Breathlessly bubbled

There were quite a few stories about the dangers of tar pits; some involved the victim misjudging the depth and sinking below the surface to drown, others would have them underestimate the temperature and end up with major burns. A certain chubby blue T-rex, Chomper, didn't believe a word of any of them. Most of those stories involved people he'd never heard of, and the few he'd actually met he hadn't known before hearing the story from someone else, so he had no way to know if they were just making it up. One story in particular was certainly fake, and been so ridiculous Chomper had outright laughed at it: Someone who'd tried to relax in a tar pit had been attacked by bubbles. How could bubbles even be at all dangerous?
Disregarding every "cautionary" story, Chomper stood nude on the edge of a tar pit, testing its depth with a stick that happened to be about as long as he was tall, finding it would come up to about his knees. He'd already tested the temperature as well, which was just below the point that it would be uncomfortably hot. Normally Chomper wasn't a fan of hot pools of any kind, but this was mostly out of spite and wanting to disprove those that had tried to scare him with those dumb stories. Slowly, he stepped into the tar, letting himself get used to the heat gradually instead of just jumping in like a moron. Once he was fully in, he sat down, the tar now up to the base of his neck. The heat actually felt better than he'd expected, and he closed his eyes with a smile on his face. Maybe there was more to this whole thing than he thought.
Some time later, as Chomper relaxed and would occasionally reach up to spread some tar onto the back of his neck, something bumped against his head from behind! He yelped in surprise and slight fear, bolting to his feet and whipping around to see an ordinary tar bubble. It slowly expanded, but stayed in place, and certainly didn't attack him like in that stupid story. Chomper laughed at the fact he'd actually been scared by a bubble, and swatted at it to pop it. It stayed intact, though, and after a couple more smacks, he decided to just leave it and go to another spot. Moving a few feet away, he sat down again and tried to continue his relaxation, but it wasn't long before he was bumped again. Another large bubble had formed next to him, and he could see that the previous bubble was still intact, and seemed to now be slightly bigger than him. He rolled his eyes, starting to get annoyed. "Oh no, I'm being attacked by freakin' bubbles. Whatever will I do?", he said, before poking the bubble next to him with a claw. Surprisingly, the bubble stayed intact, and as Chomper looked around he could see more large tar bubbles forming. Maybe this was what really happened in that "attacking bubbles" story, and the one who'd told it had just overreacted?
Still, there was clearly no danger, and Chomper sat down and watched the bubbles for a while. They each seemed to stop expanding once they were about as big as him, and would sort of slide around the surface after that. He went back to ignoring them after that and tried to relax again, before being bumped once more. Sighing, he got up again, but quickly realized he'd been surrounded by several large bubbles. "Ugh, cut it out, already..." He tried to push some aside, but he couldn't manage to move them for some reason, just making them jiggle around. Now Chomper was starting to get nervous, especially since it seemed like the group of bubbles was slowly moving closer, but there was no way that was actually happening. Except, fairly soon, he could feel another starting to press against him, and he hadn't moved. It wasn't looking like a coincidence any more; these bubbles were moving toward him as if they were alive!
Chomper started to panic and tried again to push his way out, but it still wasn't working, and they all continued their approach. He tried to at least hold them back after that, and while he could stop a couple of them, the rest were still getting closer, multiple now pressing on him at once, starting to restrict his movement. "What the hell is happening!? Someone help!" Eventually, despite Chomper's struggles, he couldn't move any more, and he tilted his head back to get a deep breath just before the bubbles squished against his face, cutting off his breathing!
For some time, he was forced to hold his breath, his body slowly being squeezed tighter and tighter, and as his lungs started to strain... Plop! All the bubbles merged together, leaving Chomper standing in the middle, panting as he finally had the chance to catch his breath. The inside of this huge bubble was a strange place; the sound of his breathing echoed off the sides, and his view of the outside was extremely distorted, making it almost impossible to make anything out. Despite how scared he'd been, the bubble's interior was actually rather cozy, and Chomper decided he wouldn't mind staying here a while, laying down on its bottom. While the bubble itself was devoid of tar, it was just as warm as if he'd been submerged in it, and he could still breathe fine. Finally, he could relax again...
Maybe about 20 minutes passed, Chomper just beginning to fall asleep, when he started to notice something. The bubble's inside seemed to be warmer than before, and he had to put a little more effort into breathing to continue getting the proper amount of oxygen. He thought little of either for now, but over the next few minutes, the air continued getting warmer and breathing seemingly less effective. Was some kind of gas getting in? Regardless, it seemed that Chomper would need to get out fairly soon. Disappointed that he couldn't stay longer, he got up and poked the bubble's edge with a claw, getting no result just like before. Sighing again, he jammed the claws of both hands against the bubble, and still, nothing happened. His eyes went wide, and he tried to grab the edge, which took a few attempts, and bite it, once again not accomplishing anything. He was trapped in this bubble! Not only that, he finally realized why he was having trouble breathing. It wasn't from some gas getting in, but rather the opposite; the bubble was airtight, and the air was slowly being used up!
Even as the lack of oxygen started to get him aroused, Chomper beat on the edge of the bubble. "Someone get me the hell out of here!", he yelled, but he already knew no one would hear him. He had to step forward a bit partway through his attack, which led him to realize he could get the bubble to move. With some effort, he pushed it along, and eventually rolled it out of the tar pit. But, where would he even go? Chomper's view of outside was so distorted that all he could tell was that he'd moved, there was no way he'd be able to find any specific thing. He couldn't just give up, though, and kept rolling the bubble, hoping he'd find something helpful eventually.
For another few minutes, he pushed the bubble around, panting from both exertion and the increasingly stale air. Despite the situation, he occasionally needed a break, with them getting slightly longer and more frequent. All the exertion was using up the limited air more quickly, and after having been in the bubble for 40 minutes, Chomper now panted even during his breaks, a purple blush on his face. His dick was fully erect and visibly throbbing, and he couldn't resist giving it a few strokes to try to calm down. As much as he wanted to cum, he couldn't spare any more time, and went back to pushing the bubble. Soon, his panting turned to gasping, his blush becoming blue, and his aching dick refused to be ignored, throbbing heavily and spurting pre as arousal built up even without any contact.
A full hour, Chomper had been in this tar bubble, and he was starting to lose his composure. His lungs were constantly burning no matter how heavily he'd gasp, and the oxygen deprivation combined with the now sweltering interior of the bubble made him dizzy and lightheaded. It wasn't much longer before he couldn't coordinate himself enough to keep pushing the bubble, weakly smacking at it a few more times before landing on his butt and grabbing his neck. As Chomper's strength started to fade, his dick started to throb even harder before shooting cum in front of him several times in a hands-free orgasm spurred by his dire lack of oxygen. "Ghih! Hah, huuah, hah..."
His gasps started to die down as he shot a bit more cum, now back to simply panting. Starting to go numb, his blue blush now covered his whole face, and he fell backward, laying on his back with a slight splash from landing in a small puddle of his own sweat. For minutes on end, he lied there, dick still fully erect and pointing almost straight up as his breathing very slowly decelerated. His body was starting to shut down from the lack of oxygen, which led to it slowly using less oxygen over time, leading to it taking quite a long time for what little usable air remained to be used up.
Another half hour passed as Chomper slowly weakened, his panting eventually becoming calm breathing, the blue on his face beginning to spread over the rest of him. Nearly depleted of oxygen, he started having trouble staying awake, his dying body being rocked by another much weaker orgasm, spurting some cum onto his fat belly. Some time later, his breaths began to be spaced out, and his body went almost entirely numb. Chomper's lidded eyes slowly rolled back and he shuddered, finally losing his grip on consciousness.
He still clung to life for a while after that, his breaths becoming sporadic and almost looking like weak hiccups. His suffocated body held on for several more minutes, his heartbeat slowing and slowing, until it and his breathing both finally came to a stop. For a while, the tar bubble held the dead dino, subtly swaying from a slight wind, before the bubble finally destabilized and popped on its own, dumping Chomper onto the ground, his body even more blue than normal.