The carbonate ramp facies model was proposed by Ahr^[1] and accepted widely by most geologists. It refers to a large carbonate sedimentary system with a gentle slope from shore to broad sea and usually with a slope of less than 1°. In a low-energy environment between shallow and deep water areas, there is no obvious slope-break belt, and wave-disturbance belts (high-energy belts) are generally in the nearshore zone. There is a lack of gravity flow deposits transported from shallow water and high energy environments to deep water areas, which is different from rimmed carbonate platform facies ^[1,2]. Read ^[3] further divided ramps into uniformly inclined ramp and remotely steepened ramp based on the study of the Middle Ordovician carbonate ramps in Appalachian Virginia, North America. Since the 21^st century, carbonate oil and gas fields related to ramp deposits have been discovered successively ^[4,5,6], such as the Lower Carboniferous car-bonate ramp of Brabant in Wales, the Lower Cambrian Longwangmiao Fm. ramp gas reservoir in the Sichuan Basin and Ordovician carbonate ramp in Gucheng area of Tarim Basin. Carbonate ramp deposits have attracted broad attention. However, little efforts have been put on the sedimentary microfacies characteristics of carbonate ramps and their impacts on favorable reservoirs.

Located in the transition area from the Cambrian- Middle Ordovician carbonate platform in the central and western Tarim Basin to the deep-water basin in the eastern part of the Tarim Basin (hereinafter referred to as the eastern Tarim Basin), the Gucheng area has developed carbonate ramp deposits ^[7]. A significant breakthrough has been made to deep Ordovician oil and gas exploration^[8]. In 2012, a high-yield industrial gas flow of 26.4×10⁴ m³/d was obtained from the lower dolomite in the Ordovician Yingshan Fm. between 6144 m and 6169 m in Well GC6. Subsequently, high production was obtained from the dolomite of Yingshan Fm. in GC8 and GC9 Wells, and gas flow of 107.8×10⁴ m³/d was obtained after acid-fracturing in Well GC9 ^[9]. However, in recent years, the exploration results in this area have not been very satisfactory ^[10]. Up to now, 18 exploratory wells have been drilled in the Gucheng area, including 5 industrial gas wells and 4 low-yield gas wells. The proportion of failed wells is high, and the exploration discoveries appear as ‘local but not consecutive’. Dolomite reservoirs have not been drilled in the Yingshan Fm. in some wells such as CT1 and CT2 ^[10,11]. The possible causes are as follows: (1) The fine divisions of sequence stratigraphy in the Ordovician carbonate has not been realized and there is a major controversy on the division of frequent sequence in the 3^rd member of the Yingshan Fm. (hereinafter referred to as Ying 3 Member). (2) There is no fine characterization on the sedimentary microfacies. Although some scholars proposed that the Ordovician carbonate rocks are ramp deposits, no deep research has been carried out on the types and development characteristics of the ramp sedimentary microfacies and their impacts on favorable reservoirs ^[12,13]. (3) Little efforts have been put on the distribution of favorable reservoir facies belts and oil and gas accumulation, so that no proven oil and gas reserves have been released. The main steps in this study are as follows: first, the types and assemblage characteristics of carbonate rocks in the Gucheng area are summarized. Secondly, the sedimentary microfacies type and development model of carbonate ramps are clarified. Finally, the middle-lower Ordovician sequence stratigraphic framework is delineated based on large amounts of the latest geological and geophysical data, core observation and thin slice identification results as well as comprehensive analysis of seismic and logging data. By investigating the controlling effects of frequent sequences and sedimentary microfacies, the dolomite reservoirs in this area are predicted as favorable reservoirs. These findings will provide guidance for future exploration in the Gucheng area, eastern Tarim Basin.

The Gucheng area is on the Gucheng Low Uplift in the Tarim Basin ^[14]. It is adjacent to the East Uplift in the east and the Manjiaer sag in the north, and bordered by the Tazhong I Fault to the Tazhong Uplift in the southwest, and the Cheerchen Fault in the southeast (Fig. 1a). From bottom to top, there are Ordovician strata developed in the Penglaiba Fm. (O₁p), Yingshan Fm., Yijianfang Fm. (O₂yj), Tumuxuke Fm., and Queerchok Fm. The Yingshan Fm. is subdivided into O₁y₄, O₁y₃, O₂y₂, and O₂y₁ members (Fig. 1c). At present, a major breakthrough has been made to the exploration of the Ying 3 (O₁y₃) dolomite reservoir, which is the major gas-bearing interval in the Gucheng area, with intercrystalline pores, intercrystalline dissolved pores, and dissolved pores as the dominant reservoir space ^[15].

Affected by the broken Rodinia supercontinent, in the Tarim Basin, deep-water argillaceous carbonate ramps were developed in the Early Cambrian, then evolved into rimmed carbonate platforms from the late Early Cambrian to the Late Cambrian, and finally transformed into carbonate ramps in the Early and Middle Ordovician ^[16,17]. From the Middle Ordovician, the Tarim Plate was gradually transformed from an early extensional to a compressional and convergent structure. The Tazhong I fault was reversed to a thrust (the Tazhong area was strongly uplifted out of water). But the Gucheng area was still in water because it was in the down-thrown side, so that development of shallow open platform deposits was noted in that area ^[10]. In the Late Ordovician, the strong subduction of the southeastern margin of the Tarim plate mitigated, while the Gucheng area subsided again. The sedimentary environment was in a gradual transition from platform to deep-water basin, so that extremely thick terrigenous clastic deposit was developed and covered on the Middle-Lower Ordovician carbonates, forming a favorable cap region ^[15] (Fig. 1). By then, the history of carbonate deposition ended in the Gucheng area.

At the end of the Late Ordovician, affected by the Cheerchen faulting activity, a tilting movement occurred in the Gucheng area, which made the early structure with west platform (high) and eastern basin (low) transform to low and gentle nose uplift. The latter was high in the southeast and low in the northwest (the prototype of the Gucheng low bulge). Moreover, the Gucheng area was cut by the NNE-trending fault into blocks of horst and graben. At the end of the Devonian, the Gucheng area was uplifted strongly by the early Hercynian movement, and the Silurian and Devonian strata were strongly denuded. In the Middle Permian (Late Hercynian), intense magmatic activity occurred, and the upwelling of deep hydrothermal fluid along faults played an important role in reforming the Early Ordovician reservoirs and primary oil and gas reservoirs ^{[18, 19]}. At the end of the Triassic, the Indosinian movement uplifted the southeastern part of the Gucheng area (near the Cheerchen Fault) and the east of the Tarim Basin, which uplifted the structural relief of the Gucheng area from the southeast to the northwest ^[18]. At this point, the low bulge of the Gucheng area was finalized (Fig. 1b).

Based on the description of nearly 1000 m long cores from 15 wells in the Gucheng area and the observation of a large number of thin sections, detailed analysis of their rock types, sedimentary structures and the performance of original sedimentary fabrics as well as the combination of differences in hydrodynamic conditions and rock types, the Yingshan Fm. carbonate ramps are divided into 4 subfacies, namely back ramp, inner shallow ramp, outer shallow ramp, and deep ramp, and further divided into 10 microfacies, namely tidal flat, dolomitic shoal, top dolomitic flat, medium-high energy shoal, storm shoal, and finally inter-shoal (Table 1).

The back ramp subfacies is located between the highest sea level and the mean sea level, and periodically submerged by sea water and has the low water energy, mainly fine-grained sediments and strong quasi-contemporaneous dolomitization. According to the type of its rock assemblage, its logging, and seismic response characteristics, the back ramp is further divided into tidal flat and dolomitic lagoon microfacies.

The tidal flat microfacies is located on the most landward side of the ramp, and the hydrodynamic force is mainly from intermittent storm tides. The lithology is dominated by micrite dolomite and silt-fine crystalline dolomite. Horizontal laminae were observed on the cores (Fig. 2a). Local presence of bird eye-pane structures and silt-crystalline, as well as micrite-micro crystallite structures and alternating laminae crystallized to different degrees were observed on the SEM (Fig. 2b). This microfacies is developed vertically with thickness varying from 1 m to 5 m, and it is mostly associated with micrite dolomite of dolomitic lagoon microfacies. It is characterized by low-to-medium tooth-like GR (Fig. 3a) and continuous events with medium-strong amplitude (Fig. 4a).

The dolomitic lagoon microfacies is located in a relatively low-lying area on the back ramp and is between shoal bodies or high landform. Due to limited water circulation, its lithology is dominated by dark micrite-micro crystalline dolomite, with dark horizontal laminae, local bacteria, algae, and pyrite scattered occasionally (Fig. 2c). It is characterized by thin layers, medium tooth-like GR (Fig. 3a), and continuous events with medium-strong amplitude (Fig. 4a).

The inner shallow ramp is located between the mean sea level and the lowest sea level and is characterized by strong hydrodynamics, high-energy grain shoal, and periodic exposure to the water surface, strong dolomitization in the quasi-contemporaneous period. The dolomitic shoal, top dolomitic flat, and inter-shoal dolomitic flat were developed (Fig. 3b).

The dolomitic shoal microfacies is located in a relative bulge belt in the shallow water area near the mean sea level, in a semi-restricted environment, and has strong hydrodynamics. The lithology is dominated by crystalline dolomite with residual grain structure. After the original rock was restored, the sand or oolitic structure was visible, and the residual intergranular pores, intercrystalline pores, and dissolved pores were developed (Fig. 2h). Vertically, it is usually deposited alternately with the dolomitic flat of the back ramp and the high-energy sandy shoal of the outer ramp. The dolomitic shoal has a thickness varying from several meters to tens of meters and is characterized by medium-low tooth-like GR (Fig. 3b), and chaotic hilly reflection with weak amplitude (Fig. 4a).

The top dolomitic flat microfacies is usually located at the top of the dolomitic shoal. Because intermittently exposed to the sea surface, it is prone to karstification by atmospheric freshwater, resulting in a large number of dissolved cavities (Fig. 2e). The lithology is dominated by coarse-middle crystalline dolomite, with intercrystalline pores and intercrystalline dissolved pores (Fig. 2f, 2g). It is characterized by low GR (Fig. 3b), low density, and high compensated neutron porosity. The seismic response is similar to those of the dolomitic shoal (Fig. 4a).

The inter-shoal dolomitic flat is located in a relatively low-lying area between dolomitic shoals, with limited water circulation and weak water energy. The lithology is dominated by powdered crystalline dolomite (Fig. 2d). In the vertical direction, there are dolomitic shoals or top dolomitic flats alternating each other with medium jagged GR (Fig. 3b), high resistivity and continuous sub-parallel reflections with strong amplitude.

The outer shallow ramp is between the lowest sea level and the fair wave base, with unobstructed water circulation and well developed grain shoals. According to the hydrodynamic strength, the outer shallow ramp is divided into three microfacies: middle-high energy shoal (including high-energy shoal and middle-energy shoal), low-energy shoal and inter-shoal sea (Fig. 3c).

Medium-high energy shoals are strongly affected by wave action. The lithology is dominated by sparry granular limestone. The grains are made of more sand debris, followed by oolites and a small amount of gravel debris and silt debris, etc. (Fig. 2i-2k). Their thicknesses vary greatly from several meters to tens of meters. Vertically, the medium-high energy shoals are usually overlapped with low-energy shoals and inter-shoal sea. They have low GR (Fig. 3c), low uranium-to-thorium ratio, and mound-like chaotic reflections with weak amplitude (Fig. 4b).

Low-energy shoals are mostly associated with high- energy shoals. They were developed under relatively weak hydrodynamic conditions. The lithology is dominated by micrite-sparry granular limestone. The grains are dominated by fine sand and silt debris, and a small amount of medium sand debris (Fig. 21). The thickness of the shoal is generally less than 5 m. The logging and seismic responses are similar to those of the high-energy shoal with a much smaller scale (Fig. 4c).

The inter-shoal sea is located at the low-lying area between the medium and high energy shoals. It has low hydrodynamic energy, which is not conducive to the accumulation of granular shoal. The lithology of the inter-shoal sea is dominated by micrite limestone, and wavy and horizontal laminae are observed in the core (Fig. 2m). Microscopic observation shows the presence of bioclastics (Fig. 2n). Compared with the medium-high energy shoal, the inter-shoal sea is characterized by high GR (Fig. 3c), high uranium-to-thorium ratio, and parallel-subparallel reflections with medium-strong amplitude and continuous events (Fig. 4b).

The deep ramp is between the fair wave base and the storm wave base. It was developed in relatively deep water. Micrite limestone is dominant, with local storm shoal deposits. Its deposition characteristics are as follows:

Static mud facies is widely distributed in the deep ramp. The lithology is dominated by micrite limestone and silt- bearing micrite limestone (Fig. 2o). It is characterized by relatively high GR (Fig. 3d), medium uranium-to-thorium ratio, and continuous or sub-parallel reflections with moderate to strong amplitude along the bedding (Fig. 4d).

The storm shoal is in the shape of low gentle hills, which is the result of storm wave disturbance. The lithology is dominated by micrite-sparry conglomerate limestone, and the conglomerate is dominated by micrite-sparry sandy limestone of medium-low energy shoal facies (Fig. 2p). Due to the sporadic nature of storms, the storm shoals are distributed in static mud like lenses. Compared with the static mud, the storm shoals are characterized by low and medium GR (Fig. 3d) and low uranium-to-thorium ratio.

In addition, according to the seismic profile with flattened horizons in the study area, there is a Middle-Lower Ordovician steep-end slope in the east of the Gucheng area. Together with the Taxi ramp, they constitute a carbonate ramp with a far steep end. In other words, to the east of the deep ramp subfacies, there is a development of outer ramp-basin subfacies which is distributed below the storm wave base and hardly affected by storm waves. The deposits are mainly of low-energy in-situ carbonate and semi-deep sea deposits, dominated by micritic limestone of static mud microfacies and semi-deep-sea mudstone (Fig. 5).

According to the lithology of the Ordovician carbonates, δ¹³C content, seismic and logging curves in the study area, the Ordovician in the Gucheng area can be divided into 6 third-order sequences, which are namely SQ1, SQ2, SQ3, SQ4, SQ5 and SQ6 from the bottom to the top. SQ3 is sub-divided into 3 fourth-order (frequent) sequences including SQ3-1, SQ3-2 and SQ3-3. From the bottom to the top, the GR, U, and U/Th curves at the sequence interface change from a funnel-shaped low value to a tooth-shaped medium-high value. The δ¹³C value changes from a negative bias to a positive bias The seismic reflection event near the interface are continuous. The correlation and division of sequences are simple. Each sequence is composed of a transgressive system tract (TST) and a highstand system tract (HST) and is dominated by the HST deposits. Taking Well GC9 as an example, the division and characteristics of sequence interfaces are introduced (Fig. 6).

The bottom of SQ1 (6722.3 m) is the interface between the Ordovician and the Cambrian. Below this interface, there is an interval of coarse-middle crystalline dolomite deposited in a relatively limited sedimentary water environment. Above the interface, there is TST micrite limestone deposited in an open water environment. The δ¹³C value above the interface gradually changes from a negative bias to a positive bias. The δ¹³C value reaches the maximum at 6679.8 m, where the GR and U values reaches their peaks, indicating the maximum transgression, slow depositional rate and high content of accumulated radioactive elements, so marl deposits were developed. The δ¹³C value gradually decreases upwards and remains at a relatively high level, and the GR and U curves are characterized by low values in box and tooth shapes, indicating frequent water level turbulence, strong energy, so HST particle shoals were developed, such as dolomitic shoal and medium-high energy shoal. At 6460.1 m, the δ¹³C value reaches the minimum and shows an increasing trend again, and the U, U/Th curves show sharp peaks, indicating another rise of sea level. The TST of SQ1 is at 6722.3-6679.8 m, and it is dominated by inter-shoal micrite limestone. HST is at 6679.8-6460.1 m, where medium- high energy shoal, dolomitic shoal and top flat microfacies were developed, with coarse and thick sediments.

The bottom of SQ2 (top of SQ1) is at 6460.1 m. Below the interface, there is coarse-middle crystalline dolomite on the top flat, and the GR and U curves show peak clusters with high values. Above the interface, there is a SQ2-TST inter-shoal dolomitic flat with powdered-fine crystalline dolomite, the GR and U curves have low values in a box shape, and the δ¹³C value increases successively, indicating the rising water body. At the depth of 6391.5 m, all the U, GR, U/Th and δ¹³C are abnormally high, indicating the flooding surface and the interface between TST and HST. In the TST period, inter-shoal flat and dolomitic flat were developed in the inner shallow ramp. In the HST, there developed dolomitic shoal and top flat within the inner shallow ramp.

The bottom of SQ3 is at the depth of 6291.8 m. Below the interface, there is a top flat with coarse-middle dolomite. Above the interface, there is an inter-shoal dolomitic shoal with powdered-fine crystalline dolomite, indicating slow transgression. The maximum transgression occurred at the depth of 6267.3 m, forming the sea flooding surface, with peak values of GR and U curves and the extremely positive δ¹³C value. At 6166.9 m and 6075.1 m in the upper part of SQ3, there are two signs of flooding surface, i.e. two small-scale sea level rise and fall cycles. Therefore, according to the cycle characteristics of the lithology combination, δ¹³C content and logging curves, SQ3 can be sub-divided into 3 fourth-order sequences, including SQ3-1, SQ3-2, and SQ3-3, with interfaces at 6187.7 m and 6094.8 m, respectively. The TST of SQ3-1 mainly developed inter-shoal dolomitic flats with powdered-fine crystalline dolomite 24.5 m thick. The HST mainly developed dolomitic shoals and top flats with fine-coarse crystal dolomite 79.6 m thick. The SQ3-2 deposit has the total thickness of 92.9 m, 77.6% of which is in the HST. The TST mainly developed inter-shoal dolomitic flats, and the HST mainly developed dolomitic shoal and top flats. The SQ3-3 deposit has a total thickness of 99.6 m. The TST mainly developed inter-shoal sea microfacies, and the HST mainly developed mid-high energy shoals. The HST is obviously thicker than the TST. The inner shallow ramp dominated by dolomite is mainly in the SQ3-1 and SQ3-2. The outer shallow ramp dominated by limestone is in the SQ3-3. They reflect the whole transgressive process.

The bottom of SQ4 is at 5995.2 m. Below the interface, there is sparry sandy limestone, while above the interface, there is sparry silty and sandy limestone. The TST developed medium-energy shoal microfacies, and the HST developed high-energy shoal microfacies. The bottom of SQ5 is at 5909.1 m. Below the interface, there is sparry sandy limestone of high-energy shoal microfacies, while above the interface, there are micrite limestone and mudstone of inter-shoal sea microfacies. 14.9 m thick inter-shoal deposits was developed in the TST period. 121.8 m middle-high energy shoal sediments was developed in the HST period. The bottom of SQ6 is at 5766.4 m. Below the interface, the formation is dominated by sparry silty and sandy limestone. Above the interface, there is a presence of micrite limestone. The top of the Middle Ordovician is at 5680.2 m). Static mud was developed in the deep ramp in the TST period. Middle-high energy shoal deposit was developed in the outer shallow ramp in the HST period.

The sequence stratigraphic framework and sedimentary microfacies in the study area were analyzed based on well-seismic data, the entire stratigraphic framework, and key wells. The Middle-Lower Ordovician sequence was well developed in the study area. The thickness slightly decreases from the west to the east (which is consistent with the paleo-tectonic pattern which is high in the east and low in the west). The deposition is continuous, and multiple types of sedimentary microfacies were developed (Fig. 7).

Vertically, SQ1-SQ3-2 corresponds to the inner shallow ramp around GC13, GC9 and GC8 Wells in the western part of the study area. At the bottom of the sequence, it is inter-shoal dolomitic flat microfacies, and upward gradually transforms to dolomitic shoal and top flat microfacies. The shoal thickness increases after superimposition in the sequence. In the eastern part of the study area, there are alternating middle-high energy shoals and inter-shoals in the outer shallow ramp around CT1 and CT2 Wells. From SQ3-3 to SQ6, the study area is dominated by outer shallow ramp deposits, and deep ramp deposits occur gradually eastward. For example, there are multiple sets of middle-high energy shoals in the outer shallow ramp around Well GC13, and multiple sets of storm shoals and static mud in the deep ramp, and alternating middle-high energy shoals and inter-shoal sea deposits in the outer shallow ramp around Well CT2.

Horizontally, SQ1 is in a gradual transition from the back ramp-inner shallow ramp to the outer shallow ramp from the west to the east in the study area. Around Well GC8, there are multiple sets of dolomitic shoals and top flats in the inner shallow ramp, up to 120 m thick. When SQ2 deposited, as seawater invaded westward, the sedimentary range of the shallow ramp shrank westward to around GC13 and GC9 Wells. The upper part of the sequence has dolomitic shoals which are thin and poorly continuous. Around GC8, CT1, and CT2 Wells, there are middle-high-energy shoals and inter-shoal deposits in the outer shallow ramp. During the deposition of SQ3-1, as seawater receded eastward, the inner shallow ramp expanded eastward to the surroundings of Well GC8. In the west of Well GC8, two sets of stable dolomitic shoals and top flats were developed on the top of HST. In the east of Well GC8, lenticular middle-high energy shoals were developed in the outer shallow ramp. During the deposition of SQ3-2, as the inner shallow ramp receded westward to the surroundings of Well GC9, 98 m thick superimposed deposits of dolomitic shoal and top flat were developed in the inner shallow ramp, and thin middle-high energy shoal deposits and inter-shoal deposits were developed in the outer shallow ramp. During the deposition of SQ3-3, as the inner shallow ramp continued to recede westward, a small amount of dolomitic shoal deposits were developed at the bottom of the HST in Well GC13, while the outer belt of the shallow ramp was still dominant in the east of Well GC13. During the deposition period of SQ4-SQ6, the outer belt of the shallow ramp and the deep ramp were developed, and the deep ramp expanded gradually to the west.

The seismic calibration results show that the iterative RMS amplitude can indicate the range of the high-energy grain shoal. Taking sequences as the mapping units and considering the ramp sedimentation model, we compiled the sedimentary microfacies plans of different system tracts based on single-factor maps, i.e. the sequence thickness map, the distribution of grain shoal, and the paleo-geomorphology, through single-factor analysis and multi-factor comprehensive mapping ^[14]. Taking SQ3 as an example, the planar distribution of the sedimentary microfacies in the frequent sequence framework is introduced as follows (Fig. 8).

During the deposition period of SQ3-1-TST, the inner shallow ramp and the outer shallow ramp were developed in the study area, with scattered grain shoals. During the deposition period of SQ3-1-HST, as the sea level decreased slowly, the back ramp occurred in the northwestern part of the study area, and the shallow ramp gradually moved eastward. More and more grain shoals appeared, and dolomitic shoals and mid-high energy shoals were widely developed. Shoal individuals are fusiform, and distributed discontinuously on a belt in the nearly NS direction. The cumulative area of dolomitic shoals is 225.6 km², and that of medium-high energy shoals is 68.9 km² (Fig. 8a). The transgressive scale in the SQ3-2-TST period was larger than that in the SQ3-1-TST period, so the interface between the inner shallow ramp and the outer shallow ramp in the study area migrated westward. During the deposition period of SQ3-2-HST, the shallow ramp migrated westward (namely the back ramp receded westward). The cumulative area of dolomitic shoals in the inner shallow ramp is 165.1 km², and that of the middle-high energy shoals in the outer shallow ramp is 78.4 km² (Fig. 8b). During the deposition period of SQ3-3-TST, when large-scale transgression occurred in the study area, the shallow ramp moved westward again, and deep ramp deposits occurred in the eastern part of the study area. During the deposition period of SQ3-3-HST, the inner shallow ramp, the outer shallow ramp, and the deep ramp were developed in turn from west to east. Middle-high energy shoals increased in the outer shallow ramp, and their cumulative area is up to 128.9 km², while dolomitized shoals decreased to only 96.3 km² in the inner shallow ramp (Fig. 8c). It should be noticed that top flats on variable scales appeared in most dolomitized shoals (at the top). As the body of water became shallower westward, the dolomitic shoal became larger, and accordingly larger top flat.

In summary, in the period from SQ1 to SQ6, the sea level rose gradually. As seawater invaded westward areas, the shallow ramp in the Gucheng area receded westward, and the deep ramp expanded gradually westward. In the period from SQ2 to SQ3-1, a short regression occurred (Fig. 7), resulting in the inner shallow ramp to expand eastward consecutively, and a large area of dolomitic shoal developed (Fig. 8a). In the period from SQ3-2 to SQ3-3, transgression continued, so the area of dolomitic shoal within the shallow ramp decreased gradually (Fig. 8b, 8c). Since SQ4, few dolomitic shoal deposits were developed in the study area. As a result, there is a sedimentary combination of thick dolomitic shoals and top flats which are superimposed on each other in the middle and lower parts of the Ying 3 Member (SQ3-1 and SQ3-2) and distributed in an almost continuous zone adjacent to the sequence interface.

The analysis of cores and thin slices shows that the storage space in the Ordovician carbonate reservoirs in the Gucheng area is dominated by intercrystalline dissolved pores, dissolved vugs, and intercrystalline pores (Fig. 2e-2h). The reservoir physical properties of 208 samples from 14 wells show significantly different porosities of different types of rock. The fine-middle crystalline dolomite has the highest porosity, which is 4.06% on average in 42 samples. The second is the coarse powdered- fine crystalline dolomite, which has a porosity of 2.76%. The granular limestone has lower porosity values, which have an average of 0.85% in 87 samples. The granular marl-micrite limestone has the lowest porosity of 0.42%.

Based on the above data, the statistics by the type of sedimentary microfacies shows that the top flat reservoirs dominated by middle-coarse crystalline dolomite have dissolved pores and inter-crystalline dissolved pores extensively developed and the best physical properties. The porosity is 4.36 % on average and up to the highest value of 18.6%. The dolomitic shoal reservoirs dominated by fine-middle dolomite with residual sand debris have inter-crystalline pores and a small amount of dissolved cavities. Their porosity is in average 3.09% and up to the highest value of 9.1%. The inter-shoal dolomitic flat reservoirs dominated by powdered crystalline dolomite have intercrystalline pores and locally developed fractures. Their porosity is in average 1.61%. The medium-high energy shoal reservoir dominated by granular limestone has few fractures and the porosity is less than 1%. The micrite dolomite and micrite limestone in the back ramp and the deep ramp are tight and hardly have storage spaces.

Different sedimentary microfacies have different lithologic combinations and different reservoir physical properties. Favorable reservoirs are mainly distributed in the middle-coarse crystalline dolomite top flats with dissolved pores and the middle-fine crystalline dolomitic shoals with intercrystalline pores and intercrystalline dissolved pores.

Sedimentary microfacies control the primary pore structure of the reservoir, and affects the preservation and transformation of late pores. The dolomitization degree and the conditions of freshwater dissolution in the quasi-contemporaneous period are controlled by stratigraphic sequences, especially the frequent sequences ^[20]. Grain shoals deposited in the high-energy environment are usually on the relative bulges in the shallow ramp. As the sea level decreased slowly, the grain shoals were exposed gradually and prone to freshwater leaching, dissolution, and dolomitization. These phenomena developed into dolomitic shoal reservoirs with residual sand debris and dissolved pores. The dolomitic shoal top is often exposed out of water which resulted into its strong freshwater dissolution, and further developed into top flat reservoirs with dissolved pores (Fig. 9).

Gas logging anomaly was observed in the Well GC601 at 6041.8 m, and the TOC increased from 1.5% to 20%. Cores at 6042.0-6155.2 m show 2 sets of SQ3-2-HST dolomitized shoal and top flat reservoirs with intercrystalline pores, intercrystalline dissolved pores and dissolved cavities, at 6044.8-6064.1 m and 6069.2-6086.7 m, respectively. According to the measurement of the physical properties, the porosity is between 3.5% and 4.2%, and the permeability is between 3.7×10^-3 μm² and 5.9×10^-3 μm², respectively. This indicates that the lower reservoir has better physical properties than the upper reservoir, and the lower reservoir has better gas logging show than the upper reservoir. Comprehensive geologic interpretation shows a poor gas layer which is 8.2 m thick in the lower reservoir, and 1.9 m thick in the upper reservoir. In comparison, in the dolomite section of the SQ3-2-TST dolomitized shoal, core observations show few intercrystalline pores and cracks, but almost no dissolved pores. The porosity is less than 1.5% and the permeability is less than 1×10^-3 μm². Comprehensive interpretation only found a poor gas layer a thickness of 4.1 m. In the SQ3-1-HST dolomitic shoal, the porosity of the dolomite section was tested and showed a value of 3.8%, higher than that of the dolomite section in the SQ3-2-TST dolomitic shoal and comparable to that of the SQ3-2-HST dolomite reservoir. Comprehensive interpretation shows a poor gas layer with a thickness of 8.9 m. In the lower part of SQ3-1-HST, there is a suspicious gas layer 19.6 m thick and a poor gas layer 8.1 m thick.

In conclusion, the Ordovician carbonate reservoirs in the Gucheng area are mainly controlled by the sedimentary microfacies and frequent sequence interfaces. The quasi-contemporaneous dolomitization and freshwater dissolution play the key role in developing favorable reservoirs.

Based on the above understanding, and considering the latest drilling data and oil and gas shows, we stacked and merged the dolomitized shoals (including top flats) in the HSTs of SQ3-1, SQ3-2 and SQ3-3 (Fig. 8) into three nearly NS trending dolomitized shoal belts more than 200 m thick each, along GC6-GC9, GC18-GC13 and GC16, respectively (Fig. 10). From east to west, the scales and thicknesses of the dolomitic shoals in the belts increase successively. By now, wells with gas flow (at high or low rate) and gas show (gas logging show) in the study area are all located in the dolomitic shoal belts. For example, GC6, GC8, GC9 and GC17 Wells with 4 high yield drilled in the high horst are all located in the eastern GC6-GC9 belt. 3 wells drilled in the central GC18-GC13 belt (at the structural low) have found more than 100 m thick dolomite reservoirs each. Although these reservoirs are water layers, they reflect sufficient liquid supply and large reservoir space. In Well GC16 situated in the western belt, nearly 200 m thick dolomitic reservoirs were drilled, which are poor gas layers and gas-bearing water layers. Gas production was high at the early stage, but water production became high at the later stage. This also indicates a hydrocarbon potential in the western belt. Maybe late structural transformation is responsible for missing industrial gas flow ^{[9, 13]}.

Thus, affected by frequent oscillation and slow decline of the HST sea level, a discontinuous narrow dolomitized shoal belt with small thickness (including top flat) was developed in the inner shallow ramp. As sea water continued intruding, the dolomitized shoals migrated laterally. Vertically, they superimposed on each other, while laterally they merged with each other, resulting in many thick and large high-quality dolomitized shoal and top flat reservoirs distributed in a belt like ‘platform margin’. For example, the central and western dolomitized shoal belts which are less controlled by wells in the Gucheng area have many excellent Ordovician dolomite reservoirs. They are targets for future hydrocarbon exploration. In addition, to reduce exploration risks, more efforts will be put on hydrocarbon transport and preservation conditions.

The Early Middle Ordovician in the Gucheng area of eastern Tarim Basin is a typical carbonate ramp which is divided into four subfacies (i.e. back ramp, inner shallow ramp, outer shallow ramp and deep ramp), and 10 microfacies (i.e. argillaceous dolomite flat, dolomitized shoal, dolomitized top flat, medium-high energy shoal, storm shoal, inter-shoal, etc.) from west to east. There are six third-order sequences from bottom to top, and SQ3 is divided into 3 fourth-order frequent sequences.

Carbonate reservoirs are controlled by both sedimentary microfacies and frequent sequence. Sedimentary microfacies control their primary pore structure, and frequent sequence controls the intensity of early exposure to freshwater dissolution and dolomitization. The middle-fine crystalline dolomite with residual sand debris in the shallow ramp was formed with the large-scale dolomitization of the grain shoal during the quasi-contemporaneous period, so intercrystalline pores were widely developed. The middle-coarse crystalline dolomite in the top flat below the sequence interface was often exposed out of water. Therefore, a large number of dissolved cavities were developed. The granular limestone in the medium-high energy shoal in the outer shallow ramp was always below water, so it was less dolomitized, more cemented and tighter.

The frequent SQ3-HST dolomitic shoals and top flats which are superimposed vertically and merged laterally constitute a large-scale continuous shoal reservoir belt. They are primary reservoirs in the carbonate ramp and will be the targets for exploration evaluation. In addition, more effort should be put on the accumulation factors to reduce exploration risks.

GR—natural gamma, API;

P_e—photoelectric absorption eross-section index;

R_LLD—deep lateral resistivity, Ω·m;

R_LLS—shallow lateral resistivity, Ω·m.