Pacific Section AAPG, SPE and SEPM Joint Technical Conference

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Depositional Facies And Geometry Of Repetto Formation Turbidites In Ventura Avenue Field, Ventura Basin, California

Abstract

Pliocene Repetto Formation turbidites were deposited rapidly in the continuously subsiding trough of the Ventura/Santa Barbara Basin. We used detailed stratigraphic correlation and cores from one of the reservoir intervals of the Ventura Avenue Field to map depositional geometries and interpret stacking patterns. The geometries are unique because of the elongate basin shape and the large volume of sediment contributed to the basin.

The turbidite facies consist of thickly bedded amalgamated sandstone, thinner inter-bedded sandstone and mudstone units, and intervals that are predominantly mudstone. The 600-foot-thick interval is sand-rich, with a net-to-gross over 60 percent. Individual sedimentation units range from a few meters thick for some of the amalgamated beds to centimeter-thick interbeds of sandstone and mudstone. The thicker units are typically poorly sorted with coarsest grains of pebbles to very coarse sand and a matrix of medium to fine sand. These are interpreted to represent high-density turbidites. Sometimes finer-grained caps of Bouma-structured sands or mudstone separate the thicker sandstone beds. Thinner-bedded sands are generally finer-grained and often contain Bouma-structured sandstone interbedded with mudstone.

Based on our correlation framework and core observations, we interpret individual flows to be generally unconfined. However, the elongate trough geometry constrains the larger-scale lobate depositional elements. These lobate units are generally topographically controlled. The basin-trough geometry limits their ability to migrate laterally, so they are forced to also migrate up and down depositional dip.

These compensationally stacked bodies have implications for geologic modeling and geostatistical distributions.