Abstract Porosity evolution and reservoir quality are strongly affected by mechanical compaction. However, the compaction-induced porosity reduction process in sandstone reservoirs during the postaccumulation period and its effect on gas saturation remain poorly understood. By studying the sandstones in the Bayan'aobao area, we were able to quantify the porosity evolution and gas saturation variations caused by mechanical compaction during the postaccumulation period through fluid inclusion analysis and mathematical modeling. Moreover, we verified our findings by compaction experiments and geological statistics. Fluid inclusion microthermometry was employed to determine the hydrocarbon accumulation periods, and the highest homogenization temperature of the two-phase aqueous fluid inclusions coexisting with natural gas fluid inclusions, i.e., ~115 °C (with a corresponding geological age of ~140 Ma), was regarded as the beginning of the postaccumulation stages. With these data, we constructed an inversion evolution model (based on the geologic statistical model of sandstone mechanical compaction) for the postaccumulation period and determined the gas saturation changes (based on compaction porosity evolution). The modeling results indicate that the sandstone porosity decreased greatly during postaccumulation subsidence and increased slightly during late-stage uplift, while the gas saturation increased with both the porosity decrease during the postaccumulation subsidence and the porosity increases during the later uplift. The modeling results of subsidence are concordant with the compaction experiments, which also show that the larger the initial porosity, the greater the porosity reduction. The uplift modeling results demonstrate that an erosion-induced decrease in burial depth allowed the sandstone porosity to increase, which is supported by some scholars’ findings (e.g., Neuzil and Pollock, 1983; Jiang et al., 2004; Zhang, 2013) and by the positive correlation between sandstone porosities and restored geologic unit thicknesses in the study area. However, the inversion modeling indicates that the reason for the porosity rebound in litharenites is that the increase in elastic porosity slightly exceeds the decrease in viscoplastic porosity, which would have been different if the uplift-induced erosion rate had been distinctly lower. Gas production test data and restored geologic unit thicknesses indicate that uplift and erosion may be able to enrich sandstone gas reservoirs to a certain extent, consistent with the calculated gas saturation variation results, but a full understanding of gas saturation evolution awaits further studies. The inversion model can provide a powerful tool for quantitative analysis of the sandstone compaction process during the postaccumulation period, which could be used for reference in other similar regions.

中文翻译：

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机械压实成藏后砂岩孔隙演化及其对含气饱和度的影响——以鄂尔多斯盆地巴彦敖包地区下石河子组为例

摘要 孔隙度演化和储层质量受到机械压实作用的强烈影响。然而，在成藏后阶段压实致砂岩储层孔隙度降低过程及其对含气饱和度的影响仍然知之甚少。通过对巴彦敖包地区砂岩的研究，我们通过流体包裹体分析和数学建模，量化了成藏期机械压实引起的孔隙度演化和含气饱和度变化。此外，我们通过压实实验和地质统计验证了我们的发现。采用流体包裹体显微测温法确定油气成藏时期，以及与天然气流体包裹体共存的两相水性流体包裹体的最高均质温度，即 ~115°C（相应的地质年龄为~140 Ma），被认为是后积累阶段的开始。利用这些数据，我们构建了成藏期反演演化模型（基于砂岩机械压实地质统计模型），确定了含气饱和度变化（基于压实孔隙度演化）。模拟结果表明，砂岩孔隙度在堆积后沉降过程中大幅度降低，在隆升后期略有增加，而含气饱和度随着堆积后沉降过程中孔隙度的降低和后期隆升过程中的孔隙度的增加而增加。沉降模拟结果与压实实验结果一致，也表明初始孔隙度越大，孔隙率降低越大。隆起模拟结果表明，侵蚀引起的埋藏深度减小使砂岩孔隙度增加，这得到了一些学者的研究结果的支持（例如，Neuzil 和 Pollock，1983；Jiang 等，2004；Zhang，2013）和通过研究区砂岩孔隙度与恢复地质单元厚度之间的正相关关系。然而，反演模型表明，岩屑岩孔隙度反弹的原因是弹性孔隙度的增加略超过粘塑性孔隙度的减少，如果隆起侵蚀速率明显较低，则情况会有所不同。产气测试数据和恢复的地质单元厚度表明，抬升和侵蚀可能在一定程度上富集砂岩气藏，与计算的含气饱和度变化结果一致，但对含气饱和度演化的全面了解有待进一步研究。该反演模型可为后堆积期砂岩压实过程的定量分析提供有力工具，可供其他类似地区借鉴。