Characteristics and Genetic Mechanisms of Oil and Gas Phases in the Paleogene of Huizhou 26 Sub-sag in Huizhou Depression

Huizhou 26 sub-sag is rich in oil and gas resources. The oil and gas phases are complex and variable, with unknown origins. Based on the data of crude oil properties, well fluid components, and organic geochemistry, empirical statistical method and PVT phase diagram were used to identify the phase types in the study area. The spatial distribution characteristics and genesis mechanisms were analyzed. The results indicate that the crude oil from Huizhou 26 sub-sag has the characteristics of low density, low viscosity, low sulfur content, and high wax content. It also has three types of hydrocarbon phases: black oil, volatile oil, and condensate gas, among them black oil is the most widely distributed, volatile oil and condensate gas only distribute in the Huizhou 26-6 and 21-8 structural zones. Vertically, the phase types of Enping Formation are mainly volatile oil and condensate gas, while black oil, volatile oil, and condensate gas coexist in Wenchang Formation. Some wells exhibit a characteristic of vertical distribution of oil and gas alternation. The spatial differences of oil and gas phases are mainly controlled by the thermal evolution degree of source rocks and the filling process of oil and gas. Among them, the maturity of oil and gas corresponding to the main filling period and the proportion of late high maturity oil and gas mixing are the main reasons. Based on a comprehensive analysis of the formation stages and processes of different types reservoirs in the study area, three phase evolution models were established, namely "First stage oil phase filling", "Two stages oil phase superposition", and "Early oil phase, late gas phase". This study seeks to provide theoretical guidance for efficient exploration of deep hydrocarbons.

Keywords: Huizhou depression; oil and gas phases; condensate gas; differential filling of oil and gas