Introduction
Dolomite reservoirs account for about a quarter of the total global oil and gas reservoirs [1], and have been receiving attention from geologists [2⇓⇓-5]. Predecessors have carried out a lot of research on the basic characteristics [6], diagenesis [7] and reservoir controlling mechanism [8⇓-10] of platform dolomite reservoirs. It is generally believed that the marginal platform shoal with large thickness and good connectivity is the favorable facies belt conducive to the development of dolomite reservoirs [11⇓-13]. However, in recent years, high-quality fractured-vuggy dolomite reservoirs have been found in the intra-platform shoals of the Qixia Formation in the Gaoshiti-Moxi area, which are thin, only 1 to 6 m a layer. The cumulative gas production from the first production test well G18 has exceeded 1×108 m3. In June 2023, the exploration and development well G045-H1 of the Middle Permian Qixia Formation in Gaoshiti area of the Anyue Gas Field was tested and obtained a high-yield industrial gas flow of 136.72×104 m3/d and an open flow of 293.33×104 m3/d which is the third high-yield well with a million cubic meters after the G001-X45 and G045-H2 wells. Major exploration progress has been made in the thin-layer dolomite reservoir of the Qixia Formation. It demonstrates a new special exploration field of dolomite.
The previous studies about the intra-platform shoal reservoirs focused on the limestone reservoirs superimposed with karst or fracture transformation, such as the Middle-Upper Jurassic in Amu Darya Basin [14], the Carboniferous in the Tarim Basin, NW China [15], the Upper Cambrian Xixiangchi Formation in the central and southern Sichuan Basin [16], while the intra-platform shoal dolomite reservoirs are rarely systematically discussed. In recent years, scholars have carried out fruitful research on dolomite reservoirs of the Qixia Formation in the Gaoshiti-Moxi area [17⇓⇓⇓-21]. Hao et al. [18] believed that the residual paleogeomorphology of the Caledonian paleo-uplift in the central Sichuan Basin controlled the macroscopic sedimentary pattern of the Qixia Formation. Duan et al. [19] proposed that the dolomite reservoir is mainly developed in the high-stand systems tract of the Qixia Formation. Vertically, the thickness of the single layer is thin and multi-layer superimposed. Laterally, it has good continuity locally. The genesis of dolomite is mainly penecontemporaneous dolomitization. However, He et al. [20] believed that burial dolomitization is the main genesis of the dolomite. In addition, Lu et al. [21] systematically summarized the karstification effect on the dolomite reservoirs, and believed that the pores in dolomite were inherited from the primary grain shoal and karst system, and the early atmospheric freshwater karstification was the main cause of the formation of high- quality reservoirs. In summary, the development of the high-quality dolomite reservoirs is affected by many factors such as micro-paleogeomorphology, sedimentation and diagenesis. As of 2023, the macro-paleogeomorphology pattern before the deposition of the Qixia Formation in the Sichuan Basin has been basically clear, but the influence of micro-paleogeomorphology on the intra-platform shoal is unclear. The origin of the dolomite has two different directions: penecontemporaneous and buried dolomitization. However, how intra-platform shoal and different stages of dolomitization controlled the development of the dolomite is still unclear, and the reservoir formation model should be investigated. This paper first analyzes the distribution and characteristics of the thin-layered dolomite reservoirs, and investigates the controlling factors in the Qixia Formation intra-platform shoal in the Gaoshiti-Moxi area based on typical cores, systematic samples and lab experiments, and finally establishes the reservoir formation models. The findings provide a basis for future reservoir prediction and exploration deployment.
1. Regional geology and sedimentary
The gas reservoir of the Qixia Formation in the Gaoshiti-Moxi area is located in a gentle belt in the central Sichuan Basin. Its south is connected to the Southern Sichuan low steep belt and the Southwestern Sichuan low fold belt. Its east is adjacent to a high steep belt. Geographically, it is a vast area from the eastern Hechuan District of Chongqing to the western Anyue County, and from the northern Suining City to the southern Tongliang County (Fig. 1a ). The Qixia Formation in the study area experienced Caledonian, Hercynian, Indosinian, Yanshanian and Himalayan tectonic movements [22]. The paleo-uplift formed during the Caledonian movement controlled the sedimentary pattern of the Qixia Formation [23-24].
Fig. 1 Well location in Gaoshiti-Moxi area (a) and stratigraphic column diagram of Permian Qixia Formation (b). |
Before the Permian, affected by the Caledonian Movement, the western-central part of the Sichuan Basin was uplifted as a whole. The Caledonian paleo-uplift appeared, but it was eroded by Hercynian uplifting. Then a large-scale transgression occurring in the Permian caused seawater to invade from southeast and gradually submerged the residual paleo-uplift [25⇓-27]. The Liangshan Formation, the earliest sedimentary formation in the Permian, was a thin and light gray bauxite mudstone. Going upward, it transited to swamp deposits composed of black carbonaceous shale with coal seams. The Liangshan Formation in the Gaoshiti-Moxi area was 0 to 10 m thick. As large-scale transgression happened, the deposits evolved into carbonate platform facies in the shallow sea. The sea was open and the organisms were abundant. Vertically, multiple transgressive-regressive sedimentary cycles were formed in the Qixia Formation and the Maokou Formation. The thickness of the Qixia Formation in the Gaoshiti-Moxi area was 100-120 m, mainly medium-thick layered gray limestone with thin-layered dolomite.
Considering the petrological, electrical and geochemical characteristics, the Qixia Formation in the Gaoshiti- Moxi area can be divided into the first member (Qi1) and the second member (Qi2). During the depositional period of the Qi1 Member, the sea level rose rapidly and maintained at a high level, so the deposit was dominated by deep-water open-sea micritic limestone. During the sedimentary period of the Qi2 Member, the sea level began to decline, the sea water gradually receded eastward, and the water body became shallow, then shoal bodies grew in a suitable environment, and the microfacies combination of intra-platform shoal and inter-shoal sea was developed, so the deposit was almost medium- thick bioclastic limestone. The Qi2 Member can be subdivided into two secondary shallowing cycles: Qi21 and Qi22. Both of them gradually changed from micritic bioclastic limestone to sparry bioclastic limestone intercalated with thin-layered dolomite from bottom to top (Fig. 1b ).
The intra-platform shoals were mainly distributed near the top of the two secondary cycles. They were divided into high-energy and medium-low-energy shoals. The high- energy shoals grew in shallow and circulating water. Long- term waves selection brought plenty of bioclastics, so the high-energy shoals were mainly light-colored sparry bioclastic limestone (Fig. 2a ). In the burial diagenesis stage, some bioclastic limestone underwent dolomitization and evolved into intra-platform shoal dolomite (Fig. 2b ). The medium-low energy shoals are mostly distributed near the wave base where the energy was relatively low, so micritic-sparry bioclastic limestone was common (Fig. 2c ). The inter-shoal sea deposit was dominated by low-energy micritic bioclastic limestone (Fig. 2d ).
Fig. 2 Typical petrological characteristics of the Qixia Formation in the Gaoshiti-Moxi area. (a) Sparry bioclastic limestone with fossils of red algae, fusulinid, foraminifer and other organisms; 4 479.80 m; Well M151; plane-polarized light; (b) Medium-fine dolomite; 4 530.80 m; Well M151; plane-polarized light; (c) Micritic-sparry bioclastic limestone; 4 525.41 m; Well M151; plane-polarized light; (d) Micritic bioclastic limestone with fractures filled with two stages of calcite; 4 529.45 m; Well M151; plane-polarized light. |
The shoals in Qi22 are mainly distributed in the middle and upper parts, and they are discontinuous and superimposed. The thickness of a shoal is 1-6 m (Fig. 3a ). On the plane, the shoal cores are concentrated and contiguously distributed in the Gaoshiti area, generally thicker than 10 m, and locally over 14 m, while the shoal edge expands around the shoal core (Fig. 3b ). There are 1-2 sets of low-energy shoals with good continuity in the middle-lower part of Qi21, 2-4 m thick each. In the middle-upper part, there is a set of large and continuous high-energy shoals 6-10 m thick (Fig. 3a ). Affected by micro-paleogeomorphology, the shoals in Qi21 gradually migrated to the northern Moxi-Longnüsi area. The shoal cores are thicker than 14 m, and distributed as strips or masses found in wells M117, M42 and N1 (Fig. 3c ).
Fig. 3 Sedimentary microfacies of the Qixia Formation in Gaoshiti-Moxi area (the section location shown in |
2. Reservoirs of Qixia Formation in Gaoshiti-Moxi area
The reservoir of the Qixia Formation in the Gaoshiti-Moxi area is mainly intra-platform shoal thin-layer dolomite of Qi2 Member (Fig. 1b ).
2.1. Reservoir petrological characteristics
The light gray or gray medium-thin dolomite is fine crystalline, fine-medium crystalline, and medium-coarse crystalline. The fine and fine-medium crystals are generally anhedral crystals with non-flat surface which is dirty and unclear in shape and profile, shows wave-like extinction characteristics and more residual biological debris and locally residual sand debris, and reflects by dark light in cathodoluminescence test. It is worth noting that the cathodoluminescence characteristics of the fine dolomite is similar to that of bioclastic limestone, indirectly reflecting that the diagenetic fluid is mainly marine fluid from early diagenesis (Fig. 4a-4f ). The medium and medium-coarse crystals have subhedral-euhedral, straight and flat crystal surfaces. The crystal core is dirty and the color of the rim edge is brighter. A turbid core and bright rim structure can be seen, and it is bright red in cathodoluminescence. Cycle and strip characteristics are obvious (Fig. 4g-4j ). In addition, saddle dolomite is common in the caves and fractures. They are coarse crystals with largely bending surfaces, and show wave-like extinction characteristics under cross-polarized light (Fig. 4k, 4l ). The medium, medium-coarse and saddle dolomite are red and bright red in cathodoluminescence, and there are multiple stages of dark and bright cycles and strips, indicating multiple stages of burial recrystallization.
Fig. 4 Typical lithologic slices and cathodoluminescence characteristics of the Qixia Formation in Gaoshiti-Moxi area. (a) Sparry bioclastic limestone; 4 570.70 m; Well M117; plane-polarized light; (b) It is the same horizon as Fig. a: dark light in cathodoluminescence test; (c) Anhedral fine-medium dolomite with visible residual bioclastic structure; 4 690.46 m; Well M42; plane-polarized light; (d) It is the same horizon as Fig. c: dark light in cathodoluminescence test; (e) Anhedral fine-medium dolomite (bottom) and medium-coarse dolomite (top); 4 577.15 m; Well M117; plane-polarized light; (f) It is the same horizon as Fig. e: dark light (bottom), and turbid core and bright rim structure (top); cathodoluminescence; (g) Medium-coarse dolomite; 4 575.16 m; Well M117; plane-polarized light; (h) It is the same horizon as Fig. g: turbid core and bright rim structure; cathodoluminescence; (i) Medium-coarse dolomite; 4 523.23 m; Well M151; plane-polarized light; (j) It is the same horizon as Fig. i: turbid core and bright rim structure; cathodoluminescence; (k) Saddle dolomite with bending crystal surface; 4 373.14 m; Well M117; plane-polarized light; (l) It is the same horizon as Fig. i; bright red rim; cathodoluminescence. |
2.2. Reservoir pore type
Observation of cores and thin sections shows that the most important reservoir pores in the dolomite reservoirs of the Qixia Formation in the Gaoshiti-Moxi area are intergranular pores, intergranular dissolution pores and karst caves, and fractures play the role of communicating pores and improving seepage capability. The intergranular pores are mostly developed among medium-coarse subhedral-euhedral dolomite crystals, generally triangular or polygonal, and they are regularly distributed when dolomitization is more complete (Fig. 5a ). The intergranular dissolution pores are the result of dissolution of intergranular pores, usually bay-like (Fig. 5b ), and mostly filled with asphalt and crystal fragments (Fig. 5c, 5d ). Pinhole dolomite can be formed when intergranular dissolution pores are intensively developed. Karst caves are relatively common in the dolomite reservoir. The pore-type karst caves are formed by the enlargement of primary intergranular pores and intergranular karst pores after dissolution, and they are usually partially filled by medium-coarse euhedral dolomite (Fig. 5e ). The macroscopic surface porosity may exceed 15% locally. Fracture-type karst caves are formed when acidic or hydrothermal fluids move along fractures and unselectively dissolve the rock composition around the fractures. The fractured space is expanded, and on the other hand, the bedrock around the fractures is recrystallized to medium-coarse dolomite. Karst caves and karst ditches of different width are common along the fractures (Fig. 5f ), as well as bead-like dissolution pores which are often semi- filled with calcite, dolomite and asphalt. In addition, weak and semi-filled fractures are common in cores and sections, which connect with the primary pore network (Fig. 5g, 5i ).
Fig. 5 Pores in the dolomite reservoir of the Qixia Formation in Gaoshiti-Moxi area. (a) Crystalline dolomite with intercrystalline pores; 4 531.35 m; Well M151; plane-polarized light; (b) Crystalline dolomite with bay-like intercrystalline dissolution pores; 4 500.63 m; Well M150; plane-polarized light; (c) Crystalline dolomite with intercrystalline dissolution pores filled with a small amount of asphalt; 4 500.18 m; Well M150; plane-polarized light; (d) Crystalline dolomite with intercrystalline dissolution pores filled with a small amount of asphalt; 4 523.12 m; Well M151; plane-polarized light; (e) Grey crystalline dolomite with centimeter-level karst caves semi-filled with early calcite and late euhedral dolomite; 4 531.26 m; Well M151; (f) Grey crystalline dolomite with fracture-type karst caves; 4 523.12 m; Well M151; (g) Crystalline dolomite with structural fractures and dissolved fracture; 4 687.18 m; Well M42; plane-polarized light; (h) Crystalline dolomite; dissolution along the fracture; 4 501.01 m; Well M150; plane-polarized light; (i) Crystalline dolomite with micro-fractures communicating intergranular pores; 4 502.33 m; Well M150; plane-polarized light. |
2.3. Reservoir physical properties and types
The statistical results of core physical properties show that the porosity of the dolomite reservoir is mainly distributed in the range of 2%-5%, the average value is 3.87%, and the median value is 3.37%, and the median permeability is 0.022×10−3 μm2, indicating low porosity and extra-low permeability reservoir. The median porosity of the fine dolomite is 2.65%, and the median permeability is 0.008×10-3 μm2. The median porosity of the fine-medium dolomite is 3.45%, and the median permeability is 0.022× 10-3 μm2. The median porosity of the medium-coarse dolomite is 4.09%, and the median permeability is 0.091× 10-3 μm2. The porosity and permeability of the medium- coarse dolomite are obviously better.
The dolomite reservoirs can be divided into three types: fractured-vuggy, vuggy and porous. The fractured-vuggy reservoir is the best with a large number of karst vugs and caves developed along fractures, which are shown by "high interval transit time, high compensated neutron porosity, high density, low gamma and low resistivity", and irregular black dark spots and strips on FMI. The characteristics of the vuggy reservoir are similar to those of the fractured-vuggy reservoir, but the former has fewer fractures, and the storage capacity is smaller than the latter. The porous reservoir is relatively tight, and with dissolution pinholes locally.
3. Controlling factors and reservoir formation model of high-quality dolomite reservoirs
Micro-paleogeomorphology, the distribution of intra-platform shoals, early dolomitization and late hydrothermal reconstruction are four factors controlling the formation of high-quality dolomite reservoirs. The special reservoir formation model is characterized by "micro-paleogeomorphology controlling the deposition of shoals, shoals controlling the deposition of dolomite, penecontemporaneous dolomitization benefiting preservation of pores, and late hydrothermal action effectively improving the reservoir quality ".
3.1. Controlling factors
3.1.1. Micro-paleogeomorphology controls the distribution of intra-platform shoals.
The distribution of intra-platform shoals in the Qixia Formation is obviously controlled by the micro-paleogeomorphology before the deposition of the Qixia Formation. The Liangshan Formation is the product of early Permian transgression, and the residual thickness can indirectly reflect the micro-paleogeomorphology before the deposition of the Qixia Formation. The residual thickness map (Fig. 6a ) of the Liangshan Formation in the Gaoshiti- Moxi area shows that the thickness of the Liangshan Formation in Wells G18 and G19 is the thinnest, almost zero, and there is a significant thickening trend in the north-south direction. According to the impression method, the micro-paleogeomorphology before the deposition of the Qixia Formation can be divided into first-order high zones, second-order high zones and third-order gentle slopes. The first-order high zones are mainly distributed in the Gaoshiti area. The micro-paleogeomorphology of the Liangshan Formation continues into Qi1. The Gaoshiti area is a successive high zone that is thin, but significantly becomes thicker towards northern and southern low parts (Fig. 6b ). From Qi22 to Qi21, the difference of the micro-paleogeomorphology before the deposition of the Qixia Formation gradually disappeared, and only leaving the relatively low northern Moxi and Longnüsi areas (Fig. 6c, 6d ).
Fig. 6 Stratigraphic thickness of Liangshan Formation and Qixia Formation in Gaoshiti-Moxi area. |
On the stable platform, shoal deposits first appeared on the high zones. And the high deposition rate enlarged the difference of micro-paleogeomorphology, resulting in relatively thick intra-platform shoals [28]. The high-thickness-value zone in Qi22 is mainly distributed around Well G18 in the Gaoshiti area where the micro-paleogeomorphology was high before the deposition of the Qixia Formation. When Qi21 deposited, the paleogeomorphology and sedimentary thickness in the Gaoshiti and Moxi areas tended to be consistent, but the paleogeomorphology of the northern Moxi and Longnüsi areas was relatively low, so shoals gradually migrated northward (Fig. 7 ).
Fig. 7 Development and evolution models of intra-platform shoals of the Qixia Formation in Gaoshiti-Moxi area (The section location is shown in |
3.1.2. Shoals control the distribution of dolomite
The original grain limestone in the high-energy intra-platform shoal was very permeable, and provided channels for magnesium-rich fluid and other dissolving fluids [29]. That is the material basis for the development of high-quality dolomite reservoirs and conducive to early dolomitization in the high paleogeomorphic zone. Observation to thin sections found original grain limestone in the transitional rock not completely dolomitized, and biological residue illusion with clear outlines in some crystalline dolomite, indicating that the original rock is grain limestone.
Vertically, there are more intra-platform shoals in the upper part of Qi22 and the middle and upper parts of Qi21. They are thick and continuous. The distribution position of dolomite is basically consistent with the shoals. On the plane, the dolomite in Qi22 is more distributed in the first-order high Gaoshiti zone and G19-G103 areas, followed by the Moxi area (Fig. 8a ). The dolomite in Qi21 is more distributed in the Moxi-Longnüsi area (Fig. 8b ). The distribution of dolomite is highly consistent with intra-platform shoal cores, which is also the direct evidence that shoal controls the distribution of dolomite (Fig. 3 ).
Fig. 8 Thickness contour map of dolomite of the Qixia Formation in Gaoshiti-Moxi area. |
3.1.3. Two-stages dolomitization controls the development of high-quality dolomite reservoirs.
The dolomite reservoirs of the Qixia Formation in the Gaoshiti-Moxi area have experienced various types of diagenesis. Among them, penecontemporaneous dolomitization, burial hydrothermal dolomitization and dissolution have significant influences on the formation of the reservoirs.
Penecontemporaneous dolomitization provided the basis for the formation of dolomite reservoirs. On the one hand, dolomitization builds the anti-compaction reservoir framework and preserves the pores to a large extent. On the other hand, at relatively low temperature, dolomite is more insoluble than limestone, which can avoid the calcium ions caused by pressure solution to block pores, and also provides advantageous channels for hydrothermal fluids and acidic fluids. The results of laser micro-area experiment show that in the study area, the fine dolomite has the δ18O values of −7.60‰ to −6.92‰ and the δ13C values of 2.08 ‰ to 2.61‰; the sparry bioclastic limestone has the δ18O values of −7.02‰ to −6.23‰ and the δ13C values of 1.85 ‰ to 2.86‰ (Fig. 9 ). The δ13C and δ18O values of the fine dolomite are similar to those of sparry bioclastic limestone. Compared with the seawater in the same period, the 87Sr/86Sr values of the limestone, dolomitic limestone and fine dolomite in the Qixia Formation mostly fall in the same range of the seawater (Fig. 10 ). In addition, from the perspective of rare earth element distribution pattern, the rare earth element distribution pattern of the fine dolomite is similar to that of matrix limestone (Fig. 11 ). The above geochemical characteristics indicate that the dolomite succeeded the limestone, and it is the product of penecontemporaneous marine fluid reconstruction.
Fig. 9 δ18O and δ13C values distribution of different types of carbonates in the Qixia Formation in Gaoshiti-Moxi area. |
Fig. 10 87Sr/86Sr of different types of carbonates in the Qixia Formation in Gaoshiti-Moxi area [20]. |
Fig. 11 Distribution pattern of rare earth elements in dolomite and limestone of the Qixia Formation in Gaoshiti-Moxi area. |
The transformation and dissolution of hydrothermal dolomitization during the burial period are key factors influencing the formation of dolomite reservoirs [30-31]. The homogeneous temperature of inclusions in medium- coarse dolomite and saddle dolomite crystals covers two obvious ranges: 150 °C to 170 °C and 180 °C to 200 °C, which are significantly higher than the normal geothermal gradient (Fig. 12 ). The saddle dolomite has the δ18O values ranging from −9.46‰ to −9.15‰, and the medium-coarse dolomite has the δ18O values from −8.22‰ to −7.72‰. These obviously negative values indicate the effect of hydrothermal fluid (Fig. 9 ). In addition, the REEs distribution pattern of the medium-coarse dolomite shows a certain differentiation compared with limestone. The heavy REEs are rich, and the europium (Eu) value is abnormally high (Fig. 11b ). The 87Sr/86Sr value of the medium-coarse dolomite is higher than those of the limestone and the dolomitic limestone (Fig. 10 ). The above geochemical characteristics are different from those of penecontemporaneous dolomite, indicating that the dolomite of the Qixia Formation was affected by hydrothermal action during the burial period. Although the space in the fractured-vuggy dolomite was filled with hydrothermal minerals to different degrees, the residual space is still large (Fig. 5 e, 5f). Observation to thin sections also shows that the intergranular pores and intergranular dissolution pores in the medium-coarse dolomite modified by hydrothermal fluids are more developed, and the reservoir performance is significantly better than the medium-fine dolomite not modified or weakly modified (Fig. 13 ).
Fig. 12 Statistical histogram of homogeneous temperature of the inclusions in medium-coarse dolomite and saddle dolomite of the Qixia Formation in Gaoshiti-Moxi area. |
Fig. 13 Hydrothermal dolomitization, dissolution and transformation to the Qixia Formation in Gaoshiti-Moxi area. (a) Crystalline dolomite with abundant pores (the upper right is fine-medium dolomite, and the left is medium-coarse dolomite modified by hydrothermal fluids); 4 501.01 m; Well M150; plane-polarized light; (b) Crystalline dolomite with abundant pores (the lower right is medium-coarse dolomite modified by hydrothermal fluids); 4 500.54 m; Well M150; plane-polarized light; (c) Crystalline dolomite with abundant pores (the upper is tight fine dolomite, and the lower right is medium-coarse dolomite modified by hydrothermal fluids; 4 506.88 m; Well M151; plane-polarized light. |
3.2. Reservoir formation models
Based on the study of the controlling factors of reservoir development, the reservoir formation model of intra-platform shoal thin-layered dolomite reservoirs of the Qixia Formation in the Gaoshiti-Moxi area is proposed (Fig. 14 ).
Fig. 14 The development and evolution model of high-quality dolomite reservoirs of the Qixia Formation in Gaoshiti-Moxi area (The section location is shown in |
During the sedimentary period of the Qixia Formation, intra-platform shoals were developed in the relatively high paleogeomorphic zone. The shoals are mainly sparry bioclastic limestone and micritic-sparry bioclastic limestone which have high original porosity and are conducive to the entry of subsequent diagenetic fluids. In the relatively low paleogeomorphic zone, micritic bioclastic limestone or bioclastic micritic limestone was developed, which is relatively tight. As the sea level in the high-stand systems tract declined, grain shoals in the high zone were likely to be exposed, and dissolved by atmospheric fresh water, leaving dissolution pores (vugs). At the same time, the decline of the sea level led to local semi-restriction of water in the high zone. Mg2+ in seawater was gradually rich after evaporation and concentration. Mg2+-rich fluid entered the interior of shoals along the abovementioned dissolution pores and underwent early dolomitization, forming fine dolomite. This kind of penecontemporaneous dolomite was more resistant to compaction and dissolution, which was helpful for preserving primary pores and secondary dissolution pores. During the burial period, deep hydrothermal fluid entered the dolomite reservoir of the Qixia Formation along the fractures associated with the Emeishan basalt eruption event [32], providing a large amount of Mg2+ for the primary fine dolomite around the primary pores and secondary karst pores, causing it to recrystallize or grow at the rim edge to form medium-coarse dolomite. A large number of associated intergranular pores and intergranular dissolution pores, plus hydrothermal dissolution pores and vugs along the fractures, made the intra-platform shoal thin-layered dolomite reservoir excellent.
4. Conclusions
The Qixia Formation in the Gaoshiti-Moxi area is divided into the Qi1 and Qi2 members. During the sedimentary period of the Qi1 Member, the sea level rose rapidly and maintained at a high level, and open-sea micritic limestone deposited. During the sedimentary period of the Qi2 Member, the sea level began to decline, and the lithology evolved into a microfacies combination of intra-platform shoal and inter-shoal sea, and medium-thick bioclastic limestone was developed and intercalated with thin-layered dolomite. The intra-platform shoals are mainly distributed near the top of the two secondary cycles of the Qi2 Member. On the plane, the main body of the shoal core migrated from south to north from Qi22 to Qi21. The reservoirs of the Qixia Formation are mainly thin-layered dolomite with different crystal sizes in Qi2 intra-platform shoals. The reservoir space is mainly composed of intergranular pores, intergranular dissolution pores and karst caves (vugs). And fractures improve the seepage capacity. The reservoirs can be divided into three types: fractured-vuggy, vuggy and porous. Fractured-vuggy reservoir is the best, followed by vuggy reservoir.
Micro-paleogeomorphology, the distribution of intra-platform shoals, early dolomitization and late hydrothermal transformation are four factors controlling the reservoir development. Intra-platform shoals were developed in the high micro-paleogeomorphic zone before the deposition of the Qixia Formation. The original granular limestone was permeable and conducive to early dolomitization. Penecontemporaneous dolomitization built an anti-compaction framework that protected some primary pores from being destructed, and also provided advantageous channels for hydrothermal fluid transformation. The distribution of intra-platform shoals and dolomite reservoirs is highly consistent both vertically and laterally. This further confirms that the shoal facies is the material basis of early dolomitization. Then the shoal-controlled penecontemporaneous dolomite underwent hydrothermal dolomitization and dissolution, resulting in a large number of intergranular pores and intergranular dissolution pores. These pores together with large-scale hydrothermal dissolution pores and vugs along fractures ensure high-quality intra-platform shoal thin-layered dolomite reservoirs.
Nomenclature
AC—acoustic contrast, μs/m;
GR—natural gamma ray, API;
ϕϕCNL—compensated neutron porosity, %;
δ13C—13C isotope value of carbonate cements, ‰;
δ18O—18O isotope value of carbonate cements, ‰.