The day prior to the Ixtoc blowout, the drill bit hit an area of soft strata. As a result of the strata, the circulation of drilling mud was lost resulting in a loss of hydrostatic pressure in the shaft. Instead of the mud returning to the surface, the drilling mud escaped into fractures that had formed in the rock at the bottom of the drilling hole. Pemex officials at the time decided to remove the drill bit, run the drill pipe back into the hole and pump materials down this open-ended drill pipe in an effort to seal off the fractures that were in effect creating the loss of circulation.
During the process of removing the pipe on the Sedco 135F, the mud suddenly began to flow back up towards the surface, and by removing the drill-string the well was swabbed leading to a kick. Normally, this flow could have been prevented by activating shear rams contained in the blowout preventer (BOP). These rams are designed to sever and seal off the well on the ocean floor; however in this case the drill collars had been brought in line with the BOP and the BOP rams were not able to sever the thick steel walls of the drill collars leading to a catastrophic blowout.
The drilling mud was followed by a large quantity of oil and gas which were increasing in volume. The fumes from the oil and gas ultimately exploded on contact with the operating pump motors, which began a fire and ultimately led to the demise of the Sedco 135F drilling tower. The collapse in turn created damage to the underlying well structures as well which led to the release of significant quantities of oil into the Gulf, with estimates ranging from 139.818 to 147.84 million gallons of oil, or 3.329 to 3.52 million barrels of oil.
According to local residents at the time, the oil covered the reefs and washed up on the shores. Fish were killed and octopuses were buried under the oil that filled the gaps between the rocks where they lived. Where it washed ashore, it was up to a foot deep in some locations. The prevailing winds sent the oil spill north where it covered nearly 170 miles of US beaches. Fishing was either restricted or altogether banned by the Mexican authorities in the contaminated areas both north and south of the well. Fish and octopus commercial catch levels lowered 50 to 70% from the previous year. Some of the larger aquatic species with longer life spans took years to recover from the Ixtoc I spill.