Source Within the Seal: Organic-Rich Shales Within the Onion Creek Diapir, Moab, UT

Abstract

The Pennsylvanian Paradox Formation near Moab, Utah is composed of interbedded evaporites, shales, and limestones. Sediment loading caused by the uplift of the Uncompahgre orogeny caused the Paradox salt, a ‘dirty’ salt containing a substantial volume of non-evaporitic strata, to form a series of diapiric salt walls parallel to the NW-SE orogenic front. At present, the Onion Creek diapir, one of the most proximal diapiric salt structures in the basin, is exposed at the surface. Exposures show that appreciable amounts of the Paradox shale were incorporated within the salt diapir, both along the margins and within the interior. Some of the stringers of non-evaporitic strata preserved within the diapir are of substantial size – the largest blocks of interbedded shale and limestone observed within the diapir are upwards of 20 meters thick and over 100 meters along bedding. A large sample suite of the Paradox shale has been collected along the margins and within the interior of the exposed caprock at Onion Creek, and samples have been analyzed using source rock pyrolysis techniques. Pyrolysis results show that samples typically have high Total Organic Content (TOC) values (upwards of 20% TOC), and are Type II to Type II/III oil-prone source rocks. Tmax values and PI ratios from the sample suite show that samples have passed through the early to main oil window, indicating hydrocarbons have been generated. In multiple localities throughout the diapir, hydrocarbon staining can be seen in proximity to shale exposures, and analysis of the altered strata shows that there are hydrocarbons associated with this staining. This alteration can be seen throughout the diapir from the margins to the center, and within all Paradox Formation lithologies including salt, suggesting that some expulsion has occurred even in instances where these isolated packages of shale are fully encapsulated within salt. These observations have implications for systems where a large amount of organic-rich shale is interbedded within evaporites, and perhaps broader implications for the numerous salt basins worldwide in which salt is invoked as a hydrocarbon seal.

AAPG Datapages/Search and Discovery Article #90259 ©2016 AAPG Annual Convention and Exhibition, Calgary, Alberta, Canada, June 19-22, 2016