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Dense Zones of the Kharaib Formation (Lower Cretaceous), United Arab Emirates
- Publisher: European Association of Geoscientists & Engineers
- Source: Conference Proceedings, 81st EAGE Conference and Exhibition 2019, Jun 2019, Volume 2019, p.1 - 5
Abstract
The so-called “dense zones” (intervals of very low porosity) separating the thicker, high-porosity reservoir intervals of Lower Cretaceous limestone strata in the Abu Dhabi subsurface have been studied in cores from a giant onshore oilfield. Although holding no oil reserves, the dense zones are important elements of reservoir architecture and essential components of any sequence stratigraphic interpretation, yet little substantive work has been reported regarding their description and origin. The present study of the two dense zones enclosing the upper reservoir zone of the Kharaib Formation reveals that they have similar ranges of bulk chemical composition, with much higher aluminum, iron, potassium, thorium, and uranium than the intervening reservoir zone, but are very different from one another in depositional texture. The upper dense zone consists mainly of peloid-orbitolinid packstone, whereas the next-lower dense zone consists mainly of orbitolinid mudstone. Both dense zones display cycle-like gamma ray variations that correlate throughout the field, reflecting varying content of clay and pyrite. The high clay content of the dense zones accounts for their extreme early loss of porosity and is explained as the result of their stratigraphic position, associated with (either preceding or immediately following) third-order sea-level lowstands.