四川盆地上三叠统生物扰动环境分析及与储集性能的关系

近些年来的勘探实践表明，四川盆地须家河组存在“厚砂薄储”的问题，优质储层的形成主控因素复杂。前人研究认为优质储层主要受沉积相、岩石相和成岩相控制，然而，对其他因素考虑较少。通过系统的岩心、露头、岩石薄片等资料研究认为，生物扰动构造对储层物性的影响也较大，生物扰动强度与储层物性成正比。研究认为，四川盆地上三叠统可以划分出4种岩性—生物扰动组合:①厚泥层扰动，主要形成于厚层黑色泥岩中，反映滨湖沼泽或浅湖沉积环境；②砂泥边界扰动，发育于厚层深灰色泥岩夹薄层浅灰色粉—细砂岩中，扰动指数为2~3，反映远端砂坝下部环境；③砂泥薄互层扰动，主要发育在砂泥薄互层中，砂层单层厚度为2~8cm，纵向上砂泥薄互层，扰动指数为4~5，反映三角洲前缘远端砂坝上部环境；④厚砂层扰动，主要发育于厚层砂岩中，扰动指数为1~2，表明其形成于水体能量相对较强的水下分支河道环境。生物扰动构造与储层物性呈正相关，即生物扰动越强，储层物性越好；而弱扰动或未扰动部位原生孔隙胶结致密，孔渗性都非常差。这主要是因为生物扰动可以提高岩石的均质性和岩石颗粒的分选性。
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Environmental Interpretation of Upper Triassic Bioturbation Structures and Correlation with Petrophysical Properties of Reservoirs in Sichuan Basin

Recent exploration of Sichuan basin manifests that there exists thick sand bodies but thin reservoirs in Xujiahe Formation and controlling factors of favorable reservoirs are complex.Most researchers considered the sedimentary facies,lithofacies and diagenetic facies to be key factors of favorable reservoir,but paid little attention to other factors.Based on observation and measurement of outcrops,cores and rock slices,we consider that bioturbation give certain contribution to improve the reservoir properties and is also an important factor.Based on the bioturbation distribution in the cores of upper Triassic Sichuan basin,four types of lithology-bioturbation assemblage can be subdivided as follows: ① Bioturbation produced by single trace fossils,developing mainly in thick bedded mudstone,and presents shallow lake and shore lake swamp sedimentary environments;② Bioturbation near boundary between sandstone and mudstone,which occurs mainly in sediments of thick-bedded mudstone interbed with thin-layered siltstone,with bioturbation index between 2 and 3 reflecting the upper distal sand bar sediments;③ Bioturbation interbedded mudstones and sandstones,which exists in thin-bedded sandstone and mudstone,with sandstone thickness of 2-8cm and bioturbation index between 4 and 5,representing the middle shallow lake depositional environment;④ Bioturbation produced by single trace fossils,which develops predominantly in thick-bedded sandstone,with bioturbation index between 1 and 2,indicating shore lake sedimentary environments with high wave energy.Based on bioturbation,sandstone diagenesis and porosity structures,there suggests that the more strong the bioturbation is,the better the reservoir petrophysical property is,namely a positive correlation between the bioturbation structures and the reservoir petrophysical properties.The primary porosity in the place of strong bioturbation,is preserved greatly to form the high porosity and high permeability reservoirs,since the bioturbation increases the homogeneity and grain gradation of rocks.