KEROGEN CHEMISTRY AND ITS CONTROL ON HYDROCARBON GENERATION IN SULFUR-RICH ORGANOFACIES

Hydrocarbon gases generated from source rocks are related to the changing composition of the kerogen within the rock as it is thermally converted into bitumen and hydrocarbons. Kerogen loses hydrogen relative to carbon during this transformation resulting in a progressive decrease in hydrocarbon volatility. However, correlating this declining volatility to the generated gas collected during the drilling of a source rock reservoir can be challenging, particularly when the source rock is in the latter stages of condensate maturity, as drier gases become more prevalent from oil to gas cracking. This is especially difficult for source rocks composed of kerogen with higher variable sulfur concentrations which lower the activation energy of kerogen for generating hydrocarbons. As a result, the type and volume of gaseous hydrocarbons produced from individual intervals of a source rock reservoir can vary significantly, leading to uncertainty over its potential productivity and maturity. Presented is an integrated study of whole-rock pyrolysis data with elemental concentrations of kerogen and compound specific isotopes analyzed from the extracted bitumen from sulfur-rich source rocks compared to the mud gas chemistry collected from the same formation. The source rock of late condensate maturity as computed from pyrolysis variables is not easily correlated to differences in the chemical wetness or the balance and character computed from the generated gas. Instead, differences in the elemental concentrations of the kerogen are shown to correlate with these gas parameters, related not only to maturity, but also according to the former cyclic anoxia that preserved the organic matter during deposition. These cycles are evident from the increases in uranium measured by spectral gamma ray from the rock, the hydrogen index, and from the differences in the isotopic fractionation of hydrocarbon compounds measured from the bitumen extracted from those same rock intervals. As the stage of transformation of the source rock progresses from oil to gas maturity, the sensitivity of pyrolysis analysis begins to decline and the ability to distinguish slight changes in maturity become less certain, suggesting the elemental chemistry from kerogen may be more diagnostic for predicting the potential changes in productivity.