Print this pageAbstract: Genetic Characterization of Natural Gases in NW Siberia
SCHOELL, MARTIN, FRANK PICHA, ALLA ROVENSKAYA, and NIKOLAI NEMCHENKO,
Gases in NW Siberia can be stratigraphically grouped into Cenomanian age gases which are (with few exceptions) dry gases (C2+ = approx. 0.04 - 18%; delta13C1 = approx. -65.4 to approx.-50 o/oo; deltaD1= approx. -250 to approx. -210 o/oo) and Pre-Cenomanian gases (C2+ = approx. 0.3 - 30%; delta13C1 = approx. -49 to approx. -32 o/oo; delta13C2 = approx. -30.5 to approx. -26.6 o/oo; delta13 C3,4 = approx. -29.8 to approx. -25.3 o/oo; deltaD1 = approx. -258 to approx. -206 o/oo; deltaD2 = approx. -251 to approx. -213 o/oo; deltaD3 = approx. -221 to approx. -182 o/oo). Associated CO2 from pre-Cenomanian gases in the Urengoy area has delta13 CCO2 = approx. -27 o/oo. Nitrogen in N2-rich gases in Urengoy and Yurkharovskoe is isotopically similar to air nitrogen.
Only two of the analyzed gases on the northern Yamal peninsula (Maliginskoe and Zapadno-Seyakhinskoe) have carbon and hydrogen isotope values of -65.4 and -61.4 o/oo and -249 to -241 o/oo, respectively, both consistent with a bacterial origin via the CO2-reduction pathway involving waters that are depleted in deuterium compared to marine waters. However, the majority of the Cenomanian gases in NW-Siberia is isotopically different from these bacterial gases, not only in carbon but also in hydrogen isotopes of methane which is inconsistent with a bacterial origin, unless the associated CO2 is isotopically enriched in 13C and the waters involved were different from those of the Yamal gases. However, the CO2 we analyzed, although not from Cenomanian gases, did not indicate any 13C-enrichment.
The pre-Cenomanian gases in NW Siberia have carbon isotope patterns typical for mature to post-mature gases with very little isotopic differences between ethane, propane and butane, suggesting that these gases are primary cracking products of kerogens. One exception to this rule is the gas from Tarasovskoe south of Urengoy, an oil-related wet gas with isotope patterns that are typical for incipient oil-related gases (delta13C1 =-50.6 o/oo; delta13C2 = -37.6 o/oo1; delta13C3 = -34.8 o/oo). However, C2+ hydrogen isotope patterns of the pre-Cenomanian gases, including Tarasovskoe, are similar to those of gases from coaly organic matter, when compared to a world-wide database. We propose that the deep gases in NW Siberia, are possibly sourced to a large part from coaly sections in the Jurassic or pre-Jurassic and are not genetically related to the oils.
The origin of the Cenomanian gases in NW Siberia, in particular those from the Urengoy megagroup, remains enigmatic. It is unlikely that they are migrated deep gases because they are isotopically significantly different from deeper gases at Urengoy. Yet, criteria for a bacterial origin are similarly inconclusive. Analyses on associated waters, condensates and other gas components will be necessary to resolve this complex issue.
AAPG Search and Discovery Article #90942©1997 AAPG International Conference and Exhibition, Vienna, Austria