Ravva - Cairn’s first development success story has been the bedrock of innovation, and the foundation of our success story in the country and the region. Ravva, which in Sanskrit and Telegu means “diamond” showcases the journey of growth that Cairn has been able to achieve in its business. Incidentally, Ravva is the only field in India to get such a unique name indicating the belief of the nation in it.

1. RAVVA
Celebrating 16 Years of Technical Excellence
Innovating Development | Seismic Interpretation

2. 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,
SImpedance,
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 G&G workflows. The visualisation environment allows
the display of different volumes and attributes simultaneously, which enhances the quality of
interpretation.

3. 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.

4. 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.