Subsurface Evaluation of Source Rock and Hydrocarbon Potential of the Anambra Basin, South Eastern Nigeria

Abstract

Exploratory activities carried out in the Anambra Basin have revealed that the basin has the potential of generating hydrocarbon. Subsurface core samples obtained from Enugu 1325 and 1331 wells within the Anambra Basin were utilized in this study with the aims of characterizing the organic matter as well as determining the maturity in order to deduce the hydrocarbon potential of the basin. The study was also aimed at determining the palaeo-depositional environment as well as highlighting the source input of the organic matter.
The analytical procedure for total organic matter and rock eval pyrolysis was achieved by the use of LECO 600 analyzer. Soluble organic matter (SOM) was determined by the use of Soxhlet Extractor while whole rock analysis and biomarker distributions were determined by the use of gas chromatography (GC) and gas chromatography/mass spectrometry (GC-MS) respectively.
The lithologic sequence of both wells consists of coals, shales and siltstones. The coals are black while the shales range from light to dark grey with some coal intercalation. The coals and shales are fissile while the siltstones with light grey colour are medium grained. Results of the Total Organic Carbon (TOC), Soluble Organic Matter (SOM) and Generic Potential (GP) ranged from 1.59 - 70.33wt%, 238.1 - 4095.2 ppm and 2.34 - 177.36 respectively. These imply that the source rocks are moderately to fairly rich in organic matter. Cross plots of hydrocarbon potential versus TOC, SOM against TOC indicated that the source rock is Type III and gas prone. Tmax value ranges from 426 - 435 degrees C and Bitumen ratio is from 22.4 - 106 which indicate low maturation level for the source rock. The ratios of C₂₉ hopane (beta)(alpha)/(alpha)(beta), C₃₀ (beta)(alpha)/(alpha)(beta), and 22S/22S+22R C₃₂ hopane ranged from 0.32 to 0.57, 0.20 to 0.59, and 0.49 to 0.56 respectively thus suggesting immature organic matter. Cross plot of hydrogen index (HI) versus Tmax, production index (PI) versus Tmax both suggest that the source rock is immature. Methyl Phenanthrene Index (MPI-1), Methyl Dibenzothiophene ratio (MDR) and calculated vitrinite reflectance (Rm), showed ranges of 0.14-0.76, 0.99-4.21, 0.62-0.82 respectively. These further indicate immature to marginally mature status for the sediments. Values of C₂₄ tetracyclic/C₂₄ tricyclic terpanes and the C₁₉/C₂₀ tricyclic terpane ratios, show respective ranges of 1.54-2.25 and 0.74-1.34 respectively, which are indicative of terrigenous organic matter. The dominance of C₂₉ over C₂₈ and C₂₇ further indicate higher terrigenous input. The abundance of 1,2,5 TMN(trimethyl naphthalene) suggests a significant land plant contribution to the organic matter. The Pr/Ph ratio values of 7.2 - 8.9 point to terrestrial organic input under oxic conditions. A cross plot of Ts/Ts+Tm versus dia/(dia + reg) C₂₇ steranes and high ratio of C₃₀/C₂₉Ts suggests suboxic depositional condition. The presence of C₂₇ to C₂₉ steranes and diasteranes indicates mixed sources (marine and terrigenous) with prospects to generate both oil and gas.
It can be deduced that the sediments were deposited in a suboxic, low Eh environment, and contain moderately to fairly rich organic matter with a substantial terrigenous input. The source rock has the potential to generate gas rather than oil given sufficient maturity.