How facies and diagenesis affect fracturing of limestone beds and reservoir permeability in limestone–marl alternations

Abstract

Facies and diagenetic heterogeneities in carbonate reservoir rocks affect both, fracture distribution and fracture permeability. Many studies focussed on fracture patterns in limestone–marl alternations, as e.g. fluid flow models, are based on laterally continuous beds. Here we examine 4010 fractures in multiple layers of limestone–marl alternations using a modified scan-line method. The studied successions belong to the Blue Lias Formation (Hettangian–Sinemurian), exposed on the coast of the Bristol Channel, United Kingdom. We combine methods of sedimentology and structural geology with rock physics to gain a better understanding of the role of facies, diagenesis and petrophysical properties (tensile and compressive strength, hardness, porosity) on the distribution of fractures (fracture orientation, density, spacing and height). Fracture distribution varies significantly despite similar bed thicknesses, indicating that planar bedding planes (i.e. well-bedded limestones, WBL) and beds with bedding plane irregularities (i.e. semi-nodular limestones, SNL) must be distinguished. SNL show higher percentages of non-stratabound fractures (67%) while they are more stratabound in WBL (57%). Additionally, beds with variable bed thicknesses (in scale of 15 m long beds) exhibit a wide range of fracture spacing, whereas fractures in beds with more continuous bed thicknesses are more regularly spaced. Considering all lithologies, the percentage of non-stratabound fractures increases proportionally with CaCO3 content. Three subsections studied in detail reveal different main sedimentological and diagenetic features (from early lithified over differentially compacted to physically compacted). All of them are characterised by dissimilar percentages of stratabound and non-stratabound fractures in limestone beds and marl interbeds. Our findings demonstrate that the distribution of fractures in individual well-bedded limestones is not necessarily representative for successions of limestone–marl alternations; multiple layers should therefore be studied in outcrop analogues as basis for fluid flow models of reservoirs composed of such lithologies.