异常高压对储集层压实和胶结作用的影响——以渤海湾盆地渤中凹陷西北次凹为例

Influence of overpressure on reservoir compaction and cementation: A case from northwestern subsag, Bozhong sag, Bohai Bay Basin, East China

根据岩心和薄片资料,定量分析渤海湾盆地渤中凹陷西北次凹异常高压对古近系东营组储集层压实和胶结作用的影响。异常高压对压实具有明显抑制作用,异常高压带砂岩压实强度及压实减孔率低于上覆常压带砂岩,异常高压带地层压力每超过静水压力4 MPa,可保存约1.1%原生孔隙;异常高压对胶结物发育有控制作用,异常高压带及邻近内侧常压带内,胶结较强烈,而远离异常高压带的外侧常压带,胶结强度迅速降低。在常压带,碳酸盐胶结带的厚度比自生黏土矿物胶结带厚度小。碳酸盐差异胶结存在"细粒聚集效应",即碳酸盐优先在粒度较细的砂岩中形成胶结物,使得异常高压带内砂岩储集层物性有好有差。外侧常压带是原生孔隙保存最为有利带,异常高压带是原生孔隙保存较为有利带,而内侧常压带原生孔隙保存相对较差。

Based on data from core analysis and thin section, the influence of overpressure on the compaction and cementation of the Paleogene Dongying Formation reservoir is studied quantitatively in the northwestern subsag of the Bozhong sag, Bohai Bay Basin. Reservoir compaction is inhibited obviously by overpressure because the compaction strength of sandstones in overpressure setting is weaker than that of its overlying sandstones in normal setting. The primary porosity of sandstones is preserved about 1.1% as pore pressure is above hydrostatic pressure every 4 MPa in overpressure setting. Moreover, reservoir cementation is affected by overpressure： cementation strength is strong in overpressure setting and the adjacent inner-pressure setting, while it declines rapidly in outer-pressure setting far away from the overpressure. The thickness of zone with strong carbonate strength is thinner than that with strong authigenic clay strength. Differential carbonate cementation presents ＂build-up effect in fine grain＂, which means that carbonate is prior to generate in sandstones of fine size, and causes that physical properties of sandy reservoir are different in the same overpressure setting. The results show that outer-pressure setting is the most favorable zone for preserving primary porosity, overpressure setting is the secondary, and iner-pressure setting is relatively poor.