The oil and gas industry is usually divided into three major sectors: upstream (or exploration and production - E&P), midstream and downstream. The upstream sector includes searching for potential underground or underwater crude oil and natural gas fields, drilling exploratory wells, and subsequently operating the wells that recover and bring the crude oil or raw natural gas to the surface.
The upstream industry has traditionally experienced the highest number of Mergers, Acquisitions (M&A) and Divestitures. M&A activity for upstream oil and gas deals in 2012 totaled $254 billion in 679 deals. A large chunk of this M&A, 33% in 2012, was driven by the unconventional/shale boom especially in the US followed by Russia and then Canada.
The aggregate value of Upstream E&P assets available for sale (Deals in Play) reached a record-high of $135 billion in Q3 2013. The value of Deals in Play doubled from $46 billion in 2009 to $90 billion in 2010. With ongoing M&A activity, the level remained almost the same, reaching $85 billion in December 2012. However, the first half of 2013 saw approximately $48 billion of net new assets coming on the market. Remarkably, the total value of Deals in Play in Q3 2013 nearly tripled over 2009 to $46 billion, in less than four years.
This categorization comes from value chain concepts, even before formal development of Value Chain Management.
- Drilling Contractor Company
A company that owns and operates drilling rigs that are contracted by an operator to drill wells. Examples include Patterson UTI, Trans-ocean, Nabors, Independence Contract Drilling and Cactus.
- Integrated Oil & Gas Company:
- Independent Oil & Gas Company:
- Oil Service Company:
A company that provides products and/or services to the oil and gas industry. Usually a combination of labor, equipment, and/or other support services. Examples include Schlumberger, Halliburton, Saipem and Baker Hughes.
- Oil Equipment Manufacturer:
Many major security companies take part in securing the industry.
Any other oil and gas related business not defined above, such as software companies providing necessary IT services for easy execution in the oilfield. 
- Example: Offshore oil/gas production facilities, drilling rigs, intervention vessel.
- Upstream, midstream & downstream Archived 2014-01-07 at the Wayback Machine
- Industry Overview Archived 2017-05-08 at the Wayback Machine from the website of the Petroleum Services Association of Canada (PSAC)
- Coalbed Methane Basic Information Archived 2012-12-05 at the Wayback Machine
- "Oil and Gas M&A; Values Reach Record $254 Billion in 2012". Archived from the original on 2013-11-05. Retrieved 2013-11-05.
- "$135 Billion Worth Oil and Gas Assets on the Market". Archived from the original on 2013-11-05. Retrieved 2013-11-05.
- American Petroleum Institute Archived 2011-11-26 at the Wayback Machine Definition Of Upstream Company and description
- "Eyal Mesika - Security Issues in the Upstream Oil Industry - OilVoice". www.oilvoice.com. Archived from the original on 2017-04-20. Retrieved 2017-04-20.
- ISO 14224