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Journal of Petroleum Geology, vol.
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: [email protected]
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 q13CTOC values (> -27%thou); and an
upper, less rich interval (mean TOC 1.4%, mean HI 83, q13CTOC approximately -28%thou). 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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