angle stacks illuminate the
effects of the brine fluids. ERG attribute is
generated using these brightening aspects by
simultaneously illuminating the bodies
of hydrocarbon as well as those filled with brine.
This attribute was generated after forward
modelling as well as understanding the effects of
AVO vis-à-vis the reservoir rocks of middle
Miocene and sands of lower late Miocene
sequences. After delineating the channel
morphology of the sands, the fluid
characterisation is carried out by generating ERG
attribute.
The attribute identifies the probable locations
of hydrocarbon filled geobodies in the sequence
for further volume estimates to be candidates for
exploratory/development drilling.