Using Petroleum Systems Analysis to Evaluate Play Fairways in the Bakken Shale

Jay E. Leonard
Platte River Associates, Inc.

Petroleum systems and play analysis in conventional plays consists of a geological evaluation of the petroleum system elements (source, reservoir, trap, seal, and charge access). These elements are then translated into a “probability of geologic success” or some other measure of risk. The unconventional petroleum system should be evaluated with exactly the same conceptual approach. However, unconventional systems differ from their conventional counterparts. These differences include the nature of the geologic proxies used to evaluate the petroleum and play systems, and their relative contribution to play risk.

Source: An example is source rock evaluation. In a conventional setting, source rocks are judged by their capacity for generation and expulsion of hydrocarbons. In many unconventional settings, where the source rock and reservoir are the same, hydrocarbon expulsion is undesirable and unnecessary. In fact, the salient source rock characteristic in such cases is its ability to retain petroleum. Ironically, an improved understanding of source rock expulsion may be more critical for shale oil play evaluation than for conventional plays, despite the fact that shale oil plays do not require expulsion to be successful. In common with conventional systems, maturity indicators, such as Transformation Ratio, are key proxies (see Figure 1). However, for unconventional systems, other attributes may be significant, such as Rate of Oil Generation, and Initial Total Organic Carbon.

Reservoir: Another example of the differing nature of geologic proxies is reservoir storage. In a conventional play, reservoir storage is a function of primary porosity, reservoir thickness, and trap closure geometry. These data are readily available from seismic mapping and reservoir quality prediction. In a shale gas play, reservoir storage is a function of; (1) three different porosity types (primary, fracture, and nano-), (2) the shattered rock volume, and (3) the adsorption capacity, which is in turn a function of organic carbon content, pressure, and temperature. Shattered rock volume will be controlled, in part, by rock brittleness (Figure 2).

Trap and Seal: In conventional plays trap geometry can be mapped using seismic data. In unconventional plays, it can be difficult to define trap geometry and sealing elements, especially at the prospect level. The reservoir volume accessed by an individual well is determined by the well design and the reservoir’s response to hydraulic fracturing.

Charge Access: There are no simple proxies for charge access uncertainty. It requires an understanding of hydrocarbon expulsion timing and volume, trap timing, basin geometry through time, and an estimate of migration pathway and velocity.

The relative importance of individual elements to play risk varies with play type. In conventional plays charge access is often the most critical risk, and the most difficult to quantitatively evaluate. Column capacity is also important and difficult to evaluate. In unconventional plays, where coupling of source and reservoir is common, charge access can often be neglected. Reservoir storage and effectiveness are often the high risk elements.

In this talk, we develop these ideas with examples from the Bakken Shale Formation (Williston Basin, North Dakota) and other plays. We will show that the new geologic proxies and the types of data they require can be integrated into an established conceptual framework of a petroleum and play system. Our examples demonstrate a workflow that includes geologic evaluation and common-risk segment mapping to find sweet spots within unconventional plays.

Figure 1. (top) Geochemical proxies for the determination of productive wells within the Bakken Formation; (bottom) geographic extent of above proxies to determine sweet spots.