Diagenesis, Dewatering, and Source Rock Potential of Ordovician Shales from the High Atlas, Morocco

I. Jonathan Evans

The Ordovician shales of the High Atlas are extremely well exposed throughout their 1,000-m thickness. They provide an exceptional opportunity to study shale sedimentology and diagenesis on a large scale.

The shales are interpreted as shelf sediments. They contain uncommon thin, wave-rippled, fine sandstone layers which record rare high-energy events and are attributed to winter storm action. The shales coarsen upward and the sandstone horizons become more common toward the top of the sequence, reflecting increasing proximality up the sequence from an outer to a mid-shelf setting.

In places the shales contain large ferroan carbonate concretions. These have been analyzed together with the shales to determine the diagenetic and dewatering history of the sequence. Comparison of the chemistry of the noncarbonate fraction within the concretions with that of the host shales has led to quantitative mass-transfer models for the system. These suggest the sequence was an important source of cementing components (e.g., Ca, Si) for adjacent potential reservoir rocks. Other elements (e.g., Al, Mg, K) have not moved out of the system; the shales were not therefore a source for authigenic aluminosilicates as is often supposed. The major transport/dewatering pathways were the interbedded storm-sandstone horizons. These are now extensively quartz cemented. Local synsedimentary aults were probably also important conduits. These results have important implications regarding the relationship between shale and sandstone diagenesis.

The shales presently contain up to 1% TOC. In the study area, however, they are thermally postmature with respect to hydrocarbon generation. Equivalent rocks which have experienced less burial should be considered potential source rocks.

AAPG Search and Discovery Article #91032©1988 Mediterranean Basins Conference and Exhibition, Nice, France, 25-28 September 1988.