Facies control on diagenesis and fracturing in peritidal carbonates : an outcrop based study of lower cretaceous deposits from central east Oman

Abstract

The links between depositional fabrics, diagenesis and fracture patterns are explored for the Barremian-Aptian Jurf and Qishn formations of central east Oman. Field, petrographic and analytical (oxygen and carbon isotopic ratios, clumped isotopes, strontium isotopes, major and trace elements, tensile strength) data were acquired. The excellent outcrop exposures allow the investigation of the geometries, dimensions and spatial distributions of sedimentary bodies, diagenetic bodies and fractures at the inter-well scale (one kilometre). It is suggested that in the Late Barremian, storm and wave reworking and transport were the dominant sedimentary processes operating in the peritidal environment of the southern Tethyan platform. The stratigraphic distribution of the parasequences and discontinuity surface types suggest some regional sea level control on the peritidal carbonate cyclicity. Pervasive dolomite is restricted to the Jurf Formation transgressive systems tract, which is characterized by a peritidal environment rich in microbial mats. Other system tracts, despite being composed of peritidal facies with evidence for high salinities, were not dolomitized, suggesting that the presence of microbial mats, i.e. the depositional fabric, exerted a major control on dolomite distribution. Spacing and orientation of fractures in limestones also depend strongly on the depositional fabric. Grain-supported facies, such as grainstones and rudstones, develop closely and evenly spaced northwestsoutheast oriented stratabound fractures; whereas mud-supported facies develop widely and unevenly spaced throughgoing fractures predominantly oriented north-south. It is suggested that fractures developed until fracture saturation in early cemented grain-supported facies during the Late Cretaceous. As burial increased, the bedding interfaces become stronger allowing for the development of throughgoing fractures that cross-cut mud-supported beds, which remained undersaturated with respect to fractures. This study shows that diagenetic heterogeneities of the Jurf and Qishn formations are strongly linked to depositional heterogeneities, and that both the depositional fabric and early diagenesis control the distribution of fracture heterogeneities.