Abstract: The Use of Biomarkers and Gas Chromatography to Imply a Potentially New Source Rock in the Paleozoic Sequence of Saudi Arabia

Gary A. Cole, Adnan A. Al-Hajji, Henry I. Halpern

During the drilling of a Paleozoic stratigraphic test in central Saudi Arabia, geochemical evaluation of the cuttings and core identified two sections of potential exploration importance. The upper sequence in the well contained 175 feet (53 m) of Silurian age marine, clastic source rock. This organic-rich (1-3% total organic carbon), oil-prone (average hydrogen index = 400 mg hydrocarbons/g TOC) source rock was immature to incipiently mature based on graptolite fluorescence, TAI, and biomarker isomerization parameters (hopanes and steranes were not equilibrated). The second sequence encountered that had exploration significance was a relatively clean Ordovician age sandstone 100 feet (30 m) below the source rock zone. An extract from this sandstone indicated the presence of a mature ydrocarbon derived from a marine carbonate (marl) sequence deposited under anoxic-reducing conditions.

Comparison of the sandstone extract to the Silurian source rock extract interval resulted in a poor match. The Silurian source rock had a biomarker and gas chromatographic (GC) signature consistent with a marine, clastic source deposited under dysoxic conditions. Biomarkers and GC clearly showed non-correlation between these two extracted hydrocarbons.

Comparison of the GC and GCMS data of the Ordovician sandstone "oil" extract to the InfraCambrian Huqf and to other potential Paleozoic source rocks and oils suggested that the Ordovician extract was not derived from these source rocks. Because most of the GC and biomarker data differed significantly between the Ordovician extract and all other samples examined, a different source rock derivation is likely. Possible source rock origins for this hydrocarbon could be a more deeply buried Ordovician (or possibly Cambrian) marine sequence, or even a different InfraCambrian source rock than those characterized from Oman since there are large InfraCambrian rift basins directly beneath this area.

AAPG Search and Discovery Article #90986©1994 AAPG Annual Convention, Denver, Colorado, June 12-15, 1994