Geochemical characterization of Jurassic source rocks from Cuba: 2. Constancia Formation in onshore Varadero oils fields

Abstract

Samples collected from the Upper Jurassic Constancia Formation in the Varadero oilfields, located in the Northern Marginal Basin of Cuba were analyzed to evaluate the type and origin of the organic matter and their source rock potential. This study is based on geochemical analyses of whole rock (total organic carbon content (TOC), Rock-Eval pyrolysis), petrographical analyses (vitrinite reflectance, spore colouration index (SCI), palynofacies) and gas chromatography/mass spectrometry (GC-MS). Results obtained from 25 core samples collected from four petroleum exploration wells indicate that these rocks have organic carbon contents ranging from 0.13 wt% to high 5.84 wt%. The average hydrocarbon generation potential as determined by Rock-Eval pyrolysis potential (S2) is between 0.34-5 mg HC/g rock for samples with low TOC, suggesting good to excellent potential to generate oil and 5-49.9 mg HC/g rock for samples with high TOC. Based on palynofacies analyses three organic assemblages have been identified. Quantitative and qualitative analyses of palynofacies show that the Constancia Formation of Jurassic age, represent oxic proximal and suboxicanoxic/marine restricted environment. The Amorphous organic matter is the dominant element of the particulate organic matter in the assemblages I, followed by structured terrestrial organic matter and spores/pollen in the assemblages III. The results of Rrandom and SCI suggest a maturity level of the organic matter ranging from immature to marginally mature. The Constancia extract rock are immature to early maturity evaluated from the C29-αα20S/(S + R) and C29-20R ββ/(ββ + αα) sterane indices.