New Insights of the Cretaceous Source Rock Potential in the Central Atlantic Ocean Using a New End-Member Workflow Based on Palynofacies and Rock Eval Data

Abstract

In the Central Atlantic Ocean, large amount of organic-rich sediments commonly referred as black shales are deposited and preserved from the shelf to the abyssal plain during the Cretaceous. Although the presence of these organic-rich sediments is widely observed in the Central Atlantic, the review of these black shales reveals many differences about the origin of the organic matter (marine and terrestrial) and their total organic carbon (TOC) up to 40% (e.g. Arthur et al. 1984; Summerhayes, 1981; Trabucho et al. 2010).

In this area, the OM type was generally assessed using Rock-Eval data (i.e. Hydrogen Index, HI). However, if pure terrestrial (Type III) and marine (Type II) OM have well defined and distinct HI values, numerous samples show intermediate values between these 2 end-members, suggesting a mixture of both sources. In these settings, palynofacies observation is the best method to verify the OM types and to determine the proportion of terrestrial and marine contributions in these samples. Eventually, this will help to assess, for each formation, the source-rock potential and the type of hydrocarbon that will be mainly produced.

Here, we have applied this combined approach to characterize the OM of Cretaceous sediments sampled from deep sites of the Northwest African Margin, including DSDP sites 367 and 369 from leg 41. Palynofacies data on 20 samples proved the presence of 3 end-members (i.e. Types II, III and IV) with specific ranges of HI values. They also highlighted a fourth end-member: a marine degraded OM with intermediate HI values between Types II and III, indicating that intermediate HI values are not related to marine and terrestrial mixtures in these wells.

The determination of specify end-members for the Northwest African Margin were then used to characterize and quantify their proportions using the available Rock-Eval data in order to evaluate the source-rock potential evolution throughout the early Cretaceous in the distal domain. This method highlighted: (1) the evolution of the contribution of terrestrial OM in the southern part of the margin from Albian to Turonian; (2) the impact of the global Oceanic Anoxic Event 2 (OAE 2) on the quality of the source-rock potential; and finally (3) a leaner northern domain with high contribution of terrestrial OM. This study thus proposes an updated review of the source rock potential of Cretaceous organic-rich sediments deposited in the eastern Central Atlantic Ocean.

AAPG Datapages/Search and Discovery Article #90323 ©2018 AAPG Annual Convention and Exhibition, Salt Lake City, Utah, May 20-23, 2018