Diagenesis and petroleum potential of the Mowry shale in the Powder River Basin, Wyoming, The

Abstract

The Mowry Formation has long been considered an important source rock in the Rocky Mountain Region, and particularly in Wyoming’s Laramide Basins. However, it has barely been targeted as a self-sourcing reservoir since the development of modern drilling and completion technologies. Up until 2018, production results had been mixed, but recent interest has resulted in a 50% increase in the number of Mowry targeted wells. This study’s aim is to characterize the role diagenesis has played in the reservoir quality, and to evaluate the petroleum potential of the Mowry. The Mowry is an organic-rich siliceous mudstone with numerous interbedded altered volcanic ash layers. On the eastern side of the Western Interior Basin, the Mowry was deposited as a distal marine accumulation and is predominantly characterized by the evidence of reworked storm and current deposition, hemipelagic settling, and high productivity. The Mowry has experienced extensive diagenesis, particularly early diagenesis, which has had a significant influence on the reservoir quality. Key diagenetic processes include biogenic silica dissolution and precipitation, mineralization, compaction, illitization of smectite, organic matter maturation, and secondary porosity development. The relatively early stabilization and lithification of the matrix resulting from the transition of opal-A to opal-CT to microcrystalline quartz is a key diagenetic process that contributed significantly to the preservation of pore space. These preserved pore spaces accommodated the pre-oil bitumen, which likely formed a connected bitumen network. With increasing depth, mineral matrix porosity declined as a result of mechanical and chemical compaction while organic matter hosted pores developed and became the chief pore type deep in the basin. Overall, the Mowry Formation, in eight study wells, has an average TOC of 2.71 wt.%, which meets the minimum TOC threshold requirement of 2 wt.% for effective petroleum source rocks. The middle Mowry is more organically rich than the upper and lower Mowry and has an average TOC of 3.03 wt.%. Pyrolysis S1 is 33% higher and pyrolysis S2 is 42% higher in the middle Mowry relative to the upper and lower Mowry. Along the basin axis, the Mowry has reached the late oil maturity window with Tmax values measuring 458°C. Three hotspots in the Powder River Basin have yielded higher measured maturity values.