Geochemical Characteristics and the Origin of Superdeep Condensates in Tarim Basin, China

Abstract

A series of trace compounds (diamondoids, ethanodiamondoids, and thiadiamondoids) were detected through two-dimensional gas chromatography/time-of-flight mass spectrometry (GC × GC-TOFMS) analysis of Ordovician condensate samples from the Tazhong area. Gas chromatography-mass spectrometry (GC-MS) analysis showed that the biomarker parameters are less effective for high-maturity oils. Carbon isotope and geochemical features suggested that the gas is a high-temperature cracking gas when its temperature is higher than 170 °C. The H2S content is 8.27%, suggesting that it is affected by thermochemical sulfate reduction (TSR). However, the geological analysis indicated that the Ordovician reservoirs do not satisfy the conditions for TSR. The high-maturity oil in the Ordovician reservoirs may generate diamondoids and ethanodiamondoids when cracking, while TSR and severe cracking occur in deep Cambrian source rocks and produce a large number of diamondoids, ethanodiamondoids, organic sulfur compounds (OSCs), etc. The secondary geochemical products that are carried up by the dry gas and migrate upward through faults and are enriched in Ordovician crude oil reservoirs. On this basis, we proposed that the condensate presented was formed by the mixing of dry gas from Cambrian oil that was altered by cracking and TSR into Ordovician in situ slightly cracked oil, therefore speculating that the favorable reservoir–seal assemblages in this area may contain abundant oil and gas resources. Consequently, improved knowledge of secondary alteration effects on the reservoir and underground fluids is vital for oil and gas prediction and exploration development in the next step.

Hydrocarbon Phase State Evolution and Accumulation Process of Ultradeep Permian Reservoirs in Shawan Sag, Junggar Basin, NW China

“…The hydrocarbon molecular composition and reservoir temperature and pressure have important effects on the hydrocarbon phase state. , Variations in hydrocarbon components can lead to significant differences in phase states. The higher the molar percentage of C 1 -C 5 , the lower the cricondentherm and critical temperature, and the higher the cricondenbar .…”

“…The empirical parameter discrimination methods for hydrocarbon phase determination include triangular fluid component diagram discrimination, block diagram, Z value, and Z 1 -Z 2 factor. 29,30 The PVT phase diagram discriminant method is a relatively accurate method for determining hydrocarbon phase states. 52−54 3.4.…”

“…19−21 It is of great importance for understanding the current compositional distributions in the reservoir, the mechanism of the migration and accumulation of hydrocarbons in ultradeep reservoirs, and explaining the distribution of hydrocarbons in different phase states in a basin. 22−25 Studies on hydrocarbon phase states have mainly focused on the identification of fluid phase states, 26,27 the main controlling factors, 10,28,29 and the formation mechanism of reservoirs with different hydrocarbon phase states. 30 The empirical parameter discrimination method and PVT phase diagram discrimination method can be used to determine the hydrocarbon phase states when the reservoir fluids are well mixed.…”

“…Studies on hydrocarbon phase states have mainly focused on the identification of fluid phase states, , the main controlling factors, ,, and the formation mechanism of reservoirs with different hydrocarbon phase states . The empirical parameter discrimination method and PVT phase diagram discrimination method can be used to determine the hydrocarbon phase states when the reservoir fluids are well mixed .…”

Synthetic Doping of Diamondoids through Skeletal Editing

“…In summary, we report an efficient skeletal editing procedure for the displacement of a CH 2 group by a heteroatom in diamondoids. While thiaadamantane was identified and isolated a long time ago from raw oil, thiadiamantane and thiatriamantanes were discovered only over the past decade through GC-MS analyses, ,, but no structural proofs or mechanism of formation were provided. The present approach not only supplies reference compounds but also opens avenues to explore the essentially unknown chemistry of higher aza- and thiadiamondoids.…”