The fluid inclusion characteristics and petroleum accumulation of Chang 8 tight sandstone in northern Shaanxi, Ordos Basin

This paper is the result of oil and gas exploration engineering.

Objective The Yanchang Formation 8 in the northern Shaanxi region of the Ordos Basin has a wide oil distribution range and is an important target area for future exploration. However, the relationship between the evolution of hydrocarbons and the distribution of oil reservoirs is still unclear.

Methods Based on the analysis of the occurrence characteristics, composition and homogeneity of fluid inclusions, geological and geochemical methods are used, the oil source and reservoir forming characteristics of Chang 8 tight sandstone are discussed, and the formation time of Chang 8 reservoir and its relationship with hydrocarbon accumulation are revealed.

Results The fluid inclusions in Chang 8 sandstone reservoir are mainly gas-liquid hydrocarbon inclusions and gasliquid two-phase brine inclusions, which are distributed in quartz enlarged edge or fractures of fine sandstone, and can be divided into early and late stages. The homogenization temperature of brine inclusions associated with gas-liquid hydrocarbon inclusions mainly has two peak ranges of 85-105℃ and 115-135℃, which shows that oil and gas are continuously filled. The history and evolution of reservoir formation shows that the main filling period of Chang 8 oil is 110-135 Ma, during which the early filling in late Jurassic and large-scale filling in early Cretaceous occurred. The source rocks of Chang 7 in the region are mainly developed, and the Ro value is close to 1.0% in the maximum hydrocarbon generation and expulsion stage, reaching the hydrocarbon generation threshold and generating a large number of hydrocarbons. The source rocks of Chang 9 are locally distributed in Zhidan area.

Conclusions Combining the characteristics of inclusions with the formation of hydrocarbons, the Chang 8 reservoir in Northern Shaanxi is controlled by factors such as source rock, reservoir and hydrocarbon charging. The oil filling degree is different, and the lithologic reservoir is developed differently, which shows two sets of source rocks hydrocarbon supply and differential accumulation.