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Statistical Analyses Of Confined Turbidite Bedding Thickness In Monarch-Spellacy Sand Equivalents, Midway-Sunset Field, CA


Bedding thicknesses hold the key to identifying turbidite complexes in the subsurface. Statistical best-fit relationships for bedding thicknesses in confined turbidite systems can help differentiate channel-levee systems from proximal fans. Statistical analyses will also help increase discovery of new petroleum reservoirs and enhance geological understanding for further petroleum recovery in old fields. The Midway-Sunset Oil Field, located on the southwest side of San Joaquin Basin, CA, provides a unique setting where it is possible to study the differences among channel-levee, proximal fan, and marginal fan turbidite deposits. Frequency distributions of bedding thicknesses measured from whole cores, taken in the Midway-Sunset Field, will be analyzed. These cores encompass three systems within a turbidite complex: channel-levee, intraslope fan, and marginal fan systems. Observed bedding thickness distributions are compared to theoretical cumulative frequency distributions to determine whether they follow power law, log-log, inverse log, polynomial, and exponential relationships. Variation in bedding thickness frequency distributions may relate to depositional setting, such as differences between channel-levee systems and the basin floor, amount of sediment from tectonic triggered deposition, and paleo-sea levels within deepwater basins. Grain size and amalgamated surfaces may also show variations in bedding thickness frequency. Preliminary results from the marginal fan system indicate that the frequency distribution of bedding thickness for this locality is statistically distinct from an exponential distribution (Kolmogorov & Smirnov test, p-value <0.05), but not from a lognormal distribution (Kolmogorov & Smirnov test, p-value >> 0.05). When bedding thicknesses are sorted by grain size, the medium sands (1/4-1/2 mm) and the coarse sands (1/2-1mm), follow lognormal distributions with p-values > 0.10. Fine grain sands (125-250µm) also follow a lognormal distribution with p-values > 0.20. While the measured bed thickness distributions contain more thin beds than thick, the medium and coarse sand beds compose most beds greater than 100 cm thick. Thicknesses vary depending on structural location and will be assessed by comparing bedding thicknesses along strike to bedding thicknesses along dip. Dip section measurements will also be compared and tested against each other as will strike section to strike section. Comparisons among different localities and parts of the turbidite system provide opportunities to explore the effects on bedding thickness of depositional settings, sediment supply, and paleo-sea levels.