渤海湾盆地沾化凹陷BYP5导眼井古近系沙河街组三段下亚段岩心富氢气逸散气特征及其地质意义

Characteristics and geological significance of escaping gas rich in natural hydrogen from pilot well BYP5 cores of lower sub⁃member of third member of Shahejie Formation in Zhanhua Sag,Bohai Bay Basin

BYP5导眼井是为探索渤海湾盆地沾化凹陷渤南深洼带较高热演化区古近系沙河街组三段下亚段(沙三下亚段)含油气性而部署的一口取心井,取心段深度介于4267.0~4338.1 m。为揭示取心段含油气性特征,开展了典型样品冷冻密闭碎样热解和岩心逸散气的收集定量与组分分析。研究结果认为:取心段总体是一套富有机质、富碳酸盐矿物的优质烃源岩,成熟度(Ro)约1.2%,热演化过程中应曾发生了高效生排烃作用,导致其现今游离烃(S1)和氢指数(IH)均较低;岩心逸散烃气含量总体不高,主要介于0.001~0.01 cm^(3)/g,均值为0.005 cm^(3)/g,逸散烃气相对高值段与热解游离烃(S1)相对高值段基本一致;逸散气组分主要由CH_(4)、CO_(2)、H_(2)、C_(2)H_(6)组成,其中H_(2)摩尔百分数介于1.08%~19.23%,平均7.09%,具有富氢气特征。H_(2)与CO_(2)具有明显正相关性,与CH_(4)具有明显负相关性;逸散气应属原位滞留的气体,氢气的形成可能与有机质热解过程中异质键的裂解和去甲基化作用有关,建议加强有机质热裂解成因天然氢气的形成机制、地质勘查与评价研究,为该类型天然氢气的勘探部署决策提供依据。

The pilot well BYP5 is a cored well drilled to explore the oil and gas bearing properties of the highly thermally evolved lower sub⁃member of the third member of the Paleogene Shahejie Formation in the Bonan deep sag of the Zhanhua Sag,Bohai Bay Basin.The coring interval depth ranges from 4267.0 to 4338.1 m.To reveal the oil and gas bearing properties of the cored interval,pyrolysis of frozen,sealed fragments was conducted on typical samples and the escaping gas from the core was collected and quantified for composition analysis.The results show that the cored interval is a high⁃quality hydrocarbon source rock rich in organic matter and carbonates,with a maturity(Ro)of about 1.2%.Efficient hydrocarbon generation and expulsion likely occurred during thermal evolu⁃tion,causing the current low free hydrocarbon(S1)and hydrogen index(IH)values.The content of the hydro⁃carbon gas from the core was generally low,ranging from 0.001 to 0.01 cm^(3)/g,with an average of 0.005 cm^(3)/g.Segments with relatively high levels of escaping hydrocarbon gas corresponded to those with relatively high pyrolysis S1values.The escaping gas was mainly composed of CH_(4),CO_(2),H_(2),and C_(2)H_(6),with mole percentages of H_(2) ranging from 1.08%to 19.23%,with an average of 7.09%,indicating hydrogen⁃rich characteristics.H_(2) showed a signifi⁃cant positive correlation with CO_(2)and a negative correlation with CH_(4).The escaping gas from the core was likely trapped in⁃situ,and the formation of H_(2) might be related to the cleavage of hetero⁃bonds and demethylation during the pyrolysis of organic matter.Further research is suggested on the formation mechanism,geological exploration,and evaluation of natural hydrogen released during organic matter pyrolysis,so as to provide a basis for the decision⁃making in the exploration and development of this type of natural hydrogen resource.