Petroleum Systems of the Greater Liberia Basin, West Africa

The greater Liberia basin of offshore Liberia and southern Sierra Leone has, until recently, been neglected by the petroleum industry due to i.) a perception that it is incapable of generating and expelling abundant hydrocarbons, ii.) a perception that traps are compromised due to their stratigraphic nature. The 2009-2010 discoveries of trapped, multi-phased hydrocarbons in the deepwater basin suggest the presence of one or more petroleum systems.

The Cretaceous history of the greater Liberia basin is important to understanding the petroleum system(s). Basin shape, segmentation, age of oceanic crust, location of the ocean-continent boundary, paleo-temperature, and erosion were inputs used to create a series of basin models.

Trapped, sampled oils are likely early-middle maturity if derived from Turonian marine kerogens. Late maturity oils and gases are characteristic when derived from Cenomanian and Albian marine and marginal-marine kerogens.

In the deepwater basin, basal heat flow values below Tertiary-Upper Cretaceous overburden are best represented by a range of values that vary between purely continental and oceanic crustal types; these are tied to temperatures gradients in the deepwater wells. Most shelf penetrations, by contrast, reveal a shallow paleo-oil window that is not present today. This observation suggests either high heat flow during the Lower Cretaceous rifting event, or more recent uplift and erosion.

A geologic risk prior to drilling in deepwater was the presence and maturity of Ceno-Turonian sources, which would be in the early oil maturity window (Ro = 0.70-0.80) at present-day burial. However, evidence suggests that this, and other organic intervals above this proposed source interval were buried more deeply in the past.

Oil expulsion relative to reservoir and seal deposition is favorable in most, but not all, cases modeled. Cool thermal models for the youngest marine source intervals suggest Tertiary expulsion and migration. Warm thermal models for the oldest marine source intervals suggest Middle-Late Cretaceous expulsion and migration. Later gas charge has introduced at least one, dual-phased petroleum system in the northwest part of the basin. Primary risk rests with timing of erosion and breach, followed by later gas charge.

AAPG Search and Discovery Article #90142 © 2012 AAPG Annual Convention and Exhibition, April 22-25, 2012, Long Beach, California