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Sequence Stratigraphy and Origin of Silurian Organic-Rich “Hot Shales” of Arabia and North Africa


Grabowski, George, ExxonMobil Exploration Company, Houston, TX


The sequence stratigraphy of “hot shales” of the Silurian, determined from well-log cor­relation, provides a framework for interpretation of the origin of these organic-rich rocks.

The Silurian is a thick wedge of marine shale, with nearshore sandstones mainly in the upper part. Several intervals of organic-rich “hot shales” occur within the Silurian. A wide­spread basal “hot shale” up to 30 meters thick lies directly above and thins by onlap onto an erosional surface, or more rarely above an organic-poor shale. The overlying unit of organic-lean shale and siltstone thins as it progrades in a basinward direction, forming a subtle clinoform geometry. Above this lies a basinally restricted interval of shale, sometimes organic rich, which laps onto the underlying progradational shales. This pattern is repeated many times.

The basal “hot shale” and basinally restricted shales are interpreted as lowstand-trans-gressive units above sequence boundaries. The basinward-thinning progradational shale­siltstone units are interpreted as highstand deposits. As the highstand units thin, the basi­nally restricted shales come closer together and distally merge to form thicker units of shale. These merged “hot shales” are interbedded with limestones that formed due to the reduc­tion in detrital sedimentation in a distal setting.

Organic enrichment is related to the restricted basinal setting, presumably allowing for better preservation of organic matter, as well as to diminished detrital sedimentation, thus avoiding dilution of the organic matter in the sediment. Several intervals of “hot shale” occur in the oldest basinally restricted shales (early Llandoverian), suggesting greatest restriction at this time.