Hydrocarbn explusion threshold (HET) is the critical condition for hydrocarbon expulsion in separate phase from a source rock when the generated hydrocarbon amount has satisfied all needs for ahsorpBon by minerals, solution in water, and blocking of capillary pressure. Research results show that the HET varies mainly with three geological parameters: total organic carbon content (C%), kerogen type index (KTI) and thermal maturation degree (R0). Source rocks with low C% and KTI cross the HET at a high level of maturation degree (larger R0); source rocks with lower R0 and C% can also cross the HET if the kerogen has a larger KTI. Under general geological conditions, a source rock first crosses the methane expulsion threshold (HETgl), then the heavw hydrocarbon gas threshold (HETgn), and finally the liquid hydrocarbon expulsion threshold (HET0). In this paper the concept of HET, and its critical conditions, are applied to establish the scientific validity of the concept and grade the source rocks, to study the phases of hydrocarbons in migration and the mechanisms of hydrocarbon acvumulation, and to divide the hydrocarbon expulsion into stages. Applications to different basins in China show that HET provides an accurate and efficient method to guide oil and gas prospecting.

Geological analysis and physical analogy exponments indicate that, under geological conditions, hydrocarbon tends to migrate along a path of least resistance and attempt to follow the largest buoyancy component There are four generalized modes of possible transport. First. hydrocarbons rends to migrate along the pathways with high porosity and permeability, and with a large grade difference relative to surrounding rocks (grade difference predominance): second, hydrocarbons rends to migrate in the opposite direction in oxverlying formations to the nadir of the sedimentation centre (divided syncline predominance): third. hydrocarbons tends to migrante in the direction of lower fluid pressure (fluid pressure predominance); fourth, hydrocarbons tends to migrate in the direction vertical to buoyancy (flow direction predominance). This paper reports on field observations in the Daqing oilfield area of China and also on physical analog experiments used to illuminate the four basic modes of transport Under geological conditions, the hydrocarbon migration pathways are controlled by these four basic modes, which can he used to predict the directions of hydrocarbons migration and select favourable exploration locations.

油气在生排运聚成藏过程中须经历生烃排烃、运烃、聚烃和资源等5个门限。某一确定的油气运聚成藏系统进入任一个门限都将损耗一部分烃量。在实际地质条件下，源岩生成的烃量只有超过进入各个门限时损耗烃量之和才能大规模聚集成藏，并构成油气勘探的远景区。根据物质平衡原理研究油气生成量和损耗量之间的平衡关系，建立了相应的地质模型。在此基础上，提出了门限控烃作用及其计算模型。文中阐明了各门限的地质含义、判别标准及其控油气作用机理，给出了应用实例。

The chemical compositions of 28 representative crude oils and oil sand samples collected from the Bamianhe region of the Bohai Bay Basin, eastern China were studied in detail in order to understand their genetic relationships, biodegradation ranking and thermal maturity level. Molecular geochemical parameters indicate the presence of one single oil family in the Cenozoic reservoirs. A number of biomarker features support deposition of the source rocks for these oils under conditions of enhanced salinity/stratified water columns. The extremely low maturity levels of these "immature oils", as indicated by sterane isomerization ratios, were not supported by other maturity parameters. The genetic relationship between the oils and the Es4 and Es3 members of the Eocene-Oligocene Shahejie Formation present in the deeper part of the Niuzhuang Sag, but not those along the Niuzhuang South Slope was suggested using a wide range of molecular parameters. Variation in the sterane concentrations observed from potential source rocks with different maturity levels in the study area suggests a high probability of migration contamination of mature oils by immature bitumens along migration pathways or from intra-reservoir shales. Variation in the level of oil biodegradation could also create anomalies in the use of gross oil compositions as an indicator of oil maturity. Although the recent rapid subsidence of the Bohai Bay Basin and the saline depositional environment of one of the potential source rocks favor oil generation at relatively low maturity levels, most of the economic oils discovered in the Bamianhe region appear to have derived from conventionally mature source rocks but contain lower maturity fluids admixed with them.

采用地质与地球化学原理与方法对渤海湾盆地八面河油气区进行系统测试和分析，定量估算该油田的油源分配，提出了相应的运移通道和成藏机理。研究结果表明，虽然八面河油田附近埋藏较浅的沙四段烃源岩提供的“未熟-低熟油”在八面河地区原油中占一定的比例，但这些原油的主体应来自相邻的生油洼陷--牛庄和广利洼陷内埋深超过2700m的沙四段成熟烃源岩。定量计算表明，八面河地区未熟-低熟油的含量不超过20%，其余为正常原油。来自生烃洼陷的油气的运移受断层、砂体运载层等优势通道共同控制，八面河油田只是运移优势通道的指向之一。新提出的原油来源与运聚模式已为近期油气勘探实践所证实。