We carried out a detailed organic facies study on shale cutting samples from the Devonian Pimenteira Formation in the São Luís Basin, northwest Brazil, to reconstruct the depositional paleoenvironment and origin of the organic matter. Samples were collected from the stratigraphic well 2-ANP-5-MA and were analyzed for total organic carbon (TOC), total sulfur (%S), insoluble residue (RI), spore coloration index (SCI), vitrinite reflectance (%Ro), palynofacies, and biomarkers. The TOC values of the analyzed sediments range from 0.19 to 0.51%, %S from 0.17 to 0.85%, and the total carbonate (CaCo3) is low (<15%); SCI ranges from 4.0 to 5.0, %Ro from 0.45 to 0.47%, and chitinozoan reflectance from 0.42 to 0.51%. The prasinophyte algae content exhibits a strong correlation (r2 = 0.73) with the TOC content; this suggests that this organic component is the main factor controlling TOC and that variations in prasinophyte content could reflect changes in the sea level associated with marine transgressions and regressions. Two types of palynofacies (PI—phytoclasts, amorphous organic matter [AOM], and spongiophyton-dominated—and PII—palynomorphs and zoomorphs-dominated) were identified. PI is characterized by a palynomorph association dominantly composed of marine palynomorphs like acritarchs and prasinophytes, whereas PII is characterized by phytoclasts and an association with AOM. Chitinozoan, conodonts and spongiophyton are common throughout this sedimentary sequence. The TOC (wt%) vs %S relationship indicates that the organic matter is dominantly marine but has some freshwater influence. The ternary graph including AOM, phytoclasts, and palynomorphs+zoomorphs suggests a depositional environment ranging from oxic proximal shelf to dysoxic proximal-distal shelf with deltaic influence. The presence of marine microplankton and land plants was corroborated by the distribution of the steranes components (C27, C28, and C29). The thermal maturity indicators, including %Ro, chitinozoan reflectance, SCI, Ts/(Ts + Tm) ratios, and C29 20S/(20S + 20R) and C29 ββ/(αα + ββ), indicate that the organic matter is immature.