Stephanie B. Gaswirth and Kristen R. Mara

Transcription

1 U.S. Geological Survey Assessment of Undiscovered Resources in the Bakken and Three Forks Formations, Williston Basin, North Dakota, Montana, and South Dakota, 2013 Stephanie B. Gaswirth and Kristen R. Mara This talk on the reassessment of the Bakken petroleum resource was presented to RMAG on August 7th. Key objectives, as discussed by Ms Gaswirth, were to explain why an assessment was performed only 5 years after the assessment in 2008 and also to describe the methodologies that were employed. The present assessment began in fall At a meeting in Bismark in April 2011, legislators and companies working in the Williston concluded that another analysis of the Bakken and also the ThreeForks Formation was justified due to new information and drilling that had occurred since Some background information puts the reassessment in context. According to the Energy Information Administration, a part of DOE, the US consumes 7 BBO/yr. Based on DOE data, as of April 2013, production in the Williston Basin was over 3 BBO from all formations, of which 673 MBO was produced from the Bakken. The Three Forks production was 50 MBO, composing approximately 22 percent of the historical production from the Williston Basin. Considering production from the past 4 or 5 years, these two formations have accounted for well over 80 percent of the production in the basin. As to why to assess so soon after the last evaluation in 2008, the Three Forks was not included in the previous study and there has been a substantial amount of drilling in this formation since Also, there is much more drilling information from both formations since There is additional well production history to assist in calculating estimated ultimate recovery (EUR) and new completion techniques that are changing the ways that these wells are produced. For the latest assessment, there was more input from industry, although industry may not agree with the results, as well as from state agencies from North Dakota and Montana and also the Colorado School of Mines Bakken consortium. There have been a number of estimates of Bakken oil over the past decades. In 1974 the estimate was 10 BBO. Continental in 2011 calculated 24 BB of ultimate recoverable oil. In 2008 the USGS estimate of technically recoverable undiscovered resource was 3.65 BBO just from the Bakken. In 2008 (the cut off for data was 2007) there was substantial amount of development in the Elm Coulee area discovered in 2001; production along the Nesson anticline; the Antelope Field; the Parshall field just had six or seven wells completed prior to the last assessment; and there was some production from the Bakken Fairway area. About 150 MBO had been produced from the Bakken according to USGS. Fast forward to the data that were available for the 2013 assessment and there has been extensive development of the central basin; extensive production in Divide County; Burke County; Parshall and Sanish fields have been very filled in at this point; and the Pronghorn play has come on line. USGS estimates 673 MBO have been produced from the Bakken and 46 MBO from the Three Forks at the time of the present assessment. Very little had been produced from the Three Forks at the time of the 2008 Assessment. 1

2 How does USGS assess technically recoverable undiscovered resources? USGS is mandated by Congress to assess the resources of Priority Basins. Two different types of accumulations, conventional and continuous, are examined individually as they present differently geologically. Conventional resources are characterized as having migrated away from the source, have structural or stratigraphic traps, and are produced generally from vertical wells. A continuous resource tends to lie within the source rock, and/or has migrated only very limited distances from the source rock; there is generally no water drive; the reservoirs are tight with low porosity and low permeability. The Bakken and Three Forks are classic examples of a continuous resource. Most of the assessment units were analyzed by USGS using continuous resource methodology. For continuous resources USGS looks at features such as the assessment unit area (AUA), defined by unique geologic characteristics as well as production features; an area within the AUA known as a sweet spot, delineated by optimal geology and optimal production for various geologic reasons and a non sweet spot outside of the sweet spot; cell sizes in which can be fit a non interfering horizontal well; and the number of cells that can fit within a given area in a sweet spot or non sweet spot. USGS also calculates estimated ultimate recovery (EUR) for all wells where there is access to the data. All of this information feeds into the methodology. The stratigraphy of the Bakken petroleum system includes the lower shale member source rock; the middle Bakken member, which is a reservoir for oil sourced by the upper and lower shale members; the upper shale member source rock; and the lower part of the Mississippian Lodgepole Formation. The Sanish sand was renamed the Pronghorn member in a 2011 paper by Julie LeFevre and others, and USGS used that terminology for the purpose of this assessment. The Pronghorn member of the Bakken is the first transgression of the Bakken Formation. Underlying the Pronghorn, where it is present, is the Three Forks Formation. It also sits laterally adjacent to the Bakken where the Bakken Formation subcrops. Therefore the Three Forks has a greater extent in the majority of the assessment area. To summarize, the source rocks for the total petroleum system are the upper and lower Bakken shales. The reservoir rocks conventional in this total petroleum system are the Lodgepole Formation, which was assessed in 2008 as part of the Williston Basin Province assessment, and the Bakken middle member outside of the continuous area, where it is producing conventionally in Canada and in some of the fringe areas of North Dakota and Montana. The continuous reservoirs include the Bakken middle member within the continuous area, the Pronghorn member of the Bakken, as well as the Three Forks Formation. In incorporating the Three Forks in the present assessment, USGS adjusted the total petroleum system (TPS) boundary from the 2008 assessment. The TPS outline is defined by the distribution of fluids in the petroleum system. The upper and lower shales, which are the source rock for the system, have generated oil that has stayed close to the source or migrated out into reservoirs, and a TPS boundary was drawn that covers all of the area where the fluids may be distributed. The 2008 TPS, which did not include the Three Forks, was based on the upper Bakken shale, which was the most laterally extensive unit, and a bit of the Lodgepole Formation. The 2013 TPS boundary has extended farther to include the Three Forks. The other new piece of data that was heavily used in the 2013 assessment was updated thermal maturation data and this work was performed in the geochemistry labs at USGS. A series of kerogen 2

3 isolation experiments were performed using samples collected both at the USGS and in North Dakota from a large number of cores from the upper Bakken. It was determined that instead of the 400 hydrogen index number that was used in the 2008 assessment, a hydrogen index (HI) of 450 was more appropriate to define the area of the Bakken that is mature for oil. This determination affected where USGS placed some of the boundaries for the assessment units. USGS also defined sweet spot in each of the assessment units based on factors such as thermal maturation, structural enhancements, and other characteristics. An area that was adjusted quite a bit was the northeast corner of the eastern transitional continuous oil assessment unit. There has been some drilling up in Burke County that did not exist when USGS assessed in 2008, and this was incorporated using both resistivity information as well as HI information. EUR curves were constructed by Troy Cooke, now with DOE. EURs vary among AUs. The eastern transitional oil AU, which contains the Parshall field and part of the Sanish field, has a mean EUR of 363. The Northwest transitional oil AU had the lowest mean EUR, based on well production and extrapolation of the production curves. The USGS website has an open file report called Input Data Form, and in those forms can be found all of the information for every AU that fed into this study. One of the variables is the EUR for each of the AUs, including the ranges and F5 F95 for these data. For the Bakken Formation, each of the AUs has a mean volume in billions of barrels of technically recoverable oil. The largest AU is the Nesson Little Knife continuous oil assessment unit. A hypothetical middle Bakken conventional AU was assessed that sits on the fringe of the continuous oil AUs. USGS totaled the resource for the assessment units to be 3.65 BBO and put a range on that of a probability F95 to F5. One of the questions that has arisen is that this is the same number as in 2008, but one thing to take into account is the fact that over 500 MBO have been produced since Therefore, it can be said that the estimated total has actually increased. The Three Forks Formation can be classified in different ways. Some classifications from North Dakota differentiate the Three Forks into units, from the lowermost Three Forks unit 1 up through 6, with the Pronghorn or Sanish Sand lying on top of the upper part of the Three Forks. Some companies have referred to the breaks as benches, with the upper part of the Three Forks considered the first bench, the second bench as the middle part of the Three Forks, and the third and fourth in the lower part of the Formation. USGS used a classification that divided the member into upper, middle and lower Three Forks. The entire Three Forks Formation was assessed. The Pronghorn unit is stratigraphically and structurally a Bakken member. However, it was assessed with the Three Forks formation for the following reasons: the Pronghorn sits below the lower Bakken shale, which acts as a barrier, i.e., when a horizontal well is placed into the Pronghorn or upper Three Forks it is believed that the lower Bakken acts as a barrier to production and interference into the middle Bakken. The other reason is that companies were providing information that when a well is placed into the upper Three Forks and fracked, the produced oil may be from the overlying Pronghorn member. For those two reasons the Pronghorn and the Three Forks were lumped together for assessment. However, it is recognized as being part of the Bakken Formation. 3

4 The Pronghorn member is somewhat similar to the Middle Bakken in terms of lithology: dolomitic siltstones, sandstones and shales. It is the first transgression of the Bakken Formation and it is sourced from the overlying shales of the Bakken Formation. Isopachs from Julie LeFevre show the area to the north as the distal facies of the Pronghorn with the proximal facies to the south; they overlap in some of the areas that are being drilled agressively. The Three Forks is lithologically quite distinct from the Bakken Formation in that it is composed of dolomite, silty dolomite, dolomitic limestone, and siltstone, and deposited in subtidal and intertidal environments. Core samples characteristically show an interbedded tan pink dolomitic mudstone that is interbedded with green mudstone laminae. In the lower part of the Three Forks anhydrite becomes more pronounced and the core looks very oxidized. For the assessment, the cut off for data was at the end of The production in the Three Forks was primarily from the upper bench. There were a few wells that were beginning to be completed in the second bench; Continental had completed a test in the third bench and testing was in progress in the fourth bench. Until additional wells are drilled, it will not be known if there will be interference between benches if multiple laterals are installed. USGS will monitor the situation and is not opposed to reassessing, perhaps in another 4 years. Two assessment units (AU)for the Three Forks Formation were defined based on the formation extent of the Three Forks and also the Upper Bakken shale, the uppermost source rock for the Three Forks; the thermal maturation of the Upper Bakken Shale; and structural enhancements and sweet spot areas. The first AU is conventional and it is hypothetical because there is no production or drilling in this area. The USGS considered it conventional because it sits outside of the area in proximity to the source rocks, the upper and lower Bakken shales. There is also the Three Forks continuous oil assessment unit, which is defined by the extent of the upper Bakken shale. The sweet spot area mainly sits in this area of the Nesson and Little Knife anticlines: there is structural enhancement, high thermal maturity of the Bakken shales, and to date there has been drilling and a substantial amount of production in the Three Forks. The Three Forks EUR curve is comparable to the Bakken EUR if all of the Bakken AUs were taken into account. For the Three Forks Formation the major portion of the resource is believed to be within the continuous AU, with a minor amount in the conventional AU. The technically recoverable undiscovered oil resource is estimated to be 3.7 BBO. In response to a question from South Dakota, very little Three Forks resource was allocated to that state as current data do not support that reservoirs have been created. The source would also be an issue as is seems that most of the oil generated from the Bakken shale either remains in the area or migrates to the north. As a result South Dakota was relegated very little resource in this assessment. In summary, there are six continuous oil resources, five from the Bakken and one from the Three Forks Formation. The technically recoverable undiscovered oil resource was estimated to be a mean 3.65 BBO for the Bakken and 3.7 BBO for the Three Forks, for a total of 7.4 BBO. The F95 is 4.4 MB and the F5 is 4

5 11.4. USGS also assessed associated gas and natural gas liquids. This makes the Bakken and Three Forks the largest continuous oil accumulation assessed by USGS in the US to date. The full report is available from the USGS Website. 5