关键词: 油气演化阶段, 原油裂解烃, 深层油气, 轻质油, 凝析油气, 成因类型, 资源潜力

Abstract: The subdivision of hydrocarbon evolution stage and the resource potential evaluation of source rocks are of great significance for deep conventional and unconventional petroleum exploration and studies on deep basic petroleum geology. The hydrocarbon evolution of deep source rocks can be divided into four stages, i.e., light oil (volatile oil), condensate, wet gas and dry gas, corresponding to four types of deep hydrocarbons that may be sourced from the source rocks in crude oil reservoirs. Using thermal simulation, this paper evaluates the hydrocarbon generation potential of deep source rocks, and then puts forward indexes for the division of four hydrocarbon evolution stages. In view of the fact that the evaluation of resource potential of deep source rocks needs to consider whether normal crude oil is expelled and how much the expulsion is, an experimental scheme of first performing oil expulsion at peak of normal oil generation and then starting heating of oil expelled source rock in a confined pyrolysis system is adopted to establish the hydrocarbon evolution mode of deep source rocks. This mode can be used to roughly evaluate the hydrocarbon resource potential of deep source rocks. Based on the experience of reservoir classification according to the gas over oil ratio in reservoir (GORr), the gas over oil ratio in source rock (GORs) and the methane content of source rock pyrolysis simulation products are used as the classification indexes of hydrocarbon evolution stage under laboratory thermal simulation conditions. The rapidly rising GORs values, i.e., 142 m³/m³ (800 scf/bbl), 890 m³/m³ (5 000 scf/bbl) and 3 562 m³/m³ (20 000 scf/bbl), and 95% methane content were taken as the upper GORs limits of light oil, condensate, wet gas and dry gas, respectively. Considering that GORs cannot be obtained directly from component analysis of core samples, these limit values cannot be used for dividing the hydrocarbon evolution stages of actual sections. Since the vitrinite reflectance (R_o) or equivalent vitrinite reflectance (R_oE) is commonly used by explorers to classify hydrocarbon generation stages of source rocks, the Ro range of the above GORs limits can be obtained by converting the laboratory temperature into R_o using the oil inhibition R_o model. It is worth noting that the R_o value obtained from the thermal simulation experiment in a confined pyrolysis system is higher than the R_oE value measured in the actual formation. Light oil and condensate can be divided into four categories according to their geneses. Among them, type A is formed by type Ⅰ to type Ⅱ organic matter after oil expulsion, type B is formed by type Ⅱ to type Ⅲ organic matter without hydrocarbon expulsion, type C is formed by crude oil cracking, and type D is formed by secondary alteration. At present, the researches on primary light oil and condensates (type A, B and C oil and gas) are insufficient and need to be strengthened. Deep light oil and condensate resources are not only affected by the organic matter content, type and maturity of source rocks, but also related to the following geological factors of deep strata:(1) hydrocarbon expulsion efficiency of normal oil (black oil); (2) large-scale oil cracking in reservoir; (3) mixture of oil and gas from different sources. Various genetic types of light oils and condensates are found in China, showing broad exploration prospects in the fields of light oil and condensate resources.

Key words: hydrocarbon evolution stage, oil cracking hydrocarbon, deep oil and gas, light oil, condensate, genetic type, resource potential