Thin-bedded turbidites: Overview and petroleum perspective

Among the broad range of transport and depositional processes that operate in deep- water, turbidity currents are widely recognized as one of the most common. Turbidite deposits, therefore, are a common facies in many deep-water systems, and within these the thin-bedded (TBTs) and very-thin-bedded turbidites (VTBTs) are most abundant. In many hydrocarbon- bearing turbidite systems, they represent a huge and relatively neglected potential for exploration and production; hence we have used data gathered from modern, ancient, and subsurface systems to synthesize their principal characteristics. They are generally deposited from low-concentration flows (0.25-50 kg/m3 [0.02-3 lb/ft3]) that have reached relatively low velocities (0.15-0.5 m/s [0.5-1.6 ft/s]) with individual beds deposited between 1 hour (thin sandy turbidites) and several days (silt-mud turbidites). The standard Bouma facies model best represents TBTs, and the Stow model typifies VTBTs. Turbidites typically show many partial-sequence variations from the ideal complete sequence. Distinctive facies associations are characteristic of turbidites in channel, lobe, open slope, and basinal settings, whereas vertical sequences of bed thickness are less easily attributed to specific environments. Small- scale (micro-sequences), typically of three to seven beds arranged in symmetric to asymmetric patterns, are the most commonly observed. For quantitative characterization of TBTs and VTBTs, we have developed a series of attribute indices-including facies net- to-gross, sand connectivity, facies ratio, and sediment textural indices. These serve as input data for reservoir modeling and simulation and for characterizing associated architectural elements.