SILURIAN — LOWER DEVONIAN BLACK SHALES IN MOROCCO: WHICH ARE THE ORGANICALLY RICHEST HORIZONS?

Abstract

Lowermost Silurian (Rhuddanian, lower Llandovery) black shales represent the main Palaeozoic petroleum source rock throughout North Africa and Arabia. The unit also occurs in parts of Morocco with recorded TOC values of up to 10.5%. However, in contrast to many other North African and Arabian countries, the Silurian‐Lower Devonian shale‐dominated succession in Morocco also contains a number of other horizons with elevated organic contents. In order to evaluate the organic richness and better understand the depositional mechanisms of this shale succession in Morocco, samples were collected from petroleum exploration wells, from the spoil heaps of shallow water wells and from outcrops, and were subsequently analysed. Graptolite biostratigraphy provided a high‐resolution correlation framework. The data was integrated with that from published and unpublished studies, and the results may help in improved predictions of the source quality of Silurian—Lower Devonian strata in Morocco.An Aeronian (middle Llandovery) shale sample with a high organic content (4.35% TOC) was recovered from the NE margin of the Tadla Basin, central Morocco. Based on gamma‐ray data from the subsurface, however, this horizon appears to be laterally discontinuous within the basin. Secondly, late Telychian‐Wenlock shales from the eastern Atlas Mountains were found to have TOC values of around 2.5%, and may be correlated with age‐equivalent organic‐rich strata in the Ghadames Basin (eastern Algeria, western Libya, southern Tunisia) and Iraq. Late Silurian shale‐limestone alternations in Morocco apparently do not contain major amounts of organic mattel; although comparable deposits in parts of western Algeria are believed to be organic rich. Early Devonian graptolitic black shales from the Tadla Basin and its margins contain high amounts of organic matter (around 5% TOC); however; the lateral continuity of this unit is at present unclear.We propose that high primary productivity during the Silurian‐Early Devonian provided the basis for the formation of these organic‐rich shales in Morocco. Sea level changes may have been an important additional factor. For example, the Rhuddanian organic‐rich shales were deposited during the initial stages of a transgression, when circulation was still restricted due to the pronounced pre‐Silurian relief. At other times during the Silurian‐Early Devonian in Morocco, deposits with elevated organic content seem to have been formed during periods of high sea level, which may have been associated with high primary productivity and/or a rise in the oxygen minimum zone onto the shelf.