The Cretaceous stratigraphy, Songliao Basin, Northeast China: Constrains from drillings and geophysics

3.1 Drilling technology of SR1

The SR1 adopts hydraulic vertical spindle core drill instrument type XY-5, which is produced by the Lianyungang drilling machinery factory, Lianyungang, China. The bit adopts diamond bit with a diameter of 127 mm. During the rope core drilling, the inner drill pipe is designed with single action, core blocking alarm, and straightened technology, which can guarantee the core integrity and the rock samples recovery rate [36].

3.2 Magnetotelluric method

At a 8 km northwest to southeast, 500 m dot pitch magnetotelluric (MT) profile comprising 40 (0.001–10 s) stations across southern SLB (location shown in Figure 4) was surveyed to image the two dimensional resistivity distribution of the Cretaceous sedimentary. Magnetotelluric field data were collected by the American crystal earth company instrument Aether type, and the data were processed using prMT software, which could eliminate the serious interference data in the original time series. To get higher quality impedance tensor and apparent resistivity, the time field signal was converted to the frequency domain data by the Fast Fourier transform and selective iteration technology. The signal-to-noise ratio of the electromagnetic signal determines whether the correct response of the magnetotelluric sounding can be obtained [37]. Figure 5 shows the apparent resistivity and phase curves of some magnetotelluric sounding points. From these few examples, it can be seen that the quality of the magnetotelluric sounding data obtained in field was good and the near-field interference did not exist in the data. Then, the data adopt the two-dimensional nonlinear conjugate gradient inversion algorithm to obtain the apparent resistivity of different geological bodies in tensor electricity (TE) and tensor magnetism (TM) models by using different inversion parameters [38]. Since the actual dimensions of geological bodies are three-dimensional structures, the TE model is susceptible to induce the magnetotelluric sounding data distortion. Therefore, in this article, we adopt the TM model and the regularization factor τ is 10, which not only takes into account the smoothness of the model but also has a good fitting relationship with the original magnetotelluric sounding data [19]. Initial model resistivity is set to 100 Ωm. The error value of apparent resistivity and impedance phase are 10% and 5%, respectively. The horizontal and vertical smoothness ratio is 1 and the number of iterations is 500 times. The final root mean square (RMS) fitting difference is reduced to 1.7%, which proves the reliability of the inversion result (Figure 6).

Figure 4

Simplified geologic map of the southern Songliao Basin. (1) Holocence series low washland-riverbed alluvium; (2) Holocence series high washland-riverbed alluvium; (3) Holocence series big deposit; (4). River; (5) Highroad; (6) City; and (7) MT site locations.

Figure 5

The MT apparent resistivity and impedance phase curve of #1 and #7 points. The blue dot is the tensor electricity model, and the red dot is the tensor magnetism model.

Figure 6

Relationship between RMS versus roughness in L-curve. In this graph, τ = 10 is the compromise parameter.

4.1 The SR1 results

The SR1 program has some research purposes: (1) to study the stratigraphic correlation among the SK-1, SK-2, and SR1 boreholes; (2) to provide some constrains on the tectonic setting of SLB; (3) to understand the sedimentary facies and palaeoclimatology evolution. The SR1 project consists of drilling and coring of Quaternary to Cretaceous strata [35,36]. The drilling depths of 2,010 m were completed, and core samples of 1,895 m were recovered. The detailed description on the SR1 core are some changes of environment within sedimentary microfacies, such as the change of colors, contents, water cut evaluation, and structure. Table 2 presents the columnar section of the SR1.

4.2 MT results

The MT is one of the effective methods that have been widely used to study the lithospheric structure and tectonic evolution history in various terranes of the world [19]. In this article, the variation in the MT resistivity has successfully reflected the change of SLB sedimentary paleoenvironments. The shape of MT resistivity curve is the “high-low-high” H-type. Figure 5 shows the resistivity and the impedance phase curve of #1 and #7 points.

Within the MT profile, overall apart from the top 0–100 m, the MT resistance value is 10³ Ωm, which indicated that the geological body is Quaternary silt. Sites from #1 to #7 in the shallow (100–800 m), the range of low resistance (10⁻¹–10¹ Ωm), from northwest to southeast gradually expanding can be associated with the thickening of upper Cretaceous sedimentary strata and the range of high resistance (10²–10³ Ωm) gradually narrowing can be associated with the thinning of lower Cretaceous sedimentary strata. In the sites from #8 to #18, the MT results show a reverse trend. Thus, both the thickest sediment of the upper Cretaceous and the thinnest of the lower Cretaceous position are in #7 sites.

On the basis of the SR1 results on the #7 point, we may reasonably draw the conclusion of the MT profile that 0–100 m horizontal highest resistance is the Quaternary silt, the 100–140 m horizontal high resistance is the Mingshui formation siltstone, the 140–160 m horizontal high to middle resistance is the Sifangtaizi formation mudstone, 160–260 m horizontal middle resistance is the 4 unit of the Nenjiang formation mudstone, which is the waterproof layer, the 260–430 m horizontal low to middle resistance is the 3 to 1 unit of the Nenjiang formation black shale, the 430–530 m horizontal low resistance is the Yihe formation argillaceous siltstone, the 530–670 m horizontal low to middle resistance is the Qingshankou formation mudstone with a small amount of black shale, the 670–1,710 m the horizontal middle to high resistance is the Quantou formation argillaceous siltstone and mudstone, the 1,710–1,900 m horizontal middle resistance is the Denglouku formation mudstone interbedded with siltstone, and the 1,900–2,010 m is the Indosinian coarse grain texture granite (Figure 7).

Figure 7

The magnetotellurics nonlinear conjugate gradient inversion results. The yellow bar in SR1 is coring intervals; SR1, China Cretaceous Continental Scientific Drilling Project; T03-T3, seismic horizons.

5.1 Stratigraphic correlation

On the basis of SK1n and SK2e results, we present the stratigraphic correlation with the SR1 and the MT section located in the south central depression. The stratigraphy in SR1 is basically consistent with that in SK1n and SK2e; however, there have some differences in definition the geological boundary between of the late Cretaceous Mingshui formation and the Paleogene Yi’an formation. The Paleogene Yi’an formation is at the depth of 318 m [11,13] while in SR1, the depth is 115 m. What is more, the sedimentary sequences of SK1n, SR1 in the Nenjiang formation show some differences: the maker layer lacustrine black shale was found at the depth interval of 1,586–1,800 m in the SK2n borehole, while that was at the depth interval of 352–427 m in SR1 (Figure 3a and b). The total length of Nenjiang formation unit 1 and 2 in SK1n borehole is 214 m, while that in the SR1 borehole is 75 m, implying that the depositional thickness is thinner in SR1 (Figure 8).

Figure 8

Stratigraphy of the north borehole of the Cretaceous Continental Scientific Drilling SK1n, east borehole SK2e, and the geothermal borehole SR1.

In addition, this trend can be found in the Quantou Formations: the maker layer mudstone was found at the depth interval of 1,473–1,553 m in SK2e, while that was at the depth interval of 2,090–2,537 m in SR1. The total length of Nenjiang Formation unit 1 and 2 is 80 m in SK1n borehole, while that is 447 m in the SK2e borehole. These results indicate that the depositional thickness is thinner in SR1 than that in SK2e [35].

5.2 Constrain on the tectonic setting of SLB

The nature and the geotectonic setting of SLB have divided the geologists for many years. Some believe that SLB was a back arc basin during Late Jurassic to Cretaceous [24,26]. Opponents of this view argued that SLB was an intracratonic compound rift basin. The data presented in this article as well as the data published in different disciplines such as geothermics, sedimentology, and tectonic are consistent with the second view [39]. Based on the previous achievements, we conclude that the evolution of the SLB consists five stages: (1) mantle uplift stage: during the late Permian to Triassic, the Eurasian plate collides with the paleo-Pacific plate and triggered large-scale folds. (2) Early rift-depression stage: during the middle to late Jurassic, the initial continental tension formed different scales rift basins. (3) Late rift-depression stage: during the late Jurassic to early Cretaceous, the continuous intensive tension caused SLB rift enlarged and developed the Huoshiling, Shahezi, and Yingcheng Formations. (4) Down-warping stage: during the early to late Cretaceous, the thermal subsidence driven SLB integral settlement and developed the Denglouku, Quantou, Qingshankou, Yaojia, and Nenjiang Formations. (5) SLB shrinkage and regional compression stage: during the late Cretaceous to Quaternary, regional stress became compressional and developed the Sifangtaizi and Mingshui formations (Figure 9).

Figure 9

Cretaceous paleoclimate evolution of the Songliao Basin [7].

The MT cross section across the south central depression area reveals that the subsurface resistance image is the typical “steer’s-head” geometry, which coincides with petroleum industry seismic inference. The Denglouku formation was deposited in the downwarping stage overlap on the graben basins and in the successor extensive overlap. A regional unconformity occurs near the base of the Upper Cretaceous strata, equivalent to seismic horizon T2. This unconformity divides the thermal subsidence tectonostratigraphic unit into two parts, each with distinct architecture and depositional features [31]. The Denglouku Formation, which began to form at the beginning of the Early Cretaceous, is related to left-lateral movement of the Paleo-Pacific plate along the Paleo-Asian continental margin.

Specifically, the MT resistivity anomaly areas are corresponding with the unconformities, such as the unconformities between Mingshui and Sifangtai Formation at the depth of 140 m, the Nenjiang and Yaojia Formation at the depth of 430 m, the Qingshankou and Quantou Formation at the depth of 670 m, and the Quantou and Denglou formation at the depth of 1,710 m, which is equivalent to seismic horizon T03, T1, T2, and T3, respectively [13].

5.3 Sedimentary facies and environmental evolution

The Cretaceous continental sedimentary records of SLB enable us to understand how the terrestrial geologic and ecologic systems responded to past climate fluctuations [6]. The Sk1e, SK1n, SK2e, SK2w, and SR1 boreholes provide significant rock records of global climate changes under the conditions of greenhouse climate.

The occurrence of terrestrial red beds indicates that the climate changed rapidly, and the red beds would be the indication of climates that are warm and dry or seasonal with respect to rainfall [40,41]. Combining with the data of the spore/pollen relative abundances, we argue that the red siltstone intermittently appeared in the Mingshui, Sifangtaizi, Yaojia, Qingshankou, and Quantou Formations in the borehole of SK1n, SK1s, and SR1 indicate that the climates during the middle Cretaceous to the Late Cretaceous were warm and dry or seasonal with respect to rainfall [42,43]. In addition, according to the oxygen isotope data, and the broad-leaved evergreen forest widely spread in SLB and Far East, some experts argued that the temperature is generally above 5°C and the highest more than 25°C [7]. Thus, we argue that the Mingshui, Nenjiang, and Yaojia Formations indicate a semi-humid monsoon warm environment, the Sifangtaizi and Quantou Formation indicate a semi-humid subtropical environment, and the Qingshankou formation indicate a humid subtropical-tropical environment. The absent of red siltstones in Denglouku Formation indicates the climate did not change rapidly or sharply, and the environment was semi-arid semi-humid subtropical [43].

In addition, coal is distributed in Huoshiling–Yingcheng Formation of SLB in Lower Cretaceous, indicating a relatively well-developed plant ecosystem, while, in the SR1 borehole, the coal is virtually absent in the deepest stratigraphic units, implying a less humid climate and less-developed plant ecosystem [42]. Moreover, the main terrestrial petroleum reservoirs in the Qingshankou formation unit 1 and the Nenjiang formation units 1 and 2 indicate different climate zones. The climate zones were shifted from tropical-southern subtropical zone to the northern subtropical zone.

The atmospheric temperature of SLB in Cretaceous changed rapid and can be divided into four cooling events and three warming events. In detail, the cooling events were the Shahezi Formation units 1 and 2, the Denglouku Formation unit 4, and the Nenjiang Formation and the Mingshui Formation unit 2. The warming events were the Denglouku Formation unit 1 and 2, the Qingshankou and Yaojia Formations, and the Sifangtai Formation.

Based on the aforementioned information and the fact that the SLB has the same trends with the Far East in the spore/pollen data, oxygen isotope data, and the Sikhote-Alin orogenic belt in the isotopic data [43], we infer that the paleoclimate and paleoecology of SLB was in humid to semi-humid subtropical environments in the Cretaceous (Figure 9).