Organic geochemistry of kerogen from La Luna Formation, Western Venezuelan Basin, using diffuse reflectance – Fourier transform infrared spectroscopy (DRFTIR)

Abstract Twelve samples of marine type II kerogen from La Luna Formation (Maracaibo sub-Basin, Western Venezuelan Basin) have been characterized via diffuse reflectance – Fourier-transform – infrared spectroscopy (DRFTIR). The samples were selected from different regions in the sub-Basin and correspond to different maturity levels with subtle variations in organic facies. For the analysis, the kerogen samples were mixed with potassium bromide (KBr) at a 5% w/w concentration. Spectra were obtained using a Varian 640-IR spectrometer operating in Fourier Transform mode and coupled with an EasiDiff diffuse reflectance accessory. Each spectrum was generated within the 400–4000 cm−1 frequency interval, at 4 cm−1 resolution, after 52 scans. IR parameters known to distinguish organic facies, such as the A factor and the C factor, separated the kerogen from anoxic environment (samples from the Maraca Creek) versus the kerogen from suboxic/disoxic conditions (samples from La Molina and San Antonio). Organic facies indices by Chen et al. (1998) showed to be influenced by maturity effects. Higher thermal maturity of samples from La Molina creek was supported by increasing aromaticity (indices by Lis et al., 2005), as well as decreasing aliphaticity as expressed by a new parameter termed here in as AII. This parameter compares aromatic signals in the 700–900 cm−1 interval with the aliphatic bands centered at 2953, 2923, and 2856 cm−1. This is the first study that applies DRFTIR compositional parameters to characterize kerogen type II from marine carbonates.