Niobrara source rock maturity in the denver basin: a study of differential heating and tectonics on petroleum prospectivity using programmed pyrolysis

Abstract

The Niobrara Formation of the Denver Basin is an unconventional oil and gas drilling target composed of alternating chalk and marl units. These units act as the source, seal and trap for hydrocarbons generated in the organic rich marl beds of the Niobrara. Organic geochemical data, such as oil-to-source rock correlations, oil biomarkers and gas isotopes, indicate that the fluids accumulated within the reservoirs closely match the source rocks in type and maturity; therefore, this is a system of in-situ generation and accumulation as well as short-distance migration. In a system where migration is minimal, proximity to an effective source rock is a prerequisite for a productive well. With such a prerequisite, play delineation should begin with regional source rock maturity assessment. Historically, source rock maturity has been studied through programmed pyrolysis (such as Rock-Eval(TM)), vitrinite reflectance, log interpretation or basin modeling. Recently, a new pyrolysis instrument called the Source Rock Analyzer(TM) has come to market. With this new addition, questions of data congruence have arisen between the Rock-Eval(TM) and the Source Rock Analyzer(TM) that preclude combined data sets without further study. This work establishes the veracity of the data from the Source Rock Analyzer(TM) and compares its results to those of a Rock-Eval(TM) instrument using a suite of 103 source rock samples. The data trends between the two machines are compared and then applied to a regional source rock maturity evaluation of the Niobrara Formation in the Denver Basin. The test of data veracity shows that the S2 and Tmax parameters from the Source Rock Analyzer(TM) are comparable to those from the Rock-Eval(TM), showing good correlation and a nearly one-to-one relationship. The other parameters, S1 and S3 show similar trends but there is significant scatter in the data. The calculated parameters (hydrogen index, oxygen index, production index, and total organic carbon) are correlative but deviate significantly from a one-to-one relationship. In the regional source rock evaluation, new samples are analyzed deep within the basin as well as along a northeast-southwest trend stretching from Wattenberg Field to the Colorado-Nebraska border. The new analyses show that maturity along these trends is affected by a thermal anomaly that is evident in the modern-day thermal gradient. Through crossplotting, mapping and modeling it is shown that the onset of hydrocarbon maturity in the Niobrara is 432 degrees C Tmax and that hydrocarbon expulsion occurs between 438 degrees C and 443 degrees C Tmax. The study also shows that Niobrara production can be predicted by mapping the thermal maturity as well as free hydrocarbon anomalies within the basin.