Tricyclic Terpane Geochemistry Of Source Rocks From The Northwestern And Central Niger Delta
Abstract
This study investigated the use of tricyclic terpanes in geochemical characterization of source rocks from two oil wells in the northwestern and central Niger Delta. The source rocks and oils from the central Niger Delta were also correlated. This was with a view to determining the source rock potential, type, depositional environment, and thermal maturity of the organic matter in the source rock.
Forty-three shale samples were selected from a suite of ditch cuttings from two oil wells in the northwestern and central Niger Delta. These samples were pulverized and Soxhlet extracted for 24 hrsusing dichloromethane as solvent. The soluble organic matter(SOM) was separated into saturates, aromatics and NSO fractions using open column liquid chromatography. The stationary phase used was activated silica gel while the eluents were n-hexane, toluene and methanol. The saturated hydrocarbon fraction was analysed for biomarkers using gas chromatography-mass spectrometry (GC-MS).
The average concentration of SOM and the total hydrocarbons for well X source rock samples were 3065.83 ppm and 2029.17 ppm and that for well Y source rock samples were 8365.26 ppm and 3140.53 ppm, respectively.The values observed in both cases exceeded the minimum 500 ppm and 300 ppm, respectively required for a potential source rock. The source rock samples had very good to excellent potential to generate petroleum. The tricyclic terpane source parameters showed that well X samples consisted of mixed marine/terrestrial organic matter while well Y samples consisted of organic matter largely from terrestrial origin. This deduction was evidenced in the abundance of hopanes, C27-C29 steranes distribution, isoprenoid/n-alkane ratios and cross plot, and hopane/sterane ratio observed in shale samples from both wells. The average Pr/Ph ratio of 1.32 and 1.79 for shale samples from well X and well Y, respectively, suggested that the organic matter present in samples from both wells were deposited under suboxic conditions with most of the samples from well Y showing more oxidizing depositional conditions than those from well X. The results of sterane and hopane biomarker maturity parameters indicated that the thermal maturity status of organic matter were at onset of oil generation and main oil window. Most of the samples from well X were in the main oil window and were thermally more matured than those from well Y. The results of tricyclic terpane maturity parameters indicated low thermal maturity status for the organic matter in samples from both wells with samples from well X thermally more maturedthan those from well Y. The oils and shale samples from well Y were comparable at molecular level but showed an overall poor geochemical correlation.
The study concluded that the shale samples from well X were from mixed source organic matter deposited under suboxic conditions with thermal maturity in the main oil window, while those from well Y were derived largely from terrestrial source deposited under suboxic but more oxidizing influence compared with depositional conditions for well X.
Description
118p
Keywords
tricyclic, terpanes, geochemical, characterization, northwestern