Pre-discovery seismic modelling and prediction of the Deep Panuke Late Jurassic carbonate bank gas discovery, offshore Nova Scotia

Abstract

Abstract Deposition of the Abenaki Formation carbonate complex began during the Middle Jurassic, shortly after the interpreted onset of western Atlantic sea-floor spreading, and continued through Late Jurassic time, resulting in thick carbonate deposits along the Jurassic shelf edge. This carbonate complex was the exploration target of 16 wells drilled from 1970 to 1989. Although there were no discoveries in the carbonate objectives, oil was discovered in Cretaceous siliciclastics in structural drape closures overlying the carbonate margin. The Province of Nova Scotia decided to undertake a technical study to further the interest in the carbonate bank objective while the shallow oil play was being developed. Subsequently, in 1989, two studies were undertaken to investigate a porosity play in the Jurassic Baccaro Member of the Abenaki Formation because significant porosity had been encountered in a number of the dry holes that targeted the carbonate margin, particularly the Demascota G-32 well. The first study modelled porosity in several wells utilizing one-dimensional (1D) and wedge models, and amplitude v. offset (AVO) models. The Demascota G-32 well was used to seismically identify and model several types of potential hydrocarbon-bearing prospects along the bank. Modelling indicated that zones of 11–14% porosity with a minimum thickness of 10 m could be detected. However, a thickness of 50 m was required to resolve the top and base of the porosity. The AVO model for the Demascota well demonstrated that an increase in amplitude with offset (AVO effect) in the presence of gas and fracturing (which dramatically decreases Poisson’s ratio) should be detectable and may be useful in identifying prospects in this type of play. The assumption of a fractured reservoir was reasonable due to the number of seismically identified faults at the front of the carbonate bank and the description of fractures in conventional cores taken in the Baccaro Member. The second study involved acquiring a pair of dip and strike seismic lines adjacent to the Demascota well and reprocessing them to preserve relative amplitudes while providing a high-resolution, stacked section, especially at the Baccaro Member level. Interpretation of these lines was undertaken in the AVO domain. An increase in amplitude with offset was observed up-dip of the Demascota G-32 well that indicated a fractured, gas-filled porous zone could exist up-dip of this well. Following these studies, 3D seismic data was acquired over the shallower Panuke Oil Field and similar studies were conducted on that dataset in 1997. As a result, Encana drilled a structural high containing bright amplitude reflectors and encountered over 100 m of net gas pay in vuggy and cavernous limestones and dolomites in 1999. This validated the earlier studies, confirming that these reservoirs could be seen on seismic data and proved that economic gas reservoirs exist in the carbonate bank. These results should encourage exploration along the extensive, sparsely drilled Late Jurassic carbonate margins bounding offshore eastern North America and its conjugates in the Central Atlantic Region.