Mapping porosity anomalies in deep Jurassic sandstones – an example from the Svane-1A area, Danish Central Graben

Hydrocarbon-bearing Upper Jurassic sandstone reservoirs at depths of more than 5000 m may form a future exploration target in the Danish Central Graben (Fig. 1). The Upper Jurassic sandstone play in the Danish sector has historically been less successful than in the neighbouring Norwegian and British sectors of the North Sea. This is mainly due to poor reservoir quality of the sandstones. However, the discovery in 2001 of an oil accumulation at a depth of more than 5000 m in the Svane-1 well has triggered renewed interest in the Upper Jurassic High Temperature – High Pressure (HTHP) sandstone play in Danish waters. The Jurassic plays comprise sandstone reservoirs deposited in a variety of environments, ranging from fluvial to deep marine. This paper presents a study of a minor area around the Svane-1A well in the Tail End Graben (Fig. 1). The objective was to map acoustic impedance variations and hence to identify porosity anomalies associated with Jurassic sandstone units. Interpretation in a tectonic setting such as the Jurassic HTHP petroleum system in the Danish part of the Central Graben is hampered by low seismic vertical resolution. However, by combining regional seismic mapping with inversion results and petrophysical log analysis, such obstacles can be tackled by mapping acoustic impedance variations. Application of seismic inversion techniques for porosity prediction in sandstone is a standard geophysical tool (Dolberg et al. 2000). Petrophysical analysis of well-log data from the upper part of the Jurassic sandstones encountered in the Svane1A well shows a relationship between acoustic impedance (AI) and total porosity (PHIT), see later. This log-derived AI-PHIT relationship can be applied to transform acoustic impedance variation into porosity variation, when the acoustic impedance is predicted from seismic inversion of a 2D profile, and can be used to locate porosity anomalies associated with sandstone intervals in the area around the Svane-1A well.