Pliocene Repetto Formation Turbidites, Ventura Avenue Field, Ventura, California: Linking Reservoir Properties, Lithofacies, and Stratigraphic Elements
Authors: Schwalbach Jon R. , Holloway James W. , Steel Elisabeth , Coldewey Ralph 
Published: May 2020

The Pliocene Pico and Repetto Formation turbidites of the Ventura Basin were the subject of early research on deep-water sedimentation. We focus on the subsurface Repetto Formation, primarily using cores and well logs to characterize the sandstone reservoirs and describe sand-body geometry and stratigraphic stacking. The generally poorly sorted, coarse-grained sandstone facies deposited in this elongate basin dominated by longitudinal flows have been recognized as differing from those in traditional submarine–fan–lobe facies models. They have been variously described as ribbon or shoestring sands, compensationally stacked elongate-lobe elements, and components of a braided-lobe complex. Our study utilizes recently acquired subsurface data from cores and well logs and new analytical techniques to improve our understanding of facies distributions and rock properties. Our subsurface data, however, come from only a small area of the basin and therefore afford only a limited perspective on basin-wide depositional systems.

An important tool for our work is a well-log–based petrofacies model that ties directly to lithofacies. This technique enables us to delineate in significant detail lithofacies stacking patterns and lateral facies variations in wells without core. The petrofacies enable us to improve the geologic context of well-log correlation. Linked with core observations, petrofacies determination helps to define the geometry of architectural elements on the vertical scale of 50 to 100 ft (15–30 m) and laterally on the scale of typical well spacing (hundreds of feet/meters). Facies stacking and lateral relations also enable us to identify important stratigraphic surfaces.

In the coarsest-grained elements, we identify two lithofacies stacking patterns that likely represent different depositional settings. Both have pebble-to-gravel–sized clasts in a sandy matrix and angular mudstone clasts at their bases. One element exhibits a fining-upward stacking pattern often characteristic of channel-fill sequences and potentially represents a distributary channel system that eventually migrates or is abandoned. The second coarse-grained element has a blocky stacking pattern consisting predominantly of amalgamated sandstone beds and remains coarse-grained nearly to the top of the unit. This depositional geometry could be consistent with a braided-lobe complex, characterized by multiple broad and low-relief channels with intervening bar forms. Both coarse-grained elements likely represent axial positions in their respective depositional settings and are relatively easy to map between wells because of the abrupt facies change at the base of the units and their relative thickness (tens of feet/meters).

Thinner-bedded, finer-grained sandstone bedsets show more variability in stacking pattern and are more difficult to map over larger distances. The correlation geometry of these stratal elements is generally planar and subparallel, but individual sandstone beds are less continuous. The sandstone beds are often interbedded with mudstone and sometimes slurry deposits. We interpret these units to represent off-axis or distal deposition.

Petrofacies are key for predicting reservoir properties, since porosity and permeability are closely correlated to lithofacies. For example, the petrofacies technique enables us to differentiate the coarsest-grained bedsets, deposits of high-density turbidity currents (generally poorly sorted sandstone with lower matrix porosity and permeability), from thin-bedded sandstone (generally finer-grained and better sorted, with higher matrix porosity and permeability).