Permo-Carboniferous plays of the Silver Pit Basin

Abstract

The Silver Pit Basin is situated in the northern part of the southern Gas Basin, and is separated from the Sole Pit Basin to the south by the Outer Silver Pit Fault. Over much of the basin up to 400 m of Silverpit Formation interbedded shales and evaporites are present. The absence of the Leman Sandstone reservoir over much of the basin has meant that gas discoveries predominate in the Carboniferous (e.g. Caister, Murdoch) rather than in the Permian, as in the Sole Pit Basin. In the Silver Pit Basin and its immediate margins, hydrocarbons have been discovered in Paleozoic reservoirs ranging from Dinantian sandstones to Zechstein carbonates. Within the Carboniferous the most significant reservoirs are fluvio-deltaic sandstones within the Westphalian and Upper Namurian. The best quality reservoirs are generally found in the coarsest sandstones, though locally the development of fracture porosity and secondary porosity due to leaching can be significant. The reservoir potential of the Permian is only significant around the margins of the Silver Pit Basin. On the southern margin, at the feather edge of the main Rotliegend Leman Sandstone fairway, significant gas accumulations are present in aeolian and mixed aeolian/sabkha facies. On the northern margins the presence of Zechstein intra-basinal highs resulted in the local deposition of carbonates which may also have reservoir potential. The main source rocks are coals and shales within the Carboniferous, predominantly in the Westphalian, but also to a lesser extent in the Namurian and Dinantian. The main kitchen area is in Quadrant 44, where significant gas generation has occurred since the Cretaceous. Migration and hydrocarbon charging is relatively straightforward for the Leman Sandstone and Carboniferous sandstone reservoirs. For Zechstein carbonate reservoirs migration via basin margin faults facilitates hydrocarbon charging. The most significant regional seal is provided by the Permian sequence of Silverpit shales and overlying Zechstein evaporites. Carboniferous shales probably also act as seals on a local or semi-regional scale, supplementing the regional seal and contributing to deep intra-Carboniferous traps. Most discoveries have been made in reservoirs immediately beneath the Base Permian Unconformity and generally the prospective areas are best identified by mapping at Base Zechstein/Base Permian horizons. The most prospective structures are associated with the major northwest-southeast-trending structural highs, often as ‘pop-up’ features. Most accumulations are predominantly structurally closed; however, for the ‘feather-edge’ Rotliegend play, closure to the northwest can have a stratigraphic component.