Constructive Effect of Tuffaceous Filling Dissolution in Clastic Reservoir—A Case Study from Kuishan Sandstone in Permian of Gaoqing Buried Hill in Jiyang Depression, Bohai Bay Basin

Abstract

Tuffaceous fillings are a significant component of the Permian Kuishan sandstone in the North China Platform, and their complex diagenetic processes have a notable impact on the development of clastic rock reservoirs. This study, based on microscopic analysis of reservoirs and combined with quantitative analytical techniques such as electron probe microanalysis, homogenization temperatures of fluid inclusions, micro-area carbon-oxygen isotope analysis, and laser Raman spectroscopy, investigates the influence of tuffaceous interstitial material dissolution on reservoir development in the Permian Kuishan sandstone of the Gaoqing buried hill in the Jiyang Depression, Bohai Bay Basin. The results indicate that the dissolution intensity of tuffaceous interstitial materials can be classified into three levels: strong, moderate, and weak. In the strong dissolution zone, associated fractures and dissolution pores significantly contribute to reservoir porosity, with a positive correlation between dissolution plane porosity and total plane porosity. The reservoir space is characterized by a network of dissolution pores and fractures. The moderate dissolution zone is marked by the development of authigenic quartz, feldspar, and clay minerals, which do not effectively enhance porosity and permeability. The weak dissolution zone contains well-preserved volcanic glass shards, crystal fragments, and clay minerals, representing non-reservoir development sections. Lithology, sedimentary facies, diagenesis, and fractures collectively control the quality of the Permian Kuishan sandstone reservoir in the Gaoqing buried hill of the Jiyang Depression, Bohai Bay Basin. The advantageous zones for reservoir development in this area can be effectively predicted using thickness maps of the Kuishan sandstone, planar distribution maps of sedimentary facies, and fracture prediction maps derived from ant-tracking and coherence algorithms.

1. Introduction

The tuffaceous interstitial material in clastic rocks primarily consists of extremely fine-grained volcanic ash (typically less than 0.01 mm) [1]. Under the microscope, it is characterized by an irregular shape and intergranular distribution. Chemically unstable, this material often undergoes alteration and dissolution, forming reservoirs. Over the past two decades, in several basins in China, oil and gas fields where the primary reservoir space is formed by the dissolution pores of tuffaceous interstitial material have been discovered. In the Nantun Formation of the Beier Sag in the Hailar Basin, tuffaceous conglomerate sandstone has developed, with intergranular and devitrification pores as the main reservoir spaces, reaching a maximum porosity of 20% [2]. The tuffaceous sandstone in the Wuerhe Formation of the Wuxia Fault Zone in the Junggar Basin has an average porosity of 14%, and due to well-developed fractures, the average permeability reaches 149 mD, with tuffaceous dissolution pores also being the main reservoir space [3]. In the Permian coal-derived gas reservoirs of the Sulige Temple area, the impact of tuffaceous dissolution on the reservoir is dual: the secondary pores generated improve reservoir storage capacity, but the clay mineral cementation damages reservoir connectivity [4]. In summary, the dissolution of tuffaceous interstitial material in clastic rocks can enhance reservoir storage capacity, but its “easily transformable” nature makes its diagenesis highly complex, making it a hot topic in recent research on the formation mechanisms of secondary pores in reservoirs.

Previous studies on tuffaceous sandstone reservoirs have yielded rich results: (1) Tuffaceous interstitial material can enter reservoirs in various ways, either as volcanic ash “falling” into the reservoir or being transported by water. Both scenarios are often characterized by large-scale development of tuffaceous interstitial material, with its content having little relationship with hydrodynamic conditions [5,6]. (2) The reservoir spaces in tuffaceous rocks can be categorized into the following: cooling contraction primary pores, intergranular primary pores, devitrification pores, dissolution pores, altered clay matrix pores, and fractures, often characterized by low porosity and low permeability; original sedimentation, later diagenesis, and tectonic activities all influence reservoir properties. (3) The diagenesis of tuffaceous rocks is more complex than that of conventional sedimentary rocks, with welding, devitrification, and tuffaceous alteration and dissolution being unique to these reservoirs [7], and these three diagenetic processes often have constructive effects on the reservoir. (4) The formation of zeolite minerals and dawsonite during tuffaceous alteration can serve as markers to distinguish them from conventional sedimentary rocks [8]; the formation of analcime and laumontite is usually related to intense alteration of volcanic materials, and the formation of zeolites negatively impacts the reservoir by reducing porosity through cementation and causing volume expansion, further damaging the reservoir. Dawsonite forms in an alkaline fluid environment rich in Na⁺, Al³⁺, and high CO₂, which can degrade reservoir properties. It has been reported that mantle-derived CO₂ gas reservoirs have developed in the study area, and although dawsonite was not found in the reservoirs in this study, previous studies have identified dawsonite and dolomite mineral assemblages in wells near the Gaoqing-Pingnan fault [9].

Due to the complex mineral composition, mineral transformation, and diagenesis of tuffaceous clastic rock reservoirs, reservoir heterogeneity is strong, constraining the evaluation and exploration of these reservoirs. Building on previous research, the author utilized electron probe microanalysis, homogenization temperatures of fluid inclusions, micro-area carbon-oxygen isotope analysis, and laser Raman spectroscopy to study the diagenesis of the Permian Kuishan sandstone reservoir in the Gaoqing buried hill of the Jiyang Depression in the Bohai Bay Basin. The study explores the alteration processes and mineral transformations of tuffaceous interstitial material in sandstone reservoirs, aiming to provide references for the exploration and development of tuffaceous clastic rock reservoirs.

2. Geological Background

The Gaoqing buried hill is located in the southwestern part of the Dongying Sag within the Jiyang Depression of the North China region (Figure 1a). It is bounded to the east by the Gaoqing Fault, which connects it to the Boxing Sub-sag and also serves as the controlling fault for the Gaoqing buried hill (Figure 1b) [10]. The Boxing Sub-sag is a hydrocarbon-rich area within the Jiyang Depression. Drilling data reveal that the Late Paleozoic strata in the study area primarily consist of the Benxi, Taiyuan, Shanxi, Lower Shihezi, Upper Shihezi, and Shiqianfeng Formations [11]. Among these, the Kuishan Member of the Upper Shihezi Formation is the target layer of this study, the position marked with an star is the development section of the Kuishan sandstone. (Figure 1c). It is a product of epicontinental seas, composed of abundant volcanic and coastal deposits (Figure 1d) [12]. The tidal channel deposits exhibit sedimentary structures formed under high-energy hydrodynamic conditions, such as massive bedding, trough cross-bedding, wedge-shaped cross-bedding, and parallel bedding. Intertidal zones are characterized by gray, gray-white, and red mudstones and tuffaceous mudstones, reflecting weaker hydrodynamic conditions in these areas. The Kuishan sandstone is a set of thick-bedded quartz sandstone developed in the middle-upper part of the Kuishan Formation, primarily distributed in western Shandong and southern North China regions. It consists of white to pale yellow thick-bedded medium- to coarse-grained quartz sandstone. The Kuishan sandstone exhibits various sedimentary structures, including large-scale wedge-shaped, planar, and trough cross-bedding in its middle-upper sections, with swash cross-bedding and parallel bedding particularly developed. Swash cross-bedding forms through the shoreward and seaward reciprocating swash action of waves on beach surfaces. Characterized by wedge-shaped bedding sets with low-angle intersecting set boundaries, adjacent sets may show either consistent or opposite dip directions of laminae. The laminae are straight and extensively continuous, with eroded tops and intact bases of bedding sets. The good grain sorting within laminae (sometimes showing grain size variations) commonly occurs in backshore–foreshore zones and longshore bar environments, reflecting strong hydrodynamic conditions during deposition. The gravel components are predominantly pure quartz with moderate to good sorting and subrounded to well-rounded morphology, indicating relatively high maturity. These characteristics result from long-term reworking by frequent swash–backwash movements of breaking waves in coastal environments, demonstrating typical shallow marine sedimentary features (Figure 1e).

3. Materials and Methods

Samples were collected from the HG 102 and HG 4 wells in the Gaoqing buried hill, totaling over 60 pieces. The preparation and identification of cast thin sections were conducted at China University of Petroleum (East China). Following the Petroleum Industry Standard of the People’s Republic of China-Standard for Thin Section Identification of Petroleum Industry Rocks (SY/T 5368-2000) [13], the “point-counting” method was used to identify detrital components, interstitial materials, rock textures, and microstructures. The instruments employed included a Leica polarizing microscope, a Leica Scanning Stage SCANplus 300 × 300 with an integrated measuring system (Leica Microsystems, Wetzlar, Germany), and a CL8200MK5 cathodoluminescence microscope (Leica Microsystems, Wetzlar, Germany), all operated at room temperature.

Analytical methods: Scanning electron microscopy (SEM) observation and description were performed at China University of Petroleum (East China) under an accelerating voltage of 20 kV and a beam spot diameter of 3.5 nÅ. Electron probe microanalysis (EPMA) was conducted at the Shandong Provincial Academy of Geological Sciences using a JEOL JXA-8230 instrument (JEOL, Tokyo, Japan), with varying voltages applied depending on the composition of grains, cements, and tuffaceous interstitial materials. Fluid inclusion microthermometry was carried out at China University of Petroleum (East China) using a Leica polarizing microscope equipped with a heating–freezing stage (Leica Microsystems, Wetzlar, Germany). During homogenization temperature measurements, the heating rate was set at 10 °C/min below 60 °C and 5 °C/min above 60 °C. Laser Raman spectroscopy related to fluid inclusions was conducted in the darkroom of the same laboratory under ambient temperature conditions (25 ± 1 °C). Microscale in situ stable carbon and oxygen isotope analysis was performed at Zhongmeng Energy Technology (Tianjin) Co., Ltd., using a DELTA V isotope ratio mass spectrometer (Thermo Fisher Scientific, Waltham, MA, USA). The input parameters of Well HG102, including strata top depth, present thickness, lithology, and missing erosional thickness, were acquired from the well log profiles provided by Shengli Oil Company, SINOPEC. Initial porosity, matrix density, matrix thermal conductivity, and matrix heat capacity were adopted from the default values in BasinMod 2012 software, based on which the burial history curve of Well HG102 was reconstructed. The tectonic evolution process was sourced from the well-established geological data of Shengli Oil Company, SINOPEC.

4. Results

4.1. Reservoir Rock Types and Fundamental Characteristics

Data obtained from the “point-counting” method on cast thin sections and X-ray diffraction analysis reveal that the quartz content in the Kuishan sandstone ranges from 86% to 98%, with an average of 92.1%, with subordinate feldspar (avg. 4.1%) and lithic fragments (avg. 6.3%). The reservoir rock types are predominantly quartz sandstone, with a minimal and negligible amount of lithic quartz sandstone. (Figure 2a). The porosity of the Kuishan sandstone reservoir in the study area ranges from 2.1% to 9.8%, with an average porosity of 7.11%. The horizontal permeability ranges from 0.03 mD to 1.12 mD, with an average horizontal permeability of 0.25 mD. There is a good correlation between porosity and horizontal permeability. According to the Petroleum Industry Standard of the People’s Republic of China SY/T 6285-2018—Evaluation Method for Oil and Gas Reservoirs [14], the Kuishan sandstone in the study area is classified as an ultra-low-porosity and ultra-low-permeability reservoir (Figure 2b). The reservoir lithology is dominated by pebbly coarse sandstone, pebbly medium sandstone, pebbly fine sandstone, and fine sandstone, accounting for 92.75% of the reservoir (Figure 2c).

4.2. Pore Types

The main pore types developed in the reservoir of the study area include the following: feldspar dissolution pores (Figure 3a), intergranular dissolution pores formed by the dissolution of tuffaceous interstitial materials (Figure 3b), intragranular dissolution pores formed by quartz dissolution (Figure 3c), microporosity between clay minerals such as kaolinite (Figure 3d), moldic pores formed by feldspar dissolution (Figure 3e), and fracture-related pores (Figure 3f). Additionally, there are minor amounts of primary pores (Figure 3g), intergranular dissolution pores formed by calcite cement dissolution (Figure 3h), and dissolution pores in metamorphic quartzite fragments (Figure 3i). In the tuffaceous sandstone reservoirs of the Gaoqing buried hill, microfractures are well developed. Core observations reveal fractures ranging from 20 mm to 45 mm in length, often filled with siliceous cement (Figure 3j). Microscopic analysis shows healed fractures within brittle grains such as quartz, formed by grain fragmentation. In fracture-developed reservoirs, tuffaceous matrix dissolution is more pronounced, creating a network of dissolution pores and fractures that form effective reservoir spaces (Figure 3k,l). In contrast, intervals with only tuffaceous matrix development and no fracture networks exhibit significantly lower pore connectivity (Figure 3m).

By ranking the 37 samples based on their pore area ratio, it was found that as the total pore area ratio increases, the largest contributions to this increase come from tuffaceous interstitial material dissolution pores and fracture-related pores (Figure 3n). Furthermore, the ratio of tuffaceous interstitial material dissolution pores to the total pore area shows a strong correlation with the total pore area ratio (Figure 3o). Therefore, investigating the genesis of tuffaceous interstitial material dissolution pores in the study area can provide theoretical guidance for understanding the formation of high-quality reservoirs in the Permian Kuishan sandstone of the Gaoqing buried hill.

4.3. Authigenic Minerals Associated with Tuffaceous Alteration

The complex chemical composition of tuffaceous interstitial materials results in equally complex diagenetic processes. Through observations of cast thin sections, cathodoluminescence (CL) thin sections, and scanning electron microscopy (SEM), combined with X-ray diffraction (XRD) analysis of clay minerals and laser Raman spectroscopy, the types of authigenic minerals in the study area were identified. The main mineral types associated with tuffaceous alteration in the Kuishan sandstone reservoir include carbonate minerals, zeolites, authigenic clay minerals, siliceous cements, ferruginous cements, and sericite. The content of these cements varies significantly, with specific characteristics as follows.

4.3.1. Carbonate Minerals

Authigenic carbonate minerals are relatively abundant in the study area, primarily concentrated in dissolution pores of tuffaceous interstitial materials within the Kuishan sandstone. Their extensive development is attributed to the alkaline environment formed during tuffaceous alteration. The carbonate cements in the study area are mainly divided into three types: calcite, ferroan calcite, and siderite. Calcite and micritic ferroan calcite coexist within pores, with the ferroan calcite spatially distributed outside of the calcite. Calcite crystals reach up to 150 μm in size (Figure 4a). In CL images, calcite exhibits bright yellow luminescence (with a Raman shift at 1085 cm⁻¹, Figure 4b) [15], while ferroan calcite shows red luminescence (Figure 4c). Authigenic siderite is less abundant, occurring as euhedral rhombic grains within larger pores formed by interstitial material alteration, and is sometimes associated with ferroan calcite and kaolinite (Figure 4d). The proportions of these carbonates vary significantly: calcite accounts for 51% of the total carbonate cements, ferroan calcite for 32%, and siderite for 17%.

4.3.2. Zeolites

Zeolite minerals are commonly found during the devitrification of volcanic glass and are diverse in type. In the Kuishan sandstone of the study area, zeolite minerals are developed near 2350 m in the HG 102 well, primarily laumontite. They typically occur as pore fillings, exhibit low relief, and show partial dissolution (Figure 4e). Under fluorescence microscopy in the same field of view, they emit yellow light, indicating hydrocarbon charging after their formation (Figure 4f) [16].

4.3.3. Authigenic Clay Minerals

Authigenic clay minerals include kaolinite, illite, illite–smectite mixed layers, and chlorite. These minerals are closely related to tuffaceous alteration. XRD data reveal that clay minerals constitute 3%–13% by volume, with significant variations in their content. Kaolinite ranges from 3% to 83%, with an average of 19.27%; illite ranges from 9% to 96%, with an average of 69.62%; illite–smectite mixed layers range from 0% to 53%, with an average of 9.62%; and chlorite ranges from 0% to 4%, with an average of 1.48%. Authigenic kaolinite often fills dissolution pores of tuffaceous interstitial materials, appearing as plate-like or book-like aggregates under SEM (Figure 4g) and exhibiting dark blue luminescence in CL images (Figure 4c). Illite and illite–smectite mixed layers typically show filamentous or honeycomb structures under SEM (Figure 4h), with distinct features. Authigenic orthoclase grains transitioning to illite are also observed (Figure 4i). Chlorite is poorly developed in the study area, occurring only in a few dissolution pores of tuffaceous interstitial materials, and appears as flaky aggregates under SEM (Figure 4j).

4.3.4. Authigenic Quartz

Authigenic quartz is well developed in the Kuishan sandstone of the study area and can be classified into two types based on its occurrence: microcrystalline quartz with well-formed crystals, often observed under SEM growing on the surface of clay minerals (Figure 4k). The maximum thickness of quartz overgrowths can reach 40 μm. In cathodoluminescence (CL) images, these overgrowths can be divided into two phases: Phase I appears dark brown but is incompletely preserved and developed only along the edges of some grains; Phase II exhibits dark blue coloration and is thick and well developed, with sharp boundaries against the host mineral grains (Figure 4l).

4.3.5. Authigenic Pyrite

Authigenic pyrite is locally enriched in the Kuishan sandstone, particularly near 2384 m in the HG 102 well, where it occurs in large quantities (Figure 4m). It forms continuous pore-filling cements, exhibits distinct reflective characteristics, and often appears as floating particles, significantly degrading reservoir quality.

4.3.6. Sericite (Altered Tuffaceous Material)

After tuffaceous interstitial materials enter the syndepositional stage, rapid changes in temperature and chemical conditions render their components highly unstable. Consequently, the alteration of tuffaceous interstitial materials is widespread in the Kuishan sandstone of the study area. Sericite formed by tuffaceous alteration consists of fine scaly muscovite and illite, displaying a satin-like interference color under the microscope and replacing the original tuffaceous interstitial materials in situ between grains (Figure 4n). In addition to sericitization, tuffaceous interstitial materials can also undergo silicification and clay alteration, each with distinct characteristics: Silicified tuffaceous materials exhibit felsitic textures and first-order gray-white interference colors (Figure 4o). Clay-altered tuffaceous materials show wavy extinction, with some fully transformed into clay minerals such as illite or kaolinite (Figure 4p).

4.4. Characteristics of Diagenetic Fluids in Tuffaceous Sandstone Reservoirs

4.4.1. Fluid Inclusion Characteristics

Saline fluid inclusions in quartz overgrowths and microfractures are predominantly elliptical and irregularly elongated, ranging in size from 1 to 9 μm. They are colorless, transparent, and characterized by a gas–liquid ratio of 4%–13% (Figure 5a,b). Hydrocarbon inclusions are mainly elliptical and irregularly elongated, ranging in size from 1 to 4 μm, appearing dark brown and fluorescing yellow and blue under UV light, with a gas–liquid ratio of 6%–40% (Figure 5c,d). Homogenization temperatures of two-phase (gas–liquid) inclusions in quartz overgrowths, measured by microthermometry and laser Raman spectroscopy, fall into two ranges: 60–80 °C and 110–120 °C (Figure 5f). Combined with burial and thermal history, these inclusions formed during the shallow burial period of the Gaoqing buried hill in the Mesozoic and the main hydrocarbon accumulation period of the Jiyang Depression in the Cenozoic (Figure 5h). Inclusions from the shallow burial period are associated with authigenic quartz formed during tuffaceous alteration, while those from the main accumulation period are linked to authigenic quartz formed in an organic acid-rich fluid environment. This is further supported by the Raman scattering peaks of CO₂ in coeval saline inclusions adjacent to hydrocarbon inclusions (Figure 5e).

Homogenization temperatures of two-phase (gas–liquid) inclusions in authigenic carbonate fall into two ranges: 40–60 °C and 95–115 °C (Figure 5g). Combined with burial and thermal history, these two stages of calcite cementation correspond to the shallow burial period of the Gaoqing buried hill in the Mesozoic and the main hydrocarbon accumulation period of the Jiyang Depression in the Cenozoic (Figure 5h). The first stage represents authigenic calcite formed in an alkaline environment during tuffaceous alteration, while the second stage represents calcite formed by organic acid decarboxylation [17].

4.4.2. Microscale Carbon and Oxygen Isotope Characteristics

Using laser in situ microanalysis of carbon and oxygen isotopes, the isotopic compositions of two types of calcite in the Gaoqing buried hill were determined. The δ¹³CPDB values of the first-stage calcite range from −6.97‰ to −1.95‰, and the δ¹⁸OPDB values range from −16.91‰ to −9.16‰. For the second-stage ferroan calcite, the δ¹³CPDB values range from −6.47‰ to −5.10‰, and the δ¹⁸OPDB values range from −10.74‰ to −9.13‰.

4.4.3. Electron Probe Microanalysis (EPMA) of Tuffaceous Materials

Tuffaceous interstitial materials exhibit poor chemical stability. Electron probe microanalysis (EPMA) of tuffaceous interstitial materials and associated minerals in the reservoir reveals that the primary components of unaltered tuffaceous interstitial materials are SiO₂ and Al₂O₃, followed by metal oxides such as sodium, potassium, iron, manganese, and calcium. The SiO₂ content ranges from 30.79% to 80.40%, with an average of 49.44%; Al₂O₃ content ranges from 9.39% to 32.12%, averaging 20.90%; FeO content ranges from 0.09% to 36.29%, averaging 6.58%; K₂O content ranges from 0.021% to 8.92%, averaging 2.85%; Na₂O content ranges from 0.009% to 12.52%, averaging 1.95%; MgO content ranges from 0% to 11.43%, averaging 3.11%; and CaO content ranges from 0% to 2.69%, averaging 0.42%.

4.4.4. Unique Features of Diagenetic Fluids in Tuffaceous Sandstone Reservoirs

During the initial stage of tuffaceous alteration, hydrolysis and alteration of tuffaceous materials release large amounts of metal ions, maintaining an alkaline environment in reservoir pores for an extended period. This alkaline environment favors the precipitation of carbonate minerals, while the activation of Na⁺, K⁺, and Ca²⁺ ions, along with high alkalinity and salinity, promotes the formation of zeolite minerals. Layered silicate minerals, such as clay and mica groups, are the most common secondary products of tuffaceous alteration [18], with sericite (a mixture of muscovite and illite) being the most developed in the study area. Subsequently, as the Gaoqing Fault became active and laterally connected source rocks matured, the diagenetic environment turned acidic, leading to the crystallization of quartz and calcite formed by organic acid decarboxylation. Simultaneously, soluble materials within the reservoir underwent dissolution (Figure 5i). Under the influence of compaction, tuffaceous interstitial materials rapidly filled the pores, causing the porosity to decrease sharply to 11.65%. Alkaline fluids generated from tuffaceous alteration led to extensive calcite and quartz cementation, reducing the porosity by 8.5% to 3.15%. Subsequently, organic acids reformed the reservoir, increasing the porosity by 3.93% to 7.11%. Compared to conventional clastic reservoirs, the early formation of authigenic minerals in tuffaceous sandstone reservoirs is more controlled by the hydrolysis and alteration of volcanic materials, while later stages may be influenced by external formation fluids.

5. Discussion

5.1. Tuffaceous Alteration and Dissolution Processes

EPMA mapping visually contrasts the elemental differences between unaltered and altered tuffaceous materials through color intensity (Figure 6a), summarizing the elemental migration patterns during tuffaceous alteration. Na⁺ and K⁺ exhibit strong similarity, with their contents significantly higher in unaltered tuffaceous materials compared to altered ones. This is attributed to the dissolution or replacement of highly reactive metal ions like Na⁺ and K⁺ by H⁺ during alteration [19], removing them from the original water-rock reaction system (Figure 6b) and resulting in an alkaline diagenetic environment, leading to the enrichment of Fe²⁺ and Ca²⁺ (Figure 6c).

During the tuffaceous alteration process, the color, occurrence, and physical properties of tuffaceous interstitial materials allow the alteration and dissolution to be divided into four stages. The original tuffaceous interstitial materials appear as yellowish clumps (Figure 6d), containing minor glass and crystal fragments, primarily composed of volcanic ash filling intergranular spaces less than 0.01 mm. This stage typically occurs in poorly sorted sand bodies, where compaction rapidly fills intergranular spaces with tuffaceous materials (Figure 6e), which remain largely unaltered and completely destroy porosity, negatively impacting the reservoir. Most tuffaceous interstitial materials undergo significant changes in temperature and chemical conditions during burial, leading to silicification-dominated alteration, characterized by felsitic textures and first-order gray-white interference colors (Figure 6f). At this stage, alteration is evident, but dissolution is insufficient, with only partial formation of clay mineral intercrystalline pores, still generally detrimental to the reservoir (Figure 6g). With further alteration, especially in reservoirs where fractures connect diagenetic fluids, tuffaceous interstitial materials undergo dissolution, accompanied by “dirty” kaolinite clumps (Figure 6h). Dissolution pores and associated clay mineral intercrystalline pores become the main pore types in such reservoirs, providing some constructive effects. In fully open systems, such as field samples from the Boshan area in Shandong, near-complete dissolution of tuffaceous interstitial materials is observed, with only minor altered remnants at grain edges (Figure 6), significantly benefiting the reservoir.

5.2. Petrophysical Response to Tuffaceous Matrix Dissolution

The alteration and dissolution of the tuffaceous matrix are accompanied by the formation and dissolution of authigenic minerals, leading to varying mineral assemblages at different stages, which directly impact reservoir properties. Based on the alteration products of tuffaceous material and the transformation of secondary minerals, the intensity of tuffaceous matrix alteration is classified into three levels: strong alteration (Type I), moderate alteration (Type II), and weak alteration (Type III), each exerting distinct effects on reservoir properties.

Type I: Exemplified by the interval 2370~2390 m in Well HG 102, this reservoir section is 20 m thick, primarily composed of pebbly quartz sandstone. The porosity ranges from 4.8% to 9.8%, with an average of 7.5%, and permeability ranges from 0.06 mD to 1.12 mD, averaging 0.28 mD, making it a high-quality reservoir in the study area. The authigenic mineral assemblage associated with tuffaceous alteration includes quartz, clay minerals, mica, and carbonates. The activation of Na⁺, K⁺, and Ca²⁺ ions in the tuffaceous matrix, along with silica precipitation, leads to the crystallization of feldspar (mainly K-feldspar in the study area). This reservoir type is characterized by coarse grain size and good sorting. Authigenic feldspar is subsequently dissolved by organic acids, forming kaolinite, as observed in backscattered electron images showing altering K-feldspar.

Type II: Represented by the interval 2347.5~2352.5 m in Well HG 102, this reservoir is 5 m thick, mainly consisting of pebbly quartz sandstone and quartz sandstone. Compared to Type I, it has thinner sand bodies and finer grain size, with porosity ranging from 2.7% to 5.4% (average 3.8%) and permeability from 0.032 mD to 0.053 mD (average 0.032 mD). The authigenic mineral assemblage includes quartz, feldspar, clay minerals, mica, and carbonates. The key difference from Type I is that K-feldspar is only partially dissolved, with some residual feldspar remaining. Backscattered electron images reveal the illitization of K-feldspar. The presence of abundant authigenic and clay minerals limits the effectiveness of tuffaceous alteration in enhancing porosity and permeability.

Type III: Exemplified by the interval 2352.5~2370 m in Well HG 102, this section is primarily composed of tuff, mudstone, and muddy siltstone, with a reservoir thickness of 9.5 m. It has finer grain size compared to the previous types, with porosity ranging from 2.1% to 2.8% (average 2.45%) and permeability from 0.03 mD to 0.031 mD (average 0.03 mD). Well-preserved silt-sized quartz, feldspar, glass shards, crystal fragments, and clay minerals are observed, indicating a non-reservoir section in the study area. Based on these findings, a model for tuffaceous matrix alteration in the study area is established (Figure 7).

5.3. Controlling Factors of Tuffaceous Matrix Dissolution

Tuffaceous matrix is widely present in the reservoirs of the study area. Lithology, sedimentary facies, and fractures collectively control the quality of the Permian Kuishan sandstone reservoirs in the Gaoqing buried hill of the Jiyang Depression, Bohai Bay Basin. Among these factors, lithology and sedimentary facies are relatively stable in the study area, with thick quartz sandstones (up to 30 m) deposited in tidal channels being the dominant lithology and facies for high-quality reservoir development. However, fracture development varies and is comparable across the study area, making the study of fractures particularly important.

Imaging logging data (FMI) provide clear responses to fractures, the brighter yellow parts generally indicate that the formation has a higher electrical conductivity or a higher porosity, the darker yellow parts usually represent that the formation has a lower electrical conductivity or the rock is relatively dense, and the pink curves represent the fractures around the wellbore. Statistical analysis of fracture orientations in Well HG 102 shows that fractures predominantly dip east and southeast, consistent with the dip direction of the Gaoqing Fault. Fracture angles are mainly medium-high to high (greater than 50°), aligning with the steep nature of the Gaoqing Fault, suggesting a genetic link to fault activity (Figure 8a,b). Previous studies on the Gaoqing Fault’s activity rate indicate that it initiated during the Paleogene Kongdian–Shahejie 4 depositional period, intensified during the Shahejie 3–2 period, and gradually weakened after the Shahejie 1 period, ceasing activity by the Neogene–Quaternary (Figure 8c) [20,21,22]. Fluid inclusion temperatures and CO₂ in the fluid inclusions in healed fractures within quartz grains further support this, indicating that the second phase of quartz cementation occurred during the Cenozoic Shahejie Formation.

On a planar scale, coherence and ant-tracking attributes were used to predict fractures at the top of the Kuishan sandstone in the study area. Low coherence between adjacent seismic traces indicates discontinuous strata and fracture development (Figure 9c). Similarly, ant-tracking algorithms applied to 3D seismic coherence volumes in the Gaoqing area delineate fractures by clustering “ants” along fracture locations (Figure 9b).

By integrating the structural map of the Upper Shihezi Formation, Kuishan sandstone thickness map (Figure 9a), sedimentary facies distribution map (Figure 9b), fracture predictions from ant-tracking, and coherence-based fracture predictions, favorable zones for Kuishan sandstone reservoir development in the Gaoqing buried hill of the Jiyang Depression, Bohai Bay Basin, were identified. These zones are characterized by significant fracture development, thick sandstone bodies, and tidal channel microfacies (Figure 9e).

6. Conclusions

(1)

In the Permian Kuishan sandstone reservoirs of the Gaoqing buried hill and surrounding uplifts in the Jiyang Depression, Bohai Bay Basin, tuffaceous matrix dissolution pores and fracture pores contribute most significantly to increased pore connectivity. The ratio of tuffaceous matrix dissolution pores to total pore space shows a strong correlation with total porosity. The reservoirs also exhibit diverse authigenic minerals, including carbonate cement, zeolite cement, authigenic clay minerals, siliceous cement, ferruginous cement, altered tuffaceous material, and sericite.

(2)

Tuffaceous matrix dissolution generally enhances clastic reservoir quality. Based on alteration products and secondary mineral transformations, the tuffaceous matrix alteration intensity in the Kuishan Member of the Upper Shihezi Formation is classified into three levels: strong alteration (Type I), moderate alteration (Type II), and weak alteration (Type III). These types exhibit significant differences in reservoir properties and authigenic mineral assemblages, with strong alteration corresponding to high-quality reservoir facies.

(3)

The tuffaceous matrix is widely distributed in the study area’s reservoirs, with lithology, sedimentary facies, and fractures collectively controlling reservoir quality. By integrating structural maps, sandstone thickness maps, sedimentary facies distributions, and fracture predictions from ant-tracking and coherence algorithms, favorable zones for Kuishan sandstone reservoir development in the Gaoqing buried hill of the Jiyang Depression can be predicted.