鄂尔多斯盆地中生界延长组特低渗-致密砂岩储层成藏动力学特征

Pool-forming dynamic characteristics of extra-low permeability and tight reservoir in Yanchang formation,mesozoic erathem,Ordos basin

鄂尔多斯盆地延长组特低渗-致密储层孔隙小、喉道细。高压压汞和恒速压汞实验显示,渗透率越低,排驱压力越大,中值半径越小,不同渗透率级别的储层,其渗透率往往由不同级别的喉道半径所控制。特低渗-致密储层具有非达西流的渗流特征,启动压力梯度大,尤其是渗透率小于0.3mD的致密油储层,其启动压力梯度一般大于1MPa/m,石油运移需要较大动力。由于喉道细微,毛细管阻力大,加之储层孔隙形状不规则,矿物颗粒表面以及喉道表面粘土矿物发育,比表面积大,石油通过时摩擦阻力较大,因此特低渗-致密储层中油水难以依靠自然浮力发生重力分异,使得石油无法因重力差异发生游离相"自发"运移,油气运移需外力驱动。长7优质烃源岩具有高产烃率和高排烃率,生烃体积膨胀作用明显,产生超压,是鄂尔多斯盆地延长组特低渗-致密储层石油运移的主要动力。

The extra-low permeability and tight reservoir of the Yanchang formation in Ordos Basin has small pore and tiny throat. As we all know, the lower the permeability the dis-placement pressure is greater and the median radius is smaller. The permeability of differentpermeable grade reservoir is often controlled by the levels of throat radius. The extra-low permeability and tight reservoir has non-Darcy flow seepage characteristics which means high start-up pressure gradient. Especially in the tight reservoir whose permeability is less than 0.3 mD, the start-up pressure gradient is generally greater than one MPa per meter, so the oil need larger displacement pressure to migrate. As a result of tiny throat and high cap-illary resistance, additionally the pore shape is irregular and the clay was enriching on min-eral particle and throat surface, and the specific surface area is large, and the frictional re-sistance was large when the oil flow through, so the gravitational differentiation of oil and water which relied on natural buoyancy is difficult occur in the extra-low permeability and tight reservoir and the oil can’t free migrate in separate phase that relied on gravity differ-ence, then hydrocarbon need to be driven to migrate by external force. The Chang7 high quality source rocks has high rate of hydrocarbon production and hydrocarbon expulsion. Be-cause the hydrocarbon volume expansion during hydrocarbon generation process was obvious, so superpressure generated, which was the primary driving force for migration of the extra-low permeability and tight reservoir of the Yanchang formation in Ordos Basin.