Fracture networks in Rotliegend gas reservoirs of the Dutch offshore: Implications for reservoir behaviour

Abstract

Fracture systems of Rotliegend gas fields in and at the margins of the northern Broad Fourteens Basin in the Dutch offshore are described in terms of orientation, frequency, origin and type, and in relation to larger-scale structures. First, fracture data collected from core and image logs have been corrected to account for the bias related to the 1-D sampling. Second, these results were integrated with data on fracture cements and diagenesis in order to assess the timing of the fracture network development. On the basis of their regional extent three phases of fracturing and four orientation trends can be distinguished in the basin: (1) at Triassic times and related to early diagenesis and burial, NW-SE to NNW-SSE and NE-SW to ESE-WNW particulate-shear fractures developed; (2) during the Mid-Kimmerian and related to the main burial stage, shear-related and dilational-shear-fault-related fracturing occurred parallel with larger-scale faults; (3) during the Cretaceous and related to uplift, NW-SE and NE-SW joints propagated; a regional joint system developed outside the Jurassic rift basin, preferentially oriented E-W to ESE-WNW; these joints have not been dated accurately. The fault-related shear fractures tend to compartmentalise the reservoirs, whereas the regional joints tend to enhance reservoir flow properties. These fracture systems are thought to play a negative or positive role, respectively, but only in fields with poor reservoir quality. Consequently, in such cases small-scale fractures should be taken into account in field development planning.