Predicting API in Basin Modeling: New Kinetic Models

Abstract

Predicting the API gravity of an oil using basin modeling source rock reaction kinetics displayed poor matches between modelled predictions and observed data (Baur et al., 2011). Poor API gravity predictions result from erroneous primary and secondary source rock cracking schemes combined with unsuitable adsorption models used within the same kinetic framework. Previous reaction kinetics research has focused primarily on GOR and fluid phase predictions. Many publicly available kinetics do not predict increasing API gravity trends with increasing maturity, but instead predict no or inverse correlations. When bulk kinetics are used in basin modeling, predefined relationships between API gravity and source rock thermal maturity can be applied to predict API gravity. Ideally, a source rock kinetic uses at least two oil components of different densities, which are generated and expelled from the source rock, so that the API gravity prediction is a consequence of the relative mixing. Tang (2011) offers multi-component Type I, II and III source rock kinetics, which correctly predict API gravity trends, but these kinetics cannot be adjusted to match specific source rock characteristics due to very complex secondary cracking schemes.

Five new kinetics datasets were developed, each one representing a standard source rock type (I, II, IIS, III and IV). Each kinetic uses two oil pseudo-components and two gas pseudo-components. The ratio between the pseudo-components at 100% transformation ratio represents average ratios from public and proprietary kinetics data. The oil and gas pseudo-components are based on the Gaussian distributions of their activation energies. Peak generation occurs at lower activation energies for the heavier oil pseudo-component, and at higher energies for the lighter oil pseudo-component. This systematic shift of activation energies allows a constant change in the API gravity predicted for primary generated oil. The default API gravity ranges for the five kinetics represent observed averages, but can be adjusted by changing the molecular weight of the two oil pseudo-components. The kinetics have been successfully applied to case studies with complex, vertically stacked API gravity distributions. An adsorption scheme is applied where the adsorbed and expelled hydrocarbons have the same API gravities. The total amount of adsorbed hydrocarbons can be independently controlled, which makes the kinetics ideal for unconventional resource assessments.

AAPG Datapages/Search and Discovery Article #90323 ©2018 AAPG Annual Convention and Exhibition, Salt Lake City, Utah, May 20-23, 2018