High-resolution sequence stratigraphy and diagenesis of mixed carbonate/siliciclastic successions.

Abstract

The aims of this thesis are to evaluate whether the distribution of diagenetic features in nearshore successions can be explained within a sequence stratigraphic concept; to assess whether the identification and analysis of these features can be used to aid in making sequence stratigraphic interpretations; and to evaluate whether sequence stratigraphy can be used to predict the distribution of diagenetic heterogeneities at the field and inter-well scale. High resolution sequence stratigraphic frameworks have been established for two nearshore mixed carbonate/siliciclastic successions, using facies and early diagenetic analyses. These are the Upper Jurassic Corallian Group of south Dorset and the Lower Cretaceous lower to middle Ericeira Group of west central Portugal. Early diagenetic analyses (including petrographic, CL, XRD and stable isotope work), was performed on 143 samples extracted from concretions and cemented beds at key horizons within these two successions. The early diagenetic results from within the majority of these cemented bodies generally supports the proposed facies-based sequence stratigraphic interpretation of parasequences, systems tracts and sequences. At a parasequence scale, early diagenetic analyses indicate that concretionary growth is controlled by an initial phase of rapid burial, equating to the period of parasequence progradation, followed by a period of prolonged residence time within a single diagenetic zone. This equates to a period of non-deposition/marine flooding at a parasequence boundary. The results also indicate that the application of sequence stratigraphic theory can be used to predict the presence and location of early diagenetic concretions within similar successions. Analysis of early diagenetic features within carbonate cemented beds (such as marine hard-ground surfaces) also supports the facies-based identification of parasequences. Such features are closely associated with parasequence boundary formation and the available data indicates that it is possible to predict their distribution within systems tracts of similar successions. At t?e systems tract scale, analysis of upward increasing or decreasing trends in the volume of pore-mling authigenic phases relates to subtle changes in the rate of sedimentation versus the rate of accommodation creation. A general upward increase in the volume of early diagenetic products (particularly dolomite) is seen to Occur within highstand systems tracts, which is attributed to an increase in residence time within early diagenetic zones as the rate of progradation/burial increases and the rate of non-deposition/marine flooding at parasquence boundaries remains constant. Similarly, an upward increase in the a180 isotope values of carbonate cements (to a more marine value) occurs within shelf-margin or lowstand systems tracts where rates of ~e1ative sea-level are rising increasingly quickly and, a dominance of marine cements occurs within coarser gramed beds of transgressive systems tracts. At the sequence scale, early diagenesis is controlled by the degree of relative sea-level fall and subsequent subaerial exposure at the end of the highstand systems tract. If a sequence is bounded above by a type-t sequence boundary and there is evidence of a period of sub-aerial exposure then the effects of surface related diagenesis (dissolution, replacement and further cementation) can be identified. However. if a sequence is bounded above by a type-2 sequence boundary, the effects of any meteoric diagenesis are likely to be confined to in-extensive fresh-water lenses originating from an up-dip/sub-aerially exposed area of the basin. Consequently, primary early diagenetic cements contained within pore spaces of the existing systems tracts will be preserved. The approach taken in this research has demonstrated that early diagenesis is a useful tool in refining high resolution sequence stratigraphic interpretations and in the prediction of the distribution of early diagenetic heterogeneities within reservoir units. It also shows that for nearshore successions the distribution of diagenetic heterogeneities can be predicted at a range of scales.