Fractal characteristics of artificially matured lacustrine shales from Ordos Basin, West China

Nanometer scaled pores are critical to studying gas shale reservoirs. In order to obtain the information of the evolution mechanism of nanoscale pore within lacustrine organic-rich shales, artificially matured shale samples from the Ordos Basin were treated using hydrous pyrolysis experiment. Low-temperature nitrogen adsorption, inductively coupled plasma atomic emission spectrometry (ICP-AES), and field emission scanning electron microscopy (FE-SEM) experiments were used to investigate the nanopore evolution with migration and precipitation of materials. The results show that the pore sizes were distributed from 1.1 to 500 nm, and the overall porosity tends to increase first and then decrease. The micropores and fine mesopores (< 10 nm) increased gradually from the 250 to the 350 °C, calcite appeared dissolution following a small peak of feldspar dissolution at this stage, and the CO2 reaches a partial pressure peak at 350 °C. The micropores, mesopores and macropores increased steeply from the 370 °C to the 450 °C. Organic pores were not developed until 350 °C, and well developed at 370 and 400 °C. Organic pores, intergranular pores of clay and intragranular pores of pyrite were well developed at 370 °C. The cumulative specific surface areas increased at 400 °C caused by the dehydration and transformation reaction of clay minerals. This study could provide a reference for the exploration of shale gas in lacustrine shales with different thermal maturities.


Introduction
Unconventional hydrocarbon resources are 8.3 times as large as conventional oil and gas resources (USGS 2000(USGS , 2007IEA 2008IEA , 2009Zou et al. 2012). Great efforts toward exploring unconventional tight oil and gas has been devoted and has been made significant breakthroughs in China, especially in the Sichuan Basin and Ordos Basin in recent years (Chen and Xiao 2014;Cui et al. 2013;Tang et al. 2015;Wu et al. 2015;Xue et al. 2015). The "three dessert zone" of oil and gas reservoirs are mostly distributed in the source reservoir interlayer symbiotic or close to the tight sandstone reservoir of the Triassic Yanchang formation with relatively good physical property (Yao et al. 2013).
Numerous laboratory experiments have been performed to investigate the static characterization of porosity in geological shale reservoirs (Alessa et al. 2021;Chalmers et al. 2012;Clarkson et al. 2013;Liang et al. 2014;Loucks et al. 2009;Sun et al. 2016;Yang et al. 2015a, b). The shale pore structure is the dominant factor influencing the capacity of an unconventional reservoir (Sakhaee-Pour and Li 2016; Gregg and Sing 1982), and the nanometer pores is the main channels of shale gas migration and storage within a shale (Yang et al. 2013;Xue et al. 2015). Nanoscale pores in shale are the carrier of free gas and adsorbed gas (Loucks et al. 2009;Ambrose et al. 2010;Sakhaee-Pour and Bryant 2012;Curtis et al. 2012), Besides, scholars have carried out related studies on how the pore structures changed with thermal evolution (Hu et al. 2015;Chen and Xiao 2014;Kuila et al. 1 3 2014; Tiwari et al. 2013;Curtis et al. 2012;Mastalerz et al. 2012;Clarkson et al. 2013). The methods of low-temperature nitrogen adsorption, argon ion polishing and scanning electron microscopy are widely used to directly observe the pore size, pore size and distribution of nanoscale under the microscope, and yielded substantial results regarding the evolutionary process of their pore type, size, shape, spatial distribution and connectivity (Alessa et al. 2021;Mastalerz et al. 2012;Sun et al. 2015;Xue et al. 2015;Yang et al. 2017).
However, the evolution law of nanopore is quite different between thermal simulation samples and actual geological samples with maturity differences in Ordos (Yin et al. 2018).
The porosity of geological samples shows a trend of decreasing and then increasing, and the whole range of porosity is smaller. While the porosity of the thermal simulation sample presents a rule of increasing first and then decreasing, and the porosity is too large (Sun et al. 2015;Xue et al. 2015). Most of the thermal simulation experiments are columnar samples (Sun et al. 2015;Xue et al. 2015). Considering the influence of the lattice mechanical properties of columnar rock on the simulation experiment, we selected fragmented samples for thermal simulation experiments. Given the abovementioned facts, we chose the immature oil shales of the Yanchang Fm. in the Ordos Basin as an objective to investigate the evolution of pores. Artificially matured samples were obtained through hydrous pyrolysis experiments, which were conducted on the same samples with different lithostatic pressure, hydrodynamic pressure, heating rate, time and temperature. The evolution of the nanopore structure was analyzed using low-temperature N2 adsorption and field emission scanning electron microscopy (FE-SEM).

Geological setting
The study area is a part of the Weibei uplift region tectonically, which is located at Tongchuan city, south of the Ordos Basin (Fig. 1). According to the analysis of sedimentary records and lithofacies paleogeographic evolution history, the Ordos Basin experienced multiphase evolutionary stages from Paleozoic sedimentary marine facies, Mesozoic sedimentary continental facies, to Cenozoic basin peripheral fault-depression activities (Jiang et al. 2014). Mesozoic includes Upper Triassic Yanchang Formation, Lower Jurassic Fuxian Formation, and Yanan Formation (Ming 2006), which constitute 2 cycles and 3 oil and gas-bearing combinations vertically. According to lithology, electric and oilbearing properties, Yanchang Formation was divided into 10 members (Chang1-Chang10 from top to bottom). The block samples were collected from Chang 7 Member, which was deposited at the stage of Lake Basin expansion with an area of 5 × 10 4 km 2 , and maximum water depth of 150 m (Hua 2010).

Samples
The samples in this study were selected from the Triassic 7th Member of the Yanchang Formation in the Hejiafang area of the Ordos Basin. The bulk block samples were collected from an outcrop in an oil shale mine. Table 1 lists the TOC, RockeEval,and Vitrinite reflectance data of original shale. Before pyrolysis, the source rock was divided into seven aliquots to conduct parallel experiments, with one sample (HJF2-0) retained for comparison.

Pyrolysis
The pyrolysis experiments were conducted in a WYMN-3 HTHP simulation instrument at the Lanzhou Institute of Geology, Chinese Academy of Sciences (LIGCAS), as used by Sun et al. (2015), Wu et al. (2016) and Li et al. (2017). According to the buried history of the Upper Triassic Yanchang Formation, the fluid pressure set in the experiment is calculated. The experiments in this study investigated six different pressures: 16.9, 22.1, 32.5, 37.7, 42.9 and 52.2 MPa. Every experiment at a different pressure was conducted for 72 h. Table 2 lists the relevant conditions of the experiments.

Nitrogen adsorption
The low-pressure nitrogen adsorption measurements for quantified the changes of micropore were conducted with an ASAP 2020 HD88 surface area analyzer at the LIGCAS. The specific instrument parameters were described by Sun et al. (2015) and Yang et al. (2018) in detail.

Results and discussion
Nitrogen adsorption isotherms Figure 2 shows the nitrogen adsorption-desorption isotherms of the shale samples at different simulation temperatures and pressures. The hysteresis loops for the original and thermal simulation samples are Type H3 indicating that intragranular pores with shapes of parallel and tubular holes 1 3  .18 and 6.57 cm 3 /g. Determine the pore volume size in the adsorption capacity of P/P 0 ≈ 1 the size of the sample , indicating that more samples and large pore development, it can be seen that the sample HJF2-5 has the maximum pore volume, while the sample HJF2-3 has the smallest.
Response of pore structure parameters to temperature Table 3 summarizes the detailed pore structure parameters at the different simulation condition. The cumulative specific surface area ranged from 0.587 to 6.2449 m 2 /g with an average of 2.0507 m 2 /g. Figure 3 plots the relationship between the specific surface area and the average diameter, and cumulative specific surface area. We can roughly divide the temperatures into two stages. From the unheated sample to 400 °C (Fig. 3a), the specific surface area increased rapidly with a peak of 1-2 nm and a peak of 2-50 nm (Fig. 3b). From 400 to 450 °C, the specific surface area reduced gradually.  1 3 Figure 4 shows the distributions of the pore volumes and cumulative pore volumes. The cumulative pore volume ranged from 0.004442 to 5.569 cm 3 /g with an average of 1.1216 cm 3 /g. In these samples, the pore size distributions ranged from 1.7 to 500 nm with an average of 29.27 nm.
However, before 350 °C, the pore volume of simulated samples are lower than that of original sample (Fig. 4b). The main reason for this may be that the pressure plays a leading role in the diagenesis of this stage. After 350 °C, the volume of the sample pore is larger than that of the original sample, which is caused by hydrocarbon generation and dissolution of organic matte. At 400 °C, the volumes of micropores, mesopores and macropores increased steeply, especially for the macropores, the cumulative volume of which was much higher than those of the previous samples. When the temperature and pressure exceeded 374 °C, 22.1 MPa, respectively, the water will be vaporized. The gasification water has great influence on the pore volume. The gasification exists only in theory instead of geological condition. After 400 °C, even with further increases of the simulated temperature and pressure, the cumulative pore volume reduced gradually. Figure 5 shows the linear relationships of the cumulative pore volume and specific surface area with the average diameter at different final simulation temperatures. There is a significant correlation-ship between BET surface area and micropore, mesoporous, macropore, and total pore volume, with correlation coefficients (R 2 ) of 0.89 (Fig. 5a), 0.88 (Fig. 5b), 0.84 (Fig. 5c) and 0.81 (Fig. 5d), respectively. It is shown that in the process of buried depth, the pore volumes are changed with diagenesis. The changes of specific surface area are the result of compaction effect and organic matter generating hydrocarbon, the main contributors to the specific surface area are micropores and fine mesopores

Morphological and distribution features of pore characteristics
The seven samples (original sample and simulated samples) were observed by scanning electron microscope (SEM). The morphological changes of the pores are varied during the process of thermal evolution (Fig. 6). The overall porosity tends to increase first and then decrease. Scanning electron microscopy (SEM) showed that the amount of pores developed in the shale were mainly inorganic pores, relatively few organic pores, and a few microfractures. Inorganic pores mainly include intergranular pores, pyrite microspheres, intergranular pores and intragranular pores. The micropore types of original samples are mainly primary residual pores, secondary dissolution pores (Fig. 6a), clay mineral intergranular pores (Fig. 6b), and pyrite intergranular pores. Meanwhile, some shrinkage pores formed by epigenetic action are developed. The organic pores were poorly developed at 250 °C and 300 °C (Fig. 6a-c). As the increase in simulated burial depth, the hydrocarbon generation of organic matter was strengthened and the organic pore increased. Organic pores began to develop at 350 °C (Fig. 6d) and were well developed at 370 and 400 °C (Fig. 6e, h). The main reason for the rapid increase in the porosity was the intense hydrocarbon generation causing organic pore development at 350 °C, after which the porosity decreased gradually and the transformed organic matter began to stabilize, resulting in a reduction in the rate of pore development at 370 °C.
The intragranular pores of pyrite were not developed in the unheated sample (Fig. 6a) and were well developed at 400 °C (Fig. 6g, h). The smectite mixed layer would 1 3 disappear, and that the amount of illite would increase after 370 °C (Fig. 6g). Thus, organic pores are not as good as inorganic pores, especially in the sample of HJF2-5 at 400 °C, the interlayer pores of clay minerals are obviously increased, and the pores are much larger than the volume of the organic pores ( Fig. 6e-h). Many studies claimed that the TOC content is the main factor that controls the development of nanoscale pores (Zeng et al. 2014;Loucks et al. 2009). Jarvie et al. (2007) held that the porosity would increase 4.9% by per consumption of 35% of the TOC for shale with a TOC content of 7.0%. Before 300 °C, the thermal evolution was low maturity stage, which was a small amount of organic reduction, the pores developed are mainly inorganic pores. After 350 °C, localized organic pores were began developed and then increased gradually with the changed of TOC from 20.3% to 12.1%.

The generation and migration of pyrolysis inorganic minerals
Dissolved Na, Mg, K, Ca, Al, Mn and Si of minerals in mass exchange were determined (Fig. 4). The contents of Na + , K + , Ca 2+ , Si 4+ and Mn 2+ increased at 300 and 370 °C, respectively, reaching the maximum at 370 °C and then rapidly declining. The contents of Al 3+ , Mg 2+ and P 4+ are relatively low and remaining stable. The curves K + Na + and Ca 2+ are similar, but the content of Na + is obviously higher than others. Mg 2+ has a small sudden increase at 370 °C, and then becomes stabile.
The acidic solution products experienced dissolution of feldspar minerals and partially transformed into other minerals. Some of them existed in the form of ions. The dissolutions of calcium feldspar and albite played a leading role at 250 °C (Fig. 6a). More feldspar minerals will release the corresponding ions in the dissolution process [Eq. (1)]: Following the peak of feldspar dissolution, a small peak of calcite dissolution appeared. The dissolution of calcite belongs to the dissolve congruently [Eq. (2)]. Through the XRF element analysis (Fig. 7), the content of HJF2-2 presented a low Ca 2+ concentration, but a high total ions concentration value, which indicates that the water-rock equilibrium transfer was easy to happen.
The reason for the observed behavior is the influence of calcite dissolution from other minerals in the reaction system [Eq. (3)]. For HJF2-3 samples, the corrosion degree of feldspar minerals reduced to a minimum value because of (1) Feldspathoid + H 2 O + H + → K + , Na + , Ca 2+ + Al 3+ + H 4 Si O 4 (aq) Fig. 5 The relationship between the pore volume and BET surface area 1 3 1 3 carbonate cementation. At 350 °C, as decarboxylation of acetic acid can result in CO 2 points pressure, theoretically this diagenetic environment favors the dissolution of carbonate rocks. But the balance of pH (such as feldspar and kaolinite petrochemical) had a buffer. Therefore, CO 2 could not produce organic acid dehydroxy reducing the pH, it caused the precipitation of carbonate, and leaded to the densification of the reservoir. For sample HJF2-3, the result of XRF analysis shows that the content of calcium oxide had a small peak, and indicated that the temperature point is influenced by the CO 2 partial pressure.
The clay minerals transformation is affected by dissolution of feldspar minerals. After feldspar minerals are corroded, a large amount of K + , Al 3+ and Si 4+ will be released. The released dissolution product of feldspar precipitate into kaolinite, smectite and illite which will promote the transformation process of clay minerals, which is a reduction in volume of the reaction (Eqs. 3.4-6). The pore observation of illite indicated that it was the highest at 400 °C (Fig. 6g).
At 400 °C, the sample shows the highest values of K, Ca, Mg, Al, Si, Fe, S, Cl and P (Fig. 7). The dehydration reaction of clay minerals is consistent with the graphs of interlayer seams of clay by the electron microscope and the N 2 adsorption. The dehydration of clay was adsorption water to constitution water in turn. (2)

Conclusions
Lacustrine artificially matured shale samples were treated using hydrous pyrolysis. Low-temperature nitrogen adsorption, X-ray fluorescence, and field emission scanning electron microscopy (FE-SEM) experiments were used to investigate the nanopore evolution of artificially matured shale samples. The following conclusions were obtained: (1) The hysteresis loops corresponding to temperature ranges from unheated to 450 °C were Type H3, indicating that the pores developed were irregular and open with good connectivity between intragranular pores with shapes of parallel, and slit-like and open-ended tubes.
(2) The pore sizes were distributed from 1.1 to 500 nm.
From the 250 °C to the 325 °C, the micropores and fine mesopores (< 10 nm) increased gradually, but less than the unheated sample; at 370, 400 and 450 °C, the micropores, mesopores and macropores increased significantly.
(3) The cumulative specific surface area and cumulative pore volume presented the same trend on the whole. The overall porosity tends to increase first and then decrease. The main contributors to the specific surface area are micropores and fine mesopores. (4) Organic pores were not developed until 350 °C, and well developed at 370 and 400 °C. Organic pores, intergranular pores of clay and intragranular pores of pyrite were well developed at 370 °C. (5) Calcite appeared dissolution following a small peak of feldspar dissolution, and influenced by the CO 2 partial (6) Smectite + K + + Al 3+ → Illite + Na + + Ca 2+ + Fe 3+ + Mg 2+ + Si 4+ Fig. 7 The enrichment of element concentrations at different simulated conditions 1 3 pressure at 350 °C. The clay minerals transformation have affected by dissolution of feldspar minerals. The smectite mixed layer would disappear, and that the amount of illite would increase at 400 °C.