The Influence of Sedimentary Environment on the Elastic Characteristics of Shale Reservoirs: A Case Study of the Longmaxi Formation in the Zigong area, Southern Sichuan

Key words: 
• Sedimentary environment /
• elastic properties /
• organic shale /
• Longmaxi Formation /
• Zigong area /
• Sichuan Basin

Abstract: [Objective] The direct relationship between sedimentary environment and seismic elastic response of organic shale is very important for reservoir quality evaluation and geological modeling of unconventional oil and gas reservoirs. [Methods] The sedimentary characteristics and lithofacies of the Longmaxi Shale in the Zigong area are characterized, and four depositional units (I-A, I-B, I-C and II) are identified, the influence of sedimentary environment on the elastic characteristics of shale of Longmaxi Formation in Zigong area, Sichuan Basin is analyzed. [Results] The results show that the sedimentary environment mainly controls the elastic characteristics of the shale of the Longmaxi Formation in two aspects: first, the difference of rock structure caused by hydrodynamic action; The elastic characteristics of shale in the deep water shelf environment are obviously different from those of argillaceous siltstone in the overlying turbidite environment, the variation of elastic characteristics is mainly controlled by the precession system and water depth.In addition, it is found that quartz and TOC content have a competitive relationship in influencing the elastic characteristics of the reservoir. Finally, the prediction ability of pre-stack AVO inversion to seismic elastic attributes is used to trace the sedimentary evolution process, which provides an insight for directly characterizing sedimentary facies of unconventional shale reservoirs by geophysical attributes. [Conclusions] The sedimentary environment affects the elastic characteristics of shale reservoirs by controlling the structure and composition of rocks. For the influence of quartz and TOC content on reservoir elasticity, the softening effect caused by organic matter is dominant at low quartz content, and the hardening effect of quartz grains is dominant at high quartz content.