Biomarkers applied to the assessment of organic matter sources, thermal maturation and depositional environment of the Devonian Pimenteiras Formation, Parnaíba Basin (Brazil)

Abstract

The potential for hydrocarbon generation of Devonian shales from Pimenteiras Formation has been the object of study in many research works. In this study, the focus is to evaluate the organic matter sources, thermal maturation and depositional environments of the Pimenteiras Formation based on aliphatic and aromatics biomarkers. Thereby, a total of 30 samples from three wells were analyzed using gas chromatography and mass spectrometry (GC/MS). The n-alkanes, isoprenoids, and terpanes data revealed that shales from Pimenteiras Formation results of a mixture of marine, transitional and terrestrial organic matter. These findings are consistent with C27, C28 and C29 steranes distribution and abundance of methylphenanthrene and methyldibenzothiophene. The Pr/n-C17 and Ph/n-C18 ratios in the BP-77 well are greater than 1.0 (immature), while the most samples from BP-59 and BP-22 wells, the Pr/n-C17 and Ph/n-C18 ratios are less than 1.0 (mature). Findings are consistent with CPI, M30/H30, S/(S + R) (C29 ααα) and C29 ββ/(αα+ββ ratios. Additionally, the Ts/(Ts + Tm) ratios, MPI-1 versus Ro, 2-MP, 3-MP, MPI-1, MPR-1, dimethylnaphthalenes and dimethyldibenzothiophene indicate that the organic matter varies from immature to mature, and reaches the overmature stage in terms of thermal maturity. The Pr/Ph ratio, Pr/n-C17 vs Ph/n-C18 plot, high H35/H34, C29/C30, C27 Dia/C27 ααα (R + S) ratios and high concentration of dibenzothiophene suggests that the organic matter was deposited under oxic, suboxic and anoxic conditions. On the other hand, the low abundance of gammacerane and low Gam/H30 ratio suggests a low salinity and/or water column stratification. Additionally, the data showed that the organic matter was deposited in marine, marine carbonate platform and terrestrial environments.