The qualitative and quantitative classification of modern clastic marginal-marine depositional systems, Trinidad

ABSTRACT Trinidad's modern coastal depositional systems are for the first time documented by (traditional) descriptive and (modern) semiquantitative and quantitative classification methods. The processes controlling the morphology and distribution of these systems are also investigated. Google Earth™ satellite images, together with published basinal processes data (e.g., significant wave height, surface littoral currents, and tidal cyclicity) are used to describe system morphology, map depositional elements, and to determine the processes responsible for system distribution, respectively. Coastal depositional systems along Trinidad's east, south, and west coasts comprise deltas, estuaries, tidal-inlet complexes, strandplains, and tidal flats. Their distribution is controlled by the wave regime, although they are also influenced by tidal and fluvial processes. The wave regime changes from wave-dominated along the open east coast, to mixed-energy wave-dominated along the semi-sheltered south coast, and to mixed-energy tide-dominated along the sheltered west coast. Distribution of depositional systems related to this regime are such that the wave-dominated east coast sees the development of wave-dominated estuaries. The south coast has a series of wave-dominated, tide- and river-influenced deltas. The sheltered west coast is the most morphologically diverse with river- and wave-dominated deltas, strandplains, and tidal flats. The application of semiquantitative and quantitative classification methods on modern systems has showed their competence in classifying mixed-influence systems from their morphology and sedimentary record. Quantitative classification using depositional elements lends an appreciation to the morphological signatures of mixed-influence systems. Quantitative classification for sedimentary sections gives an appreciation of the preservation of mixed-influence physical processes. The combination of both allows the correlation of the impact of physical properties on morphology and sedimentary record (i.e., possible disparities between morphology and processes). The combination of semiquantitative and quantitative methods may also hold the key to unlocking the mixed-processes classification of depositional elements.

Visualisation of geomorphological features and interpretation of the depositional system of the Brewster Member, Ichthys Field

The Brewster Member of the Early Cretaceous Upper Vulcan Formation is one of the main reservoirs of the Ichthys Field. The Brewster reservoir is characterised as massive sandstone, 150–200 m thick, deposited in a deep marine environment. It has a high net sand-to-gross ratio (over 90%) throughout the field, but heterogeneous reservoir quality between the wells. Visualisation of various geo-morphological features from seismic data is critical for the better understanding of the reservoir. Due to thick overburden, frequency content of the seismic volume is limited at reservoir level. Since the reservoir primarily consists of high net sand-to-gross sandstone, and seismic data has limited resolution, it is difficult to identify the various depositional elements using conventional seismic amplitude interpretation. Seismic attribute visualisation techniques were applied to Ichthys 3D seismic and inverted P-impedance data, enabling identification of several geo-morphological features in this otherwise massive sandstone interval (e.g. feeder channel belt, channel-lobe complex, channel-lobe fringe). Core and log scale sedimentological interpretation were tied to the identified feature to understand the depositional architecture of the Brewster Member, which could lead to a better understanding of the distribution of reservoir properties.

Seismic expression of depositional elements associated with a strongly progradational shelf margin: northern Santos Basin, southeastern Brazil

ABSTRACT: Seismic facies analysis and seismic geomorphology are important tools for the analysis of depositional elements in subsurface. This paper aimed to investigate the character and genesis of depositional elements and erosive features associated with an Eocene progradational shelf margin in northern Santos Basin. Identified seismic facies are interpreted as shelf-margin deltas/shoreface deposits, tangential (oblique) clinoforms, sigmoidal clinoforms, topset reflectors, mass-transport deposits and turbidites. These facies are grouped into four associations representing periods of relatively constant environmental conditions. Association 1 is composed of shelf-margin deltas/shoreface deposits, tangential clinoforms and extensive sand-rich turbidites disposed as submarine channels and frontal splays. A progressive increase in clinoform angle within this association has been identified, culminating in high-relief sigmoidal clinoforms with less voluminous turbidites of facies association 2. Association 3 is composed by subparallel to divergent topset reflectors, interpreted as continental to shelfal deposits placed during base-level rises. These are always truncated basinward by slump scars, formed as a consequence of sediment overload at the shelf margin during aggradations. Association 4 is composed of sigmoidal clinoforms, mass-transport deposits and turbidites. Early clinoforms are steeper as a consequence of the topography of the slump scars. Subsequently, dip angles become progressively gentler as the system approach to the equilibrium profile. The steep physiography was favorable for canyon incision, which played an important role in turbidite deposition. Mass-transport deposits, formed subsequent to slope collapse, are composed of mud-rich diamictites, and show strong internal deformation.