川中合川气田须二段致密砂岩储层甜点研究

Research on tight sandstone reservoir sweet spots of the second member of Xujiahe formation of upper triassic in Hechuan gas field, central Sichuan basin

以川中合川气田须二段为研究对象,利用大量钻、测井资料、岩心、铸体薄片、扫描电镜、压汞及物性分析数据,系统研究了致密砂岩储层特征及储层甜点形成的主要控制因素.认为须二段储层甜点纵向上受高频层序界面控制,其平面分布受有利沉积微相、溶蚀成岩相控制.基于沉积相、成岩相、孔隙度、有效砂岩厚度、测试产量的量化加权评价,建立了储层甜点定量评价指标.研究结果表明:储层甜点发育于水下分流河道、河口坝微相、绿泥石衬边-强溶蚀相、微裂缝-强溶蚀相,具有孔隙度较高(9%~12%及以上)、有效厚度较大(15~20 m及以上)、测试产量较高(4×10^4~10×10^4 m^3/d或更高)的特点.须二段被划分为2个半高频旋回、5个砂层组,优质储层主要分布在层序界面之上的基准面上升的早期,须二段下部的X2-1砂层组储层最发育,连续性最好;有利沉积微相为三角洲前缘水下分流河道、河口坝,有效储层主要为中粒、中细粒岩屑长石石英砂岩,发育残余粒间孔、粒间溶孔、粒内溶孔和微裂缝;须家河组煤系地层生烃高峰期酸性地层水所引起的长石、岩屑和杂基的溶蚀作用是形成次生溶孔,改善储层物性的主要原因;早期绿泥石衬边胶结提高砂岩的抗压强度并抑制石英加大边的形成,从而使得较多的原生粒间孔隙得以保存;在雷口坡组古残丘顶部的须二段砂岩中,由于差异压实而发育微裂缝,这种微裂缝大幅改善了储层的渗透能力.归纳出7种成岩-储集相组合类型,基于沉积相-成岩相耦合,结合储层物性、有效厚度、测试产量定量评价,筛选出3个储层甜点发育区.Ⅰ类储层分布于三角洲前缘主河道和汇流区的水下分流河道-河口坝叠置砂体与绿泥石衬边-强溶蚀相或微裂缝-强溶蚀相耦合区域,绿泥石衬边-强溶蚀相呈片状分布,范围较大,而微裂缝-强溶蚀相分布局限,微裂缝发育部位与古残丘有关.

Focusing on the second member of Xujiahe formation of upper triassic, central Sichuan basin, systematical studies were conducted on tight sandstone reservoir features and the controlling factors of reservoir sweet spots formation, based on analysis of borehole, well logging, core observation, casting thin section, scanning electron microscope, mercury intrusion and physical property. This paper argues that the spatial distribution of reservoir sweet spots of the second member of Xujiahe formation is controlled by high resolution sequence boundaries in vertical, and by favorable sedimentary microfacies and dissolved diagenesis facies in horizontal. Quantitative evaluation index of reservoir sweet spots were established based on quantitative weighted evaluation of sedimentary microfacies, diagenesis facies, porosity, effective sand thickness and gas test production. The results show that reservoir sweet spots were developed in subaqueous distributary channel or mouth bar microfacies, chlorite rims intensively dissolved diagenesis facies or microfracture intensively dissolved diagenesis facies, with characteristics of relatively higher porosity(9%-12% or above), larger effective sand thickness(15-20 m or above), and higher gas test production(4×10^4-10×10^4 m^3/d or above). The second member of Xujiahe formation of upper triassic can be divided into two and a half high resolution stratigraphic sequences or five sandstone groups. High quality reservoirs with best continuity were developed in the early base-rising phase, in layer X2-1 of the lower section of the second member of Xujiahe formation. Effective sandstones were mainly developed in sedimentary microfacies of deltaic front subaqueous distributary channel and mouth bar, composed of medium grain and medium-fine grain lithic feldspathic quartz sandstone with residual intergranular pore, intergranular dissolved pore, intragranular dissolved pore and microfractures. Formation of secondary dissolved pores which improved the reservoir physical property was mainly attributed to the dissolution of feldspar, debris and matrix, resulted from acid water in hydrocarbon generation peak of coal bearing strata of Xujiahe formation. The early cementation of chlorite rims improved the compaction resist strength of sandstone, restrained the formation of quartz overgrowths, and thus more primary intergranular pores were conserved. Microfractures were developed due to differential compaction over Leikoupo paleo-monadnock which greatly improved the permeability of the second member of Xujiahe formation. Seven diagenesis-reservoir facies were concluded, moreover, reservoir sweet spots were quantitatively evaluated and three relatively high permeability areas were screened out based on the coupling of diagenesis and sedimentary facies, combined with physical property, effective reservoir thickness and test production of natural gas. First class reservoirs were distributed in the main channel and the conflux area of the deltaic front subaqueous distributary channels or in the channel-mouth bar superimposed sandstone complex, coupling with chlorite rims intensively dissolved diagenesis facies or microfracture intensively dissolved diagenesis facies. Chlorite rims intensively dissolved diagenesis facies were distributed in a large area, while the microfracture intensively dissolved diagenesis facies were distributed sporadically over the Leikoupo paleo-monadnock.