SOURCE ROCK EVALUATION OF TOTAL ORGANIC CARBON, MINERALOGY, AND ALIPHATIC HYDROCARBON BIOMARKERS (THE ANZA BASIN, KENYA)

Abstract

One of the essential steps in hydrocarbon exploration and exploitation is to understand the association of organic matter and minerals in petroleum reservoirs which is a vital component of the hydrocarbon-generation process. Understanding how and where hydrocarbons origi- nated, helps to comprehend and predict where they can be discovered, particularly in uncon- ventional reservoirs such as source rocks. The purpose is this study is to characterize the or- ganic matter content of seven source rocks collected from three wells (Chalbi-3, Sirius-1, and Ndovu-1) located in the Anza Basin in the north eastern part of Kenya. These were character- ized using: (i) elemental (CHN) analysis for organic matter components; and (ii) X-ray diffrac- tion (XRD), X-ray fluorescence (XRF), Fourier transform infrared spectroscopy (FTIR), scan- ning electron microscopy (SEM-EDX), thermogravimetric and differential scanning calorime- try (TG/DSC) for mineralogical characterization, and gas chromatography-mass spectrometry (GC-MS) analysis for saturated aliphatic biomarkers. This evaluation revealed, for all the wells and the depths studied, the source rocks have TOC values of 0.54%, 1.46%, 1.54%, and 1.99 % for Sirius-1, Chalbi-3, and Ndovu-1, respectively. The high quartz content coupled with a fair to good %TOC content, and low H/C values, have demonstrated the potential of the source rocks as being good and capable of oil and/or gas expulsions from the wells. High organic richness has been measured in each source rock sample. The XRF, XRD, and FTIR results revealed that the source rocks were comprised mainly of quartz, followed by silicate-clay min- erals and calcite minerals. While Thermogravimetric Analysis (TG/T∆G) and Differential Scanning Calorimetry (DSC)–under a controlled (O2/air) oxidizing atmosphere and at a heating rate of 10 °C min−1, showed there was a strong correlation between TG and DSC, and that for both processes, the source rocks underwent thermal decomposition in three steps. In addition, solvent-extractable organic matter investigation revealed a unimodal distribution of bi- omarkers: n-alkanes and isoprenoids (low pristane/ phytane ratios). Thus, the hydrocarbons in the source rocks were derived from algae and bacteria, which had been deposited under weakly anoxic and low oxic environmental conditions with least contributions from terrestrial organic matter sources. Hydrogen Indices are low (HI= H/C: 0.19–0.60 atomic ratios), indicating a predominance of mixed type III/IV (oil/gas-prone). The drilled well depths (1066-3100 m) demonstrated good reservoir rock properties with fair to good hydrocarbon generation potential (TOC values between 0.5–2%) with the existence of commercial hydrocarbon production.