鄂尔多斯盆地陇东地区二叠系太原组铝土岩系储层特征及勘探意义

Reservoir Characteristics and Exploration Significance of the Bauxite Rock Series of Permian Taiyuan Formation in the Longdong Area of the Ordos Basin

近期,鄂尔多斯盆地西南缘陇东地区二叠系太原组铝土岩系天然气勘探获重大突破,开辟了盆地找气新领域。为系统探究铝土岩这一新的储层类型,基于岩心观察、测井、地震、岩石薄片、X射线衍射全岩分析、扫描电镜—能谱、高压压汞、低温氮气吸附等方法,详细研究了铝土岩系储层岩石学特征和孔隙结构特征,进而阐述了盆地铝土岩系储层的主控因素和勘探意义。结果表明:(1)岩性垂向上具有三段式的结构特征,上部和下部以铝土质泥岩为主,中部以铝土岩为主,溶蚀孔隙发育,是铝土岩系主要储层发育段;(2)太原组铝土岩系矿物组成以铝质矿物和黏土矿物为主,主要矿物为一水硬铝石、高岭石、伊利石及绿泥石,矿物晶体细小,以隐晶、微晶及隐晶质团块为主;(3)铝土岩储集空间主要是颗粒内溶孔、基质溶孔、粒间溶孔、晶间孔隙及微裂隙等,孔径主要介于20~200μm;储层主流喉道孔径介于150 nm~4μm,孔隙结构较好,退汞效率高,整体上以亚微米—微米级孔喉为主;储层物性孔隙度平均为10.6%,渗透率平均为4.04×10^(-3)μm^(2),大于0.3×10^(-3)μm^(2)以上的占到36%,储集条件较好;(4)沉积环境和古地貌控制了铝土岩储层的分布,潜坑和阶地型微古地貌富集铝土岩,是主要的勘探有利区。该研究成果可为华北等地区铝土岩天然气勘探提供借鉴。

Recently,a breakthrough has been made in the gas exploration of the bauxite rock series from the Permian Taiyuan Formation in the Longdong area,southwestern margin of the Ordos Basin,introducing a new field of gas res⁃ervoir exploration.As a new type of reservoir,the study of the bauxite rock series reservoir has significant research value.In this study,based on core observation,well logging,earthquakes,rock slice analysis,whole rock analysis by X-ray diffraction and scanning electron microscope-energy spectroscopy,high pressure mercury injection,low temperature nitrogen adsorption and other experimental methods,the petrological and pore structure characteristics of bauxite reservoirs were systematically studied.Furthermore,the main control factor and exploration significance of bauxite reservoirs in the basin is expounded.The results show that:(1)the lithologic sequence is characterized by three segments in the vertical direction.The upper and lower parts are composed of bauxitic mudstone,the middle part is composed of bauxite,and the solution pores are well developed in the main reservoir section of the bauxite rock series;(2)The main mineral composition of the bauxite rock series from the Taiyuan Formation is bauxite and clay minerals,the main minerals are diaspore,kaolinite,illite and chlorite,and the crystals are small,primarily cryptocrystalline,microcrystalline and cryptocrystalline masses;(3)the bauxite reservoir spaces are primarily com⁃posed of intragranular solution,matrix solution,intergranular dissolution,and intercrystalline pores and microfractures.The pore size ranges from 20 to 200μm.The pore size of the mainstream throat ranges from 150 nm to 4μm,and has better pore structure and high mercury removal efficiency.Overall,the pores are primarily sub-micron and micron pores.The average porosity and permeability of the reservoir are 10.6%and 4.04×10^(-3)μm^(2),36%of which are larger than 0.3×10^(-3)μm^(2),and the reservoir performance is good.(4)Sedimentary environment and paleogeomor⁃phology control the distribution of bauxite reservoir.Buried pit and terrace micro-paleogeomorphology enrich bauxite and are the main favorable exploration areas.The bauxite reservoir distribution is controlled by the sedimentary envi⁃ronment and palaeogeomorphology,and bauxite is enriched by buried pits and terrace micro-paleogeomorphology,and favorable areas are relatively developed.The research results can provide reference for the exploration of bauxite natural gas in North China and other areas.