Chalk lithology-fluid characterization using regional broadband elastic attributes: an integrated study from the North Sea Central Graben
The chalk play of the North Sea Central Graben spans the Norwegian, Danish and UK sectors and its reserves approach 10 billion barrels of liquids and almost 5 billion boe of gas. As opposed to clastic reservoirs, the quantitative interpretation of carbonates and chalk reservoirs requires careful rock physics analysis integrated with reliable pre-stack elastic attributes. Such attributes need to be correctly calibrated with well log information to understand the variability of the chalk reservoir facies. The acoustic impedance attribute correlates well with porosity in carbonates, low acoustic impedance equating to high porosity (Landrø et al., 1995; Santoso et al., 1995; D’Angelo et al., 1997; Li and Downtown, 2000; Abramovitz et al., 2011). However, a number of recent wells drilled in Norwegian waters have encountered high porosity in dry, non-hydrocarbon-bearing chalk reservoirs (Gennaro and Wonham, 2014) which contradicts the well-established mechanism of porosity preservation through the presence of hydrocarbons. This has triggered the development of new theories (Oxnevad and Taylor, 1999; Mork et al., 2018) and highlights the importance of having additional independent attributes such as Vp/Vs to improve the detection of hydrocarbons and to optimize well placement and derisking in near-field exploration. A large regional GeoStreamer multi-sensor dataset covering 18.000 km2 in the central North Sea Central Graben has recently been depth processed as a contiguous dataset (GeoStreamer PURE). One of the main objectives of the reprocessing was to better image the main producing chalk fields in the Upper Maastrichtian and Danian layers (Ekofisk and Tor Formations) such as Ekofisk, Eldfisk, Valhall, Dan-HalfDan, Tyra and Gorm, using advanced depth imaging techniques. A bespoke Quantitative Interpretation (QI) workflow for chalk characterization has been developed and implemented, demonstrating the value of high-quality pre-stack broadband seismic data integrated with extensive well data analysis for the mapping of porosity variations and for predicting hydrocarbon accumulations in these very challenging chalk reservoirs.