Grain-Size Characteristics of Unconfined Deep-Water Deposits in the Quaternary Santa Monica Basin, California: Implications for Reservoir Quality in Distal Turbidite Systems

Romans, Brian; Fildani, Andrea; Clark, Julian; Power, Bruce; Sullivan, Morgan

As exploration and development moves farther offshore and into ultra-deep water, the likelihood of encountering distal parts of turbidite systems in the subsurface increases. Variability in turbidite sandstone reservoir quality (porosity-permeability) is a consequence of numerous factors, including burial depth and compaction, fluid flow and diagenetic history, structural/deformational history, and depositional facies. In some basins, all of these factors are at play, confounding the ability to determine their relative effects and, thus, to make accurate predictions. Here, we use sediment cores from a modern (latest Pleistocene to Holocene) submarine fan system in the Santa Monica Basin, offshore California, to study the influence of depositional facies and architecture on sediment texture and grain-size. Grain-size distributions of samples from modern deep-sea fans are a valuable proxy for depositional controls on reservoir quality because the sediments are not yet deeply buried nor have undergone significant diagenesis. The Santa Monica Basin was chosen because it combines sediment cores from the deepest, most distal, part of the basin (ODP Site 1015) with seismic-reflection data to evaluate stratigraphic architecture.

Bathymetry and seismic-reflection data demonstrate that ODP 1015 penetrates tabular, sheet-like architecture indicative of deposition of sediment gravity flows in an unconfined setting. Sediment cores from the seafloor to ~120 m depth are characterized by medium- to thick-bedded, normally graded sands that are visually structureless with dewatering features. The upper parts of some beds have subtle horizontal structures and/or ‘banding' at mm- to cm-scale. Laser particle size analysis of 55 samples from eight individual beds was conducted to investigate quantitative grain-size distributions. Results show that these sand beds contain 20-30% silt in the basal parts, >50% silt in the upper part, and an overall mean of 40%. Distributions are dominantly fine-skewed with the most normal distributions derived from sands with obvious stratification. We hypothesize that the poorly sorted nature of these turbidite sands is a function of the distal environment in which sediment-laden flows are unable to effectively sort grain sizes during rapid deposition. These processes are a direct consequence of the very low gradients and absence of channels that define distal settings.

AAPG Search and Discovery Article #90163©2013AAPG 2013 Annual Convention and Exhibition, Pittsburgh, Pennsylvania, May 19-22, 2013