鄂尔多斯盆地安塞南地区延长组长8段油藏富集规律

Enrichment rules of Chang 8 Member of Yanchang Formation in southern Ansai area,Ordos Basin

鄂尔多斯盆地安塞南地区延长组长8段发育低孔、超低渗储层,以安塞南丹东-河庄坪地区长8段致密油藏为例,利用大量的岩心、薄片、电镜、物性、地化及测井资料,系统地开展了长8油层组储层特征、油藏发育控制因素及富集规律研究,并构建了长8油层组的成藏模式。研究区长8油层组属三角洲前缘亚相沉积,主要岩性为细粒长石砂岩及长石岩屑砂岩,且长82亚段的分选要好于长81亚段。长8油层组储层内部的储集空间主要为粒间孔及粒间溶孔,孔隙组合包括粒间孔+溶蚀孔类型、溶蚀孔+粒间孔类型两类;镜下薄片观察结果显示,长8段的面孔率总体较低,平均总面孔率仅为3.3%。沉积微相与产能耦合关系分析认为,水下分流河道和河口坝为优势沉积微相。为此制定了研究区长8油层组储层分类评价标准,其中,Ⅰ、Ⅱ类储层为研究区主要油气富集区域,而Ⅲ类储层所占的比例最大;Ⅲ类储层被细分为Ⅲa及Ⅲb亚类,其平均试油产量分别为3.5和0.6 t/d。长8段致密油藏为典型的岩性油藏,油藏的分布主要受沉积相带和储层物性的联合控制,裂缝、高渗砂体、较高的过剩压力为油气成藏提供了运移通道和动力来源。

Low porosity and ultra-low permeability reservoirs are developed in Chang 8 Member of Yanchang Formation in southern Ansai area,Ordos Basin.For better understanding the enrichment rules of tight oil reservoirs,Chang 8 tight oil reservoir in Dandong-Hezhuangping,southern Ansai area is comprehensive studied in reservoir characteristics,hydrocarbon enrichment control factors and distribution rules of Chang 8 oil layer group.Furthermore,Chang 8 oil reservoir enrichment modes are constructed by the integration of different data types including large mumbers of cores,thin sections,electron microscopy,physical properties,geochemical and well logging data.The results show that Chang 8 oil formation in the study area belongs to the delta front subfacies,and the lithology is dominated by feldspar sandstone,followed by feldspar lithic sandstone.The sorting property of Chang 82 is better than that of Chang 81.The reservoir space of Chang 8 reservoir is dominated by intergranular pores,followed by intergranular dissolved pore.There are two main types of pore assemblages:intergranular pores+dissolved pores,and dissolved pores+intergranular pores.The surface porosity of target layer is generally low,with an average value of 3.3%.Predominant microfacies,such as underwater distributary channel and estuary bar,have relatively high production test oil production,while the test oil production in channel flank is obviously low.The classification and evaluation criteria for Chang 8 oil layer group in study area were formulated.TypesⅠandⅡreservoirs are the main reservoir type for hydrocarbon accumulating,while typeⅢreservoirs have the maximum are aproportion.The average test oil production of TypeⅢa andⅢb reservoirs 3.45 and 0.62 t/d respectively.The structure of study area has no obvious control over the hydrocarbons enrichment,and the distribution of hydrocarbons is mainly controlled by sedimentary facies belt and physical properties of the reservoir.Fractures,high-permeability sand bodies,and high excess pressure provide channels for oil-gas migration and driving forces for hydrocarbon accumulation.