We present the results of a microstructural study of oil window organic shales, with samples from the Avalon, Bakken, and Woodford shale plays. The results of this study are placed within the framework of an overview of shale microstructure in both the oil and gas windows. Results show shales in the oil window possess a significantly different microstructure than those in the gas window with a general lack of porosity in the organic matter of oil window shales. While this fits the current idea of porosity development in the organic matter occurring during the generation of gas, several deviations to this rule of thumb are observed. Some oil window Avalon samples exhibit porosity in the organic matter. Other Avalon samples exhibit fractures contained solely within the organic matter that have the appearance of hydraulic fracturing possible due to oil generation within the organic matter. In addition, several of the gas window Woodford samples exhibit a mixture of porous and non-porous regions of organic matter within microns of each other. From this two important observations are made:
The microstructure of the organic matter in oil window shales is complex; however, many times porosity is absent in the organic matter. This raises the question of how oil is stored and transported in these shales.
Other factors beyond thermal maturity complicate the development of porosity in the organic matter.
The knowledge gained from this study has implications as an integral component in determining what makes a good shale reservoir. In addition, it illuminates the need to rethink the storage and transport mechanisms in oil window shales as compared to gas window shales.