The Bakken Formation is a typical tight oil reservoir and oil production formation in the world. Pore structure is one of the key factors that determine the accumulation and production of the hydrocarbon. In order to study the pore structures and main controlling factors of the Bakken Formation, 12 samples were selected from the Bakken Formation and conducted on a set of experiments including X-ray diffraction mineral analysis (XRD), total organic carbon (TOC), vitrinite reflectance (Ro), and low-temperature nitrogen adsorption experiments. Results showed that the average TOC and Ro of Upper and Lower Bakken shale is 10.72 wt% and 0.86%, respectively.
Yuming Liu1, Bo Shen2, Zhiqiang Yang3 and Peiqiang Zhao4
1State Key Laboratory of Petroleum Resources and Prospecting, China University of Petroleum, Beijing, China. 2Geophysics and Oil Resource Institute, Yangtze University, Wuhan, China. 3Exploration & Development Research Institute of Liaohe Oilfield Company, Petrochina, Panjin, China. 4Institute of Geophysics and Geomatics, China University of Geosciences, Wuhan, China.
Received: 20 September 2018 / Accepted: 23 October 2018
The Bakken Formation develops micropores, mesopores, and macropores. However, the Upper and Lower Bakken shale are dominated by micropores, while the Middle Bakken tight reservoir is dominated by mesopores. The total pore volume and specific surface area of the Middle Bakken are significantly higher than those of the Upper and Lower Bakken, indicating that Middle Bakken is more conducive to the storage of oil and gas. Through analysis, the main controlling factors for the pore structure of the Upper and Lower Bakken shale are TOC and maturity, while those for Middle Bakken are clay and quartz contents.
The unconventional reservoirs such as shale gas and tight oil have been paid more attention since the production of conventional oil and gas decreased [1,2,3]. It is difficult to obtain economic oil flows without horizontal well drilling and fracturing technology, as these unconventional reservoirs are impermeable or extremely low permeable [4,5]. In unconventional reservoirs, the development may be more important than the exploration. However, the exploration of unconventional resources cannot be neglected.
A series of geochemical and petrophysical properties are required to characterize and evaluate [5,6]. Among these parameters, pore structures are important factors affecting the fluid transport both in the hydrocarbon accumulation and production in porous media and are also key parameters for reservoir grading and productivity evaluation [7,8,9,10]. Therefore, it is necessary to study the pore structure characteristics such as pore size distribution and pore types, and the controlling factors of the unconventional reservoirs.
According to pore sizes, the pores can be classified into three types: Micropore (<2 nm), mesopore (2–50 nm), and macropore (>50 nm) . Compared with the conventional reservoirs, the unconventional shale oil and gas reservoirs always develop more micropores and mesopores, and show more complex pore systems with strong heterogeneity . Thus, it is necessary to utilize proper methods to characterize the pore structure. A variety of advanced experimental methods can be used to analyze the pore structure of porous media. The pore types and sizes can be qualitatively observed by CT scan , scanning electron microscope (SEM) , field emission scanning electron microscope (FESEM) , and transmission electron microscope (TEM) .
Meanwhile, quantitative characterization methods include high pressure mercury injection (PMI) , constant rate mercury injection (RMI) , nuclear magnetic resonance (NMR) [18,19], low-temperature N2 and CO2 adsorption [20,21], small-angle and ultra-small-angle neutron scattering (SANS and USANS) , etc. Each method has its own advantages and disadvantages. For example, it is difficult for qualitative methods providing pore size distributions. NMR can capture almost of all sizes of pores but it requires other quantitative methods (PMI) to scale . Low-temperature N2 adsorption (LTNA) is suitable for capturing the pores with a size of approximately 1 to 150 nm. Hence, it is suitable for obtaining nano-scale pore structure parameters.
In recent years, a growing number of studies have applied low-pressure N2 adsorption to explore the pore structure characteristics of unconventional shale and tight sandstone, such as pore volume, specific area, and pore size distributions [23,24]. Kuila and Prasad  investigated the specific surface area and pore-size distributions in shales with a nitrogen gas-adsorption technique. Wang et al.  studied the pore structure of shale gas of Longmaxi Formation, Sichuan Basin, China using LTNA data. Su et al.  combined the LTNA and low-temperature CO2 adsorption method to characterize the pore structure of shale oil reservoirs in the Zhanhua Sag, Bohai Bay Basin, China.
The Bakken formation is a typical tight oil reservoir which underlies parts of Northern USA and Southern Canada. This formation is divided into three members: The Upper Bakken, the Middle Bakken, and the Lower Bakken. The previous geological studies include petroleum source rocks , systemic petroleum geology , lithofacies and paleoenvironments , diagenesis and fracture , and petrophysics properties [32,33].
Studies on the pore structure of the Bakken Formation have been reported. Liu and Ostadhassan  characterized the microstructures of Upper/Lower Bakken shales with the aid of SEM images and observed that micropores developed extensively in those shale samples. Li et al.  measured the permeability of Middle Bakken samples including Kinkenberg permeability and PMI-based permeability.
Kinkenberg permeability is commonly higher than PMI-based permeability, indicating smaller pores cannot be detected by the PMI method. Saidian and Prasad  reported the pore size distribution of Middle Bakken and Three Forks formations provided by PMI and LTNA methods. Pore size distributions are affected by clay content. The greater the clay content, the higher the amplitude of the pore size distribution. Liu et al.  investigated the fractal and multifractal characteristics of pore-throats of Upper/Lower Bakken shale using the PMI method. Liu et al.  reported the pore structure and fractal characteristics of the Bakken formation in North Dakota, USA.
It was observed that the pore structure of the Middle Bakken and the Upper/Lower Bakken are significantly different. However, the differences in the pore structure of the three Bakken members and their main controlling factors are not thoroughly investigated.
In this paper, 12 samples were derived from the Bakken Formation for experimental tests. The X-ray diffraction (XRD), total organic carbon (TOC) analysis, vitrinite reflectance (Ro) and low-temperature N2 adsorption experiments were conducted on these samples. Pore structure parameters, such as pore morphology, specific surface area, and pore size distribution, were then analyzed based on low-pressure N2 adsorption curves. The controlling factors of pore structures were determined by analyzing the correlations of the pore structures with the mineral composition, TOC, and thermal maturity.
Samples and Methods
Geological Background and Samples
The Williston Basin is located in the Northern United States and in Southern Canada with an area of 40,000 × 104 m2, see Figure 1 . This basin occupies portions of North Dakota, South Dakota, Montana USA, and Alberta, Saskatchewan, and Manitoba Canada [38,39]. It was deposited on the Superior and Wyoming Craton, and Trans-Hudson orogenic belt from the Cambrian to Carboniferous (mainly Mississippian) system.
Figure 1. Location of the Williston Basin and study area.
Subsidence and basin filling were most intense during the Ordovician, Silurian, and Devonian Periods, when thick accumulations of limestone and dolomite, with lesser thicknesses of sandstones, siltstones, shales, and evaporites were laid down . The Bakken Formation overlies the Upper Devonian Three Forks Formation and underlies the Lower Mississippian Lodgepole Formation. However, the Bakken Petroleum System (BPS) included the Bakken, lower Lodgepole and upper Three Forks formations .
As mentioned in the introduction to this paper, the Bakken Formation in the Williston Basin is divided into three different members: The Upper, Middle, and Lower Bakken, see Figure 2 . The lithology of Upper and Lower Bakken members mainly consists of dark-gray to brownish-black to black, fissile, slightly calcareous, organic-rich shale, which was deposited in an offshore marine environment during periods of sea-level rise .
Figure 2. Basin schematic diagram and the typical log curves (modified after Liu et al. ).
They serve as the main source rocks and seal rocks and have a thickness of about 8 m and 13 m, respectively. The total organic carbon (TOC) content of the Upper and Lower Bakken is between 12% and 36% with an average of 11.33% . The Middle Bakken member consists of the products of the continental shelf and the shallow foreshore environment. The lithology of the middle member is highly variable and consists of a light-gray to medium-dark-gray, interbedded sequence of siltstones and sandstones with lesser amounts of shale, dolostones, and limestones rich in silt, sand, and oolites [31,39].
The oil and gas of the Bakken Formation are mainly produced from the middle member with a thickness of about 15 m. It is easy to identify the three members using well logs. For the black shale, the gamma ray (GR) log curve shows a large value, generally greater than 200 API, while the resistivity logs are generally higher than 100 Ω·m. For the Middle Bakken, the GR log values are very small and resistivity log readings are lower than those in Upper and Lower Bakken shale.
The study area is located in North Dakota, near the center of the Williston Basin. In this part of North Dakota, the Bakken Formation reaches its maximum thickness of approximately 46 m, which is conducive to our research. In this study, we respectively chose four samples from the Upper, Middle, and Lower Bakken members (a total of 12 samples) for pore structure and main controlling factors. A series of experiments including: (1) Low-pressure N2 adsorption; (2) X-ray diffraction (XRD); (3) total organic carbon (TOC) analysis; and (4) vitrinite reflectance (Ro) were conducted.
We used the Dmax-2500 X-ray diffraction analyzer for the X-ray diffraction (XRD) experiments following the Chinese national standard SY/T5163-2010 (SY/T5163-2010). The Cornerstone™ carbon-sulfur analyzer was used for the TOC analysis and the MPV-SP microphotometer was used for the vitrinite reflectance (Ro) measurement under the condition of 22 °C and 35% humidity.
We used the JWBK-200C surface area analyzer for the low-pressure N2 adsorption experiments following the Chinese national standard GB/T21652-2008. Prior to the adsorption measurement, approximately 3 g of 40 to 80 mesh samples were first dried under vacuum for 12 h at high temperature (110 °C) to remove bound water and residual volatile compounds. The nitrogen adsorption was performed with a surface area and pore structure analyzer at 77 K. The adsorbed volume was measured at different relative equilibrium adsorption pressure ( P 0 / P ) which ranges from 0.01 to 0.99, where P is the gas vapor pressure in the system and P0 is the saturation pressure of gas.
Brunauer-Emmett-Teller (BET) method  by multipoint calculation was used to calculate the surface area (SA). For the pore volume determination, it was calculated as the total volume of nitrogen adsorbed at the relative pressure of 0.99. The pore size distribution (PSD) was determined based on Barrett-Joyner-Halenda (BJH) model .
These experiments were performed in the Wuxi Department of Petroleum Geology, Research Institute of Petroleum Exploration and Development, SINOPEC.
Mineralogical Compositions and Geochemical Characteristics
Table 1 shows the mineral composition and geochemical characteristics of the studied samples. The minerals in the samples of the Upper and Lower Bakken are dominated by clay minerals and quartz, followed by feldspar and pyrite. A small amount of dolomite and calcite is contained. The clay content ranges from 24.07% to 50.59%, with an average value of 42.23%, and the quartz content varies between 38.04% and 63.59%, with an average of 45.28%. The mineral composition of the Middle Bakken samples is also dominated by clay minerals and quartz. However, the difference between Middle Bakken and Upper/Lower Bakken is that the dolomite and calcite contents of Middle Bakken are higher, and pyrite and feldspar content of Middle Bakken are relatively smaller.
Table 1. The mineral composition and geochemical characteristics of the Bakken samples.
The clay content is in the range of 13.27% to 36.18% with an average of 25.76%, which is less than that of the Upper/Lower Bakken samples. The Upper and Lower Bakken are abundant in organic matter, and the TOC content is between 6.58% and 15.86 wt%, with a mean value of 10.72 wt%. According to previous studies, the kerogen type of this area is mainly type II [39,42]. The Ro ranges between 0.62% and 1.11%, with an average of 0.86%, indicating the Bakken shale belongs to the low mature and mature stages in thermal maturity.
Pore Structure Characteristics
The adsorption/desorption models of studied samples can be divided into two stages, see Figure 3. The first stage is the single-multiple layer adsorption stage. In this stage, the adsorption curve coincides with the desorption curve, and the adsorption quantity increases gradually with the increase of relative pressure, which suggests the presence of micropores (<2 nm) in the samples.
Figure 3. The nitrogen adsorption-desorption curves of the Bakken samples. (a) Upper Bakken; (b) Middle Bakken; (c) Lower Bakken.
The second stage is the capillary condensation stage, i.e., separation of adsorption and desorption curves, namely, the hysteresis loop exists. The initial point of the hysteresis loop indicates that the minimum capillary begins to show capillary condensation, and the end point of the hysteresis loop suggests that the maximum capillary is filled with nitrogen. Additionally, the adsorption/desorption curves of the samples have no obvious “platform segment” at high relative pressure, indicating that the Bakken samples contain the macropores (>50 nm) beyond the measurement range of low-pressure N2 adsorption .
The adsorption/desorption isotherms of porous media often separate under certain conditions, producing the hysteresis loop. The morphology of the hysteresis loop is closely related to the pore structure of the samples . The adsorption/desorption curves and pore types of Bakken samples can be divided into three models, according to whether the hysteresis loop exists or not and the morphology of the hysteresis loop, see Figure 4. The first type has no hysteresis loop. This kind of adsorption/desorption curves results because the relative pressure of the same pore is identical during capillary condensation and evaporation and the adsorption curve coincides with the desorption curve.
Figure 4. Three types of adsorption-desorption curves of Bakken samples. (a) Type A; (b) Type B; (c) Type C.
It generally corresponds to cylindrical pores, parallel plate-like pores, or wedge pores. The second type has a hysteresis loop without an inflection point, which belongs to types H1 or H3 based on the classification of IUPAC (International Union of Pure and Applied Chemistry) . The types H1 or H3 adsorption/desorption isotherms correspond to cylindrical pores with openings at both ends or plate-shaped pores with openings on all sides, namely the open, permeable pores . The hysteresis loop of the third type has an inflection point and belongs to type H2 based on the classification of IUPAC . There is a sharp drop of the inflection point in the desorption curve in type H2, corresponding to ink bottle-like pores.
The adsorption/desorption curves of sample 3 and sample 10 belong to the first type, indicating the development of impermeable pores dominated in the two samples. The adsorption/desorption curves of sample 1, 4, 5, 6, 7, 8, 9, 11, and 12 belong to the second type, developing the open, permeable pores that are conducive to the flow of oil and gas, see Table 2. The curve of sample 2 belongs to the third type. It indicates that the sample contains more ink bottle-like pores but cannot deny the existence of the impermeable pores closed at one end and the open, permeable pores.
Table 2. Ideal pore model division of Bakken formation samples.
That may be because the effect of these two kinds of pores on the adsorption/desorption curve is obscured by the ink bottle-like pores. The existence of ink bottle-like pores is beneficial to the adsorption of shale oil and gas. However, the closeness of the pores is not conducive to the desorption and diffusion of shale oil and gas. The condensed liquid in the bottle would evaporate and flow out quickly as the relative pressure drops to a certain extent. Therefore, it is necessary to prevent shale gas from bursting out when the relative pressure drops to a critical pressure.