Bradley J. Huizinga

The molecular fossil record of oleanane and its relation to angiosperms.

Oleanane has been reported in Upper Cretaceous and Tertiary source rocks and their related oils and has been suggested as a marker for flowering plants. Correspondence of oleanane concentrations relative to the ubiquitous microbial marker 17α-hopane with angiosperm diversification (Neocomian to Miocene) suggests that oleanane concentrations in migrated petroleum can be used to identify the maximum age of unknown or unavailable source rock. Rare occurrences of pre-Cretaceous oleanane suggest either that a separate lineage leads to the angiosperms well before the Early Cretaceous or that other plant groups have the rarely expressed ability to synthesize oleanane precursors.

Chemostratigraphic reconstruction of biofacies: Molecular evidence linking cyst-forming dinoflagellates with pre-Triassic ancestors

New data from numerous detailed mass-spectrometric studies have detected triaromatic dinosteroids in Precambrian to Cenozoic rock samples. Triaromatic dinosteroids are organic geochemicals derived from dinosterols, compounds known in modern organisms to be the nearly exclusive widely occurring products of dinoflagellates. We observed the ubiquitous occurrence of these dinosteroids in 49 Late Triassic through Cretaceous marine source rocks and the absence of them in 13 Permian-Carboniferous source rocks synergistic with the dinoflagellate cyst record. However, finding dinosteroids in lower Paleozoic and Precambrian strata presents challenging results for molecular paleontologists, evolutionary biologists, palynologists, and especially for those concerned with the food web at various times of biological crisis. Other than the few species known as parasites and symbionts, many other dinoflagellate species are important as primary producers. The presence of Precambrian to Devonian triaromatic dinosteroids gives chemostratigraphic evidence of dinoflagellates (or other organisms with similar chemosynthetic capabilities) in rocks significantly older than the oldest undisputed dinoflagellate fossils (dinoflagellate cysts from the Middle Triassic, ∼ 240 Ma), and older than the putative Silurian ∼ 420 Ma) dinocyst, Arpylorus antiquus (Calandra) Sargent, from Tunisia. This systematic chemostratigraphic approach can shed light not only on lineages of dinoflagellates and their precursors, but potentially on many other lineages, especially bacteria, algae, plants, and possibly some metazoans.

Multiple Oil Families in the West Siberian Basin

Two major oil families are identified in the West Siberian basin. Twenty-six of 32 analyzed oils occur in Jurassic and Cretaceous reservoirs and are derived from anoxic marine Upper Jurassic Bazhenov source rock, based on geochemical comparison of oils and source rock extracts. These oils are widely distributed both north and immediately south of the Ob River, and their biomarker ratios indicate a wide range of source rock thermal maturity from early to middle oil window (Van-Egan, Russkoye, Samotlor, Sovninsko-Sovyet, Olyenye, Ozynornoye, and Kogolym), to peak oil window (Srednekhulym, Yem-Yegov, Vostochno-Surgut, Khokhryakov, Fedorov, and Urengoi), to late oil window (Salym). Some of these oils have been mildly (e.g., Fedorov 75) to heavily (e.g., Russkoye) biodegraded n the reservoir. The Bazhenov-sourced oils show different compositions that support regional variations of organic facies in the source rock. For example, the anoxic marine Bazhenov facies, which generated the oils in the Samotlor, Fedorov, and nearby fields, was particularly clay poor and sulfur rich. Six nonbiodegraded, highly mature oils show geochemical characteristics that suggest they were derived from clastic-rich lacustrine or nearshore marine source rocks dominated by terrigenous higher plant input like those in the Lower to Middle Jurassic Tyumen Formation, although no correlation was observed between the oils and a single rock sample (Yem-Yegov 15) from the formation. The six oils occur in the Tyumen (Taitym, Geologiche, and Cheremshan) and fractured basement/Paleozoic (Gerasimov, Yagyl Yakh, and Verchnekombar) reservoirs in positions readily accessible to any oil migrating from the Tyumen source rock. For example, at the Gerasimov location, the Tyumen Formation lies unconformably on weathered basement-Paleozoic reservoir rocks. Most of the probable Tyumen-sourced oils ar from south of the Ob River, but the occurrence of Geologiche oil to the north suggests that related oils may be widespread in the basin.

Geochemistry of Selected Oils and Rocks from the Central Portion of the West Siberian Basin, Russia

Six analyzed oils, produced from Middle Jurassic to Upper Cretaceous strata in the Middle Ob region of the West Siberian basin, show biomarker and stable carbon isotope compositions indicating an origin from the Upper Jurassic Bazhenov Formation. The chemical compositions of these oils are representative of more than 85% of the reserves in West Siberia (Kontorovich et al., 1975). Bazhenov-sourced oil in Cenomanian strata in the Van-Egan field underwent biodegradation in the reservoir, resulting in a low API gravity, an altered homohopane distribution, and the appearance of 25-norhopanes without alteration of the steranes. High API gravity oil from the Salym field has surpassed the peak of the oil window, consistent with abnormally high temperatures and pressures in the Bazhenov source rock from which it is produced. The remaining oils are very similar, including samples from Valanginian and Bathonian-Callovian intervals in a sequence of stacked reservoirs in the Fedorov field. Bazhenov rock samples from the study area contain abundant oil-prone, marine organic matter preserved under anoxic conditions. Organic matter in a Bazhenov core from the Pokachev 58 well was compared with the oils because it is thermally mature and shows total organic carbon (TOC = 13.8 wt.%) and hydrogen index values (HI = 489 mg HC/g TOC) representative of the average for the formation (10.7 wt.% for 840 samples; 420 mg HC/g TOC for 75 samples). Other rocks in the Middle Ob region are far less likely to have generated these oils than the Bazhenov Formation. No prospective source rocks older than Middle Jurassic were available. Geochemical analyses indicate a lack of thermally mature Hauterivian or younger source rocks in the study area. Core samples from the Lower Cretaceous Frolov Formation either are thermally immature (Hauterivian) or show little oil-generative potential (Berriasian-Valanginian). The Upper Jurassic Vasyugan Formation shows lower oil-generative potential than the Bazhenov Formation. The average TOC for Vasyugan rocks is 3.20 wt.% (240 samples). Average atomic H/C ratios for Vasyugan and Bazhenov kerogens are 0.90 (10 samples) and 1.13 (25 samples), respectively. The Vasyugan Formation cannot be excluded as a source rock because insufficient sample was available for biomarker analysis. Core from the Lower to Middle Jurassic Tyumen Formation in the YemYegov 15 well was compared with the oils because it is thermally mature and shows TOC and HI values (2.78 wt.% and 137 mg HC/g TOC, respectively) indicating slightly more favorable oil-generative characteristics than the average for the formation (2.75 wt.% for 720 samples; 95 mg HC/g TOC for 25 samples). The core contains terrigenous, gas-prone organic matter that shows no relationship with the analyzed oils.