Exercise

The following exercise lets you try your hand at constructing a sequence-stratigraphic framework using well-log-curve suites from five wells. The wells penetrate a middle Miocene succession from Vermilion Block 50 field, offshore Louisiana. The cross section is oriented along depositional strike, and well spacing is about 1,500 ft. The succession, composed of sandstone, siltstone, and shale, represents deposition during the Miocene on the northern Gulf of Mexico shelf. Laterally aligned major shale-bearing intervals in the cross section are approximately equivalent.

There are three steps in this exercise: (1) identify major stratal stacking patterns on well logs, (2) delineate key sequence-stratigraphic surfaces and correlate them among the well logs, and (3) designate the systems tracts bounded by these key surfaces. A description of each step follows.

1) Identify Major Stratal Stacking Patterns
First, identify each of the three types of large-scale stacking patterns on each well log: progradational, retrogradational, and aggradational (fig. 2). These log patterns are the rock record of major depositional processes during the cyclic rise and fall of relative sea level. Focus on the gross vertical successions of sandstone and shale that characterize each stacking pattern and determine where the progradational, retrogradational, and aggradational trends are represented in each well log. Sandstones exhibit pronounced leftward deflections relative to shale intervals on both the SP and GR logs. Note that each of the three types of stacking patterns is not necessarily developed in all the logs in this exercise.

2) Identify and Correlate Key Sequence-Stratigraphic Surfaces
The next step is to correlate these stacking patterns among the well logs of the cross section. Mark and correlate the key sequence-stratigraphic surfaces that bound them. Mark all sequence boundaries, maximum flooding surfaces, and transgressive surfaces on the cross section.

Sequence boundaries occur at the top of principal upward-coarsening, progradational successions and at the bases of blocky and blocky-serrate, fluvial incised-valley fills. The maximum flooding surface is found at the top of principal upward-fining, retrogradational well log patterns. It coincides with the marine condensed section at the position of maximum GR value. Transgressive surfaces occur at the bases of the retrogradational successions.

3) Designate Systems Tracts
The final step in this exercise is to identify the lowstand, transgressive, and highstand systems tracts defined by the key surfaces that you have already correlated. Use the paint tool to color these zones on the cross section.