HYDROCARBON SOURCE ROCK ASSESSMENT AND REVISED BIOSTRATIGRAPHY OF DSDP SITE 369A, OFFSHORE NORTHWEST AFRICAN MARGIN

Abstract

Deep Sea Drilling Project (DSDP) Site 369A, drilled off Cape Bajador, NW Africa, provided an opportunity to examine the source rock potential and biostratigraphy of the offshore Aaiun‐Tarfaya Basin. Within the Albian‐Campanian interval, total organic carbon (TOC) contents were found to be highly variable and cyclic, ranging from less than 0.5% to nearly 12%. Variations in hydrocarbon generation potential (S1 + S2) versus depth in general paralleled variations in TOC. Organic matter was generally marine in origin and oil‐prone. The newly generated data reported in this paper expand the range of TOC values which have been reported from the study region. However, they also illustrate the kinds of problems which can be introduced as a result of sampling bias. Sampling for this study tended to focus on darker‐coloured, more organic‐rich lithologic core intervals, contrasting with previous shipboard analyses which appeared to have focused on lighter‐coloured intervals. Based on the abundance of calcareous nannofossils, the elevated TOC contents appear to have been, at least partly, driven by elevated surface productivity which in turn increased biological oxygen demand within the sediment. New biostratigraphic data was used to identify an unconformity between the late Albianl early Cenomanian and the early Campanian successions, consistent with previous interpretations. Integration of the geochemical and biostratigraphic datasets indicates that these organic‐rich sediments are in general equivalent to Cretaceous Oceanic Anoxic Events 1B, 1D and 3. Considering the apparent net thickness of these oil‐prone sediments and the possibility that they may be widely distributed throughout the basin, they may have the potential to generate commercially significant volumes of oil where buried sufficiently deeply.

Geochemical Evidence for Large‐Scale Drainage Reorganization in Northwest Africa During the Cretaceous

“…With the exception of well CM1 located on the continental shelf, the sedimentary rocks from the DSDP sites were deposited in deep pelagic environments occasionally influenced by distal turbidity currents (Jansa et al, ). Samples from DSDP sites were dated according to the biostratigraphical scheme defined for each DSDP site (Cepek, ; Cool et al, ; Foreman, ; Hayes et al, ; Lancelot et al, ; Lancelot & Winterer, ; Pflaumann & Čepek, ; Williams, ). The time resolution of samples is at the substage level (i.e., ∼2 to 3 Ma) for the Late Albian‐Turonian interval and at the stage level (i.e., ∼5 to 10 Ma) for other intervals, which is sufficient to identify long‐term trends in sedimentary provenance (Figure ).…”