四川盆地东南缘下志留统龙马溪组页岩基本特征

Basic characteristics of Lower Silurian Longmaxi Formation in SE margin of Sichuan Basin

下志留统龙马溪组是四川盆地东南缘最为重要的烃源岩和页岩气勘探层位之一。在岩石学研究的基础上,利用岩石热解、色谱—质谱等分析技术,对研究区龙马溪组页岩地球化学特征进行了系统分析。龙马溪组厚为60～420 m,主要由8种岩性构成:层状—非层状泥/页岩、白云质粉砂岩、层状钙质泥页岩、泥质粉砂岩、层状—非层状粉砂质泥/页岩、粉—细砂岩、钙质结核、富含有机质非层状页岩,自下而上沉积环境由闭塞盆地—深水陆棚—浅水陆棚过渡。龙马溪组岩心平均孔隙度为0.58%～0.67%,渗透率为0.01×10^(-3)μm^2～0.93×10^(-3)μm^2。微孔隙发育,主要为晶间孔和粒内孔。在川东南地区,龙马溪组TOC值为0.55%～4.41%,仅下部为优质烃源岩,中上部为中等烃源岩。在鄂西渝东地区,石柱剖面TOC值高,为0.55%～5.88%,其它地区一般在0.3%以下。干酪根类型主要为Ⅰ型—Ⅱ_(1-2)型,以Ⅱ类为主。龙马溪组成熟度基本处于过成熟晚期阶段,仅在局部构造高位和龙马溪组上部Ro值不及2.0%。饱和烃色谱—质谱特征表明,龙马溪组下部主要为沉积具有一定盐度的滞留还原沉积建造,生物多为低等水生的菌藻类,但向上逐渐趋于氧化。龙马溪组页岩具有较强非均质性,下部粉砂岩夹层在建深1井密度大,页岩储层在建深1井较丁山1井更发育。

Lower Silurian Longmaxi Formation is one of the most important source rocks and shale gas exploration beds. Using the rock eval, chromatography-mass spectrometry, and other analytical techniques, the geochemical characteristics of Longmaxi Formation has been systematically analyzed based on the studies of petrographic features. The thickness of Longmaxi Formation ranges from 60 m to 420 m, and is comprised by eight lithofacies, including laminated and nonlaminated mudstone/shale, dolomitic siltstone, laminated lime mudstone/ shale, argillaceous sihstone ,laminated and nonlaminated silty mudstone/shale ,fine grained silty sandstone, calcareous concretions and organic matter enriched noncalaceous shale. The depositional environment gradually changes from cutoff basin, deep shelf, and to shallow shelf bottom-up. Porosity measured on core samples of the shale from the Longmaxi Formation in exploratory wells ranges from 0.58% to 0.67%. The microporosity observed in the thin sections of the shale is dominated by the intercrystal and intragranular pores. TOC of Longmaxi Formation ranges from O. 55% to 4.41% in SE Sichuan Basin,but only the lower part is high quality source rocks, the mid-upper with general source rocks. TOC is very high in Shizhu profile, east Chongqing-west Hubei Province,with range of 0.55% - 5.88% , but which is lower than 0.3% in other regions. Organic matter range from type Ⅰ -kerogen to type Ⅱ1-2-kerogen in Longmaxi Formation, but is dominated by the latter. The maturity of organic matter in Longmaxi Formation is very high,with late stage of post-maturation, and Ro is just lower than 2.0% in high point of local structure or upper part of Longmaxi Formation. The lower part of Longmaxi Formation （mainly aquatic algae ）is deposited in restricted deoxidation conditions, which is revealed by chromatography-mass spectrometry features of saturated hydrocarbon, But becoming more and oxidation up. The heterogeneity of Longmaxi Formation shale is very strong, and siltstone bedded in lower part shale is more denser in Jianshen #1 well. Shale reservoir in Jianshen #1 well is more developed in Dingshan #1 well.