ABSTRACT

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. In order to better understand folding, methods are presented which separate the contribution various folding mechanisms to fold geometry. The folding mechanisms recognized are: mechanical folding, or folding due to faulting; drape compaction; and differential compaction. As a consequence of separating the various folding mechanisms, important geometric aspects of folding were recognized at Nordheim. They include: a) the upward movement of folds relative to regional dip, b) the shift of fold crests along dip and strike at various depths and c) the role of compaction in the final fold geometry. Upfolding is the term used to define the upward movement of folds relative to regional dip. Up folding is recognized where intervals thicken off a fold crest in all direction, and not just in the direction of the growth fault. For example the deepest interval in the lower Wilcox, the Migura, has over 200 ft of isopach relief and about 250 ft of closure. Shallower intervals and horizons show similar relationships between structure and isopach but with less relief. Upfolding is the dominant mechanism of folding in the Nordheim area. Most of the folding not explained by upholding is explained by either drape or differential compaction.

Mechanisms of folding related to growth faulting proposed by previous authors have only incorporated two dimensions. Upfolding is a three dimensional concept, and it is believed to be caused by material moving down a concave shaped 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.