Upstream, Midstream, and Downstream: A Guide to Petroleum Industry Terminology by Sector

The Three Sectors Explained

Upstreamcovers exploration and production — finding hydrocarbons in the subsurface and bringing them to the surface. Upstream activities include seismic surveys, exploratory and appraisal drilling, reservoir characterisation, field development planning, well completions, and production operations. The upstream sector's commercial unit is the barrel of oil or the thousand cubic feet of gas produced.

Midstream covers the gathering, processing, transportation, and storage of hydrocarbons between the wellhead and the refinery or end-user. Midstream assets include gas processing plants that separate natural gas liquids from the gas stream, pipelines, LNG liquefaction and regasification terminals, and crude oil storage facilities. The midstream sector earns its revenue on throughput volumes and tariffs rather than commodity price, which gives it a more stable cash flow profile than upstream.

Downstreamcovers refining and petrochemicals — converting crude oil and natural gas into the finished products used by consumers and industry. Downstream assets include crude distillation units, conversion units (crackers, reformers), blending facilities, and petrochemical plants. The downstream sector's profitability is driven by the “crack spread” — the margin between crude oil input costs and refined product prices.

Upstream Terminology

E&P(Exploration and Production) is the standard abbreviation for the upstream business, used in company names, division titles, financial reports, and regulatory filings. An E&P company — or an E&P division of an integrated major — holds licences to explore for and produce hydrocarbons in specific geographic areas.

The legal and commercial framework for an E&P licence is typically governed by either a concession agreement or a PSC(Production Sharing Contract). Under a PSC, the host government retains ownership of the hydrocarbons in the ground; the contractor covers all exploration and development costs and is repaid in “cost oil” (or cost gas) from production, with the remaining “profit oil” split between the contractor and the government according to the negotiated terms. PSCs are the dominant contractual model in many producing countries including Azerbaijan, Angola, and Malaysia.

Where multiple companies share a licence, their relationship is governed by a JOA (Joint Operating Agreement). The JOA specifies each partner's WI (Working Interest) — the percentage share of costs and revenues. A partner with a 25% WI pays 25% of all costs and receives 25% of gross production. The NPI (Net Profits Interest) is a variant in which one party receives a share of profits rather than gross revenues, typically used to carve out an interest for a government or a non-operating partner who does not contribute to costs until the project reaches payout.

Major expenditures in the upstream are governed by AFE(Authorization for Expenditure) documents. An AFE sets the approved budget for a specific activity — drilling a well, building a facility, acquiring seismic — and provides the financial authorisation for the operator to commit the partnership's funds. AFEs are central to the financial governance of joint ventures and are a standard deliverable in any well planning process.

CAPEX (Capital Expenditure) and OPEX (Operating Expenditure) are the two fundamental categories of spending in any petroleum business. CAPEX covers investments in long-lived assets — drilling wells, building platforms, constructing pipelines. OPEX covers the ongoing costs of operating those assets — lifting costs, maintenance, well intervention, tariffs. The distinction matters enormously for accounting treatment (CAPEX is capitalised and depreciated; OPEX is expensed immediately) and for economic modelling of field life extension decisions.

G&G(Geology and Geophysics) refers collectively to the earth-science disciplines — geology, geophysics, petrophysics — that characterise the subsurface before and during drilling. A G&G team generates the prospect inventory, interprets seismic data, and integrates well data to update the reservoir model. G&G spend is a significant component of upstream CAPEX in the exploration and appraisal phases.

Midstream Terminology

Produced natural gas contains heavier hydrocarbon components beyond methane — ethane, propane, butane, pentane, and heavier fractions. These are collectively called NGL (Natural Gas Liquids) and are separated from the gas stream at a gas processing plant. NGLs are valuable products in their own right: ethane feeds steam crackers in petrochemical complexes; propane and butane together form LPG (Liquefied Petroleum Gas), which is widely used for domestic heating and cooking, particularly in markets without pipeline gas infrastructure; pentane and heavier fractions are used as natural gasoline blendstock.

LNG (Liquefied Natural Gas) is natural gas that has been cooled to approximately minus 162 degrees Celsius, at which point it liquefies and its volume shrinks to roughly 1/600th of its gaseous volume. LNG allows gas to be transported by ship to markets that are not connected by pipeline — a capability that has fundamentally changed the global gas market. LNG projects require enormous capital investment in liquefaction trains (at the export terminal) and regasification terminals (at the import end), which makes FEED (Front End Engineering Design) — the detailed pre-investment engineering study that defines scope, cost, and schedule before a Final Investment Decision (FID) — especially critical.

Large midstream infrastructure — pipelines, processing plants, LNG terminals — is typically built under EPC (Engineering, Procurement and Construction) contracts. Under an EPC arrangement, a single contractor takes responsibility for designing the facility, procuring all equipment and materials, and constructing and commissioning it to a specified performance standard. EPC is also called lump-sum turnkey (LSTK) contracting. The alternative is an EPCM (Engineering, Procurement and Construction Management) arrangement, where the EPCM contractor manages the process but the owner holds the procurement contracts directly.

T&D (Transmission and Distribution) describes the pipeline network that moves gas from processing plants and gathering systems to end users. Transmission pipelines carry gas at high pressure over long distances; distribution networks deliver it at reduced pressure to homes and businesses. The distinction matters for regulatory purposes — transmission and distribution are regulated differently in most jurisdictions — and for metering, tariff structures, and safety requirements.

Downstream Terminology

A petroleum refinery is essentially a series of separation and conversion processes that transform crude oil into saleable products. The entry point is the CDU (Crude Distillation Unit), an atmospheric distillation column that separates crude oil into fractions by boiling point: gases, naphtha, kerosene, gas oil, and atmospheric residue. The residue from the CDU then goes to the VDU (Vacuum Distillation Unit), which operates under reduced pressure to distil heavier fractions that would decompose if heated to their normal boiling points at atmospheric pressure. The VDU produces vacuum gas oil and vacuum residue, which are feedstocks for downstream conversion units.

FCC(Fluid Catalytic Cracking) is the most important conversion process in most refineries. The FCC unit breaks large, heavy hydrocarbon molecules from vacuum gas oil into smaller, lighter molecules — primarily gasoline, light cycle oil, and LPG — using a hot fluidised catalyst bed. FCC gasoline typically makes up the largest single component of a refinery's gasoline pool, and the unit's performance is central to refinery profitability.

The quality of the gasoline produced is measured by its octane rating. RON (Research Octane Number) is measured under relatively mild engine test conditions; MON (Motor Octane Number) is measured under more severe conditions. The pump octane rating sold at petrol stations in many countries is the arithmetic average of RON and MON, designated as (R+M)/2. In Europe, pump grades are typically labelled by RON alone. Refinery blending operations are carefully managed to hit octane targets while minimising the use of high-octane components, which carry a cost premium.

The petrochemicals sector — which converts refinery and gas plant streams into polymer feedstocks — adds another layer of acronyms. LDPE (Low-Density Polyethylene) and HDPE (High-Density Polyethylene) are the two main grades of polyethylene, differentiated by their molecular structure and mechanical properties. PP (Polypropylene) is produced from propylene and is one of the most widely used plastics in packaging, textiles, and automotive components. PVC (Polyvinyl Chloride) is produced from ethylene and chlorine and is ubiquitous in construction, pipes, and electrical insulation. The global demand for these polymers is one of the primary long-term drivers of petroleum feedstock demand even as transport fuel demand evolves.

Cross-Sector Acronyms

Some acronyms appear in upstream, midstream, and downstream contexts alike because they describe universal concepts — safety management, process engineering documentation, and control system architecture — that apply wherever hydrocarbons are handled.

HSE (Health, Safety and Environment) is the universal label for the safety and environmental management function. Every sector of the petroleum industry has HSE requirements, HSE management systems, and HSE professionals who speak the same fundamental language regardless of whether they work on a drilling rig, a pipeline compressor station, or a refinery.

P&ID (Piping and Instrumentation Diagram) is the engineering drawing that shows all equipment, piping, instrumentation, and control systems in a process unit, including every valve, sensor, and control loop. P&IDs are produced in every sector and are the foundation document for operations, maintenance, and hazard analysis. PFD(Process Flow Diagram) is a higher-level document that shows the main process streams and equipment without the detail of all instrument connections — it is the starting point for understanding a process before diving into P&IDs.

FEED(Front End Engineering Design) is the engineering phase that bridges conceptual design and detailed design. A FEED study defines the technical scope of a project, produces preliminary P&IDs, equipment lists, and cost estimates, and provides the basis for a FID. FEED studies are conducted for upstream facility projects, midstream infrastructure, and downstream refinery upgrades alike.

The DCS (Distributed Control System) is the computer-based system that monitors and controls process operations in real time. Originally developed for refineries and chemical plants, DCS technology has been adopted across all sectors. Modern DCS systems integrate with SCADA (Supervisory Control and Data Acquisition) networks to provide operators with a complete view of facility operations from a central control room. Understanding DCS architecture and nomenclature is essential for process engineers, instrumentation engineers, and operations personnel across all three sectors.

Written by Habib Huseynzade, a petroleum industry professional with upstream oil and gas experience. Habib founded Petroleum Acronyms to provide a fast, reliable reference for industry terminology encountered in daily operations.