柴达木盆地英雄岭页岩纹层结构特征及石油地质意义

Characteristics of laminated texture and petroleum geological significance of Yingxiongling shale in Qaidam Basin

柴达木盆地英雄岭构造带干柴沟地区下干柴沟组上段(E32)Ⅳ—Ⅵ油组发育一套咸化湖相页岩,碳酸盐矿物含量高,纹层发育。该层系页岩油勘探取得了重大进展,纹层结构特征、沉积机理、有利岩相组合等急需深入研究。岩心描述、薄片鉴定、扫描电镜、X—射线衍射等分析结果表明:干柴沟地区页岩纹层结构可分为水体分层型纹层结构(Ⅰ型)、季节型纹层结构(Ⅱ型)、洪水阵发型纹层结构(Ⅲ型)3类,其中Ⅰ型纹层结构为典型纹层结构,相对占比为70%,由黏土质纹层与方解石纹层构成偶纹层,单纹层厚度为0.1~0.5mm,为咸水湖水体季节性分层条件下沉积的纹层结构。夏秋季水体分层,上层水体有利于微生物繁衍;冬春季水体分层消失,有利于有机质沉积和保存,同时,Ⅰ型纹层结构沉积速率慢,有利于有机质富集,TOC平均为1.33%。Ⅱ型纹层结构相对占比为20%,由黏土质纹层与粉砂质纹层组成,单纹层厚度为0.5~1mm,沉积与陆源输入物的季节性变化有关,沉积速率快,TOC平均为0.47%。Ⅲ型纹层结构相对占比为10%,由黏土质纹层与粉砂质纹层组成,单纹层厚度为0.5~2mm,发育微冲刷面、透镜状粉砂脉等,反映沉积时具有一定水动力条件,非静水沉积,与重力流、湖流相关,沉积速率快,TOC平均为0.27%。干柴沟地区页岩主要分布于半深湖区,沉积单旋回自下而上为泥岩→页岩→泥质白云岩→白云岩,期间偶发砂质重力流沉积。Ⅰ型纹层结构页岩沉积序列上与白云岩叠置分布,白云岩孔隙度平均为9.8%,微观尺度上形成良好的源储组合,有利于页岩油成藏,为干柴沟地区页岩油有利岩相组合;宏观尺度上该套页岩层系TOC高,物性好,脆性矿物含量高,是柴达木盆地页岩油勘探开发的理想层系。Ⅱ型纹层结构页岩、Ⅲ型纹层结构页岩生烃能力差,对油气成藏贡献小。

A set of saline lake facies shale was deposited inⅣ-Ⅵoil group of the upper member of Lower Ganchaigou Formation(E32)in Yingxiongling Structural Belt in Qaidam Basin,with high carbonate mineral content and well-developed laminae,in which great progress was made in the shale oil exploration.Therefore,the in-depth research should urgently be conducted on the laminated texture,depositional mechanism,and favorable lithofacies combination.The experimental results of core description,thin section identification,scanning electron microscopy and X-ray diffraction analysis show that the shale laminated texture in the study area is divided into three types,i.e.,water stratified type laminated texture(TypeⅠ),seasonal type laminated texture(TypeⅡ)and flood surging type laminated texture(TypeⅢ).Type I shale shows a typical laminated texture deposited in seasonal stratified water body in saline lake,accounting for 70%of the total laminae,which is composed of clayey laminae and calcite laminae,with a single lamina thickness of 0.1-0.5 mm.In summer and autumn,the water body was stratified,and the upper water body was favorable for microbial proliferation.While in winter and spring,the water body stratification disappeared,which was beneficial to the deposition and the preservation of organic matters.In addition,TypeⅠlaminated texture was featured by a low deposition rate,which was beneficial to the enrichment of organic matters,with an average TOC of 1.33%.TypeⅡlaminated texture accounts for 20%of the total laminae,which is composed of clayey lamina and silty lamina,with a single lamina thickness of 0.5-1 mm.The deposition of TypeⅡshale was related to the seasonal variation of terrestrial inputs,showing rapid deposition rate,with an average TOC of 0.47%;The proportion of TypeⅢlaminated texture is 10%,which is composed of clayey lamina and silty lamina,with a single lamina thickness of 0.5-2 mm,micro-scouring surface and lens-shaped siltstone veins developed,and an average TOC of 0.27%,indicating certain hydrodynamic conditions related to gravity flow and lake flow rather than non-hydrostatic environment,as well as rapid deposition rate.The shale in the study area was mainly developed in semi-deep lake,with the single sedimentary cycle including mudstone→shale→argillaceous dolomite→dolomite from bottom to top,and occasional sand gravity flow deposits.For TypeⅠlaminated texture,the shale deposition sequence was superimposed on dolomite,with the average porosity of dolomite of 9.8%,forming good source rock and reservoir assemblage in micro scale,which was beneficial for shale oil accumulation,showing the favorable lithofacies combination of shale oil in Ganchaigou area.Macroscopically,this set of shale has high TOC,good physical properties,and high content of brittle minerals,which is an ideal interval for shale oil exploration and development in Qaidam Basin.The hydrocarbon generation potential of TypeⅡandⅢshales is poor,which have low contribution to hydrocarbon accumulation.