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.
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%.