多阶段成岩作用对深层碳酸盐岩岩石物理性质的影响——以四川盆地震旦系灯影组四段为例

Influence of multi-stage diagenesis on the rock physical properties of deep carbonate rock-reservoirs:a case study of 4th Member of Sinian Dengying Formation in Sichuan Basin

国内深层-超深层碳酸盐岩储层普遍经历多阶段、多期次的成岩作用改造,非均质性极强,致使脱离地质背景的岩石物理分析方法难以准确地给出岩石物理变化规律与储层特征之间的联系.本文通过对合川地区震旦系灯影组四段深层白云岩岩石物理性质的测量,并结合岩石学特征分析、多尺度孔隙结构分析及沉积环境的划分,在成岩作用过程框架下分析岩石物理特征的变化规律.研究结果表明,目标层段白云岩差异性成岩过程形成了不同的岩石孔隙结构特征与微观结构特征.丘滩相高能微生物白云岩分别经历同生期微生物白云岩化与渗透回流白云岩化作用、准同生期选择性溶蚀作用、埋深白云岩化作用以及构造-热液白云岩化,形成的晶粒白云岩主要为致密"焊接"型和紧密型白云石晶体接触边界,溶蚀孔隙发育,微裂隙与孔喉共同作为流体渗流通道.滩间海低能硅质、泥质与灰质云岩主要经历弱毛细管浓缩白云岩化作用、强机械压实致密化与埋深白云岩化作用,形成的晶粒白云岩存在致密"焊接"型、黏土型、石英胶结型与方解石型晶体接触边界,孔隙以残余原生粒间或晶间孔为主,以微裂隙作为主要的流体渗流通道.晶粒边界性质与孔隙结构共同控制了样品的宏观物性及地震弹性性质,相同晶粒边界的样品具有一致的纵波-横波速度和纵波速度-速度比变化关系.微裂隙等软孔隙对纵、横波速度影响程度的差异造成有效压力-速度、纵波-横波速度、纵波速度-速度比、孔隙度-速度关系以及孔隙尺度流体相关频散作用的强弱发生明显变化.研究结果可为深层碳酸盐岩的储层特征地震预测提供岩石物理依据,并可作为示例形成在地质过程中研究岩石物理特征演化的新方法.

The deep-ultra deep carbonate reservoir in China,commonly subjected to modification of multi-stage diagenesis,has extremely high heterogeneity.Therefore,it is difficult to accurately identify the relationship between the rock physics variation pattern and reservoir characteristics via a rock physics analysis that fails to sufficiently incorporate the geological setting.In this research,the rock physics properties of the deep dolomite of the 4th Member of the Sinian Dengying Formation in the Hechuan area are experimentally measured,and subsequently,the regularity of the rock physics characteristic variation is investigated within the framework of the diagenetic processes,via the integration of the analysis of the rock physics and petrologic characteristics,multi-scale pore structure characterization,and identification of sedimentary environments.The results show that the differentiated diagenesis of the dolomite in the target interval results in different pore structure characteristics and micro-texture characteristics of the rock.The high-energy microbial dolomite of the algal mound-grain beach facies is subjected to the contemporaneous microbial dolomitization and seepage-reflux dolomitization,penecontemporaneous selective dissolution,burial dolomitization,and hydrothermal dolomitization,successively.The grain dolomites formed are mainly compact"welded"and compact dolomite crystal contact boundaries,and the dissolution pore is extensive development with the micro-cracks and pore throat as seepage path.The low-energy siliceous,muddy,and limy dolomite of the inter-beach sea environment mainly experiences the weak capillary concentration dolomitization,intensive mechanical compaction-induced densification,and burial dolomitization.Such crystalline dolomite is observed with four types of contact boundaries,namely the compact"welding"contact,clay contact,quartz cementation,and calcite contact boundaries.Pores are mainly composed of residual primary inter-granular or crystalline pores,and micro-crack are the main fluid seepage path.The macroscopic seismic elastic properties of the samples are controlled jointly by the crystal boundary property and pore structure.The samples with the same crystal boundary condition have consistent correlations between the compressional-and shear-wave velocities,and between the compressional-wave velocity and the velocity ratio.The varied effects of soft pores like micro-cracks on the compressional-and shear-wave velocity causes considerable changes in the relationships between acoustic velocity and effective pressure,compressional-and shear-wave velocities,compressional-wave velocity and velocity ratio,porosity and acoustic velocity,and the intensity of pore-scale fluid-related dispersion.This research is an attempt to demonstrate a novel method for investigating the petrophysical variation of rock during the geological process,and the obtained findings can provide the petrophysical basis for seismic prediction of the characteristics of deep carbonate reservoirs.