Thermal maturity is an important parameter for shale play evaluation. It impacts many reservoir properties including kerogen composition, kerogen density, bitumen content, the type of producible hydrocarbon, porosity, pore-size distribution, water saturation, and clay type. Accounting for thermal maturity is of prime importance while estimating formation volumes in place, because kerogen and hydrocarbon properties (density, hydrogen and carbon weight fractions) required for accurate volumetric estimates depend on it. Presently, analysts fix those properties based on a priori knowledge and experience. Maps of play maturity exist, but they are approximate and usually do not encompass vertical variations. In plays with thick organic-rich intervals, like Vaca Muerta, Argentina, the resulting accuracy of volumetric analysis is questionable.
Thermal maturity is usually estimated by direct inspection of organic matter (vitrinite reflectance) or by pyrolysis of rock samples. It can also be inferred from actual hydrocarbon production. Several authors already recognized observable patterns on logs and proposed ad hoc methods to estimate maturity. A new interpretation method is proposed in this contribution. It is an evolution of a volumetric solver such as ELAN where kerogen and hydrocarbon properties are not fixed, but are inverted for at each depth, under certain constraints. Natural variability of these properties, both vertically and laterally, can then be captured and a more accurate volumetric estimation is achieved. Several innovations enable this method, including: a downhole TOC measurement, a new hydrogen index measurement, and—most importantly—the development of bulk kerogen and hydrocarbon property models that provide quantitative relationships between thermal maturity, density, hydrogen and carbon weight fractions of these components, among others. The property models supply additional constraints for the volumetric solver, such that unknown parameters (volumes and maturity indices) can be fully determined from a set of downhole logs.
Application of the new framework is illustrated in wells of variable thermal maturity in the Vaca Muerta formation, Argentina. A quick-look technique that allows an approximate estimation of fluid type is first presented. It highlights the inherent fluid type information content of some particular logs. The presentation of the full inversion results follows, providing a quantitative evaluation of formation component volumes together with kerogen and light hydrocarbon properties.