Dawson Informal Research Talk (DIRT) via Zoom - Carla Skinner
Dawson Informal Research Talk
Overpressure in the Sable Subbasin
In the Sable Subbasin, there is a knowledge gap around pressure distribution in sediments. Fluid pressure have been observed in Jurassic and Early Cretaceous reservoirs in over 200 wells and generally increase with depth in unexpected increments from hydrostatic pressure to immense excess pressure – a systems known as “stepped excess pressure”. The pressure systems of the Scotian Margin were extensively studied from 1989 to 2003 by several researchers without reaching a consensus on mechanisms and timing of excess pressure generation and dissipation (Mudford and Best 1989; Wielens 2003; Williamson and Smyth 1992). Hydrocarbon generation during the Cretaceous was cited as a pressure source, supported by basin modelling results. Recent thermal modelling (South Venture O-59 well) suggests ongoing hydrocarbon generation (Wong et al. 2016). In the Sable region, excess pressured sections are lithified with thin intra-formational seals, and high net-to-gross ratios of porous sandstone to tight sandstones, shales, and low permeability limestones. It is difficult to rationalize current excess pressure without late hydrocarbon generation (or another late pressure source) within the region given micro-nano Darcy flow across thin imperfect seals that should allow for pressure equilibration over hundreds of thousands to several million years.
In this study, the pressure distribution in the Sable Subbasin was investigated with a sub-regional 3D static reservoir model of the reservoirs associated for five gas fields: South Venture, Venture, Arcadia, Citnalta, and Uniacke. The model was populated with excess pressure data and lithologies interpreted from wireline logs and well tests and inspected to test the hypothesis that excess pressure distribution is controlled by reservoir connectivity, which is ultimately controlled by permeability.
Pressure and fluids were interpreted to be currently entering the system by ongoing generation of hydrocarbons. At each of the fields, a similar arrangement of reservoir connectivity and pressure distribution were observed, although the ages of the rock units involved change from field to field due to progradational advance of the shelf and progressive formation of “expansion trends”. Expansion trends are isolated, high accommodation space depocentres formed in the hanging walls of down-to-basin listric faults, which formed because of depositional loading and salt mobilization. These listric faults also set up low-relief, hanging wall, fault-bend folds - the principal hydrocarbon traps of the Subbasin. In the “deep” section, where reservoirs are stratigraphically and structurally isolated, the fluids and pressures are interpreted to be actively dissipating by mechanical leak. In the “intermediate” section, pressure equilibration within pressure cells occurs where the displacement of minor crestal faulting exceeds the thickness of minor intra-formational seals allowing for “stair stepping” up juxtaposed permeable units. In the “shallow” section, which is above the listric fault system, the reservoirs are contiguous and hydrostatically pressured.
Topic: Carla Skinner DIRT Talk
Time: Apr 29, 2021 11:30 AM Atlantic Time (Canada)
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Meeting ID: 819 9270 1820
With a new academic year comes a new line-up of DIRT Talks! These Dawson Informal Rock Talks will be taking place via Zoom in a new time slot (every second Thursday at 11:30 am AST), alternating with the research seminars. Unfortunately these will feature a lot less beer, but we still look forward to everyone coming to support the department and learn about some interesting people, research and opportunities throughout the year.
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