The equipment used in oil fields is very specialized. It is not just a matter of digging for oil. The first specialized equipment is required to search for, find and test locations for the oil. Then once it's confirmed oil work really begins. Most oil fields are in areas not near civilization. Civilization has to be brought to the field. This is all of the deck, heating and electricity basic to specialized equipment used to drill oil wells and harvesting. The team just needs to live in comfort. Different equipment is needed to operate the oilfield. Then there is the equipment designed to extract and process oil safely.
There are over 40,000 oil fields that require specialized equipment work. Some are in the ground, others are offshore. No matter where is located the field requires the oil field equipment. The oilfields are regions with a large number of oil wells. Most of the wells are pumping crude oil. There must be some way of processing or transporting oil. Some oil fields oil processed and stored for shipping. Other areas have pipelines carrying oil. Each step of the process requires the tools to use. There is always support facilities such as offices and testing laboratories. Workers need places to eat, sleep and play. That's quite a bit of necessary equipment. Most need to be brought in, be sent on or being blown inside.
As the gold rush in recent small oil fields create autonomous cities. Get the inside team takes time. It is better to have what is needed and a little more. Every oil field needs to be in place. With spare parts for repairs to prevent delays. It's a full time job to maintain and monitor. Some equipment is standard and easy to install maintain and operate. For example what is needed to generate electricity. The other team specializes extremely specific training required to operate and maintain. For example it needed to heat a pipe, drill well and pump the oil out. The drilling fluid also specializes in the area and the work they are facts. All of it needs to be sent inside.