Reservoir conditions and its oil-controlling effect of Chang7-Chang9 in the Southern Zhouchang area, Ordos Basin

Deep beneath the rolling hills of northern China lies a vast geological treasure chest known as the Ordos Basin. For decades, geologists have hunted for oil here, moving from easy-to-reach pockets trapped in ancient rock folds to more elusive targets hidden in layers of rock that are incredibly dense and hard to penetrate. These difficult targets are called tight reservoirs. Imagine a sponge that has been crushed so tightly that its holes are microscopic; oil can exist inside, but it struggles to move through the rock to reach a drill. Finding oil in these conditions requires a precise understanding of the rock's texture, its tiny internal spaces, and the specific conditions that allow oil to gather there. The challenge is not just finding the oil, but figuring out which specific layers of rock are thick enough and porous enough to yield a profitable amount of fuel, rather than just a trickle mixed with water.

In the southern part of a region called Zhouchang, a team of researchers turned their attention to three specific layers of rock known as Chang7, Chang8, and Chang9. These layers sit deep within the basin, sandwiched between thick sheets of mudstone that act as natural lids. The scientists wanted to understand the physical nature of these rocks and determine exactly what conditions make them good places for oil to accumulate. They gathered data from drilling logs, core samples, and production tests to build a detailed picture of the underground environment. Their work reveals that while these rocks are indeed tight, they are not uniform; some sections are far more promising than others, and the key to finding the best spots lies in a specific combination of rock thickness, pore size, and mineral content.

The researchers discovered that the rock in this area is primarily a fine-grained sandstone, composed mostly of feldspar, a common mineral, with smaller amounts of quartz and rock fragments. Under a microscope, the rock reveals a complex history of formation and alteration. The spaces where oil lives are not large, open caverns but rather tiny pores created when acidic fluids dissolved parts of the rock grains, leaving behind small gaps between the remaining particles. On average, the rock has a porosity of 6.9 percent, meaning that less than one-tenth of the rock's volume is empty space available for fluids. The ability for oil to flow through these spaces, known as permeability, is equally low, with a median value of 0.33 times 10 to the negative 3 power micrometers squared. Because of these low numbers, the researchers classify these formations as tight reservoirs. They further divided the reservoirs into three categories based on their quality: Type I, which has the best flow properties; Type II, which is moderate; and Type III, which is the poorest.

The distribution of oil in these layers does not follow the traditional pattern of a large, continuous pool sitting on top of water. Instead, the oil is trapped in lens-shaped pockets, isolated by layers of mudstone and siltstone that prevent it from spreading out. This creates a situation where oil and water often exist together in the same rock layer, flowing out simultaneously when a well is drilled. The team found that the oil is spread across the area in a quasi-continuous manner, meaning that while there is no single giant reservoir, the oil is present in a dense, overlapping network of these small lenses. This makes the exploration strategy different from traditional oil fields, as the goal is to target the specific "sweet spots" within this dense network rather than drilling into a single large accumulation.

Through their analysis, the team identified the specific factors that control where the oil is most likely to be found in high concentrations. The most important factor is porosity; when the rock has more than 7 percent porosity, the oil saturation and production rates increase significantly. The second most critical factor is the amount of shale, or fine clay, mixed into the sandstone. If the shale content exceeds 12 percent, the rock becomes too tight and the oil production drops sharply. Permeability also plays a vital role, with wells producing the best results when the permeability is greater than 0.5 times 10 to the negative 3 power micrometers squared. Interestingly, the thickness of the sandstone layer itself was found to be a less significant factor. While thicker layers are generally better, the study showed that once a layer reaches a thickness of about 10 meters, adding more thickness does not guarantee higher production. The quality of the rock matters far more than the sheer volume of it.

The findings suggest that the most productive wells in the Chang7 to Chang9 layers will be those drilled into sandstone that is thicker than 10 meters, has a porosity greater than 7 percent, a permeability exceeding 0.5 times 10 to the negative 3 power micrometers squared, and a shale content below 12 percent. By focusing on these specific criteria, oil companies can better target their drilling efforts in the Ordos Basin, moving away from guesswork and toward a more scientific selection of wells. This approach not only helps in maximizing the recovery of oil from these difficult reservoirs but also provides a clearer roadmap for exploring similar tight oil formations elsewhere. The study confirms that even in the most challenging geological environments, a detailed understanding of rock properties can unlock significant energy resources.