东濮凹陷古近系沙河街组烃源岩生烃底限及其差异探讨

随着油气勘探不断向地层深部拓展,弄清烃源岩层生烃底限对于确定含油气盆地有效勘探领域范围和评价油气资源潜力都具有十分重要的意义。为此,以渤海湾盆地东濮凹陷古近系沙河街组烃源岩为研究对象,根据烃源岩层生排烃机理,采用有机元素变化分析法、生烃潜力法、残留烃量法和排烃量法4类7种方法对该区烃源岩的生烃底限进行了研究。结果表明:1用不同方法确定的沙河街组烃源岩生烃底限相差不大,全区生烃底限对应的镜质体反射率值(Ro)介于3.23%~3.97%,平均为3.71%,全区生烃底限对应的深度H值介于5 244~5 525m,平均为5 433 m;2凹陷北部生烃底限Ro介于3.61%~3.97%,平均为3.81%,其生烃底限H值介于5 415~5 545m,平均为5 488 m;3凹陷南部生烃底限Ro介于3.20%~3.74%,平均为3.48%,其生烃底限H值介于5 265~5 375m,平均为5 278 m。结论认为:该区南、北地区生烃底限存在差异,北部地区生烃底限对应的热演化程度(Ro)值和深度值明显高于南部地区,这与凹陷北部沙河街组发育巨厚膏盐层有关,主要体现为南、北地区在有机质类型、地层温度和压力等方面存在着差异性。

Hydrocarbon generation thresholds of Paleogene Shahejie Fm source rocks and their north–south differences in the Dongpu Sag,Bohai Bay Basin

As oil and gas exploration extends to deeper and deeper strata, to clarify the hydrocarbon generation threshold of source rocks is of great significance to defining the effective exploration range of petroliferous basins and evaluating the potential of oil and gas resources. In this paper, the Paleogene Shahejie Fm source rocks in the Dongpu Sag, Bohai Bay Basin, were taken as an example. Their thresholds of hydrocarbon generation were investigated by using 7 methods in 4 categories (i.e., the organic element variation analysis method, hydrocarbon generation potential method, residual hydrocarbon method and hydrocarbon expulsion method). It is shown that the hydrocarbon generation thresholds of Shahejie Fm source rocks determined by different methods are similar, with the corresponding vitrinite reflectance (Ro) and the burial depth (H) being in the range of 3.23–3.97% (AVG 3.71%), 5 244–5 525 m (AVG 5 433 m), respectively; and that in the north, Ro and H are in the range of 3.61–3.97% (AVG 3.81%), and 5 415–5 545 m (AVG 5 488 m), while in the south, they are in the range of 3.20–3.74% (AVG 3.48%), and 5 265–5 375 m (AVG 5 278 m). To sum up, the hydrocarbon generation thresholds in the south are different from those in the north. The corresponding thermal evolution degree (Ro) and burial depth (H) in the north are much higher than those in the south, just because thick gypsum salt layers are well developed in the Shahejie Fm in the north. The difference in tectonic settings between the south and the north is mainly reflected in organic type, formation temperature and pressure, etc.