## 4 Results
### Micro-Sealing Ability of Carocks  
When assessing the micro-sealing ability of the Cambrian caprock in the Tarm Platform Area, a breakthrough-pressure test is the most direct and mature method. The higher the breakthrough pressure, the stronger the sealing ability of the caprocks. Therefore, breakthrough pressure is often used as the main parameter for evaluating the sealing ability of caprocks. The testing methods include mercury intrusion, step-by-step, displacement, and continuous methodologies. A total of 64 core samples from four wells drilling the Cambrian in the study area were selected, and then the displacement method was conducted to test their breakthrough pressure. In order to eliminate the inaccurate results caused by the clay-mineral dilatation after sucking the water and the influence of the water on the gypsum properties, the saturated solvent of the rock samples for this breakthrough-pressure test used kerosene instead of traditional saline water.  
The experimental results show that the Cambrian lithology in four wells are mainly mudstone, gypsum-salt rocks and dolomite. In well ST 1, the sampling depth of the Avatage Formation is from 1673.10 m to 1675.40 m. Its lithology, identified by microscopic thin section, is gypysiferous powdery crystalline dolomite, with the minimum breakthrough pressure of 0.7 MPa and the maximum of 4 MPa. The sampling depth of the Shayilike Formation is from 1674.90 m to 1675.40 m, with a lithology of light-gray gypsum rocks. The minimum breakthrough pressure is 2.1 MPa and the maximum is 14 MPa, and it has not broken through for 48 h. In well ZS 5, the sampling depth of the Avatage Formation is from 6189.0 m to 6223.4 m. The results of thin-section identification are mainly gypsum, mixed with gypysiferous powdery crystalline dolomite. The breakthrough pressure is a minimum of 0.5 MPa and the maximum is 14 MPa, and it has not broken through for 48 h. However, the breakthrough pressure of most samples has not exceeded 4 MPa. The sampling depth of the Shayilike Formation is from 6545.4 m to 6553.5 m, and its lithology is mainly dolomite with powder crystal gypsum. The breakthrough pressure is a minimum of 0.5 MPa and the maximum is 14 MPa, and it has not broken through for 48 h. In well ZS 5, compared to the breakthrough pressure in the Avatage Formation, which is mainly lower than 4 MPa, the results in the Shayilik Formation are mostly higher than 9 MPa. The samples of well Y 10 belonged entirely to the Avatag Formation, with the sampling depth range of 6173.9-6448.6 m. The lithology is almost all dolomite and its breakthrough pressure ranges from 0.1 MPa to 14 MPa. Also, it has not broken through for 48 h, and the breakthrough-pressure span is large and irregular. There are relatively few samples of well YM 36, with only three collected, from 5597.7 m to 5598.0 m in the Avatage Formation. The lithology is powder crystal dolomite; its minimum breakthrough pressure is 0.7 MPa and the maximum is 12.5 MPa. By organizing and analyzing the above data, it was found that the higher the permeability, the lower its breakthrough pressure.  
The breakthrough pressure of most samples from the four wells is greater than 5 MPa. According to the classification evaluation criteria for breakthrough pressure in China, there is good sealing performance when the breakthrough pressure is greater than 5 MPa. Therefore, it is generally believed that when the breakthrough pressure is greater than 5 MPa, the sample can be regarded as the caprock. However, there are still many samples with breakthrough pressure below 5 MPa, some of which are only 0.1 MPa. Therefore, such samples do not have good sealing ability.