Predicting Petroleum Quality in Unconventional Resource Assessments: New Kinetic Models and Workflow
Petroleum quality predictions using published reaction kinetics in combination with basin modeling have displayed poor matches in the past between modeled output and field observations. This mismatch is because existing kinetic schemes do not predict increasing API gravities with increasing maturity. This is a result from erroneous primary and secondary source rock cracking schemes combined with unsuitable adsorption models used. To overcome this issue predefined relationships between API gravity and source rock thermal maturity can be applied. Ideally, a source rock kinetic model 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. This relative mixing must be understood for primary generation within the source rock, for the effects of in‐source rock sorption, oil to‐gas cracking and finally for the expelled hydrocarbons. Five new kinetic datasets were developed, each representing a standard source rock type (II‐S, II, I, II‐III, III‐IV), which provide geologically reasonable API gravity trends and ranges derived from naturally observed averages. Each kinetic model uses two liquid pseudo‐components and two vapor pseudo‐ components to ensure API gravity and CGRs can be calculated. The relative ratios between the pseudo‐ components at full kerogen transformation are average ratios available from public and proprietary kinetic datasets. The primary generation follows published activation energies including minor shifts, which allow peak generation to occur at lower activation energies for the heavier liquid pseudo‐component, and at higher energies for the lighter pseudo‐component. This systematic shift of activation energies results in a constant change in API gravity as primary generation progresses. Different sorption models can be combined with the new kinetic models without compromising the API gravity predictive capacity. This provides greatest flexibility when assessing unconventional shale gas and shale oil plays and their potential, especially when viscosity is linked to API gravity. However, based on learnings from in‐house unconventional resource assessments the preferred sorption model is controlled by component concentration only. Desorption of gas is typically not a process, which is important for unconventional shale plays and thus the Langmuir approach is not standard in the new default kinetic models. However, the sorption container size needs to be quantified to consider the impact of in source rock alteration on the expelled GOR. The suggested concentration‐based sorption model ensures that the generated, sorbed and expelled petroleum quality is always in equilibrium and changes with maturation. The new API gravity kinetic models can be integrated into basin modeling software tools, which link rock properties with pressure prediction and hence provide a significant improvement for petroleum quality prediction not only for unconventional resource assessments but also for conventional systems.
AAPG Datapages/Search and Discovery Article #90349 © 2019 AAPG Hedberg Conference, The Evolution of Petroleum Systems Analysis: Changing of the Guard from Late Mature Experts to Peak Generating Staff, Houston, Texas, March 4-6, 2019