Natural gas washing induces condensate formation from coal measures in the Pinghu Slope Belt of the Xihu Depression, East China Sea Basin: Insights from fluid inclusion, geochemistry, and rock gold-tube pyrolysis
Natural gas washing induces condensate formation from coal measures in the Pinghu Slope Belt of the Xihu Depression, East China Sea Basin: Insights from fluid inclusion, geochemistry, and rock gold-tube pyrolysis
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DOI：
10.1016/j.marpetgeo.2020.104450
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发表时间：
2020-08
影响因子：
4.2
通讯作者：
A. Su;Honghan Chen;Jian-xin Zhao;Tongwei Zhang;Yue‐xing Feng;Cunwu Wang
中科院分区：
文献类型：
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作者：
A. Su;Honghan Chen;Jian-xin Zhao;Tongwei Zhang;Yue‐xing Feng;Cunwu Wang
利用流体包裹体分析、石油地球化学、岩石金管热解等方法，对东海盆地西湖凹陷平湖斜坡带凝析油的成因和可能的形成过程进行了系统研究。颗粒含油包裹体（GOI™）技术、测井和地层测试共同表明，现今的凝析气层是由古油层演化而来的。石油包裹体生物标志物表明，这些古石油生成于中等成熟度的煤系地层。根据有机显微组分，干酪根的元素比，岩石评价热解，并从金管热解实验的结果，煤的措施进行了评价，作为源岩容易产生气体和能够在中等成熟度的石油形成。该成熟度水平与根据一系列分子成熟度比（镜质组反射率~ 0.7-1.0%Ro）确定的现今凝析油成熟度一致。与特定来源有关的生物标志物表明，凝析油也是煤衍生的。这些结果表明，凝析油是古油藏形成后的次生蚀变产物，与低成熟度下富含树脂的有机质的热降解或高成熟度下干酪根的热裂解无关，部分凝析油具有明显的低碳数正构烷烃损失和较高的甲苯/正庚烷（Tol/nC 7）比值。这表明古油藏经历了不同程度的气洗，形成凝析油，而不是高温下的石油裂解过程。天然气和古油之间的后续相互作用导致油包裹体显示从近黄色到蓝色的荧光变化。在同一原油组合中测得的不同的蒸汽体积分数是油气两相圈闭的结果。储层孔隙和含沥青油包裹体中沥青（沥青质）的普遍存在被认为是天然气脱沥青作用的结果，伴生水包裹体的均一温度（Th）和埋藏-热曲线表明古油藏形成于中新世晚期。过剩天然气主要由局部成熟气和非均质高-过成熟气组成，晚于第四纪运移进入古油藏。这一过程不仅导致了凝析油的形成，而且导致了储层中密度和含蜡量的垂向分异。
A systematic study into the origin and potential formation processes of condensates in the Pinghu Slope Belt of the Xihu Depression, East China Sea Basin, was conducted using fluid inclusion analysis, petroleum geochemistry, and rock gold-tube pyrolysis. Grains containing Oil Inclusions (GOI™) technique, well logging, and formation testing together indicate that the present-day gas condensate layer has evolved from a paleo-oil layer. Biomarkers from oil inclusions indicate that these paleo-oils were generated from coal measures at a moderate maturity level. Based on organic macerals, elemental ratios of kerogens, Rock-Eval pyrolysis, and results from gold-tube pyrolysis experiments, the coal measures are evaluated as source rocks prone to gas generation and capable of oil formation at moderate levels of maturity. This maturity level coincides with present-day condensate maturities determined from a series of molecular maturity ratios (vitrinite reflectance ~ 0.7–1.0%Ro). Biomarkers related to specific sources suggest that the condensates are also coal-derived. These observations suggest that the condensates are derived from secondary alteration following paleo-oil accumulation, rather than being related to the thermal degradation of resin-rich organic matter at a low maturity level, or the thermal cracking of kerogen at a high maturity level.Some condensates are characterized by a significant loss of n-alkanes with low carbon numbers and relatively high toluene/n-heptane (Tol/nC7) ratios. This indicates that the paleo-oil reservoirs have been subjected to varying degrees of gas washing, which resulted in condensate formation, as opposed to the process of oil cracking under high temperatures. Subsequent interactions between gas and paleo-oils resulted in oil inclusions showing fluorescence varying from near-yellow to blue. Variable vapor volume fractions measured in the same oil assemblage are the result of two-phase trapping of oil and gas. The widely observed presence of bitumen (asphaltene) in the reservoir pores and bitumen-bearing oil inclusions is considered to be due to gas deasphalting.Homogenization temperatures (Th) of associated aqueous inclusions and burial-thermal curves suggest that the initial paleo-oil reservoirs formed in the late Miocene. Excess gas, composed of local mature gas and heterochthonous high-to over-mature gas, migrated into the paleo-oil reservoirs later, in the Quaternary. This process not only resulted in the formation of condensates, but also led to vertical differentiation of density and wax content in the reservoirs.