Multiple fluid flow events from salt-related rifting to basin inversion (Upper Pedraforca thrust sheet, SE Pyrenees)

Abstract

Fault-related dolomitisation is responsible for the development of numerous hydrocarbon reservoirs hosted in diagenetically-altered carbonates and is therefore critical to hydrocarbon exploration, subsurface storage (i.e. CO₂), the formation of associated mineralisation (<em>i</em>.<em>e</em>. MVT-deposits) and for understanding the key controls on subsurface fluid flow. Multiple dolomitised outcrop analogues have been characterised in recent years, but uncertainty still remains as to the controls on dolomitisation in terms of dolostone geobody size and geometries, their distribution and how they impact reservoir quality. Late Tithonian shallow-marine carbonates at Serra Esparreguera in the Maestrat Basin (E. Spain) were partially dolomitised on the seismic scale, resulting in a spectrum of geobodies with varying degrees of spatial connectivity. Dolostone predominantly replaces Polpís Fm wackestones and packstones, and bioclastic grainstones of the Bovalar Fm. Dolostone geobodies transition through vertical stratigraphy from being massive and spatially extensive to localised stratabound bodies as textural heterogeneity increases. Irregular dolostone geometries occur in the Polpís Fm, which is texturally homogenous relative to the overlying Bovalar Fm, cross-cutting bedding in areas with high abundance of faults. Faults occur adjacent to dolostone and constrain its lateral extent across the outcrop. Dolomitisation fronts are typically sharp with morphologies affected by small-scale faults and bedding-parallel stylolites. Dolomitisation occurred under burial conditions and dolostones were later overprinted by phases of calcite and saddle dolomite cementation. The spatial distribution of dolostone is strongly influenced by the depositional heterogeneity and faults, while smaller structures (<em>i</em>.<em>e</em>. metre-scale fractures and stylolites) and bedding surfaces controlled the dolomitisation front geometry. Dolostone geobodies at Serra Esparreguera provide new insights into the structural, depositional and diagenetic controls on dolomitisation at a seismic scale, which can be used as a predictive guide to improve the understanding of carbonate reservoirs with complex paragenetic histories.