Evidence of Multiple Vitrinite Populations in the Niobrara Formation of the Denver Basin and Their Implications for Petroleum Exploration and Development

David Thul and Stephen Sonnenberg

Defining the extent of effective Niobrara source rock in the Denver Basin has proven challenging, not only because of the limited data, but also because of discrepancies between various analysis types. Vitrinite reflectance maturity parameters (Ro) and pyrolysis maturity parameters (Tmax, Vitrinite Reflectance Equivalence-VRe) do not always agree with one another. At a single location within the basin, the available data could characterize a source rock to be anywhere between marginally mature and wet gas mature, depending on the technique used to assess the sample. By examining sets of samples analyzed by various petrographers, as well as programmed pyrolysis, we have identified what we believe to be a systematic mischaracterization of indigenous vitrinite populations resulting in the over-estimation of maturity in certain areas. We show this mischaracterization may occur due to the presence of multiple, reworked vitrinite populations present in the formation. Typically this mischaracterization leads to assessing the source rock to be peak oil mature (0.9 %Ro) when in fact it is near onset oil mature (0.6 %Ro). In one extreme case, an immature source rock (0.3 % VRe) is identified as being near peak oil maturity (0.8 % Ro). The differences in maturity measured by each technique are significant for exploration because misidentifying maturity can add risk to a prospect. This is amplified in the Denver Basin because much of the acreage is already leased. The acreage that does become available may be on the play margin and evaluating source effectiveness will be critical. On the production side, the maturity is tied to oil quality and in order to maximize returns the highest quality product is desired (lighter and sweeter).

AAPG Search and Discovery Article #90156©2012 AAPG Rocky Mountain Section Meeting, Grand Junction, Colorado, 9-12 September 2012