A new resource assessment workflow to achieve gas risk and phase-specific source potential indices

Abstract

Accurate estimation of source rock potential has been a challenge, especially when a mineral matrix effect (MME) may influence the source rock potential estimation process. A recent study has proposed a new workflow based on custom organofacies classification to obtain a more accurate evaluation of lacustrine source rock potential. However, their workflow was not demonstrated on more commonly occurring marine source rocks. This study aims to better characterize marine source rocks, and additionally presents phase-specific Source Potential Index (SPI), SPIoil and SPIgas, and a new parameter called the Gas Risk Index (GRI). The methodology outlined in this study begins with an organofacies characterization based on both Rock-Eval pyrolysis and palynofacies analyses on samples from an offshore northwestern African well. Then, empirical laws are proposed to correct the MME to attain an improved source rock potential and calculate the new indices SPIoil, SPIgas, and GRI. The results demonstrate that most of the samples with HI values between 400 and 500 mg/gC, usually associated with a terrestrial-marine mixture (gas- and oil-prone), are actually composed of an altered marine organic matter (oil-prone) in the basin studied. It was observed that the MME lowered the S2 and HI values of samples with TOC up to 7 wt% in the study well, thereby underestimating the overall generative potential of the samples. This led to misidentification of organofacies based on lowered HI values, resulting in an inaccurate determination of liquid and gas prone nature of the source rock, which directly impacts the SPI values, increasing the risk of incorrect assessment. Therefore, the use of both palynofacies and pyrolysis data is recommended in differentiating terrestrial-marine mixtures from altered marine organic matter, and in correcting the MME to obtain reliable SPI values. This study presents a case for a new sample analysis workflow, which includes correction for the MME, refined organofacies classification, and determination of gas risk.