Abstract

Based on differences in mineralogy, paleovalley morphology, hydrocarbon production history and crosscutting Stratigraphic relationships, the subsurface Basal Quartz in the study area is interpreted to comprise four paleovalley fills consisting of braided-fluvial strata overlain abruptly by meandering fluvial strata. Significantly these strata accumulated in an accommodation-limited part of the basin where net sedimentation rate was only about 1 m/m.y. Accordingly, stratal characteristics contrast markedly with those in more extensively studied nonmarine successions that accumulated in high net-accommodation basins, and include (1) each unconformity bounded sequence is significantly thinner, (2) the upward change from braided-fluvial to meandering-fluvial or tidal strata occurs abruptly, and (3) a general lack of finegrained floodplain mudstone, which improves significantly the lateral connectivity of reservoir sandstones. A long-term effect of limited net accommodation, and the consequent slow aggradation of the sedimentary pile, is the repeated cannibalization of older valley-fill deposits by younger paleovalleys. This creates a stratigraphy consisting of laterally and vertically consistent stratal assemblage dominated by a single, typically the youngest, paleovalley fill. In places, however, parts of older paleovalley fills are preserved as localized remnants surrounded on most sides by younger strata. On a local scale, this results in the development of a significantly more complicated stratigraphy marked by rapid lithological changes, and similarly reservoir quality, even over short horizontal distances. On a more regional scale, the complex crosscutting of different-aged paleovalley fills, in addition to the tendency for each paleovalley to incise from a similar Stratigraphic level, makes Stratigraphic correlation difficult. Together, these characteristics can have significant Stratigraphic and economic ramifications.

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