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Practical Sequence Stratigraphy in Onshore Exploration and Development, With Examples from the Yegua and Cotton Valley Trends

By

EWING, Thomas E.

Venus Exploration, San Antonio TX

 

Seismic stratigraphy can be confusing and jargon-rich. However, five major principles of sequence stratigraphy are of practical use to the geoscientist working in the onshore Gulf Coast Basin. Examples are taken from the Yegua and Cotton Valley trends.

1) Sequence stratigraphy focuses on unconformities. We must appreciate depositional unconformities (generated by relative sea level (RSL) change or shelf-margin failure) as one defining element of the stratigraphic architecture. Unconformities make traps, bound isolated sands, and truncate structures. Find them and study them.

2) Regional unconformities on passive margins form by falling RSL. They indicate that sand may be transported downdip into lowstand prograding wedges (PWs) and deep-water environments, bypassing the sand-poor highstand shelf in incised valleys. Predict downdip sandstone accumulations from incised valleys, incised valleys from downdip sandstones, and downdip deep-water sandstones from slope failure unconformities.

3) Other unconformities form by erosion during rising RSL (transgression), and transgressive sandstones overly them. Recognizing transgressive unconformities is essential to reservoir development; transgressive sandstones have different geometries and reservoir properties.

4) If basinwide RSL changes drive sequences, then high-resolution stratigraphic correlation using flooding surfaces and unconformities, and guided by paleontology, can target little-explored areas for exploration. One key to trend exploration is recognition of RSL history.

5) Lowstand sandstones usually form in different depositional systems and environments than highstand sandstones. Lowstand PWs may build into deeper water, are more subject to gravity failure, and are more exposed to waves and currents than highstand deltas. Study each system separately.