Geoquímica inorgânica e orgânica dos folhelhos da Formação Pimenteiras: implicações para um sistema petrolífero não convencional

Abstract

The Pimenteiras Formation records the marine ingression in the Meso/Neo-Devonian of the Parnaíba Basin, and it shales are the main source rock interval for its conventional petroleum system. The aim of this paper is to integrate geological and geochemical data in order to provide the potential on shale gas production of the Pimenteiras Formation, based on the outcrop sampling. Nine depositional facies were identified: shale with ostracods (Fo), micaceous silty-shale (Fm), tabular shale (F), massive fine or very fine sandstones (Af), sandstones with hummocky cross lamination (Ah), interbedded clay-silt couplets with linsen (Hl) and wavy (Hw) hetherolitics, dark siltstone with sulfide levels (S) and clast-supported breccias (Bc), observed on top of the outcrops. Clay minerals dominate the mineralogy, followed by quartz. Weathering processes are evidenced by the high contents of lateritic iron and kaolinite. The scanning electron microscopy allowed to classify the pores into macropores (> 50 μm), and also clusters of organic matter and clay minerals with various habits were seen, highlighting the massive and the vermicular ones. The total organic carbon (TOC) values range from 0.02 to 1.31%. Pyrite crystals occur in the rock with the highest TOC value in response to the anoxic behaviour of the S facies. Additionally, the composition of the clay minerals is dominated by smectites and mixed-layer clay minerals. The preliminary results indicate a prospective difficulty for the hydraulic fracturing due to the major presence of clay minerals. The primary porosity has favourable dimension, but unfavourable geometry. The low thermal maturity and inexpressive TOC values points out to the low potential production. Thus, the rocks of this study are unfavourable for the shale gas production under the current conditions required by the economic market.