Geochemical Significance of Discovery in Cambrian Reservoirs at Well ZS1 of the Tarim Basin, Northwest China

Abstract

Gas condensate and natural gas samples from wells ZS1 and ZS1C in the Cambrian strata of the Tarim Basin, NW China are analyzed geochemically to assess thermal maturity, extent of oil cracking and secondary alteration. Hydrocarbons are derived from the Cambrian source rocks with gas condensate and solution gas reservoired in the Middle Cambrian Awatage Formation and dry gas in the Lower Cambrian Wusonggeer and Xiaoerbulake formations, where caverns from dolomitization and local fractures provide spaces for petroleum accumulation. Gas and gas condensate in the Middle Cambrian are mainly generated from kerogen thermal degradation at high maturity stage (condensate -wet gas window) rather than from in-reservoir thermal cracking of

Geochemistry of Paleozoic marine petroleum from the Tarim Basin, NW China: Part 5. Effect of maturation, TSR and mixing on the occurrence and distribution of alkyldibenzothiophenes

“…The d 13 C values of the whole condensate, saturated hydrocarbons, aromatic hydrocarbons and resins in ZS1C condensate are À30.0‰, À30.9‰, À30.5‰ and À29.4‰, respectively, whereas those in the ZS1 condensate are À32.8‰, À33.4‰, À30.9‰ and À32.3‰, respectively (Zhu et al, 2015). Systematically enriched 13 C in ZS1C condensate is caused by TSR, which kinetically favors the removal of 12 C (Krouse et al, 1988).…”

“…The highest value of 2.34 is from the ZS1 condensate. This is the first commercially produced liquid hydrocarbon from a Cambrian reservoir and derived from a Cambrian source rock at very high maturity (Zhu et al, 2015). The second highest value of 1.69 is from TZ62(S) oil, commonly regarded as the Cambrian-Lower Ordovician end member source due to its unusually heavy isotopic composition (Tian et al, 2012;Li et al, 2012Li et al, , 2015.…”

“…This condensate is very different from the others with an API gravity of 20.7°at 20°C and gas/condensate ratio as high as 25,195 m 3 /m 3 . The associated gas contains 8.3% H 2 S and the condensate is enriched in sulfur (2.06%) (Li et al, 2015;Zhu et al, 2015). Extremely low API gravity in ZS1C results from the consumption of hydrocarbons by TSR and possible incorporation of sulfur into liquid hydrocarbons, which not only increases the bulk sulfur content but also forms a large quantity of DBTs which are dominant in the total ion chromatogram of the aromatic fraction (Fig.…”

“…Unusually high amounts of DBTs are present in oils and gas condensates from the Tarim Basin, especially in the Tazhong Uplift (Cai et al, 2009;Zhu et al, 2011;Li et al, 2012Li et al, , 2015. Either in situ TSR in the Cambrian reservoirs or migrated H 2 S from an external occurrence of TSR is responsible for the high DBT concentrations in the Lower Ordovician oils from the Tazhong Uplift (Li et al, 2015;Zhu et al, 2015). Reservoir petrology, hydrothermal flow and the link between dissolution of anhydrite in the Cambrian strata and TSR have been investigated in previous studies (Cai et al, 2001(Cai et al, , 2008(Cai et al, , 2009.…”

Application of the monoterpane ratio (MTR) to distinguish marine oils from terrigenous oils and infer depositional environment in northern Tarim Basin, China

“…Large amounts of Paleozoic marine oils ranging from condensates to heavy oils have been discovered in the Yingmaili sub-uplift, Halahatang sub-depression and Lunnan sub-uplift. These marine oils are reservoired in Ordovician carbonates, and Triassic and Cretaceous sandstones and are derived from Ordovician-Cambrian source rock systems which were deposited under anoxic conditions Zhang et al, 2000Zhang et al, , 2002Sun et al, 2003;Zhang and Huang, 2005;Zhu et al, 2015). Oils from the Luntai sub-uplift, located in the northern Tabei uplift ( Fig.…”