Abstract
Since 2006 wastewater has been injected below the Val d’Agri Quaternary basin, the largest on-land oilfield in Europe, inducing micro-seismicity in the proximity of a high-rate injection well. In this study, we have the rare opportunity to revise a massive set of 2D/3D seismic and deep borehole data in order to investigate the relationship between the active faults that bound the basin and the induced earthquakes. Below the injection site we identify a Pliocene thrusts and back-thrusts system inherited by the Apennines compression, with no relation with faults bounding the basin. The induced seismicity is mostly confined within the injection reservoir, and aligns coherently with a NE-dipping back-thrust favorably oriented within the current extensional stress field. Earthquakes spread upwards from the back-thrust deep portion activating a 2.5-km wide patch. Focal mechanisms show a predominant extensional kinematic testifying to an on-going inversion of the back-thrust, while a minor strike-slip compound suggests a control exerted by a high angle inherited transverse fault developed within the compressional system, possibly at the intersection between the two fault sets. We stress that where wastewater injection is active, understanding the complex interaction between injection-linked seismicity and pre-existing faults is a strong requisite for safe oilfield exploitation.
Highlights
The VDA is a Quaternary basin developed within the southern Apennines thrust-and-fold belt (Fig. 1A), when an extensional phase replaced a Mio-Pliocene compression[13] among others
Consisted of two distinct phases with different tectonic style (Fig. 1B): i) a Neogene thin-skinned phase, where a pile of thrust sheets consisting of Meso-Cenozoic sedimentary units with associated foredeep flysch deposits stacked over the Inner Apulian Platform (API); ii) a subsequent Late Pliocene-Early Pleistocene thick-skinned phase during which high angle N- to NW-trending thrusts were deeply rooted originating large-wavelength anticlines within the API14–16
The VDA extensional basin is shaped by two active and opposite dipping, high-angle range-bounding fault systems: the Eastern Agri (EAFS) and the Monti della Maddalena (MMFS) (Fig. 1A)[18,19]. Both the EAFS and MMFS consist of arrays of separated normal fault segments, a typical feature of a set of immature fault systems
Summary
The VDA is a Quaternary basin developed within the southern Apennines thrust-and-fold belt (Fig. 1A), when an extensional phase replaced a Mio-Pliocene compression[13] among others. The VDA extensional basin is shaped by two active and opposite dipping, high-angle range-bounding fault systems: the Eastern Agri (EAFS) and the Monti della Maddalena (MMFS) (Fig. 1A)[18,19]. Both the EAFS and MMFS consist of arrays of separated normal fault segments, a typical feature of a set of immature fault systems. Low-magnitude seismicity was related to the seasonal variations of the Pertusillo water impoundment located a few kilometers to the South of the oilfield[22,23] (Fig. 1A)
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