Abstract: Organic Facies and Geochemistry of Petroleum Source Rocks in a Middle Cretaceous Transition-Drift Sequence in Northern Gabon

Kuo, Lung-Chuan - Conoco Inc.

An Aptian-Albian transgressive sequence deposited during a transition from continental rifting to drifting in northern Gabon contains petroleum source rocks with large variations in organic richness and quality. The TOC content (0.04-10.71 wt.%), hydrogen index (86-691 mgHC/gTOC), and Rock-Eval S1+S2 (0.08-73.91 mgHC/gRock) of 224 samples from 8 wells all exhibit log-normal distribution. In the Dentale and Gamba Formations (fluvial-deltaic), shales deposited in swampy lakes usually have high TOC content, contain algal organic matter, and have potential of generating waxy and non-waxy oils, whereas those deposited in abandoned channels, oxbow lakes, crevasse splays, levees, point bars and floodplains have lower TOC content and hydrogen index, contain larger amount of woody organic matter, and have mixed potential of oil and gas generation. The Vembo shales (lagoonal) contain bacterial and terrigenous organic matter and vary from oil-prone to gas-prone. The Ezanga Salt (evaporitic) has no hydrocarbon source potential. The Namina shales (shallow marine) contain mainly marine algal organic matter and are prone to generating non-waxy oils. The organic facies variation from deltaic, fluvial, shallow marine, to lagoonal are clearly defined by a systematic increase in gammacerane index and decrease in pristane/phytane ratio (Fig. 1) which reflect an increase in water salinity and decrease in the redox potential of the source sediments.

The north-south trending Atlantic Flexure divides two major petroleum systems in northern Gabon. Oils in the pre-salt system to the east are generated mainly from the Melania and Kissenda (freshwater lacustrine) source rocks, and those in the post-salt system to the west are generated from the Azile-Anguille and Namina (marine) source rocks. Basin modeling suggests that the separation of these two systems is controlled by the maturation history of the basin and the presence of the Ezanga salt. In the Ogooué Delta area, the distribution of Azile-Anguille-sourced and Namina-sourced oils appears to be related to the migration of the depocenters during the evolution of the delta. From the Ogooué Delta to the Atlantic Flexure, the Azile-Anguille source rocks are early mature, whereas the pre-salt oils and gases are trapped below the Ezanga salt, thus the Namina shale is likely the major source of oils. Between the Atlantic Flexure and the Lambarene Horst, the Ezanga salt prohibits oil and gas migration from the pre-salt source rocks to the post-salt reservoirs. In the Interior Basin, oil generation and expulsion is limited only to the deepest portion of the Kissenda source rock. To the north of the Interior Basin, oil generation from the pre-salt source rocks predated seal development and the post-salt source rocks are immature (Fig. 2).

The exploration results to date suggest that the post-salt petroleum systems off shore northern Gabon have good potential for future oil and gas discoveries. Basin modeling will be useful for studying the timing and phase of hydrocarbon generation and migration from the Namina and Azile-Anguille source rocks and its relation to reservoir and seal development.

AAPG Search and Discovery Article #90933©1998 ABGP/AAPG International Conference and Exhibition, Rio de Janeiro, Brazil