Using spectral decomposition as a hydrocarbon indicator tool in the Norwegian Sea

Abstract

Investigation oftwo deepwatergasaccumulationsint he NorwegianSea indicatethatp hasea nd amplitude spectraderived usingspectraldecomposition canbe used ashydrocarbon indicators. Specifically,this techniquee nhancest he ability to evaluatephasea nda mplitude anomaly conformance to structurea nd complements the moretraditionalutilization ofspectraldecomposition for thinbed analysis.The 6707/10-1Luvawell on the NykHigh encountered a156mthick column ofdry thermogenic gasandwas the first deepwaterdiscovery inNorway. Despitethe factt hatt he accumulation ischaracterized byawell developed flatevent,e xamination ofpartials tack broadband3Dreflectivity dataproved inconclusivei n substantiatingthe presence ofa namplitude anomaly conformant withstructurea tall offsetr anges. Amplitude andphasetuningcubeswerederived usingspectraldecomposition from near,farandfull offset-stack3Dseismic data.Strongamplitude anomaliesmost notably atfrequenciesless than10Hz,structurally conformant withthe observed gas/watercontact,a reobserved atall offsetr anges. Phasei nstability,a ssociated withrock property differencesbetweent he gascharged andb rinesaturated portions ofthe reservoir,results inananomaly int he phasetuningcube thatisalso most evident atlow frequenciesandisalso conformant withthe observed gas/water contact.Similarspectraldecomposition evaluation ofseismic dataoverthe OrmenLange gasdiscovery gaveequally dramatic results. Traditionalamplitude extraction methodologieso nt he full stack broadbandd atasets how an ambiguous amplitude anomaly withlimited conformance to structure.Spectraldecomposition ofthe samef ull stack datasets how well defined anomaliesatfrequenciesl ess than1 2Hz,i nbotha mplitude andphasetuning cubes,conformant withthe gaswatercontactprovenbydrilling.3Df orwardmodels built usingrock propertiesfrom the Luvaa ndO rmenLange wells haveb eenp rocessed withthe samespectraldecomposition softwareandsubstantiateobserved results. Spectraldecomposition analysis ofuntested prospects ino therareaso fthe NorwegianSea show bothphasea nda mplitude anomaliesatl ow frequenciesthatappearconformant withstructure.