Abstract

The Muddy Sandstone in the Powder River Basin (PRB), northeast Wyoming, is a promising reservoir for CO2 sequestration because: (1) existing wells for hydrocarbon production can be used for CO2 injection when a field is depleted; and (2) data are available to assess the ability and capacity to trap CO2. Here, I provide new data and compile published results to: (1) characterize four oil and gas fields (the Amos Draw, Kitty, Hilight, and Sand Dunes fields) in the PRB with respect to lithofacies, sedimentary environment, sandstone composition, sand-body geometry, and porosity and permeability; and (2) assess the controls on reservoir heterogeneity and the CO2 sequestration potential.

Five lithofacies are recognized based on core description and log responses. They are interpreted as offshore, lower–middle wave-dominated shoreface, weathering zone, fluvial incised-valley, and tide-influenced estuarine depositional environments. The Muddy Sandstone contains predominantly quartz, with total quartz higher than 70 percent of the total framework grains. The percentage of feldspar is generally less than 5 percent, except for the Rozet Member in the Amos Draw Field, which is up to 22 percent. Sandstone petrographic examination also shows that the Muddy Sandstone can be divided into four groups based on the relative abundance of pore space, carbonate cement, and matrix. Sandstone with high porosity up to 23 percent is found in the shoreface lithofacies in the Amos Draw and Hilight fields and is also found in the estuarine lithofacies in the Kitty Field. The incised-valley lithofacies is of particularly low porosity due to high matrix content and carbonate cementation. The measured porosity in the sandstone varies between 1 percent and 23 percent, and the permeability is generally less than 10 millidarcys (mD). The variation of porosity is consistent with the observation in thin sections. XRD results show that the pore-filling clay minerals include kaolinite, chlorite, illite, and smectite. Core and well log correlation show that sandstone formed in lower–middle shoreface environments is laterally extensive and of uniform thickness, whereas sandstone of fluvial and estuarine origins is more variable in lateral extent and thickness. Based on examination of lithofacies, sandstone geometry, and thin section petrography, I suggest that the best reservoir interval for CO2 sequestration in the Amos Draw Field is the lower Rozet Member, in the Sand Dunes and Hilight fields is the Springen Ranch Member, and in the Kitty Field is the Ute Member. Variables examined in this study provide important inputs for calculating CO2 capacity potential and predicting chemical reactivity after CO2 injection.

Reservoir quality of the Muddy Sandstone is highly heterogeneous, and the complexity may be attributed to a combination of depositional environment, history of relative sea-level change during deposition, and type and extent of diagenetic alteration. The Muddy Sandstone, along with the overlying Mowry Shale, represents one third-order depositional sequence. Diagenetic processes include feldspar and lithic dissolution, secondary clay formation, quartz overgrowth, and four stages of carbonate cementation, which are early dolomite overgrowth, secondary calcite filling, dolomite replacement, and spotty siderite cementation. Carbonate cementation is interpreted as early diagenetic products formed during marine transgressions.

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