Several important breakthroughs have been made in the Upper Sinian Dengying Formation of the Sichuan Basin in recent years. The Weiyuan gas field in the southwestern Sichuan Basin and the Anyue gas field in the central Sichuan Basin have been discovered successively, showing giant exploration potential of highly evolutive natural gas pools in this formation^[1,2]. Currently researches on the Dengying Formation mostly have focused on the exploration regions with breakthroughs, and have reached the finding that mound- beach facies and dissolution by meteoric water are the main factors controlling the development of high-quality reservoirs in this formation^[3,4,5]. However, research findings are few in a broad region, restricting further exploration and development in the whole basin. The Sichuan Basin was in the depositional environment of shallow-water carbonate platform during the depositional period of Dengying Formation^[6], the predominant reservoir facies (mound-beach bodies) were extensively developed in the whole area, and leaching caused by the second episode of Tongwan movement lasted for ten million years^[7], so the Dengying Formation in the whole basin has a geological setting to form high-quality reservoirs. By using field section, 3D seismic data, and related analysis and test data, development characteristics of diverse reservoirs of the fourth member of the Dengying Formation in Yuanba, Northeast Sichuan Basin and adjacent areas are discussed, genesis and controlling factors of reservoirs are analyzed, and distribution of favorable reservoir zone is predicted.

Geographically, the study area is located in the Langzhong city, Tongjiang county of Bazhong city in the Sichuan province, and Hanzhong city of Shaanxi province. Tectonically, it belongs to the low gentle tectonic belt of northern Sichuan, adjacent to the Micang-Daba fault-fold belt on the north, gentle belt of central Sichuan on the south, high angle tectonic belt of eastern Sichuan on the east, and Longmenshan thrust belt on the west, and strikes north-east east on the whole (Fig. 1). Only a few wells drilled in the study area aimed at Dengying Formation, and most of them are concentrated in the northwestern Micangshan fault-fold belt, for example, wells Z1, Q1 and Tx1, in which the fourth member of Dengying Formation is at the depth of about 2000 m. Only one well was drilled in Tongjiang county (well Ms1), in which the fourth member of Dengying Formation is at the depth of 8050 m (top of the Dengying Formation). Because the burial depth of the fourth member of Dengying Formation is too deep, only a small amount of core of 7.44 m in total was taken, mainly from the middle-lower part of the member (only 2 m of core from the upper part). The other wells drilled are relative old, and no core from Dengying Formation in wells Z1 and Q1, and the core taken from the middle part of the fourth member of Dengying Formation in Well Tx1 is only 4.39 m. As limited core data of study area available, we used field profiles and seismic data mainly in this study.

The fourth member of the Dengying Formation at top of the Upper Sinian in the northeastern Sichuan Basin is in conformable contact with the underlying third member mixed tide-flat facies and in parallel unconformable contact with the overlying Lower Cambrian Series^[7]. The lithology of the fourth member of the Dengying Formation in study area is quite complicated, mainly consists of microbial dolomite (including clotted dolomite, microbe-bond framework dolomite, microbe- laminated dolomite, and stromatolite dolomite), dolarenite and a little sandy dolomite and muddy dolomite. By using well data and measured data of field profiles, thickness distribution map of the fourth member of the Dengying Formation in northern Sichuan has been plotted (Fig. 1). It can be seen from the figure that the fourth member of the Dengying Formation in the study area is 220-380 m thick, thicker in western Guangyuan to Yuanba belt, and thinner towards the middle part and east. Under the influence of third order sea-level fluctuations, two transgressive-regressive cycles happened during the deposition of the fourth Member of Dengying Formation, a set of low-energy (muddy) micritic dolomite deposited from the end of the first stage of regression to the second stage of transgression, thus the fourth member of the Dengying Formation is divided into the first submember and the second submember from bottom to top, and the reservoirs are mostly microbial dolomite and grain dolomite in the upper part of the first submember and upper-middle part of the second submember (Fig. 2).

Accurate description of reservoir characteristics is helpful for finding out the formation mechanisms and quality evaluation of the reservoir^[8,9,10]. Mound-beach bodies of the fourth member of Dengying Formation in northeastern Sichuan experienced differential reformation of multiple diagenetic processes after deposition, so they may have different combination characteristics of reservoir space types in different sections and regions, and these different combination characteristics control the reservoir quality. Through analysis of 11 field profiles and drilling core data, it is concluded that reservoir rocks of the fourth member of the Dengying Formation are mainly clotted dolomite, stromatolite dolomite, and microbe- bond framework dolomite related to microbial-mound buildup and (microbial) dolarenite related to grain beach buildup.

2.1.1. Reservoir rocks related to microbial mound buildup (microbial dolomite)

Influenced by the paleoclimate and paleosalinity, the Sichuan Basin mainly developed microbes like cyanobacteria during the depositional period of the Dengying Formation. After the transgression deposition of the third member of Dengying Formation, the microbes thrived with opulent sunshine and proper temperature as the sea level fell during the fourth member. They could cohere and capture lime-mud particles during growth, finally giving rise to carbonate buildup with dome feature, which is called microbial mound^[11]. During the process of microbial mound buildup, the microbe growth could form abundant frame pores and cave system, and the microbial dolomite related to the microbial mound can act as favorable reservoir rock^[4].

2.1.1.1. Microbe-laminated dolomite

In the low energy zone below the wave base with quite water and little wave disturbance, the mircrobes would grow in horizontal laminated or tiny ripple shapes, finally forming microbe-laminated dolomite (Fig. 3a). The laminae are variable in thickness, and consist of dark grey and light grey ones stacking over each other. The dark grey lamina is mainly micrite dolomite formed by microbe filament cohering tiny dolomite-mud particles. The light color lamina has less microbe and is mainly dolosparite. Accompanied with the development of microbe-laminated dolomite, some microbe filaments could form bird-eye pores or fenestrule during their growth or death. These early pores could provide dissolution channels for later fluids, so the laminated dolomite may be reformed into reservoirs by karstification. This kind of laminated dolomite in the research area has an average porosity of 2.05%.

2.1.1.2. Stromatolite dolomite

Along with the fall of sea level, the wave disturbance became stronger, the microbe laminae evolved from horizontal-tiny ripple shapes to wave-mound shapes, finally creating stromatolite dolomite (Fig. 3b-3d). On one hand, with more abundant microbes, stromatolite dolomite has more birdeye pores or fenestrule developed, on the other, due to the stronger water kinetic energy, the microbes could form frame pore-cave system during their intertwining growth. The stromatolite dolomite is one important type of reservoir rocks in the research area, with an average porosity of 4.44%.

2.1.1.3. Clotted dolomite

Along with the further fall of sea level, the microbes became increasingly prosperous due to enough sunshine and higher temperature. Microbes can stick and capture clastic particles lime-mud material or spherulites to form clots or masses, which would turn into clotted dolomite after diagenesis (Fig. 3e, 3f). Clotted dolomite is the most characteristic and developed reservoir rock type in the research area, which has inter- frame pores and caves similar to intergranular dissolution pores as main storage space, and with average porosity of 4.89%.

2.1.1.4. Microbe-bond framework dolomite

Similar to clotted dolomite, the microbe-bond framework dolomite is also microbe dolomite with anti-wave structure (including cohering framework and growth framework) formed by the baffle and calcification or growth of microbes^[12]. Similar to clotted dolomite in macroscopic shape to some extent, it differs from the clotted dolomite distinctively in that it has obvious signs of microbe filament growth (Fig. 3g). With inter-frame pores and caves also as storage space, this kind of reservoir rock in the study area has the highest average porosity of 6.17%, but is lower in development degree than clotted dolomite.

2.1.2. Reservoir rocks related to grain beach buildup

When sediment interface was located in high energy belt above wave base, early formed microbial dolomite or low energy micrite-crystal power dolomite would suffer from the crash of wave, and have intraclasts formed. If the depositional environment was unsuitable for microbial growth, typical dolomite of grain beach would be formed (Fig. 3h). Major grain rock in the study area is dolarenite, and oolitic dolomite is rarely seen. The arenite content of dolarenite is 70%-90%, and grain size is 0.3-2.0 mm and mostly 0.4-1.0 mm, and good roundness and sorting. Due to good sorting, it is uniform in color on cores. It has pinholes developed most and an average porosity of 3.12%. It is noteworthy that because of the special conditions suitable for microbe growth during the depositional period of Dengying Formation, pure dolarenite takes a small proportion in the fourth member of the Dengying Formation. During deposition of the dolarenite, growth of microbes may stick and capture arenite, thus the grain beach formed often has microbe-bond structure^[4,13], and is called microbe-bond dolarenite (Fig. 3i). From the macroscopic distribution of grain dolomite, the development of grain beach in the study area is also closely related to microbial mound, the grain dolomite, clotted dolomite and stromatolite dolomite often appear as high frequency interbeds (Fig. 2), making them difficult to be told from each other, so grain dolomite and microbial dolomite often combine into mound-beach complex, representing the most important reservoir type in the study area.

During the 570 million years after the deposition of Dengying Formation, the Sichuan Basin experienced seven tectonic movements successively, including Tongwan, Caledonian, Hercynian, Dongwu tectonic movement and so on^[14], and the superimposed modification of multi-stage complex diagenetic processes has made the fourth member of Dengying Formation in northeastern Sichuan diverse in reservoir space. According to the differences in formation mechanism and morphology, the storage space of the fourth member of Dengying Formation in northeastern Sichuan can be divided into three kinds: pore, karst cave and fracture.

2.2.1. Pores

According to the formation mode, pores in the fourth member of Dengying Formation can be divided into primary and secondary types. The primary pores formed at the same time as the rocks can be mainly divided into two types, residual intergranular pores and residual frame pores in the grain dolomite and clotted dolomite or microbe-bond framework dolomite respectively. The pores formed in the original rock that have not been completely filled by cementing material, nor enlarged by dissolution in the later period are the residual pores. As most of the primary pores would suffer dissolution in the later period, primary pores occur at a low frequency in the northeastern Sichuan Basin. Secondary pores refer to those generated by various diagenetic processes (mainly dissolution, recrystallization, etc.) after the formation of the sediments, or those enlarged by dissolution on the basis of primary pores, such as inter-frame dissolution pores (Fig. 3c, 3e), intercrystalline (dissolution) pores (Fig. 4a) and intergranular dissolution pores; or the newly generated pores, such as the intragranular dissolution pores (Fig. 4b) and moldic pores. Intergranular dissolution pore and inter-frame dissolution pore are the most important pore types in the northeastern Sichuan area and closely related to the construction of mound-beach body.

2.2.2. Karst caves

Karst caves are another type of important reservoir space in the fourth member of Dengying Formation in northeastern Sichuan. The pores more than 2 mm in diameter are all called caves. The karst caves are mainly the products of dissolution of the remnants of the original biological frame holes after the early cementation and filling, and the size of such caves is controlled by the size of the original frame holes and the early cementation strength primarily, and later karstification secondarily. The caves in the fourth member of Dengying Formation of the study area are generally 5-15 mm in diameter, with irregular edges. Some of the caves are partially filled with medium-coarse crystal dolomite, quartz and bitumen. Since most of the caves are in stromatolite, clotted and microbe- bound granular dolomites suffered strong dissolution, thus the caves are mostly in elongated and irregular oval shapes and distributed nearly parallel to the layer or microbial lamina (Figs. 3d, 4c, 4d). The caves are good in horizontal connectivity, poor in vertical connectivity, and isolated in local parts.

2.2.3. Fractures

Fractures, as a special type of reservoir, can act as storage space and percolation channels for fluid migration. The fractures in the northeastern Sichuan are dominated by horizontal interlayer fractures and high angle fractures (Fig. 4d-4f). Fractures are interwoven into network in local parts. Some fractures are directly connected with pore (cave) system (Fig. 4d, 4e). It is speculated that the dissolution fluid entered the early pores along the fractures, and dissolving and improving the porosity and permeability of the early formed sediments (the fractures themselves also have dissolution enlargement), finally, an effective fracture cave reservoir system is formed. It is worth noting that although there are a large number of fractures in the fourth member of Dengying Formation in the northeastern Sichuan Basin, most of the fractures are partially filled or fully filled by dolomite, calcite, quartz and so on (Fig. 4f), so the fractures make little contribution to the increase of the total pore volume of the reservoir, but are crucial for the improvement of the early porous layers.

Physical parameters can reflect reservoir performance and show the storage and permeability capacity of reservoir rock directly^[15,16]. According to the porosity distribution histogram, 192 samples from the fourth member of Dengying Formation in the northeastern Sichuan region have a porosity range of 1.10%-13.83% (on average 4.38%), and a main frequency distribution range of 2%-6%, accounting for 71.9%. The samples with porosity greater than 8% account for 8.3%, reflecting the characteristics of local high porosity (Fig. 5a). The histogram of permeability distribution shows that 190 samples from the fourth member of Dengying Formation of the northeastern Sichuan Basin have a permeability range of (0.00008- 20.9) × 10^-3 μm², an average permeability of 1.3 × 10^-3 μm², and a main permeability distribution frequency of less than 0.001 × 10^-3 μm² (52.6%). The samples with a permeability of more than 1×10^-3 μm² account for 23.7% (Fig. 5b). In general, the reservoirs of the fourth member of Dengying Formation in the northeastern Sichuan Basin are highly heterogeneous, and low in porosity and permeability, with local high porosity and permeability sections. The scatter diagram of porosity-permeability relationship shows that most of the samples have a positive correlation between porosity and permeability, showing the characteristics of the pore-cave type reservoir; affected by fractures, some of the samples show low porosity and high permeability (Fig. 5c), so it can be inferred that the reservoirs of the fourth member of Dengying Formation are fracture- pore-cave type.

Soon after deposition of the fourth member of Dengying Formation, the second episode of the Tongwan movement made the Dengying Formation in the Sichuan Basin overall lift and expose. Subjected to dissolution by meteoric freshwater, the formation had high quality reservoirs developed^[3,4,5]. Lots of previous researches on the main reservoir controlling factors of Dengying Formation showed that the epigenic karstification was the most critical factor for the formation of high quality reservoir. However, wells in the Gaoshiti-Moxi area show that the reservoir development in Dengying Formation is obviously controlled by sedimentary facies. For example, the clotted dolomite and stromatolite dolomite representing microbial mound and doloarenite representing grain beach have much better physical properties than dolomite in other facies (Fig. 6). Wells encountering the fourth Member of Dengying Formation have much lower frequency of drilling break (4.3%) and lost circulation (43.5%) than wells encountering traditional karst reservoirs^[17,18], indicating that the epigenic karstification is not the main factor controlling reservoir formation in Dengying Formation. This phenomenon is very similar to what happened in Danian eogenetic karst limestone reservoirs in Urbasa-Andia plateau, Navarra, Northern Spain^[19].

Based on the karst research on the world-wide islands and coasts environment in areas like Caribbean and Kangaroo Island of Australia, Vacher and Mylroie^[20] proposed a karstification model different from the traditional sense, named eogenetic karstification. This new concept of karstification is mainly based on the difference of material basis when karstification occurs. In traditional classical karstification, named telogenetic karstification, the bedrock is stable in mineral composition and tighter after deep burial and late diagenesis, fluid can only make reformation along the channels of fractures, joints and bedding planes, which results in large-scale groove and cave systems. But in the case the bedrock is low in maturity and only experiencing shallow burial or short period of shallow burial, the bedrock has good porosity and permeability, when subjected to karstification, the karst channels are the porous layers in the bedrock itself^{[19, 21-22]}, this kind of karstification is the eogenetic karstification. In eogenetic karstification, the main karst channels are pores, and the fluid does sheetflood dissolution in the bedrock, giving rise to reticular solution channels in 3D space. If there were microcracks in the early porous carbonate rocks, dissolution would first enlarge these dominant channels first, then the enlarged fractures and bedrock reticular solution channels interweave in space, showing piebald and spongy dissolution features^[21]. Due to the mechanism of sheetflood dissolution, the fluid flows in the intergranular space and dissolves matrix grains. Limited by the intergranular flow path, the dissolved materials mainly fill the adjacent pore system, showing an increase of dissolution degree from matrix to the edge of the dissolving pore (cave) and to dissolving pore (cave), that is the friable halo feature surrounding the karst cave^[22].

The fourth member of Dengying Formation in the northeastern Sichuan Basin was uplifted and exposed shortly after deposition due to the second episode of Tongwan movement, without medium-deep buried experience during diagenetic stage, and had the geological background of eogenetic karstification. Features of piebald and spongy karst system are found in core samples and thin sections (Fig. 7a-7c) of this member, and the space in the piebald karst system is filled by breccia dolomite and dissolved carbonate sand (Fig. 7d), showing the same features of eogenetic karstification. Affected by the sheetflood dissolution inside the porous layer, three different types of dissolution-filling phenomena, bedrock zone, half-dissociation zone and mixed filling zone, can be identified in the grain dolomite of the fourth member of Dengying Formation (Fig. 7e, 7f); the dissolution degree increases from matrix to dissolving caves, and the bedrock gradually dissociates and almost disappears at last. The half- dissociation belt is very similar to the "fragile halo" pointed out by Moore, which is also one of the most representative identification marks of eogenetic karstification^[22].

Based on the above understandings, we believe that the karstification in the northeastern Sichuan Basin belongs to eogenetic karstification^{[23,24,25,26]}. The dissolution process is controlled by the early porous layers, in other words, by the sedimentary facies, thus the reservoirs are characterized by facies control. This is especially evident in the reservoir under the weathering crust on Gucheng profile, Wangcang county. On this profile, influenced by the karstification during the second episode of Tongwan movement, there is a weathering crust layer about 3 m thick at the top of the fourth member of Dengying Formation (Fig. 8), and a 1 m thick dissolution pore-cave asphalt layer under the weathering crust. The existence of asphalt layer indicates that the karstification during the second episode of Tongwan movement has some direct contribution to the reservoir development. Due to the facies variation, a 2.5 m thick very tight siliceous dolomite layer with pure lithology turns up under the asphalt layer, and in the clotted dolomite under the siliceous dolomite, karst reservoirs come up again in large scale. Within 5 meters below the weathering crust, the difference of reservoir development in the siliceous dolomite and clotted dolomite shows that the karstification during the second episode of Tongwan movement does not directly contribute to the reservoir development, but is superimposed reformation on the dominant sedimentary facies belt, that is facies controlled karstification. The caves in the study area are mostly elongated and elliptical shapes, and distributed along the microbial laminae (Figs. 3i, 4c, 4d). According to statistics on physical properties of reservoirs in the northern Sichuan Basin, the microbial mound and grain beach facies with more original pore systems have much higher porosity and permeability, indicating that sedimentary facies is the most critical factor for reservoir development (Fig. 9). After the deposition of Dengying Formation, the characteristics and spatial distribution of reservoir were basically fixed under the dual control of sedimentary facies belt and facies-controlled karstification. Yuanba in the northeastern Sichuan Basin and its neighbouring areas have been located in the junction of the North Sichuan depression and the Central Sichuan gentle belt for a long time, the connection part of three large-scale positive structures, sheltered by these three structures, this area has gentle strata occurrence and weak later-structural transformation^[27]. In addition, due to the ultra-deep burial depth (the burial depth of more than 3500 m since the Silurian, and the current burial depth of more than 8000 m), the Dengying Formation has been in a high-pressure closed system for a long time, and the material can’t be carried out of the system by fluids. Although the reservoirs have also undergone the superimposed reformation of Caledonian-Himalayan multi- stage structural fracturing and buried dissolution, these fracturing and dissolution have contributed little to the increase of the total pore volume of the reservoirs, but are conducive to the improvement and adjustment of the early porous layers.

During early stage of Dengying Formation deposition, affected by regional tension activities, a NS-striking Mianyang- Changning taphrogenic trough was formed in western and central Sichuan area. The taphrogenic trough and platform on its east side constituted the depositional model of “rimmed platform at rift margin”, and there was a high energy facies belt of platform margin mound-beach east of Gaoshiti-Langzhong-Guangyuan (Fig. 10)^[28,29,30]. First, 3D seismic data of Yuanba area also show clear platform shelf differentiation. The mound-beach deposits at the platform margin are thick and in mound shape. The mound-beach bodies with frame pores were reformed by eogenetic karstification under facies control into dissolved pore-cave reservoirs, showing clutter medium-strong interrupted reflections with amplitude variations (Fig. 11). All these indicate that Yuanba area had favorable reservoir development conditions. Second, close to the hydrocarbon generation center with high quality Lower Cambrian source rock in the Mianyang-Changning taphrogenic trough, the Yuanba area has the advantage of near source rock hydrocarbon accumulation, and relatively weaker tectonic deformation in the later period and no faults developed, which are favorable for natural gas adjustment and preservation. If exploratory wells are deployed in this area and make breakthrough, the exploration scope of Sinian in the Sichuan Basin will be expanded significantly.

Recent evaluation shows that there are abundant deep and super-deep oil and gas resources and large exploration potential in carbonate strata of Sichuan Basin and surrounding areas. However, except in central Sichuan Basin, the Sinian strata in the Sichuan Basin and surrounding areas are generally large. The target strata in the Yuanba area are over 8000 m deep. If new exploration breakthrough is made, it will enrich super-deep and extra-deep reservoir generation and maintaining theories in marine carbonate rocks, and provide new cases of eogenetic karstification reformed reservoir under facies control.

Meanwhile, it will drive evaluation research and exploration of super-deep strata in other areas, which is of important practical significance.

The reservoir rocks of the fourth member of Dengying Formation in Yuanba and its surrounding areas in the northeastern Sichuan mainly include stromatolite dolomite, clotted dolomite, and microbe-bond framework dolomite related to microbial-mound building, and dolarenite related to grain beach building. The reservoir space mainly includes intergranular dissolution pores, inter-frame dissolution pores and karst caves. The reservoirs show large scale and strong heterogeneity, and are fracture-pore-cave type.

There is a piebald karst system and half-dissociation belt (friable halo structure) in the fourth member of Dengying Formation in northeastern Sichuan area, indicating the karstification is eogenetic karstification and is controlled by sedimentary facies belt. Mound-beach facies is the main controlling factor of high-quality reservoir development in the fourth member of Dengying Formation.

Controlled by the Mianyang-Changning taphrogenic trough, high energy mound-beach belts of platform margin developed in the west of Yuanba. On 3D seismic section, the mound- beach belts show mound shape and clutter and medium-strong interrupted reflections with variable amplitudes. It is concluded from interpretation that two mound-beach facies belts developed in the Yuanba area are favorable exploration targets.