Here is the formula for seeing what is the chance that a Pokemon will be captured:
CatchValue = ((( 3 Max HP - 2 HP ) (Catch Rate Capture Rate ) / (3 Max HP) ) Status Modifier
CatchValue is the chance that we want to know
Max HP is the maximum HP that the Pokemon has at the time. Here, I took HP IVs as 18 (average) and EVs as 0, and a neutral nature. This ends with Max HP as 159.
HP stands for the amount of HP the Pokemon has at that very moment. Since this is the first turn, the Articuno was not damaged, and the HP is the same as Max HP i.e. 159
Catch rate is the rate at which a Pokemon is usually captured. Articuno's is 4
Status Modifier is the status the Articuno has at that time. Here we take that as 1, as it is the first turn.
Most importantly, Capture Rate is the chance at which a ball can capture a Pokemon. Since this is a Quick Ball and it is the first turn, it's value is 3
After doing all the math,
We get the chance as 4.
So that means it is pretty low, and you got very lucky indeed.
Hope I helped!