Cementation of chalk and fractures in a naturally fractured reservoir: Petrographic and isotopic implication the Kraka

The present study aims at characterizing the types of fracture and the timing of phases of cementation taking place during chalk diagenesis within the Kraka Field. Three types of fractures are identified in sedimentary cores: Partially cemented, brecciated and sheared fractures. Whereas the brecciated fractures are cemented by blocky calcite cement, partially-cemented fractures can be divided into two groups. The first group shows an early, polyphase quartz cementation followed by calcite cementation. The second group contains one single phase of calcite cementation. Isotopic analysis of d13C and d18O were carried out using bulk rock samples and calcite cement that precipitated in intraparticle porosity and fractures. The d13C and d18O values range from +0.53 to +1.31 and -9.86 to -3.88, respectively. The isotopic values of calcite cement are more depleted in oxygen than bulk rock samples. In addition, calcite cement in intraparticle porosity is slightly more depleted in carbon and oxygen than in fractures, suggesting that it precipitated slightly later during burial. The resemblance in isotopic compositions in all fracture-filling cements sampled indicates similar diagenetic conditions during fracture cementation, despite a significant difference in the style of deformation recorded by the types of fractures described.