The carbonate build-up composed of microbialite generated from trapping and binding of detrital sediments by bacteria and algae microorganisms and related grainstone is named mound-shoal complex. It is a new type of important carrier of oil and gas resources, and has shown great potential in exploration globally. With special sedimentary characteristics and reservoir forming process, it has become a hot spot of study^{[1,2,3,4,5,6,7,8,9]}. Abundant achievements have been made in studies on its fabric characteristics, sedimentary sequences and reservoir controlling factors^{[10,11,12,13,14,15]}, and some researchers have paid attention to the role of paleogeomorphology in controlling its formation, size and migration^[16,17,18]. However, the coupling relationship between the sedimentary patterns of various types of mound-shoal complexes and paleogeomorphology has been rarely reported.

The Cambrian platform margin mound-shoal complex in the Gucheng area is a new target for ongoing exploration of deep ancient carbonate rocks in the Tarim Basin. The exploration practices in recent years reveal that the mound-shoal complex is large in size, good in reservoir properties and active in hydrocarbon shows in gas logging^[19,20], showing promising exploration prospects. In addition, its complicated sedimentary-diagenetic process leads to strong reservoir heterogeneity. The mound-shoal complex is the important basis of reservoir formation and evolution, but its sedimentary development models and controlling factors have remained uncertain, making it difficult to define the distribution range of the mound-shoal sedimentary microfacies and sort out the favorable reservoir areas. Based on observation of cores of the Gucheng area and outcrops in northwestern Tarim area, thin section identification, fine description of seismic facies, and restoration of paleogeomorphology by residual thickness method etc., the development models of various types of mound-shoal complexes coupled with different ancient landforms were figured out. From the perspective of ancient landform control, we try to find out the microfacies distribution in various types of mound-shoal complexes, and the complex types favorable for development of large-scale reservoirs. This provides a geologic basis for prediction of favorable reservoirs and selection of exploration targets.

Controlled by the Sinian-Cambrian continental rifting and extensional environment, the Tarim plate didn’t change much in overall paleogeographic pattern during the Cambrian, but the Tarim platform and other isolated carbonate platforms became larger due to progradation and accretion^[21]. The Gucheng area is part of the Gucheng low uplift, a second-order structural unit in the eastern Tarim Basin. It is located on the southeast margin of the platform in the West Tarim (Fig. 1a), with an area of nearly 3000 km², and is adjacent to the Tadong uplift to the east^[19]. During the Cambrian-Ordovician period, the Gucheng area experienced the evolution from carbonate platform to mixed shelf^{[20,21,22,23]}. The Lower Cambrian strata are mainly grain-bearing dolostone of the middle-outer ramp and marl of the carbonate ramp platform. During the Middle-Upper Cambrian, with the continuous regression, progradation took place on the platform margin in the West Tarim area, and a series of progradational platform margin mound-shoal complexes, which mainly consist of bacteria and algae microbial carbonate rocks and related granular dolostones, developed in the Gucheng area, forming rimmed carbonate platform. In Lower Ordovician, weakly rimmed-steepening distally semi-restricted ramp platform developed, inside which dolomitized shoal and tidal flat facies deposited. The Middle Ordovician is dominated by open platform granular limestone; and the Upper Ordovician is dominated by marine clastic rocks of over-compensation condition (Fig. 1b).

At present, in the 3D seismic work area in the Gucheng area of 2000 km², four stages of platform margin mound-shoal complexes progradating seawards laterally have been identified in the Middle-Upper Cambrian (Fig. 1c), with an overlapped area of about 1600 km² and a buried depth range from 6700 m to 8200 m. The buildup height of each stage is generally 300-700 m, 420 m on average. Wells Chengtan 1 and Chengtan 2 revealed the third and fourth stages of platform margin mound-shoal complexes in the Upper Cambrian have a thickness of approximately 1000 m. The data acquired from a new round of well drilling has laid a foundation for further geological understanding on the complex.

Observation under microscope shows the dolostone samples from the complexes have clear original sedimentary structures. According to the structures, the dolostones can be classified into seven types, the dendrite dolostone and spongiostromata biostrome dolostone related to the framework growth of microbial communities, the stromatolite dolostone, laminated dolostone, clotted dolostone, pellet dolostone and other microbialites related to microbial trapping and binding^{[4, 10-13]}, and the related granular dolostone. Among them, microbialites are the main components of the mound, and granular dolostone related to microbialite is the main rock type of the shoal. Various textures of rocks in mounds and shoals indicate different sedimentary environments with different water depths and energy. Their characteristics are the main basis for sedimentary microfacies identification. According to the distribution of lithofacies associations revealed by the drilling data and outcrops, the mound-shoal complex is divided into mound base, mound front, mound core, mound back and mound flat microfacies.

The mound base is located at the very bottom of the complex, and is composed of bedded deposits in medium thickness bed (Fig. 2), including calcarenitic, pellet dolostone with granular structures and some microbial pellet structures (Fig. 3a, b). Well Chengtan 1 reveals that the mound base microfacies is mainly composed of fine sand and finer silt detritus with micrite-sparite and sparry cements between them; the pellets are dispersive, and those related to microbial aggregation have relatively vague and irregular edges, while those of biological excretion origin are dark in color and oval or ellipsoid in shape, with higher organic matter content. Sedimentary characteristics of the shoal in the mound base reflect the environment at the initial development stage of the mound-shoal complex had deep water and medium-weak energy, unfavorable for microbial growth.

The mound core, located above the mound base, is a set of thick mound-shaped deposits in the middle of the complex (Fig. 2b, c), and has not been drilled in the Gucheng area. The study of Sugetbulak outcrop etc. in the Northwest Tarim shows that the mound core mainly develops stromatolite, spongiostromata biostrome, dendrite and thrombolite dolostones with various microbial structure types^{[10,11,12,13]}. The dendrite dolostone mainly develops in-situ epiphyte cryptocrystalline dolomite. Under the microscope, it appears as bifurcated bush-like aggregates, or densely agglomerated globular micro-clusters, with sparry cements, and primary framework pores between dendritic frameworks (Fig. 3c). This kind of dolostone is formed in high hydrodynamic environment. The thrombolite dolostone shows cyanobacteria bonding between thrombolites with unclear edges, micrite-sparite cements and primary pores between thrombolites. The thrombolite dolostone mainly developed in the upper part of the deep and turbulent subtidal zone (Fig. 3d). Macroscopically, the stromatolite dolostone is generally lamellar, with alternate bright and dark layers or dome-shaped microstructures, and primary pores between layers (Fig. 3e). It commonly occurs in high-temperature and high-salinity shallow water environment of the supratidal zone. The spongiostromata biostrome dolostone is generally spongy and faveolate, with developed microbial visceral pores (Fig. 3f). It is symbiotic with sand-gravel detritus, indicating relatively strong hydrodynamic environment. The mound core microfacies with active microbial activity constitute the main part of the mound in the complex, and is mainly deposited in shallow water high-energy environment^[24].

The mound back, located behind the mound core (Fig. 2c), has not been drilled in the Gucheng area. On the Aksui-Yushi outcrop section, it is mainly composed of dark gray laminated dolostone^[13], with laminar structure of bacteria laminae, grain layers and sparry cements seen under microscope (Fig. 3g). The undulating bacteria laminae have higher micrite content, and are formed by the microbial calcification, and trapping and binding of microorganisms. The grain layers have higher sphericity, even ring structure, and sparry cements, and are formed in hydrodynamic conditions stronger than the bacteria laminae. The mound back blocked by the mound core shows the sedimentary characteristics of shoal microfacies with alternate low water energy and high water energy.

The mound front is located in the forelimb of the mound core (Fig. 2a, b), and composed of thrombolite dolostone, dolorudite and dolarenite, and some pellet dolostone. Drilled wells reveal the mound front facies has cyanobacteria growth traces in particles, poor sorting and roundness of detritus within various particles, and mainly sparry cement. In the sediment, a small number of pellets have pelletoid structure and are scattered as mentioned above (Fig. 3h, i). It is a result of re-sedimentation in the forelimb of the mound core through transportation by mechanical action such as wave or gravity differentiation, and has the sedimentary characteristics of deep-water medium-high energy shoal facies.

The mound flat is dome shaped deposit in medium thickness layer at the top of the complex (Fig. 2a, c), mainly consisting of oolitic dolostone, dolorudite and dolarenite. In Wells Chengtan 1 and Chengtan 2, the sand detritus and ooids cemented by sparite are mainly larger than fine sandstone in granularity, and are fairly-well sorted and rounded. The gravel detritus have dendritic structure of epiphyte and other microbial structures remained, and the microbial primary pores are partially or completely filled with later siliceous minerals or calcite.

Different from the tuft aggregates of dendritic stones in the core, these gravel detritus of epiphyte are mixed with sand detritus and oolites (Fig. 3j-l). The mound flat is formed by re-sedimentation of the incompletely consolidated microbialite near the wave base after washed and broken repeatedly by strong wave action, and has the typical characteristics of shallow-water high-energy shoal.

The Cambrian platform margin mound-shoal complex in the Gucheng area belongs to the generalized organic reef sedimentary formation^{[4, 13-24]}, and has a large deposition rate in its isochronous strata and obviously greater thickness than the surrounding contemporaneous sediments. In Wells Chengtan 1 and Chengtan 2, 428 m and 398 m thick mound-shoal complexes were drilled respectively (Fig. 2a), which show reflection characteristics of mound-shape buildup on the seismic section (Fig. 4). The 3D seismic data of the Gucheng area show that the complexes have complete mound-shape envelope and clearly identifiable stages. The top and bottom of each stage complex are characterized by seismic reflections of strong amplitude and good continuity. The next stage of mound-shoal complex has onlap characteristic to the envelope boundary of the slope break of the former stage, or downlap to the sedimentary layer at the foot of the front of the envelope boundary of the former stage of mound-shoal complex (Fig. 4). The onlap and downlap relationships between these layers represent the characteristics of sedimentary transition surface. Through seismic event tracking and meticulous depiction of their contact relations, four stages of mutually superimposed platform margin mound-shoal complexes prograding toward sea have been identified from west to east in the Cambrian strata in the Gucheng area, and the isochro-nous stratigraphic units in each stage have been divided. The isochronous stratum of the first stage is above the Lower Cambrian, and the second stage overlies the first stage. These two stages of complexes belong to the Middle Cambrian. The isochronous stratum of the third stage developed later in the Upper Cambrian. The fourth stage overlaps on the slope break zone of the third stage, without intra-platform sediments (Fig. 4).

At present, the commonly used methods of paleogeomorphology restoration include mold method, residual thickness method, layer flattening method, sedimentology analysis and sequence stratigraphy method^[25,26]. The study areas have differences in data base and geological conditions, and each method has its limitation and applicability. In the Gucheng area, only two wells drilled the target interval and are distributed in a limited region (Fig. 1c). Obviously, sedimentology analysis and sequence stratigraphy analysis aren’t suitable for paleogeomorphology restoration of this area. The extensive 3D seismic data in the study area has laid a foundation for paleogeomorphology restoration by residual thickness method, mold method and layer flattening method. These methods can restore paleogeomorphology more rapidly and directly and have a higher planar resolution. Both mold and layer flattening methods select the layer with filling characteristic above the adjacent target interval as the base level and flatten it to achieve restoration of paleogeomorphology. However, their limitation lies in the difficulty of necessary correction of de-compaction, paleo-water depth and differential settlement and the base level selection. Residual thickness method is to flatten the stable marker bed below the adjacent target interval to restore paleogeomorphology, and its main limitation is that it does not take the influence of differential erosion and the selection of marker bed into consideration^[25,26,27].

The layer with filling characteristic overlying the Cambrian strata in the Gucheng area is the marine clastic rock of the Upper Ordovician Charchag Formation^[20]. But this layer is far away from the target interval, and has no marker bed that is easily recognizable and stably distributed. Moreover, the compaction correction of the Cambrian-Ordovician clastic rock, limestone and dolostone are difficult. The Cambrian in the Gucheng area is dominated by dolostone, which has not experienced extensive denudation and has strong resistance to compaction. In addition, the Yuertusi Formation at the bottom of the Lower Cambrian is the largest marker bed of flooding with a stable distribution throughout the whole Tarim Basin^[28]. In the Gucheng area, the Yuertusi Formation shows strong amplitude, continuous seismic events on the seismic section (Fig. 4). Its seismic reflection is clear and traceable regionally, thus, the Cambrian possesses the proper characteristics for paleogeomorphology restoration by residual thickness method.

After flattening the Yuertusi Formation on the basis of meticulous horizon interpretation of 3D seismic data, the thickness distribution from the top of Yuertusi Formation to the top of Lower Cambrian represents the ancient landform before deposition of the first stage of platform margin mound-shoal complex. The thickness distribution from the top of Yuertusi Formation to the top of the isochronal stratigraphic unit of the first stage represents the ancient landform before deposition of the second stage. In the same way, the ancient landform before deposition of complex of each stage can be recovered. Areas with large sedimentary thickness are the paleogeomorphologic high-potential areas.

The paleogeomorphological features of the Gucheng area recovered by residual thickness method in the Early Cambrian (before deposition of the first stage of mound-shoal complex), the early Middle Cambrian (before deposition of the second stage of mound-shoal complex), the late Middle Cambrian (before deposition of the third stage of mound-shoal complex) and the early Late Cambrian exhibit the evolution process form gentle slope to rimmed platform (Fig. 5), which is consistent with the features of Cambrian lithofacies paleogeography evolution of the whole basin^{[21-22, 29-30]}. This indicates the results of residual thickness method are reliable. Three geomorphological types, uniform gentle slope, folded steep slope and monoclinic steep slope, were identified before deposition of the four stages of mound-shoal complexes in the Gucheng area (Fig. 5a-e). In the Gucheng area, with progradation of complex in each stage in sequence, the Cambrian S-N rimmed platform margin gradually migrated from west to east toward the sea, reflecting the continuous fall of the overall sea level since the Middle Cambrian.

As shown in the seismic section of the flattened Yuertusi Formation, the Lower Cambrian in the Gucheng area decreases in thickness gradually from west to east, showing the paleogeomorphologic characteristic of gentle slope from west to east, with a slope angle generally less than 1°. On the plane, the slope has weak slope break zones developing stably in the north-south direction, generally showing the characteristics of gentle slope platform on the whole (Fig. 5a, b). During the early Middle Cambrian period, the first stage of mound-shoal complex began to develop on the Lower Cambrian weak slope break zone. The carbonate rocks formed by microorganisms were significantly larger in deposition rate and thickness than other strata in the same period, appearing as a positive convex buildup on the seismic section in mound shape^{[14,15,16,17,18,19,20]} with the characteristics of rimmed platform (Fig. 5a). At this point, the buildup had large height and steep slope on the rimmed platform margin, while the terrain was relatively gentle at the foot of the steep slope of platform margin, forming a north-south trending folded steep slope platform margin (Fig. 5a, c). At the end of the Middle Cambrian, in the folded steep slope of platform margin where the first stage of mound-shoal complex developed, the second stage developed, showing the characteristics of monoclinic steep slope (Fig. 5a, d). In the Late Cambrian, the third stage came up on the monoclinic steep slope where the second stage developed, also forming monoclinic steep slope platform margin. Then the fourth stage overlapped on the monoclinic steep slope, in band distribution along the north-south direction (Fig. 5a, e).

The mound-shoal complex inside has distinct differences in seismic reflection characteristics, showing four types of seismic facies (Table 1). (1) Mound base microfacies is characterized by strong amplitude, relatively continuous-continuous, parallel-subparallel sheet seismic reflection with medium-low frequency. (2) Formed by the mound-shoal microbial growth and the binding or trapping of microbes, the mound core microfacies has no sedimentary bedding and shows weak amplitude, chaotic-discontinuous mounded seismic reflection characteristics. (3) The mound front microfacies and mound back microfacies, located before and behind the mound core, have similar seismic reflection characteristics of moderate-strong amplitude, relatively continuous, medium-high frequency, broom-like convergent-subparallel features. (4) The mound flat microfacies is characterized by moderate strong amplitude, continuous to fairly continuous, low and medium frequency dome-shaped or S-shaped progradation seismic reflection.

The carbonate platform margin with strong hydrodynamic force is the most favorable site for the development of mound-shoal complex, and the sedimentary formations of mound-shoal complex belonging to biological reefs are an extremely sensitive indicator of paleogeomorphology^[31,32]. The depositional characteristics of mound-shoal complex mainly depend on paleogeomorphology, sea level fluctuation, and hydrodynamic conditions. The hydrodynamic conditions are closely related to the slope and shape of platform margin and the structural characteristics of paleogeomorphy^[31]. When the slope is gentle, the waves reduce in energy, and the hydrodynamic force is weak, in contrast, when the slope is steep, the hydrodynamic force is strong at the platform margin. The configuration of sea level fluctuation and paleogeomorphology determines the size and structure of accommodation space at the platform margin^[32,33]. Therefore, among the various factors influencing the depositional model of the Cambrian platform margin, in the case that hydrodynamic conditions and sea-level fluctuation are difficult to be directly recovered, paleogeomorphology visible features recorded in the long geological history can reflect directly the integration of these factors. The paleogeomorphology controls distribution of mound-shoal complex facies and shapes different types of platform margin complexes through controlling the hydrodynamic conditions, accommodation space size and structure.

Based on the structural style of internal sedimentary facies under the constraints of the gentle slope and steep slope before deposition of each stage of mound-shoal complex in the Gucheng area, 3 types of mound-shoal complexes, gentle slope symmetric accretion type, steep slope asymmetric accretion type and steep slope asymmetric progradation type have been identified (Table 1).

The first stage of complex is gentle slope symmetrical accretion type. After the large-scale transgression in the early Early Cambrian in Tarim Basin, the major source rock stratum, Yuertusi Formation, was formed in the Tarim Basin, and then the sea level began to drop^{[21, 30]}. At the location where the topographic height difference changed slightly in the Lower Cambrian gentle slope platform, the mound base microfacies of the foundation stage of mound-shoal complex in the Middle Cambrian began to develop, lying flat on the gentle slope in sheet. With the further decline of relative sea level, the mound-shoal complex entered pioneering stage and flourishing stage. But still in the early decline stage of relative sea level, the water was still deep and the hydrodynamic force under the background of gentle slope was weak, which was not conducive to the large-scale and rapid growth of anti-wave microorganisms. Hence, the mound core dominantly grew vertically catching up with the sea level, with a small horizontal distribution. But the configuration of uniform gentle slope and high sea level led to sufficient accommodation space in vertical and horizontal directions (Table 1). Under the joint action of wave reformation and microbial trapping, the mound front and mound back microfacies fully developed in a large size, forming a symmetric mound-shaped structure staggering with the mound core microfacies. When the complex grew close to the sea level or exposed to the air, the accommodation space disappeared, and the complex entered into decaying stage. The strong hydrodynamic force near the wave base reformed the microbialite which had not consolidated before, and in the relatively balanced accommodation space on the mound-shaped structure composed of the mound back, mound core and mound front, extensive mound flat microfacies turned up, marking the end of deposition of this stage of complex. The microfacies is characterized by symmetric vertical accretion structure of "mound base-mound back+(small) mound core+mound front-(large) mound flat", where the mound of mound core is small in area, the shoal of mound flat is larger in area, and the shoals of mound front-mound back- mound base are large in area. On the whole, it shows the characteristics of small mound and large shoal.

The second stage of complex is steep slope asymmetric accretion type. At the flat foot of the folded steep slope in the front of the first stage of complex buildup, mound base microfacies, foundation of the second stage complex developed. Under the background of steep slope, the strong hydrodynamic conditions promoted the rapid flourishing of microbialite with framework growth, and there was sufficient accommodation space in the front of and above the folded steep slope (Table 1), leading to large scale mound core microfacies. Since the active microbial processes restricted the wave reformation, the mound front microfacies formed in its forelimb was smaller in scale. Limited by the earlier platform margin steep slope, the accommodation space in its back limb was insufficient, thus, mound back microfacies barely developed. Similarly, with the rapid vertical growth of the mound-shoal complex, its top was reformed by wave, and the mound flat microfacies with a large scale deposited in the large horizontal accommodation space in mound core-mound front microfacies, marking the end of the second stage of mound-shoal complex deposition. The microfacies of this complex is characterized by asymmetric vertical accretion structure of "mound base-(large) mound core+mound front-mound flat". Compared with the ancient gentle slope, the steep slope had stronger hydrodynamic force. Therefore, the mound of mound core is large, the shoal of mound flat is slightly larger than that in the gentle slope, the shoal of mound base is larger, and the shoal of mound front is smaller. Generally, the second stage complex is characterized by large mound and large shoal (Table 2).

The third and fourth stages of mound-shoal complex are steep slope asymmetric progradation type. They developed in the monoclinic steep slope on the front of the former stage complex with strong hydrodynamic force. The mound core grew rapidly upwards on the basis of mound base (which could be the mound front of the former stage). Due to the lack of sufficient accommodation space above (Table 1), it was forced to grow forward. In addition, the intense wave action at the steep slope and instability of the slope restricted the stable development of microbialite, leading to smaller size of mound core. The large accommodation space in the forelimb of mound core was filled with a large amount of intraclast formed by wave breaking, resulting in large-scale mound front. The mound back was not developed either due to the lack of backward accommodation space. With the decline of relative sea level, the mound-shoal complex entered decaying stage, and the mound flat was also small due to the lack of accommodation space. Restricted by the accommodation space of monoclinic steep slope, the rapidly growing mound-shoal complex under strong hydrodynamic conditions shows a series of typical characteristics of progradation structure, recording a strong decline process of relative sea level. The microfacies of this stage complex is characterized by lateral progradation structure of "mound base-(small) mound core+ mound front-mound flat". This stage of complex has the smallest mound area in mound core among the three types of complexes, large shoal area in mound flat but also the smallest among the three types of complexes, and large shoal area in mound base-mound front. In general, it is characterized by small mound and large shoal (Table 2).

A lot of studies show that the main influencing factors of sedimentary reservoirs in ancient microbial mound- shoal complex include paleogeomorphy, sedimentation, and various dissolution processes controlled by sedimentation and paleogeomorphy^[34,35,36]. Hence, the differential sedimentary models of mound-shoal complexes under different paleogeomorphic backgrounds are bound to have profound impacts on the development of different types of complex reservoirs.

First of all, sedimentary facies is a prerequisite for development of primary pores. The primary pores of mound-shoal complex are mainly related to microbialite^{[35, 37]}, including the framework pores in and between the dendrite and thrombolite microbial frameworks, visceral pores of spongiostromata biostrome, fenestral pores in the stromatolite, laminite and so on. Microbial action is most active in the mound core microfacies, so the mound core has many types of microbialite, making it the favorable zone for development of primary pores in the complex.

Secondly, sedimentary environment is the basis of reservoir reconstruction by karstification. Meteoric fresh water dissolution, organic acid dissolution and hydrothermal dissolution etc. reform the mound-shoal complexes into high-quality reservoirs, among which meteoric water dissolution is the key factor^{[19, 34, 36-39]}. The mound core microfacies was favorable for microbial proliferation, and had well-developed primary pores due to the shallow, high-energy sedimentary environment, providing essential seepage conditions for water-rock reaction of multi-media fluids for reservoir reconstruction by atmospheric water, hydrothermal fluid and organic acid dissolutions. The mound core has a porosity of 3%-10% generally^[13], and mound front, mound back and mound base have a porosity of about 2% generally. In addition, the upward-shallowing sedimentary sequence makes the mound flat microfacies with residual microbial structure at the mound-shoal complex top more prone to penecontemporaneous dissolution of atmospheric fresh water. As revealed by Well Chengtan 1, the mound flat microfacies at the top of the third stage of progradation type mound-shoal complex has centimeter-scale dissolution pores and a porosity of up to 11.2%. The shallow, high- energy environment of mound core and mound flat microfacies, which are conducive to the formation of high-quality karst reservoirs, are the advantageous reservoir facies in mound-shoal complex, and the karst reservoirs in mound flat have better physical properties (Table 2).

Finally, the differential sedimentary models coupled with ancient landforms determine the reservoir development law. The results of 3D seismic reservoir prediction show that the three types of differential sedimentary models of complexes under the constraints of the three paleogeomorphologic types lead to the scale difference of favorable reservoir facies belts. In the case that the favorable reservoir facies belts are roughly equal in predicted thickness, the total area of core and flat of the steep slope asymmetrical accretion type characterized by large mound and large shoal is at least 100 km² larger than that of the other two types (Table 2). Apparently, the steep slope asymmetrical accretion type complex has the most developed high-quality reservoirs, and the gentle slope symmetric accretion type complex characterized by small mound and large shoal take the second place. In general, the two accretion type complexes have larger scale of favorable facies belts than the progradation type complex.

The Cambrian platform margin of the Tarim Basin has distinct segments^[40,41], among which, the whole Lungu-Gucheng platform margin in the east is of steep slope type and shares similar structural characteristics, and develops the steep slope accretion-progradation platform margin complex^[41]. Mound-shoal complex similar to the second stage of mound-shoal complex in the Gucheng area characterized by large mound and large shoal is likely to occur in this area, which is considered the favorable facies belt for development of large-scale reservoirs. In addition, the deep Cambrian mound-shoal complexes in Lungu-Gucheng platform margin have a variety of reservoir-cap rock assemblages, showing great exploration potential.

The Cambrian platform margin mound-shoal complexes in the Gucheng area are mainly composed of microbial dolostone and related granular dolostone, representing microbial carbonate sediments. According to the different combinations of seven structural types of dolostone indicating sedimentary environments, the complex is divided into five sedimentary microfacies, namely, mound base, mound core, mound front, mound back and mound flat.

The paleogeomorphology controlled the sedimentary model of mound-shoal complex through accommodation space size and structure. In the four stages of overlapped mound-shoal complex prograding toward sea developed under the background of three types of ancient landforms, three types of differential sedimentary models were identified. The first stage is gentle slope symmetric accretion type, with five types of microfacies developing completely, which is characterized by symmetric vertical accretion structure with mound core as the axis. The second stage is steep slope asymmetric accretion type without mound back microfacies, which is characterized by asymmetric vertical accretion structure. The third and fourth stages are steep slope asymmetric progradation type without mound back microfaices, and are characterized by lateral progradation structure. The gentle slope accretion type and steep slope progradation type complexes have the characteristics of "small mound and large shoal", while the steep slope progradation type has the characteristics of "large mound and large shoal".

Paleogeomorphology controls the distribution of sedimentary facies belts inside mound-shoal complex. Sedimentary facies belts provide conditions for the development of primary pores, and the sedimentary environment indicated by them is the basis of reservoir reconstruction by various karstification processes. In summary, the differential sedimentary models coupled with paleogeomorphology determine the development scales of complex reservoirs. The accretion complex has large favorable reservoir facies belts, and the second stage of complex characterized by "large mound and large shoal" in Gucheng area is inferred to have the most favorable reservoir facies belts. Finding out the relationship between sedimentary models of different types of mound-shoal complexes and paleogeomorphology provides a geological basis for selection of deeper favorable reservoir belts in the Gucheng area. Moreover, as exploration of deep and ultra-deep carbonate reservoirs goes on, for the mound-shoal facies belts similar to the Cambrian ones with low exploration degree, the analysis of paleogeomorphic structure can provide reference for preliminary selection of favorable exploration targets.