Genetic Origin of Gas and its Effect on Hydrocarbon Property in the Ordovician of Tazhong Area Tarim Basin, NW China

Abstract

Abstract

Although abundant natural gases have been discovered in Ordovician reservoirs in the Tazhong area, central Tarim Basin, their genetic origins is still an open question because of significant variations in chemical and carbon isotopic compositions. Gas samples from Ordovician in the Tazhong area have been analyzed for molecular and isotopic composition together with associated condensate oil samples for wax and adamantane content, and fluid inclusions for chemical composition, to elucidate the origin of deep strata gas. The Ordovician gases have a methane content of 54.3 %-94.6 %, gas/oil ratio of 45-62,393 m³/m³, and the dry coefficient of 0.61-0.99 (C₁/C_1-5). The methane, ethane, propane and i-butane isotopic values for the Ordovician gases range between -54.8 ‰ and -31.9 ‰, between -45.0 ‰ and -29.4‰, between -37.9 ‰ and -24.2 ‰, and between -34.7 ‰ and -23.7 ‰, respectively. The MDI of condensate oils indicate relatively high maturity, corresponding source Ro > 1.7-2.1 %. According to the empirical relationship between marine gas methane isotopes and corresponding source rock maturity Ro (d13C1 = 15.80 lg Ro-42.21), the source rock Ro when Tazhong gas-condensate reservoir formed would be > 2.0 %, indicating the natural gas belongs to high temperature cracking gas. The (C₂/C₃) values and (C₂/iC₄) values of gases are relevantly > 2 and 10, respectively, ln(C₁/C₂) values shows a slightly wider range than ln(C₂/C₃) values, and ln(C₂/C₃) values stated a slightly wider range than (δ¹³C₂-δ¹³C₃) values, suggesting the gases derived from the high temperature oil-cracking gas at depth. Two types of inclusions in the Ordovician are commonly developed: liquid-hydrocarbon inclusions in the first period and gaseous hydrocarbon inclusions in the second period, suggesting that oil filled at an earlier time and gas filled at a later time. The charge and mixing of the dry gas into the Ordovician pools may have resulted in anomalies of hydrocarbon properties on the gas charging faults. With increasing distance from the gas charging faults, the anomalies of oil density, wax content, dry gas coefficient, hydrogen sulfide contents, gas/oil ratio, well production, 4-/1-MDBT ratio of crude oil weakened, and carbon isotope compositions of natural gas mainly became lighter.

AAPG Datapages/Search and Discovery Article #90260 © 2016 AAPG/SEG International Conference & Exhibition, Cancun, Mexico, September 6-9, 2016