The Cretaceous Niobrara Formation is a prolific organic-rich hydrocarbon reservoir throughout the greater U.S. Rocky Mountain region. Resistivity mapping is a common tool used for maturity mapping of the Niobrara because a good correlation exists between increased resistivity and increased thermal maturity. Niobrara resistivity is low in thermally immature areas on basin margins. Resistivity progressively increases as maturity increases from the oil window into the wet-gas window. However, this trend of increasing resistivity with increasing maturity is reversed as maturity increases from the wet-gas to dry-gas window. Examples of this anomalous resistivity reversal are present in the Piceance, Sand Wash, and DJ. Possible mechanisms to explain decreasing resistivity with increasing maturity include the existence of a conductive petroleum phase (pyrobitumen) and the increase in residual water salinity driven by water vapor solubility in the produced gas. However, the current study concludes that the development of petroleum-expulsion fractures combined with changes in wettability in the Niobrara provides the best explanation for the process of decreasing resistivity with higher thermal maturity. Low-maturity Niobrara is water-wet and has low resistivity. In the oil-generation window, generated polar organic compounds (resins and asphaltenes) compete with water to coat grain surfaces and the rock changes from water-wet to oil-wet, resulting in a significant resistivity increase. In the advanced stages of maturation, liquid and solid hydrocarbons are cracked to gas. As the resins and asphaltenes become cannibalized, the oil-wetting behavior is reversed, releasing grain surfaces to be rewetted by connate water. The conductive water phase, even at low water saturations, is restored and resistivity drops.
This study reveals that the rate of porosity creation during oil-expulsion fracturing can reach 5.0 percent per 50 m.y. More volume expansion during gas generation is assumed to increase total porosity by additional 1.0 percent. Along with changes in the Archie saturation component (n), the decrease in resistivity can reach 80 percent.