Bulk Rock Major and Trace element Chemostratigraphy of the Cretaceous Lower Eagle Ford Formation in South Texas

The Eagle Ford Formation is an unconventional hydrocarbon source rock play in South Texas which produces oil, condensate, and gas from its organic-rich shales deposited in the Late Cretaceous. Twenty-three whole rock samples of the Lower Eagle Ford from a 120.85 ft core from La Salle County, Texas were studied to evaluate major and trace elements to examine the environment at time of deposition, cyclicity, and source rock potential. Major and minor element analyses were conducted by ICP-OES, trace elements were analyzed by QQQ-ICP-MS, LOI was analyzed by muffle furnace, and TOC was analyzed by LECO at the University of Houston. Average enrichment factors of productivity proxies Cu (1.61), Ni (1.41), Zn (1.53), and Cd (24.96) are greater than one indicating the depositional environment had elevated productivity. High average values of degree of pyritization (0.79), V/(V+Ni) (0.85), S wt% (1.53 wt%), and enrichment factors greater than one for Mo (35.96), V (3.90), and U (2.07), and Se (65.01) and depletion of Mn (0.57) indicate euxinic conditions. Basin restriction is supported by Mo/TOC of ~11 and that the samples plot in particulate shuttle range in the Mo EF vs U EF discriminatory diagram. The samples have depleted detrital elements (Sc (0.32), Y (0.66), Zr (0.35), Nb (0.47), Th (0.43), Hf (0.38), and W (0.24)). The samples are heavily diluted with high carbonate content (CaCO3 = 52.80 wt%) compared to average shale (PAAS). Sample EFRD-S43 is an ash layer with low CaO (0.95 wt%), low TOC (0.50 wt%), and enrichment factors greater than one for detrital elements (Y (1.21), Zr (1.97), Th (2.30), and Hf (3.11)). Three major geochemical cycles were identified at this level of sampling. The samples were deposited in a semi-restricted basin between the Edwards and Sligo reefs which were sills during Eagle Ford basin evolution. The interval of 109.87 ft (EFRD-S41) to 76.91 ft (EFRD-S29) is the most favorable for unconventional petroleum targeting due to the high TOC (6.09 wt%) and organic matter (8.15 wt%), kaolinitic clay reflected by low K/Al ratios (0.24), and high brittleness index (0.83) related to carbonate rich adjacent units.