Shrinkage of Shale Cores in Shale Domes


Introduction		The most universal characteristic of shale domes, as contrasted to salt domes, is that the shale cores lose water, and thus shrink after emplacement. The shrinkage results in local sags in the overlying beds and, in some basins, in downward-towards-the-shale tilting of adjacent beds.
Shale domes showing shrinkage		None of the Texas domes exhibit indisputable down-toward-the-core tilting of adjacent beds; however, seismic cross sections indicate that such tilts are common around the shale domes in the Frontera region of southern Mexico. The shale domes in the Caspian Sea reportedly exhibit crestal sags and high pressures in the cores and in the peripheral low permeability sheaths but rarely exhibit down-toward-the-core tilting in adjacent beds. Shrinkage of the shale cores may result in a system of ring faults dipping into the shale. This has been well established in the Reconcavo Basin. A shale dome in the far offshore of the Eugene Island area of Lousiana also exhibits this type of faulting.
Crestal sag example one		South Taft dome area of San Patricio County, Texas, displays the effects of shale shrinkage on the overlying section of sediments. A structure map and cross section are shown below.
Crestal sag example two		The map and cross section below are of the North Laward shale dome, Jackson County, Texas. The map is a structure contour map on the Greta (Frio) Sand. Shrinkage of the shale diapir under the dome caused deformation in the overlying section.
Ring faults example		The Carijo diapir of the Reconcavo basin, Brazil, is flanked by ring faults dipping into the diapir that resulted from shrinkage of the shale mass. The cross section below shows these faults.
Table of Contents		Traps Associated with Shale Domes