Biomarkers in Cretaceous sedimentary rocks from the Codo Formation - Parnaiba Basin: Paleoenvironmental assessment

摘要

Earlier geological works showed that Cod6 Formation - CF (Parnaiba Basin, Brazil) has been deposited in the Aptian, and suffered the influence of various transgressive-regressive marine cycles, thus suggesting various depositional environment possibilities from saline lacustrine deltaic to shallow marine during sedimentation. Cretaceous potential source rock samples from the CF were investigated to identify diagnostic biomarkers in order to ascertain paleoenvironmental conditions of deposition and thermal evolution of organic matter. The TOC results showed low organic matter (OM) contents and Rock-Eval analysis implying low thermal evolution of organic matter. GC-MS analysis showed a great diversity of biomarkers within the five studied samples. The main features of organic matter composition were: low content of n-alkanes, Ph Pr, presence of i-25, i-30 (squalane) and gammacerane thus suggesting saline to hypersaline depositional environment. The biomarkers fingerprint showed a full series of beta beta hopanes and Delta(13(18)) hopenes, alpha alpha alpha R and beta alpha alpha R steranes, and Delta(5) and Delta(8(14)) sterenes, pointing out immature OM. Several C-30 steranes have been detected, among them 4 alpha(methyl)- and 4 beta(methyl)-24-ethylcholestane, 24-n-propylcholestane and 24-isopropylcholestane (all identified by co-injection) and dinosteranes, which some of them are indicative of a marine depositional system.