--> --> Characterization of Pliocene Pico and Repetto Formation Turbidites at Ventura Avenue Field, Ventura, California

AAPG Pacific Section and Rocky Mountain Section Joint Meeting

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Characterization of Pliocene Pico and Repetto Formation Turbidites at Ventura Avenue Field, Ventura, California

Abstract

The Pliocene Pico and Repetto Formation turbidites of the Ventura Basin were the subject of some of the early research on deep-water sedimentation. Combinations of cores, subsurface well logs, and outcrop data have been used to characterize the sandstone reservoirs and interpret depositional setting and sand-body geometry. The sandstones deposited in this broad, elongate-trough basin have been recognized as differing from turbidites that make up the lobes of a traditional submarine fan setting. They have been variously described as ribbon or shoe-string sands, compensationally stacked elongate-lobe elements, and components of a braided-lobe complex. The majority of the sands are the result of high concentration flows and are poorly sorted. Our study utilizes recently acquired subsurface data from cores and well logs, and new analytical techniques that link these data types, to progress our understanding of depositional environments, facies distributions and rock properties. A key element is a well-log-based petrofacies model that directly ties to lithofacies. This technique enables us to delineate lithofacies stacking patterns and lateral facies variations in wells without core. Well-log correlation, linked with core observations and petrofacies determination helps to define the geometry of stratigraphic elements on the scale of 50 — 100 feet (15-30 m) and identify key stratigraphic surfaces. Petrofacies also are key for predicting reservoir properties, since porosity and permeability are closely correlated to lithofacies. In the coarsest-grained stratigraphic 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. We interpret these elements to have been deposited in axial positions within the depositional system. One element exhibits a fining upward stacking pattern and potentially represents a distributary channel system that eventually migrates or is abandoned. The second coarse-grained element has a blocky stacking pattern and remains coarse-grained nearly to the top of the unit. Overall deposition of the amalgamated sands was likely very rapid. This depositional geometry could be consistent with a braided-lobe complex. Thinner-bedded, generally finer-grained sandstones show more variability in stacking pattern and are more difficult to map over larger distances using well-log correlation. The correlation geometry of these stratal elements is generally planar and subparallel, but the individual sands are less continuous or change thickness. The sands are often interbedded with mudstones and sometimes slurry deposits. We interpret these units to represent off-axis or distal deposition. The coarsest-grained bedsets are deposits of high-density turbidites. They are generally amalgamated, poorly sorted sands, with lower matrix porosity and permeability. The tops of graded bedsets, and the thin-bedded sandstones, tend to be finer-grained and better sorted. They usually have higher matrix porosity and permeability.