| After more than 40 years of oil and gas exploration, it has not yet made a industrial breakthrough in Ulungu
Depression, northern Junggar Basin. In 2016, the first natural gas reservoir was found in the Triassic system of Shibei Sag.
Therefore, it is of great significance to clarify the origin of natural gas and gas source rocks for evaluating the potential
and scale of oil and gas exploration. The geochemical data of natural gas in Ulungu Depression and its surrounding areas
are systematically collected. The characteristics and types of natural gas in Well Zhunbei-1 are recognized based on the
analysis of compositions of natural gas, carbon isotope and light hydrocarbon. Based on the results of closed system
hydrocarbon generation simulation experiment of three Carboniferous and one Permian source rock samples, the gassource
rocks correlation in Well Zhunbei-1 is carried out with the carbon isotope compositions of methane, ethane and
propane, and the genetic mechanism of natural gas is analyzed in combination with seismic profile, regional magnetic
anomaly and other data. The results show that: (1) The natural gas of Well Zhunbei-1 is characterized with low methane
content (54.5%- 73.9%), low drying coefficient (＜0.95), high nitrogen content (11.1% - 32.7%), heavy ethane carbon
isotope (-26‰), and high content of methyl cyclohexane (MCC6) in light hydrocarbons (63.6%), belonging to humic
pyrolysis gas. (2) Several regional large faults are developed in the north of Shibei Sag. Influenced by the diorite
intrusions, organic matter was thermally catalyzed, and nitrogen-rich natural gas was formed by magmatic degassing.
Combined with regional analysis, it is considered that the magmatic degassing caused by diorite intrusions developed
along the faults is the main reason for high nitrogen content in natural gas of Well Zhunbei-1. (3) The carbon isotope
characteristics of the natural gas in Well Zhunbei-1 are similar to those of the hydrocarbon generation simulation products of
the source rocks of the Lower Carboniferous Dishuiquan Formation (with δ13C2 of - 27.6‰ to - 24.0‰), indicating that
they have certain affinity. It is inferred that the natural gas mainly comes from the marine terrestrial transitional mature
source rocks of the Dishuiquan Formation. The traps adjacent to the source rock of Dishuiquan Formation are important
plays for the exploration of natural gas reservoirs in Ulungu Depression. |
