Geology topics
Source-linked reports with geographic coverage including Bakken Formation.
Using a geology-based assessment methodology, the U.S. Geological Survey estimated means of 220 million barrels of continuous oil and 1 trillion cubic feet of continuous gas in the Bakken Formation of northwest Montana.
This paper identifies the drivers of the phenomenal growth in productivity in hydraulically fractured horizontal oil wells producing from the middle member of the Bakken Formation in North Dakota. The data show a strong underlying spatial component and somewhat weaker temporal component. Drivers of the spatial component are favorable reservoir conditions. The temporal component of well productivity growth is driven by increasing the number of fracture treatments and by increasing the volume of proppant and injection fluids used on a per fracture treatment basis. Random Forest, a non-parametric modeling procedure often applied in the context of machine learning, is used to identify the relative importance of geologic and well-completion factors that have driven the growth in Bakken well productivity. The findings of this study suggest that a significant part of the well productivity increases during the period from 2010 to 2015 have been the result of improved well-site selection. For the more recent period, that is from 2015 through 2017, part of the improved well productivity has resulted from substantial increases in the proppant and injection fluids used per stage and per well.
Samples of Bakken Formation core from the Fleckton 1-20 well in Ward County, North Dakota, were analyzed using the Strontium Residual Salt Analysis (SrRSA) method to assess pore-water communication among the upper, middle, and lower sections of the unit by analyzing 87 Sr/ 86 Sr in pore-water salts leached from the core. Major and trace element analyses were also conducted on bulk-rock samples and leachates (Elemental Residual Salt Analysis or ERSA). The middle part of the Bakken Formation, a calcareous and dolomitic siltstone to fine sandstone, is encased between two black shales—the upper and lower sections of the formation. These units are informal and herein named the upper, middle, and lower Bakken members. Strontium isotope measurements and concentration of some solutes in leachates indicate that pore water in the black shale of the upper Bakken member has remained isolated from pore water in the middle Bakken member except for within a few feet immediately near the contact where elemental profiles indicate diffusive mixing. The SrRSA 87 Sr/ 86 Sr values from the middle Bakken member are consistent with produced water collected from 28 wells in Montana and North Dakota. In contrast to the similarity in 87 Sr/ 86 Sr values, ratios of concentrations such as K/Rb, Ca/Sr, Ca/Mg, and Na/Cl are slightly different between the pore-water leachates and produced water values. The differences in K/Rb and Ca/Sr are probably due to selective adsorption of Rb leading to larger K/Rb ratios and minor dissolution of carbonate minerals leading to larger Ca/Sr ratios in the leachates.
The level to which data are aggregated can impact analytical and predictive modeling results. In this short paper we discuss some of our findings regarding the impacts of data aggregation on estimating change points in the production profiles of horizontal hydraulically fractured Bakken oil wells. Change points occur when production transitions from one flow regime to another. Change point determination is important because it governs calculation of ultimate recovery from these and similar wells drilled in shale plays.
The quality and age of shallow groundwater in the Bakken Formation production area were characterized using data from 30 randomly distributed domestic wells screened in the upper Fort Union Formation. Comparison of inorganic and organic chemical concentrations to health based drinking-water standards, correlation analysis of concentrations with oil and gas well locations, and isotopic data give no indication that energy-development activities affected groundwater quality. It is important, however, to consider these results in the context of groundwater age. Most samples were recharged before the early 1950s and had 14C ages ranging from 30,000 years. Thus, domestic wells may not be as well suited for detecting contamination associated with recent surface spills as shallower wells screened near the water table. Old groundwater could be contaminated directly by recent subsurface leaks from imperfectly cemented oil and gas wells, but horizontal groundwater velocities calculated from 14C ages imply that the contaminants would still be less than 0.5 km from their source. For the wells sampled in this study, the median distance to the nearest oil and gas well was 4.6 km. Because of the slow velocities, a long-term commitment to groundwater monitoring in the upper Fort Union Formation is needed to assess the effects of energy development on groundwater quality. In conjunction with that effort, monitoring could be done closer to energy-development activities to increase the likelihood of early detection of groundwater contamination if it did occur.
No abstract available.