Feedback of balanced cross sections and gravity modeling: numerical estimation of horizon mislocations. A case study from the Linking Zone (Northeastern, Spain)

Abstract:

The lack of subsurface information in the Linking Zone (between the Iberian and the Catalan Coastal Ranges) where no seismic sections and few boreholes are available, together with the need to perform an evaluation of a potential CO2 reservoir, have motivated us to carry out a combined structural and geophysical study. The reservoir is located in the Bunt/Muschelkalk facies (Triassic in age) just underneath the Keuper evaporites (regional detachment). The expected density contrast between cover/basement/detachment rocks represent a suitable setting to apply gravity modeling.

Therefore, we designed the location of eight serial and radial cross sections over 1.50000 available geological maps, we also include bedding data (field work) and thickness and depth information from wells and previous stratigraphic profiles. Besides, gravity data were acquired along the sections to build up 2.5D models and thus, to constrain the geometry of the basement and the thickness of the sedimentary cover. Density values used in the modelling come from a database with 1470 sites (compiled and acquired).

Initially we build the balanced sections using the available geological information and applying standard geometric techniques. Regional knowledge and previous sections were also taken into account. Then, we took these sections into Oasis Montaj to fit the real and expected gravimetric signal. In this work we present the comparison of the location of certain horizons before and after that feedback. In some cases, mislocation of some horizons may reach up to 0.4 km, which represents up to 50% of the expected depth. After fitting the gravity data with balanced cross-sections we carried out a stochastic inversion that allowed reducing the uncertainty to a maximum of 0.15 km, i. e. c. 20% . Further error analysis may be focused on the double-checking with seismic section information from the industry, if and when available.

Attached figure displays an example of one of the performed sections. There, extrapolation of subsurface structures under the Ebro foreland basin based on lateral information cannot be supported by the measured gravimetric signal. The mislocations of the basement top in A and B zones reach -0.4 and + 0.8 km respectively, with critical implications for any potential CO2 storage.