Petroleum Geology

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PG- Petroleum Geology
Topic: Unconventional resources and their appraisal methods

1. Introduction
The term “unconventional resources” (UHC) refers to those hydrocarbons obtained from natural petroleum reservoirs that are deemed unusual for oil and gas companies to produce due to physically limiting factors such as low permeability and high hydrocarbon viscosity. The most popular unconventional fuels come from gas shale, tight gas sands, and coal bed methane; however a more complete list would include oil shale, tar sands, heavy oil reservoirs, and methane hydrates as well. Since there is a wide variety of factors that would make a resource play unconventional and since these factors are sometimes unique to particular reserve types, there are different ways to define unconventional hydrocarbons. For example, the NPC defines unconventional gas as natural gas that can only be recovered after using a combination of several advanced recovery technologies whereas oil sands are defined as reservoirs of semi-liquid hydrocarbons with a viscosity in excess of 10000cP. These definitions are often incomplete by themselves. The key point is that unconventional resources are lower quality fuel sources and are not as economically viable as crude oil and conventional gas. To further understand the importance of unconventional resources this it is useful to also understand the resource triangle concept.

2. Resource Triangle
The Resource Triangle concept illustrates a trend where resource deposits that are technologically complex to exploit are more ubiquitous than those deposits in which resources can easily be extracted from. For example crude oil reservoirs, at the tip of the triangle, are simple to develop but short in supply; whereas, methane hydrates, at the base of the triangle, are practically infinite in supply but several technical barriers prevent their extraction. A recent evaluation of unconventional and conventional hydrocarbons in 7 United States basins confirmed that this relationship holds true for hydrocarbon reservoirs throughout the country. There are several characteristics of unconventional hydrocarbons that exemplify the resource triangle trends. They are as follows: • Unconventional hydrocarbon deposits are of lower quality but higher prevalence than conventional hydrocarbon deposits. • As technology improves unconventional hydrocarbons will become more feasible to pursue. • As demand for oil increases unconventional reserves will become more profitable to develop. • As the availability of conventional reserves reaches a peak value a smooth transition of unconventional resources entering the market will supply the demand gap. The Resource Triangle concept is useful in two ways. It provides a Hierarchical arrangement of resources with the higher quality reserves at the thinner tip of a pyramid (indicating the rarity of the reserve) and the lower quality reserves at the broader base of the pyramid (indicating the extensiveness of the reserve). The concept can also be used as a rough approximation to correlate the total recoverable conventional resources in a basin to the total recoverable unconventional resources in that basin. 3. Classification & Evaluation

The purpose of classifying hydrocarbon resources is ultimately to facilitate communication of key concepts. Resource classification elucidates the commercial potential of a given accumulation of petroleum. The SPE, AAPG, WPE, and SPEE have funded the development of the Petroleum Resource Management System (PRMS) in an effort to standardize the definitions of petroleum resources and how they are estimated. According to this classification scheme all petroleum resources, conventional and unconventional, fall under the overall category of "Total Petroleum Initially in Place (PIIP)". This category is then subdivided into "Discovered PIIP" and "Undiscovered PIIP". Discovered PIIP is divided into "Commercial" and "Sub-commercial". Commercial PIIP includes...
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