One of the cited conditions that leads to the development and persistence of a low permeability gas accumulation (basin-centered gas) is that the rate of thermal gas generation exceeds the bleed-off or up-dip migration rate. It is thought that the supply of gas, relative to the rate of loss can lead to the creation of over-pressured cells.

Burial history modeling of the Jonah Field / Pinedale anticline area (NW Green River basin, Wyoming) suggest that the majority of gas generation for late Cretaceous source rocks (Mesaverde and Rock Springs equivalents) occurred between ~70 to 35 Ma (Figure 1 a-f). The main gas generation pulse begins at ~50 Ma following a rapid increase in sedimentation rate. Gas generation culminates approximately 5 Ma after maximum burial and continues for 5 to 10 million years after reaching maximum gas generation rate (Figures 1 b,c,e,f). The source rocks continue to generate at a low rate until the late Miocene.

Multiple lines of evidence point to a late Miocene (10-5 Ma) regional uplift in the NW Green River basin that resulted in the erosion of 3000 to 6000 feet of sediment. Providing that heat flow did not increase during this time frame, source rocks would have experienced a reduction of temperature of 50 to 65 deg F. This cooling, coupled with relatively high transformation ratio levels (at Rock Springs level in the deep basin, Figure 1f) causes hydrocarbon generation from the modeled type III kerogen to essentially cease. Therefore, the prevailing basin-centered gas model that requires generation to be greater than migration is not supported by the burial and thermal history modeling.

An additional cited condition to support a ‘basin-centered gas accumulation’ states that the generated gas has difficulty both in migrating and simultaneously displacing water from the system due to the extremely low permeability of that system. Basin models suggest that when hydrocarbon generation first begins, the reservoir unit immediately above or below a maturing source bed may have porosity approaching 17 to 18 percent (Figures 2 a & b). Although modeled reservoir porosity may vary significantly depending on the original mineralogy and diagenetic history of the reservoir, our models indicate that the Lance and other upper Cretaceous formations would have sufficient reservoir quality to allow gas migration and water displacement.

This presentation will focus on evaluating some of the possible causes of low permeability gas accumulations using basin modeling techniques and scenarios to test whether the accepted causative agents can effectively coincide to form and maintain the long-term persistence of these accumulations. From a basin modeling perspective, we will also review the current level of understanding of high uncertainty petroleum system elements such as; thermal history, source rock kinetics and expulsion.

Figure 1. Comparison of burial histories at Jonah vs. Pinedale and resulting gas generation timing.

Figure 2. a) SHB 13-27 – Mesaverde gas generation compared to compaction curves for modeled reservoirs; b) SHB 13-27 – Rock Springs gas generation compared to compaction curves for modeled reservoirs.

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