鄂尔多斯盆地三叠系延长组长7段页岩气储层特征及其勘探开发前景

Characteristics of Chang 7 shale gas reservoirs in Triassic Yanchang Formation of Ordos Basin and its exploration and development prospects

鄂尔多斯盆地作为我国的第二大沉积盆地,油气勘探潜力巨大。盆地内三叠系延长组7段富有机质烃源岩发育广泛,分布范围达(4~5)×10^(4) km^(2)。该套烃源岩有机质丰度高,有机质类型为Ⅰ—Ⅱ_(1)型,R_(o)主要分布在0.9%~1.2%之间,处于热演化成熟阶段,滞留烃含量大,为大规模页岩油气藏的发育奠定了重要物质基础。长7_(3)亚段富有机质泥页岩层段虽具备良好的含气性,但却缺乏深入的含气特征系统分析。以盆地西缘南段地区为例,利用岩石地球化学、有机地球化学、同位素分析等多种分析手段,查明了该区三叠系延长组长7段烃源岩储层地质及地球化学特征,分析了长7段页岩含气性特征、页岩气赋存状态,初步计算了长7段页岩气资源量。研究区内三叠系延长组长7_(3)亚段黑色页岩含气性良好,其组成以页岩油溶解气为主,含少量干酪根黏土矿物吸附气及游离气,计算平均解吸气量为1.91 m^(3)/t。长7_(3)亚段赋存页岩油、气资源,二者资源量相当,在后期勘探开发研究中建议考虑油气并举的可能性。湖盆中部厚层长7段黑色页岩发育区含气量达2 m^(3)/t,储层刚性矿物及微孔裂隙发育,含气丰度高,资源量大,为页岩气有利勘探区域。初步计算长7页岩气总资源量约为4.25×10^(12) m^(3),具有良好的勘探前景,勘探有利地区为姬塬—固城—正宁一带。

As the second-largest sedimentary basin in China,the Ordos Basin has enormous potential for oil and gas exploration.The Chang 7 member of the Triassic Yanchang Formation in the basin is extensively distributed with organic-rich source rocks,covering an area of 40000 to 50000 km^(2).These source rocks are characterized by high organic content,with organic matter types mainly being typeⅠtoⅡ_(1).The R_(o) values mostly range from 0.9%to 1.2%,indicating that they are in thermal maturity stage.The high content of retained hydrocarbons provides a strong material basis for the development of large-scale shale oil and gas reservoirs.Although the organic-rich mud shale layers in the Chang 7_(3) sub-member have good gas-bearing properties,a systematic analysis of their gas-bearing characteristics is lacking.Using the southern part of the basin’s western margin as a case study,multiple analytical methods such as rock geochemistry,organic geochemistry,and isotope analysis were used to identify the geological and geochemical characteristics of the source rock reservoirs in the Chang 7 member.The gas-bearing characteristics and shale gas occurrence states of the Chang 7 member were analyzed,and the shale gas resource potential was preliminarily calculated.The results indicated that the black shale of the Chang 7_(3) sub-member in the study area exhibited good gas-bearing properties,with dissolved shale oil gas being the main compo-nent,along with minor amounts of adsorbed gas of kerogen clay minerals and free gas.The average volume of desorption gas was calculated to be 1.91 m^(3)/t.The Chang 7_(3) sub-member contained both shale oil and gas resources,with approximately equivalent quantities.It is recommended to consider both oil and gas development in future explo-ration and development research.In the central part of the lake basin,where thick black shale of the Chang 7 member occurred,gas content reached 2 m^(3)/t.These reservoirs contained rigid minerals,micropores,and frac-tures,with high gas abundance and substantial resource potential,making this a favorable area for shale gas exploration.The total shale gas resource of the Chang 7 member was preliminarily estimated to be about 4.25×10^(12) m^(3),indicating promising exploration prospects.The favorable exploration areas were identified in the Jiyuan,Gucheng and Zhengning area.