Geochemical record for the depositional condition and petroleum potential of the Late Cretaceous Mamu Formation in the western flank of Anambra Basin, Nigeria

• 作者: Gbenga O.Ogungbesan Taofik AAdedosu
• 来源期刊: 绿色能源与环境:英文版
• 学科: 地球科学
• 关键词: Anambra BASIN Mamu FORMATION PALEOENVIRONMENT KEROGEN Thermal maturity
• 年，卷（期）: 2020,（1）
• 页码范围: 83-96
• 页数: 14页
• 分类号: X70

摘要

Carbonaceous shale exposures of the Late Cretaceous Mamu Formation along Ifon-Uzebba road in western arm(Benin Flank) of Anambra Basin, southwestern Nigeria, were analyzed for bulk organic geochemical, molecular biological and poly-aromatic hydrocarbon(PAH) compositions to investigate the organic matter source, paleo-depositional condition, thermal maturity and petroleum potential of the unit. The bulk organic geochemistry was determined using Leco and Rock-Eval pyrolysis analyses while the biomarkers and PAH compositions were investigated using gas chromatography-mass spectrometer(GC-MS).The bulk organic geochemical parameters of the shale samples showed total organic carbon(TOC)(1.11-6.03 wt%), S2(0.49-11.73 mg HC/g Rock), HI(38-242 mg HC/g TOC) and Tmax(425-435 C) indicating good to excellent hydrocarbon source-rock. Typical HI-Tmax diagram revealed the shale samples mostly in the gas-prone Type Ⅲ kerogen region with few gas and oil-prone Type Ⅱ-Ⅲ kerogen. The investigated biomarkers(n-alkane, isoprenoid, terpane, hopane, sterane) and PAH(alkylnaphthalene, methylphenanthrene and dibenzothiophene) indicated that the carbonaceous shales contain mix contributions of terrestrial and marine organic matter inputs that were deposited in a deltaic to shallow marine settings and preserved under relatively anoxic to suboxic conditions.Thermal maturity parameters computed from Rock-Eval pyrolysis, biomarkers(hopane, sterane) and PAH(alkylnaphthalene, alkylphenanthrene, alkyldibenzothiophene) suggested that these carbonaceous shales in Anambra Basin have entered an early-mature stage for hydrocarbon generation. This is also supported by fluoranthene/pyrene(0.27-1.12), fluoranthene/(fluoranthene + pyrene)(0.21-0.53) ratios and calculated vitrinite reflectance values(0.49-0.63% Ro) indicative that these shales have mostly reached early oil window maturity, thereby having low hydrocarbon source potential.