Seismic Sedimentology by Stratal Slicing of Fluvial and Shallow-Marine Depositional System in Pliocene, Offshore Louisiana

Hongliu Zeng, Lesli J. Wood, and Tucker F. Hentz1

ABSTRACT

In Vermilion Block 50-Tiger Shoal fields, Offshore Louisiana, Pliocene depositional history is illustrated by stratal slicing in an 800-km2 3-D seismic volume. With limited bandwidth, sand bodies (5–30 m thick) are poorly resolved and difficult to pick in vertical seismic section, especially at regional scale. Most fourth-order sequence boundaries cannot be traced without well control. However, stratal slices picked at phantom geologic-time surfaces help to effectively solve the problem. Facies classification is based on analysis of amplitude patterns on stratal slices. Instantaneous attributes and coherency cube provide additional information. Recognized depositional elements include numerous fluvial channels (0.1–3 km wide) in a coastal plain setting, highly sinuous delta-plain distributary channels (0.1–3 km wide) and crevasse splays, neritic shale, and variable-size (1–40-plus km wide) incised valleys.

Viewing stratal slices in relative geologic time (movie) enables one to identify many depositional cycles (fourth-order sequences), with each one typically starting with an incised-valley system (LST), followed by shallow-marine facies (TST), and ending with a delta plain or coastal plain (HST). Incised-valley fills and the relict HST in the previous sequence commonly share a single seismic event (10–30 m thick). In many cycles incised valleys are headward eroded into a delta plain/coastal plain in the updip direction.

 

1Bureau of Economic Geology, The University of Texas at Austin, University Station Box X, Austin, Texas 78713; e-mail: hongliu.zeng@beg.utexas.edu; lesli.wood@beg.utexas.edu; tucker.hentz@beg.utexas.edu.