Oil Trapping Characteristics of Turbidites: 4. Character of the Sediments

In the cycle of basin filling, gravity-laid reservoir strata (turbidites and associated slide and slump deposits) are deposited during the regressive phase. Such strata usually are separated from normal reservoir rocks (shallow marine and nonmarine strata) by an envelope of pelagic sediments. The latter can serve both as a permeability barrier between normal and gravity sediments and as a source of hydrocarbons. An example of the temporal and spatial relations of these three major facies during one cycle of basin filling is provided by upper Cenozoic strata of the Ventura basin. Gravity strata can be subdivided into proximal and distal facies. This distinction is based largely on electric-log correlatability, sand-body morphology, and predictability of sand percentages. In the proximal facies, electric-log correlations are uncertain, normal to the direction of sediment transport, but are good parallel with that direction. Sandstone bodies wedge out rapidly, so predicting sand percentages is difficult even with abundant well control. Individual sandstone bodies have a channellike shape and commonly exhibit a complex distribution pattern. Distal-gravity sediments in the Los Angeles and Ventura basins provide examples of the progressive onlap of this facies against preexisting basement highs where differential compaction can promote updip closure for these sediments. In the distal facies, electric-logs are easily correlated both parallel with and normal to the direction of sand transport. Individual sand bodies are widespread and sheetlike, and sand-percentage values are predictable with a minimum of well control. Defining the structural history of a prospect area aids in predicting on- and off-structure sand bodies both in the proximal and distal gravity-sediment facies. Successful oil exploration in gravity sediments also is facilitated by paleogeographic reconstructions which define both their source area and their transport direction.