碎屑岩孔隙演化定量计算方法的改进和应用

Improvement and application of quantitative calculation of porosity evolution of clastic rock

孔隙度演化是油气藏成藏机制研究的重点和难点。在利用"反演回剥法"恢复孔隙度时会存在一些缺陷,包括将面孔率等同于孔隙度、将机械压实损失孔隙度全部归结为成岩早期、不考虑岩石表观体积的变化等。基于此,将压实作用造成的岩石表观体积变化考虑在内,并将压实作用分期列入计算,推导了每一期成岩作用中对应不同岩石表观体积的压实和胶结损失孔隙度和溶蚀增加孔隙度、对应古埋深孔隙度的计算公式。以鄂尔多斯盆地西南部延长组长7和长63小层致密砂岩为例,结合本区的成岩相,建立了5种孔隙度演化模式,并应用不同方法分别计算出孔隙度演化曲线。不考虑岩石表观体积变化和压实不分段时,恢复的孔隙度存在很大的偏差,可达13.8%。

Porosity evolution of reservoirs is a key and difficult point of the study on hydrocarbon accumulation mechanism. But the present research methods of "porosity inversion and back stripping"have some flaws,including equating thin section porosity with porosity,attributing the porosity loss by compaction to early diagenetic stage,and neglecting rock bulk volume's reduction in the compacting process. In this study,compaction was corrected in every stage of diagenesis according to the normal compaction curve. Equation of porosity variation through history is derived,accounting for the porosity loss through compaction and cementation,and also porosity increase by corrosion,also taking into account of rock bulk volume variation in every stage of diagenesis. A case study on Chang 7 and Chang 63 tight oil reservoir of the southwest Ordos Basin was carried out,integrating the diagenetic facies of the research area and the derived formula. We reconstructed the porosity evolution from inversion and back stripping under the constraint of diagenetic sequences,and built five porosity evolution pattern and porosity evolution curve by different methods. If compaction is not corrected and rock bulk volume change is not considered,large error of the recovered porosity exist and it may be up to 13. 8% .