Nature and origin of collapse breccias in the Zechstein of NE England: local observations with cross-border petroleum exploration and production significance, across the North Sea

Abstract

Hydrocarbon reservoirs hosted in Permian strata were some of the first to have been discovered in Europe. With discoveries in the Zechstein carbonates of Norway in recent years, and with exploration of Zechstein prospects both onshore and offshore UK, as well as in Dutch, Danish and Norwegian offshore sectors, understanding the architecture of the Zechstein carbonates remains very relevant. Here we study outcrops of Roker Formation carbonates (Z2, Ca2) in NE England to better understand geological processes associated with deformation following evaporite dissolution, with implications for exploration and production.Collapse of Z2 Roker Formation strata in NE England, following the dissolution of c. 100 m or more of the Z1 Hartlepool Anhydrite, resulted in fundamental changes to the architecture of the succession. Complete dissolution of the anhydrite removed an effective regional seal and dramatically enhanced matrix and fracture permeability of the overlying Roker Formation. The collapsed Roker Formation can be vertically divided into three zones, based upon the degree of deformation. The lower zone and vertical collapse-breccia pipes that can extend across all zones have the highest permeabilities. The process of collapse was gradual, with local variations in the degree of brecciation. We derive a schematic sequence of collapse, recognizing the impact of mechanical barriers within the succession in retarding deformation up-section and it is this that ultimately leads to the vertical zonation.Timing of evaporite dissolution is poorly constrained: it could have occurred soon after deposition, at the end of the Permian or during Tertiary uplift. It is known that evaporite dissolution has occurred offshore, with the oil fields Auk and Argyll (UK Central North Sea) given as examples of dissolution collapse-brecciated reservoirs. Reservoir quality is typically improved, with both matrix and fracture porosity and permeability enhanced. Complete evaporite dissolution could in some cases lead to the potential breach of the seal.

Origin of Oil in Upper Permian (Zechstein) Carbonate Reservoir Rocks at the Jarvis Structure Underlying the Ettrick Field, Outer Moray Firth, Uk North Sea

“…At the wells studied, broadly outer-to mid-ramp facies containing storm beds and reworked units pass up into shoreface oolitic-bioclastic facies overlain by microbialites, above which are sabkhalagoonal gypsum-anhydrite, before the next flooding and initiation of another carbonate platform. The diagenesis of the Jarvis Zechstein carbonates is also similar to their well-documented equivalents in the Southern Permian Basin (Clark, 1980;Daniels et al, 2020) in that they are pervasively dolomitised with common loss of original microscale features. The common coarse fabrics and presence of baroque dolomite, plus the negative oxygen isotope data, support burial recrystallisation of the dolomite (see Daniels et al, 2020).…”

“…The diagenesis of the Jarvis Zechstein carbonates is also similar to their well-documented equivalents in the Southern Permian Basin (Clark, 1980;Daniels et al, 2020) in that they are pervasively dolomitised with common loss of original microscale features. The common coarse fabrics and presence of baroque dolomite, plus the negative oxygen isotope data, support burial recrystallisation of the dolomite (see Daniels et al, 2020). Positive to very positive carbon isotope values are typical of Zechstein carbonates generally.…”

A new and working petroleum source rock on the UK Continental Shelf (Upper Permian, offshore Yorkshire)

“…These sediments form broadly off-lapping packages of carbonate and anhydrite, with one major phase of basin-filling halite (EZ2E, Fordon Formation evaporite = Stassfurt evaporites in the SPB). There is evidence in County Durham for some synsedimentary fault movements which controlled deposition of the EZ1Ca reef (Ford Formation = Zechstein Limestone in the SPB, Daniels et al, 2020).…”

Seismic and Petrographic Characterisation of the Zechstein Hauptdolomit Platforms Around the Elbow Spit High, Dutch Offshore

“…Although the overall map expression does not resemble circular karstification features such as dolines, which are known to occur in the offshore UK (Grant et al, 2019), the rugosity could have been caused be a similar karstification process. Daniels et al (2022) described so-called collapse breccias in the Roker Formation (i.e. the equivalent of the Hauptdolomit in onshore NE England) which are present at outcrop just north of Sunderland.…”