ABSTRACT 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.
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.
ABSTRACT 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.
A new development in the application of sequence stratigraphic concepts in marine as well as continental basins is the recognition of high-frequency cyclic patterns in rock successions in the subsurface. Studies of six wells from the northern, central, and southern parts of the Gulf of Suez show the presence of well-preserved, high-frequency cycles with periodicities similar to the orbitally forced Malankovitch parameters. Subsurface rock successions, third-order sequences, and high-frequency cycles were compared with outcrops. After establishing the biostratigraphic framework for the above-mentioned wells, a sequence analysis was performed. Sequence boundaries and maximum flooding positions in each well were calibrated with the occurrences and evaluation of the high-frequency cycles. It became obvious that there is an intimate relationship between these high-frequency Milankovitch cycles and sequence organization. In addition, a close relationship can be observed in the subsurface as well as in outcrops between high-frequency climatic changes (connected to the Milankovitch cycles) and (litho)facies variability. Quantitative evaluations of each sequence and/or systems tract can be computed with the International Geoservices' cyclicity analysis tool (MILABAR). The results are summarized in a well composite chart, rate (NAR), and ratio of preserved time. In correlations between the wells, an accuracy of 500-100 Ka can bemore » obtained. The quantitative evaluation of the sequence and high-frequency cycle analysis gave some new aspects concerning the (litho)facies and geodynamic development during the pre- as well as the synrift stages of the Gulf of Suez Basin.« less
Applying depositional sequence stratigraphic concepts to the interpretation of siliciclastic depositional systems is becoming an increasingly important tool in petroleum geology. After a succession of breakthroughs during the 1970s and 1980s, sequence stratigraphic concepts now have entered a phase of intense application and documentation, especially with regard to successful implementation in the field of petroleum geology. Workers have applied these concepts to a variety of databases, ranging from outcrop to cores to electric logs and to multifold seismic data. Clearly, sequence stratigraphic concepts embody–not a rigid model or template–but rather a way of looking at geology. This volume has two purposes: to compile some recent applications of siliciclastic sequence stratigraphic concepts, and to present new studies focused on refining conceptual models. This memoir grew out of a 1991 symposium, "Variations in Depositional Systems Within a Sequence Stratigraphic Framework: Applications to Exploration," organized by the authors at the AAPG annual meeting at Dallas. Robert Loucks and Rick Sarg have edited a companion volume, also published by AAPG,"Entitled Carbonate Sequence Stratigraphy: New Developments and Applications."
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