Diagenetic Signatures in Incised Valley Fills: Differentiation of Sequence Boundaries from Diastems and Implications for Potential Reservoir Quality

Sharon A. Stonecipher

Determining whether a channelized sediment package represents an incised valley fill or a simple prograding system in an area of low accommodation and high sediment supply can be very difficult, especially in proximal or onshore regions where seismic resolution is insufficient to reveal diagnostic indicators of baselevel change such as incision or erosional truncation. However, making this determination is important not only for evaluating the geologic history of a basin, but also for predicting factors such as the existence of associated downdip lowstand sediments, plumbing systems, and reservoir quality. An increasing body of data suggests that it is possible to differentiate between valley fills and channelized sediments resting on a diastem on the basis of early diagenetic pattern .

The accepted definition of incised valley fills includes them as part of the lowstand system, however diagenetic evidence emphasizes that these sediment packages were deposited largely while relative baselevel was rising. The basal portion of incised valley fill commonly exhibits the characteristics of braided stream to tidally influenced fluvio-estuarine sedimentation and a diagenetic signature indicative of marine pore waters. Deposits underlying the incised valley show evidence of meteoric exposure. A highstand fluvial system resting on a diastem typically exhibits a meteoric diagenetic signature only. Some incised valley fills also contain later stage highstand fluvial deposits which appear to be amalgamated with the underlying fluvio-estuarine sequence, but which exhibit distinct y different diagenetic characteristics. Examples of several Interior Cretaceous Seaway sands (Frontier, Fall River, Muddy) from varying basins, ages, depths, and provenances but with similar depositional and diagenetic traits will be shown and compared with an example from the Miocene of Egypt to illustrate these patterns and their effect on reservoir quality.

AAPG Search and Discover Article #91019©1996 AAPG Convention and Exhibition 19-22 May 1996, San Diego, California