Douglas W. Waples

The art of maturity modeling; Part 2, Alternative models and sensitivity analysis

The sensitivity of exploration decisions to variations in several input parameters for maturity modeling was examined for the MITI Rumoi well, Hokkaido, Japan. Decisions were almost completely insensitive to uncertainties about formation age and erosional removal across some unconformities, but were more sensitive to changes in removal during unconformities which occurred near maximum paleotemperatures. Exploration decisions were not very sensitive to the choice of a particular kinetic model for hydrocarbon generation. Uncertainties in kerogen type and the kinetics of different kerogen types are more serious than differences among the various kinetic models. Results of modeling using the TTI method were unsatisfactory. Thermal history and timing and amount of hydrocarbon generation estimated or calculated using the TTI method were greatly different from those obtained using a purely kinetic model. We strongly recommend use of the kinetic R[o] method instead of the TTI method. If we had lacked measured R[o] data, subsurface temperature data, or both, our confidence in the modeling results would have been sharply reduced. Conceptual models for predicting heat flow and thermal conductivity are simply too weak at present to allow us to carry out highly meaningful modeling unless our input is constrained by measured data. Maturity modeling therefore requires the use of more, not fewer, measured temperature and maturity data. The use of sensitivity analysis in maturity modeling is very important for understanding the geologic system, for knowing what level of confidence to place on the results, and for determining what new types of data would be most necessary to improve our confidence. Sensitivity analysis can be carried out easily using a rapid, interactive maturity-modeling program.

A Simpler Kinetic Model of Vitrinite Reflectance

We have developed SIMPLE-Ro as a simpler but conceptually compatible alternative to existing kinetic models for vitrinite reflectance, such as VITRIMAT and EASY%Ro. Where calculations are done using the isothermal-segment approach, SIMPLE-Ro is much faster. Where the constant-heating-rate-segment approach is used, SIMPLE-Ro and EASY%Ro are of similar speed. SIMPLE-Ro is based on a single activation energy in each time step, rather than on a range. This activation energy changes as a function of Ro, thus simulating the maturity-dependent change in activation-energy distribution observed in nature. SIMPLE-Ro can be calibrated to any desired distribution of activation energies, or can be calibrated against measured Ro data independent of any other kinetic model.

Quantitative Evaluation of Lower Cretaceous Mannville Group as Source Rock for Alberta's Oil Sands

The Western Canada sedimentary basin hosts about 12 billion bbl of conventional oil in Devonian to Cretaceous reservoirs. Lower Cretaceous oil sands contain an additional 1.3 trillion bbl in place. The oil sands represent the biodegraded remnants of supergiant conventional oil deposits, the source for which has been thought to be mature rocks of the equivalent-age Mannville Group. This work shows, however, that the known Mannville rocks alone are incapable of generating the required volume of hydrocarbons. The volume of hydrocarbons generated in the Mannville beneath central Alberta was calculated by combining measured geochemical and geologic data with Lopatins method for thermal maturation. Original hydrocarbon-generative capacity of the Mannville rocks was calculated from geochemical analyses of immature samples. Using average values for total organic carbon (TOC) (1.3%) and Rock-Eval hydrogen index (100 mg HC/g TOC), maximum hydrocarbon generation per unit volume of source rock was calculated. The maturation model was then employed to estimate the extent to which maximum yield has actually been achieved. Total volume of source rock in the basin was obtained from isopach maps of Mannville shale. Multiplication of actual oil generation per unit volume by source rock volume gave a generated volume of 450 billion bbl. These calculated values are exceedingly optimistic, however, because they ignore inefficiencies in expulsion and migration. Inclusion of oil generated in Mannville-equivalent source rock in the Foothills belt would less than double this quantity, assuming that source potential there is similar to that of the central Alberta rocks. It is clear that the Mannville Group of the Western Canada sedimentary basin cannot be the major source of Albertas oil-sand hydrocarbons. Either the hydrocarbons were generated in as yet undocumented, very organic-rich Mannville-equivalent rocks in the Foothills, or they are derived from multiple sources throughout the basin.

Petroleum generation kinetics: Single versus multiple heating-ramp open-system pyrolysis: Discussion

Peters et al. (2015) criticize the one-run kinetics method that my colleagues and I developed and promoted (Waples et al., 2002, 2010; Waples and Nowaczewski, 2013), and further claim that source-rock kinetics derived by the standard multirun methods are superior. Specifically, they state that Arrhenius ( A ) factors determined mathematically from multirun kinetics are significantly more accurate than those obtained using other methods. They also claim that those A factors and the associated activation-energy ( E a) distributions are sufficiently accurate in an absolute sense to be used, with confidence and without any further quality control, in modeling hydrocarbon generation. I disagree strongly with those various statements. Any discussion of the kinetics of hydrocarbon generation must begin with an understanding that the parallel first-order kinetic description used by almost everyone is simply a convenient construct, and it is clearly not a mechanistic description of what happens in nature (e.g., Stainforth, 2009). Most researchers in this field believe that the reactions involved in hydrocarbon generation are mainly chain reactions rather than simple decompositions, that reaction schemes do not involve a group of discrete parallel processes, and that generation yields various intermediate products on the way to final products (e.g., Stainforth, 2009; Tang and Ma, 2009). We therefore seek a useful kinetic system, rather than a correct one, and we must be prepared to make compromises and simplifications to achieve this goal. That said, not all compromises and simplifications are equally valid or acceptable. The parallel first-order kinetic model has been remarkably successful for three decades in modeling the evolution of the S2 peak during Rock-Eval–type pyrolysis of the great majority of kerogens, although exceptions have been noted where the S2 peak is narrower than first-order kinetics can explain (e.g., Burnham et al., 1996; Stainforth, 2009). Because generation of the …

Mechanisms for generating overpressure in sedimentary basins: A reevaluation: Discussion

In response to a previous article by Osborne and Swarbrick (1997), Bitzer (1999) and Osborne and Swarbrick (1999) have briefly debated the importance of sediment compressibility (grain response) in controlling overpressure. In an article that appeared too late for consideration by those authors, Waples and Couples (1998) showed that compaction can be viewed as a sequence of steps, including (1) application of stress through sediment accumulation, (2) response of the grain framework to the applied stress, (3) …

Was the Mannville Group the Source for Alberta's Heavy Oils?: ABSTRACT

The Western Canadian basin hosts about 12 billion bbl of conventional oil in Devonian to Cretaceous reservoirs. Lower Cretaceous heavy-oil sands contain 1,300 to 2,600 billion bbl in place. They represent the biodegraded remnants of supergiant conventional deposits, the source for which has been thought to be mature rocks of the equivalent-age Mannville Group. This work shows, however, that the Mannville rocks alone are incapable of generating the required volume of hydrocarbons. Volume of hydrocarbons generated in the Mannville under central Alberta was calculated by combining measured geochemical and geologic data with a model (modified from Lopatins method) for thermal maturation. Original hydrocarbon generative capacity of the Mannville rocks was calculated from geochemical analyses of immature samples. Using average values for TOC (1.3%) and Rock-Eval Hydrogen Index (100 mg HC/g TOC), maximum hydrocarbon generation per unit volume of source rock was calculated. The maturation model was then employed to estimate the extent to which maximum yield has been achieved. Total volume of source rock in the basin was obtained from isopachs of Mannville shale. Multiplication of actual oil generation per unit volume by source rock volume gave a generated volume of 450 billion bbl. Inclusion of oil generated in the Foothills belt would less than double this number. These calculated values are exceedingly optimistic, however, because they ignore inefficiencies in expulsion and migration. It is therefore clear that the Mannville Group cannot be the major source of the heavy oils. Dominant contributions probably come from Paleozoic and other Mesozoic rocks. End_of_Article - Last_Page 610------------