Rock and reservoir parameters from pre-stack inversion of surface seismic data

Hüseyin Özdemir, Jesper Wissendorf Hansen, and Emma Tyler provide an example of mapping of oil-water contacts where pre-stack AVO inversion to rock physics parameters provided improved delineation whilst post stack inversion procedures including elastic inversion gave inconclusive results. A Paleocene field was discovered in year 2000 in the Danish North Sea on the flank of a salt dome. The field was further appraised by a sidetrack discovery well and two subsequent vertical wells through years 2000 and 2001. Full logging suites were acquired in all wells including shear sonic data. VSP data were acquired in the three vertical wells. The wells were positioned using a 3D seismic data set, which was not specifically processed for Paleocene targets and no AVO processing was performed. Hence it was decided to re-process the data for AVO analysis and as part of this to perform forward modelling and fluid substitution in order to calibrate the seismic anomalies to the results of the forward modelled well data and use the results as an exploration/appraisal tool. The main exploration and development challenge is to find a seismic attribute that can differentiate between hydrocarbon and water zones. Conventional and pre-stack AVA inversion analysis was also made for lithological and pore fluid determinations. The latter analysis resulted in reservoir rock properties such as Poisson’s ratio (m), hl and +l. h and + are the Lamé constants and l is the density. Other combined and weighted attributes have been computed for more accurate pore fill identification. This paper is based on our EAGE presentation (Özdemir, Hansen, and Robinson/Tyler, 2003), which also includes a brief discussion of pre-stack inversion to acoustic impedance (AI), shear impedance (SI), and density using full Aki-Richards equations (Aki and Richards, 1980; Stewart 1990; Fatti, Smith, Vail, Strauss and Levitt, 1994; Larsen, Margrave, and Lu, 1999). This inversion methodology was first introduced by Özdemir, Ronen, Olofsson, Goodway, and Young (2001) for the joint inversion of multicomponent seismic p-wave and converted wave data.