Where Did the Oil Come From?
In layman’s terms, the Oil in the Niobrara is self sourced from the organic matter (predominantly microscopic phytoplankton) deposited in the formation that was cooked to just the right temperature (Thermal Maturity) to produce oil and gas. In more scientific terms: “Regional studies by Landon and others (2001) and Meissner and others (1984) indicate that organic matter in the Niobrara Formation is Type-II, oil-prone kerogen. The sediments were deposited during a major marine transgressive cycle known as the Niobrara cyclothem (Kauffman, 1977)” (Finn & Johnson, 2005)
“The reservoirs are interbedded with shales, and in the Denver (DJ) Basin there are four main chalk intervals and three main source bed intervals in the Niobrara,” (Durham, AAPG Explorer 2010) In what is a serendipitous occurrence, we have multiple oil source shale layers from which oil has migrated into a reservoir host rock (limestone – Fort Hays and Smoky Hill units) all in one formation.
Why is the oil accessible in rocks that are normally low in permeability (flow)?
“The formation has a unique fault and fracture pattern mainly because it’s so brittle, according to Ray. He likened it to a brittle sandwich, with a limestone facies between organically rich shale zones that source the crude.” (Durham, AAPG Explorer 2010)