Sequence Stratigraphic Division and Associated Cement Distribution: Upper Jurassic Fulmar Formation, Central North Sea, UKCS

F. E. Burns, R. L. Gawthorpe, S. Burley, and J. Lasoki

The Fulmar Formation is an important reservoir interval in the Central North Sea, UKCS. Because of the highly bioturbated nature of the sediments, primary sedimentary structures are rare. This has limited previous sequence stratigraphic interpretations. Detailed and integrated ichnofabric analysis allows the development of a superior sequence stratigraphic interpretation in which a relatively predictable distribution of early diagenetic cements is observed.

Within the cored intervals examined, five recurring ichnofabric associations are recognised, and these reflect a range of environments, from offshore transition to lower shoreface. In the two fields studied, a series of sharp-based sandstones are recognised. Analysis of the ichnofabrics associated with these sharp-based sandstones indicates an abrupt shallowing, typically with a change from offshore transition to a proximal lower shoreface environment. The base of each sandstone is marked by an erosional surface commonly with a shell/pebble lag. A common ichnofabric expression of such an abrupt shoaling is the superimposition of clean sandstones with Ophiomorpha on silty sandstones with Cylindrichnus. Internally, the sandstones display a progradational stacking pattern. Ichnofabric ana lysis therefore suggests that these abrupt basinward shifts in facies represent deposition in response to punctuated falling sea-level i.e. forced regression.

Within this sequence stratigraphic framework, abundant carbonate and pyrite cements are observed. Although the sedimentological relationship of these cements is unclear, their distribution is relatively systematic, being best developed at the tops of parasequences. This evidence suggests that they are related to minor flooding surfaces, and has important implications for predicting reservoir heterogeneity.

AAPG Search and Discover Article #91019©1996 AAPG Convention and Exhibition 19-22 May 1996, San Diego, California