A petroleum system is designated by the name of the source rock, followed by the name of the major reservoir unit and the confidence factor (Magoon, 1992). Three levels of confidence are acknowledged: (!) for known, (.) for hypothetical, and (?) for speculative. This paper concentrates on the geochemical aspects of the Maikop/Diatom-Productive Series (!) Petroleum System.

Analysis of more than 200 potential source rocks and 100 crude oils concludes that the primary hydrocarbon source rock is the Maikop/Diatom sequences of Miocene and Oligocene age. This dual source rock assignment is justified on molecular and isotopic evidence, along with stratigraphic complexities observed in outcrop intervals. The single genetic oil family is divided into three subtypes with a combined biomarker-isotope correlation technique. These subtypes correspond to subtle variations in the source rock facies that contribute to the hydrocarbon charge; the distribution of oil subtypes aids in the recognition of migration trends. Specific parameters used in the correlation include a terpane fraction with abundant pentacyclics that exponentially decrease with increasing carbon number, accompanied by minor to moderate amounts of gammacerane, oleanane (including des-A variety), 29,30-bisnorhopane, and the C24 tetracyclic terpane. The C30 diahopane, C29Ts, 2a(methyl)hopanes, and resinous biomarkers (e.g., bicadinanes) occur in minor to trace amounts. The tricyclic terpanes are dominated by the C23 and C24 members, whereas the C15 and C16 regular and rearranged drimanes dominate the bicyclic terpanes. The samples also contain minor amounts of the C25 highly branched isoprenoid (the diatom biomarker), ·b-carotane, and aryl-isoprenoids. The carbon number distribution of the sterane fraction shows nearly equal numbers of C27, C28, and C29 members, but relatively low abundance of the C30 and rearranged series. The latter observation is also extended to include the ring-C monoaromatic steranes as well. The carbon isotopic composition of the oil saturate fraction range between -24.8 and -28.0 per mil, whereas the aromatic fraction ranges from -23.4 and -27.4 per mil. On the modified Sofer-plot, these oils plot below the best-fit line, consistent with oils of marine origin. In general, the oils produced from onshore fields are isotopically depleted compared to those produced from offshore fields. This data combined with variation on the molecular level, form the basis of the biomarker-isotope definition of three oil subtypes.

Variation in crude oil chemistry provides additional clues to the secondary alteration events that influence crude oil quality in the region. The crude oils are commonly altered by biodegradation and water washing (76% of dataset), and complications related to mixing of oil phases are common (54% of dataset). Secondary charging of reservoirs, phase separation, and gravity segregation are interpreted to occur within various fields. Analysis of crude oil maturity indicates that the expulsion from the source rock occurs relatively early (0.7-0.8 vitrinite reflectance equivalence); the expulsion event is promoted by aquathermal overpressure (enhanced by rapid burial) and the intercalation of source rock/migration conduit couplets. Migration models are constructed for various regions. Most of the onshore fields have a strong lateral component, whereas the offshore fields have a strong vertical component requisite with the fault/fracture systems and disruption by mud diapirs. Giant fields generally charged by combined vertical-lateral components, whereas the migration pathways in the Kura Depression are strongly influenced by paleo-channels that flowed along the river valley strike.

The bulk of the reserves are hosted in Middle Pliocene sandstone reservoirs (Productive Series); these are part of the fluvial-deltaic depositional systems ass elated with a paleo-Volga provenance. Smaller hydrocarbon accumulations are noted with reservoirs younger and older than the Middle Pliocene, and with increased distance from the source rock depocenter. Giant fields contain oils with biomarkers that are closer to equilibrium values; the implication is that the source rocks in the drainage area for the trap experienced higher oil generation levels. In cases where diverse maturities are observed in a single field (e.g., Bibi Eibat), the reservoir that is responsible for the giant field classification contains the oil that is near equilibrium. The giant fields are also in a position to receive a hydrocarbon charge from multiple expulsion events; this is favored where growth faults are present and/or are associated with mud diapirs.

The source rock for the oil is dominated by organic matter input from marine sources that was deposited in suboxic to anoxic conditions, with lessor contributions from terrestrial sources. Overall, the kerogen composition of the effective source rock facies is defined as a fairly uniform Type II/I organic matter (algal/bacterial). The contribution of organic matter from terrestrial sources (Type III) is higher in the onshore outcrop sequences, relative to samples collected from the offshore sequences; a relationship that generally covaries with source rock potential and molecular indicators. The source rock analyses also allow regional thermal maturity gradients to be constructed. The low thermal gradient is attributed to the depositional history and contributes to the interpretation that peak oil generation occurs between 7000 and 8000 meters in burial depth (based on crude oil maturity analysis; Wavrek et al., 1996). In areas with effective source rocks, peak oil generation was initiated during the middle Pliocene and continues to the present. The interpretation that effective gas generation occurs at 8000 to 10000 meters of burial depth (Wavrek et al., 1996) is confirmed by the results of the isotopic gas analyses reported by Abrams and Narimanov (1997). Finally, it is noted that the samples from the Pliocene Productive Series that contained good to excellent amounts of total organic carbon are documented to be stained with migrated hydrocarbons; indigenous organic matter in these rocks is devoid of hydrocarbon source potential.

AAPG Search and Discovery Article #90937©1998 AAPG Annual Convention and Exhibition, Salt Lake City, Utah