The Niobrara was deposited during a period of large variations in sea level, in the in the middle of North America during late Cretaceous Age (145.5 – 65.5 million years ago). With a descending Western Interior Seaway crust, a major marine transgression occurred and created conditions ideal for carbonate deposition.
Much of the Niobrara exploration activity has been guided by resistivity mapping. In general, higher resistivity correlates with higher maturity and better production. It has been identified that resistivity is typically low in thermally immature regions of the Niobrara. Resistivity gradually increases with increased maturity, as the production progressively moves from the oil window into the wet-gas window. However, when referring to increasing maturity of wet-gas to dry-gas window, this trend reverses. Recent studies have indicated that the best explanation for this process of decreasing resistivity with higher thermal maturity is likely due to changes in wettability combined with development of petroleum-discharge fractures.
The process is as follows:
Low maturity Niobrara is water-wet and therefore has low resistivity.
In the oil-generation window, organic compounds and water are competing to coat grain surfaces and eventually the rock changes from water-wet to oil-wet, causing a substantial increase in resistivity.
Further on, hydrcarbons (solid and liquid) go through a process known as “Thermal Cracking” and are converted to gas. Essentially, as these organic compounds (resins and asphaltenes) are consumed the oil-wetting behavior is reversed, thus releasing grain surfaces to be rewetted by connate water. The conductive water phase presence is re-established, resulting in resistivity decreases.
The figure below shows a regional map of maximum resistivities in the Niobrara. Resistivities are low (<10 ohm-m) in the shallow eastern part of the basin and increase westward into the deeper part of the basin. The trend of increasing resistivity with increasing maturity is reversed in the area of Wattenberg field where the Niobrara maturity is highest. This area is indicated on by the 20,000 GOR contour.
The Niobrara has had completions operations performed since the early 1980’s. By 2009, the introduction of horizontal drilling and multi-stage frac intervals had identified a viable individual target area in the Niobrara chalks. Certain companies have utilized completion designs that were driven from their earlier experiences in Bakken field operations.
D-J Basin - Active horizontal drilling continues to present day, with about 50 rigs running in early 2014.