The Main Controlling Factors of the Cambrian Ultra-Deep Dolomite Reservoir in the Tarim Basin

Abstract

The genesis of deep-to-ultra-deep dolomite reservoirs in the Tarim Basin is crucial for exploration and development. The Cambrian subsalt dolomite reservoirs in the Tarim Basin are widely distributed, marking significant prospects for ultra-deep reservoir exploration. Based on big data methodologies, this study collects and analyzes porosity and permeability data of carbonate reservoirs in the western Tarim Basin, specifically targeting the Cambrian deep-oil and gas-reservoir research. Through an examination of the sedimentary evolution and distribution of carbonate–evaporite sequences, and considering sedimentary facies, stratigraphic sediment thickness, fault zone distribution, and source-reservoir assemblages as primary reference factors, the study explores the macro-distribution patterns of porosity and permeability, categorizing three favorable reservoir zones. The controlling factors for the development of Cambrian carbonate reservoirs on the western part of the Tarim Basin are analyzed from the perspectives of sedimentary and diagenetic periods. Factors such as tectonic activity, depositional environment, microbial activity, and pressure dissolution are analyzed to understand the main causes of differences in porosity and permeability distribution. Comprehensive analysis reveals that the porosity and permeability of the Series2 carbonate reservoirs are notably high, with extensive distribution areas, particularly in the Bachu–Tazhong and Keping regions. The geological pattern of “Three Paleo-uplifts and Two Depressions” facilitated the formation of inner-ramp and intra-platform shoals, creating conducive conditions for the emergence of high-porosity reservoirs. The characteristics of reservoir development are predominantly influenced by diagenetic and tectonic activities. The Miaolingian is chiefly affected by diagenesis, featuring high permeability but lower porosity and smaller distribution range; dolomitization, dissolution, and filling processes under a dry and hot paleoclimate significantly contribute to the formation and preservation of reservoir spaces. In the Furongian, the Keping and Bachu areas display elevated porosity and permeability levels, along with substantial sedimentary thickness. The conservation and development of porosity within thick dolomite sequences are mainly governed by high-energy-particulate shallow-shoal sedimentary facies and various dissolution actions during diagenesis, potentially indicating larger reserves.

1. Introduction

The genesis of deep and ultra-deep (>6000 m) dolomite reservoirs has become a focal point of research within the domain of oil and gas exploration in recent years, such as the Cambrian dolomite reservoirs in the Tarim Basin [1,2,3] and the Dengying Formation dolomite reservoirs in the Sichuan Basin [4]. The Tarim Basin’s platform-basin area is distinguished by significant depth (burial depth > 6000 m) and ancient stratigraphic ages, predominantly featuring Lower Paleozoic marine facies assemblages [5,6]. Oil and natural gas resources in the Tarim Basin, with burial depths exceeding 6000 m, constitute 83.2% and 63.9% of the nation’s total reserves [7], respectively, positioning it as China’s most extensive deep terrestrial oil and gas enrichment zone.

The Cambrian subsalt dolomite in the Tarim Basin is the first reservoir-cap assemblage above the main hydrocarbon source rock in the platform basin area, providing fundamental geological conditions for large-scale oil and gas reservoirs [8,9]. The hydrocarbon source rocks of the Cambrian Yuertusi Formation, alongside thick evaporites and deep sub-salt dolomite reservoirs, compose a vital source–reservoir-cap combination, marking a strategic petroleum exploration target in the Tarim Basin. Investigations into the deep-buried (>8000 m) Yuertusi Formation’s hydrocarbon source rocks revealed characteristics typical of self-storing unconventional reservoirs, highlighting their potential as tight reservoirs. The formation and preservation of reservoir spaces are significantly influenced by primary porosity, organic matter, hydrothermal solvation, silicification, and dolomitization [10]. In deeply buried carbonates, reservoir quality is largely determined by the extent of early cementation and dissolution, transport pathways, and formation fluid flux [11,12,13]. The genesis of deeply buried (>8000 m) high-quality dolomite reservoirs is intimately linked to microbial-associated microfacies types and dolomitization [14], with dolomitization crucial for enhancing and maintaining the quality of these carbonate units [15]. Dolomitized subtidal sediments are the most productive reservoir sections, with associated evaporites typically acting as lateral and top seals. Studies have shown that upward-shallowing carbonate sequences are typically overlain by evaporite rock, leading to dolomitization of the carbonates through brine backflow caused by evaporation [13]. Reports indicate that arid climate is a significant factor in the development of many major dolomite bodies [16]. Additionally, various diagenetic processes play a key role in reservoir development, enhancing reservoir quality through different dissolution mechanisms, including organic acid dissolution [17], early diagenetic meteoric dissolution [18], hydrothermal dissolution [19,20,21], and thermal sulfate reduction [22,23], or a combination thereof [24]. Understanding the genesis of deeply buried dolomite reservoirs is essential for identifying promising exploration targets. Current exploration reveals substantial hydrocarbon potential beneath gypsum-salt formations, especially within the Middle Cambrian strata of the Tarim Basin, where Cambrian dolomite spans an area of 1.5 × 10⁵ km² and exceeds thicknesses of 1000 m [25,26,27]. Over 30 carbonate oil and gas fields have been discovered in the Tarim Basin to date, boasting proven oil reserves of more than 2.1 × 10⁹ tons and natural gas reserves up to 5 × 10¹¹ m³. A three-stage evaluation method for assessing the quality of gypsum-salt rock cap rocks, based on rock type, burial depth, and fracture density [28], has been proposed, highlighting its significance for deep oil and gas exploration.

Porosity and permeability are crucial determinants of favorable reservoirs, with variations in dolomite porosity and permeability typically associated with original sedimentary structures, grain types, stratigraphic positions, or paleogeographical locations [29,30]. While many studies have investigated the factors controlling pore development and distribution within carbonate reservoirs, few have managed to macroscopically explore favorable reservoir distribution patterns and seek spatial correlations with factors influencing porosity and permeability development, necessitating extensive data support. Due to the limited outcrop of Cambrian strata in the Tarim Basin, extensive drilling is required to obtain rock information from these layers. Drilling provides point data, but new visualization techniques can transform this point data into surface distributions, accurately representing the subsurface reservoir distribution, aiding precise oil and gas exploration decisions.

In recent years, global data on stratigraphy, chronology, sedimentology, and tectonics have proliferated, with increasing precision. Big data analysis and multiple reconstruction methods now offer new spatial and temporal insights. Artificial intelligence methods for calculating and predicting carbonate reservoirs are also emerging [31,32,33,34].

This study is based on big-data techniques to integrate well logs and outcrop samples for quantitative reconstruction, using new 3D visualization methods to characterize Tarim Basin reservoirs. The methodology includes the following: (1) collecting literature on Cambrian porosity and permeability in the Tarim Basin, and extracting 656 carbonate porosity and permeability data points; (2) organizing and classifying data by time, well location, and carbonate type; (3) using statistical methods to analyze the data, identifying temporal and spatial variations in porosity and permeability; (4) reconstructing the spatial distribution of reservoir porosity and permeability, utilizing ArcGIS to map the 3D spatial and temporal distribution patterns of porosity and permeability. Spatially, we generate continuous distribution values of porosity and permeability within specified areas through statistical analysis of sample data from concurrent periods, overlaying these values on 3D paleogeographic and residual-thickness maps to produce comprehensive distribution pattern maps. Vertically, we examine porosity and permeability formation patterns across three times scales, investigating different developmental stages and overall trends; and (5) By establishing classification criteria for favorable oil and gas areas based on sedimentary evolution and carbonate-evaporite series distribution, we predict potential oil and gas zones, analyze and restore the primary controlling factors during reservoir-formation sedimentation and diagenesis processes, offering new insights into the distribution of ultra-deep dolomite reservoirs.

2. Geological Setting

The Tarim Basin is located in the interior of the Eurasian continent, bordered by the Tianshan Mountains to the north and the Kunlun Mountains to the south, and bounded by the Altyn Tagh Zone to the east [35,36]. The Tarim Basin, with an area of 56 × 10⁴ km², is the largest inland petroliferous superimposed basin developed on the Archaean–Early Mesoproterozoic metamorphic crystalline basement and metamorphic fold basement. [14]. The tectonic pattern of the basin is “four uplifts and five depressions”. The four uplifts are the Tabei Uplift, the Bachu Uplift, the Tazhong Uplift, and the Tadong Uplift, and the five depressions are the Kuqa Depression, the North Depression, the Southwest Depression, the Southeast Depression, and the Tanggu Depression (Figure 1).

The main study areas of this paper are the Keping, Bachu–Tazhong, and Tabei regions. In the Cambrian, these areas are classified into four systems and six groups based on well-logging, drilling, and paleontological data (Figure 2). From the bottom up, the formations and sedimentary stages developed in the Tarim Basin are as follows: the Terreneuvian Yuertusi Formation (Є1y), characterized by mud-rich gently sloping deposits; the Series2 Xiaoerblak Formation (Є1x) and Wusongger Formation (Є1w), featuring a weakly rimmed carbonate-platform sedimentary stage; the Miaolingian with the Shayilik Formation (Є2s) and Awatag Formation (Є2a), distinguished by a strongly rimmed evaporite platform; and the Furongian with the Lower Qiulitage Formation (Є3xq), known for its weakly rimmed platform [38].

3. Distribution Characteristics of Porosity and Permeability

In terms of evaluating favorable zones, Cambrian carbonate reservoirs in the Tarim Basin are classified into four levels, based on the carbonate reservoir classification scheme proposed by the Tarim Oilfield Company. These levels range from favorable to unfavorable, designated as Class I, II, and III for reservoir zones, and Class IV for non-reservoir zones. The distribution of these classes is visually represented on 3D models, facilitating a more intuitive observation and analysis of the correlation between the sedimentary environment and the development of porosity and permeability. This approach also enables more effective acquisition of reserve information by analyzing the residual thickness of high-quality reservoir areas. It serves as a reference for the exploration and prediction of deep-to-ultra deep carbonate reservoirs.

3.1. Series2

The sedimentary stage of the Xiaoerblak Formation marks a transitional period from a ramp platform to a restricted platform. In the Keping area, medium ramp mound–shoal deposits predominantly developed, while the Bachu area, characterized by gentle and low-relief landforms, mainly saw the development of middle ramp mound–shoal and inner ramp depression deposits. The distribution and scale of the inner ramp mound–shoal belt during the sedimentary stage of the Xiaoerblak Formation were controlled by three intra-platform uplifts. The three paleo-uplifts were the Wuqia paleo-uplift, the Tanan paleo-uplift, and the Keping–Wensu paleo-uplift [38]. The lithology of the Xiaoerblak Formation comprises, from bottom to top, micrite dolomite, fine-crystallized dolomite, microbial dolomite, and argilliferous dolomite. A significant number of cyanobacteria microbial reefs with favorable physical properties have developed within the thick layered microbial dolomite, constituting the main reservoirs of the Cambrian. These reservoirs cover an area of 10.6 × 10⁴ km² and possess substantial exploration potential [41,42,43]. In the Keping area, the sea level rises toward the northeast, leading to a weakening of the dolomitization process [44]. The dolomite reservoir spaces vary, categorized into pores, dissolved vugs, and fractures, with pore-vug dolomite reservoirs being the predominant combination in the Xiaoerblak Formation. Examination by casting thin sections from outcrop samples in the Keping area and cores in the Bachu area revealed that secondary dissolution pores are mainly developed in the Xiaoerblak Formation dolomite. The primary types of pores include intercrystalline dissolution pores, interparticle dissolution pores, intraparticle dissolution pores, granular-mold pores, and non-fabric selective dissolved pores [45].

The Wusongger Formation consists of two third-order sequences, corresponding to two lithological segments [46,47]. The lower segment, developed in a restricted-to-semi-restricted sedimentary environment, mainly features tidal-flat facies and intra-platform mound–shoal. The upper segment, deposited in an evaporative environment, is characterized primarily by evaporative-lagoon and sabkha microfacies and tidal-flat facies [46]. The Wusongger Formation comprises sparry dolarenite, gypsum-bearing very-fine-to-fine crystallized dolomite, and mud dolomite flat-facies sediment. The platform-margin facies sediment in the Lunnan-Tazhong32 well and the intra-platform shoal sediment form the main material basis for the development of the Wusongger Formation’s reservoir, with the low-energy shoal sediment developed in tidal-flat facies around the three major paleo-uplifts also contributing. The platform margin zone spans an area of approximately 7080 km², while the intra-platform shoal in the west covers about 3200 km² [40].

The Series2 strata exhibit high porosity in the Keping, Bachu–Tazhong, and Tabei areas, with permeability being relatively high in the Bachu–Tazhong area and a limited range of high permeability found in the Keping area. The Xiaoerblak section in the Keping area has shown high porosity and high permeability, with an average porosity of 12.00% and a maximum permeability of 45.52 mD, while the average porosity of other sections is around 4.20% (Figure 3). Class I favorable reservoirs have been developed in the Shutan1 well, Batan5 well, Zhongshen1 well, Zhongshen5 well, and Kang2 well in the Bachu area. Although there are no gypsum rocks overlaying the Yingmai and Xinghuo well-blocks in the Tabei area, reservoirs here are classified as Class I favorable due to the excellent reservoir physical properties of the reef-shoal on the platform margin and the effective oil and gas supply from the underlying Yuertusi Formation. Class II favorable reservoirs are mainly developed in other intra-platform areas covered by gypsum rocks and the platform-edge zone. Class III favorable reservoirs are primarily distributed within the platform interior, platform margins, and peripheral carbonate platform regions.. High-quality reservoirs are mainly concentrated in the inner-ramp slope facies zone and the intra-platform shoal facies zone, with relatively thick sedimentary rocks formed in the Tazhong area (Figure 4), indicating substantial reserves.

3.2. Miaolingian

There are two types of large-scale reservoirs, sabkha dolomite and brine reflux dolomite, which are widely developed during the Miaolingian. The sedimentary stage of the Shayilik Formation was marked by the development of a strongly rimmed platform, dominated by evaporative lagoons. The carbonate-platform environment transitioned from a restricted and evaporative platform to an open platform. A comparison of the paleogeography between the Shayilik Formation and the Awatag Formation revealed a continuous expansion of the gypsum-salt lake over time. The interbedding of intra-platform gypseous dolomite flats, grainstone shoals, and gypseous salt rocks forms an effective inter-salt reservoir–seal combination [78]. On the platform margin, reef (mound)–shoal facies reservoirs and infiltration–reflux dolomite reservoirs behind the reef (mound)–shoal facies have developed, covering an area of up to 1.41 × 10⁴ km² [47,79].

Furthermore, the Miaolingian features two types of high-quality sealing layers: gypsum-salt lakes and gypsum dolomitic flat-facies evaporites, as well as mud dolomitic flat-facies argillaceous dolomites. The gypsum-salt-rock cap rocks are primarily located in the central region. Well-developed gypsum-salt-rock cap rocks are found in the mid-western part of the northern depression, most of the mid-western part of the central uplift, and the northern slope of the southwestern depression’s western part. Moderate gypsum-salt-rock cap rocks are found in some areas of the central uplift and the northern part of the northwestern structural belt in the eastern part of the southwestern depression. The central part of the Tabei Uplift and the southern part of the northwestern structural belt in the southwestern depression exhibit poorly developed gypsum-salt rock sealing layers [28]. The evaporite sealing layer has a thickness ranging from 300 to 800 m and covers an area of 14.3 × 10⁴ km². The dense argillaceous dolomite sealing layer has a thickness ranging from 100 to 300 m and spans an area of 13.7 × 10⁴ km². The upper section of the Awatag Formation predominantly consists of brown dolomite and limestone, and gypsum dolomite, while the middle and lower sections are chiefly composed of brownish-gray salt rock and gypsum-salt rock, interspersed with some dolomite and gypsum mudstone. The Awatag Formation and the lower part of the Shayilik Formation consist of a set of stably distributed evaporites, forming a regional sealing layer. The reservoirs and cap rocks in the Miaolingian, combined with the source rocks in the Series2 and Terreneuvian, constitute an effective hydrocarbon system in the region [80,81].

The Miaolingian exhibits significant porosity distribution in the Tazhong, Tabei, and Keping regions, although the average overall porosity does not exceed 12.00%. However, permeability shows substantial values, primarily concentrated in the Bachu, Tazhong, and Keping areas. Generally, the evaporation platform in Tazhong and the reef flat facies in the Keping area display higher porosity and permeability values (Figure 5). Thanks to the synergistic combination of source rock, in-platform beach reservoirs, and the gypsum-salt-lake evaporative sealing rock in the Awatag Formation, Class I favorable areas have developed around the Zhongshen1 well and Zhongshen5 well in Tazhong and Yaha7X-1 well in Tabei. Class II favorable areas are mainly distributed in the gypsum rock-covered areas within the platform, including Mabei1 well in the Bachu area and the Yingmai and Yaha areas in Tabei. Class III favorable areas primarily develop in other carbonate rock areas within the platform and all platform-margin reef beach areas. The residual strata thickness in the Keping area is thin, suggesting the Bachu–Tazhong area possesses more exploration potential (Figure 6). This may be associated with the development of high-quality salt-rock sealing layers in the Miaolingian.

3.3. Furongian

Thick dolomite reservoirs are extensively developed in the Lower Qiulitage Formation, with the Keping area characterized by sedimentation on a gently sloping carbonate ramp. The Lower Qiulitage Formation features sedimentation across a range of environments within the carbonate ramp system, including middle shallow subtidal, restricted shallow subtidal, intertidal, and supratidal zones. These lithofacies are vertically stacked in repeated shallowing-upward meter-scale cycles. The dolomite of the Lower Qiulitage Formation exhibits a diversity of pore types, including primary intergranular, intragranular, fenestral, intercrystal, and diagenetically altered pores. Agglomerates and stromatolites of microbial origin generally yield relatively high porosity, whereas high-energy oolitic rocks typically exhibit very low porosity. In this context, microbialite-based intertidal cycles and those dominated by high-stand (or regressive) sequences feature relatively extensive pore spaces, despite being highly variable (or vertically heterogeneous). Consequently, subtidal cycles dominated by grainstones and those marked by transgressive sequences tend to exhibit exceptionally low porosity. This pattern indicates that the development and preservation of porosity in the thick dolomite sequence are predominantly governed by sedimentary facies, which are influenced by fluctuations in sea level of various magnitudes and by late-diagenetic processes [83].

The Tabei area primarily consists of crystalline dolomite and granular dolomite, with reservoir spaces mainly comprising intergranular pores, intergranular dissolution pores, dissolution cavities, and fractures, showcasing a variety of types and combinations. The Bachu area is characterized by crystalline dolomite and micritic dolomite [84]. Dolomite in the Keping area can be classified, based on its structure, into micritic dolomite, algal dolomite, granular dolomite, and crystalline dolomite [85].

The overall porosity in the Furongian is higher than in the Miaolingian, averaging 20.00%, predominantly found in the Keping and Bachu areas. The average porosity in wells such as Mabei1, Hetian1, Batan5, and Tong1 in the Keping and Bachu areas reaches up to 8.00%, and the average porosity in Tabei’s Xinghuo2 and the Yingmai–Yaha area is as high as 7.52%, with permeability peaking at 37.00 mD, forming Class I favorable areas. During this period, the Keping area primarily developed reef beach and platform-margin facies with high water energy, which facilitated the development of carbonate deposits and pores, also exhibiting good permeability. The Tabei area, developed on the platform margin, also displayed high permeability values (Figure 7), but overall lower than earlier values, potentially due to compaction effects in the diagenetic environment (Figure 8). The Bachu–Tazhong area, transitioning from a restricted platform margin to an open platform, developed Class II favorable areas, with Class III favorable areas developing in other carbonate reservoir regions.

4. Main Controlling Factors of Favorable Reservoirs in the Tarim Basin

4.1. Unconventional Reservoirs

The secondary tectonic units of the Yuertusi Formation in the Tarim Basin, formed during a rapid marine transgression, represent a continental-shelf carbonate platform within a rift and extensional-tectonic environment. The distribution of this black shale series is influenced by uplift depression alternation, with upwelling currents and hydrothermal activity enhancing nutrient enrichment, fostering high primary productivity. This productivity provided favorable conditions for forming organic-rich hydrocarbon source-rock layers at the base of the Yuertusi Formation in the Lower Cambrian of the Tarim Basin.

The distribution of hydrocarbon source rocks in the Early Cambrian is controlled by the late Neoproterozoic paleogeomorphology of the Tarim Basin, characterized by uplifted highlands in the Bachu–Tazhong region. Yuertusi Formation source rocks are prevalent in the southern part of the northern uplift, the eastern Awati depression, and the Maigaiti slope. Outcrop and drilling data reveal two rift basins separated by the central ancient uplift. Initial deposition of Lower Cambrian source rocks occurred in the northern rift basin, with thickness and distribution linked to Ediacaran source rocks. The southern rift basin has a thinner Yuertusi Formation, with no source rocks in wells Batan5 and Mabei 1, and was later deformed by orogenic events, with Devonian rocks overlying Ediacaran strata. Widespread carbonate rocks of the Xiaoerblak Formation in the Early Cambrian eventually connected the two rift basins. Therefore, differential rift tectonics and basin evolution control the distribution and thickness of Lower Cambrian source rocks in the Tarim Craton [90].

The lower part of the Terreneuvian in the Cambrian of the Tarim Basin developed the Yuertusi Formation, a black-shale rock series with high TOC content and excellent hydrocarbon generation potential, considered one of China’s most important hydrocarbon source rocks. The northwestern Tarim Basin contains two sets of black shales (upper and lower) within the Yuertusi Formation, both influenced by silica-rich hydrothermal activity and upwelling flows, with hydrothermal activity ceasing during the deposition of the upper shales. Terrestrial input was higher during the deposition of the upper shales, leading to intense TOC enrichment. The global transgression in the early Cambrian led to extensive carbonate platforms in the northwestern Tarim Basin, resulting in a warm and humid climate. Elemental geochemical studies suggest the Yuertusi Formation sedimentary strata formed under a semi-humid to semi-arid climate, with fluctuating water depth and oxygen levels. This environment led to high-quality hydrocarbon source rocks, represented by the lower black mudstones of the Yuertusi Formation.

The lower shale is primarily composed of quartz, feldspar, barite, and clay, with high SiO₂ content and low TOC content (averaging 1.85%). Enrichment of trace elements and rare earth elements indicates hydrothermal activity’s influence, promoting biomass proliferation. Upwelling flows supplied abundant nutrients, enhancing ancient productivity and expanding the vertical range of the minimum oxygen zone, conducive to organic matter preservation. Enrichment factors of redox-sensitive elements (such as U, V, and Mo) suggest that the deposition of the lower shale occurred under anoxic conditions, with intense hydrothermal activity triggering the enrichment of reducing gases (H₂S) and minerals (SiO₂), while weak weathering led to low and insufficient nutrient input. Although the former created benthic conditions conducive to organic matter preservation, high concentrations of silica diluted primary productivity, resulting in low TOC content and small hydrocarbon potential [91,92].

The upper shale, primarily composed of quartz, feldspar, ankerite, and clay, with minor apatite and hematite, has low SiO₂ content and high TOC content (averaging 7.88%), deposited under suboxic conditions. Termination of hydrothermal activity and increased carbonate content led to declining ancient productivity. The upper black shale may have generated and expelled large quantities of hydrocarbons, contributing to abundant Lower Paleozoic petroleum resources. Intense weathering transported nutrients and terrestrial detritus, promoting productivity and organic matter preservation. Organic matter accumulation was controlled by high productivity and low oxygen levels, leading to high TOC content and significant hydrocarbon potential [91,92] (Figure 9a,b).

Research indicates the organic-matter abundance of the hydrocarbon source rocks in the study area is high, with the Yuertusi Formation in a mature-to-highly mature evolution stage, primarily producing liquid petroleum and wet gas. Vitrinite reflectance (Ro) values and kerogen analysis confirm the presence of Type I and Type II organic matter, indicating a predominance of liquid petroleum generation in marine environments.

4.2. Main Controlling Factors for Conventional Reservoir

4.2.1. Controlling Factors during the Deposition Stage of Dolomite Reservoirs

• Tectonism

The Xiaoerblak Formation is influenced by the paleogeographic pattern of “three paleo-uplifts and two depressions.” The three paleo-uplifts were the Wuqia paleo-uplift, the Tanan paleo-uplift, and the Keping–Wensu paleo-uplift, and the two depressions were the Southwest Depression and the North Depression. The Tazhong grain beach belt, Bachu beach-algae debris belt, and Wensu mound–shoal complex collectively form the material foundation for the development of dolomite reservoirs in the Xiaoerblak Formation [40,93]. During the sedimentary period of the Wusongger Formation, the inner uplifts of Tanan, Wuqia, and Keping–Wensu continued to exist and dictated the platform’s differentiation. With further sea-level decline, weakly rimmed-platform margin sediments started to develop in the Lunnan area, characterized by larger-scale biological mounds (reefs) and granular shoals at their margins [60,62,94]. During the Shayilik Formation’s depositional period, the distinction between inner uplifts and depressions essentially vanished, with a general decrease in sea level. This led to the development of strongly rimmed platforms dominated by evaporative lagoons. Sedimentation in the Furongian remained relatively stable [95,96]. The Lower Qiulitage Formation adhered to the depositional pattern of the Miaolingian, characterized by the development of platform facies in the west and basin facies in the east. Subsequently, the western Tarim Basin ceased the development of evaporative platforms, leading to the extensive re-emergence of restricted platforms throughout the region.

2.

Sedimentary environment

Sedimentary cycles and exposure dissolution contribute to the formation of high-quality reservoirs. Controlled by hydrodynamic energy and periodic sea-level changes, these cycles influence carbonate platform sedimentation [97,98]. High-energy intra-platform shoal environments foster high-quality dolomite reservoirs, with bright crystalline–bioclastic limestone and micritic bright crystalline–bioclastic limestone exhibiting high original porosity, facilitating diagenetic fluid entry. Sea-level drops in the highstand-systems tract result in freshwater dissolution of grain shoals, forming dissolution pores. Concentrated Mg²⁺ from seawater enters these pores, leading to early dolomitization and fine-grained dolomite formation, which resists compaction and dissolution, preserving primary and secondary pores [99].

The Xiaoerblak Formation, overall, represents a sedimentary sequence with water depth gradually becoming shallower, and hydrodynamic conditions transitioning from weak to strong and then back to weak again [40,41]. As the sea level gradually declined, water energy significantly increased, coupled with relatively stable tectonics and flat topography, fostering the development of a carbonate-ramp depositional system during the sedimentary period of the Xiaoerblak Formation. The Xiaoerblak Formation dolomite formed during the transgression of the Yuertusi Formation and the regression of the Xiaoerblak Formation. Periodic sea-level drops facilitated short-term exposure and dissolution. In the Tazhong area, moderate exposure to atmospheric water dissolution occurred. During the Xiaoerblak Formation’s sedimentation, intertidal facies were widespread, providing optimal conditions for atmospheric water dissolution and forming high-quality reservoirs. Thus, the syngenetic stage is crucial for porosity formation, with reservoir quality depending on the intensity of atmospheric water dissolution. The upper parts of sedimentary cycles are most susceptible to exposure, forming high-porosity zones, while the lower parts are less affected, resulting in poorer porosity [75].

The primary sedimentary facies include restricted and open platforms, with marginal development of platform margins and slope facies. The dolomite reservoirs within the restricted-platform and platform-margin facies exhibit relatively good reservoir performance, making them the dominant reservoir development zones. However, the carbonate grains within the open platform and slope facies are finer, leading to comparatively poor reservoir development. The Wusongger Formation transitioned from an evaporative environment, primarily developing evaporative lagoon–sabkha microfacies and tidal-flat facies, to later stages featuring restricted-to-semi-restricted environment sedimentation, primarily consisting of tidal-flat and intra-platform shoal bodies [46,78]. The paleoclimate during the Miaolingian was drier and hotter than during the Xiaoerbulak Formation’s sedimentation period [100], with salt lakes continuously expanding.

During the Furongian, there was a general trend of rising sea levels, leading to a gradual deepening of water bodies, and the paleoclimate shifted from the dry and hot conditions of the Middle Cambrian to warmer and more humid conditions [89]. This shift adversely affected the development of early diagenetic pores [101]. Drilling data from wells such as Well Tong1 and Fang1 in the Bachu area revealed the development of high-energy shoal facies in localized areas within the Lower Qiulitage Formation. In the Tazhong area, drilling data from wells like Tacan1 and Zhongshen1 indicated a gradual increase in grain content from bottom to top in the Lower Qiulitage Formation, reflecting the gradual enhancement of water energy in the Tazhong area during the Furongian. In the late Furongian, high-energy grain-shoal facies were relatively well developed in the Tazhong area [89,102,103]. Typically, high-energy grainy rocks exhibit very low porosity. Despite vertical fluctuations, tidal cycles dominated by microbialites and tidal cycles lead to relatively abundant pore spaces in the transgressive sequences. Consequently, grain-dominated tidal cycles and transgressive sequences typically exhibit extremely low porosity [83]. The development and preservation of porosity in thick dolomite sequences are primarily controlled by sedimentary facies, influenced by sea level fluctuations of various magnitudes and late-diagenetic overprinting.

According to sedimentary-facies studies, carbonate reef–shoal facies at the platform margin and evaporative-platform tidal-flat facies are conducive to reservoir development, such as the biogenic framework pores formed during microbialite formation, while gypsum provides the basis for the development of dissolution pores during synsedimentary and post-burial periods, thus forming good primary porosity.

3.

Microorganism

A significant presence of microorganisms was found in the Wusongger Formation, with microbial-induced dolomitization being key to the formation of dolomite in this formation [104]. The Miaolingian dolomite and evaporite coexisted to form sabkha-style dolomite during the same sedimentary period. Evaporation and microbial mediation are two significant factors influencing dolomite formation. Evaporation provides the necessary concentration of Mg²⁺ for dolomite formation, and microbial mediation helps Mg²⁺ overcome kinetic barriers to incorporate into the calcium carbonate lattice, forming dolomite [105]. The porosity in the dolomite of the Lower Qiulitage Formation is usually phase-selective, with microbial-derived agglomerates and stromatolites typically producing relatively high porosity. Evidence of microbial-mediated primary dolomite formation in ancient sabkha environments has been discovered [106,107,108,109]. The metabolic activity of microorganisms plays a catalytic role, primarily through microbial metabolism increasing the saturation of dolomite in the extracellular microenvironment. Additionally, the negatively charged cell wall carried by microorganisms can serve as a nucleation site for dolomite crystals [110,111].

4.2.2. Controlling Factors during the Diagenetic Stage of Dolomite Reservoirs

• Diagenesis

Burial dolomitization includes both the recrystallization of early-formed dolomite and the replacement of precursor limestone. During replacement and recrystallization in the burial environment, external carbonate and magnesium ions are introduced. However, the recrystallization of early-formed dolomite leads to different porosity evolution compared to dolomite formed by replacing limestone. Recrystallized dolomite often contains fewer calcite inclusions, has superior compaction resistance, and retains greater primary porosity. Therefore, inner-to-mid-ramp shoals are favorable areas for early dolomitization and porosity preservation. In contrast, dolomite formed by directly replacing limestone during burial lacks significant primary porosity, due to intense cementation and pressure dissolution before dolomitization [112].

During the burial diagenesis of the Xiaoerblak Formation, the predominant diagenetic fluids were primarily inherited seawater, engaging in metasomatism with calcite and triggering recrystallization in early-formed micritic limestone. Subsequent hydrothermal activities further facilitated the dolomitization process. The dolomitization within the Xiaoerblak Formation encompasses a range of processes including sedimentary and penecontemporaneous sabkha dolomitization, brine reflux dolomitization, and post-burial dolomitization, with the latter being the most dominant. Syngenetic dolomitization creates a compaction-resistant framework, preserving some primary porosity and providing favorable pathways for hydrothermal fluid migration. Shoal facies provide the material basis for early dolomitization. Subsequent hydrothermal dolomitization and dissolution of the syngenetic dolomite within these shoals form abundant intergranular and dissolution pores. These pores, along with large-scale hydrothermal dissolution cavities along fractures, create high-quality, thin-layer dolomite reservoirs in the intra-platform shoals [99]. The Xiaoerblak Formation likely formed during a syngenetic shallow-burial period [44]. Early dolomite formation increases reservoir porosity and deep-burial compaction resistance, laying the foundation for reservoir development.

The diagenetic phase of the inter-salt dolomite reservoirs in the Shayilik Formation is predominantly influenced by dolomitization, dissolution, and filling processes. Dolomitization and dissolution play constructive roles in the development of the dolomite reservoirs, while the filling process has a primarily destructive effect. The dolomitization is characterized mainly by evaporation-pump dolomitization and burial dolomitization, both indicative of incomplete dolomitization [113,114].

The Lower Qiulitage Formation experienced various stages of dolomitization, ranging from contemporaneous/penecontemporaneous to shallow, mid-deep burial, and hydrothermal- alteration dolomitization. During its shallow-burial phase, in a semi-open diagenetic system with a limited external CO2-3 supply, dolomitization most likely occurred through volume replacement of calcite by dolomite. Despite significant porosity adjustments before and after dolomitization, the overall porosity remained largely unchanged, yet pore structures underwent modifications, suggesting that moderate dolomitization is beneficial for the preservation of pore spaces within grain shoal dolomite [51,115]. High-temperature and high-pressure conditions in deep-burial environments promote dolomite dissolution and recrystallization, leading to enhanced cation ordering, larger crystal sizes, and well-formed crystal shapes [116]. Recent studies show extensive dolomite recrystallization occurs during late diagenesis and/or deep burial. Early dolomite in the lower Qiulitage Formation, initially metastable, recrystallizes into more stable fine-to-coarse-crystalline phases under shallow-burial conditions via near-surface marine fluids (Figure 9c,d). It is then compacted under deep-burial conditions [117]. Thus, the lower Qiulitage Formation primarily interacts with meteoric and marine fluids, experiencing strong burial dolomitization, resulting in large crystals and poorly preserved original sedimentary structures.

2.

Structure and fracture development

Tectonic movements during the diagenetic period, especially the influence of the Himalayan orogeny, had a profound impact on the Bachu area, causing significant fracturing of the rock layers. This fracturing increased the connectivity of reservoirs. The Cambrian strata in the Tarim Basin have been subject to multiple tectonic movements, leading to the development of numerous faults. The extensive development of fractures creates a network that plays a critical role in the migration of formation fluids.

The fractures within the dolomite reservoirs of the Xiaoerblak Formation are primarily of structural, diagenetic, and dissolution origins. Modifications by later acidic fluids further expanded the reservoir space. Additionally, compaction and dissolution processes during burial also played a role in modifying the reservoirs. The lithology of the lower tidal-flat gypsum-dolomite reservoirs in the Shayilike Formation is mainly characterized by gypsum dolomite, mud crystalline dolomite, and gypsum dolomite conglomerate [118], significantly influenced by lithofacies and fractures, which govern subsequent modifications and petrophysical improvements of the reservoirs.

The sealing mechanism and quality of cap rocks are predominantly determined by the lithology of the cap rocks. In the case of gypsum rocks, their sealing capacity is primarily affected by petrophysical properties, such as non-connected pores and small pore radii, leading to low porosity and permeability. The salt interval is sealed by gypsum layers above and below, with a limited supply of diagenetic fluids and Mg²⁺, constraining the overall extent of dolomitization. Sedimentary components and structural features, as well as early-diagenetic characteristics, are well-preserved, resulting in low intensity of dolomitization [119]. Tectonic activity is another vital factor influencing rock deformation and is the direct cause of fracturing. Generally, once rocks fracture, the quality of the cap rocks diminishes. The Miaolingian, in particular, developed cap rocks mainly in the Wusongger Formation. The regions such as the Amam Zone, Awati Depression, Keping Uplift, western Bachu Uplift, Maigaiti Slope, and southern Tazhong Uplift, primarily composed of salt or gypsum rocks with deep burial and low fracture density, are identified as favorable areas for gypsum cap-rock development. Conversely, areas like the eastern Bachu Uplift, and northern Tazhong Uplift, where gypsum or salt rocks are prevalent but with deep burial and high fracture density, are considered as regions with moderate gypsum cap-rock development. Other marginal areas with shallow burial and high fracture density are identified as poor areas for gypsum cap-rock development [23].

3.

Dissolution and filling effect

In the Xiaoerblak Formation, a multitude of micro-scale porosities, including intercrystalline, intergranular, and intragranular dissolution pores, mold pores, algal framework pores, and variably sized dissolution cavities, have emerged. Isotopic studies conducted on Well Zhongshen 1 have identified two phases of negative carbon-isotope shifts within the middle and upper parts of the Xiaoerblak Formation. These shifts signify sea-level regression and stratal exposure, likely leading to freshwater leaching and dissolution processes, which, in turn, result in the formation of karst reservoirs. Following sedimentation, due to sea-level changes, these formations were periodically exposed at the surface, undergoing freshwater leaching and dissolution, thus enhancing reservoir connectivity and forming favorable reservoir spaces. Consequently, in the western region of the Tarim Basin, the Xiaoerblak Formation may have extensively developed sub-layer dissolution cavities, creating high-quality reservoirs.

Meteoric dissolution during the penecontemporaneous stage is evident in well logs, such as in Well Zhongshen5, where angular gypsum dolomite, commonly found in well logs with good physical properties, mainly consists of micro-pores between mud-crystalline dolomite and gypsum, related to gypsum dissolution by atmospheric freshwater leaching. Burial dissolution, predominantly involving organic acids as the dissolution fluid, further contributes to the modification of reservoirs [113,118].

The Lower Qiulitage Formation, subjected to evaporative processes, experienced an increase in salinity and density as low-salinity seawater entered the platform area. Driven by density differences, this high-density seawater moved downwards, interacting with carbonate grain deposition along the platform-margin shoals and leading to the formation of residually structured dolomite. This process is predominantly controlled by various dissolution actions: contemporaneous dissolution typically occurs in the upper-middle part of microbial mounds or grain shoals, influenced by sedimentary paleogeography and high-frequency sea-level changes, exhibiting certain structural selectivity. Electron probe analyses reveal lower Fe and Mn contents and slightly higher Sr content between grains, suggesting that the impact of atmospheric freshwater was brief and did not significantly alter the geochemical characteristics of diagenetic fluids. Likewise, there were no marked changes in C-O isotopes, which remained generally similar to contemporaneous seawater [51]. Penecontemporaneous dissolution, mainly governed by the dual effects of sea-level rise and fall and atmospheric freshwater, also shows some structural selectivity. Epigenetic karst, influenced by tectonically controlled paleogeography, typically lacks structural selectivity [120]. Burial dissolution is mainly influenced by faults and the hydrological environment of underground fluids, with a diverse fluid composition including organic acids, CO₂, H₂S, and hydrothermal fluids related to faults [113]. The fracture systems formed by fault zones and fault actions provide favorable pathways for fluid migration and rock dissolution. Additionally, atmospheric freshwater dissolution, jointly controlled by epigenetic karst and deep-seated faults, represents a significant dissolution process within the study area.

5. Conclusions

Temporal Distribution: Through the analysis of porosity and permeability distribution patterns in relation to paleogeography and residual thickness, comprehensive conclusions can be drawn: (1) the carbonate reservoirs of The Series2 exhibit high porosity and permeability across extensive areas, suggesting promising exploration prospects in the Bachu–Tazhong and Keping regions. (2) The Miaolingian is primarily influenced by diagenetic processes, characterized by high permeability but lesser porosity and distribution range. (3) The Furongian shows notably higher porosity and permeability in the Keping and Bachu areas, with significant sediment thickness, indicating potential for higher reserves. The Keping area demonstrates high porosity and permeability in both Series2 and Furongian periods, while the Bachu area is favorable in the Series2 but has lower porosity values in the Miaolingian. Despite high porosity in the Keping and Bachu areas during the Furongian, permeability values are comparatively lower. The Tazhong area exhibits relatively high porosity and permeability in both Series2 and Miaolingian, yet both values decrease in the Furongian. The Tabei area shows an increase in permeability, featuring a broader range of high permeability in the Furongian compared to other regions.

Spatial Distribution: Class I favorable reservoir areas of Cambrian carbonate reservoirs are located in reef flat facies belts, inner-ramp-platform shoals, and areas covered by gypsum-salt sedimentation in the Bachu–Tazhong and Tabei regions, as well as zones with developed fractures. Class II favorable areas are primarily found around the periphery of Class I areas and in platform margin facies. Class III favorable areas are mainly distributed in marginal zones where platform-type source rocks are present, including restricted platform margins and open platforms.

Main Controlling Factors of Reservoirs in the Tarim Basin: (1) during the early stage of the Xiaoerblak Formation, the primary hydrocarbon source rocks developed in the Yuertusi Formation, mainly consisting of black shale rich in organic matter. Reservoir development was chiefly governed by the interbedded sedimentation of carbonate and evaporite during the rising- and intermittent-stagnation phases of the ancient sea level, along with the development of inner ramp and platform shoals in the Keping and Bachu areas under the paleogeographic pattern of “three paleo-uplifts and two depressions,” developing an environment conducive to high-porosity reservoirs. The characteristics of reservoir development are mainly controlled by diagenetic and tectonic processes. (2) In the Miaolingian sedimentation period, evaporite sedimentation predominated during the late highstand and regressive stages of the ancient sea level, with dolomite deposition favored by the dry and hot paleoclimate, making dissolution and filling processes under diagenesis the main controlling factors for the development of high porosity and permeability in the Bachu–Tazhong area. (3) During the Furongian sedimentation period, the development of high-energy grain shoals was facilitated by the rising sea level, with the development and preservation of porosity in thick dolomite sequences mainly controlled by sedimentary facies. During the diagenetic period, various dissolution processes played a significant role in the development and preservation of high porosity.

In summary, the Series2 of the Tarim Basin and the Miaolingian period mainly have good exploration prospects in the Tazhong area, while the Furongian period mainly has good exploration prospects in the Keping area. Due to favorable sedimentary environments and early diagenesis, the Series2 and Furongian played a favorable role in maintaining porosity and permeability. The combination of such sedimentary environments and diagenesis processes needs to be taken seriously and is a factor for consideration for the exploration of high-quality dolomite reservoirs.