Abstract: Niger Delta Petroleum Systems

Haack, R. C.; P. Sundararaman; J. O. Diedjomahor - Chevron Nigeria Limited; N. J. Gant - Chevron Overseas Petroleum Inc.; J. Dahl - Stanford University

Regional integration of results from conventional exploration geochemistry, gravity-magnetics and structural analysis provide a comprehensive new understanding of Niger Delta petroleum systems.

Nigeria is the twelfth largest producer of crude oil in the world. Daily oil production from the Niger Delta is 2.2 million barrels, and recoverable reserves are estimated to be around 21 billion barrels. Historically, structural play-types have dominated, although large stratigraphic traps have been discovered. The basin has matured through one cycle of successful exploration, and future success depends on linking the geology of shelf/onshore areas to deep water as well as new play types in older producing areas.

Three petroleum systems are present in the Niger Delta and delta frame, including Lower Cretaceous (Lacustrine), Upper Cretaceous-Lower Paleocene and Tertiary Deltaic. One biodegraded seep oil from Nigerian Tar Sands along the northern flank of the Dahomey Embayment has been correlated to Neocomian source rocks in Ise-2. A source-rock extract and pyrolyzate of the seep are similar to the "Bucomazi system" in the Lower Congo Basin. Oil recovered from Paleogene sands in Shango-1 is inferred to be derived from Upper Cretaceous-Lower Paleocene source rocks identified in Epiya-1, consisting of Types II and II-III kerogens. The principle source for oil and gas in the Niger Delta, however, is the Tertiary Deltaic petroleum system, consisting of Types II, II-III and III kerogens. Source-facies variation, inferred on the basis of oils and source rocks, is regionally mapped and responsible for controlling, along with burial, the complex distribution of gas and oil across the Delta.

For example, regional variation in organic matter input, determined by mapping the relative abundance of C₂₉ steranes, demonstrates effectively how source facies change across the Delta. The dark gray areas with higher C₂₉ sterane content (Fig. 1) have the highest organic matter contribution from land plants and represent proximity to paleodrainage systems during source-rock deposition. The light gray areas have the lowest land-plant contribution, which probably indicates source-rock deposition under more marine-influenced conditions.

A simple episodic hydrocarbon-migration model (Fig. 2), that incorporates source-facies variation, provides a new framework for studying the complex GOR distribution across the Delta, within fields and on a sub-regional scale. The model predicts changes in the composition of generated hydrocarbons through time, which occur within the geological framework of an evolving and dynamic petroleum system, including hydrocarbon generation, migration pathways, reservoirs and traps. Secondary processes and variable gas solubilities, beyond the scope of this paper, also play important roles in understanding distribution of gas and oil in the Niger Delta.

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