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AAPG GEO 2010 Middle East
Geoscience Conference & Exhibition
Innovative Geoscience Solutions – Meeting Hydrocarbon Demand in Changing Times
March 7-10, 2010 – Manama, Bahrain

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

Aamir Siddiqui1; Mitsuyoshi Kaneko2

(1) Reservoir Geology, Core Laboratories International, Abu Dhabi, United Arab Emirates.

(2) Abu Dhabi Oil Company, Abu Dhabi, United Arab Emirates.

The Upper Jurassic Diyab Formation was deposited during a marine transgression resulting from regional subsidence. The Diyab Formation consists of argillaceous lime mudstones and wackestones that change laterally eastward into peloidal packstones, grainstones, and dolomitic packstones. In Abu Dhabi, the Diyab Formation is subdivided into three informal members: lower, middle, and upper based on the lithology and gamma ray signatures.

Diagenetic silica occurrences in the Diyab intervals have been identified by conducting visual inspection of cores as well as petrographic analyses on some core samples. Silicification of carbonate host rock involves the precipitation of silica in the form of pore-filling silica cement as well as the replacement of carbonate by chert. Early mechanical compaction and sediment dewatering played important role in the siliceous skeletal particles dissolution, migration of silica rich fluids and the consequent precipitation of chert. Nodular chert is the most common diagenetic silica form observed in the Diyab Formation, whereas selective replacements and silica cement within carbonate samples also observed.

Most occurrences of nodular chert are encountered near and inside dolomitic layers of upper zone of the Diyab reservoir. Chert in carbonate rocks is generally known to be biogenic origin. However, within the Diyab reservoir occurrence is closely associated with the infiltrated brine which also caused dolomitization in the carbonate rocks. These minor chert replacements have not significantly affected reservoir quality, but their recognition is important in calibration of wireline log responses for lithology. Microfractures also developed within the nodules due to the brittle nature of chert and these fractures may also aid in the fluid transport within the reservoir.