ORGANIC FACIES OF EARLY CRETACEOUS SYNRIFT LACUSTRINE SOURCE ROCKS FROM THE MUGLAD BASIN, SUDAN

A. A. Mustafa* and R. V. Tyson*

*Fossil Fuels and Environmental Geochemistry (Postgraduate Institute), Drummond Building, University of Newcastle, Newcastle upon Tyne, NE1 7RU.

e-mail: r.v.tyson@ncl.ac.uk

Cuttings samples from eight wells in the Muglad Basin have been analysed using a combination of organic geochemistry and palynofacies. The lacustrine Aptian-Albian shales of the Abu Gabra Formation, previously identified as the main source rock, have an overall mean TOC of 1.43% (n = 146), with those from the NW part of the basin (Sharaf area) being approximately twice as organic-rich as those from the SE (Heglig area). The Abu Gabra Formation contains two distinct organic facies: a lower interval dominated by higher TOC values (1.5-2.3%), higher measured hydrogen indices (338-546), higher amorphous kerogen contents (>80%), and heavier d 13CTOC values (> 27); and an upper, less rich interval (mean TOC 1.4%, mean HI 83, d 13CTOC approximately 28). The isotopic contrast between the upper and lower units may potentially be of stratigraphic use. The organic facies differences appear to reflect deteriorating preservation of the organic matter (higher dissolved oxygen, possibly due to shallower conditions resulting from lower rates of subsidence). Use of S2 v. TOC plots suggests corrected true mean hydrogen indices of around 800 in the richer facies (corresponding to a Type I kerogen). Mean random vitrinite reflectance, pyrolysis Tmax values, and visually determined fluorescence colours indicate that the samples studied are mostly immature, or at most in the earliest part of the oil window (<0.7% VRo); biomarker data suggest that the vitrinite reflectance values may be suppressed by up to 20%.

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