鱼卡凹陷石门沟组上段泥页岩地球化学及储层特征

Geochemistry and reservoir characteristics of shale in the upper member of Shimengou Formation,Yuka sag

鱼卡凹陷位于柴达木盆地北缘中部,为青海省陆相页岩气勘探的重点区域。中侏罗统石门沟组上段为半深湖-深湖相沉积,暗色泥页岩厚度10~170 m,平均为55 m;为页岩气勘探的主力层系。运用液氮吸附、有机碳含量、有机质成熟度、干酪根元素、全岩、黏土矿物X衍射、能谱扫描电镜等测试方法,对泥页岩样品的有机地球化学及储层特征进行了研究,结果表明:中侏罗统石门沟组泥页岩TOC含量为1.34%~12.84%,均值为5.3%;干酪根H/C和O/C比范式图解上显示有机质类型主要为Ⅱ2型;R o为0.45%~1.00%,总体热成熟度表现为凹陷西部高于东部。黏土矿物含量为34%~58%,平均46.1%;石英含量42%~55%,平均48.3%。泥页岩孔隙结构复杂,根据吸附回线及孔径分布曲线划分为两类:第1类以一端不透气性孔和四边开放的板状狭缝孔为主,孔径主要集中在3~5 nm,呈单峰状分布;第2类以一端不透气性孔和开放性倾斜板狭缝孔为主,孔径主要分布在3~5 nm和6~45 nm,呈双峰状分布。由于泥页岩成熟度较低,有机质纳米孔隙不发育,TOC含量与介孔、总孔体积具有弱正相关性,与微孔体积相关性不大;脆性矿物含量与孔隙度成正相关,黏土矿物含量与微孔体积相关性不大,与介孔、总孔体积呈正相关。黏土矿物是石门沟组上段泥页岩纳米孔隙的主要提供者,是孔隙发育的主要控制因素;孔隙结构及孔径分布和沉积环境有关,TOC含量及热演化程度也会影响泥页岩孔隙发育的程度。

Yuka sag,located in the middle of the north margin of Qaidam basin,is a key area for continental shale gas exploration in Qinghai province.The upper member of the Shimengou formation of the middle Jurassic is a semi-deep lacustrine to deep lacustrine facies deposit with a dark shale thickness of 10-170 m with an average of 55 m for major formation in shale gas exploration.The organic geochemistry and reservoir characteristics were studied by using nitrogen gas adsorption,total organic carbon content,organic matter maturity,kerogen element,X-ray diffraction analyses and SEM.The results show that the TOC content in Shimengou formation of middle Jurassic is 1.34%-12.84%,with an average of 5.3%.Kerogen types are mainly typeⅡ2;R o ranges from 0.45%to 1.0%,which is higher in the western area than that in the eastern area.Clay mineral content ranges from 34%to 58%with an average of 46.1%.Quartz content ranges from 42%to 55%,with an average of 48.3%.The nanopore structure in the Shimengou shale are complex,which can be divided into two types according to the nitrogen adsorption-desorption curves and pore diameter distribution.The first type is dominated by the airtight pores with one end closed and the open parallel plate-slit pores,and the pore diameters are in a range of 3-5 nm with a single-peak distribution.The second type mainly consists of the airtight pores with one end closed and open inclined slit pores,peaking in a range of 3-5 nm and 6-45 nm with a bimodal distribution.Due to the low maturity,the nano pores of organic matter are not developed,TOC content has a weak positive correlation with mesoporous and total pore volume,but not with micropore volume.Brittle mineral content is positively correlated with porosity,clay mineral content is not correlated with micropore volume,but positively correlated with mesoporous volume and total pore volume.The clay content is a key factor controlling the nanopore development in shale,the pore structure and pore diameter distribution are related to sedimentary environment,and the TOC content and thermal evolution also have some influence.