Lithology and U – Pb Geochronology of Basement of Cenozoic Yitong Basin in Northeastern China : Implication for Basin Architecture and New Horizon of Deep Natural Gas Exploration

The lithology and formation age of basement rocks are significant for the understanding of the nature of basin architecture, evolution and the potential of hydrocarbons of a basin. In this study, the basement lithology of the Cenozoic Yitong Basin is investigated through the petrological analysis of cores, cuttings, and thin sections. The results suggest that the basement rocks of the Yitong Basin are mostly composed of unique igneous rocks that are different from nearby basins’ sedimentary and metamorphic basement. The igneous rocks are dominated by intrusive monzonite granite and alkali feldspar granite. Additionally, U–Pb zircon geochronology of basement samples by LA-ICP-MS and the geological interpretation of apparent resistivity data indicate that the igneous basement in major part of the basin was mainly formed by a lateral intrusion of granite into the Permian sedimentary stratum in the Yanshanian period from 177 to 170 Ma. The results also reveal the two-layer basin architecture with coal-bearing Carboniferous–Permian strata below the igneous basement covered with Tertiary sediments, thus providing a new geologic horizon for deep natural gas exploration in the older coal-bearing sedimentary rocks beneath the current igneous basement.


Introduction
An increasing number of studies have revealed that, during the Late Jurassic, the tectonic activity in eastern Asia changed drastically from a regime of strong intracontinental compressional orogenesis and crustal thickening to one of the strong intracontinental extensional rifting and lithospheric thinning [1].This transformation of tectonic deformation was closely related to the subduction of the Pacific Plate under the Asian continent [2][3][4].In the eastern region of China, this tectonic transition is characterized by a series of faulted basins that were developed during the Early Cretaceous/Late Jurassic, Cretaceous-Cenozoic, and Cenozoic periods.These basins are also the principal regions where hydrocarbon plays are located [5].Oil and gas resources and their production have been proven in the Songliao and Bohai Bay Basins, which are adjacent to the Yitong Basin and have been consistently ranked in the first and second places for reserves and production, respectively, among all the petroliferous basins in China from 1959 to 2002.Although the hydrocarbon production of anticlinal reservoirs has gradually decreased since 2003, a large number of buried-hill hydrocarbon reservoirs in the bedrock have been discovered.For example, more than 100 bedrock buried-hill hydrocarbon reservoirs have been discovered in the Bohai Bay Basin thus far [6], and they have since become major oil and gas reservoirs in this area.In addition, oil and gas reservoirs have been discovered in the carbonite and metamorphic basement rocks of the Songliao Basin [7].Some experts have predicted that the buried-hill reservoirs in the bedrock located in the faulted basins in eastern China will trigger a future revolutionary breakthrough in oil and gas exploration and production.However, no natural gas was found in the 14 wells drilled into the buried hills of the Luxiang fault depression in the middle of the Yitong Basin (YB), and they thus appeared to be a failure [8].This failure was not only unexpected but also raised doubts about the lithology and formation age of the bedrock in the YB.Therefore, it is necessary to determine the lithology and formation age of this bedrock, which will help reveal the basin architecture and new exploration potentials.

Geological Setting
The Yitong Basin (YB), which is located in northeastern China and distributed within a long, narrow region trending toward the northeast direction at 45-55 • , is approximately 140 km long and 5-20 km wide and spans a total area of 2400 km 2 [9].This basin contains three tectonic units: The Chaluhe Fault Depression (CFD), Luxiang fault depression (LFD), and Moliqing fault depression (MFD) (Figure 1).The YB is separated from the Songliao Basin by the Daheishan fault uplift in the southeast, lies next to the Bohai Bay Basin in the southwest, and is adjacent to the Nadanhada Hill fault uplift.
Minerals 2018, 8, x FOR PEER REVIEW 2 of 20 principal regions where hydrocarbon plays are located [5].Oil and gas resources and their production have been proven in the Songliao and Bohai Bay Basins, which are adjacent to the Yitong Basin and have been consistently ranked in the first and second places for reserves and production, respectively, among all the petroliferous basins in China from 1959 to 2002.Although the hydrocarbon production of anticlinal reservoirs has gradually decreased since 2003, a large number of buried-hill hydrocarbon reservoirs in the bedrock have been discovered.For example, more than 100 bedrock buried-hill hydrocarbon reservoirs have been discovered in the Bohai Bay Basin thus far [6], and they have since become major oil and gas reservoirs in this area.In addition, oil and gas reservoirs have been discovered in the carbonite and metamorphic basement rocks of the Songliao Basin [7].Some experts have predicted that the buried-hill reservoirs in the bedrock located in the faulted basins in eastern China will trigger a future revolutionary breakthrough in oil and gas exploration and production.However, no natural gas was found in the 14 wells drilled into the buried hills of the Luxiang fault depression in the middle of the Yitong Basin (YB), and they thus appeared to be a failure [8].This failure was not only unexpected but also raised doubts about the lithology and formation age of the bedrock in the YB.Therefore, it is necessary to determine the lithology and formation age of this bedrock, which will help reveal the basin architecture and new exploration potentials.

Geological Setting
The Yitong Basin (YB), which is located in northeastern China and distributed within a long, narrow region trending toward the northeast direction at 45-55°, is approximately 140 km long and 5-20 km wide and spans a total area of 2400 km 2 [9].This basin contains three tectonic units: The Chaluhe Fault Depression (CFD), Luxiang fault depression (LFD), and Moliqing fault depression (MFD) (Figure 1).The YB is separated from the Songliao Basin by the Daheishan fault uplift in the southeast, lies next to the Bohai Bay Basin in the southwest, and is adjacent to the Nadanhada Hill fault uplift.exploration.Magmatic rocks distributed widely around the Yitong Basin (YB) are mostly characterized as batholiths or stocks.As a result, some researchers have proposed that a large batholith formed in this area prior to the formation of the YB [11].However, exploration data have indicated that natural gas originated from Permian coal strata accumulated in the granitic bedrock inside the Chaluhe Fault Depression [12].In contrast, large-scale carbonate and Archean metamorphic bedrock oil and gas reservoirs have been discovered in the Songliao and Bohai Bay Basins [13], which are in close proximity to the YB.Therefore, studying the bedrock lithology and its genetic texture will not only facilitate a more in-depth understanding of basin genesis and its architecture but also reveal the formation mechanism of the accumulation of natural gas in granitic bedrock reservoirs and new exploration potentials.

Methods
Samples were collected from the three fault depressions in the Yitong Basin (YB), where 90 wells have been drilled into the bedrock.In this study, 62 bedrock core samples were collected from 30 of the 90 wells that were drilled.Sample identification and analysis were carried out at the State Key Laboratory of Geological Processes and Mineral Resources at the China University of Geosciences (SKL of CUG).First, 62 thin sections were prepared for rock and mineral identification and analysis to determine the basement lithology of the YB.U-Pb zircon dating was performed on 10 samples that were collected from the bedrock cores of 10 different wells.These 10 samples covered the major lithological types of bedrock and were collected from the slope, uplifted zone, and concave area of the basin.Thus, the results of this study likely reflect the age of the basement of the entire basin.
The samples for zircon dating were crushed and prepared at the geological laboratory of the Hebei Institute of Geological Survey.First, the core samples were crushed and sorted to separate the zircons.Then, a binocular microscope was used to select representative and transparent whole zircon grains that were free of inclusions and cracks.These zircons were then mounted in epoxy resin.After the resin was cured, the zircons were ground and polished to expose their interior structures.Finally, the grains were carbon-coated and imaged using cathodoluminescence (CL) scanning electron microscopy.Both the CL imaging and U-Pb isotopic dating were completed at the SKL of CUG.Zircons with greater transparency and clearer oscillatory zoning were selected based on the analyses of reflected light, transmitted light, and CL images.The U-Pb isotopic dating of zircons was carried out using laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS).The laser ablation system used for U-Pb zircon dating was a GeoLas 2005, and the inductively coupled plasma mass spectrometer used was an Agilent 7500a.The spot diameter used in the analysis was 30 µm.Helium (He) was used as the carrier gas during laser ablation, with argon (Ar) being used as the compensation gas to adjust the sensitivity; 29 Si was used as an external standard.A small amount of nitrogen was added to the plasma gas (Ar + He) to improve instrumental sensitivity and analytical precision [14].Each instance of the data analysis contained a blank signal that lasted for approximately 20-30 s and a sample signal that lasted for approximately 50 s, and the international zircon standard 91500 was used for fractionation correction [15].Data collected in this experiment were processed using ICP-MS DataCal in the laboratory.The standard errors of both the isotopic ratios and ages were 1σ.All the diagrams were produced using Isoplot V3.0 (Redmond, Washington, DC, USA, 2003).

Results
The sampled cores are observed under the microscope to identify the mineralogical compositions and granite rock types.The U-Pb zircon dating method is employed to investigate the ages of the granites from different depressions of the basin.These together will help understand the rock properties and nature and age of intrusion in the process of basin forming.

Basement Lithology
Sixty-two sampled thin sections from the subsurface core were prepared and identified under microscope using cross-polarized and plane polarized lights.Figure 2 shows the mineral composition of the major rock types and secondary mineral characteristics of the metamorphic rocks.The sample at the depth of 2786.8 m from well X12 is medium-coarse-grained biotite granite.There exists a large hypidiomorphic-granular plagioclase particle (plg) with polysynthetic twining and an indistinct zonal structure.The grains are intensively sericitized (ser) and limonitized (lmt), and Np'Λ(010) = 13 • .Xenomorphic granular quartz (qtz) is scattered throughout, with grains containing multiple cracks and a small amount of feldspar.The sample at depth of 2529 m from well C11 is micrographic microspherulitic K-feldspar granite.It is mainly composed of mixed micrographic spherulites generated by the eutectic crystallization of K-feldspar (k-fsp) and quartz (qtz).Scattered xenomorphic granular plagioclase (plg) that has been sericitized (ser) and xenomorphic quartz (qtz) are sparsely distributed throughout.The sample at depth of 2122.9 m from well Y46 is microcline (mc) with tartan twinning.It is xenomorphic and sparsely distributed.There exists a small amount of fine-grained plagioclase (plg) and large grains of limonitized (lmt) biotite.The sample at depth of 3066.6 m from well C27 is porphyaceous moyite.The main mineral types are xenomorphic granular perthite (per), xenomorphic quartz (qtz), and plagioclase phenocrysts.The groundmass is mainly composed of fine-grained quartz (qtz), scattered limonite (lmt), and agglomerated sericite.Sample at depth of 1945.3 m from well Y9 is medium-fine-grained monzogranite with hypidiomorphic plagioclase (plg).It is characterized by clear polysynthetic twins, sericitization (ser) at the surface, and a grain size of 0.5-2.5 mm.Np'Λ(010) = 12 • .Oligoclase and xenomorphic granular quartz (qtz) are sparsely distributed.Undulatory extinction is observed.Multiple-grain aggregation is primarily observed.Scattered schistic biotite is completely limonitized (lmt).Sample at depth of 1945.3 m from well Y9 is medium-fine-grained monzogranite.Xenomorphic granular orthoclase (or) with Kaner twining and sparsely distributed hypidiomorphic plagioclase (plg) that has been sericitized (ser) at the surface are present.Xenomorphic granular quartz (qtz) mostly exhibits multiple-grain aggregation.Schistic biotite is completely limonitized (lmt).Sample at depth of 3153.5 m from well C17 is microcline (mc) and perthite (per).They are xenomorphic with multiple cracks.Xenomorphic granular quartz (qts) exhibits strong undulatory extinction, strong fragmentation, and minor fractionation.Xenomorphic granular plagioclase (plg) has polysynthetic twins that are slightly bent and has multiple cracks.Mineral mylonitic bandings (myl) are irregularly distributed between large feldspar-quartz grains.Sample at depth of 1746.6 m from well Y13 is quartz-muscovite schist with andalusite.It is mainly composed of muscovite mica flakes.The c-axis is obviously aligned.It primarily exhibits banding aggregation.Fine-grained xenomorphic crystalline andalusite (and) is scattered in muscovite, with the long axis slightly aligned.There exists a small amount of medium-and fine-grained quartz that is lenticular and sparsely distributed in bands of muscovite, with close intragranular contact.
The lithology identification results based on thin section observation suggested that the bedrock of the Yitong Basin (YB) was mainly composed of igneous rock, with a small amount of metamorphic rock found in the Moliqing fault depression (MFD).Microscopic observations of the igneous rocks indicated that they contained common minerals such as quartz, plagioclase, potassium feldspar, albite, and microcline; in addition, plagioclase with lamellar twinning, potassium feldspar-quartz graphic textures, microcline with tartan twinning, perthite and quartz, plagioclase phenocrysts, and orthoclase with Carlsbad twinning were observed in these thin sections.Schist, which reflected a relatively higher degree of metamorphism, was discovered in a minor region of bedrock in the MFD (Figure 2).Table 1 indicates that the bedrock was mainly composed of various minerals, including quartz (20-50%), alkali feldspar (30-60%), and plagioclase (20-50%) and that the bedrock mostly contained fine-to medium-grained granite with inequigranular structures.By applying the International Union of Geological Sciences plutonic classification system [16] (Figure 3) for classification, we determined that nearly 70% of the samples were classified as monzogranite and syenogranite, 20% of the samples (area 3 in Figure 3) as alkali feldspar granite, approximately 5% of the rocks (area 6 in Figure 3) as granodiorite, and a small fraction of the rocks as monzonite and gabbro.The above study of the bedrock lithology of the YB yielded the conclusions that the bedrock of this basin almost completely consist of intrusive rocks, considering that nearly 70% of the samples were classified as monzogranite and syenogranite.
(20-50%) and that the bedrock mostly contained fine-to medium-grained granite with inequigranular structures.By applying the International Union of Geological Sciences plutonic classification system [16] (Figure 3) for classification, we determined that nearly 70% of the samples were classified as monzogranite and syenogranite, 20% of the samples (area ③ in Figure 3) as alkali feldspar granite, approximately 5% of the rocks (area ⑥ in Figure 3) as granodiorite, and a small fraction of the rocks as monzonite and gabbro.The above study of the bedrock lithology of the YB yielded the conclusions that the bedrock of this basin almost completely consist of intrusive rocks, considering that nearly 70% of the samples were classified as monzogranite and syenogranite.

U-Pb Zircon Ages
The results of U-Pb zircon dating are listed in Table 2.As indicated by the CL images of zircon from the bedrock in the Yitong Basin (YB) (Figure 4), most of these zircons were light grey, although a few were dark grey.These zircons were mostly long euhedral prismatic crystals with clear internal structures and typical magmatic oscillatory zoning, suggesting that they were produced by magmatic crystallization [18].Extensive studies have revealed that zircons with different origins vary in their concentrations of Th and U as well as in their Th/U ratios; therefore, the Th/U contents of these zircon grains could also be used to indicate their magmatic origins.For example, magmatic zircons contain higher proportions of Th and U and larger Th/U ratios (generally, >0.4), whereas metamorphic zircons contain lower proportions of Th and U and smaller Th/U ratios (generally, <0.1) [19].According to the results of the LA-ICP-MS and U-Pb zircon isotopic analyses (Table 2), the Th and U concentrations in the igneous rock samples from the Chaluhe Fault Depression (CFD) were 34.4-1232 and 78.1-1562 ppm, respectively, and the Th/U ratios of the four zircon samples collected from samples C11, C15, C17, and C24 were 0.59-2.5,0.44-0.72,0.39-0.69,and 0.45-1.3,respectively.The Th/U ratios of the six samples collected from both the Luxiang fault depression (LFD) and Moliqing fault depression (MFD), i.e., samples L2, X5, X10, Y3, Y18, and Y46, were 0.31-0.75,0.42-2.41,0.89-2.26,0.52-0.86,0.68-1.68,and 0.17-0.70,respectively.It was evident that the Th/U ratios of these samples also lay within the Th/U range of typical magmatic zircons.In summary, all the zircons we dated in this study were found to have magmatic origins.The distributions of the entire LA-ICP-MS data obtained from the 10 zircon samples in this study indicated that these zircons were densely distributed on the concordant curve (Figure 4), with only a minority of the zircons deviating slightly from the curve.However, the deviating values still lay within the error range of the concordia, suggesting that the U-Pb isotopic systems of these zircons remained in a closed state after formation and that the formation age of the basement in the

Discussion
A series of faulted basins were developed in eastern China during the Yanshanian period, such as the Songliao and Bohai Bay Basins, which closely neighbor the Yitong Basin (YB).Over the past few years, a large amount of fossil energy has been discovered from the bedrock of these basins, but their basement lithology is completely different from that of the YB.Some studies have indicated that the basement rocks of the Songliao and Bohai Bay Basins are mainly composed of sedimentary rocks with a small proportion of metamorphic and igneous rocks.The fact that these basins are considerably larger than the YB has facilitated the development of bedrock buried-hill reservoirs.In contrast, the observation result of a large amount of thin sections suggests the basement of the YB is mainly composed of tight and strong-competent granite, which is not favorable for the development of the buried-hill reservoirs in bedrock.
On the other hand, although the bedrock of the YB is mainly composed of intrusive rocks, the basement itself is not a single batholith.The apparent resistivity profile from the electrical-resistivity data inversion indicates that the bedrocks of the Chaluhe Fault Depression (CFD) and Luxiang fault depression (LFD) within the YB are essentially rock masses resulting from the lateral intrusion of granite (Figure 6) [20,21].This is consistent with the development of the Yanshanian age granite based on the U-Pb dating of the basement samples of CFD, which suggests the lateral granite intrusion into a Permian sedimentary layer during Yanshanian period from 177 to 170 Ma.Following this intrusion, the upper layer of the bedrock was eroded because of the uplift.Then, the YB was formed and underwent a new period of sedimentation during Cenozoic period.Similar to the bedrock, regional metamorphism was not observed in the Paleozoic carbonate and Carboniferous-Permian coal strata beneath the intrusive sheet.A major breakthrough in natural gas exploration was previously made in the granitic bedrock inside Chang Well 37 (C37) (not a buried-hill reservoir).These all suggest that natural gas is derived from the deep Carboniferous-Permian coal strata buried more than 6000 m beneath the intrusive sheet, thus opening up a new horizon of natural gas exploration in the YB.
Minerals 2018, 8, x FOR PEER REVIEW 18 of 20 based on the U-Pb dating of the basement samples of CFD, which suggests the lateral granite intrusion into a Permian sedimentary layer during Yanshanian period from 177 to 170 Ma.Following this intrusion, the upper layer of the bedrock was eroded because of the uplift.Then, the YB was formed and underwent a new period of sedimentation during Cenozoic period.Similar to the bedrock, regional metamorphism was not observed in the Paleozoic carbonate and Carboniferous-Permian coal strata beneath the intrusive sheet.A major breakthrough in natural gas exploration was previously made in the granitic bedrock inside Chang Well 37 (C37) (not a buried-hill reservoir).These all suggest that natural gas is derived from the deep Carboniferous-Permian coal strata buried more than 6000 m beneath the intrusive sheet, thus opening up a new horizon of natural gas exploration in the YB.For the Mesozoic-Cenozoic basins with underlain Paleozoic and even Proterozoic sediments in east China and northwest China, Mesozoic and Cenozoic granite intrusions are very common [22], an intrusive sheet could exist if lateral intrusions of Mesozoic to Cenozoic granite occurred.At the same time, the diapir of the older source rocks below the intrusive sheet resulting from the heating and tectonic compression could cause the granite sheet to arch up; these arched rocks will then form the anticline or fractured granite to trap the hydrocarbons.This kind of fractured granite reservoirs are widely distributed in hydrocarbon basins in the Songliao, Hailar, and Bohai Bay Basin in northeast and east China [23].The Carboniferous to Permian strata with coal measure are widely distributed in basins in northeast China, east China, north China, northwest, and southwest China [24], the lateral intrusion of granite rocks may occur in many basins due to the widely developed Mesozoic to Cenozoic intrusions, which indicates deep coal-bearing Carboniferous-Permian sandstone and limestone below the lateral intrusive sheet could also be exploration target for deep natural gas.For the Mesozoic-Cenozoic basins with underlain Paleozoic and even Proterozoic sediments in east China and northwest China, Mesozoic and Cenozoic granite intrusions are very common [22], an intrusive sheet could exist if lateral intrusions of Mesozoic to Cenozoic granite occurred.At the same time, the diapir of the older source rocks below the intrusive sheet resulting from the heating and tectonic compression could cause the granite sheet to arch up; these arched rocks will then form the anticline or fractured granite to trap the hydrocarbons.This kind of fractured granite reservoirs are widely distributed in hydrocarbon basins in the Songliao, Hailar, and Bohai Bay Basin in northeast and east China [23].The Carboniferous to Permian strata with coal measure are widely distributed in basins in northeast China, east China, north China, northwest, and southwest China [24], the lateral intrusion of granite rocks may occur in many basins due to the widely developed Mesozoic to Cenozoic intrusions, which indicates deep coal-bearing Carboniferous-Permian sandstone and limestone below the lateral intrusive sheet could also be exploration target for deep natural gas.

Conclusions
In this study, we investigated the lithology and zircon age of the basement in the Yitong Basin (YB).The results have considerably improved our understanding of the nature and architecture of the basin and the value of any fossil energy reserves that may exist below the bedrock.The following conclusions are drawn:

•
The basement of the Cenozoic Yitong Basin is almost entirely composed of igneous rocks, especially intrusive rocks.The bedrock is not a single batholith but is instead composed of sheeted rock masses in the Chaluhe Fault Depression (CFD) in the northern part of the basin and in the Luxiang fault depression (LFD) in the middle of the basin.Both rock masses were produced by the lateral intrusion of granite.

•
The Yitong Basin was not formed by one tectonic event.Although the CFD in the north and the LFD in the middle of the basin were both formed during the Yanshanian period, the bedrock of the Moliqing fault depression (MFD) was formed during the Indosinian and Hercynian periods.

•
The Yitong Basin is located close to the Mesozoic Songliao and Cenozoic Bohai Bay Basins, but has completely different bedrock properties compared to the nearby basins.The Yitong Basin does not have favorable conditions for the development of the buried-hill reservoirs because of its smaller basin area and tighter bedrock.

•
Two-layer basin architecture has been revealed.The Carboniferous-Permian sedimentary rocks that have not undergone metamorphism still exist below the batholith (laterally intrusive sheet) within the CFD.Therefore, these newly revealed deep coal-bearing Carboniferous-Permian strata represent a new horizon for future clean natural gas energy exploration in the deeper subsurface besides the Tertiary sedimentary cover above the intrusive sheet.

Figure 1 .
Figure 1.Location and division of tectonic units in the Yitong Basin.

Figure 1 .
Figure 1.Location and division of tectonic units in the Yitong Basin.Paleozoic to Cenozoic granitic rocks are widely developed in the Northeast China and have been used to characterize the regional tectonics of Paleozoic central Asian Orogenic Belt (CAOB) and Paleo-Pacific subduction[10], but the results have not provided insights relating to basin evolution and fossil energy

Figure 3 .
Figure 3. Classification of the intrusive rocks in the bedrock.Classification grid and rock names are from Streckeisen [17].

Figure 4 .
Figure 4. Cathodoluminescence (CL) images and U-Pb concordia plots for samples from the Chaluhe, Luxiang, and Moliqing fault depressions.Sample numbers and burial depths are annotated in the bottom left corner of each image.

Figure 4 .
Figure 4. Cathodoluminescence (CL) images and U-Pb concordia plots for samples from the Chaluhe, Luxiang, and Moliqing fault depressions.Sample numbers and burial depths are annotated in the bottom left corner of each image.

Figure 6 .
Figure 6.Apparent resistivity section of the Chaluhe Fault Depression (CFD) through the Yitong Basin, showing basin architecture controlled by the lateral intrusive sheet.

Figure 6 .
Figure 6.Apparent resistivity section of the Chaluhe Fault Depression (CFD) through the Yitong Basin, showing basin architecture controlled by the lateral intrusive sheet.

Table 1 .
Lithologic characteristics of core samples from the igneous basement of the Yitong Basin.