龙马溪组页岩微观结构、地震岩石物理特征与建模

Microtexture,seismic rock physical properties and modeling of Longmaxi Formation shale

龙马溪组页岩是目前国内页岩气勘探的主要层位之一.由于岩石物理实验结果具有区域性,龙马溪组页岩的岩石特征与其地震弹性性质的响应规律需要开展相关的实验和理论研究工作予以明确.本研究基于系统的微观结构观察(扫描电镜和CT成像技术)和岩石物理实验来分析龙马溪组页岩样品地震弹性性质的变化规律,并依据微观结构特征建立相应的地震岩石物理表征模型.研究结果表明,石英含量对龙马溪组页岩的孔隙度以及有机碳(TOC)含量具有一定的控制作用,TOC和黄铁矿主要赋存于孔隙中;岩石骨架组成亦受控于石英或粘土含量,在石英含量大于40%(对应粘土含量小于30%)时,以石英、粘土共同作为岩石骨架,而粘土含量大于30%时,则以粘土作为岩石的骨架.因此,岩石骨架组成矿物、TOC含量、孔隙度共同制约龙马溪组页岩的地震弹性性质,富有机质储层岩石通常表现出低泊松比、低阻抗和低杨氏模量的特征,但由于支撑矿物的转换,某些富有机质页岩亦可表现为高阻抗特征.粘土矿物的定向排列仍然是造成页岩样品表现出各向异性的主要原因,各向异性参数与粘土含量具有指数关系.基于龙马溪组页岩的岩性特征及微观结构特征,可以利用自洽模型(SCA)、微分等效模量模型(DEM)和Backus平均模型的有效组合较为准确地建立龙马溪组页岩的地震岩石物理模型,实验结果和测井数据验证了模型的准确性.研究结果可为龙马溪组页岩气储层的测井解释和地震"甜点"预测提供依据.

Longmaxi Formation shale of Sichuan Basin is a typical shale-gas system in which the rock is the source,reservoir,and seal,and is also the most important horizon for gas shale exploration in China.The elastic properties of shale rocks vary significantly between reservoirs and within a reservoir due to the wide variety of material composition and fabric anisotropy.The rock properties and corresponding seismic rock-physics properties are not completely known for the Longmaxi Formation shale,thus related theoretical and experimental investigations are crucial for seismic exploitation of shale gas reservoirs and corresponding hydraulic fracturing.The mineralogy and the organic content were determined through powder XRD analysis.Seismic rock-physics properties and their impact factors of the gas shale samples from Longmaxi Formation are analyzed based on acoustic experiments and microtexture analysis with the combined application of SEM and CT scanning,and the rock-physics model for shale is also proposed.Cylindrical cores of a suite of shales from this formation with different organic matter contents were cut and oriented carefully in the direction normal,parallel,and at 45°to bedding planes,and velocities were measured using the ultrasonic transmission technique at a range of confining pressures,which can be used to determine the stiffness and anisotropy properties of rock samples.The results show that the TOC（total organic content）and porosity are controlled to some degree by the quartz content,and TOC and pyrite are mainly located in the pore space,and are not the load-bearing grains.Shale samples can be classified as silica-rich rock.The constitution of the rock frame is also governed by the clay or quartz content,and the rock samples are claysupported when the clay content is larger than 30% or the quartz content lower than 40%,and quartz combined with clay becomes load-bearing material when clay content is lower than this value.Thus,the seismic properties of those shale samples are controlled by load-bearing grains,TOC and porosity.We observe a ＂V＂shaped trend of P-wave impedance as function of quartz content：P-wave impedance decreases as the quartz content increases till about 50%,after the turning point,the P-wave impedance increases again.Organic-rich shale shows low P-wave impedance and VP/VSratio due to relatively higher TOC and quartz content.Nonetheless,high Pwave impedance is also observed when the quartz content is very high resulting from the variation of load-bearing grains.Preferential alignment of mineral grains is the dominant factor to create the elastic anisotropy of these shale samples at high confining pressure.A strong positive correlation is found between the anisotropy parameters（ε,γ）and the clay content,which follows an empirical exponential relationship：ε=0.0016V1.32 clay,γ=0.0015V1.33 clay.Samples with low clay content show negativeεandγdue to the vertically aligned microcracks.C11,C33,C44,C66 and C13are five elastic stiffness constants of typical transverse isotropic media,which also show apparent empirical correlations.Among these,C13 and C33,C44 satisfy an empirical relationship：C13 =1.11C33-2C44,and C13 and C12 satisfy an empirical relationship：C13 =1.13C12.By combined usage of the Self Consistent Approximation（SCA）,Differential Effective Medium and the Backus average method to describe the microtexture of shale sample properly,the prediction results of the model fit very well with those measured data at lab and well-logging data,which proves the practicality of the model.Longmaxi Formation shale is variation in mineral composition,organic content and microtexture due to complex depositional and digenetic evolution,which is likely to attributed to the significant difference in seismic rock physics and mechanical properties.Our results can also provide basis for well-logging interpretation and ＂sweet spot＂discrimination by the seismic method for Longmaxi Formation shale.