The Crati Basin is a Pliocene‐Holocene extensional basin filled by the progradation of different types of marine fan‐delta systems. Coarse‐grained Gilbert‐type fan‐deltas developed during the Pleistocene. They represent the sedimentary response to a strong differential uplift involving the basin margins and the basin itself. The differential uplift was responsible for the fragmentation of the basin into several sub‐basins, into which these fan‐deltas prograded. The protection and lateral confinement by structural highs, steep coastlines and the absence of strong tidal action in the adjacent Ionian Sea, allowed the regular progradation of these fan‐deltas in the restricted gulfs and narrow embayments of the Crati Basin. For the classical Gilbert‐type (fan) deltas in lacustrine settings, homopycnal inflow favours a rapid mixing of water masses and deposition of sediment close to the river mouth. In the case of the example described here, the density contrast between the sea water and the inflowing river water caused the separation of the muddy fraction from the coarse sandy and conglomeratic part of the sediment. This allowed the development of steep mud‐poor coarse‐grained delta foresets. Slope instability features (slump scars, conglomeratic flow slides) are fairly scarce in the proximal parts of the San Lorenzo del Vallo system. Towards the north, where protection from the Ionian Sea was less, they increase in importance.

Tidal bundle sequences are the characteristic large‐scale cross‐bedded sets with mud drapes deposited by strongly asymmetrical bidirectional tidal currents. By means of time series analysis of the bundle thickness of such sequences, the palaeohydrodynamic processes can be reconstructed. This technique involves: (1) Fourier analysis to test the periodicity of the bundle‐thickness sequence and to estimate the periods and the phases of the most important periodic components; (2) filtering analysis to resolve the bundle‐thickness sequence into different components (i.e. diurnal components, random variations, neap/spring components and longer period variations) and also to estimate the relative importance (amplitude) of each component. From these analyses, useful information can be derived as to the palaeotidal regime (whether semidiurnal or mixed) and also about the possible influence of non‐tidal processes, such as storms (their strength, duration, frequency and direction). Results from a subrecent and an ancient example show that tidal currents and storm‐induced currents are the most important hydrodynamic processes in the transport and deposition of sediment in these shallow marine environments.

No abstract

No abstract

The Sognefjord Formation in the Troll Field area was deposited during a general rise in sea-level during the Late Callovian to Early Volgian. A detailed study including palynology, sedimentology, response modelling and petrography of carbonate cement has been performed on 15 wells primarily from the western part of the field. All wells have been extensively cored through the reservoir enabling detailed palynological and sedimentological study of the depositional sequences in the wells. The Sognefjord Formation is interpreted, in this study, as a shoreline-attached tidally-influenced shelf complex. The study is based on the general consistency found between the eustatic sea-level curve published by Haq et al. in 1987 and the biostratigraphical zonation for the Troll Field described here. The response to sea-level fluctuations in a shallow marine environment is discussed and a response model is established. A theoretical assessment of the criteria for recognizing important events is presented. The maximum flooding surfaces (mfs) and sequence boundaries (sb) are easiest to recognize. Theoretically there is a higher probability of forming carbonate cements in connection with maximum flooding surfaces. The stratigraphically important dinocysts on Troll are classified according to their relation to sea-level fluctuations. Most of the palynozone boundaries are related to maximum flooding surfaces, except for a few which coincide with sequence boundaries. A stratigraphical subdivision of the Troll Field reservoir is presented based on a comprehensive palynological study. Lithofacies analysis in conjunction with palynofacies interpretation have been applied to delineate the depositional environment. As an aid to the interpretation a theoretical model was established. The predicted succession and variability of sequences and facies types outlined by the theoretical model could be recognized in all wells. Petrographical analysis shows that early diagenetic, near-surface cementation has occurred in connection with maximum flooding surfaces and sequence boundaries on Troll. With the help of the stratigraphical framework the presence or absence of sequence boundaries, transgressive systems tracts, maximum flooding surfaces, and highstand systems tracts in each cycle has been defined in each well. This has given the detailed geometry of the reservoir sands of the field, and also allowed correlation of carbonate cemented horizons which are associated with bounding surfaces of the systems tracts. Knowledge of the distribution and extent of these cemented horizons has considerable significance for oil production strategy, particularly the siting of horizontal wells.