Dolomitization impact on fracture density in pelagic carbonates: contrasting case studies from the Gargano Promontory and the Southern Apennines (Italy).

The results of combined field and laboratory analyses carried out in well bedded, partly dolomitized successions of Mesozoic pelagic carbonates outcropping in the Gargano Promontory and Southern Apennines (Italy), revealed that dolomitization can exert opposite roles in modulating fracture density of carbonate successions. The coarsely crystalline dolomites of the Gargano Promontory, which are characterized by planar-S to planar-E textures and some intercrystalline porosity (3.6–18.4%), are less densely affected by stratabound fractures (mostly joints) than the precursor micritic limestones. In contrast, the finer-crystalline, no porous dolomites outcropping in the Southern Apennines, which are dominated by non-planar-A textures, are more densely fractured than the precursor micritic limestones. Therefore, intrinsic textural parameters of dolomites, such as crystal size, texture and porosity played a prominent role in modulating stratabound fracture density. In addition, dolomites of the Gargano Promontory are thicker bedded than the precursor limestone beds due to their diagenetic homogenization, which is enhanced by absence of strongly stylolitized bed surfaces and marly interlayers. This contributed to reduce the overall density of the stratabound fractures formed later. As density of stratabound fractures is considered a prominent factor affecting the large-scale porosity of carbonate rocks, the results of this study provide new insights on how dolomitization may either increase or reduce the quality of fractured reservoirs of geofluids through its multiple controls on density of such fractures.