Controls on the geometry and distribution of carbonate cements in Jurassic sandstones: Bridport Sands, southern England and Viking Group, Troll Field, Norway

Abstract

Summary The petrography and diagenesis of calcite cements in the Lower Jurassic, Bridport Sands (southern England) and Upper Jurassic, Viking Group sandstones (Troll Field, offshore Norway) have been investigated in order to assess their geometry and effect on hydrocarbon recovery. In the Bridport Sands, sediment texture and mineralogy controlled carbonate cementation. Clay-rich fairweather sediments were weakly cemented and are now compacted. Bioclast-rich storm deposits were stabilized mechanically by early fringing cements. During burial bioclasts and fringing cements were replaced or dissolved, and pores were filled by simultaneously precipitated ferroan calcite. Thus, cemented beds are laterally continuous for several kilometres in the Bridport Sands as a consequence of the sheet-like geometry of the storm beds in which they developed. In the Viking Group sandstones, carbonate cementation was controlled by rate of burial. Fringing cements formed locally during non-deposition or emergence. Cementation continued with non-ferroan calcite incorporating bacterially derived bicarbonate generated during prolonged residence in near surface zones of bacterial activity. Cement geometries will reflect the distribution of emergent surfaces and the longevity of residence near the surface. These two cases demonstrate the potential for laterally extensive carbonate cements to develop in shelf sandstones. The cements in these examples have different origins but in both cases their distribution is related to the episodic nature of deposition in the shelf environment.

Clay Diagenesis in the Sandstone Reservoir of the Ellon Field (Alwyn, North Sea)

“…Calcite cementation is an important factor limiting the diagenetic reactions (Kantorowicz et al, 1987;Saigal and BjCrlykke, 1987;Walderhaug et al, 1989). According to Bjcrlykke et al, (1992), the carbonate cement intervals in the Brent Group are generally thin, usually <1 m, and usually more abundant in the marine part of the Brent Group (i.e., Rannoch, Etive) than in the deltaic environment of the Ness Formation.…”

Origin of clasts and matrix within the Milano and Napoli mud volcanoes, Mediterranean Ridge accretionary complex

“…However, the ankerite or complex carbonate may be of diagenetic origin. In marine sedimentary basins, ankerite commonly forms during late diagenesis, under combined lithostatic/hydrostatic loads of several kilometers, in the presence of fluids characterized by light oxygen isotopes, and in some cases, associated with oil migration (Kantorowicz et al, 1987). The main mode of formation is by "ankeritization," in which Fe or Mn substitutes for Ca or Mg in carbonates.…”

Berthierine from the Lower Cretaceous Clearwater Formation, Alberta, Canada

“…In general though, marine and brackish water occurrences considerably outnumber those known for freshwater. While berthierine most often has been reported as a component of ooids, it also has been described as an authigenic grain-coating or pore-lining in sands (Dean and Nahnybida 1985;Kantorowicz et al 1987;Prentice and Wightman 1987;Longstaffe et al 1989aLongstaffe et al , 1989bLongstaffe et al , 1992Longstaffe 1993).…”

“…Ancient berthierine is almost exclusively reported as ooids consisting of concentric sheathes around a nucleus of clay, ferric oxide, other ooids, fossil fragments or quartz grains. Reports of well-developed, authigenic grain-coafings or pore-linings are relatively few (Dean and Nahnybida 1985;Kantorowicz et al 1987;Prentice and Wightman 1987;Longstaffe et al 1989aLongstaffe et al , 1989bLongstaffe et al , 1992Longstaffe 1993). Similarly, odinite in Recent sediments almost always occurs as peloids of presumed fecal origin.…”