Abstract: Geometry and Mechanisms of Folding Related to Growth Faulting in Nordheim Field Area (Wilcox)

By

Lee T. Billingsley

The Nordheim area in western De Witt County, Texas has produced over 121 billion cubic ft of gas, of which over 53 billion cubic ft has come from the deep lower Wilcox. Consequently, a better understanding of folding in the Nordheim area should aid future exploration efforts, especially in the deeper (greater than 10,000 ft) Wilcox. Methods are presented which show how various folding mechanisms have influenced the present day fold geometry. The folding mechanisms recognized are: mechanical folding, or folding due to faulting; drape compaction; and differential compaction. Important geometric aspects of folding at Nordheim include: a) thin intervals overlying fold crests, b) the shift of fold crests along dip and strike at various depths and c) the role of compaction in influencing the final fold geometry.

"Upfolding" is the term used to describe the fold mechanism which causes thin intervals over fold crests. The thin intervals indicate upward movement relative to regional dip because the thickening occurs in all directions from the fold crest, and not just in the direction of the growth fault. Upfolding is the dominant mechanism of folding in the Nordheim area. Most of the folding not caused by upfolding is caused by either drape or differential compaction. For example, upfolding of the Migura horizon in the lower Wilcox caused over 200 ft of thickening off the fold crest in the overlying interval. Total closure at Migura level is about 250ft. Shallower intervals and horizons show similar relationships between structure and isopach but with less relief.

Mechanisms of folding related to growth faulting by previous authors have been limited to two dimensions. In contrast, upfolding is a three dimensional process. It is believed to originate from material moving down a concave listric normal fault. The concave shape may cause a volume problem, which is overcome by the upward movement of material.

Three specific exploration concepts have been developed as a result of this study.

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