Since the Cenozoic era, the collision between the Indian and Eurasian plates has caused the Qinghai–Tibet plateau to substantially rise [1
]. The complex intra-continental deformation process has had a profound impact on environmental climate change, sedimentary tectonic evolution, and the source-sink system in the inland region of Asia, which has considerably affected environmental resources, especially the accumulation, migration, and transformation of oil and gas in the inner basins of the Qinghai–Tibet plateau [2
]. The Qaidam Basin (Figure 1
), bounded by the eastern Kunlun, Altun, and Qilian mountains, is the largest inland basin in the Qinghai–Tibet plateau and one of the most important oil–gas basins in China [5
]. In recent years, hundreds of millions of tons of oil and gas fields have been successively discovered in the Kunbei fault step zone, Yingxiongling in the southwest margin of the basin, Dongping nose uplift structure, and Niudong slopes in the northwest of the basin [8
]. However, the central part of the northern margin of the Qaidam Basin, which had an early breakthrough in oil and gas exploration in the basin, is developing slowly with no significant discoveries. Previous studies on the sedimentary system, reservoir genesis, and diagenetic evolution have investigated the central region of the northern margin of the Qaidam Basin [9
]. However, a scarcity of studies on provenance and diagenesis and lack of scientific predictions regarding high-quality and lithologic reservoirs are key factors that presently restrict oil and gas exploration in this area.
Clay contents are widely distributed in various types of sediments and are the comprehensive products of sedimentation and diagenesis under certain climatic, hydrological, and provenance conditions, which are important for determining the sedimentary environment, paleoclimatic conditions, and diagenesis [10
]. Therefore, the study of clay mineral contents and their combination provides important information for understanding the provenance, paleoclimate, sedimentary environment, and diagenetic characteristics for reproducing depositional and diagenetic processes [12
]. The fluctuation of sediment chemistry is a common method for studying environmental change [13
]. Different environmental conditions have different effects on the decomposition, migration, and enrichment of various elements with different properties [14
]. Therefore, the change of the element contents in the sediment can largely reflect the sediment in the sedimentary source and change of environmental conditions [15
]. Therefore, it is possible to analyze the provenance properties, sedimental–diagenetic environment, and paleoclimate conditions of sedimentary rocks by studying the characteristics of clay mineral assemblages and contents of major and trace elements and their ratios in sedimentary rocks. In this study, we analyzed core samples from key drill wells in the central part of the northern Qaidam Basin margin to study their provenance properties and sediment–diagenetic environment. The results provide data for future use in oil and gas exploration.
2. Geologic Setting
Based on tectonic geological analysis of the northwest region of China, regardless of the complexity of the tectonic evolution before the Mesozoic, the subduction and reduction of oceanic crust in the entire northwest region and subsequent collisional orogenic action between the continental lithospheric plates completed by the end of the Triassic [16
]. Although the formation time and mode of the peripheral fault zones in the Qaidam Basin differ, the collisional effects of the Eastern Kunlun fault zone completed in the Permian and the main orogenic and uplift periods with relative lag ended by the end of the Middle Triassic [17
]. The South Qilian fault zone mainly represents the uplift and orogeny associated with closure during the Caledonian [19
]. Starting in the Triassic, the northwest region entered a land-evolution stage, and the Qaidam block inherited characteristics of the late Paleozoic fault-block continental apron. During the Indo-China epoch, accompanied by the closure of the circumferential trough, the fault block activity of the symmetrical continental uplift intensified. During the Yanshan movement, the Meso-Tethys Ocean continued to expand north and south, and the northern fault block belt of the Qaidam Basin in the circumference of the extrusion dynamic field was continuously uplifted. However, because of differences in the tectonic position and nature of the fault block, the uplift speed of each fault block also differed and the area with relatively slow uplift gradually developed into a depression-type fault block [23
The Cenozoic Paleogene–Neogene sandstones are mainly composed of continental sedimentary facies [24
], which were deposited mainly in braided-river delta-lacustrine environments and include fluvial, deltaic, and lacustrine sedimentary deposits (conglomerate, sandstone, siltstone, and mudstone). The lithology changed substantially over the Paleogene and has the characteristics of a progradational sequence. Interactive deposits of coarse clastic sandstones and fine clastic sandstones of unequal thickness indicate that the hydrodynamic conditions changed more frequently in the Neogene. Based on the results of previous studies on the stratigraphic framework of the northern Qaidam Basin [26
], the Paleogene–Neogene can be divided into five units. These five stratigraphic units are listed in Figure 2
3. Materials and Methods
Samples were obtained from cores of eight exploration wells (Figure 1
) from the Lulehe Formation and Xiaganchaigou Formation in the center of the northern Qaidam Basin. Sandstone is the main lithology of the samples, followed by siltstone.
X-ray diffraction (XRD) was performed following techniques outlined by the Standard of Petroleum and Natural Gas Industry of the People’s Republic of China (SY/T 5163-2010 Analysis Method for Clay Minerals and Ordinary Non-Clay Minerals in Sedimentary Rocks by the X-Ray Diffraction). XRD patterns were obtained using an Ultama IV X-ray diffractometer (Rigaku, Tokyo, Japan) equipped with a rotating anode, using radiation, operating at 40 kV and 40 mA, taking DS (divergence slit) = SS (scattering slit) = 1°, and RS (receiving slit) = 0.15 mm. The scanning angle ranged from 2° to 52° with a step interval of 0.02° at a rate of 4° (2)/min. Sample powder was placed in a 500-mL or 1-L beaker with distilled water to soak for 1 day. To remove organic matter, 15% hydrogen peroxide solution was added the beaker over 1 day. Then, dilute 0.5% hydrochloric acid was added to the beaker and stirred for 1 h. The treatment was repeated until the reaction to remove carbonate had completed. Then, the treated samples were thoroughly washed with and soaked in deionized water. A few drops of 5% sodium hexametaphosphate solution were added to the soaked samples and allowed to sit for 5–10 min. Then, an ultrasound was used to disperse and fully suspend the clay. The suspension of clay particle sizes <2 μm was drawn in the upper 2–5 cm range and centrifuged to separate the suspension (3500 r/min for 10 min) to precipitate the clay particles. The upper layer of water was removed, and the clay was set into oriented sections. After separation and extraction, 40 mg of dry clay and 7 mL of distilled water were combined, well agitated, dispersed by ultrasonic vibration for 10 min, and then applied to a glass slide to dry naturally and make a oriented section (N section). The N section was placed on a sedimentation mount for X-ray scanning. Then, the tested N section was placed in a desiccator containing ethylene glycol saturated steam and held at 40–50 °C for 7 h. After cooling to room temperature, an ethylene glycol saturated section (EG section) was prepared and subjected to X-ray diffraction analysis. After measuring the XRD pattern, the EG section was placed in a muffle furnace, heated to 550 °C, and held at constant temperature for 2 h. After cooling naturally to room temperature, a heating section (T section) was prepared, and X-ray diffraction analysis was performed immediately.
Prior to testing, a 1% dilute hydrochloric acid solution was used for drop testing. Hydrochloric acid (1% solution) was dropped on the rock in situ. Calcite reacts violently with abundant bubbles, whereas dolomite reacts slowly with only a few bubbles. The iron-bearing calcite reaction is between calcite and dolomite [29
]. The carbonate cement of Paleogene sandstone in the north Qaidam Basin margin is mainly calcite and nearly free of dolomite and iron calcite [30
]. Therefore, the carbonate (i.e., calcite) cement can preliminarily be determined according to the effervescence strength. The combination of optical microscope, scanning electron microscopy (SEM), and X-ray microanalysis ensured that samples with high carbonate cement content were selected for testing. The instrument used for determining stable carbon and oxygen isotope concentrations was a stable isotope mass spectrometer, MAT252, produced by Finnegan, Germany. The samples were gently powdered in an agate mortar, passed through a 100–200 mesh sieve, and then packed in a cavity mount for bulk mineral analyses. The phosphoric acid method was adopted for sample treatment. Trace samples were added to 100% phosphoric acid for reaction at 90 °C. Then, the generated CO2
was dried and dehydrated. Isotopes were measured with the MAT252 mass spectrometer. Carbon and oxygen isotope test results for all samples were determined with respect to the PDB standard [30
]. This analysis was completed at the Key Laboratory of Oil and Gas Resources Research of the CAS (Chinese Academy of Sciences).
The morphology of clay mineral particles was characterized by SEM (Merlin Compact, Zeiss, Oberkochen, Germany) at the Key Laboratory of Oil and Gas Resources Research of the CAS with a resolution of 0.8 nm.
Before elemental analysis, the samples were observed under a microscope for mineral composition and structure to ensure that the samples used for elemental geochemical analysis were not altered, secondarily mineralized, or secondarily weathered. Approximately 150 sandstone samples were selected and ground into thin slices. The mineral constituents, grain size, sorting, and roundness of these samples were observed and recorded using a polarizing microscope. Each thin section was analyzed using the Gazzi-Dickinson method [31
]. Medium- to coarse-grained sandstones were used for point counting to minimize the grain-size effects [32
]. A minimum of 400 points were counted per sample. All samples were ground with a pollution-free crusher and passed through 200-mesh sieves and then baked in an oven for about 3 h to remove moisture for accurate weighing. Determination of the main elements was carried out by a 3080E3X fluorescence spectrometer produced by the Japanese Science Company. Samples were sealed and dissolved by HF + HNO3
and analyzed by laser coupled plasma mass spectrometry. The major and trace element analyses were both completed at the Key Laboratory of Oil and Gas Resources Research of the CAS.
5. Clay Minerals
Authigenic kaolinite mainly forms by feldspar leaching in an acidic medium, followed by the direct precipitation of pore water in the diagenetic stage [33
], and it can also form by weathering and low-temperature hydrothermal metasomatism [37
]. It is mainly distributed in the humid tropics and subtropics [33
] and it is an indicator mineral of weak acid leaching and strong chemical weathering [37
]. Authigenic smectite usually forms in alkaline media poor in K+
but rich in Na+
], which is related to a cold climate. The contents of authigenic smectite are higher in sediments of temperate semi-humid zones [33
] and decrease with the heat of the climate [39
]. Chlorite is precipitated from Mg2+
-rich pore water in relatively high temperature and strongly alkaline environments [33
]. Generally, chlorite can only be preserved in areas with weak chemical weathering, which is completely opposite of the formation environment of kaolinite [41
]. Illite mainly forms by the weathering of aluminosilicate minerals such as potash feldspar and mica under weak leaching in a K+
-rich alkaline water medium [33
], or Fe2+
in smectite are replaced by Al3+
and dehydrated to form illite [35
], which mainly occurs in dry and cold environments [44
]. The existence of an illite–smectite mixed layer reflects the gradual transformation from smectite to illite in an alkaline environment rich in K+
, and illite becomes more stable in a water environment with a high K+
The results of the clay mineral composition analysis are shown in Table 2
. Based on the test results, the clay mineral contents in the Paleogene sandstones are relatively high, with an average of 17.11%. The clay mineralogy of the Paleogene sandstones is dominated by I + I/S + C types (I = illite, I/S = mixed-layer illite/smectite, C = chlorite) and characterized by very high illite content, high mixed-layer illite/smectite and chlorite contents, and lesser smectite and kaolinite contents (Figure 6
). Among the samples, the highest illite content is 84%, the lowest is 18%, and the average is 51.6%. For the second most dominant clay component, mixed-layer illite/smectite, the highest content is 60%, the lowest is 2%, and the average is 28.4%; the average smectite content in the illite–smectite mixed layers is 13.7%. The highest chlorite content is 56%, the lowest is 2%, and the average is 17.6%. Smectite and kaolinite were observed in only a small number of samples.
Illite is very well developed in the Paleogene sandstone samples, mostly in the form of filamentous, bridging, and curved leaf-like filling between particles and particle surfaces (Figure 7
a,e,i). The mixed-layer illite/smectite is well developed and commonly seen on the surfaces of particles or filled between particles with semi-honeycomb or cotton flock shapes (Figure 7
Flaky chlorite is also common on the surfaces of the particles (Figure 7
f). A small amount of smectite is distributed on the surfaces of the particles in honeycomb shapes as bridges between particles (Figure 7
d,g,h). Kaolinite is only observed in the shallow layer of well MX101 and distributed with a book-like habit on the surfaces of the particles (Figure 7
g). The illite contents in the lower Paleogene samples are substantially higher than those in the upper Paleogene samples, whereas the mixed-layer illite/smectite content decreases considerably from the upper to lower Paleogene (Figure 8
), indicating that the clay minerals in the sandstone are mainly authigenic. The assemblage characteristics are the result of the joint action of sedimentary climate, sedimentary environment, and diagenetic environment.
The climatic characteristics are determined to be arid to semi-arid because the Paleogene parent rocks in the Qaidam Basin were mainly weathered through shallow physical weathering and supplemented by chemical weathering [47
]. According to previous studies, inland saline lakes are conducive to the transformation of kaolinite and smectite to illite and the preservation of illite [39
]. The average illite content in the Paleogene sandstone samples from the hinterland of the northern margin of the Qaidam Basin is 51.86%, the average content of mixed-layer illite/smectite is 28.15%, and thus the cumulative content of these two components is more than 80%. Most of the samples do not contain smectite and kaolinite, which reflects the fact that the belly of the northern margin of the Qaidam Basin was dominated by a closed salinized lake basin in the Paleogene, and that the climate gradually became cold and dry. In an alkaline environment rich in K+
, large amounts of kaolinite and smectite gradually transform into mixed-layer illite/smectite and illite. The average content of authigenic chlorite in the samples is 17.33%, indicating that the diagenetic environment in the Paleogene was dominated by an alkaline environment rich in Mg2+
, and that leaching and chemical weathering were weak. Longitudinally, with the increase of burial depth, the illite content increases, whereas the mixed-layer illite/smectite content decreases, indicating that with the increase of ground temperature, the transformation of mixed-layer illite/smectite to illite, and chlorite precipitation were accelerated. Therefore, it can be inferred that the hinterland of the northern margin of the Qaidam Basin in the Paleogene mainly developed closed saline lake basin deposits, and that the overall climatic environment was arid to semi-arid. The diagenetic environment was mainly an alkaline environment rich in K+
, and leaching and chemical weathering were weak.
7. Discussion and Conclusions
Regardless of the complexity of the tectonic evolution prior to the Mesozoic, the subduction of oceanic crust and collision between continental lithospheric plates completed in northwestern China by the end of the Triassic [16
]. The formation time and formation mode of the peripheral fault zone in the Qaidam Basin differ. The collision of the Kunlun fault zone was completed in the Permian, and the relatively lagging main orogenic uplift period ended by the end of the Middle Triassic [17
]. During the Caledonian period, the Qilian fault zone was closed, uplifted, and orogenic [19
]. Starting from the Triassic, the northwest region entered a stage of intracontinental evolution, and the major tectonic events were mainly affected by the evolution of the southern Tethys tectonic domain [22
]. The Qilian Mountain orogenic belt also occurred in this large-scale tectonic background and experienced the process of a collision–suture–uplift orogeny. The results of facies analysis of the Paleogene clastic rocks in the center of the northern Qaidam Basin show that the tectonic background of the source area was mainly based on a recycled orogenic belt, magmatic island arc, collision suture, and fold-reverse fault zone, which indicates that the provenance of the center of the northern Qaidam Basin is mainly from the Qilian orogenic belt. Mixture source identification models indicate that the source of the Paleogene clastic rocks in the center of the northern Qaidam Basin is mainly quartz sediment and acidic igneous rocks with a small amount from neutral igneous rocks. The tectonic background was mainly an active continental magmatic arc, subduction accretion, or a fold-thrust belt. Through comparison of the rare earth element distribution model, it is found that the provenance area was in the region of the Yuka–Jiulong Mountains in the middle of the Qilian Mountains, which represents a moderate transportation distance.
] estimated that the mineral composition of the upper crust is about 21% quartz, 41% plagioclase, and 21% potassium feldspar (volume fraction). Feldspar is the most important parent-source mineral in the process of chemical weathering of the upper crust. Na, K, Ca, and other alkali metal elements are largely lost with surface fluids in the form of ions, and clay minerals (e.g., smectite, illite, kaolinite) form. During this process, the molar fraction of Al2
, the principal component of weathering products, varies with chemical weathering intensity. Accordingly, Nesbitt and Young [55
] put forward the CIA as an indicator to reflect the weathering degree of the source region when they studied Paleoproterozoic clastic rocks from the Huronian of Canada (higher CIA values are associated with stronger weathering). In 1989, Nesbitt and Young suggested that CIA = 50–65 reflects incipient chemical weathering under cold and dry climatic conditions, CIA = 65–85 reflects intermediate chemical weathering under warm and humid conditions, and CIA = 85–100 reflects extreme chemical weathering in hot and humid conditions [61
]. The upper crust (UCC) points to continental shale (PAAS) to represent the direction of continental weathering. The corrected CIA values are 56.3–75.7 with an average value of 66.5. Moreover, the sample points are close to the weathering trend line, indicating that the Paleogene provenance in the center of the northern Qaidam Basin mainly formed under cold and dry climatic conditions and experienced limited chemical weathering with a small amount subjected to moderate chemical weathering under warm and humid conditions. X-ray diffraction analysis of clay minerals shows that the illite content is relatively high followed by chlorite and mixed layers of illite and smectite, which reflects the climatic and environmental characteristics of arid and semi-arid areas. The characteristics of the carbon and oxygen isotopes reveal that the sedimentary environment in this period was dominated by freshwater deposition. Therefore, the sedimentary environment in the Paleozoic in the center of the northern Qaidam Basin is inferred to have been dominated by cold and dry freshwater deposition with moderate to weak weathering.
The investigation of source, sedimentary environment, and diagenetic environment of the sandstone reservoir in the lower part of the lower Ganchaigou Formation in the northern margin of Qaidam provides important information of the sedimentary system and evolution law of the reservoir rock, which provides a reference for further oil and gas exploration in this area.