Facies Analysis and Reservoir Quality of the Triassic Cormoront Formation

(O.Al-Ja'aidi MSc. project Abstract)

 

The Triassic Cormorant Formation was deposited in a geographic zone conducive to the development of semi-arid climatic conditions. Fining-upward cyclic sequences in the Cormorant Formation of the northern North Sea represent the deposits of an ancient ephemeral stream-playa lake complex analogous with some of those forming in modern semi-arid inland basins. It contains a widespread of continental sediments consisting of several hundred meters of sandstones and mudstones deposited by ephemeral streams in an extensional basin setting. From cores and logs, the Cormorant Formation can be seen as complex thick sequences of calcareous red-purple mudstones and sandstones in which root bioturbation, burrowing and color mottling are prominent. The fine member deposits are characterized by the influence of the pedogenic processes. Throughout the Cormorant Formation carbonate cements are abundant. Calcretres are commonly present mainly associated with the finer deposits but also as reworked channel lags. Conglomerates and cleaner sandstones that have also been cemented by calcite in places are interpreted to be of calcrete origin. Thin horizons of palaesols often form the tops of fining-upward sequences, the basal parts of which are formed by mudstone, siltstone, sandstone and basal lag conglomeratic deposits. These sediments are interpreted as alluvial, distal plain/sheetflood deposits. The larger sand bodies were probably deposited by braided streams.

Reservoir Quality in the sands is controlled by both depositional texture and diagenesis. In most sand bodies, reduced permeabilities occur in the bioturbated siltstones and sandstones. Calcite cementation dominates throughout the deposits, but is more concentrated in channel and sheetflood deposits.