Ravva | Seismic Interpretation


Published on

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.

Published in: Technology, Business
  • Be the first to comment

No Downloads
Total views
On SlideShare
From Embeds
Number of Embeds
Embeds 0
No embeds

No notes for slide

Ravva | Seismic Interpretation

  1. 1. RAVVACelebrating 16 Years of Technical ExcellenceInnovating Development | Seismic Interpretation
  2. 2. Ravva | Innovating DevelopmentSeismic InterpretationSeismic 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 densersampling of the subsurface strata & structures.Seismic interpretation provides valuable inputs for optimal field development by precisely mappingsubsurface structures and suitably placing producer and injector locations. Many seismic surveys, 2D and3D were acquired in the Ravva block area of which, the 3D steamer 1990 was used for phase I fielddevelopment. In 2000, Ocean Bottom Cable (OBC) 3D seismic was acquired and the data was used forthe subsequent infill development of the field.Seismic attributes like Amplitude Verus Offset (AVO) and impedance inversion and rock property volumeslike total porosity, clay volume and fluid saturation, calibrated with well information gives insights for abetter placement of infill wells and extension of field life.The advanced seismic interpretation tools use interactive workstations for large amounts of seismic databy 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 constructionof synthetic seismograms. In Ravva, with the availability of the well data, excellent well to seismic tieswere established. The identified seismic events in Ravva data are correlated blockwide in the 3D volumealong with faults to improve the structural framework and to position the infill producers optimally. Inaddition to providing excellent structural images, the surface slices in the zone of interest provide vitalstratigraphic information for the characterisation of the reservoir.A Rock Physics Analysis was conducted to understand the log and seismic responses of the reservoirsands and shales. The ultimate goal of the analysis was to gain insights into the petrophysicalproperties of reservoir such as lithology, porosity, and fluid content through AVO analysis or seismicinversion.This included depth-trend analysis, cross-plot analysis, fluid substitution modeling, AVO interfacemodeling, 2D wedge modeling and offset synthetic modeling. AVO classification was performed withmodeled responses. In Ravva, the Miocene oil bearing sand has been classified as class II/III responseswith 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, spectraldecomposition, stochastic rock properties, and enhanced restricted gradient were also used for thereservoir characterisation.The stochastic rock properties were used for the detailed reservoir characterisation work. It has provideda technically definable method to populate the properties between and away from the well points. Themethod 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 bringsgreater flexibility and power for maximum impact on G&G workflows. The visualisation environment allowsthe display of different volumes and attributes simultaneously, which enhances the quality ofinterpretation.
  3. 3. Ravva | Innovating DevelopmentThe power of 3D visualisation comes from volume rendering, which uses colour and opacity to filterseismic data attributes for selective display in three dimensions. Opacity tool allows the user to pick andchoose which amplitudes to display within volumes.The 3D visualisation also includes interpretation of seismic attributes related to rock and fluid propertiesand time-lapse seismic interpretation to trace the movement of fluids within the reservoir duringproduction.This technique was used extensively in Ravva and has helped to visualise the geobodies and channelgeometries in Pliocene and late Miocene strata. The identified geobodies were analysed for hydrocarbonpotential in the Ravva block.Seismic Attribute AnalysisRavva 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 beengenerated using state of the art technology. The attributes are used to the best of their potential todecipher, delineate, and characterise the producing reservoirs as well as the exploration targets.Seismic attribute analysis radically changes exploration for hydrocarbons. It facilitates extracting themaximum amount of value from the seismic data by providing more detail on the subtle lithologicalvariations of the reservoir. They are extracted with reference to the top of the marker or extracted windowto decipher the geological information and understand the distribution of reservoir facies for placement ofadditional development locations to recover more oil and gas from the reservoirs or to add moreresources by suitable exploration well locations in a virgin area. Most of the attributes routinely run on 3Dseismic 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 andsubsequently was proved successful by drilling. Multi trace seismic attributes are extracted using morethan 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 ananalysis window over the entire volume of the data set. The Coherence volume/variance cube helps in theinterpretation of the variations in the faults and sedimentary facies, and the delineation of the sedimentaryfacies zones within favourable hydrocarbon reservoirs. The coherence slices are helpful in thedelineation and distribution of faults, and thus, significant in the exploration and development of oil andgas.The variance cube was generated for the Ravva block to study the variance among the seismic tracesin the lower late Miocene sequence. The variance cube was flattened with reference to the mappedhorizon and horizon slices were generated. The horizon slice corresponding to1500 msec had clearlybrought out the channel morphology with associated faulting at this level.
  4. 4. Ravva | Innovating DevelopmentSpectral Decomposition of Seismic DataSpectral decomposition is an invaluable tool to identify the channel geometry and associated geologicalfeatures, especially in a fluvial environment, where morphology is the key indicator to understand thedepositional environment. There are primarily two types of software applications for SpectralDecomposition:-1) SWFFT and 2) CWT. Of late, CWT has been widely used for its frequency localisationaspects of the signal. CWT is the analysis of the frequency of the data at local level and does not requirea window to carry out the analysis. However, the data generated varies with the frequency of the volumesand is blended to highlight the anomalies associated with the sequence to understand the morphologicalevidences to arrive at the probable geometry of the reservoir sands in order to place the developmentlocations. Extensive studies were carried out in the Ravva Block to bring out the morphology of thediscontinued sands, which are hydrocarbons bearing in lower late Miocene sequence. This wasaddressed 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 outthe channel geometry and gave substantial insights into the probable depositional environment of thesesands as well as the extent of the hydrocarbon bearing sands. This helped in understanding theopportunities available for these sands to be of primary/secondary targets for exploration/development.AVOAmplitude versus offset or AVO analysis is perhaps the most commonly utilised direct hydrocarbonindicator in exploration reflection seismology. Hydrocarbon related ‘AVO anomalies’ may show increasingor decreasing amplitude variation with offset. Conversely, brinesaturated ‘background’ rocks may showincreasing 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 tohydrocarbons or lithologic factors.AVO anomalies have been observed prominently in the main reservoirs of the Ravva block. The variousattribute volumes like Lambda-Dlambda, Mu, Rho volumes have been adequately characterised by thefluid effects. The attributes derived from these volumes have successfully demonstrated the efficacy ofAVO and have been used for delineation and subsequent placement of the wells.ERG AttributeAVOS 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 whensubstituted with different fluids, and on calibrating the responses of hydrocarbons and brine fluids. Thisphenomenon is seen very clearly on angle stacks processed from full stack responses of the seismicdata. Qualitative attributes of these angled stacks will give fluid response with different angle stacks. Ithas been observed that ultra far (angles beyond 50 ) stacks indicate bright response for hydrocarbonbearing sands whereas near angle and mid angle stacks illuminate the effects of the brine fluids. ERGattribute is generated using these brightening aspects by simultaneously illuminating the bodies ofhydrocarbon as well as those filled with brine.This attribute was generated after forward modelling as well as understanding the effects of AVO vis-à-visthe reservoir rocks of middle Miocene and sands of lower late Miocene sequences. After delineating thechannel 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 furthervolume estimates to be candidates for exploratory/development drilling.