Ixtoc I was an exploratory well being drilled by Sedco 135F for PEMEX, a state-owned Mexican petroleum company, in waters about 50 meters (164 feet) deep. The well itself, however, was over 2 miles deep. When drilling mud circulation was lost, the drill string was pulled, resulting in a loss of hydrostatic pressure of the mud column and subsequent flowing of oil and gas to the surface. The BOP failed to work properly. An ROV and submersible were used to approach the BOP, but poor visibility and debris on the seafloor, including derrick wreckage and the drilling pipe, made it difficult. Although the BOP was reached and closed, it was reopened to prevent destruction due to the pressure of oil and gas. Upon reaching the platform, the oil and gas exploded, engulfing the rig in flames. The rig sank and damaged the underlying structures, including 3,000 meters of pipe, causing a massive discharge of oil and gas into the Gulf. The initial flow rate was estimated at 30,000 barrels per day; that decreased to about 4,000 barrels per day by early August when 100,000 steel, lead, and iron balls were pumped into the well. In October, a funnel-shaped device weighing 310 tons with 12 meters width and 6 meters height—a “sombrero”—was put over the well; oil and gas were pumped from under it through a flexible hose to the top of the platform. Only a small portion of the oil was recovered using this mechanism. Rough seas in December damaged the device so it was removed. Two relief wells were drilled, starting in mid-June and mid-July. After both were completed, mud was pumped through them and into the formation, eventually relieving pressure and reducing the flow of oil and gas; on March 23, 1980, the well was finally killed and was sealed with cement plugs.
Recovery efforts started in June and ended in early October. Skimmers and absorbent devices were used to recover oil while containment booms anchored to barges were used to confine the oil. Although it was theorized that these efforts would result in 20% pick up, they resulted in only 4-5% recovery. High winds and wave heights, daytime only operations, difficulty rearranging barges and booms so that oil did not drift around the sides with weather changes, and equipment failure all contributed to the small recovery. Booms were also used, with little to no effect, to protect the lagoons. Although the exact amounts are unknown, the U.S. EPA states that 1–2.5 million gallons of dispersants (COREXIT products primarily, including the now-deemed-too-toxic 9527) were sprayed during almost 500 aerial flights over the surface oil slick; this was followed in October by spraying from boats. Initially, the spraying was 20–25 miles from shore, but during the fall it included spraying close to beaches, near lagoon mouths, and around the well site. The quantity of dispersant used is unknown, but information from PEMEX indicates at least 9,000 metric tons were used. Some Mexican beaches were not cleaned. Others were cleaned using bulldozers to dig trenches and bury the oily sand.
The blowout resulted in an estimated 140–147 million gallons of oil released into the southern Gulf of Mexico. The fate of that oil was varied and subject to debate. The Mexican sandy beaches and barrier islands, as well as lagoons, were polluted with the crude oil. Although scientists observed roughly 6,000 metric tons that landed on the Mexican beaches within 4–5 days, they said that groups and individuals reported five times that amount. According to investigators, approximately 4,000 metric tons of oil and tar were deposited about ½-inch thick on the south Texas coast in a swath 30 miles long and 20–30 feet wide. An estimated 25% of the oil (120,000 metric tons/approximately 840,000 barrels) sank to the bottom of the Gulf, while biodegradation, together with photochemical and chemical breakdown accounted for 10-15% of the oil. Although PEMEX estimated that 50% of the oil burned at the well site, scientists estimated that only 1% burned. They also estimated that 4–5% was mechanically recovered. Tar mats on beaches persisted at least one year following the spill.
Effects on organisms and ecosystems:
Initially, shore organisms experienced an 80% decline.
1,400+ shore birds were oiled.
Crabs had great losses; shrimps grew tumors.
Subtidal organisms (polychaetes and amphipods) decreased 50% initially.
Fish and octopus catches in the first year dropped as much as 70% in the hardest hit areas.
Mangrove oysters, which grow on the roots of mangrove trees, were decimated and after 30 years showed no signs of recovery.
Mangrove trees experienced decline.