Temperature History And Its Impact On AVO Responses In The Halten Terrace Area

Abstract

Temperature is a fundamentally important petroleum geology parameter affecting both i) reservoir quality preservation (diagenetic evolution) and ii) source rock maturation (hydrocarbon generation). The link between temperature and porosity is of particular interest as porosity controls heat conductivity, density and thus seismic velocities. In deeply buried sandstone reservoirs, subject to high temperature exposure ( T>70 degrees celsius), porosity typically is reduced by chemical compaction in the form of quartz cementation.

The relationship between sedimentary basin temperature history, evolving sandstone compaction/cementation and corresponding rock properties opens for seismic constrained reservoir prediction. The main purpose of this dissertation was to investigate if there is a systematic relationship between the basin temperature and seismic reservoir AVO response. If so, can it be used to predict reservoir quality (porosity)? High quality seismic data and exploration wells from the Åsgard field cluster at the Halten Terrace was used as a well-constrained test dataset. The technical approach applied in this study represents a borehole-calibrated seismic based workflow. 3D-seismic derived maps (TWT and amplitude maps) are combined with modeled time-dependent temperature history, temperature dependent AVO response and borehole-calibrated AVO response. The result of this integrated analysis represents temperature dependent and seismic AVO constrained prediction of sandstone reservoir quality (porosity).

The AVO response at the top of the reservoir (Garn Formation) was classified to be Class 1, and modeled to have developed from a Class 3, through a Class 2, to a Class 1 response through increasing temperature as a function of increasing burial depth with time. By combining the modeled temperature and porosity through time with the modeled AVO dependency on temperature, it is possible (with uncertainty) to predict at which porosity value, and at which geological time, the polarity change from negative to positive AVO intercept occurs, which is useful for reservoir oriented seismic interpretation. Furthermore, present-day AVO responses along two selected inlines were modeled, providing information about porosity variation along the inlines. The discovered AVO response trend is used to predict where the porosity is highest (and indirectly where grain size coarsest) along the inline. This porosity/ grain size prediction is in turn used to indicate the proximal to distal orientation of the reservoir deposition. In an early-phase exploration context this methodology holds potential to better predict reservoir quality distribution between and beyond well control.