Statistical analysis of stratal patterns and facies changes at the terminations of â turbiditicâ sandstone bodies : the Oligocene Cengio Unit (Tertiary Piedmont Basin)

The quantitative study of outcrop analogues of confined turbiditic sandstone bodies provides constraints to the geological, geophysical and geostatistical methods of subsurface interpretation and reconstruction of this type of deep-water reservoirs. Therefore, we studied two sandstone bodies that belong to the Cengio Unit (Late Oligocene; Gelati & Gnaccolini, 1980; Cazzola et al., 1985), a well-known turbidite system of the Tertiary Piedmont Basin. The system fills a structural depression and laps out against slope marlstones, both downcurrent and parallel to the average palaeoflow trends. It is formed by several stacked, amalgamated sandstone bodies, which invariably develop above stratified units of alternating turbiditic sandstones and shales. We studied two sandstone bodies that correspond to lobes I and II of Cazzola et al. (1985). The lower sandstone body I is a sandstone-shale stratified unit. This is covered by sandstone body II, an amalgamated sandstone lithosome. The total maximum thickness is about 25 m, the length of exposure is about 2500 m in N-S downcurrent direction, and 3000 m in E-W direction. Following field facies analysis, a simple descriptive statistical approach was applied to two cross-sections that we draw after bed-by-bed measurement and field correlation of 26 stratigraphic logs. The statistical analyses considered the most influent variables on reservoir heterogeneity: number and thickness of beds, grain-size, sand/mud ratio and most relevant facies attributes. Quantitative comparison was thus obtained among the depocentral area, two different pinch-out zones (abrupt frontal onlaps oblique to palaeocurrent trends and gradual lateral onlaps, parallel to palaeo-flows), and the downcurrent zone. The lateral onlap terminations are characterised by well-separated beds, high abundance of laminated bed-sets, high frequency of water-escape structures in sandstone beds and high diffusion of mud-chips. Differently, the frontal onlaps show dominant bed amalgamation, high sandstone/shale ratio, high ratio of thickness vs. number of beds and prevalence of massive sandstones. Moving downcurrent, sandstone body I showed a positive statistical correlation between decreasing thickness of sand divisions in turbidite beds and increasing number of beds, and a statistical tendency to downcurrent transition from massive sandstones to classical Bouma turbidites. This indicates that, at this time, the proportion of basin width vs. volume of sediment flows was in favour of the downcurrent transformations of transport and depositional processes. Differently, the upper amalgamated sandstone body II showed no regular statistical trends and only minor changes in thickness, number of beds and facies, both downcurrent and towards the onlap terminations. The analysis of bed thickness distribution clarified the relationship between basin confinement and volume of flows. The striking difference between sandstone bodies I and II could be attributed to a sudden increase in volume of the sediment gravity flows entering the basin, which forced the depositional system to aggrade, forming thick beds with very minor lateral facies changes. The results of the Hurst test showed that sandstone body I and II plot in the field of turbidite lobes (Chen & Hiscott, 1999). In our case, the test showed also to be sensible to the differences between the depocentre-downcurrent sectors and the onlap regions, thus suggesting a possible application to the problem of discriminating sub-environments in subsurface studies.