Why is an unconventional hydrocarbon reservoir unconventional? An investigation into petrophysical and fluid composition controls.

非常规油气藏是世界上剩余油气藏中最大的一部分。除了低渗透率外，这些储层最显著的特征是流体分布的不寻常和不可预测的性质：缺乏明显的顶部密封，密度较小（浮力较大）的流体位于密度较大的流体之下。因此，天然气或石油可以聚集在高渗透地层下面的低渗透地层中，气藏可能位于水柱或油柱下面。岩石物理性质和毛细力明显支持非常规油气聚集中流体的不寻常分布，这在高渗透率地层中是不存在的。在多相系统中，浮力与流体界面的表面张力和流体对固体材料的润湿性相平衡。因此，页岩将碳氢化合物密封在砂岩-页岩接触层下方，即使上覆的页岩可能含水饱和。如果这适用于非常规储层，岩石结构（成分加纹理）的某种组合与流体性质（浮力、成分、表面张力）相互作用，会产生不寻常的流体分离。但流体和岩石物理性质如何控制流体分布尚不清楚。

Unconventional petroleum reservoirs represent the largest fraction of remaining hydrocarbon reservoirs in the world. The most striking feature of these reservoirs, in addition to low permeability, is the unusual, unpredictable nature of fluid distributions: the lack of an obvious top seal and less dense (more buoyant) fluids underlying more dense fluids. Thus, gas or oil can accumulate in low permeability formations that underlie high permeability formations, and gas pools may underlie water or oil columns. Petrophysical properties and capillary forces clearly underpin the unusual distribution of fluids in unconventional petroleum accumulations, which are not found in high permeability formations. In multiphase systems, buoyant forces are balanced against surface tension at the fluid interfaces and the wettability of fluids on solid materials. Thus, shales seal hydrocarbons below a sandstone-shale contact, even though the overlying shale maybe water-saturated. If this applies to unconventional reservoirs, some combination of rock fabric (composition plus texture) interacts with fluid properties (buoyancy, composition, surface tension) to produce unusual segregations of fluids. But how fluid and petrophysical properties control fluid distributions is not understood. Four reservoir units provide natural laboratories for testing models that describe unconventional reservoirs: (1) the hydrocarbon-rich Montney Formation in western Canada that contains conventional hydrocarbon accumulations where it is shallow (distinct top seal, gas over oil over water) but unconventional accumulations where it is deep (no distinct top seal, variable and unsystematic fluid distributions); (2) the Upper Cretaceous large tight-gas-sand accumulations in the U.S. Rocky Mountains where lower permeability reservoirs underlie high permeability caprocks; (3) Lower Cretaceous tight oil/gas sandstones in Alberta with complex pore systems that require hydraulic stimulation to produce; and (4) Duvernay Formation, a shale liquids reservoir where rocks are variably oil-wet, water-wet and have mixed wettability in different parts of the reservoir. I will combine field data and quantitative modeling to develop models for fluid distributions in unconventional reservoirs and to identify tipping points that distinguish conventional from unconventional accumulations. Computationally powerful software can simulate migration of hydrocarbon fluids under the influence of both capillary and buoyancy forces. Such software has not previously been applied to unconventional petroleum systems and may require additional development. This analysis will, in the end, yield new, more precise models for hydrocarbon migration, trapping and distribution in unconventional petroleum systems, tested on important petroleum systems in Canada and the United States and globally applicable.