Engines
Drilling, completion and workover trucks, rigs and equipment such as pumps typically run off of diesel-powered or gasoline engines.  The exhaust fumes from gasoline and diesel fuels can produce emissions that are noticeable to people living downwind.

Polycyclic aromatic hydrocarbons (PAHs) are found in exhaust from motor vehicles and other gasoline and diesel engines.  A long list of other air pollutants, including nitrogen oxides, carbon monoxide, BTEX, formaldehyde and metals are also contained in diesel fuel combustion products.

Flaring
Flaring is the practice of burning gas that is deemed uneconomical to collect and sell. Flaring is also used to burn gases that would otherwise present a safety problem. It is common to flare natural gas that contains hydrogen sulfide (i.e., sour gas), in order to convert the highly toxic hydrogen sulfide gas into less toxic compounds. 

Flares emit a host of air pollutants, depending on the chemical composition of the gas being burned and the efficiency and temperature of the flare. Flaring results in hydrogen sulfide emissions if hydrogen sulfide is present in large enough amounts in the natural gas. There may also be additional by-products formed if some of the chemicals used during the drilling or hydraulic fracturing process are converted to a gaseous form and are burned along with the natural gas.

The Ventura County Air Pollution Control District, in California has estimated that the following air pollutants may be released from natural gas flares:  benzene, formaldehyde, polycyclic aromatic hydrocarbons (PAHs, including naphthalene), acetaldehyde, acrolein, propylene, toluene, xylenes, ethyl benzene and hexane.  Researchers in Canada have measured more than 60 air pollutants downwind of natural gas flares.[1]

Pits
Earthen pits are often used to store or evaporate produced water and waste water from natural gas dehydration or oil/gas separation units. Additionally, prior to disposal drilling wastes (muds and cements) and hydraulic fracturing (fracking) wastes are often stored in earthen or metal pits that are open to the air. There are hundreds of different chemicals that may be used during drilling, fracking and workover procedures, including acids, biocides, surfactants, solvents, lubricants and others.

Chemical compounds that are naturally present in natural gas, or chemicals that have been injected downhole during drilling, hydraulic fracturing or well workover operations, will be present in the water or wastes that are held in pits.  Some of the lighter or more volatile chemicals and compounds, such as benzene, toluene, hydrogen sulfide, etc., will escape from the produced water pits into the atmosphere.  These chemicals may then be transported through the air, into nearby neighborhoods.  The odors associated with the natural gases or chemicals will vary, depending on the concentrations, volumes, and combinations of chemicals released.

Vehicles

Burning fuel to power trucks also emits NOx, carbon monoxide, and sulfur dioxide, as well as particulate matter.  These compounds combine with VOCs to form ground-level ozone (smog).

Venting
Venting is the release of gas to the atmosphere.  Venting occurs at a number of points in the oil and gas development process (well completion; well maintenance; pipeline maintenance; tank maintenance; etc.).  

During oil and gas development, huge quantities of gas may vented to the atmosphere.  For example, during well completion, after a well is drilled and stimulated (e.g., hydraulically fractured), the wellbore and surrounding formation must be cleaned out.  The solids and fluids from the well go into pits, while the gases are allowed to escape into the atmosphere, or they are burned off (flared).  It has been estimated that a single well Wyoming's Jonah field will emit  115 tons of VOCs, and 4 tons of hazardous air pollutants such as benzene, toluene, ethylbenzene, xylene and hexanes. If the gas is flared, rather than vented, the emissions of VOCs and HAPs are reduced to 29 and 1 ton, respectively; but flaring of completion gases also results in the release more than a ton of nitrogen oxides, and almost half a ton of carbon monoxide per well.[2] 

The primary component of natural gas is methane, which is odorless when it comes directly out of the gas well.  At gas processing facilities, chemical odorants such as mercaptans are added to methane, so that consumers are able to smell it in the event of a gas leak. In addition to methane, natural gas typically contains other hydrocarbons such as ethane, propane, butane, and pentanes.  Raw natural gas may also contain water vapor, hydrogen sulfide (H2S), carbon dioxide, helium, nitrogen, and other compounds. 

Almost all references to the odor of raw or wellhead natural gas state that it, like methane, is odorless. The Ohio Department of Natural Resources, however, advises landowners that one way to detect an abandoned oil or gas well on their property is if they smell "natural gas" odors coming from their tap water.   So, in some cases, there may be a slight hydrocarbon odor associated with venting of natural gas.

If the concentration of H2S in the gas is high enough, there may also be a "rotten egg" odor associated with the gas.