Abstract: Geochemical Evaluation of Hydrocarbons and Their Potential Sources in the CIS

ABRAMS, MICHAEL A., Exxon Production Research Company, Houston, Texas; K. O. STANLEY and DEBORAH GILBERT, Exxon Exploration Company, Houston, Texas

Introduction

The major hydrocarbon plays of the FSU owe their origin to the paleotectonic and paleogeographic histories of key continental crustal blocks and accretionary complexes formed as crustal blocks were amalgamated during Phanerozoic. Proven source rocks include Oligocene-Miocene Maikopian Formation (South Caspian and Black Sea), Oligocene-Miocene marine condensed sections (Sakhalin), Middle-Upper Eocene Kuma Formation (North Caucasus), Cretaceous (Aptian-Albian) Pokur Formation (West Siberia-gas), Kimmeridgian-Volgian Bazhenov Formation (West Siberia-oil), Lower-Middle Jurassic Tyumen Formation (West Siberia-oil), Carboniferous marine shales (Russian platform), Frasnian-Fammenian Domanik Formation (Russian platform), Silurian marine shales (Timan Pechora), and Vendian/Riphean marine shales (East Siberia). World class hydrocarbon systems are present in the Timan Pechora, Volga-Ural, North Caspian, East Siberia, Amu Darya, West Siberia, South Caspian, and Sakhalin basins. These source rocks vary greatly in age, molecular characteristics, and hydrocarbon potential.

Geologic Setting

Significant discovered oil and gas reserves occur in less than 10 of the more than 80 sedimentary basins in the former Soviet Union. These hydrocarbon-rich basins are characterized by super source rocks, excellent regional seals, or active systems where charge replenishes fluids lost to leakage; and large structures where hydrocarbons are trapped. These hydrocarbon systems owe their origins to the tectonic and paleogeographic histories of the basins which formed on ancient continents or on accretionary crust formed during Phanerozoic collision and amalgamation of Eurasia, and on accreted arc terranes at the eastern margin of the Eurasia continent. The oldest of these rift and sag basins formed during the Precambrian on the Siberian platform (Baykit, Angara-Lena) and in the mid-to-late Paleozoic on the Russian Platform (Volga-Ural, Timan Pechora, North Caspian, Dnepr-Donets/Pripyat). Younger basins are Mesozoic rift-sag basins superimposed on Paleozoic accretionary crust in West Siberia (West Siberia, Kara Sea) and on Turan-Scythia basement (Amu Darya, Middle Caspian, North Caucasus), Neogene basins formed during collision of Eurasia and Gondwana blocks (South Caspian, Middle Caspian, Terk-Caspian foreland/foldbelt) and the Neogene Sakhalin basins formed by back arc rifting and transpression of Cretaceous forarc terranes. The Neogene basins have the poorest quality regional seals. Traps in these basins are modified by numerous active faults and/or shale diapirs where hydrocarbons are actively migrating and remigrating.

Geochemistry of Hydrocarbons and Their Potential Sources for Selected FSU Basins

East Siberia-Angara Lena Basin-Precambrian: Most reservoired oils are generated from Riphean calcareous source rocks accummulated under reducing marine conditions (Kontorovich, A. E. et al., 1997). Restricted circulation in intracratonic depressions and foredeep lead to the deposition of finely laminated black shales with total organic carbon contents up to 10%. These oils contain molecular characteristics which are distinctive of a Proterozoic origin; mono-methyl branched alkanes, isotopically light (dC¹³ -32 to -36 o/oo, and abundant C₂₉ steranes relative to C₂₇ and C₂₈.

Timan Pechora-Paleozoic: Distinct oil groups from several different Paleozoic marine sources have been identified in the Timan Pechora Basin (Abrams et al, 1997). Several Upper Devonian organic-rich rocks were deposited in transgressive, restricted, shallow-marine settings on carbonate platforms. The Frasnian-Fammenian Domanik Formation and other domanikoid facies have generated abundant medium-sulfur, normal-gravity oils. Some rather unique oils may have generated from late Devonian, strongly reducing, evaporitic-carbonate marine settings containing significant amounts of bacterial organic matter. The Middle Devonian sourced oils are derived from normal marine shales deposited in a setting starved of clastic sediment input. A “Sawtooth” oil group is characterized by strong odd preference centered around nC₁₄ to nC₁₉, low isoprenoids relative to normals, and light isotopic ratios for saturate/aromatic compounds. These features could be derived from a marine source facies enriched Gloecapsamorpha prisca, which proliferated during the Ordovician and possibly into the Silurian.

West Siberia-Mesozoic: The key source horizon within West Siberia Basin is the Volgian Bazhenov. Maximum transgression within a silled basin lead to the deposition of shales with total organic carbon up to 30%. Oils sourced from Bazhenov shales are typically low in sulfur, low pristane to phytane ratios, and C₂₇/C₂₉ sterane approximately one. Non-marine lacustrine to shallow marine Lower to Middle Jurassic Tyumen and Togur Formations have also generated significant amounts of oil within the West Siberia Basin. These oils are typically very low in sulfur, high wax, high pristane/phytane ratios, and low C₂₉ steranes. The Aptian-Cenomanian Pokur Suite ranges from lagoonal to open marine. Brown coals in the basal and upper portions are believed to have sourced the majority of gas.

South Caspian-Cenozoic: Molecular characterization indicate most reservoired oils are sourced from similar organic facies (Abrams and Narimanov, 1997). The South Caspian oils are generally low in sulfur and medium to high gravity. The biomarkers are typically low in diasteranes, low hopanes relative to steranes, C₂₇ steranes more abundant than C₂₉ steranes, low tricyclics relative to homohopanes, and very low sterane isomerization ratios. The molecular characteristics are consistent with oils sourced from a Tertiary (oleanane), slightly calcareous, marine clastic facies in a rapidly depositing system (non thermal equilibrium). Molecular characteristics of oils and rocks, oil-oil correlation's, and maturation modeling studies suggest the oil is not syngenetic, but most likely generated from deeper Miocene and older marine shales. Isotopic ratio data indicate the onshore oils are derived from Oligocene-Lower Miocene sources whereas offshore oils are derived from Middle Miocene to possibly younger source rocks. We think the reservoired gases have been sourced from Upper Miocene mixed terrestrial-marine kerogen generated at conventional organic maturities and mixed with some early low maturity-biogenic gas.

Conclusions

The FSU hydrocarbon systems represents a very wide range and diverse group of hydrocarbon types and source rocks. Proven source rocks range in age from Riphean to Neogene. Each of the hydrocarbon-rich basins contain super source rocks that vary greatly in age, molecular character and hydrocarbon potential. These unique molecular charateristics reflect basinal settings and source age include: mono-methyl branched alkanes and isotopically light hydrocarbons from the Riphean source East Siberian Basin; odd preference centered around nC₁₄ to nC₁₉, low isoprenoids relative to normals, and light isotopic ratios from the Paleozoic sourced Timan Pechora Basin; low sulfur and pristane/phytane ratio hydrocarbons from the Bazhenov source in the West Siberia Basin; and lastly the low sulfur, low tricyclic, low sterane isomerization hydrocarbons from Maikopian and younger sources in the South Caspian Basin.

Acknowledgements:

We would like to thank Exxon Production Research Company, Exxon Ventures (CIS) Inc., and Exxon Exploration Company for permission to present this study. We would also like to acknowledge the contributions of George Ramsayer, Gary Isaksen, Jim Zullig, and Steve Creaney.

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