Characteristics and evaluation of the Upper Paleozoic source rocks in the Southern North China Basin

4.1 Organic matter abundance

4.2 Organic matter type

4.2.1 Organic maceral

The vitrinite of the carbonate rock in Taiyuan Formation in bore hole ZK0901 is 75.54%, dominated by euvitrinite, with 24.5% exinite and a KTI value of −44; the vitrinite of the mudstone in Taiyuan Formation is 14 and 7%, with 86 and 93% exinite and a KTI value of 33 and 44; the vitrinite of the coal rock in Taiyuan Formation is 64%, with 36% exinite and a KTI value of −30; the vitrinite of the lower mudstone in Shanxi Formation is 6%, with 94% exinite and a KTI value of 43; the vitrinite of the coal rock in Shanxi Formation is 56%, with 44% exinite and a KTI value of −20. The vitrinite of the mudstone in Lower Shihezi Formation of bore hole ZK0401 is 3%, with 97% exinite and a KTI value of 46; the vitrinite of the coal rock in Lower Shihezi Formation is 49%, with 51% exinite and a KTI value of −11; the vitrinite of the mudstone in Upper Shihezi Formation varies from 1 to 4%, with an average of 2.67% (3), and the exinite varies from 96 to 99%, with an average of 97.33% (3), and a KTI value from 45 to 49, with an average of 46.67 (3). According to the criteria for determining the organic matter type by KTI and KTI-I _H (mg/g) correlation, both the mudstone and coal rock indicated the characteristics of type II or III kerogen macerals (Table 3, Figure 3).

Table 3

Characteristics of Kerogen organic macerals of different rock types in the coal-bearing layers in the study area, based on the samples

Sample number	Depth (m)	Horizon	Lithology	Sapropelinite (%)	Exinite (%)	Vitrinite (%)	Inertinite (%)	KTI	Type
ZK0401-123	997.22	Upper Shihezi Formation	Mudstone	0	96	4	0	45	Ⅱ1
ZK0401-79	1209.93	Upper Shihezi Formation	Mudstone	0	97	3	0	46	Ⅱ1
ZK0401-36	1379.47	Upper Shihezi Formation	Mudstone	0	99	1	0	49	Ⅱ1
ZK0401-26	1442.21	Lower Shihezi Formation	Coal	0	51	49	0	−11	Ⅲ
ZK0401-25	1446.00	Lower Shihezi Formation	Mudstone	0	97	3	0	46	Ⅱ1
ZK0901-54	975.80	Shanxi Formation	Coal	0	44	56	0	−20	Ⅲ
ZK0901-45	994.56	Shanxi Formation	Mudstone	0	94	6	0	43	Ⅱ1
ZK0901-35	1019.97	Taiyuan Formation	Mudstone	0	86	14	0	33	Ⅱ2
ZK0901-28	1039.72	Taiyuan Formation	Limestone	0	24	76	0	−44	Ⅲ
ZK0901-13	1078.15	Taiyuan Formation	Coal	0	36	64	0	−30	Ⅲ
ZK0901-8	1087.16	Taiyuan Formation	Mudstone	0	93	7	0	41	Ⅱ1

Figure 3

Kerogen organic macerals of different rocks in the coal-bearing layers in the study area based on bore holes ZK0901-ZK0401. (a) Euvitrinite in limestone of Taiyuan formation; (b) Euvitrinite in mudstone of lower Shihezi formation; (c) Fungus in mudstone of Upper Shihezi Formation; (d) Phyllovitrinite in limestone of Taiyuan formation; (e) Phyllovitrinite in mudstone of Upper Shihezi Formation; (f) Sporinite in mudstone of Upper Shihezi Formation; (g) Suberinite in mudstone of Shanxi formation; (h) Cutinite in mudstone of Upper Shihezi Formation; (i) Suberinite in mudstone of Upper Shihezi Formation.

4.3 Thermal evolution degree of organic matter

The kerogen vitrinite reflectance (Ro) of carbonate rocks of Taiyuan Formation in bore hole ZK0901 varies from 1.21 to 2.36%, with an average of 1.84% (3), while that of the mudstone varies from 2.17 to 2.24%, with an average of 2.20% (2), and that of the coal rock is 2.06%; Ro of the lower mudstone of Shanxi Formation varies from 1.99 to 2.22%, with an average of 2.11% (2), while that of the coal rock is 2.50% and that of the upper mudstone is 2.16%. Ro of carbonaceous mudstones of Lower Shihezi Formation in bore hole ZK0401, meanwhile, is 2.23%, that of the coal rock is 2.00%, and that of the mudstone varies from 1.86 to 2.02%, with an average of 1.94% (2); Ro of the mudstone of Upper Shihezi Formation varies from 1.40 to 2.04%, with an average of 1.70% (11), and that of the mudstone with the higher TOC varies from 1.93 to 1.97%, with an average of 1.95% (2). According to the criterion for the determination of organic matter evolution degree by Ro, the evolution degree of the source rock is generally in the high-mature to over-mature stage and shows a trend toward gradual increase with depth.

The peak temperature (T _max) of rock pyrolysis of the mudstone of Benxi Formation in bore hole ZK0901 in the Yongcheng Area varies from 438 to 580℃, with an average of 519℃ (6); T _max of the carbonate rock of Taiyuan Formation varies from 421 to 588℃, with an average of 529℃ (7); T _max of the coal rock is 554℃ and that of the mudstone varies from 482 to 588℃, with an average of 565℃ (11); T _max of the lower mudstone of Shanxi Formation varies from 556 to 569℃, with an average of 561℃ (7); T _max of the coal rock is 568℃ and that of the upper mudstone varies from 457 to 563℃, with an average of 507℃ (4); T _max of the coal rock of Lower Shihezi Formation in bore hole ZK0401 varies from 525 to 560℃, with an average of 543℃ (2), while T _max of the carbonaceous mudstone varies from 528 to 553℃, with an average of 541℃ (2) and that of the mudstone varies from 538 to 556℃, with an average of 550℃ (4); T _max of the mudstone of Upper Shihezi Formation varies from 445 to 572℃, with an average of 517℃ (38), while T _max of carbonaceous mudstones varies from 499 to 511℃, with an average of 505℃ (2), and that of the mudstone with the higher TOC varies from 519 to 541℃, with an average of 531℃ (4). According to the criteria for the determination of organic matter evolution degree by maximum pyrolysis temperature (T _max), the source rock basically indicates the characteristics of high-mature to over-mature, which is consistent with the characteristics reflected by kerogen vitrinite reflectance.

5.1 Low hydrocarbon generation potential of the Upper Paleozoic source rocks

From the above analysis data, it can be seen that the hydrocarbon potential (S₁ + S₂) and chloroform bitumen “A” of the mudstone, carbonaceous mudstone, and coal rock at ZK0901-ZK0401 are low, and most of the samples cannot meet the standards of source rock. This major reason may be the high thermal maturity (Ro shown in Table 1), and a minor factor may be the long storage time of the core. According to the test data analysis (Table 1), which can support this finding, S₁ generally does not exceed 0.5 mg/g, remaining mostly 0 or 0.01 mg/g, and chloroform bitumen “A” is very low. In the case of deep-buried source rock with good caprock and fresh cores, higher hydrocarbon potential and chloroform bitumen “A” can be expected [8,9]. In the theory, S₁ + S₂ and chloroform bitumen “A” cannot be used to evaluate the organic matter content of source rock in this area; but according to the residual organic index and significant positive correlation between hydrocarbon potential (S₁ + S₂) and TOC (Figure 4), the hydrocarbon potential is also of significance for analyzing the quality of source rock under the same conditions.

Figure 4

TOC-S₁ + S₂ correlation diagram of different coal-bearing layers in the study area based on the samples.

Test results of the Upper Paleozoic source rocks in the Southern North China Basin [8,9] show that coal rocks, mudstones, and carbonate rocks are all possibly potential source rocks in Upper Paleozoic. These source rocks are vertically continuous in Taiyuan Formation and in the lower part of Shanxi Formation, mainly distributed nearby coal bed with small thickness in the upper part of Shanxi Formation and XiaShihezi Formation, seldom in Shangshihezi Formation. The vertical variations and quality differences of the Upper Paleozoic source rocks may be mainly attributed to the sedimentary environment. Previous studies show that the Late Paleozoic coal-bearing stratum is a complete transgressive–regressive cycle [24], and that the thick coal rock of Shanxi Formation was formed in the transition point of a transgressive–regressive cycle. This set of strata has a stable sedimentary base for a long time and thereby have superior conditions for the formation of source rock [24]. Mudstone at the bottom of this set of strata (mainly including the lower mudstone of Benxi Formation, Taiyuan Formation, and Shanxi Formation) is developed in tidal flat environments and accompanied by sudden transgression [25,26]. Sea level frequently rises and falls, resulting in the alternating occurrences of carbonate rock and tidal flat facies mudstone so as to form the superimposition of a bioclastic limestone–mudstone–coal bed (seam), with good source rock development conditions. Subsequently, large-scale transgression developed, providing the sedimentary environment to develop source rock of Lower Shihezi Formation; Upper Shihezi Formation was mainly developed in a delta plain environment. Here, valuable coal beds were formed under discarded clastic tidal flat peat bogs, lower delta plain peat bogs, and water-covered bogs marked by coal rock (or carbonaceous mudstone). However, multiple secondary transgressions hindered the sustainable development of peat bogs. Therefore, only multilayered thin coal beds were formed [27], with smaller controlling thickness and poorer vertical continuity. Especially with the continuation of regression, the development conditions of the coal rock deteriorated gradually, generally resulting in poor source rock in Upper Shihezi Formation. According to the above analysis, the Upper Paleozoic source rock in the Southern North China Basin is dominated by coal rock and mudstone, and carbonate rock is mainly distributed in the upper and lower strata of the coal bed. Therefore, II₂ coal bed and its upper and lower mudstones in Shanxi Formation, and the thin coal rock and upper and lower mudstone of Lower Shihezi Formation are the most important source rock development horizons, followed by the coal bed (seam) of Taiyuan Formation and its nearby mudstone and carbonate rock.

The organic matter types of the Upper Paleozoic source rocks in the Yongcheng Area are mainly type III or II kerogen and kerogen macerals of mainly euvitrinite. It is found that Taiyuan Formation consists of several sedimentary sequences of limestone–fine clastic rocks. The upper limestones are mostly mudstones or siltstones with thin coal beds or carbonaceous mudstone above. The limestones of the next sedimentary sequence directly contact the coal bed or carbonaceous mudstones of the previous sedimentary sequence. The carbonate rock displays an obvious scouring structure, which is considered to be caused by frequent and sudden transgressions [28]. Therefore, the sedimentary environment of Taiyuan Formation changed rapidly during the sedimentary period and there was frequent seawater transgression–regression. For transgression, the water body deepened and the hydrodynamic force was strong, causing strong scouring to the underlying coal bed or carbonaceous mudstone. It not only interrupts coal accumulation, but also leads to the deposition of a large amount of eroded carbon debris in carbonate rocks, enriching the carbon debris in limestone and rapidly depositing it. This speculation has been confirmed by the large amount of carbon debris remaining after dilute hydrochloric acid dissolution of Taiyuan Formation limestone [17]. After rapid regression, the upper mudstones and coal beds were formed. As a result, Taiyuan Formation limestone has high organic carbon content.

5.2 High thermal evolution degree of the Upper Paleozoic source rocks

The Upper Paleozoic has basically reached the high-mature to over-mature stage, with the highest Ro of around 2.5%, increasing gradually with depth. Previous studies have shown that the thermal evolution degree of the Upper Paleozoic coal measure source rock in different regions of the Southern North China Basin is quite different [29,30]. Based on the restoration of the sedimentary and burial history in the area, through geothermal history simulation of the Upper Paleozoic and by comparing the relationship between the paleogeothermal field and the thermal evolution history in different areas of Southern North China Basin and the evolution degree of the source rock [31], it can be determined that this area had a high geothermal gradient during the Yanshanian and Himalayan periods, during which the regional magmatic thermal metamorphism played an important role in the study area [32]. In fact, it has become a common understanding that the evolution degree of the Upper Paleozoic in the Southern North China Basin varies greatly in different regions, which is also attributed to the differences in the Cenozoic geothermal gradients in different regions [31,33].

5.3 Geological process and tectonic evolution

Although this study only analyzed the basic characteristics of the Upper Paleozoic coal measure source rock in two boreholes in the study area, based on small changes of Late Paleozoic epicontinental deposits in the regional sedimentary environment, the boreholes can represent the basic characteristics and vertical distribution characteristics of the Upper Paleozoic source rocks in the Yongcheng. Tectonic and magmatic activities are frequent in the study area, especially in the Mesozoic Yanshanian period [32,34], which is characterized by folds and faults. The distribution of magmatic rocks is closely related to uplift and faults. The rock masses are basically distributed in near NNE trending anticline axes, and then invade along the weak part of the structure or soft stratum of two limbs of the anticline (mainly coal beds). However, due to the irregular intrusion of magmatic rocks, the distribution of various types of magmatic rocks cannot be accurately delineated. Aeromagnetic data show that there are a large number of magnetic anomaly areas around the study area, and it is speculated that there may be larger hidden rock masses in the deep part [32]. Vertically, Lower Shihezi Formation is more strongly intruded by magmatic rocks than Shanxi Formation. In plane, the western limb of the anticline mainly intrudes into the Shanxi Formation, while the eastern limb mainly intrudes into the Lower Shihezi Formation. Its spatial distribution is controlled by tectonics [32,35]. Strong tectonic movement and magmatic activity have had a great impact on the regional coal quality and coal body structure, which may lead to a large amount of coal-bed methane loss and affect the maturity of coal measure strata system [36,37]. However, in the study of the influence of igneous rocks on the secondary hydrocarbon generation of Carboniferous-Permian coal rocks in North China [38], it is found that magmatic rocks do participate in and affect the hydrocarbon generation of coal measure source rock. With igneous rock, the gas hydrocarbon yield of coal rocks increases by 11−28%, which has an obvious promoting effect on coal-to-gas. In addition, because of the high TOC of mudstone near the coal rock, large methane adsorption capacity and developed pores [5,39,40], the brittle mineral content in Shanxi Formation and Lower Shihezi Formation is higher, which provides good conditions for coal-formed gas accumulation and exploitation.