Cenomanian-Turonian Source-Rocks in the Southern North Atlantic Ocean: Origin, Distribution, and Exploration Impact

Leyenberger, Terry

The Northeast Brazilian continental margin has several frontier basins which may hold significant exploration potential. The risk associated with source-rock presence, quality, and maturity were considered to be relatively low in this area. Low source risk is based on the existence of organic-rich black shale found regionally at the Cenomanian-Turonian stratigraphic boundary. This paper will discuss the origins of this source-rock and suggest the implications of understanding the tectonic setting, climate, and oceanographic setting of exploration plays.

Black shales have been deposited throughout the last 600 million years in the Earth's ocean basins. The existence of black shales is a function of the basin tectonic framework, climate, and oceanography. The basin tectonic framework determines where the climate and oceanographic conditions may leave a black shale record. The climate and oceanographic conditions determine when black shale is deposited.

The presence of Cenomanian-Turonian organic-rich source-rocks in the Atlantic Ocean basin have been documented by numerous IODP cores, outcrops, and exploration wells. The organic-rich facies present in these black shales is a function of both oceanic productivity and preservation. The complex interaction of oceanographic conditions, climate conditions, and tectonic events (volcanism) combine to produce ideal conditions for production and preservation of organic-material in the marine environment.

The existence of the precise climate and oceanic variables need for development of rich source-rock lasted for about 100,000 years at the transition between the Cenomanian and the Turonian, and is known as the Bonarelli Oceanic Anoxic Event II. This OAE is characterized by a large positive carbon isotope excursion, increased marine productivity, and water column anoxia.

During an OAE the marine environment is characterized by high productivity in the shallow water areas adjacent to the continental margin and in the continental slope and abyssal plain. Preservation of organic material is low in the shallow water areas, but high in the slope and abyssal plain areas. Hence the presence of potential organic-rich source-rocks is a function of both productivity and preservation.

AAPG Search and Discovery Article #90163©2013AAPG 2013 Annual Convention and Exhibition, Pittsburgh, Pennsylvania, May 19-22, 2013