The Lower Paleozoic Ordovician carbonate formations in the Tarim Basin are rich in petroleum resources, and a series of hydrocarbon reservoirs, which are mainly buried-hill limestone reservoirs, karstic fractured-vuggy reservoirs, and strike-slip fault-controlled paleo-karstic reservoirs, have been discovered in northern Tarim, central Tarim, Shunnan, and Shunbei areas. Reservoir features, forming conditions, and distribution have been extensively investigated ^{[1,2,3,4,5,6,7,8,9,10,11,12]}. Due to the small proportion of discovered beach-facies dolomite gas reservoirs to total discoveries in the Tarim Basin ^[4], more efforts should focus on beach-facies dolomite reservoir properties, gas reservoir types, and major controls.

The discovery of Ordovician dolomite gas reservoirs in Well GC6 drilled in Gucheng of the Tarim Basin witnessed the start of exploration to beach-facies dolomite gas reservoir ^[13] in a new field. Based on 3D merged seismic data interpretation combined with drilling, logging, and lab test data, we discuss the types, major controls and distribution of the beach-facies dolomite gas reservoir in Gucheng. This study will be instructive to the exploration of the Lower Paleozoic dolomite reservoir in the Tarim Basin, especially hydrocarbon exploration in the east platform margin in western Tarim area.

The Gucheng area of 6100 km² lies in the mid-south of the North Depression in the Tarim Basin. Adjacent to the Tadong Uplift on the east and Tazhong Uplift on the west, it is structurally a Lower Paleozoic nosing uplift inclining toward northwest and cut by NE faults into several fault blocks with grabens alternating with horsts (Fig. 1). Similar to the Tazhong Uplift, the Gucheng low salient went through multi-phase tectonic movements in the Caledonian, Hercynian, Indosinian, Yanshanian, Himalayan ^[14,15]. The fundamental tectonic framework was shaped during the Hercynian and Indosinian and only slightly reconstructed in later tectonic movements. The Cambrian, Ordovician, Carboniferous, Permian, Triassic, Cretaceous, Tertiary, and Quaternary Systems deposited from the bottom up in Gucheng, but the Silurian, Devonian, and Jurassic Systems were missing. Marine deposits settled in the Cambrian and Ordovician Periods, and continental sedimentation occurred after the Carboniferous. The Cambrian and Ordovician Systems extend in the whole Gucheng area, and the cumulative thickness generally exceeds 5000 m. Marin carbonate formations are the major targets for hydrocarbon exploration. Discovered gas pays turn up in the third member of the Ordovician Yingshan Formation (Ying3 Member for short) ^{[4, 14]}.

The Cambrian-Ordovician sedimentary system in Gucheng consists of carbonate platform, platform margin, slope, shelf, and basin ^[16,17]. The Lower Cambrian Series is composed of slope, shelf, and basin facies, and the Middle and Upper Series are composed of rimmed platform and platform margin facies. Mudstones and dolomites function as promising source rocks and reservoir rocks. The Penglaiba, Yingshan, Yijianfang, and Tumuxiuke Formations occurring in the Early and Middle Ordovician Epochs are deposited in restricted platform, open platform, and basin facies, which are extensive platform marginal grain beaches composed of dolomites, dolomitic limestones, grainstones, and calcipulverite as promising reservoir rocks and caprocks. A thick package of clastic rocks, which are mainly mudstones intercalated with thin silty fine sandstones, in the Charchaq Formation deposited in the Late Ordovician Epoch. In spite of later denudation, residual thickness still exceeds 2000 m. This package of clastic rocks functions as regional caprocks of hydrocarbon accumulations.

According to the studies of regional stratigraphic distribution and source rocks ^{[4, 18-21]}, there are two packages of source rocks, i.e. Lower Cambrian Yurtusi source rock of basin facies and Middle Cambrian Mohe'ershan (equivalent to Shayilike) source rocks of shelf-basin transition facies, in Gucheng. Lower Cambrian source rock, which is black siliceous mudstone, was drilled by 39, 33, 43, 57, and 61 m thick in Wells TD1, TD2, DT1, YD2, and YL1, respectively, in neighboring areas. The TOC of 0.50%-3.26% with the average of 2.67% and R_o of 1.73%- 2.91% indicate highly to over mature high-quality source rock. Middle Cambrian source rock of basin facies, which is calcareous mudstone, was drilled by 35, 31, 33, and 60 m thick in Wells TD2, TD1, DT1, and YD2, respectively, in neighboring areas. The TOC of 0.51%-2.58% with the average of 1.60% and R_o of 2.54% indicate moderate to good source rock. The analysis of inclusions acquired from the carbonate reservoirs in Gucheng shows that there are two phases of hydrocarbon accumulation; phase-1 in the Middle and Late Ordovician Epochs mainly gave birth to oil and gas reservoirs, and phase-2 in the Quaternary Period mainly gave rise to gas reservoirs ^[22].

The initial formation pressure and temperature measured in Ying3 gas reservoirs in 6 wells in Gucheng are shown in Table 1. Formation pressure of 66.83-73.8 MPa, pressure coefficient of 1.07-1.19, temperature of 164.1- 177.1 °C, and geothermal gradient of 2.73-2.86 °C/100 m indicate the gas reservoirs at normal temperature and normal pressure. Basically consistent pressure coefficient and reservoir temperature in all gas zones, with local slight differences, indicate similar geologic conditions for gas accumulation.

The methane content in Gucheng gas reservoirs ranges 65.62%-98.30% with the average of 85.25%. Drying coef-ficient of 0.995 indicates dry gas. The content of non- hydrocarbon gases, which are mainly CO₂, is 7.12%. As per temperature and pressure analysis on the formation fluid in Well GC6, in situ gases are of single-gas phase and formation temperature occurs far away from the right side of the phase envelope in the phase diagram. The point of surface separation lies outside the two-phase zone and shows the phase state of dry gas reservoir. Gas composition shows C₁+N₂ content of 94.066%, C₂-C₆+CO₂ content of 5.926%, and C₇₊ content of 0.008%, which indicate dry gas in the triangular phase diagram for gas reservoir classification.

The lab tests of GC18 and GC6 samples acquired from Ying3 gas reservoirs in Gucheng show formation water density of 1.11-1.14 g/cm³, pH value of 6.00-7.04, Cl^- content of 10 900-117 832 mg/L, Ca²⁺ content of 1111-1151 mg/L, Mg²⁺ content of 569-595 mg/L, SO₄^2- content of 160-255 mg/L, total salinity of 177.0-178.6 g/L, and water type of CaCl₂. With high salinity, subacidity and a variety of trace elements, the depositional environment may be deep and enclosed.

Natural gases pervasively turn up at the top of the dolomite reservoirs at the top of the Ordovician Ying3 Member in Gucheng, and there is a relatively uniform gas-water contact (GWC). The bottom of the gas zones is close to -5120 m above sea level, and the gas column is 222 m high at most. Active gas logging responses above the elevation highlight gas accumulation. Short-term production tests after fracturing and acidizing found economic gas flow or low- production gas flow at daily output of (0.09-107.89)×10⁴ m³. Well logging and production test results below -5230 m show water output. For example, the cumulative water production reached 24 m³ from Well GC7 and 213 m³ from Well GC18. There are mainly gas-water layers from -5230 m to -5120 m above sea level, which were interpreted as gas-water layers or gas layers alternating with water layers using logging data.

Vertical gas distribution is dominated by beach-facies dolomite reservoirs at the top of the Ying3 Member, and hydrocarbon richness is related to reservoir properties and elevation. Gas zones in a well are not interconnected, but there is a relatively uniform GWC for each gas zone. Lateral gas distribution is dominated by sedimentary facies of beach-facies reservoirs. Gas reservoir extension is related to beach distribution and beach termination in the up-dip direction or structural traps. Different beaches are not connected, and each beach constitutes a reservoir. In short, there are lithologic gas reservoirs dominated by beaches in the structural setting (Fig. 2).

Gas reservoirs in Gucheng are mainly composed of hydrocarbon gases with high gas maturity in view of methane volume fraction of 80.8%-97.9% and small heavy hydrocarbon (C₂₊) volume fraction of 0.11%-5.72%. Carbon isotopic compositions in hydrocarbon gases features δ¹³C₁ from -39.1‰ to -30.2‰, δ¹³C₂ from -38.7‰ to -32.9‰, and δ¹³C₃ from -35.0‰ to -31.3‰. The small difference between δ¹³C₂ and δ¹³C₁ which generally ranges 3‰-6‰, indicates sapropelic kerogens of marine facies^[23]. According to the empirical relation between marine methane carbon isotopic compositions and source rock maturity R_o established by Dai et al. ^[24], the gas R_o in Gucheng ranges 2.40%-3.32%, indicating overmature gas. Methane and ethane carbon isotopic compositions reversal denotes hybrid gases of different geneses in addition to those originating from Cambrian source rocks ^[25]. In accordance with the bitumen-equivalent vitrinite reflectance of 2.0% for Cambrian reservoir rocks and vitrinite reflectance above 2.5% for Cambrian source rocks in Gucheng, pyrolytic gases from early oil reservoirs contributed to Gucheng gases by 10%-50%, while the other 50%-90% from dispersed organic matter in deep source rocks ^[26]. Cambrian Yurtusi source rocks and Early Cambrian oil reservoirs drilled in Wells CHT1 and CHT2 were all discovered in the lower section of Ying3 dolomite gas reservoirs.

Sustainable sea level rise in the Tarim Basin from the Late Cambrian to the middle Early Ordovician resulted in an enlarged accommodation space and a high carbonate deposition rate ^[27]. The sedimentary environment of semi-deep basin, outer ramp, middle ramp, and inner ramp occurred in turn from east to west in Gucheng. On a ramp background high in the west and low in the east with an elevation difference of 50-100 m within a lateral distance of 50 km at the depositional stage of the Ordovician Yingshan Formation, inner ramp, middle ramp, and outer ramp facies occurred in turn from west to east (Fig. 3). Around Well GC10 in the western platform, corresponding to the shoaly low-energy inner ramp, there deposited micritic-crystal powder dolomites of tide flat and lagoon facies. Around Wells GC8-GC14 on the east, corresponding to the middle ramp that could be divided into an inner zone and an outer zone, there deposited medium to fine crystalline dolomites of dolomitized beach (with residual grain pattern) and inter-beach dolomitic flat facies and limestones of inter-beach bottom land and medium- to high-energy beach facies. Around CHT1 in the eastern platform, corresponding to the outer ramp, there deposited tempestite and deep-water micrite. Dependent on the ramp depositional setting, promising beach-facies dolomite reservoirs concentrate in the middle ramp deposited between the mean sea level and mean wave base, where beach-facies grainstone tended to be reworked by meteoric water leaching and infiltration backflow dolomitization to form large-scale dolomite reservoirs. In comparison, dolomitization tended to occur in the inner zone of the middle ramp, where water was shallow, to generate dolomitized beaches. In the outer zone with deep water and high water energy, there usually deposited with medium- to high-energy grain beaches.

According to the 106 m consecutive core taken from the Ying3 Member in Well GC601 in Gucheng, logging electrical properties and carbon and oxygen isotopic compositions show an evolution from dolomitized beaches deepening upward to medium- to low-energy beaches and inter-beach marine deposits, and positive excursion of carbon isotopic composition. Vertically, three sedimentary cycles were developed in the Ying3 Member (Fig. 4) with the rise of overall sea level during the depositional stage, and the sedimentary environment in each cycle was slightly different. Cycle-1 beaches, which are mainly muddy dolomitic flats, inter-beach dolomitic flats, and dolomitized beaches, occur in the inner ramp and middle ramp. Cycle-2 beaches, which are inter-beach dolomitic flats and medium- to high-energy dolomitized beaches, occur in the middle ramp. Cycle-3 beaches, which are mainly inter-bank marine deposits and medium- to high-energy dolomitized beaches, occur in the outer ramp and middle ramp. The common point of the 3 cycles is that medium- to high-energy dolomitized beaches with cumulative thickness of 30-60 m generally turn up in the middle and upper sections of the cycle. As per core observation, Ying3 beaches are composed of several meter-scale sedimentary cycles, and 0.10-4.27 m thick a cycle. At the top of the cycle, there are fine-medium crystalline and coarse crystalline dolomites with dissolved pores and cavities and vadose silt infilling texture. In the middle of the cycle, there are mainly fine-medium crystalline dolomites with granular ghost texture. Sparry calcarenite appears at the bottom of the cycle. A single-cycle beach is usually thinner than 2 m, but the cumulative thickness is large. Beach-to-formation thickness ratio is generally greater than 75%.

As per seismic forward modeling, Ying3 medium-fine crystalline dolomite beaches feature enlarged thickness, low-relief salient, weak energy, and lateral stacking on seismic sections, while micritic-crystal powder dolomites of lagoon/inter-beach bottom land facies feature continuous reflections (Fig. 5). Dolomite beaches were identified in the 3D Gucheng seismic survey based on above reflection features. The results show that due to sea level rise, beaches stacked in the vertical direction and migrated toward inner platform in the lateral direction. There are 3 beach belts parallel to the platform margin in the north-south direction (Fig. 6). Beach belt-1 in the east mainly deposited with cycle-1 medium- to high-energy grain beaches with low degree of dolomitization, under the control of the ancient land form in the Ying4 Member. Beaches facing the sea are relatively highly dolomitized. In generally, the beach is thin; for example, the beach- facies dolomites in Well GC8 is only 13.6 m thick. Beach belt-2 in the middle deposited with cycle-2 and cycle-3 thick moundy beaches stacking in the vertical direction and migrating in the lateral direction, under the control of the ancient land form at the early depositional stage of the Ying3 Member. The cumulative thickness of the dolomitized beaches in Wells GC6 and GC9 is over 100 m. Beach belt-3 in the west also deposited with cycle-2 and cycle-3 beaches, whose distribution is controlled by the ancient land form at the middle depositional stage of the Ying3 Member. In Well GC18 which penetrates the beach core, the cumulative thickness of the beach-facies dolomites is up to 80.8 m. The extensively superimposed dolomite beaches are the material basis of reservoir units.

Large-scale occurrence of carbonate reservoirs is geologically dependent on sedimentary facies, inter-interval and intra-interval corrosion and lixiviation, burial dolomitization, and hydrothermal activity ^[28]. Although the mechanisms for forming dolomite reservoir in Gucheng have not been completely clear, dolomitization and hydrothermal corrosion are regarded as significant factors on forming reservoir ^[29]. According to the analysis of a large number of samples, Ying3 dolomite reservoirs mainly consist of fine and medium-coarse crystalline dolomites and some coarse crystalline dolomite with the porosity of 1.8%-5.0% and permeability below 0.1×10^-3 μm². The pore space in the dolomite reservoirs, which may be porous reservoir, porous-vuggy reservoir, fractured-porous-vuggy reservoir, and fractured reservoir, is the product of multiple processes (Fig. 7). (1) Grainstone beaches in the Ying3 ramps went through multiple phases of small-scale exposure leaching when depositing or before diagenesis. Hypergenic karstification improved the reservoir properties of grainstone beaches. Vadose silt infilling in dissolved pores and cavities and collapsed rubbles could be observed at the top of individual beaches (Fig. 7a). (2) Granular ghosts could be observed in dolomite crystals after grainstone penecontemporaneous-shallow burial dolomitization. Ion exchange tended to occur in karstification-induced pores and cavities and promote beach-facies dolomitization, and intercrystalline pores formed on the basis of original pores (Fig. 7b). Such reservoirs often turn up in the middle and upper sections of individual beachs. (3) After dolomitization, beach-facies dolomite reservoirs went through faulting and hydrothermal corrosion to form high-graded reservoirs with fractures and dissolved pores and cavities in the vicinity of faults (Fig. 7c). Hydrothermal corrosion generally took place along the strike-slip faults penetrating the basement ^{[11, 30-31]}, and the range of corrosion was dominated by fractures. Hence, hydrothermal corrosion tended to appear along strike-slip faults and the intersections of strike-slip faults and other basement faults in Gucheng. Due to different pore geneses and degrees of reconstruction, there are great discrepancies in the properties of various types of reservoir. Fractured-porous- vuggy reservoirs have good properties with the porosity of 1.0%-20.3% and permeability of (0.061-19.000)×10^-3 μm². Porous-vuggy reservoirs have moderate properties with the porosity of 0.4%-3.2% and permeability of (0.024- 16.700)×10^-3 μm². Porous reservoirs have poor properties with the porosity of 0.1%-2.8% and permeability of (0.066- 3.741)×10^-3 μm².

As per gas test on Ying3 dolomite reservoirs in Gucheng, the gas deliverability is positively correlated with the density of fractures and dissolved fractures in the beach-facies dolomite reservoirs. Fractured-porous-vuggy or fractured dolomite reservoirs were drilled in Wells GC6, GC8, and GC9, where the density of fractures and dissolved fractures is estimated to be 18-52/m, and gas test shows economic gas flow or high-yield economic gas flow. Porous-vuggy and porous reservoirs were drilled in Wells GC601, GC17, and GC12, where the density of fractures and dissolved fractures is estimated to be smaller than 10/m, and gas test shows low production or dry zones. According to the relationship between gas production and the density of fractures and dissolved fractures, faulting and hydrothermal corrosion dominate the high production of the beach-facies dolomite reservoirs.

In accordance with drilling data, the overlying Ying1 and Ying2 Members and Yijianfang and Tumuxiuke Formations are composed of tight limestones with cumulative thickness of 350-500 m, which may function as the direct caprocks of Ying3 dolomite gas reservoirs in Gucheng. Ying3 gas zones or gas shows were discovered below this package of limestones. This direct caprock and thick Upper Ordovician Charchaq mudstone (of 2300- 2500 m thick) constitute double packages of caprock for gas reservoirs. The breakthrough pressure was tested to be over 8.0 MPa and up to 38.6 MPa for micrite and mostly below 4.0 MPa and even as low as 0.2 MPa for crystal powder dolomites in Gucheng. This means that micrite and beach-facies dolomite reservoirs may constitute a good reservoir-caprock assemblage ^[32]. As indicated by the diffusion coefficient of Ordovician limestone and dolomite samples acquired from 8 wells (GC16, GC13, GC14, GC8, GC9, GC10, GC12, and CHT1) in Gucheng, the upper Yingshan limestones of grade-II has good sealing performances for the preservation of the Lower Ordovician dolomite reservoirs.

Fractures connecting source rocks to gas reservoirs function as migration pathways. As shown by formation curvature at the top of the Ying3 Member, there are 3 sets of faults extending NE, NNE and NW, respectively in the 3D Gucheng survey (Fig. 8), corresponding to extension faults, tenso-shear strike-slip faults, and compresso-shear strike-slip faults identified on seismic sections. Fracture evolution mainly experienced 4 phases. Phase-1 extensional faults, small but more, grew from the Cambrian to the Early Ordovician. Phase-2 tenso-shear faults, including 5 sets of tenso-shear strike-slip faults in the NE direction, were active during the Middle Ordovician and became less active from east to west. Phase-3 compresso-shear strike-slip faults in NNE and NW directions were active for a long period from the Late Ordovician to the Devonian. There are 7 sets of NNE faults that became less active from west to east. Phase-4 compressional and compresso-shear faults were active from the Carboniferous to the Quaternary. NNE and NW faults further grew into compressional and compresso-shear strike-slip faults, and fault activity was intense in the east and weak in the west.

In terms of the relationship between fracture evolution and 2-phase hydrocarbon accumulation during the Middle-Late Ordovician and the Quaternary in Gucheng, the fractures forming after phase-2 tenso-shear faulting activities may function as the pathways for hydrocarbon migration and accumulation or adjustment. The present principal stress and open fractures are mostly NNE or NNW (Fig. 8), and phase-3 and phase-4 faults which were active for a long period also extend NNE. Gas accumulation mainly took place during the Quaternary Period, so it inferred that NNE strike-slip faults may be the preferential pathways for gas migration and accumulation in Gucheng.

The regional tectonic evolution in the Tarim Basin dominates the lithofacies paleogeography and hydrocarbon accumulation ^{[14, 33-34]}. As per the tectonic history of the Ying3 top based on the restored original depositional thickness, the Gucheng low salient evolved over the following periods. (1) Initial framework (from Middle to Late Caledonian). The extensional environment in the southern Tarim Basin changed into a compressional environment due to the influence of tectonic compression from the West Kunlun Mountains at the end of the Ordovician. The central Tarim was compressed and uplifted, and the Gucheng low salient began to take shape. (2) Final shape (from late Caledonian to the Hercynian to the Indosinian). The Gucheng area was affected by tectonic compression from the West Kunlun Mountains and Altun Mountains at the end of the Devonian Period, particularly from Altun with increased intensity. Due to tectonic compression from Altun on the southeast at the end of the Carboniferous Period, the Gucheng area was uplifted greatly. The initial tectonic framework was high in the northwest and low in the southeast, but it changed into a new one, high in the southeast and low in the northwest. The tectonic action from Altun at the end of the Triassic Period had the greatest impact on the Tadong Uplift, and the Gucheng area was affected too. Based on the early geometry generated by the tectonic action in the Hercynian, the Gucheng low salient continued to grow and was finally shaped. (3) Stable shape (from the Yanshanian to the Himalayan). Tectonic movements after the Jurassic Period mainly affected the Cherchen fault zone on the south. Early influence on the Gucheng area was weak, but late influence enhanced. The whole area on the south of the Cherchen fault zone began to subside steadily. The Gucheng low salient, which formed early and was stable at the late stage, was the destination of hydrocarbon migration and accumulation in different periods.

By now, there are 18 exploratory wells in Gucheng, 15 wells targeting Ordovician dolomite reservoirs, 4 wells (GC6, GC8, GC9, and GC17) with economic gas flow, and 4 wells (GC12, GC14, GC16, and GC11) with low-yield gas flow. These 8 wells have the following features. (1) These wells penetrated the beaches within the structural traps at -5120 m. All the gas zones and poor gas zones were interpreted at dolomite reservoir top. (2) Dolomite reservoirs were estimated to be 11.2-67.8 m thick, with the average of 25.5 m. There is a positive correlation between reservoir thickness and beach thickness. (3) Gas production is high at the intersections of beach-facies dolomite reservoirs and NNE strike-slip faults. The perpendicular distance between four wells with economic gas flow and NNE strike-slip faults is generally smaller than 0.5 km. As for the exploratory wells within the structural traps at -5120 m, there isn’t good exploration result in the case of poor beach-facies reservoirs around strike-slip faults or low fracture density in the beach-facies reservoirs. For example, Wells GC7 (with thin beach-facies reservoirs) and GC601 (with small fracture density in the beach-facies reservoirs) only have gas shows. The exploratory wells outside the structural traps at -5120 m, e.g. GC18, GC15 and GC13, drilled in dolomite reservoirs, have not gas shows. Wells GC4, CHT1, CHT2 and CHT3 drilled in limestones instead of dolomite reservoirs, have not good gas shows. Thus, Ordovician Ying3 dolomite gas reservoirs are lithologic gas reservoirs dominated by beaches and fractures on a tectonic setting. Gas accumulation and enrichment are jointly controlled by the regional tectonic setting, effective reservoir-seal configuration and late strike-slip faulting (Fig. 9).

The distribution of dolomite gas reservoirs is mainly related to two factors. (1) The lateral distribution is dominated by sedimentary facies. Gas zones or gas shows mainly turn up in 3 areas with dolomitized beaches in the Ying3 middle ramp. (2) The vertical distribution is dominated by 3 cycles in the Ying3 Member. Gas accumulation occurs in the dolomite reservoirs at each cycle top, which was uplifted from the platform margin in the east to inner platform in the west.

Gas enrichment or high production is mainly related to two factors. (1) Gas enrichment is dominated by NNE strike- slip faults and beaches. NNE strike-slip faults were active for a long period in the geologic history, along which hydrothermal fluids migrated upward and then diverted laterally where there were capping beds. Beach-facies dolomite reservoirs with relatively high porosity and permeability were affected first by secondary hydrothermal fluids. Hence, reservoir rocks in the strike-slip fault zones coexisting with beaches would have better properties for hydrocarbon accumulation. In the periods of hydrocarbon accumulation, especially in the Himalayan, natural gas migrated upward along NNE strike-slip faults into Ordovician Yingshan reservoirs. The reservoirs close to fractures were first injected with gas. Seven NNE fault zones confirmed in Gucheng are all rich in gas. (2) Natural gas tends to concentrate in structural highs. Wells with economic gas flow, e.g. GC6, GC8, and GC9, were drilled at structural highs in the area of interest. All gas shows or gas zones discovered by far lie in the intervals above -5230 m. To achieve high production from beach-facies dolomite gas reservoirs, structural highs and fault zones are two kinds of top priorities.

Ordovician Ying3 dolomite gas reservoirs in the Gucheng area are lithologic gas reservoirs dominated by beach facies and fractures on a tectonic setting. The Gucheng area is a low salient with some successive features, where structural highs are the destination of gas migration and accumulation. All gas shows or gas zones discovered by far lie in the intervals above -5230 m, but sufficient gas accumulation is dependent on gas supply, reservoir properties and trap properties.

The depositional setting of the platform marginal ramp in Gucheng was the foundation for extensive sedimentation of grain beaches. Medium- to high-energy grain beaches usually deposited at the top of a cycle. As sea level rose, grain beaches migrated laterally from the platform margin toward the inner. Three beach belts identified in the Ying3 Member are the promising targets for gas exploration in Gucheng.

Leaching and early dolomitization, which generated abundant intercrystalline pores or intergranular pores in limestones of grain beach facies, combined with strike- slip rifting and hydrothermal corrosion, may give birth to high-quality reservoirs. Seven NNE faults which were active for a long period in Gucheng had a controlling effect on high-quality reservoirs.

Natural gas in Gucheng was originated from the pyrolysis of ancient oil reservoirs and dispersed organic matter in source rocks. Reservoir rocks were deposited above source rocks. NNE faults which were active and open for a long period functioned as the preferential pathways for gas migration and accumulation. Seven NNE fault zones identified in Gucheng may be promising destination of gas accumulation.

Ying1, Ying2, Yijianfang, and Tumuxiuke tight limestones, whose thickness decreases from the platform margin to the inner, and total thickness is 500 m, may function as the direct seal of the underlying Ying3 dolomite gas reservoirs in Gucheng. This package of seal features high breakthrough pressure, low diffusion coefficient, and good sealing performance. All gas zones or gas shows discovered by far lie below this package of limestone overburden.