Nicholas B. Harris¹

(1) Department of Geosciences, The Pennsylvania State University, University Park, PA

ABSTRACT: Lacustrine Sequences And Hydrocarbon Source Rocks In Rift Bsins: Key Evidence In The [Image]13C Record Of Carbonate From The Early Cretaceous Congo Basin, West Africa

The richest hydrocarbon source rocks in lacustrine rift sequences commonly occur in the late rift stage. This investigation focuses on the Lower Cretaceous lacustrine rift sequence in the central Congo basin, West Africa, where late rift shales have generated approximately 3.8 BBO produced to date.

Deep lakes rapidly developed in the basin, following initial Neocomian rifting; basin-central locations were the site of largely shale deposition for nearly the entire rifting phase through early Aptian. The active rift phase is marked by deposition of interbedded sediment gravity flow sandstones with the shales, while the lower part of the late rift section contains interbedded carbonates,

Carbonate in lacustrine shales record a remarkably systematic trend of decreasing d13C. Values at the base of the section are relatively heavy at +6 to +8o/oo, probably reflecting input of detrital carbonate from Precambrian terranes in southern Africa. The 13C composition of carbonate decreases upward to values as low as -2 to -4o/oo in the late rift section. These low values must be due to input of light carbon from ground and surface water. The trend of decreasing d13C upsection is interpreted to reflect the progressive development of vegetation and soil thickness as rift topography degraded with passing time. As vegetation decayed, it producing isotopically light carbon dissolved in groundwater flowing to the rift lake. This suggests that deposition of rich source rocks was triggered by massive influx of dissolved carbon, derived from efficient recycling of vegetation in the waning stages of rifting.

AAPG Search and Discovery Article #90906©2001 AAPG Annual Convention, Denver, Colorado