Methane potential of upper Fort Union formation sandstones, Campbell County, Wyoming
Coalbed methane has been produced from the coalbeds of the Upper Fort Union Formation in the Powder River Basin for many years. The sandstones associated with these coals have also proven to be gas-charged, but industry has done little with the sandstones thus far. The purpose of this study is to determine the paleoenvironrnent responsible for the deposition of the coal and sandstone bodies in an attempt to locate gas-charged sandstone bodies in the Tongue River Member of the Fort Union Formation. The 30-mile by 90-mile study area was chosen because of the presence of a decrease in thick coalbed accumulations in the western half of Campbell County, Wyoming. Gamma-ray wireline logs were used to construct 22 cross sections within the study area using IHS PETRA. These cross sections were used to co1Telate the study interval and the coalbeds, and to pick the 5 ft thick or greater sandstones within the interval. Thickness values of the total study interval and sandstones were used to construct total thickness, total sandstone thickness, and sandstone ratio isopach maps of the study area. In the northern section of the study area, the upper part of the study interval crops out; therefore, a second set of maps were constructed for the northern area. The resulting maps depict an increase in sandstone thicknesses and percentages that coincides with the decrease in thick coalbeds, as well as an overall I to 2° dip to the west. This increase in sandstone forms a north-south trending swell with several east-west limbs attached to the main trunk. The isopach maps were compared to previous depositional models for the Upper Fort Union Formation in the Powder River Basin. These models include a lacustrine model with westward-prograding deltas, a fluvially dominated model with raised bogs limiting lateral migration of the fluvial system, and a low-lying swamp model. The lacustrine model consists only of sandstones greater than 40 ft thick and does not display the north-south trending main trunk. The current evidence does not support the lacustrine model. The north-south trending main trunk in the current data supports the model for fluvially dominated deposition. The east-west trending limbs represent tributary systems. This model suggests a raised bog scenario for peat accumulation considering the basin's coalbeds are anomalously thick and low in sulfur and ash content. This scenario is supported by the current data, but it is not consistent throughout the study area. The swamp model hypothesized deposition to be from a low-lying swamp environment similar to that of the Okefenokee Swamp of Florida. Coal bed thickness may reach great thicknesses as peat accumulation keeps pace with basin subsidence. The current data support the swamp model for peat accumulation. The resulting model for deposition of this study consists of a low-lying swamp environment with a fluvial system running through the study area from south to north. First order meandering of the system is present in the isopach maps. Raised bogs may be present, but are restricted to the southern section of the study area. The Oedekoven gas field is producing gas from a sandstone body within the study area and selected interval. This 1 mi2 field is located on the updip meander of the northsouth trending main sandstone swell. Other locations with similar location characteristics have been selected for further study.
Coalbed methane--Powder River Basin (Wyo and Mont); Sandstone--Powder River Basin (Wyo and Mont); Gas fields--Powder River Basin (Wyo and Mont), Geology
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