鄂尔多斯盆地延长组低渗透致密岩性油藏成藏机理

以层序地层学为指导,从成藏条件解剖入手,从孔隙演化的角度分析鄂尔多斯盆地中生界上三叠统延长组低渗透致密岩性油藏成藏机理,归纳其成藏演化模式。延长组低渗透致密岩性油藏分布于优质烃源岩范围内,湖相重力流沉积与三角洲前缘沉积为主要储集砂体类型。延长组低渗透致密砂岩在早期成藏期具有高孔高渗的特点,早期生成的油气在浮力作用下正常成藏,中后期成藏过程中经历了持续压实致密过程;后期主生烃期生成的原油在浮力和源储压差共同作用下沿先前残余路径以浸润方式运聚,达到致密条件时,油气停止运移。晚侏罗世—早白垩世中期,油气在高孔隙砂岩中近源多点面状充注大面积成藏,油气从湖盆中心向四周运移;早白垩世晚期,砂岩储集层成岩作用加强,储集层发生致密化。晚白垩世之后,受西倾东翘的构造挤压抬升运动影响,形成现今低渗透致密岩性油藏。

Hydrocarbon accumulation mechanism of low permeable tight lithologic oil reservoirs in the Yanchang Formation, Ordos Basin, China

A two-scale (Darcy scale and pore scale) continuum wormholing model is built to study the fractal nature of the acid-etched wormholes in acidizing carbonate reservoirs and the influence of acid type on the conditions for wormholes to form. This model considers convection-diffusion mass transfer and reaction on the acid-rock interface, and the fractal dimension of dissolution patterns is calculated using box-counting method. The results show that wormholes are formed when convection and diffusion are equivalent in strength; when convection dominates, uniform dissolution is formed; when diffusion dominates, surface dissolution is generated. For the zones where the porosity is greater than 0.7, the fractal dimensions of the surface dissolution, wormholes and uniform dissolution are 1.46, 1.50 and 1.44, respectively, among which, the fractal dimension of the wormholes is approximate to the experimental results obtained by Daccord and Lenormand (1.6±0.1). The reaction between weak acid and rocks are controlled by reaction kinetics, the acid-etched wormholes are wide, and more acid is consumed. The reaction between strong acid and rocks are controlled by mass transfer, the acid-etched wormholes are narrow, and less acid is consumed. According to an example calculation, it is found that acidizing treatments should be performed in an isolated short section each time when the horizontal well is long.