THE TIMING OF CEMENTATION IN THE ROTLIEGEND SANDSTONES OF THE SOUTHERN NORTH SEA: A PETROLOGICAL AND FLUID INCLUSION STUDY OF CEMENTS

Abstract

We report here on fracture and pore‐filling cements in the Rotliegend Sandstones of the Sole Pit area, UK Southern North Sea. Standard thin‐section petrography and scanning electron microscopy have been complemented by fluid inclusion microthermometry. The main objectives of this paper are to establish the stratigraphy of the sandstones' cements and to describe the cements' characteristics. In addition, the combined use of petrography and fluid inclusion microthermometry is used to define the timing of cementation and gas migration in the Southern North Sea.The geological history of the Sole Pit area is similar to that of other parts of the Southern North Sea. A period of aeolian deposition resulting in the Upper Permian Rotliegend Sandstones was ended by the Zechstein marine transgression. Rapid subsidence followed until Cretaceous times, when a phase of uplift affected the Sole Pit area as well as, for example, the Broad Fourteens Basin. This uplift continued through the Cretaceous and, following a minor phase of subsidence, uplift again took place in the Late Cretaceous and into the Tertiary.When plotted on burial‐thermal history diagrams, the fluid‐inclusion and petrographic results show that quartz cement precipitated over a range of burial depths. Quartz mineralisation was, however, more common at deep burial levels and also during the uplift period in the Sole Pit area, which caused extensive fracturing of the Rotliegend Sandstones. Dolomite precipitation occurred during deep burial diagenesis and ceased largely with the onset of fracturing. Sulphates (anhydrite and barite) were mainly precipitated during the inversion phase in the Sole Pit area. As both the anhydrite fluid inclusions, and possibly also the quartz fluid inclusions within some of these cements contain hydrocarbon gases, the migration of gas took place during the inversion period, using the fractures as migration pathways.