Optimizing Thermal Maturity Assessment in Unconventional Reservoirs: A Comparative Analysis of Different Maturity Estimation Methods

This study explores different methods of assessing the thermal maturity of source rocks in unconventional reservoirs, with a primary focus on establishing a robust workflow for accurately estimating thermal maturity in highly mature samples. Twenty-Nine source rock and seven flowback fluid samples were collected from 4 wells targeting a Lower Silurian-age Formation. All samples were analyzed using several analytical approaches including: geochemical, optical and fluid compositional analyses. Data generated include graptolite reflectance measurements, pyrolysis Tmax, aromatic hydrocarbon concentrations, and Pressure-Volume-Temperature (PVT) compositional data. To assess the applicability and limitations of each method in evaluating thermal maturity in samples at high maturity range (%VRE >1.2%), multiple published equations were applied to each dataset to derive thermal maturity values. Based on the calculated thermal maturity results, significant variation in values was observed depending on the adopted analytical approach. T-max based maturity estimates underestimate maturity levels in rock samples relative to other tested methods, leading to inaccurate representation of actual maturity levels in this area. This observation aligns with previous studies highlighting the suppression of Tmax due to the influence of fluids retention in this formation. Conversely, graptolite reflectance measurement conducted on rock samples revealed higher maturity values, with a difference of up to 0.3% in comparison to Tmax-derived estimates. For fluid samples, maturity values based on the slope factor of C3 – nC5 volatile light hydrocarbons agreed with fluid typing based on gas-to-oil ratio, thus they were utilized as a baseline to assess the variability from other methods. Interestingly, aromatic-based ratios demonstrated remarkable versatility, producing similar maturity values derived from both optical and fluid compositional data analyses, confirming the reliability and consistency of this method in multiple sample types. This study highlights both the utility and limitations of different methods for assessing thermal maturity in source rock extracts and generated fluids. Among the tested methods, aromatic hydrocarbons emerged as a reliable and versatile indicator of thermal maturity across all tested samples. Utilizing multiple methods can ultimately lead to accurate assessment of thermal maturity in shale source rocks, which is crucial in ensuring the success of unconventional exploration strategies and optimized production planning.