长武地区三角洲前缘短期基准面旋回与储层成因特征

长武地区延长组78、油组三角洲前缘储集砂体,按成因可分为三大类5种类型,各种类型砂体的成因明显受基准面升降、可容纳空间和沉积物补给量之间变化关系的影响。在基准面上升过程中,可容纳空间/沉积物补给量的比值对砂体成因特征的控制作用主要表现为:小于l时,以截削式河道砂体为主;大于1时,则为完整式河道砂岩。在基准面下降过程中,可容纳空间/沉积物补给量小于l时,以孤立式河口坝砂体为主;大于1时,则以远砂坝砂体为主;在水下坡折带沉积区,则为复合式坝砂体。基准面升降过程中,短期基准面旋回的规律性变化,影响着不同成因的砂体类型的分布特点。

Short-term base-level cycles and reservoir genesis on the delta front in the Changwu region, Shaanxi

The Chang-7 and Chang-8 delta front reservoir sandstones in the Changwu region,Shaanxi may be divided into:(1) subaqueous distributary channel sandstones including truncated distributary channel sandstones and uninterrupted distributary channel sandstones;(2) channel-mouth bar sandstones including isolated channel-mouth bar sandstones and distal bar sandstones, and(3) composite bar sandstones.The genesis of the above-mentioned types of sandstones are markedly controlled by base-level changes,accommodation space and sediment supply.In the course of the base-level rising,the truncated distributary channel sandstones are accentuated when the accommodation space / sediment supply ratios are less than 1,while the ratios are greater than 1,the uninterrupted distributary channel sandstones predominate.In the course of the base-level falling,the isolated channel-mouth bar sandstones are dominated when the ratios are less than 1,while the distal bar sandstones prevail when the ratios are greater than 1.The composite bar sandstones are developed at the depositional-shoreline break.The regular changes of the short-term base-level cycles in the course of base-level rising or falling are responsible for the types and distribution of the above-mentioned sandstones.