Subsurface Characterization of the Pennsylvanian Clare Basin, Western Ireland, by Means of Joint Interpretation of Electromagnetic Geophysical Data and Well‐Log Data

Abstract

The integration of passive electromagnetic geophysical data and well-log data for basin characterization and interpretation has been investigated in the Clare Basin, western Ireland. The Clare Basin is overmature and has a clear contrast in electrical resistivity between the Clare Shale Formation, a widespread organic rich shale unit, and the surrounding stratigraphy. The electrical resistivity distribution beneath the Clare Basin was determined by means of three-dimensional (3-D) joint inversion of three distinct and differently sensitive electromagnetic parameters: (1) the MT impedance tensor (Z), (2) the geomagnetic transfer function (T), and (3) the interstation horizontal magnetic transfer function (H). Well-log data from a local exploration well, Doonbeg-1, were analyzed by means of multivariate statistical methods identifying three groups with distinct resistivity values. The groups were propagated along the basin using the 3-D electrical resistivity model, showing those regions in the basin with significant organic content at high maturity stage. The lack of continuity of these regions supports the hypothesis of advective fluid heating as the cause of the high maturity levels. The results also help to define the geometry of the basin at depth and have identified an area within the basin, near the Loop Head, where organic-rich clay/shale is either poorly developed, and/or the organic matter is less mature and less conductive. Finally, the potential of the basin for both CO 2 storage and geothermal energy was considered, supporting the use of the Clare Basin as a potential site for geothermal energy but not for the storage of CO 2 .

Spatial gradients of geomagnetic temporal variations causing the instability of inter-station transfer functions

“…This suggests that IS-TFs should be carefully treated by checking the establishment of the plane-wave assumption or spatial homogeneity of geomagnetic temporal variations when using them for the analysis of subsurface resistivity structures. When using both MT responses and IS-TFs that are not biased by the spatial gradients of geomagnetic variations, we can interpret the subsurface resistivity structure in detail, as reported by Soyer and Brasse (2001) and Campanyà et al (2019).…”

“…The electromagnetic (EM) responses of Earth, such as magnetotellurics (MT), are often used to evaluate the subsurface resistivity structure. The inter-station transfer function (IS-TF) between horizontal geomagnetic data at two sites is one type of EM response, and has been used to analyze the subsurface structure (Egbert and Booker 1989;Soyer and Brasse 2001;Arora and Rao 2002;Campanyà et al 2019). As shown by Soyer and Brasse (2001) and Campanyà et al (2019), the advantage of IS-TFs is that they improve the consistency of the inverted model because the data from different sites are directly related.…”

“…The inter-station transfer function (IS-TF) between horizontal geomagnetic data at two sites is one type of EM response, and has been used to analyze the subsurface structure (Egbert and Booker 1989;Soyer and Brasse 2001;Arora and Rao 2002;Campanyà et al 2019). As shown by Soyer and Brasse (2001) and Campanyà et al (2019), the advantage of IS-TFs is that they improve the consistency of the inverted model because the data from different sites are directly related. IS-TFs are also used as indicators of the temporal changes of resistivity structures triggered, for example, by large earthquakes (Honkura and Koyama 1978;Honkura 1979;Beamish 1982;Hattori 2004) or crustal uplifts (Honkura and Taira 1983).…”

Controlling mechanism of shale palaeoenvironment on its resistivity and polarizability: A case study of the Longmaxi Formation in Southern Sichuan

“…As a supplementary method in oil and gas exploration, electromagnetic exploration has played a crucial role in shale gas exploration in complex terrain areas in recent years, bene ting from its advantages of low cost and short cycle. The variations in resistivity and polarizability of organic-rich shale can be effectively utilized to clarify information about shale distribution and thickness (Strack, 2014;Yang et al, 2017;Campanyà et al, 2019) and can also be employed for hydraulic fracturing monitoring (Peacock et al 2012;Thiel, 2017;Abdelfettah et al, 2018). The electrical characteristics of shale lay the groundwork for shale logging interpretation and electromagnetic exploration.…”

Geophysical Research on Rock Mass Quality Evaluation for Infrastructure Design

“…In the past years, many authors have used geophysical methods in their research (An et al., 2020; Campanyà et al., 2019; Cardarelli et al., 2007; Chambers et al., 2006; Hanafy et al., 2021; Improta et al., 2010; Kim et al., 2007; Kneisel, 2006; C. H. Lin et al., 2018; Loperte et al., 2016; Maslakowski et al., 2014; Osinowo et al., 2011; Samyn et al., 2014; Suzuki et al., 2000). Compared with the traditional geotechnical approaches, geophysical methods provide volumetric measurement via subsurface mapping without any physical disturbance in the subsurface (Demanet et al., 2001).…”