Mapping Hydrocarbons Integrating Seismic with Electromagnetic and Gravity Attributes: Application in the Barents Sea

Abstract Electromagnetic attributes have been extensively used for interpreting marine Controlled Source EM data (CSEM). One of the basic attributes is the so called NAR, Normalized Anomalous Amplitude Response. A few years ago a new CSEM attribute has been introduced for fast and accurate identification of the boundaries of resistive layers. It is based on the asymmetry between the in towing and the out towing responses. Combining NAR and CSEM asymmetry represents a relatively simple way to use jointly the benefits of both attributes. The efficiency of this methodology is improved if seismic and gravity data are included in the work flow. This integrated approach based on seismic data, combined with electromagnetic and gravity attributes has been applied in a complex exploration area in the Barents Sea. Introduction Inversion of geophysical data measured at surface represents a quantitative approach for estimating physical properties of hydrocarbon reservoirs and/or for obtaining information about the background geology (faults, structural trends, lateral geological variations and so on). This approach generally is based on the minimization of an objective function including the misfit between observed and predicted responses, plus a regularization function (Tarantola, 2005; Zhdanov, 2002). Inversion methods generally require long computation times and massive utilization of parallel computing resources (Newman and Hoversten, 2000). Moreover they can produce non unique solutions. A different, complementary approach for interpreting the geophysical observations is based on the analysis of quantities, often called attributes, extracted from measurements by simple mathematical manipulation. Examples of electromagnetic attributes recently applied in hydrocarbon exploration are the gradient of CSEM (Controlled Source Electromagnetic) magnitude vs. offset (Dell'Aversana, 2007) and the symmetry properties of the CSEM response itself (Dell'Aversana and Zanoletti, 2010). It has been demonstrated how CSEM attributes allow, at least in relatively simple scenarios, accurate and fast identification of reservoir boundaries. Also in the gravity domain attributes can work efficiently for rapid identification of important general features, such as basement trend, presence of fault systems, depth range of density anomaly, lateral extension of salt domes and basalts (Cella et Al., 2009). Attribute analysis cannot be considered substitutive of inversion especially in complex geological settings. They should be considered as complementary approaches. In fact in complicate scenarios attributes can be used as a quick and efficient tool for preliminary identification of the main features of the model (resistivity and/or density boundaries, presence of faults, approximate depth range of the target…). This preliminary model can be used as a starting guess for optimised inversion. In this paper we will show how several types of electromagnetic and gravity attributes can cooperate with seismic information for rapid delineation of the main geological trends in complex exploration areas. Moreover we will show how these attributes can be complementary also for identification of hydrocarbon reservoirs and for restricting the range of possible interpretative models. We will use both synthetic tests and real examples for supporting our integrated method based on combined seismic, electromagnetic and gravity information.