Traps Associated with Shale Domes


Introduction		Nearly all of the oil and gas associated with shale domes worldwide is in flanking beds; however, some oil and gas has been found in the rollover rims around the central sags in overlying beds.
Rollover rim example		The Encino field of San Patricio and Refugio counties, Texas has small gas and oil production in rollover rim traps. The cross section below shows this feature.
Pinchout and truncation example		The South Thornwell shale dome of Jefferson Davis and Cameron parishes, Louisiana, has associated pinchout and truncation traps. The cross section below shows these two traps. Note that the truncation trap has gas and water production against the shale diapir with gas only downdip.
Upturned and trucation traps		The West Bayou Carlin shale mass of Saint Mary Parish, Lousiana, has an upturned and a truncation trap. The cross section below shows these traps. Note the low resistivity zone in the shale next to the diapir.
Exploration mistakes		Most first wells on shale domes are located over the crest of the shale body. These wells drill through the crestal sags and directly into shale (such as Cities Service wells in the South Thornwell example on the previous page); thus they penetrate nonprospective beds all the way. It usually takes several more years before the more prospective outer flank wells are drilled. The beds below shale domes are not affected by the overlying domes, and correspondingly oil and gas prospects in deeper beds are independent of the domes. The principal production in the Reconcavo basin is from sands below the diapiric shales.
Table of Contents		Geophysical Characteristics of Shale Domes