Seismic expression of fault and fracture zones in Barents Sea petroleum reservoirs (SEISBARS)

Fault and fracture zones significantly affect the movement of hydrocarbons in subsurface reservoirs by channelling, blocking and/or baffling fluid flow. The type of impact individual fault zones have on flow is closely related to their internal structure. Mapping such fault zone architectures by using seismic methods is challenging, as structures sometimes cannot be resolved and at other times yield complex seismic responses which at present cannot be related to specific fault zone features. This, however, constitutes a potential un-tapped data source for fault zone characterization. Facilitating the extraction of information about fault zone architectures thus provides added value to already existing datasets, allowing more detailed assessment of fault impact on subsurface flow. Direct correlation between seismic responses and the geological structures and properties in subsurface fault zones is not possible. There is no other observational data with a sub-surface 3D coverage similar to seismic data that could be employed to facilitate this correlation. We therefore applied an indirect approach: Empirical databases on fault zone structure, composition and properties were compiled from outcrops observations. Spatial statistical analysis and 3D modelling tools were then used to generate models of fault zones which replicated their internal architecture and properties. The seismic signature of these models was studied by simulating seismic acquisition. Metrics from the resulting seismic images and the fault zone models were then compared in order to establish how changing model input parameters affected the seismic image The combination of having realistic 3D fault zone models and their simulated seismic images allowed us to investigate a number of fundamental questions related to the use of seismic data for fault zone characterization. A key feature is that this set-up facilitates mapping changes in seismic responses to changes in fault zone properties, thus for the first time allowing linking seismic responses to fault zone properties in 3D on reservoir scale. Our study has demonstrated that seismic data can resolve the internal structures inside fault zones. The level of detail is closely related to frequency, reaching a lower limit at less than a quarter of the dominant wavelength. At this scale of resolution, identical seismic responses can of course originate from several different structures and property configuration at finer scale, but using seismic data in the manner developed during the course of this project, provides a formerly lacking empirical constraint to the number of possible configuration, thus providing a way of significantly improving our mapping, modelling and assessment of fault zone properties in sub-surface data.

The scientific object of the project is to address seismic characterization of fault and fracture zone architectures posing a risk for hydrocarbon exploration and production. Faults and fracture zones significantly influence fluid flow in sub-surface rese rvoirs by acting as conduits, barriers or baffles. Although these features are commonly observed in seismic data, seismic data at present only yield limited information about flow-critical features and parameters of fault zones such as internal architectu re and petrophysical properties. The limited nature of direct observational data (i.e. wells) in sub-surface reservoirs makes it necessary to rely on statistical models and outcrop analogues to establish petrophysical property distributions. These model s routinely use seismic data in order to interpolate and extrapolate depositional facies architectures and properties between and away from wells, thus vastly improving our ability to forecast position and size of potential oil and gas field as well as op timizing production. At present there is no method which allows seismic data to be used in a similar way for modelling fault and fracture zones. Thus there is a significant un-tapped potential for using seismic data for this purpose. A key obstacle for doing this has been the lack of modeling tools that allow explcit modeling of fault zones as volumes. Systematic studies of how fault zone architectures and properties influence seismic response have therefore not been possible. Over the last few years Un i CIPR has developed a method for this purpose using extensive outcrop data. By forward modelling the seismic signature of these models the impact of different fault zone features can be identified and analyzed providing a new tool for mapping out fault- and fracture zone properties in sub-surface reservoirs.