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Many large oil and gas fields are producing from turbidites and associated deep-water rocks; examples include the Los Angeles and Ventura basins, many fields in the Great Valley of California, and some less obvious turbidite areas such as the Bradford field, Pennsylvania. For future exploration, and for the development of existing fields, it is important to understand the different types of turbidites, how they are related, and how they fit into an overall submarine-fan model that can be used predictively.
The basic deep-water (below storm wave base) facies consists of classic turbidite monotonous alternations of parallel-bedded sandstones and shales. As the sandstones thicken and the shales become thinner or absent, the classic turbidite facies grades into massive sandstones and pebbly sandstones. These are characterized by vertical amalgamation of sandstones, channeling, and scouring. The distribution of these facies on modern submarine fans is understood only sketchily, and hence the predictive fan models have been constructed largely on facies relations and observed channels in ancient rocks, and on subsurface data. Classic turbidites, with excellent bedding continuity, suggest a smooth seafloor, whereas the massive and pebbly sandstones suggest a channelized inner fan. Different ty es of thin-bedded classic turbidites indicate levee, interchannel, and distal-fan fringe environments.
Sequences of turbidites in which beds become thicker upward may indicate progradation, and thinning-upward sequences may indicate channel filling. Both can be recognized on electric logs.
Fan models can lead to fantasies when applied uncritically as examples from the Ventura basin and Great Valley illustrate.
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