Mapping Hydrocarbons by Integrating Seismic With Electromagnetic and Gravity Attributes
Authors: Carpenter Chris 
Published: February 2016

This article, written by JPT Technology Editor Chris Carpenter, contains highlights of paper IPTC 16879, “Mapping Hydrocarbons Integrating Seismic With Electromagnetic and Gravity Attributes: Application in the Barents Sea,” by Paolo Dell’Aversana, Stefano Colombo, Barbara Ciurlo, Johan Leutscher, and Jan Seldal, Eni, prepared for the 2013 International Petroleum Technology Conference, Beijing, 26–28 March. The paper has not been peer reviewed.

Electromagnetic attributes have been used extensively for interpreting marine controlled-source electromagnetic (CSEM) data. One of the basic attributes is called NAR (normalized-anomalous-amplitude response). Combining NAR and CSEM asymmetry represents a relatively simple way to use the benefits of both attributes jointly. 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 obtaining information about background geology (faults, structural trends, and lateral geological variations). This approach generally is based on the minimization of an objective function including the misfit between observed and predicted responses plus a regularization function. Inversion methods generally require long computation times and extensive use of parallel-computing resources. Moreover, they can produce nonunique solutions. A different but complementary approach for interpreting 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 magnitude vs. offset and the symmetry properties of the CSEM response itself. It has been demonstrated that 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 trends, presence of fault systems, depth range of density anomalies, and lateral extension of salt domes and basalts. Attribute analysis cannot be considered a substitute of inversion, especially in complex geological settings, but instead should be considered as a complementary approach. In fact, in complicated scenarios, attributes can be used as a quick and efficient tool for preliminary identification of the main features of the model (resistivity and density boundaries, presence of faults, and approximate depth range of the target). This preliminary model can be used as a starting point for optimized inversion. In this paper, the authors 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, the authors show how these attributes also can be complementary for identification of hydrocarbon reservoirs and for restricting the range of possible interpretive models. The authors use both synthetic tests and real examples for supporting their integrated method, which is based on combined seismic, electromagnetic, and gravity information.