Preliminary geochemical investigation of some crude oils from the Niger Delta Basin Nigeria

Geochemical studies on twenty-eight crude-oil samples from nine oilfields, onshore-offshore parts of Niger Delta Basin, were performed to classify the oils into various classes, determine the bulk compositions of oils, organic matter inputs, source depositional environment, thermal maturation, and in-reservoir processes, for petroleum system prediction. The oils were fractionated into individual bulk compositions of saturates, aromatics, resins, and asphaltenes (SARA), using the Iatroscan thin-layer chromatography-flame ionization detector (FID) technique. Column chromatography was utilized to separate the saturates, and were further analyzed with gas chromatography-FID for n-alkane and acyclic isoprenoid distributions. SARA results, classified the oils into: paraffinic (>72%Hc, sat/arom: 2.61-3.68); paraffinic-naphthenic (63-70%Hc, sat/arom:1.77-2.31) and aromatic-naphthenic/aromatic-intermediate (<60%Hc, sat/arom: <1.44) oils, generated from source rocks of terrigenous-deltaic origin. Pristane/phytane and n-alkane/isoprenoid ratios delineated the oils into terrigenous (Type III kerogen) and mixed (terrigenous-marine) Type II/III kerogen organic matter source, deposited in oxidizing to transitional environment conditions. Thermal maturity parameters suggest the oil originated from source rocks of early to peak oil window maturity. Additionally, n-alkane/isoprenoid and saturate/aromatic ratios established evidence of biodegradation in a few samples across the depobelts. However, SARA compositions, n-alkane, and isoprenoid distributions of the oils critically permit preliminary assessment study, consequently allows a first-hand geochemical interpretation of the Niger Delta petroleum systems.

针对尼日尔三角洲盆地陆上-海域区域9个油田的28个原油样品开展地球化学研究，旨在将原油划分为不同类别，测定原油的族组成、有机质输入特征、烃源岩沉积环境、热成熟度以及油藏内作用过程，以实现油气系统预测。研究采用Iatroscan薄层色谱-火焰离子化检测器（FID）技术，将原油分离为饱和烃、芳香烃、树脂和沥青质（SARA）族组分；通过柱色谱法分离饱和烃组分，并利用气相色谱-FID进一步分析正构烷烃与无环类异戊二烯烃的分布特征。基于SARA组分分析结果，可将原油划分为三类：石蜡基原油（烃类含量>72%，饱和烃/芳香烃比值为2.61~3.68）、石蜡-环烷基原油（烃类含量63%~70%，饱和烃/芳香烃比值为1.77~2.31）以及芳香-环烷基/芳香中间基原油（烃类含量<60%，饱和烃/芳香烃比值<1.44），上述原油均源自陆源三角洲型烃源岩。姥鲛烷/植烷与正构烷烃/类异戊二烯烃比值可将原油划分为两类有机质来源：陆源型（III型干酪根）与混合陆源-海源型（II/III型干酪根），其沉积环境为氧化至过渡环境。热成熟度参数表明，原油源自处于生油窗早期至生油高峰阶段的烃源岩。此外，正构烷烃/类异戊二烯烃比值与饱和烃/芳香烃比值证实，研究区各沉积带内部分样品存在生物降解现象。不过，原油的SARA组分、正构烷烃与类异戊二烯烃分布特征可有效支撑初步评价研究，进而为尼日尔三角洲油气系统提供一手地球化学解释依据。