Micropore Characteristics and Reservoir Potential of Deep Tight Carbonates from the Lower Cambrian Canglangpu Formation in the Northern Sichuan Basin, China

Abstract

Recent deep exploration in the northern Sichuan Basin has advanced our understanding of Lower Cambrian Canglangpu Formation carbonate reservoirs. However, the characteristics, genesis, and distribution of the reservoir, as well as future exploration targets, remain unclear. Specifically, core and thin-section analyses indicate that these reservoirs are notably tight, with virtually no visible macroporosity and low permeability (0.01–1 mD). However, helium porosity measurements reveal values of 2–5%, suggesting significant storage potential. An integrated approach utilizing optical and scanning electron microscopy (SEM), high-pressure mercury injection capillary pressure (MICP), nuclear magnetic resonance (NMR), and micro-computed tomography (micro-CT) was employed to characterize the pore systems. Quantitative thin-section analysis reveals visible areal porosity markedly lower than helium porosity, indicating predominance of micropores; mercury intrusion and NMR demonstrate that intragranular and intergranular micropores constitute most pore volume, although effectively connected throat sizes remain below 1 µm. Comparative stratigraphic evaluations show that porosity is more developed in the dolomite-rich upper and middle intervals of the depositional cycles, whereas the lower intervals are less porous. Early subaerial exposure promoted dolomitization and dissolution, which facilitated pore development. However, the influence of sediment mixing led to a reduction in porosity. And deep burial subjected the rocks to intense compaction and cementation, destroying most of the primary pore space. Consequently, reservoir quality is ultimately governed by the interplay between the original depositional environment and the later diagenetic history, with paleotopographic highs identified as the most promising exploration targets. These findings establish a predictive framework for reservoir quality in tight carbonate rocks, which holds significant implications for analogous plays worldwide.

1. Introduction

Recent studies have emphasized the scientific and exploration significance of tight carbonate reservoirs globally. Notable examples include the Triassic Feixianguan Formation grainstones in the Sichuan Basin [1,2], the Triassic Leikoupo Formation marlstones in the Sichuan Basin [3,4], the Ordovician Yijianfang Formation grainstones in the Tarim Basin [5], and the tight Lower Cretaceous carbonates of southern Texas [6]. In each case, reservoirs are characterized by pore diameters below 10 μm, classified as micropores [1,7,8,9]. The term micropore used in this study refers to all pores less than 10 μm in diameter, following the definition by Cantrell and Hagerty (1999) [7] and Lønøy (2006) [8]. The term macropore refers to pores with diameters greater than 10 μm. And the results suggest that the micropores are capable of storing considerable volumes of hydrocarbons, particularly in deep (>4500 m) and ultra-deep (>6000 m) settings [1,4,6,10,11]. Therefore, detailed description and quantification of micropores are essential for understanding their origin, distribution and implications for hydrocarbon exploration.

As burial depth increases, compaction and cementation destroy most macropores, making micropores the primary reservoir space [3,10,11,12]. Optical microscopy, scanning electron microscopy (SEM), and micro-CT imaging provide semi-qualitative insight into micropore morphology, and the diameter of the pores can be measured using SEM and micro-CT imaging [13]. Additionally, these imaging techniques can characterize both connected and non-connected porosity, depending on the adopted image processing workflow. While high-pressure mercury intrusion and nuclear magnetic resonance (NMR) deliver quantitative metrics—although gas adsorption yields precise surface area measurements—their effective range does not extend into the micropore domain [6,14,15]. In addition, mercury intrusion porosimetry and NMR provide quantitative metrics primarily for connected pore systems, which are relevant for reservoir storage and flow analysis. The origin of micropores remains debated: some attribute them to dissolution of unstable minerals or preservation of relict pores during burial [16,17], whereas others invoke a coupled influence of depositional facies and diagenetic transformation [18,19]. This uncertainty hinders a clear understanding of pore evolution and the prediction of deep carbonate reservoirs, thereby constraining effective hydrocarbon exploration [19,20].

The Sichuan Basin is a prolific hydrocarbon province, with Cambrian carbonate reservoirs below 4500 m emerging as key exploration targets [21,22]. In the northern Sichuan Basin, the Cambrian Canglangpu Formation attains burial depths exceeding 6000 m and lies immediately above the Qiongzhusi Formation, which contains high-quality source rocks. The TOC content of the source rock from the Qiongzhusi Formation mainly ranges from 2% to 5%, and the thickness mainly ranges from 400–500 m [23,24,25]. This stratigraphic arrangement provides both effective charge and robust sealing conditions, yielding exceptional hydrocarbon potential. A significant breakthrough was achieved in 2020 when Well JT1 yielded substantial oil and gas flows from the Canglangpu Formation [26,27]. Previous studies indicate that the reservoir is dominated by oolitic dolomite, with porosity largely attributed to multi-phase dolomitization—including evaporated seawater, deep burial, and hydrothermal types [28,29,30,31,32].

The minimal visible pores revealed by polarizing microscopy contrast with the significant hydrocarbon discoveries in this interval, implying micropores as the dominant storage medium. In this context, it is urgently needed to investigate the distribution, characteristics and genesis of these micropores in deep, tight carbonates so as to facilitate accurate prediction of future exploration targets. This study aims to (1) characterize micropores qualitatively and quantitatively using integrated petrographic, SEM, NMR, and micro-CT techniques; (2) interpret their genesis within the regional geological, depositional, and diagenetic context; and (3) evaluate the hydrocarbon potential of the deep tight carbonates in the Canglangpu Formation, thereby establishing a predictive model for analogous plays.

2. Geological Setting

The Sichuan Basin, covering an area of approximately 18 × 10⁴ km² in southwestern China, is one of the most prolific hydrocarbon basins in China. In its northern part, deep Cambrian carbonate reservoirs have become high-priority exploration targets, particularly following the significant oil and gas discovery in Well JT1 (Figure 1).

From the Late Sinian through the Early Cambrian, the basin developed as an intracratonic rift (i.e., Deyang–Anyue rift) characterized by deep-water shale deposition. Thick Dengying and Doushantuo Formation shales accumulated in this setting, creating the principal center for hydrocarbon generation and primary migration during the Sinian–Cambrian [21,22,33]. From the late Qiongzhusi Period to the Canglangpu Period, the Sichuan Basin experienced its first large-scale regression. During the Caledonian orogeny, the basin’s tectonic regime shifted from extension to compression, uplifting the margins and establishing a west-high, east-low paleotopography; enhanced eastward transport of terrigenous detritus under this gradient produced a marked west-to-east thinning of the Canglangpu Formation. Concurrently, the abundance of sandstone interbeds diminished eastward while carbonate facies expanded, recording a facies transition from continental through shoreface-clastic shallow shelf, mixed carbonate–clastic shallow shelf, and ultimately to intraplatform depression [34,35]. Continued sediment infill progressively narrowed the Deyang–Anyue rift, leading to its final demise during the Late Cambrian. Subsequent Caledonian–Hercynian tectonic events drove regional uplift during the Late Ordovician, resulting in relatively shallow burial depths that favored porosity preservation; by the Permian, however, renewed subsidence reburied these strata (Figure 2). Cenozoic Himalayan orogeny then imparted minor uplift, leading to the current burial depth of the Canglangpu Formation at 6000–7000 m [36,37].

Influenced by the Deyang–Anyue rift, the Lower Cambrian strata of the Sichuan Basin are in unconformable contact with the underlying Sinian Dengying Formation. The Lower Cambrian successions consist of, in ascending order, the Maidiping, Qiongzhusi, Canglangpu and Longwangmiao Formations. Within the Canglangpu Formation, two members are recognized: the lower Cang-1 Member—this study’s focus—is dominated by carbonate deposition, chiefly oolitic dolomite and oolitic limestone with subordinate clastic interbeds; the upper Cang-2 Member comprises mainly siliciclastic deposits (i.e., mudstone, sandstone and siltstone), locally interbedded with dolomitic siltstone.

3. Materials and Methods

3.1. Samples

The analytical samples for this study were obtained from five exploration wells in the Penglai gas area of northern Sichuan. All wells are located near the platform margin belt of the Canglangpu Formation, which is composed of several shallowing-upward depositional cycles; systematic sampling was conducted for each part of these cycles. A total of 90 thin sections were prepared from core samples by polishing, and petrographic observations were performed. Some of the thin sections were partially stained with alizarin red S to distinguish dolomite from calcite. Ten thin sections were selected for SEM analysis. For quantitative analysis of the pore structure of the grainstone of the Canglangpu Formation, six plug samples (diameter: 25 mm; height: 40 mm) were sequentially analyzed by NMR and high-pressure mercury intrusion. Additionally, three plug samples were used for micro-CT analysis. The plug sample was divided into several parts, and multiple thin slices were cut out for thin sectioning and SEM analysis. The remaining plug sample (approximately 25 mm high) underwent physical property analysis, CT scanning, nuclear magnetic resonance, and high-pressure mercury injection. CT scanning and nuclear magnetic resonance did not damage the sample, so we first performed CT scanning and nuclear magnetic resonance, and finally conducted high-pressure mercury injection analysis on the sample.

3.2. Methods

For thin-section preparation, the core samples were first impregnated with epoxy resin, then stained with alizarin red S, and subsequently polished for petrographic observation. Ten rock samples were polished using an argon ion beam and analyzed using a field-emission environmental scanning electron microscope (Quanta250 FEG, Thermo Fisher Scientific, Waltham, MA, USA). First, the sample needs to be cut into suitable small pieces (approximately 10 mm × 10 mm × 3 mm), and the section to be polished is selected. It is then polished using sandpaper of different grit sizes (from coarse to fine). Next, the sample is fixed on the polisher, evacuated, and the ion beam incidence angle is set at 40°. After polishing for 3 h at an acceleration voltage of 5 kv, high-energy argon ion beam polishing is performed. Imaging is carried out in high vacuum backscattered electron (BSE) mode, and analysis is conducted using a field emission scanning electron microscope (Quanta250 FEG) with a magnification range of 10 to 1,000,000 times. Each sample obtains 5–8 images.

Helium porosity and permeability of the core plug samples supplied by the Southwest Oil and Gas Field Company (Chengdu, China) were measured using a helium porosimeter and a nitrogen permeameter (SYZX-CQ), respectively. The confining pressure was 3 MPa and the Klinkenberg correction was applied. In addition, Adobe Photoshop v2020-based analysis was conducted on at least six photomicrographs per thin section to quantify the visible macroporosity (i.e., the areal porosity observable under a petrographic microscope, defined as the ratio of visible pore area to the total area). The image calibration procedure, contrast and brightness parameters refer to the method described by Zhang et al. (2014) [38]. It is generally considered that microporosity is approximated by the difference between the plug-sample helium porosity and the thin-section macroporosity.

To quantitatively characterize the micropore features, high-pressure mercury intrusion capillary pressure (MICP) analysis was performed on 13 samples using a MicAutopore9600 instrument (Micromeritics Instrument Corporation, Norcross, GA, USA). The sample was analyzed under a pressure of 183 MPa, enabling the measurement of pore sizes ranging from 5 nm to 100 μm. Additionally, the pore size distribution characteristics were obtained using the Washburn equation, with a contact angle of 140° and a surface tension of 0.48 N·m⁻¹.

NMR experiments were also conducted to determine the pore size distribution of the micropores. In the NMR experiments, samples were saturated with distilled water and analyzed using an NMR instrument (resonance frequency: 15 ± 2 MHz; magnet temperature: 35.00 ± 0.02 °C; probe coil diameter: 25 mm; experimental fluid: distilled water) with an echo count set to 10,000 and an echo spacing of 300 μs, yielding the overall pore distribution.

To quantify the three-dimensional pore network of the Canglangpu Formation carbonates, four samples were scanned using a three-dimensional rock CT scanner (model: v|tome|x m 300, Waygate Technologies, Skaneateles, NY, USA), which is a non-destructive imaging technique. Due to the proportional decrease in the field of view with increasing resolution, cylindrical samples (2 mm × 2 mm) were drilled from rock fragments, with a working resolution of 1 μm. X-rays produce shadow images: namely, two-dimensional projections of horizontal sections through the three-dimensional sample. The scanning resolution is around 9 μm. And the segmentation procedures include Perform image reconstruction and denoising. Then, Apply threshold segmentation to extract pore structure, and calculate the ratio of pore voxels to total volume, and output the porosity and pore size distribution. A series of equidistant parallel slices was obtained to reconstruct the three-dimensional pore network and to quantify the pore size distribution.

4. Results

4.1. Petrography

Petrographic analysis of the lower Cang-1 Member in northern Sichuan reveals a mixed carbonate-clastic assemblage, including oolitic dolomite, oolitic limestone and sandy dolomite (Figure 3). Dolomitization of primary oolitic limestone produced oolitic dolomite with grain contents exceeding 60%, dominated by well-sorted ooids. Intraclasts and ooids represent the dominant grain types, and granular calcite serves as the main interparticle cement (Figure 3c,f). Oolitic dolomite mainly develops in medium to high energy shoal complexes, whereas oolitic limestone reflects deposition in low energy intraplatform shoal or inter-shoal sea settings. Mixed deposition of siliciclastic and carbonate sediments led to the development of sandy and argillaceous dolomite within the Canglangpu Formation (Figure 3a,b). Detrital grains are dominated by fine to silt-sized quartz, exhibit moderate sorting and subangular to angular grain shapes, and a small amount of feldspar can also be observed, which reflects a terrestrial provenance. Furthermore, clastic material content mainly ranges from 10–15% through statistical analysis of the various component contents under the thin section. Petrographic analysis confirms that these mixed facies display negligible visible pores, with measured areal porosity below 1% (Figure 3g–i).

4.2. Porosity and Permeability

The pore type in the Cang-1 Member is dominated by intragranular dissolution pores and intergranular pores (Figure 3a–d). Intragranular pores form through dissolution within grains, whereas moldic pores develop when dissolution is so extensive that only the grain outline remains.

Petrophysical data from core samples from nine wells (e.g., JT1, PS7, PS9, and CT1) in northern Sichuan show that porosity ranges from 0.5% to 5% and permeability from 0.0006 to 0.1 mD (Figure 4). Approximately 70% of samples have porosities >2.0%, with an average porosity of 3.8% and an average permeability of 0.008 mD—consistent with low-porosity and low-permeability tight reservoirs. There are 31 oolitic dolomite samples in this study, and their porosity ranges from 0.5% to 5.7% with an average porosity of 3.4%. For the 55 oolitic limestone samples, porosity ranges from 0.3% to 4.8% with an average porosity of 1.5%. While the porosity of 30 sandy dolomite samples ranges from 0.3% to 3.4%, with an average porosity of 1.1%. The results indicate a high content of dolomite and a generally high porosity. Vertically, the Cang-1 Member exhibits shallowing-upward cyclicity with an increase in porosity upward (Figure 5), indicating that high-energy depositional environments favor pore development.

Additionally, the macroporosity observed on thin sections was compared with the total helium porosity. While the areal porosity under the microscope is nearly 0%, the helium porosity ranges from 2% to 5% (Figure 6). Thin-section observations show that most samples (points along the base of the plot) contain no macroporosity, and for the petrographic microscopes, they are approximately equal to the optical resolution limit [1]. Thus, this discrepancy between thin sections and helium porosity indicates that micropores account for over 90% of the total pore volume in the Cang-1 Member, with only a minor contribution from larger pores.

4.3. Pore Structure

Under the polarizing microscope, thin sections reveal almost no discernible pores; only a few oolitic dolomite samples show intragranular pore development within the dolomitic ooids, and intergranular pores are scarce, with pore diameters generally below 10 μm. However, high-magnification SEM clearly shows the development of micropores, predominantly as intergranular pores within the dolomitic ooids, with typical diameters around 2 μm, which mainly range from 1–25 μm according to the SEM results. Some pores appear elliptical, and most exhibit straight edges with polygonal shapes (Figure 7). Within some ooids, clastic minerals (mainly clay and quartz) are observed; organic matter associated with clay minerals has also generated organic matter pores. A small number of intergranular pores (diameter < 10 μm) are also noted. Thus, the primary reservoir space in the Cang-1 Member carbonates is constituted by micropores that are barely visible under polarizing microscopy, which is likely a result of destructive diagenetic processes during burial.

Mercury intrusion, a common method for pore structure analysis, involves injecting mercury into rock samples under increased pressure to quantitatively determine pore radii and connectivity. The results of six oolitic dolomite samples indicate that the mercury saturation in the Cang-1 Member reservoirs is relatively low, ranging from 58.7%–85.1%, with an average of 68.3%, and pore connectivity is poor. The quantitative results indicate that the average pore radius ranges from 0.03 μm to 0.3 μm micrometers, with an average of 0.12 μm, and the porosity ranges from 3.5 μm to 5.0 μm, with an average of 4.2 μm. Thus, the intrusion curves display a marked skew toward larger pore throats, indicative of poor pore sorting (Figure 8).

Nuclear magnetic resonance (NMR), a non-destructive method, was used to analyze the overall pore characteristics—including the pore size distribution for both connected and isolated pores. NMR analysis indicates that the pore radii in the Cang-1 Member are generally below 10 μm, with micropores as the dominant pore type.

The results are consistent with those obtained from high-pressure mercury intrusion porosimetry (Figure 9). Unlike mercury intrusion, NMR cannot directly measure the size of the pore radius and requires conversion. The pore size distribution results from nuclear magnetic resonance are relatively concentrated, while the pore radius distribution of mercury compression is more dispersed and does not show a bimodal feature. The difference between the two reflects that some pores are not connected.

CT scanning was employed to study pore space and distribution characteristics (Figure 10). The results show that the porosity of the Canglangpu Formation reservoir is around 7.4% according to the CT scanning, and the pores are generally small, with diameters ranging from 0.40 to 1.14 mm, suggesting moderately poor connectivity and an absence of developed fractures. Comparative analysis indicates that samples with higher porosity have a significantly greater number of pores, yet these pores remain predominantly small and isolated with radii generally below 1 mm. Conversely, samples with lower porosity have fewer and even more isolated pores.

The results from MICP, NMR, and CT scan are inconsistent. Among them, MICP mainly focuses on interconnected pores, so the overall porosity is lower compared to the results from the CT scan, but consistent with helium porosity. NMR also reflects interconnected pores, but with slightly lower overall accuracy. Moreover, it is difficult to observe micron-sized pores under the microscope, which leads to the discrepancy.

5. Discussion

5.1. Effects of Depositional Environment on Micropores

The Cang-1 Member is characterized by oolitic dolomite formed in high-energy shoal environments. In contrast, the pore-poor oolitic limestone was deposited in low-energy intraplatform shoal or intershoal settings (see Figure 1 and Figure 3). Thin-section observations reveal disordered stacking between dolomitic ooids and calcitic ooids (Figure 3f), supporting early dolomitization prior to full solidification. Reservoir development is most pronounced on paleotopographic highs, where subaerial exposure and limited terrigenous input enhanced dolomitization and pore generation, and lower overburden stress facilitated pore preservation [12,39]. Previous geochemical studies of the Cang-1 Member indicate that dolomitization occurred early, likely during the syndepositional period. Firstly, dolomite generally exhibits a low degree of order (below 0.6). And the carbon isotope values range from −2.0‰ to −0.5‰, and oxygen isotope values range from −7.8‰ to −4.8‰. These ranges are similar to the isotopic composition of Early Cambrian seawater, indicating that the dolomitizing fluid was derived from contemporaneous seawater [26,28,29].

Moreover, sea level fluctuations and rapid sedimentation imposed episodic subaerial exposure of the oolitic shoals, promoting meteoric water leaching. This also facilitates dissolution and dolomitization, resulting in relatively good original physical properties of the upper and middle grain shoals in the sedimentary cycle. Progressive burial then induced mechanical compaction and pressure solution, occluding many original pores and infilling the remainder with terrigenous detritus and calcite cement. Therefore, syndepositional dolomitization was more intense on paleotopographic highs, favoring the early preservation of pores. This process replaced calcite with dolomite, thereby increasing rock strength and resistance to pressure solution, effectively inhibiting later cement precipitation. As a result, pores were preserved over time, leading to the formation of porous grain dolomite. On the other hand, the Early Cambrian was characterized by the “Aragonite Sea,” during which metastable aragonite predominated in marine carbonate sediments. Aragonite is relatively unstable and prone to dissolution in both freshwater and marine environments, providing favorable conditions for penecontemporaneous dissolution and establishing the foundation for secondary pore development. During the deposition of the Canglangpu Formation, multiple sea-level fluctuations occurred. Penecontemporaneous meteoric leaching and dissolution, triggered by sea-level fall, were key factors in forming dissolution pores in this formation. During sea-level lowstands, shoal bodies on paleotopographic highs experienced widespread and prolonged subaerial exposure, leading to well-developed dissolution pores and consequently better reservoir quality. In contrast, shoals located at paleotopographic lows or on shoal flanks were either briefly exposed or remained submerged. These areas show poorly developed dissolution pores, stronger cementation, and thus relatively poorer reservoir conditions.

5.2. Effects of Diagenesis on Micropores

Based on the presented data, the reservoir of the Cang-1 Member is characterized by high porosity, but low permeability. For this case, it can be explained by the syndeposition process, where carbonate rocks form into peloidal limestones or porous-matrix carbonate rocks during the deposition process [40]. However, the Cang-1 Member has undergone dolomitization and dissolution to form pores, but it has also experienced cementation. The water body environment is relatively deep, lacking exposed erosion and has a strong cementation effect, which blocks the connections of the pores.

Thin sections reveal that the oolitic dolostones are the most favorable reservoirs within the Cang 1 Member. The primary reservoir space is composed of intragranular dissolved pores and intergranular pores, many of which are filled by asphalt. And fractures are extremely rare in core plugs and thin sections. In contrast, oolitic limestone is tight and essentially non-porous, showing negligible pore development and thus minimal reservoir potential. These observations indicate that dolomitization is the chief constructive diagenetic process: the transformation of calcitic ooids into dolomitic ooids generates intercrystalline porosity. Dissolution further enhances pore space in oolitic dolomite by etching grain margins, whereas dissolution is virtually absent in oolitic limestone.

Compaction and cementation represent pervasive porosity-destructive processes throughout the Canglangpu Formation. Mechanical compaction produces pronounced plastic deformation of grains, while chemical compaction creates stylolite seams infilled with argillaceous material (Figure 3g–i). Under compaction stress, subordinate calcite cements adopt bowl-shaped, stratabound morphologies that parallel bedding. In addition, cementation constitutes a major destructive diagenetic mechanism: calcite cements pervasively fill intergranular pores and locally occlude intragranular pores throughout the Cang-1 Member carbonates.

5.3. Reservoir Potential of Deep Tight Carbonates

Based on petrological and geochemical studies, it is indicated that the pore development in the Canglangpu Formation is primarily influenced by a combination of quasi-contemporaneous dolomitization and mixed sedimentation. Due to fluctuations in sea level and strong hydrodynamic forces, high-energy grain beaches were formed, and during the quasi-contemporaneous period, dolomitization occurred. The evidence is as follows: First, the thin-section observations reveal disordered stacking between dolomitic ooids and calcitic ooids (Figure 3f), supporting early dolomitization prior to full solidification. And the generally low orderliness of dolomite also indicates an early timing of dolomitization, corresponding to the penecontemporaneous stage. In addition, the carbon and oxygen isotopic composition ranges are similar to the isotopic composition of Early Cambrian seawater, indicating that the dolomitizing fluid was derived from contemporaneous seawater.

When the sea level drops, sediments are briefly exposed to atmospheric freshwater diagenetic environments. The influence of atmospheric water leads to selective dissolution, causing dissolution within the ooid or between grains, forming intragranular and intergranular dissolved pores. In the seabed environment, cementation occurs, forming fibrous cementation. Subsequently, it enters the burial stage, where it undergoes calcite cementation due to diagenetic fluid alteration, continuously reducing pore size. As burial depth increases, pressure solution intensifies, forming pressure solution fractures.

Despite the high burial depth of the Canglangpu Formation, where continuous compaction and cementation have progressively reduced pore space, this study reveals that the Cang-1 Member still contains abundant micropores. These micropores constitute the primary storage space in the tight carbonate reservoir. Additionally, the tight carbonate reservoir is primarily controlled by syndepositional dolomitization and the mixing of carbonate and terrigenous sediments. The oolitic dolomite in the Canglangpu Formation generally has high porosity. Paleotopographic highs are favorable for reservoir development. These areas featured shallow water depths and strong hydrodynamic conditions, providing the foundation for forming high-energy oolitic shoals and facilitating syndepositional dolomitization. During sea-level falls, these highs were prone to subaerial exposure, leading to dissolution and the creation of secondary pores. Furthermore, paleotopographic highs received relatively limited input of terrigenous clastics, resulting in lower degrees of sediment mixing. During burial, these areas experienced weaker cementation by diagenetic fluids, which was conducive to porosity preservation (Figure 11).

Therefore, paleotopographic highs, particularly those proximal to source rocks, represent favorable exploration fairways with excellent hydrocarbon accumulation potential and are key targets for future exploration. Similar to shallow tight sandstone reservoirs, deep tight carbonate reservoirs are also classified as unconventional reservoirs with significant exploration potential. In the northern Sichuan Basin, the Canglangpu Formation possesses favorable conditions for hydrocarbon accumulation. It is underlain by thick, high-quality source rocks of the Qiongzhusi Formation, which provide ample gas sources and sufficient charging pressure [24,25,26]. Although the Canglangpu reservoirs are tight, they are still capable of trapping hydrocarbons. Multiple wells have shown good performance in the carbonate section of the Canglangpu Formation, and have achieved high gas yields, which is of great exploration significance. It can be seen that deep, tight carbonate reservoirs also have enormous exploration potential.

6. Conclusions

The reservoir of the Cang-1 Member in the northern Sichuan Basin is predominantly composed of oolitic dolomite, with helium-measured porosity ranging from 2% to 5% and matrix permeability between 0.01 and 1 mD. Thin-section petrography and core-plug analyses show no visible pores, yet high-resolution SEM imaging reveals pervasive micropores. Intragrainular pores (~2 µm) within dolomitic ooids dominate the storage volume, while intergrainular pores (<10 µm) occur only sporadically, resulting in inherently poor reservoir connectivity.

The Cang-1 Member reservoir is mainly developed in the dolomite section. During the same sedimentary period, dolomitization and secondary intergranular dissolution constitute the main constructive diagenetic processes. The dissolution in oolitic dolomite is relatively weak and is mostly confined to the contact points of the particles. Compaction and cementation generally develop and gradually block the early formed pores as the burial depth increases, forming a uniform and dense carbonate rock matrix mainly composed of micropores, with poor connectivity.

The Canglangpu Formation exhibits favorable hydrocarbon accumulation conditions: it is underlain by the thick, organic-rich Qiongzhusi Formation source rocks that supplied an abundant gas charge. Paleotopographic highs are favorable zones for carbonate reservoir development. And deep, tight carbonate reservoirs also have enormous exploration potential.