oa Nodular Chert Occurrences in the Upper Jurassic Diyab Formation, Abu Dhabi, U.A.E

The Upper Jurassic Diyab Formation was deposited during a marine transgression resulting from<br>regional subsidence. The Diyab Formation consists of argillaceous lime mudstones and wackestones<br>that change laterally eastward into peloidal packstones, grainstones, and dolomitic packstones. In Abu<br>Dhabi, the Diyab Formation is subdivided into three informal members: lower, middle, and upper based<br>on the lithology and gamma ray signatures.<br>Diagenetic silica occurrences in the Diyab intervals have been identified by conducting visual inspection<br>of cores as well as petrographic analyses on some core samples. Silicification of carbonate host rock<br>involves the precipitation of silica in the form of pore-filling silica cement as well as the replacement of<br>carbonate by chert. Early mechanical compaction and sediment dewatering played important role in<br>the siliceous skeletal particles dissolution, migration of silica rich fluids and the consequent<br>precipitation of chert. Nodular chert is the most common diagenetic silica form observed in the Diyab<br>Formation, whereas selective replacements and silica cement within carbonate samples also observed.<br>Most occurrences of nodular chert are encountered near and inside dolomitic layers of upper zone of<br>the Diyab reservoir. Chert in carbonate rocks is generally known to be biogenic origin. However, within<br>the Diyab reservoir occurrence is closely associated with the infiltrated brine which also caused<br>dolomitization in the carbonate rocks. These minor chert replacements have not significantly affected<br>reservoir quality, but their recognition is important in calibration of wireline log responses for lithology.<br>Microfractures also developed within the nodules due to the brittle nature of chert and these fractures<br>may also aid in the fluid transport within the reservoir.