Abstract

The Point Lookout Sandstone in the southeastern part of the San Juan basin was deposited as a moderate- to low-energy mainland or barrier beach which separated an extensive tidal creek and tidal distributary system from open-marine environments. Fluvial processes dominated locally along this northeastward-prograding coastline, resulting in development of dip-aligned, deltaic sandstone bodies. This regressive sheet sandstone is composed of numerous small-scale, asymmetric transgressive-regressive (T-R) cycles, three to five of which are intersected in a typical vertical section.

Recognition and correlation of individual T-R cycles (<100,000 yr in duration) facilitate division of the Point Lookout Sandstone into genetic “shoreline blocks” and allow construction of detailed paleogeographic maps of shoreline position. Using this technique, shoreline irregularities and subtle differences between wave- and fluvial-dominated portions of the strandline are recognizable. Variability in strandline orientation and energy regime strongly influenced the accumulation of potential reservoir sandstone bodies. This technique is thus a useful predictive tool in energy exploration.

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