Stratigraphic relationships of some Tertiary lowstand depositional systems in the Central North Sea

Abstract

The application of sequence stratigraphic techniques, in conjunction with detailed palynofacies and sedimentological analyses, provides a new methodology for the prediction of stratigraphic relationships in selected lowstand depositional systems from the Tertiary of the Central North Sea. Traditional lithostratigraphy invokes facies changes to explain stratal variations within a depositional system. A sequence stratigraphic approach emphasizes the ‘temporal’ dimension to explain and predict the distribution of stratal units and their respective stacking patterns. Sequence definition requires an in-depth knowledge of chronostratigraphically equivalent stratal units in a regional basin context. A biostratigraphic zonal scheme for the Upper Paleocene and Lower Eocene is presented, and provides the chronostratigraphic constraints to the sequence interpretation. Each sequence is composed of a series of systems tracts and parasequences whose stratal expression evolved in response to accommodation space; the primary control on accommodation space being the inter-relationship between eustatic sea-level, subsidence/uplift and sediment supply. Palaeoenvironments responded to changes in relative sea-level which are reflected in the qualitative and numerical aspects of the faunal and floral assemblages at a variety of scales. These basinal lowstand depositional systems are represented predominantly by fans and channelized complexes. Alternating high and low (less turbulent) energy conditions possess characteristic palynomorph signatures. The stratigraphic relationships of several contrasting Tertiary lowstand depositional systems are examined. In each case, a series of stratigraphic methodologies are presented that integrate detailed palynofacies, sedimentologic, well and seismic data at both field and regional scales, using a database of more than 200 wells and 15 000 km of 2D and 3D seismic data. The first case study comprises the Late Paleocene (zones ET4–ET7), in places Lista Formation equivalent; type well, Gannet D, 22/21-5. The composition of the microflora and kerogen strongly infers a distal basinal setting. The lowstand system provides reservoir quality sands, whereas the overlying distal prograding highstand affords an effective intraformational seal. The second case study is in the Late Paleocene (zone ET8), in places Forties Formation equivalent; type well, Gannet B, 21/25-1. Certain features of this palynofacies assemblage contrast with the previous example, and suggest a more proximal palaeoenvironment strongly influenced by an adjacent coastal deltaic complex. A further case study, from the Eocene of the Gannet area, investigates fan and channel morphologies in areas where palaeotopographic relief is influenced by halokinesis. The theme of stratal relationships continues with discussion of an interdigitating leveed channel complex of Eocene age from the Kittiwake area. The application of these sequence stratigraphic techniques to hydrocarbon prospectivity, how they open new avenues for exploration and, at a production scale, new insights into reservoir geometry and continuity, are discussed.