鄂尔多斯盆地富县地区三叠系延长组长7页岩油特征及其主控因素

Characteristics and main controlling factors of Chang 7 shale oil in Triassic Yanchang Formation,Fuxian area,Ordos Basin

鄂尔多斯盆地富县地区三叠系延长组长7页岩段具有埋深浅、油质轻、油井产能变化大、资源潜力大的特点,明确油藏的富集主控因素是进行高效勘探的关键。根据富县地区长7页岩段铸体薄片、物性、扫描电镜等测试分析结果,结合岩心、测井、地震等资料,对该区长7段地层特征进行了研究,并对其主控因素展开讨论。研究区长7段广泛发育灰色—深灰色—灰黑色烃源岩,其岩性为泥岩和泥页岩,有机质类型主要为Ⅰ—Ⅱ_(2)型,源岩的镜质体反射率(R_(o))值为0.81%~1.1%,生烃能力强。砂岩储层以细粒长石砂岩为主,孔隙类型以粒内孔、残余粒间孔、溶蚀孔隙和原生粒间孔为主;储层致密,孔隙度主要分布在2.0%~16.0%之间,渗透率主要分布在(0.01~1.20)×10^(-3)μm^(2)之间。砂岩夹层的物性以及距离断裂的远近影响储层的含油性:储层粒度越粗、物性越好,含油气性也越好。储层物性受控于两方面因素:一是沉积微相,水下分流河道微相物性最好,河口坝微相次之;二是成岩作用,早成岩期绿泥石强胶结及方解石弱胶结有助于物性甜点的形成。断层的发育和断层性质对富县地区长7页岩油的富集至关重要。统计发现,当断层断距超过10 m且井筒距断裂在1 km以内时,难以获得工业油流;当断距小于7 m且井筒距离大断裂超过1 km以上时,容易获得工业油流。

The shale in the 7th member(Chang 7)of Triassic Yanchang Formation on the southeastern margin of the Ordos Basin is characterized by shallow burial depth,light oil quality,large variability in oil well production capacity,and significant resource potential.Identifying the main controlling factors of reservoir enrichment is key to efficient exploration.Based on analysis results of cast thin sections,physical properties,and scanning electron microscope(SEM)tests of the Chang 7 shale member in the Fuxian area,combined with core,well logging,and seismic data,its layer characteristics are described and the main controlling factors are discussed.The results show that the Chang 7 shale in the research area extensively develops gray,dark gray,and gray black source rocks.The lithology is mainly mudstone and mud shale,with organic matter types mainly classified as typeⅠ-Ⅱ_(2).The vitrinite reflectance(R_(o))values of source rocks range from 0.81%to 1.10%,indicating strong hydrocarbon generation potential.The sandstone reservoirs are mainly fine⁃grained feldspathic sandstone,with pore types primarily consisting of intragranular pores,residual intergranular pores,dissolution pores,and primary intergranular pores.Porosity ranges from 2.0%to 16.0%,and permeability ranges from 0.01×10^(-3) to 1.20×10^(-3)μm^(2),indicating tight reservoirs.The oil⁃bearing properties of the reservoir are impacted by the physical properties of the sand⁃stone interlayers and proximity to faults:coarser⁃grained reservoirs with better physical properties exhibit better oil and gas bearing potential.Reservoir properties are controlled by two factors,sedimentary microfacies and dia⁃genesis.Subaqueous distributary channel microfacies have the best properties,followed by mouth bar microfacies.Strong early diagenetic chlorite cementation and weak calcite cementation contribute to the formation of sweet spots in the physical properties.The development and nature of faults play a crucial role in the Chang 7 shale oil enrichment.Statistical analysis shows that it is difficult to obtain industrial oil flow when the fault displacement exceeds 10 m and the wellbore is within 1 km of the fault.However,when the fault displacement is less than 7 m or when the wellbore is more than 1 km away from a large fault,industrial oil flow is more easily obtained.