Understanding outcome probabilities of a prospect at or before discovery is a challenging yet crucial task in conventional offshore exploration. Geoscientists utilize often sparse geophysical and geological data to quantify how successful or unsuccessful prospects are expected to be. Geologic risk analysis assigns probabilities of geologic success to petroleum system elements such as the source rock, hydrocarbon migration, reservoir rock, trapping mechanism and seal (Figure 1). Risks are categorized as play or prospect risks. The former are basin-scale uncertainties that impact the entire play (dominantly source rocks and thermal maturity), and the latter are uncertainties surrounding specific hydrocarbon accumulations (presence of reservoir and the trapping and sealing of hydrocarbons).
In conventional plays, all five petroleum system elements must be favorable to create a hydrocarbon accumulation. In an unconventional play, the source rock is also the reservoir, trap and seal. This, along with the plethora of well data, results in reduced risk associated with a shale play when compared to an offshore conventional play.
FIGURE 1 | Petroleum System Play Elements of Conventional and Unconventional Prospects
Source | RSEG
Geologists at RSEG create geologic risk analyses for offshore projects by examining all available data and learnings from similar or nearby projects. Through understanding key variables including lithology, hydrocarbon charge, fluid migration, timing, structure, porosity, permeability, thickness, lateral/vertical variability, preservation and more, we assign a geologic chance of success to all petroleum system elements. These subsurface chances of success are then multiplied together to create the overall probability of success for a prospect (Figure 2). The probability of success is then used to risk the amount of recoverable reserves and the expected value of a prospect. As illustrated in our example, higher exploration risk means larger accumulations are targeted in order to realize comparable risked recoverable prospects.
FIGURE 2 | Example of a Geologic Risk Analysis
Source | RSEG
As offshore exploration technologies advance and more wells are drilled, geologic risks will progressively decline. Although the higher geologic risks associated with conventional plays may push operators to lower-risk unconventional shale plays, operators should not be afraid of offshore exploration, we believe. As the saying goes: “Pearls don’t lie on the seashore. If you want one, you must dive for it.”