Relation Between Facies and Fracturing In Chalk Outcrops - The Importance of Early Polygonal Fracture Systems in Understa

This study summarizes sedimentological and structural observations along the Cretaceous chalk formations of the Anglo-Paris Basin in both Yorkshire (UK) and Normandy (France) outcrops. It proposes a conceptual facies-fracture model which is subsequently challenged against the genetic mechanisms derived from numerical and analogical models. The study demonstrates the presence of early polygonal fault systems (PFS) that can occur in both pure and marly chalks but in very fine grained poorly sorted homogenous chalks. Fracturing can occur in chalk early, syn-depositional, and during compaction even at shallow depth. Providing an isotropic horizontal stress field, PFS’s will occur at all scales. This process will be also triggered by the cohesion contrast between chalk facies, resulting in differential compaction. Furthermore the presence of pressure solution can favor the process (Normandy) or not (Yorkshire). This early PFS’s will provide an ideal regular and radial fracture pattern to be potentially reactivated by the subsequent tectonic deformation phases. In chalk reservoirs, PFS’s can also provide a well connected pathway for flow whereas in top seals it could provide a risk for integrity. This study is therefore a first step toward a better sub surface chalk characterization of fracture distribution and prediction of behavior during production times.