During the Tertiary, it is postulated that third and fourth order eustatic cycles have controlled the position of an oxygen minimum layer (OML) on the continental slope of Nigeria. The occurrence and extent of an OML in the outer neritic to upper bathyal water depths along the continental margins of India, California and Peru, is well documented. The development of the anoxic layer is related to upwelling and the subsequent depiction of oxygen from the water column as organisms die and decay. Once the oxygen has been depleted, excess organics deposited within the OML are available for preservation. Data available from the National Oceanographic Data Center indicates that an OML is developed today off the coast of Nigeria in water depths of 150 to 500 meters. Similar marine conditions to the modern have existed back to the Miocene. However, oceanic conditions during the Eocene and earlier were significantly different than today and may have resulted in an expanded OML and enhanced opportunities for source rock preservation. Biostratigraphic analysis of the offshore shelf wells recognized discrete, repeated intervals of amorphous organic matter deposited in dysoxic bottom waters. Geochemical analysis of the same intervals found oil prone source rocks with total organic carbon in excess of 2.5% and hydrogen indices greater than 300 mgHC/gOC. Hydrous pyrolysis of potential Tertiary source rocks has established a possible correlation to offshore shelf oils. Additionally, hydrous pyrolysis of asphaltenes of biodegraded onshore Cretaceous seeps indicates that the source rock responsible for onshore Cretaceous seep oil is distinctly different from that which sourced many Tertiary reservoired oils in the delta. Understanding the sequence stratigraphic development of the Niger delta together with the backstripping will lead to an enhanced ability to predict the distribution of these Tertiary marine source rocks.

AAPG Search and Discovery Article #90928©1999 AAPG Annual Convention, San Antonio, Texas