Constraining Unusual Patterns of Thermal Maturity within the New Albany Shale, Illinois Basin using Source rock Pyrolysis Geochemical Parameters and Aromatic Markers and Biomarkers in Fluid and Rock Extracts
Source rock pyrolysis geochemical parameters (hydrogen index HI, S1, S2, TMAX, etc.) are used to analyze both generative capacity and thermal maturity of organic-rich intervals. These data help constrain both measured and calculated vitrinite reflectance (%VRo) values that are often used in regional assessments and burial history modeling. These methods can be compromised by high molecular weight (HMW) compounds being included in the convertible kerogen S2 pyrolysis peak, bitumen coatings retarding reflectance and color measurements, and misidentification of bitumen particles as vitrinite macerals. Transformation ratios (TR) of the convertible kerogen within organic matter use the present-day HI values determined from pyrolysis along with the original organic richness of the source interval to calculate the hydrocarbon conversion. This calculation is independent of vitrinite maturation. The kinetics that govern vitrinite evolution with increasing thermal stress are different than those which determine hydrocarbon generation from kerogen. Transformation ratios were calculated from rock samples collected from 50 wells within Illinois and western parts of Indiana and Kentucky. The general pattern of increasing hydrocarbon conversion in the deeper (southern) part of the basin was confirmed. However, irregular and unpredictable TR patterns developed in parts of the Fairfield Basin, Cottage Grove Fault System, and Wabash Valley Fault System in Illinois. An additional independent method was needed to confirm the TR results. Aromatic markers and biomarkers from both fluid and rock extracts can be used to assess maturity and thermal stress of the hydrocarbon system. Naphthalenes, phenanthrenes, dibenzothiophenes, triaromatic steroids, and saturate biomarkers are useful in characterizing the maturity range. Applicability requires an initial concentration normalization using gas chromatography/mass spectrometry (GCMS) results from samples of known, very low maturity. This process was accomplished using samples from wells in northcentral Kentucky. Initial results indicate a strong relationship between increasing thermal maturity in ethyl naphthalene, dibenzothiophene, homohopane isomerization, and triaromatic steroids. With this correlation, an independent method was developed to test problematic TR results and assess maturity. This technique directly impacts how to model and assess hydrocarbon generation from portions of the Illinois Basin.
AAPG Datapages/Search and Discovery Article #90373 © 2019 AAPG Eastern Section Meeting, Energy from the Heartland, Columbus, Ohio, October 12-16, 2019