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Influence of porosity and pore fluid on acoustic properties of chalk: AVO response from oil, South Arne Field, North Sea

¹ Geological Survey of Denmark and Greenland (GEUS), DK-1350 Copenhagen K, Denmark (e-mail: pj@geus.dk)
² Ødegaard A/S, DK-2200 København N, Denmark
³ Technical University of Denmark (DTU), DK-2800 Lyngby, Denmark
⁴ Stanford University, California 94305-2215, USA
⁵ Present address: Denerco Oil, DK-2840 Holte, Denmark
⁶ Present address: Dong A/S, DK-2970 Hørsholm, Denmark

Amplitude versus offset (AVO) inversion provides direct evidence for the presence of light oil in high-porous chalk in the South Arne Field, North Sea. The elastic properties of the chalk were estimated at three scales by analysing core data, log-readings and AVO-inversion results. The velocity–porosity relation of the core data matches a modified upper Hashin–Shtrikman model for Ekofisk Field chalk and the model is extended to 45% porosity. A small clay content reduces porosity without affecting chalk stiffness and this content can be estimated from the water saturation, which is controlled by silicate content and particle sorting in the zone of irreducible water saturation. The model is, thus, scaled according to clay content estimated by the water saturation. Based on comparison with the model and measurements on core samples, it is found that the sonic log data represent chalk characterized by forced displacement of the oil by mud filtrate and, thus, a much higher water saturation than estimated from, for example, a shallow resistivity log. Forward modelling of the acoustic properties of the virgin zone results in a characteristic pattern of Poisson ratio versus depth. This pattern agrees with inverted seismic data, whereas it is not captured by conventional fluid substitution.

KEYWORDS: acoustic properties, chalk, fluids, North Sea, porosity

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