Abstract:
Petroleum migration from source rock to the reservoir is one of the most critical geological processes responsible for the accumulation of hydrocarbon in sedimentary basins. The differential fractionation of dibenzofurans (DBFs) and benzo[b]naphthofurans during oil migration has made them useful as migration markers.The migration and filling history of hydrocarbon in the Niger Delta has not been fully understood. This study was designed to investigate the occurrence and distribution of dibenzofurans and benzo[b]naphthofurans in Niger Delta source rocks and crude oilsand their application in the determination of oil migration directions and distances in the basin.
Ninety-two source rocks and forty-one crude oil samples from five wells were collected from northern and offshore depobelts of the Niger Delta Basin. The rock samples were extracted with a mixture of dichloromethane/methanol (93/7, v/v) for 72 h in a Soxhlet extractor. The rock extracts and crude oils were fractionated using column chromatography. The total organic carbon (TOC), genetic potential (GP), maximum temperature of hydrocarbon generation (Tmax) and production index (PI) were determined by Rock-Eval pyrolysis. The maceral composition and vitrinite reflectance were determined by petrographic analyses. The biomarkers and isotope composition of the rock extracts and crude oil fractions were analyzed by gas chromatography-mass spectrometry, elemental analyzer-isotope ratio mass spectrometry and gas chromatography-combustion-isotope ratio mass spectrometry. The oil migration directions and distances were determined based on the plot of total dibenzofurans concentrations. Data were analyzed using descriptive statistics and linear regression analysis.
The TOC and GP of the source rocks ranged from 0.46 to 27.52 wt% (2.86 ± 1.10) and 0.27 to 199.12 mg/g (9.24 ± 5.70), respectively. These mean values exceeded the minimum 0.5 wt% TOC and 2.0 mg/g GP required for potential source rocks. The Tmax (295 to 446˚C), PI (0.05 to 0.61), vitrinite reflectance (0.23 to 0.50 %) and other maturity biomarker parameters indicated that the source rocks were at immature to early mature stage. The maceral analysis and source dependent parameters from biomarkers distributions indicated that the rocks were formed from type II/III kerogen. The relative concentrations of dibenzofuran, methyldibenzofurans (C1-dibenzofurans) and dimethyldibenzofurans (C2-dibenzofurans) in the source rock extracts ranged from 1.75 to 29.82 %, 27.60 to 40.67 % and 29.66 to 68.89 %, respectively. Benzo[b]naphthofurans were detected in high abundance in the rocks. The distribution and abundance of dibenzofurans and benzo[b]naphthofurans in the rock samples were not influenced by source and maturity. The concentrations (µg/g) of dibenzofuran, methyldibenzofurans,dimethyldibenzofurans and benzo[b]naphthofurans in the oil samples ranged from 1.06 to 136.71, 9.64 to 570.64, 61.50 to 1346.81 and 2.75 to 352.60, respectively. The total DBFs concentrations decreased from 2380.59 to 76.39 µg/g in the longest migrated oil. This decrease in concentration was due to migration induced fractionation effect. The migration distances estimated for the crude oil samples ranged from 1.0 to 64.0 km.
The abundance and distributions of dibenzofurans and benzo[b]naphthofurans in the oils from similar source facies and thermal maturities were found to be effective in determining the oil migration directions and distances in the Niger Delta basin.
