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

摘要

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 a 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 a al., 2005), as well as decreasing aliphaticity as expressed by a new parameter termed here in as A(II). 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.