Time lapse (4D) and AVO analysis: A case study of Gullfaks field, Northern North Sea

Abstract

A 4D seismic or time lapse survey has been used to investigate the amplitude versus offset (AVO) effects on seismic data in order to identify anomalies in the Gullfaks field for three different reservoir intervals namely the Tarbert, Cook and Statfjord reservoirs. Repeatability analysis has shown that the earlier seismic vintages are the most unreliable for amplitude anomaly analysis as normalised root-mean square (NRMS) values are greater than 50%. This is above the threshold of good and medium repeatability. Fluid substitution models show increases in both P-wave velocity and density for increasing water saturations with a maximum change of 7.33% in the Pwave velocity, and this is in line with predictions from previous work using the Biot -Gassman equations. AVO modelling for the top Tarbert Formation interface produced scenarios of increasing amplitudes with offset for the presence of hydrocarbons, which dim out with 100% brine saturation. This correlates to class III gas sands for different situations of varying Poisson's ratio across an interface, which has been previously modelled. Two anomalies were identified with one being related to increasing pressure due to water injection correlating to poor permeability around injector well 34/10-B-33. The second anomaly is a case of potential unswept hydrocarbons that displayed a consistent bright spot throughout all of the seismic vintages (in-inlines and crosslines). AVO attribute analysis of this event produced a class II anomaly. However, when comparing near and far offset seismic data, dimming effect was observed producing contrasting evidence. The dimming offset is viewed to have been as a result of poor repeatability values at far offsets. The modelling of the fluid contents in the studied formations to conform to existing literatures justifies the efficacy of the method.

Quantification of modelled 4D response and viability of repeated seismic reservoir monitoring in J-Area Field, Central North Sea

“…The Middle to Late Triassic Joanne Sandstone Member of the Skagerrak Formation is the primary producing reservoir and Middle Triassic Judy Member of the Skagerrak Formation and the Late Jurassic Fulmar Formation are secondary contributors ( Fig. 2) (Keller et al 2005;Jones et al 2015;Umoren and George 2018). The Joanne and Judy reservoirs within J-Area are believed to have been deposited in fluvial setting with channelized and sheet sands interbedded with shales.…”

“…Regional geological section of the North Sea Basin, showing the Member of the Skagerrak Formation(Umoren and George 2018) …”

Fluid substitution modelling and derived rock attribute-based discriminations of hydrocarbon and lithology: a case study of arcuate Niger Delta Depobelt, Nigeria

“…Extraction of fluid types or saturations from seismic, crosswell, or borehole sonic data requires a procedure to model fluid effects on rock velocity and density [4]. Fluid substitution modelling is done to obtain the sensitivity of the seismic response with different fluid saturation in the reservoir pores.…”

“…Although several techniques exist in the characterization of hydrocarbon field in search for probable reservoir saturated with commercial hydrocarbon for exploration, Gassmann relations are the most commonly and widely used equation worldwide. Several researchers have employed Gassmann fluid replacement modelling in the characterization of hydrocarbon reservoirs across the globe [2][3][4][5]. Since the discovery of hydrocarbon in the [20] Niger Delta, the extensive exploration activities carried out over the years have made the basin to be regarded as one of the World's major hydrocarbon provinces [2,6].…”