珠江口盆地白云凹陷储层沥青成因及其对油藏调整改造的启示

在珠江口盆地白云凹陷多个层系都发现了储层沥青。储层沥青丰度自上部的珠海组、珠江组至深部的恩平组、文昌组逐渐增加。储层沥青在物性较好的粗粒砂岩中多为零星状出现，而在细粒的粉砂岩和泥岩中主要呈脉状分布而且含量明显增加。几乎所有的储层沥青均没有显示出明显的各向异性，而且沥青反射率均小于2.0%，表明高温热裂解不是储层沥青形成的主要原因。沥青抽提物的分子地球化学组成特征显示，白云凹陷储层沥青抽提物中缺乏25-降霍烷系列，色谱基线没有明显的“UCM”鼓包、单井地层埋藏史显示储层也未遭受大幅度的抬升剥蚀，因此储层沥青也非生物降解成因。结合地质背景和油气藏特征分析认为，白云凹陷储层沥青的成因为：恩平组-文昌组烃源岩早期生成的原油聚集成藏以后，深部主要遭受了后期天然气的气洗作用。不同层位烃源岩、储层热演化的时间差异是导致白云凹陷油气藏发生调整，并转变为轻质油、挥发油等特殊油藏类型的重要原因。

Origin of reservoir bitumen and its implications for adjustment and reformation of hydrocarbon-accumulation in Baiyuan Sag,Pearl River Mouth Basin

Reservoir bitumen has been discovered in multiple Paleogene series of strata in the Baiyun Sag,Pearl River Mouth Basin.The abundance of reservoir bitumen gradually increases from the shallower Zhuhai and Zhujiang formations to deeply buried Enping and Wenchang Formations.The reservoir bitumen only sporadically occurs in coarse-grained sandstones with better permeability and porosity, while occurs in siltstones and mudstones in the form of veins.Almost all reservoir bitumen samples do not show apparent anisotropy with bitumen reflectance lower than 2.0%,suggesting that the reservoir bitumen in the Baiyun Sag doesn't mainly result from high-temperature cracking.All bitumen extracts lack 25-norhopane series according to the analysis of molecular geochemical composition.No gas chromatogram base-line "hump" was discovered in all saturated hydrocarbon of reservoir bitumen.The reconstructed burial history of representative individual wells shows that the reservoir strata have not experienced large-scale uplifting and denudation.Therefore,the reservoir bitumen in the Baiyun Sag is not of biodegradation origin.Based on the analyses of the geological setting and characteristics of oil and gas reservoirs,the origin of reservoir bitumen could be attributed to later "gas washing" for deeply buried hydrocarbon-accumulation generated by source rocks of Enping and Wenchang Formations during the early thermal maturity stage.The time differences of thermal evolution for source rocks and reservoirs of various stratigraphic horizons leads to the adjustment and altering of oil and gas reservoirs in the Baiyun Sag,subsequently resulting in the formation of light and volatile oil reservoirs.