障壁坝砂体储层特征与成岩孔隙定量演化模式 ——以鄂尔多斯盆地延长探区本溪组为例

为明确海陆变迁背景下障壁坝砂体成岩孔隙演化规律,运用岩石薄片、扫描电镜及配套能谱、阴极发光等资料,对延长探区石炭系本溪组障壁成因砂体储集特征与成岩作用进行综合分析,并结合砂岩初始孔隙度恢复模型与薄片孔隙定量分析技术,开展基于埋藏热演化历程的储层孔隙演化参数定量评价,建立该区本溪组障壁坝砂岩孔隙定量演化模式。结果表明:储层砂岩以石英砂岩、岩屑质石英砂岩及岩屑砂岩为主;孔隙度、渗透率平均值分别为4.72%和1.22×10^(-3 )μm^2;储集空间以溶蚀扩大孔、粒内孔、晶间孔为主;现今储层主体已达到晚成岩阶段;砂岩初始孔隙度平均值为38.1%,埋藏至今共经历了四个典型的成岩增/减孔阶段,即P_1～T_2(285～208 Ma)"低地温、快埋藏"的大幅减孔阶段(-30.7%)、T_2～J_3(208～153 Ma)"高地温、缓沉降"的溶蚀增孔阶段(+3.9%)、J_3～K_1(153～96 Ma)"高地温、稳埋深"的减孔定型阶段(-6.6%)、K_1至今(96 Ma～)"低地温、晚隆升"的弱改造阶段。

Characteristics and a Quantitative Diagenetic Porosity Evolution Mode of Barrier Bar Sandstone Reservoirs: A case study of the Benxi Formation,Yanchang exploration block,Ordos Basin

The Carboniferous Benxi Formation is rich in barrier bar sandstone reservoirs, and is one of the main production layers for tight gas in the Yanchang exploration block, Ordos Basin. To understand the physical properties of the reservoirs and how the porosity evolved, detailed analyses were conducted using thin sections, SEM combined with energy dispersive spectroscopy (EDS), cathodoluminescence (CL) microscopy and other techniques. Also, an integrated approach using an empirical formula for initial sandstone porosity combined with quantitative calculation of porosity enabled porosity evolution modes to be established for the whole geodynamic history of these materials. The results indicate that the reservoir lithology consists predominantly of moderately-to well-sorted quartzarenite, sublitharenite and litharenite. The mean porosity is 4.72% and the permeability is 1.22×10-3 μm2. The reservoirs contain a variety of pore types, mainly dissolved enlarged pores, intraparticle pores and intercrystal pores. Quantitative evaluation of the increase/decrease of porosity caused by different diagenetic conditions reveals that the sandstone (initial average porosity 38.1%) typically underwent four porosity evolution stages:(1) marked porosity loss (-30.7%) in conditions of low geothermal gradient and rapid burial (P₁-T₂, 285-208 Ma); (2) a slight porosity increase due to dissolution (+3.9%) in conditions of high geothermal gradient and slow burial (T₂-J₃, 208-153 Ma); (3) a stereotyping stage of the tight sandstone reservoir (-6.6%) in conditions of continuously high geothermal gradient and slow burial (J₃-K₁, 153-96 Ma); and (4) a weak reworking of the tight sandstone reservoir in conditions of low geothermal gradient and overall uplift characterized by fractures, from the Lower Cretaceous to the present time (K₁~, 96 Ma~).