Premium
Geochemical Characteristics of the Upper Triassic Xujiahe Formation in Sichuan Basin, China and its Significance for Hydrocarbon Accumulation
Author(s) -
XIE Zengye,
LI Jian,
LI Zhisheng,
GUO Jianying,
LI Jin,
ZHANG Lu,
DONG Caiyuan
Publication year - 2017
Publication title -
acta geologica sinica ‐ english edition
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.444
H-Index - 61
eISSN - 1755-6724
pISSN - 1000-9515
DOI - 10.1111/1755-6724.13414
Subject(s) - source rock , hydrocarbon , natural gas , methane , geology , coal , geochemistry , maturity (psychological) , coal measures , isotopes of carbon , carbon dioxide , mineralogy , chemistry , environmental chemistry , structural basin , total organic carbon , geomorphology , organic chemistry , psychology , developmental psychology
Abstract The alternative development of coal‐bearing hydrocarbon source rocks and low‐porosity and low‐permeability tight sandstone reservoirs of the Triassic Xujiahe Formation in the Sichuan Basin is favorable for near‐source hydrocarbon accumulation. The natural gas composition of the Xujiahe Formation in the Sichuan Basin is dominated by hydrocarbon gases, of which the methane content is 80.16%–98.67%. Typically, the C 2 + content is larger than 5% in main wet gas. The dry gas is mainly distributed in the western and northern regions of the basin. The non‐hydrocarbon gases mainly contain nitrogen, carbon dioxide, hydrogen, and helium, with a total content of <2%. The carbon isotope ranges of methane and its homologues in natural gas are: δ 13 C 1 of −43.8‰ to −29.6‰, δ 13 C 2 of −35.4‰ to −21.5‰, δ 13 C 3 of −27.6‰ to −19.8‰, and δ 13 C 4 of −27.7‰ to −18.8‰. δ 13 C 3 > δ 13 C 4 occurs in some natural gas with a low evolution degree; such gas is mainly coal‐related gas from humic‐type source rocks of the Xujiahe Formation. As for the natural gas, δ 2 H CH4 values ranged from −195‰ to −161‰, δ 2 H C2H6 values ranged from −154‰ to −120‰, and δ 2 H C3H8 values ranged from −151‰ to −108‰. The dry coefficient, δ 13 C and δ 2 H CH4 are all positively correlated with the maturity of source rocks. The higher the maturity of source rocks is, the larger the natural gas dry coefficient is and the larger the δ 13 C and δ 2 H CH4 values are, indicative of the characteristic of near‐source accumulation. The δ 2 H C2H6 value of natural gas is influenced by paleosalinity to a relatively large extent; the higher the paleosalinity is, the larger the δ 2 H C2H6 value is. The Pr/Ph value of the condensate oil ranged from 1.60 to 3.43, illustrating light oxidization–light reduction and partial‐oxidization characteristics of the depositional environment of coal‐bearing source rocks of the Xujiahe Formation. The natural gas light hydrocarbon (C 5 –C 7 ) from the Xujiahe Formation presented two characteristics: the first was the relatively high aromatic hydrocarbon content (19%–32.1%), which reveals the characteristic of natural gas with humic substances of high‐maturity; the second was the low content of aromatic hydrocarbon (0.4%–9.3%), reflecting water‐washing during the accumulation of the natural gas. The reported research outcomes indicate a potential mechanism for natural gas accumulation in the Xujiahe Formation, which will further guide natural gas exploration in this region.