四川盆地中二叠统栖霞组与茅口组烃源岩的差异性

四川盆地中二叠统栖霞组和茅口组都具有良好的油气勘探前景,目前对这2套烃源岩的横向和纵向分布特征还鲜有报道。为此,通过对17口井111块栖霞组和茅口组岩心样品实测总有机碳含量(TOC)的标定,运用分段平均值法,建立了自然伽马测井曲线与烃源岩TOC的关系式,利用该关系式对全盆地117口井的上述2套烃源岩TOC进行了测井评价。结果表明:(1)横向上,栖霞组和茅口组烃源岩在全盆地均有分布,发育以川东地区为最好;(2)茅口组烃源岩在平面厚度、有机碳含量及生气强度等方面均优于栖霞组,前者厚度介于30~220 m,TOC介于0.5%~3.0%,为中等—好烃源岩,而后者厚度介于10~70 m,TOC介于0.5%~2.0%,为差—中等烃源岩;(3)纵向上,栖霞组烃源岩主要分布在栖一段,茅口组烃源岩主要发育于茅一段和茅二c层。结论认为:(1)栖霞组生气强度很低,绝大部分地区小于10×10~8 m~3/km~2;(2)而茅口组生气强度则明显较高,为10×10~8~60×10~8 m~3/km~2且大部分区域都大于20×10~8 m~3/km~2;(3)后者具备形成大型气田的物质基础。

Differences between the Middle Permian Qixia and Maokou source rocks in the Sichuan Basin

The lateral and vertical distribution characteristics of the Middle Permian Qixia and Maokou source rocks in the Sichuan Basin, although with a bright exploration prospect, were rarely reported in previous literatures. According to the analysis of the tested TOC data of 111 source rock samples and the corresponding gamma-rays (GR) from 17 wells, the relationship between the TOC and GR was built, and on this basis, the TOC of the Qixia and Maokou source rocks from 117 wells all over the whole basin were calculated. The following findings were obtained. (1) Laterally, the source rocks of the two formations are both distributed in the whole basin, while those in the east are the best in quality. (2) The Maokou source rocks are better than the Qixia ones: the thickness and the TOC values of the former are 30–220 m and 0.5–3.0%, respectively, and source rocks can reach a medium to good level in quality, while those of the latter are 10–70 m and 0.5–2.0%, respectively, and source rocks in a medium quality. (3) Vertically, the Qixia source rocks are mainly developed in the 1st member of the formation, and the Maokou ones were mainly distributed in the 1st member and the C section of the second member of the strata. In conclusion, the gas generation intensity of the Qixia Fm, with less than 10×10⁸ m³/km² in most parts, is very low, while that of the Maokou Fm is much greater with a range of (10–60)×10⁸ m³/km², and that in most parts is more than 20×10⁸ m³/km², laying a robust foundation for the generation of a large gas pool. The source rocks of the Maokou Fm dominate the hydrocarbon contributions of the Middle Permian source rocks in the Sichuan Basin.