Cretaceous shales from the emerging gas province of the Rovuma basin are examined using well-log data from two exploration wells. The P-wave acoustic impedance (AI_p) data were estimated, and then brittleness and ductility were assessed within the framework of the Reuss–Voigt limits for acoustic impedance. We then predicted the shale consolidation using a weighting function (W_c) which varies between 0 for the case of grains in suspension (the Reuss limit) and 1 for the case of consolidated rock or cemented shale (the Voigt limit). At the Reuss limit, the formation AI_p is highly sensitive to pressure while at the Voigt limit the formation AI_p is insensitive to pressure. For the wells in this study, most data plot close to the lower bound with W_c < 0.5, hence showing a significant sensitivity to pressure. Although the Cretaceous shales in the Rovuma basin are dominated by a mechanical compaction regime, some onset of chemical compaction is observed. The rock-physics analysis showed that the same shale in these two exploration wells had very distinctive and contrasting elastic properties. The deeper well interval clearly shows a ductile shale while the shallower well shows a range of ductile to brittle behaviour matching with the onset of chemical diagenesis at temperatures >58°C. These differences in the development and rate of chemical diagenesis in the same formation are likely due to contrasts in the rate of burial and have important implications for future seismic exploration studies.

中文翻译：

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使用测井岩石物理分析确定莫桑比克近海鲁伍马盆地白垩纪页岩的压实历史

使用来自两个勘探井的测井数据检查了来自鲁伍马盆地新兴天然气区的白垩纪页岩。估计P 波声阻抗 (AI _p ) 数据，然后在 Reuss-Voigt 声阻抗限制的框架内评估脆性和延展性。然后我们使用加权函数 ( W _c )预测页岩固结，该函数在悬浮颗粒情况下的 0（Reuss 极限）和固结岩石或胶结页岩情况下的 1（Voigt 极限）之间变化。在 Reuss 极限处，地层 AI _p对压力高度敏感，而在 Voigt 极限处，地层 AI _p对压力不敏感。对于本研究中的井，大多数数据图都接近下限，W _c< 0.5，因此显示出对压力的显着敏感性。尽管鲁伍马盆地的白垩纪页岩以机械压实机制为主，但观察到一些化学压实的开始。岩石物理分析表明，这两个探井中的同一页岩具有非常独特和对比鲜明的弹性特性。较深的井段清楚地显示出韧性页岩，而较浅的井段显示出一系列韧性到脆性的行为，与温度 > 58°C 时化学成岩作用的开始相匹配。同一地层中化学成岩作用的发展和速率的这些差异可能是由于埋藏速率的差异，对未来的地震勘探研究具有重要意义。