The distribution of movable fluid and its coupling influencing factors in ultra-low permeability sandstone reservoirs: a case study of Chang 6 member, Wuqi area, Ordos Basin

Imagine the Earth's crust as a giant, ancient sponge made of rock. For decades, scientists have been trying to squeeze oil out of this sponge. But not all sponges are created equal. Some are like a kitchen sponge with big, open holes where water (or oil) flows freely. Others are like a dense brick or a very tight sponge where the holes are microscopic, and the fluid is stuck, trapped by tiny forces that act like invisible glue. This is the world of "ultra-low permeability" reservoirs. In these tight rocks, the oil doesn't just sit there; it's a tug-of-war between the rock's tiny pores and the fluid trying to move. The big question for energy companies is: how much of that oil is actually "movable"? If the oil is stuck in pores that are too small or too narrow, it's useless. If it's in the right-sized holes, it can be pumped out to power our cars and heat our homes. Understanding the size and shape of these microscopic holes is the key to unlocking the energy trapped deep underground.

This paper takes a deep dive into a specific slice of this underground world: the Chang 6 member in the Wuqi area of China's Ordos Basin. Think of this area as a specific, very tight layer of rock that holds oil but is notoriously difficult to drain. The researchers, led by Hongli Zhong, wanted to figure out exactly where the "movable" oil hides in these tiny spaces and what makes some rocks better at letting oil flow than others. They didn't just guess; they took real rock samples, crushed them, looked at them under powerful microscopes, and even used a special machine that acts like a high-pressure water gun to see how the pores are connected.

Here is what they found, and it's a bit like sorting a messy room into two very different piles.

First, they discovered that these rocks are a bit of a mix. They are made mostly of quartz and feldspar (which are like the main building blocks of sand), with some rock fragments thrown in. But the real story is in the holes. The researchers found that the oil isn't just floating in one big pool; it's trapped in a network of pores ranging from the size of a grain of sand down to the size of a virus. They measured the "movable fluid saturation"—basically, the percentage of oil that can actually move—across their samples. The numbers ranged from 31.38% to 62.67%. That's a huge difference! Some rocks had more than half their oil ready to go, while others had barely a third.

The team realized they could split all their rock samples into two distinct teams: Type I and Type II.

Type I samples are the "lucky" rocks. They are usually found in the middle or top of thick sand layers, like the main highway of the ancient river that deposited the sand. These rocks have a higher content of feldspar and, crucially, more "dissolved pores." Imagine a block of Swiss cheese where the holes have been eaten away by acid, making them bigger and more connected. In these rocks, the holes are larger, often in the "micron" range (one-millionth of a meter). Because the holes are bigger and better connected, the oil can move more easily. These samples had the highest movable fluid saturation, averaging around 54.86%.

Type II samples are the "tight" rocks. They are found in thinner layers or at the bottom of the sand bodies, often mixed with more mud. These rocks have more tiny, nano-sized pores and very narrow connections. It's like trying to push water through a dense sponge versus a mesh net. The oil here is stuck in the tiny nooks and crannies. These samples had much lower movable fluid saturation, averaging around 41.59%.

One of the most interesting discoveries was about the size of the holes. The researchers found that the amount of oil that can move is strongly linked to the presence of "micron-sized" pores (holes between 0.3μm and 3μm). If a rock has a good network of these medium-sized holes, the oil flows. If it only has tiny nano-holes, the oil stays stuck. This is a key difference between these "ultra-low permeability" rocks and even tighter "tight sandstone" rocks. In the tightest rocks, the oil is forced into the tiniest nano-holes, making it very hard to get out. But in the Chang 6 rocks studied here, the presence of those slightly larger micron-holes makes a huge difference.

The paper also looked at how the rock was formed and changed over time. They found that the rocks went through a process called "compaction," where the weight of the earth above squished the pores shut, reducing the original space by 65% to 85%. Then, "cementation" (minerals gluing the grains together) reduced the space by another 15% to 35%. Despite all this squeezing, the Type I rocks still managed to keep their larger pores open, likely because the feldspar in them dissolved over time, creating new, bigger spaces for the oil to hide in.

So, what's the bottom line? The researchers suggest that to find the "sweet spots" where oil can actually be pumped out of these tight rocks, you need to look for the rocks with the best-developed micron-sized pores. It's not just about how much oil is there; it's about whether the oil has a road to travel on. If the road is too narrow (nano-scale), the oil is stuck. If the road is wide enough (micron-scale), the oil can move. This study shows that even in very tight rocks, the difference between a rock that produces oil and one that doesn't often comes down to the size of the microscopic holes and how well they are connected.