Characteristics of Middle-Lower Ordovician Ultradeeply Buried Carbonate Reservoirs in Shunbei Area, Tarim Basin—A Case Study from Well SHBP1

Fracture-pore complexes, located at fault zones of Darriwilian carbonates, are the main exploration targets in the Shunbei area, Tarim Basin. This article focuses on the carbonate characteristics in the Lower Ordovician of Well ShunbeiPeng1 (SHBP1), the carbonate is sandwiched by two trending NW-SE deep-seated, strike-slip faults in the Shuntuoguole uplift. Based on detailed core investigation and careful petrographic examination, two main reservoir types, denoting microquartz and dolomite, are recognized. The former is mainly constrained in the lower part of the sampled core and gradually evolves to the latter with progressively decreases of silica. Integrated isotopic geochemistry (C, O, and Sr) and fluid inclusion microthermometry suggest that the fluids are hydrothermal origin and migrate along deep-seated strike-slip faults, forming favorable carbonate reservoirs. The fault-controlled hydrothermal reservoirs, characterized by the strong amplitude in the seismic profile, would be the most promising exploration area in deeply buried carbonates in the Tarim Basin, particularly in the Shuntuoguole area.


Introduction
The lower Paleozoic carbonate rocks, located at the Tarim Basin, contain abundant oil and gas resources and are the main targets for hydrocarbon exploration in the future [1,[2][3][4][5][6]. However, the characteristics, mechanism, and distribution of the promising carbonate reservoirs have not been clearly understood, probably due to the ultradeeply burial and multiple intensive tectonic alterations. As a result, it is urgent to perform investigations to reveal the inherent characteristics of ultradeeply buried carbonate reservoirs in Tarim Basin.
Shunbei area, located in the Shuntuoguole low uplift, is characterized with low exploration position. Notably, from the purpose of hydrocarbon exploration in Shunbei area, fault zone is the main target area [7][8][9][10][11]. Since the successive breakthrough of the Shunbei No. 1 Fault and subsequent No. 5,No. 7 Faults demonstrated the abundant distribution of fault-controlled carbonate reservoirs in the Middle Ordovician carbonates [12][13][14][15][16]. In light of abnormal beaded reflectors, the high-quality reservoirs may exist in the deep-buried geological layers, ranging from Florian to Dapingian successions [9,17,18]. However, these reservoirs have not yet been detailed studied, due to the scarcity of the drilling core dataset. Therefore, this article aims at determining the characteristics of the deep-burial carbonate reservoirs, and their relationships to the tectonic activity as well as evolution, and documents the main influential factors of these deepburial carbonate reservoirs finally. In order to reach the goals, in this article, a systematic petrographic and geochemical study of Well SHBP1, located at the Lower Ordovician Yingshan Formation of the Middle Ordovician, is carried out, proving siliceous is of secondary metasomatism origin. The above observations further indicate that transformation is important for the siliceous fluid reservoirs in this block. Till now, current investigation, upon Shunbei Yingshan Formation, is greatly limited as a result of low exploration and well coring data, therefore it is hard to judge the siliceous metasomatism of reservoirs, its function to transform the spatial distribution characteristics, and its scale. Accordingly, the study of the core of Well SHBP1 will be an important way to clarify the nature and source of siliceous fluid and serves as the prediction tool of the development characteristics, genesis, and main controlling factors of the fracture hydrothermal-controlled carbonate reservoir. This research will provide reference for future oil and gas exploration of deep-buried carbonate reservoirs.

Geological Setting
Shunbei area is located in the north of the Shuntuoguole low uplift, falling in the conjunction between the Awati faulted depression, Manjiaer depression, and Shaya uplift. The Shuntuoguole uplift formed in the middle Caledonian and underwent multiple tectonic activities, such as the Hercynian and Indo-china, forming the nowadays framework. A series of deep-seated and strike-slip faults, which intersect the strata from the crystalline basement through the Upper Ordovician Yijianfang Formation, are present in the Shuntuoguole uplift [7,8,19,20]. The Cambrian-Lower Ordovician in the Shunbei area is dominated by huge thick carbonate rocks, and then, the craton disappeared. In the late Hercynian period, the northern Tianshan Ocean and the southern Tettis Ocean closed orogeny, and the Tarim block became the southern margin of the Eurasian continent, and the Marine sediments were dominated by the continental littoral shallow lake and fluvial facies. Under the control of oblique and rotating stress fields, generated by the multistage activities of the two paleo-uplifts in Tazhong and Tabei, a series of intraplate strike-slip fault zones, sharing a good matching relationship with regional tectonic events, formed in the Shuntuoguole area ( Figure 1). Multistage fault zones, with NE and NW directions and large strike-slip features, are developed in the area, which cut down through the Cambrian to the basement and disappeared in the Upper Ordovician mudstone or Yijianfang Formation plastic formation [7,8]. Notably, the vertical structure has upright or flower-like shape. Moreover, analysis results from multiple wells indicate the desirable reservoir-forming conditions in Shunbei area, and the fault zone is the main favorable area for hydrocarbon enrichment.
Well SHBP1 is sandwiched between No. 5 and No. 7 strike-slip fault zone, trending NW-SE direction, meanwhile it has negative flower structure and conjugates the NEtrending strike-slip faults in the Xayar uplift ( Figure 1). In the block, there are different positions of deficiency in the middle and upper Silurian series, middle and lower Devonian series, upper Carboniferous series, lower and upper Permian series, Jurassic series, upper Cretaceous series, and Paleogene Kugelman formation, while the rest of strata are fully developed. In the block, the Ordovician strata are fully developed, including the lower series Penglaiba formation, the middle series Yingshan formation, the middle series Tongyi Jianfang formation, the upper series Qierbak formation, Lianglitage formation, and Sangtamu formation. Among them, the Tongjianfang formation and the Yingshan formation in the Ordovician system are the main target strata, and the Penglaiba formation-Yingshan formation developed NE concealed faults, and the reservoir type is fracture-cavity type.

Methods
The samples, for cathodoluminescence (CL) analysis, were processed by twin polishing, then pasted with epoxies, and tested with the help of CL8200 MK5 cathodoluminescence apparatus. The working conditions were set as high pressure at 8~10 kV, electric current at 0.4~0.6 mA, and vacuum degree at 13~7 Pa.
As for scanning electron microscope (SEM) analysis, fresh core samples were put in a 50°C oven for four hours at first and then coated with gold membrane. At room temperature, the samples were observed, using a Quanta 200 scanning electron microscope with frequency of 50 HZ and resolution of 30 nm.
In situ C-O isotope analysis was conducted on sections with thickness ranging from 50 to 100 μm, using laser Raman ablation on a Thermo Fisher MAT253 mass spectrometer. Specifically, the laser spot diameter is 200 μm, and GBW4405 and 811 standards are adopted for parallel analysis, with a resolution of ±0.22% for both δ 13 C and δ 18 O measurement.
Sr isotopic analysis was carried out on a Finnigan MAT-253 Triton TI mass spectrometer with an error of 0.002%. The samples for fluid inclusion analysis were prepared into 0.1 mm sections by twin polishing operation, and a Leica DM4500P microscope was employed for mineral observation. During a Linkam TS1400XY type cooling/heating stage, homogenization and freezing-point temperatures were measured, the resolutions can reach 2°C and 0.5 (±5%)°C, respectively.
3.1. Petrography Characteristics. The cores, sampling at the vicinity of the lower boundary of Yingshan Formation from Well SHBP1, which is the only cored individual well in study area, are badly damaged. The color of the whole rock is gray to white gray. From the bottom to the top, two units could be subdivided based on detailed core investigation as well as microscopic examination.
The lower unit, with color from light to gray, has a high content of silica with minor black quartz crystals (Figure 2(a)). The content of silica gradually decreases downwards, whereas the calcareous increases ( Figure 2(b)). From a microscopic perspective, two distinctive quartz textures are recognized, suggesting the matrix and cement quartz (Figures 2(c) and 2(d)). The matrix quartz is mainly composed of microcrystalline quartz (Figures 2(e) and 2(f)) and chalcedony. The former is very small (<5~20 μm) with equigranularity structure, whereas the latter is fibrous and/or radial with cryptocrystalline texture. In contrast, the quartz cements commonly consist of coarse to very coarse crystalline (20~2000 μm), and its shape varies from blade to columnar 2 Geofluids forms ( Figure 2(c)). In addition, crystals occur as fracture infills with mosaic texture and become coarsening from the rim towards the center (Figure 2(d)). The upper interval is mainly composed of dolomite, which is gray to light gray (Figures 3(a) and 3(b)). The existence of relict breccias can be observed locally (Figure 3(a)). Pore spaces, produced by dissolution, pervasively occur along the fractures and are partially filled by coarse calcite cements or organic materials (Figures 3(a) and 3(b)). With the utilization of the microscope, the size of dolomite crystals ranges from 5 to 20 μm, characterized by euhedral to subhedral crystals with planar to rectilinear crystal surfaces. Also, it is evident that some dolomite crystals are subject to mild corrosion. Intercrystallite pores are pervasively present and partially occluded by microquartz mineral (Figures 3(c) and 3(d)). Dolomites near fissures are broken into honeycomb pores, while fissures or caves are filled with megacrystal calcite (Figures 3(e) and 3(f)). To current knowledge, core characteristics indicate the typical fluid reforming traces. In this work, efforts have been made to identify the role of fluid in reforming reservoirs by geochemical means.  (Figure 4).

Sr Isotope
Characteristics. The micrite in Yingshan Formation of Shunbei area has a narrow range of 87 Sr/ 86 Sr ratio, varying from 0.708750 to 0.708792, which is basically consistent with the previously reported ratio of micrite in this area ( 87 Sr/ 86 Sr: 0.708899) (Li, Acta Petrolei Sinica). The pack stones yield 87 Sr/ 86 Sr ratios of 0.70881 to 0.708914, with an average of 0.70885, which is generally consistent with the isotopic ratio of Middle Ordovician age (0.7087~0.7089) [21][22][23]. Late-stage calcite fillings in fissures have 87 Sr/ 86 Sr ratios (0.709805~0.710201), greatly higher than those of limestones ( Figure 5).

Fluid Inclusion Microthermometry.
Inclusions in dolomite have diameters mostly of <10 μm; therefore, relevant work cannot be conducted. However, massive two-phase inclusions are developed in megacrystal calcite cement, and vapor-liquid two-phase saline inclusions mostly take place in the oriented bands. These inclusions are colorless under transmission light, and they have little typical secondary evolution, with sizes falling in the range of 3~15 μm, the average value is 8 μm. The minimum homogenization temperature is 124°C, and the maximum temperature exceeds the tolerance of the measuring instrument; thus, homogenization could not be achieved. All the measured temperatures are beyond 230°C. According to inclusion quantity, the average temperature was set slightly higher than 160°C, and the measured main homogenization temperatures were controlled in the 150~160°C or>200°C intervals ( Figure 6). 3 Geofluids [24], the salinities were calculated, ranging from 15.2 to 17.9 wt% NaCl equivalent, with an average of 16.2 wt% NaCl equivalent. In compliance with the definition of fluid inclusion combination [25,26], it is believed that there are two groups of inclusions. Other than that, considering the temperature difference exceeds 30°C, the hydrothermal fluids may exist.

Discussions
According to the aforementioned results, the development characteristics of reservoirs can be summarized. At first, pores are formed along with high-angle fractures and fractured breccias. Then, the reservoir space is composed of dilatant pore-caves along fissures, dolomite intercrystallite (corrosion) pores, and autogenic quartz intercrystallite pores. Aside from that, reservoir development mechanism can be summarized as the interface-controlled faultinghydrothermal activity. The four main evidences are as follows: (1) From the macroscopic view, observation of core indicates the fading thermal color along fissures. Moreover, with the help of a microscope, it can be

Geofluids
observed that thermal catacaustic phenomena appear in corrosion pore, and where quartz replacement happens. Thermal color fading is a phenomenon that the rock turns locally or totally shallow (even to white) in color, generally along fissures. This is mainly induced by thermal fluids, activating along fissures and reacting with the surface rocks [27,28]. Thermal color fading is common in Ordovician carbonate rocks in Tazhong area and mainly displays dark-gray or gray. In addition, thermal color fading is observable in the Silurian sandstone near fissures. Thermal color fading is generally small scaled and merely within several centimeters of two sides of a fissure. As for the case of cores of Well SHBP1, dolomites around fissures experience ther-mal color fading, and most of them (protolith) are not altered and their color remains grayish white. The alteration only occurs near fissures, and the after-alteration color turns into shallow gray to white Under thermal activity, a special thermal catacaustic phenomenon is discovered in Silurian sandstones of the Tahe oil/gas field [28]. For lots of sandstone grains, there is one or multiple irregular networked fissures, which are mostly isotropic. Fissures among different grains are generally discontinuous. If the fracturing activity is caused by oriented external force, fissures shall show certain orientation and continuity. The thermal catacaustic phenomenon in sandstone grains is caused by the thermal process, as pressure decreases, hydrothermal fluids swelled rapidly in (2) A typical hydrothermal mineral assemblage was discovered, denoting the quartz+zeolite. Zeolite is chiefly filled in vesicles and fissures in intermediate-basic volcanic lava, as well as intergranular fissures in volcaniclastic rocks. Zeolite accounts for 5~32% of the total rock volume, with an average of 18% [32]. Zeolite is generally white, shallow gray, and shallow red. Under a polarizing microscope, zeolite generally occurs as flaky, platy, or needlelike aggregates and displays low negative protuber-ance. In addition, the crystal surfaces are clear and characterized by gray to white interference colors or a fairly small extinction angle. SEM analysis shows that they occur as columnar or platycolumnar autogenic crystals, with size spanning from 15 to 30 μm. Interpenetration twins are observable, mostly occurring as aggregates filling in pores On the basis of SEM image observation and spectrum analysis, zeolite occurs in places where honeycomb dissolution pore is developed (Figure 4), with a content of <1%. In the samples here, two shapes of zeolite are observable: (1) the first shape has a high percentage, and the zeolite occurs as aggregates and its outline is close to a rhomb, mainly composed of Ca, Al, and Si; and (2) the second shape is regular and similar to the orthorhombic system of plagioclase, mainly composed of Ca, Al, Si, and O with minor Mg and K. Composition of zeolite is CaAl 2 Si 4 O 12 .6H 2 O, and it is a typical product of high-temperature dolomite, reacting with Si-rich fluid under an alkaline environment.
According to theoretical calculation results (Figure 7), dolomite can react with Si-bearing fluids at 150°C, forming talc [33,34]. Experimental results of dolomite vs. Si-rich fluids show that dolomite can react with Si-bearing fluids, forming CO 2 . Within this experimental temperature range, the solubility of calcite is higher than that of dolomite at the same temperature. According to the aforementioned study, dolomite and calcite can react with Si-bearing fluids at 100~200°C, forming CO 2 and Mg/Ca silicates and calcium metasilicate, including talc, zeolite, and other minerals. Besides, in the closed (dolomite-quartz-clay) mineralwater-CO 2 system, clay minerals (smectite) can transform into illite/smectite mixed-layer minerals in alkaline condition.
(3) The measured C-O isotopic values, varying from -1. 5 to 0.5‰, are similar with concurrent marine limestone. Additionally, it is discovered that their δ 13 C V-PDB characteristics of the matrix and secondary calcite are different [16,35,36], and δ 18 O V-PDB characteristics of the matrix and calcite veins were different from those of seawater at the same time. Carbon isotope is comparatively stable, the C isotope range for surrounding rock of Well SHBP1 is lower than <-1‰ of coeval sea water and higher than <-1‰ of secondary calcite, typical negative bias affected by organic carbon is not discovered. The genesis may be the influence, exerted by high temperature or freshwater. The δ 18 O V-PDB values of the buried dolomites are more negative than those of seawater and marine cements, and more negative than those of contemporaneous dolomites. O isotope shows typical negative bias. Given that δ 13 C has proved that organic carbon is lacking, δ 18 O of Well SHBP1 shows more typical negative bias characteristics than that of Shunnan area. From the C-O isotope intersection diagram, it can be demonstrated that the quasi-identical formation causes of the dolomite in Well SHBP1 and secondary calcite, further inferring that this characteristic is more affected by     7 Geofluids limited atmospheric freshwater characteristics. It is believed that seepage flow poses weak influences on Sr isotope. According to the lower formation features, it is inferred that the Sr isotopic characteristics are affected by globally widespread lower Cambrian black shale and siliceous rock [37,38]. Comprehension of C-O isotope characteristics indicate that they are affected by hydrothermal solution Evidence for the existence of hydrothermal fluids is inclusion temperature measurement. According to the core depth of Well SHBP1 (8450.00~8455.77 m), the ground temperature of the cored location is supposed to be 160°C. Temperature measurement of sparry calcite cement, filled in fissures-caves, indicates that the homogenization temperatures mostly exceed 160°C, and homogenization cannot achieve 230°C evenly. This suggests that the fluid temperature, when corroded magnesium calcium carbonates reprecipitate and form calcium carbonate, is higher by 5°C than that of surface rocks, after siliceous fluid reacting with dolomite. This agrees with the standard of hydrothermal fluid. Therefore, diagenetic, reforming at this position, can be determined to be faulting-hydrothermal fluid reforming.
In fact, similar phenomena are not rare in the middlelower Ordovician strata of the Tarim Basin. By means of observation, spectrum measurement, formation water Sr isotopic assay of massive rock thin sections from the reservoir interval of the Ordovician, and analysis of fault attributes, the activity characteristics of hydrothermal corrosion fluids and the control on high-quality reservoirs were investigated in-depth. The study implies that hydrothermal minerals are chiefly developed along with fault-associated structural corrosion fissures and pores/caves. Faults are the main pathway for hydrothermal fluid migration and the main controlling factor for forming high-quality reservoirs. Homogenization temperatures, H-O isotopes, and REE composition of fluid inclusions imply that quartz in the lower Paleozoic carbonate rocks was formed by magmatic-hydrothermal fluids. Furthermore, when quartz was precipitated from magmahydrothermal fluid, carbonate reservoirs were suffered from obvious corrosion. Li (2015) shows hydrothermal alteration and metasomatic silification phenomena in the Ordovician Yingshan Formation and cored interval of Shunnan Well, located at Tarim Basin, and corrosion fissures/caves, loose siliceous limestone/siliceous rock, hydrothermal veins, and coarse quartz clusters are widely developed. Geochemical analyses of core samples indicate that hydrothermal fluids display mixing signatures. The heat was sourced from magmatism, and formation water provided the main fluids, forming medium-low temperature, high-salinity silicified hydrothermal fluids.
As mentioned above, structure-controlled hydrothermal corrosion mainly occurs inside the fractured zone of a strike-slip fault. Due to the seismic wave disturbance, induced by formation fracturing/fault detachment, strong amplitude or "bead-like" strong reflection usually occurs in where fault hydrothermal activity takes place. Inside rock formations, due to fracturing or subsequent hydrofracturing, aroused by thermal brine fluid charging, abundant fissures and breccias are formed in carbonate formations, and wide matrix metasomatic silification happens, enhancing dolomite (or limestone) porosity (molar volume replacement effect). Recharging of episodic thermal brine fluids may dissolve dolomite or limestone, forming drusy caves which are partially filled by megacrystal calcite. If hydrothermal dissolution is strong enough, local collapse of rock formations can occur, thus aggravating the sink of rock formation. Accordingly, hydrothermal dissolution is called as hydrothermal karstification by some scholars [39]. Compared to carbonate wall rocks, these positions usually have more pores, caves, and fissures, while after late-stage fluid reforming, they can turn into potential high-quality reservoirs (Figure 7). Notably, the strong-amplitude abnormal reflector has the width of~200 m in Shuntuoguole area, and this strong-amplitude reflector can be well recognized by 3D seismic sections, transecting its axis.
In Shunbei oil/gas fields, the geometrical morphology of faults can be drawn according to seismic data amplitude and coherence volume analysis, while porosity and its space variation, inside strong amplitude anomalous bodies, can be precisely depicted by the comprehensive analysis of seismic attributes and logging data. In turn, they can constrain the scope of faulting-hydrothermal action, providing us a more sensible and objective understanding on the reservoir development features. Although multiple methods, including C-O isotope measure, Sr isotope, as well as fluid inclusion analysis, have been applied in this article, the research still suffers some deficiencies. First of all, there are little drilling cores for deep-burial carbonate reservoirs in Tarim Basin. As we know, comprehensive geological knowledge for a specific area roots in the abundant cores, which can provide massive information for analysis. In this article, the majority of the characteristics learn from the case of Well SHBP1, which may result in fairly great uncertainty. Other than that, it is really hard to examine the reliability of the conclusions, summarized on the basis of the experimental data, theoretical analysis, and observations. Finally, applicability activity of the research efforts to the realistic field is still lacking. In light of the above limitations, the authors will continue this research in the future.

Conclusions
(1) Based on the detailed core investigation and petrographic examination, two units, including the lower microquartz interval and upper dolomite interval, could be further categorized for the whole core samples. The intense diagenesis of SHBP1 Well was greatly constrained along with the vicinity of faults and/or fractures, indicating the distinctive characteristic of tectonic-controlled hydrothermal alteration (2) The deep-seated strike-slip faults in the Shunbei area could have provided preferential conduits for hydrothermal fluids migrating upward to the shallow burial carbonates, resulting in intensive dissolution and subsequent cementation. Therefore, faultcontrolled hydrothermal reservoirs, characterized 8 Geofluids by the strong amplitude in the seismic profile, are widely distribute in the interior of Yingshan Formation (3) This study provides a useful explanation of tectoniccontrolled hydrothermal alteration, which could be helpful to make the exploration strategy, aiming at finding favorable carbonate reservoirs in deeply buried carbonates in the Tarim Basin, particularly in the Shuntuoguole area

Data Availability
Data available on request.

Ethical Approval
On behalf of all the coauthors, the corresponding author states that there are no ethical statements contained in the manuscripts.

Conflicts of Interest
The authors declared no potential conflicts of interest with respect to the research, authorship, an/or publication of this article.