Reservoir Characteristics and Hydrocarbon Potential of Cretaceous Volcanic Rocks in the Shimentan Formation, Xihu Sag, East China Sea Shelf Basin

Abstract

In recent years, significant exploration successes and research progress in volcanic hydrocarbon reservoirs across China’s offshore basins have highlighted their importance as key targets for deep hydrocarbon exploration. In the Shimentan Formation of the Xihu Sag, East China Sea Shelf Basin (ECSSB), low-yield gas flows have been encountered through exploratory drilling; however, no major reservoir breakthroughs have yet been achieved. Assessing the large-scale reservoir potential of volcanic sequences in the Shimentan Formation is thus critical for guiding future exploration strategies. Based on previous exploration studies of volcanic reservoirs in other Chinese basins, this study systematically evaluates the hydrocarbon potential of these volcanic units by microscopic thin section identification, major element analysis, integrates drilling data with seismic interpretation techniques—such as coherence cube slicing for identifying volcanic conduits, dip angle analysis for classifying volcanic edifices, and waveform classification for delineating volcanic lithofacies. The main findings are as follows: (1) The Shimentan Formation is primarily composed of intermediate to acidic pyroclastic rocks and lava flows. Volcanic facies are divided into three facies, four subfacies, and six microfacies. Volcanic edifices are categorized into four types: stratified, pseudostratified, pseudostratified-massive, and massive. (2) Extensive pseudostratified volcanic edifices are developed in the Hangzhou Slope Zone, where simple and compound lava flows of effusive facies are widely distributed. (3) Comparative analysis with prolific volcanic reservoirs in the Songliao and Bohai Bay basins indicates that productive reservoirs are typically associated with simple or compound lava flows within pseudostratified edifices. Furthermore, widespread Late Cretaceous rhyolites in adjacent areas of the study region suggest promising potential for rhyolitic reservoir development in the Hangzhou Slope Zone. These results provide a robust geological foundation for Mesozoic volcanic reservoir exploration in the Xihu Sag and offer a methodological framework for evaluating reservoir potential in underexplored volcanic regions.

1. Introduction

Volcanic hydrocarbon reservoirs have been explored for over 150 years, yielding substantial achievements in both research and resource discovery [1,2]. To date, industrial-scale hydrocarbon accumulations and proven reserves have been documented in more than 40 sedimentary basins across 13 countries [1,3]. According to incomplete statistics, approximately 1.45 billion tons of oil-equivalent reserves have been confirmed globally [2,4], and high-yield hydrocarbon reservoirs have been discovered in volcanic rocks of multiple basins in China [5,6,7], which establishing volcanic reservoirs as critical targets for deep hydrocarbon exploration. Notably, China’s offshore basins have recently recorded wells with the highest single-well productivity worldwide [8], further revitalizing interest in volcanic reservoir exploration.

During early exploration stages, seismic identification of favorable volcanic reservoirs remains a major technical challenge. To address this, researchers have focused on establishing correlations among lithology, lithofacies, volcanic edifices, and seismic facies to aid in reservoir prediction. Initial identification efforts in faulted basins primarily relied on geometric characteristics, seismic reflection configurations, continuity, frequency, and amplitude of seismic signals [9,10,11,12,13]. Later studies advanced this by systematically correlating seismic facies with lithological and lithofacies characteristics. This approach involves analyzing the genetic relationships among volcanic lithofacies, emplacement environments, and seismic facies units—such as outer and inner seaward-dipping reflectors (SDRs) and outer high-amplitude seismic facies in continental margin basins—to enable more accurate interpretation of distinct volcanic lithofacies units [14,15].

In the rift sequences of the Songliao Basin, various seismic geometries—including sheet-shaped, draped sheet-shaped, shield-shaped, mound-shaped, lenticular, wedge-shaped, fan-shaped, disk-shaped, and cylindrical forms—have been systematically associated with specific lithological assemblages [16]. As exploration has progressed, seismic facies interpretation has become a vital tool for delineating the distribution patterns of favorable reservoirs and optimizing exploration efficacy. For instance, in the Songliao Basin, volcanic rocks can be classified into four seismic facies types: moundy/lenticular-subparallel, platy/sheet/shield-parallel/subparallel, dome/moundy-disordered, and mushroom-shaped-disordered. Each of these types exhibits distinct reservoir characteristics [17].

However, significant uncertainties still persist in the interpretation of seismic–reservoir relationships. To overcome these limitations, modern reservoir prediction methods incorporate geometric parameters such as aspect ratios of geological bodies and internal stacking patterns. In the Changling Fault Depression of the Songliao Basin, empirical relationships have been established between volcanic edifice slope angles and reservoir quality. High slope angles are associated with lava domes of extrusive facies, which typically exhibit poor reservoir quality, while medium slope angles correspond to lava flows of effusive facies with generally better reservoir quality [18]. Simultaneously, a correlation between seismic facies and reservoir quality has been established: massive/chaotic/low-amplitude seismic facies correspond to massive volcanic edifices, which generally have poor reservoir quality; pseudostratified/parallel–subparallel/medium-strong amplitude seismic facies correspond to pseudostratified volcanic edifices, which are associated with good reservoir quality; pseudostratified-massive seismic facies represent a transitional type, correlating with pseudostratified-massive edifices and generally poor reservoir quality; stratified/parallel/medium-amplitude seismic facies correspond to stratified volcanic edifices, typically indicating moderate reservoir quality. This genetic classification framework offers a practical approach for predicting reservoir development from seismic facies interpretation.

Volcanic assemblages have been extensively identified within the Mesozoic strata of the East China Sea Shelf Basin (ECSSB). Drilling data from 9 wells in the Xihu Sag have confirmed the presence of rhyolitic tuff, rhyolitic welded tuff, trachyte, and cryptoexplosive breccia within the Shimentan Formation [19,20]. Although only low-yield gas flows have been obtained from the Shimentan Formation in the BaoYunting gas field, these results confirm the reservoir potential and exploration significance of volcanic rocks in the region.

However, due to limited drilling data, the spatial distribution and reservoir characteristics of volcanic rocks in the Shimentan Formation remain poorly constrained. Further investigation is therefore urgently needed to clarify their distribution patterns and evaluate their reservoir potential. In this study, an integrated analytical approach is employed to assess the reservoir potential of the Shimentan Formation volcanic rocks. Favorable volcanic edifices are identified based on dip attribute and seismic facies characteristics, while favorable lithofacies are delineated using textural features and waveform classification. A synergistic interpretation of these multi-attribute data enables the construction of a predictive model for volcanic reservoir distribution, providing a foundation for optimizing exploration targets in underexplored volcanic basins.

2. Geological Setting

The East China Sea Basin (ECSB) developed along the western Pacific active continental margin during the Late Mesozoic and Cenozoic, forming a key segment of the tectonically active circum-Pacific belt. The East China Sea Shelf Basin (ECSSB) represents the western part of the ECSB and includes both the ECSSB and the Okinawa Trough. The basin is bounded by the Minzhe Uplift to the west and the Diaoyu Island Uplift to the east and exhibits an overall NNE-trending orientation. It is subdivided into several tectonic units, including the Taipei Depression, Zhoushan Uplift, and East Zhejiang Depression [21,22].

The Xihu Sag is situated in the central part of the East Zhejiang Depression and is the largest sag within the ECSSB. Structurally, the Xihu Sag displays prominent east–west zoning and north–south segmentation [23]. It can be subdivided into three east–west trending structural zones: the Western Slope Belt, the Central Structural Belt, and the Eastern Fault Belt. The Western Slope Belt is further segmented into three north–south trending subzones—Hangzhou Slope Zone, Pinghu Slope Zone, and Tiantai Slope Zone—by a series of basement faults striking NWW. The current study area is primarily located within the Pinghu and Hangzhou Slope Zones (Figure 1b).

Based on drilling data and seismic interpretation, the stratigraphy of the Xihu Sag comprises the following units: the Upper Cretaceous Shimentan Formation (a volcanic-sedimentary sequence), an undrilled Paleocene interval, the Eocene Baoshi and Pinghu Formations, the Oligocene Huagang Formation, the Miocene Longjing, Yuquan, and Liulang Formations, the Pliocene Santan Formation, and the Quaternary Donghai Group (Figure 2) [24,25].

Tectonically, the ECSSB has undergone a complex evolutionary history. During the Jurassic period, it was part of a continental marginal depression associated with the Paleo-Asian–Tethyan oceanic tectonic regime [21]. Since the Cretaceous, the region has transitioned into a post-arc rift system under the influence of the Pacific tectonic regime. The intense crustal shortening and orogenic thickening driven by the Paleo-Asian–Tethyan system prior to the Early Cretaceous gave way to continental marginal rifting and lithospheric thinning dominated by Pacific plate dynamics. The post-arc rifting stage since the Cretaceous can be subdivided into three major phases: rifting from the Early Cretaceous to the Eocene, regional subsidence from the Oligocene to the Late Miocene, and renewed rifting from the Late Miocene to the Holocene rifting. Volcanic activity, particularly during the Cretaceous period, played a significant role in sedimentary infill and hydrocarbon system development within the basin.

Figure 1. Tectonic framework of the East China Sea Shelf Basin and well locations in the study area (modified from [23,26]). Note: In (a), the red-shaded area and the yellow line represent the locations of (b,c), respectively. In (b), the blue line indicates the extent of seismic data in the Hangzhou Slope Belt. Abbreviations: ECSSB—East China Sea Shelf Basin; MZU—Minzhe Uplift; CSU—Changshu Uplift; DBS—Diaobei Sag; FLS—Fuliang Sag; FZS—Fuzhou Sag; HJU—Haiiao Uplift; HPU—Hupiiao Uplift; JJS—Jiaojiang Sag; JSBU—Jinshanbei Uplift; SNS—Jinshannan Sag; KSS—Kunshan Sag: LSS—Lishui Sag; OT—Okinawa Trough; PJYS—Pengjiayu Sag; QTS—Qiantang Sag; XHS—Xihu Sag; YDU—Yandang Uplift; YSDU—Yushandong Uplift.

Figure 1. Tectonic framework of the East China Sea Shelf Basin and well locations in the study area (modified from [23,26]). Note: In (a), the red-shaded area and the yellow line represent the locations of (b,c), respectively. In (b), the blue line indicates the extent of seismic data in the Hangzhou Slope Belt. Abbreviations: ECSSB—East China Sea Shelf Basin; MZU—Minzhe Uplift; CSU—Changshu Uplift; DBS—Diaobei Sag; FLS—Fuliang Sag; FZS—Fuzhou Sag; HJU—Haiiao Uplift; HPU—Hupiiao Uplift; JJS—Jiaojiang Sag; JSBU—Jinshanbei Uplift; SNS—Jinshannan Sag; KSS—Kunshan Sag: LSS—Lishui Sag; OT—Okinawa Trough; PJYS—Pengjiayu Sag; QTS—Qiantang Sag; XHS—Xihu Sag; YDU—Yandang Uplift; YSDU—Yushandong Uplift.

Figure 2. Stratigraphic column of the Xihu Sag (modified from [26]).

Figure 2. Stratigraphic column of the Xihu Sag (modified from [26]).

3. Data and Methods

3.1. Thin Section Microscopy

The identification of the rocks and minerals was conducted using hand specimens, mainly by observing the color and fabric of the rock. In addition, the rocks and minerals were identified under a transmission polarized light microscope. In this study, we used the OLYMPUS BX51 polarizing microscope (Olympus Corporation, Tokyo, Japan) to conduct microscopic observation on the rock thin sections. The sample is made into a thin section, and then, the mineral composition, particle size, structure, content, and secondary alteration of the rock were analyzed using a single polarizer and crossed polarizers.

3.2. Major Element Analysis

A total of 11 samples collected from drilling cores were subjected to major element analysis. The analyses were performed at the Key Laboratory of Mineral Resources Evaluation in Northeast Asia, Ministry of Natural Resources, Jilin University. Based on lithofacies identification, fresh rock samples were selected, rinsed with distilled water, dried, and crushed to 200 mesh to prevent contamination. The major element compositions were determined using X-ray fluorescence (XRF) spectrometry, with a relative analytical error ranging between 1% and 3%.

3.3. Coherence Cube Attribute Analysis

Coherence cube attribute analysis based on eigenvalue of seismic data was conducted using the SMI 5.0 software. For optimal detection of subtle stratigraphic features and minor faults, coherence was calculated using a short time window and a small spatial aperture. In this study, we selected a time window offset by 40 ms below the target horizon for calculation. The spatial aperture was defined as a rectangular area centered on the targeted trace, with a radius equivalent to twice the trace spacing (approximately 50 m). The time aperture was similarly centered to the targeted sample point, with a radius of two sampling intervals (4 ms, equivalent to approximately 10 m). The coherence attribute was used to quantify waveform similarity, with coherence values ranging from 0 (low similarity) to 1 (high similarity).

3.4. Dip Angle Attribute Analysis

Dip angle (expressed in milliseconds/trace) was defined as the inclination of a seismic event relative to the horizontal direction along seismic lines or traces. This differs from the conventional geological definition, which is expressed in degrees. Conversion to geological dip requires knowledge of seismic velocity and spatial sampling. A higher computed dip value indicates a steeper dip. Note that dip angles are only directly comparable within the same seismic survey area.

3.5. Waveform Classification

Waveform classification of seismic data was conducted using the Paradigm 2022 software suite. Seismic facies analysis, based on the seismic response of rock formations, provides a reliable approach for volcanic facies identification. Waveform classification, a key technique in seismic facies interpretation and lithologic differentiation, was performed using a self-organizing neural network algorithm [27,28]. The classification process included the following steps: (1) Selection of a 33–132 ms time window, offset by 100 ms below the target horizon; (2) division of the target interval into six equal-thickness segments to construct initial model traces; (3) utilization of 50% of the initial model traces in the second iteration and 150% in the third iteration; (4) execution of 30 iterations, comparing model sets with 5 and 15 trace types to optimize lithologic interpretation. Validation against borehole lithology confirmed that the model with 5 trace types provided the most accurate lithologic discrimination. In addition, vertical lithological identification was supplemented by integrating seismic facies with well-logging data.

4. Lithological Characteristics

4.1. Lithologic Composition

Volcanic rocks of the Shimentan Formation were encountered in nine wells (W-1, B-2, B-3, B-3P, B-7, K-1, N-25-1, N-25-2, N-25-6, L-1) within the Xihu Sag, with a total drilled thickness of approximately 410 m. Based on petrographic texture and mineralogical composition, the volcanic rocks are classified into three main categories comprising six distinct lithologies, described as follows:

Trachyte (9%): Characterized by an aphanitic texture with a trachytic groundmass, this rock contains tabular to columnar sanidine microliths arranged in a semi-oriented fashion. Sanidine is euhedral to subhedral and exhibits Carlsbad twinning with slight sericitization. Amphibole and minor pyroxene occur as interstitial minerals and are commonly associated with magnetite. Amphibole displays intense chloritization (Figure 3a).

Dacite (3%): This lithology exhibits a porphyritic texture, with phenocrysts comprising quartz (40%), plagioclase (25%), and minor alkali feldspar (5%). Quartz appears as anhedral grains, while plagioclase is euhedral to subhedral with well-developed polysynthetic twinning. Alkali feldspar is similarly euhedral to subhedral and exhibits Carlsbad twinning. The groundmass shows a mixture of felsitic and spherulitic textures, composed primarily of quartz, glass, and radiating clusters of fine-grained felsic minerals formed through devitrification (Figure 3b).

Dacitic tuff lava (3%): This rock displays a tuffisitic texture with 40% crystal fragments, 15% lithic fragments, and pumice. Quartz and plagioclase dominate the crystal fragments, both showing resorption textures. Plagioclase exhibits clear polysynthetic twinning and partial sericitization. Lithic fragments are dacitic, subangular to subrounded. Pumice clasts contain quartz phenocrysts in a felsic to pilotaxitic groundmass of quartz and minor plagioclase microlites (Figure 3c).

Trachytic cryptoexplosive breccia (25%): This lithology is marked by a cryptoexplosive breccia texture, with 2–4 mm angular andesitic clasts lacking phenocrysts, embedded in a pilotaxitic matrix. Clasts exhibit sharp angular boundaries with fractures filled by ferruginous cement. The matrix consists of euhedral to subhedral plagioclase microlites arranged in a semi-oriented, intergranular framework (Figure 3d).

Andesitic tuff (27%): Exhibiting a vitroclastic texture, this lithology comprises 40% crystal fragments, 20% lithic fragments, and 40% volcanic ash matrix. Crystal fragments, predominantly plagioclase with polysynthetic twinning, are subangular and angular. Lithic fragments are subangular and andesitic in composition. The matrix is made up of fine-grained volcanic ash.

Dacitic tuff (18%): Also displaying a vitroclastic texture, dacitic tuff consists of 50% crystal fragments, 20% vitric fragments, and 30% volcanic ash matrix. Crystal fragments are quartz and plagioclase, mostly subrounded to subangular. Plagioclase grains are subhedral and exhibit polysynthetic twinning with sericitization along cleavage planes. The matrix is cemented by volcanic ash (Figure 3e).

Rhyolitic tuff (10%): This lithology shows a vitroclastic texture with 30% vitric fragments, 20% crystal fragments, 10% lithic fragments, and 50% volcanic ash matrix. Vitric fragments are banded and blocky with incipient devitrification and weak birefringence. Crystal fragments, predominantly quartz and feldspar, are subangular to angular. Lithic fragments are subangular and rhyolitic in composition (Figure 3f).

Geochemical analysis of major elements shows that lava samples from Well K-1 contain SiO₂ (60.18–63.49 wt.%), K₂O (3.42–4.74 wt.%), and Na₂O (0.84–1.73 wt.%), while those from Well L-1 contain SiO₂ (58.60–66.23 wt.%), K₂O (3.42–4.74 wt.%), and Na₂O (0.20–6.54 wt.%) (

Table 1. Major element data of volcanic rocks in the Shimentan Formation, Xihu Sag.

Sample	SiO2	Al2O3	Fe2O3	FeO	CaO	MgO	K2O	Na2O	TiO2	P2O5	MnO	LOI	Total
K-1-1	61.40	17.77	2.74	2.56	0.48	1.98	3.74	1.13	0.71	0.06	0.04	7.13	99.75
K-1-2	60.04	18.41	2.66	2.73	0.46	2.21	3.95	0.84	0.75	0.05	0.05	7.62	99.77
K-1-3	63.14	18.72	1.70	1.76	0.25	1.95	4.18	0.86	0.78	0.04	0.03	6.45	99.88
K-1-4	63.49	17.06	3.96	1.98	0.54	1.74	3.23	1.73	0.58	0.04	0.06	5.60	100.01
L-1-1	66.14	16.13	2.87	1.53	1.47	0.63	2.72	4.26	0.97	0.15	0.08	2.93	99.87
L-1-2	62.82	15.53	5.19	1.54	1.99	1.06	1.83	6.48	1.17	0.25	0.18	1.57	99.63
L-1-3	61.91	17.41	3.72	1.41	2.05	1.35	2.85	3.38	1.04	0.13	0.10	4.38	99.74
L-1-4	63.68	15.73	4.67	1.19	2.32	0.62	2.38	6.13	1.16	0.19	0.13	1.68	99.87
L-1-5	64.55	15.57	2.93	1.41	1.95	1.17	1.93	6.46	1.25	0.09	0.21	2.11	99.64
L-1-6	58.46	23.81	1.64	0.66	0.39	1.35	3.96	0.20	0.99	0.03	0.02	8.25	99.76
L-1-7	65.03	14.92	3.99	1.50	1.80	1.01	1.63	6.52	1.19	0.13	0.16	1.86	99.73

). TAS diagram classification confirms that the volcanic rocks in the Shimentan Formation are primarily dacite and trachyte, belonging to the subalkaline series and representing intermediate to acidic compositions (Figure 4).

4.2. Well Logging Characteristics

The volcanic rocks of the Shimentan Formation in the Xihu Sag display distinct well-logging responses across gamma ray (GR), density (DEN), resistivity (RT), and acoustic (AC) curves depending on lithology. Rhyolitic and dacitic tuffs exhibit high GR (85–175 API), moderate to high DEN (2.1–2.7 g/cm³), high AC (65–120 μs/ft), and low RT (3–30 Ω·m). The GR curve typically displays a box-finger shape with micro-toothing; the RT curve shows a box-finger shape with smooth to micro-tooth variation, and the DEN and AC curves are characterized by linear-finger patterns with toothing. Trachyte and trachytic cryptoexplosive breccia are characterized by low GR (45–60 API), high DEN (2.4–2.8 g/cm³), low AC (35–65 μs/ft), and high RT (70–240 Ω·m). Trachyte exhibits box-shaped GR/DEN/AC/RT curves with micro-toothing. In cryptoexplosive breccia, the GR curve has a toothed morphology, while the DEN, AC, and RT curves present finger-shaped patterns with micro-toothing. Andesitic tuff shows moderate GR (60–100 API), lower DEN (2.3–2.5 g/cm³), moderate AC (70–90 μs/ft), and variable RT (0–60 Ω·m). Its GR and DEN curves display linear-finger forms with toothing; the AC curve appears smooth and linear, while the RT curve exhibits box-bell-finger characteristics. Due to the limited occurrences of rhyolite in the study area, no representative logging characteristics were determined in this lithology (Figure 5a,b).

5. Characteristics of Volcanic Facies

5.1. Classification of Volcanic Facies

Based on the volcanic facies classification scheme comprising 5 facies, 15 subfacies, and 44 microfacies [29], 3 facies, 4 subfacies, and 6 microfacies have been identified within the volcanic rocks of the Shimentan Formation in the Xihu Sag (

Table 2. Volcanic facies division scheme of the Shimentan Formation volcanic rocks in the Xihu Sag (modified from [29]).

Facies	Subfacies	Microfacies	Lithology	Fabric and Structural Characteristics
Extrusive	Subaerial lava dome	Core	Lava	Irregular columnar joints in the upper part; regular joints in the mid-lower sections
		In situ autoclastic accumulation	Autoclastic and volcaniclastic breccia association	Massive structure, composed of in situ fragmented material
Eruptive	Pyroclastic flow	Proximal belt	Welded volcaniclastic breccia/tuff	Well-developed cross-bedding and pseudo-flow structures
	Base surge	Proximal belt	Volcaniclastic breccia and tuff	Pseudo-flow structures, parallel bedding, cross-bedding, or graded bedding
		Distal belt	Tuff	Parallel, horizontal, and graded bedding
Effusive	Subaerial lava flow	Simple lava flow	Lava	Stratified stacking with vertical texture zoning: vesicular zone, dense core, and vesicle-rich top; accompanied by columnar and shrinkage joints

).

5.1.1. Extrusive Facies

The extrusive facies are represented by dome-shaped accumulations formed by the extrusion of viscous magma from volcanic vents. These structures are commonly associated with concentric, solidified lava around and above the conduit [29].

In the middle section of the Shimentan Formation, Well L-1 reveals trachyte with consistent “box-shaped, micro-tooth” GR/DEN/RT/AC logging responses, suggesting lithological homogeneity typical of the core microfacies of a subaerial lava dome. In the upper Shimentan Formation of the same well, trachytic cryptoexplosive breccia shows a characterized brecciated texture with “finger with micro-tooth” logging signatures across all curves. This is interpreted as indicative of the in situ autoclastic accumulation microfacies, where breccia originates from the collapse and fragmentation of dome materials near the vent.

5.1.2. Eruptive Facies

The eruptive facies in the Shimentan Formation are primarily composed of pyroclastic flow and base surge subfacies.

The pyroclastic flow subfacies refers to high-temperature density currents composed of volcanic gas and pyroclastic material [30]. In the lower member of the Shimentan Formation, dacitic tuff lava encountered in Well W-1 exhibits welded volcaniclastic textures. Drill core analysis reveals angular dacitic clasts with minor volcanic glass, indicative of deposition by pyroclastic flows transported a moderate distance from the vent, corresponding to the proximal belt microfacies.

The base surge subfacies, a specific type of pyroclastic density current, are further divided into vent-proximal, proximal, and distal microfacies based on textural and depositional features [29]. In Well B-2, coarse-grained rhyolitic tuff breccia displays well-developed “finger with micro-tooth” GR/RT/AC/DEN log signatures, reflecting lithologic heterogeneity typical of tuff-breccia associations, consistent with the proximal belt (Figure 5d). Conversely, Well K-1 contains fine-grained rhyolitic tuff characterized by “linear-tooth” GR and AC, “box with micro-tooth” DEN, and smooth bell-shaped RT curves. These responses indicate lithologic homogeneity associated with distal ash-rich tuff deposits, aligning with the distal belt microfacies.

5.1.3. Effusive Facies

The effusive facies are dominated by highly fluid, subaerial lava flows. The volcanic rocks of the Shimentan Formation are primarily intermediate to acidic in composition and are classified within the simple lava flow subfacies.

In Well K-1, the lower member contains dacite exhibiting well-developed vesicles and prominent flow structures. Logging signatures include a “finger with smooth” GR and DEN curve, a “smooth bell” RT curve, and a linear AC curve. These features reflect textural and compositional heterogeneity, corresponding to the simple lava flow microfacies, characterized by individual, non-compound lava layers.

5.2. Characteristics of Seismic Facies

Interpretation of seismic facies is based on reflection configuration, continuity, frequency, and amplitude—parameters critical for inferring lithofacies and volcanic facies distributions [10,28,31]. Among these, reflection configuration is particularly diagnostic, revealing insights into volcanostratigraphic textures and facies architecture (

Table 3. Seismic facies characteristics of volcanic rocks in the Shimentan Formation, Xihu Sag.

Facies	Geometry	Frequency	Amplitude	Continuity	Reflection Configuration Pattern	Seimic Profile
Base surge	Tabular	High	Medium to strong	Semi-continuous to continuous	Parallel to subparallel reflection
Simple lava flow	Tabular to lenticular	Low	Medium to strong	Semi-continuous	Progradational reflection
Compound lava flow	Tabular to wedge-shaped	Low to medium	Medium to strong	Semi-continuous to discontinuous	Progradational reflection
Lava dome	Mound-shaped	Medium to high	Medium to weak (occasionally strong)	Discontinuous	Chaotic reflection
Volcanic conduit	Pipe-like	Medium to high	Medium to weak (occasionally strong)	Discontinuous	Chaotic reflection

Note: In the seismic profile, red lines denote interpreted faults, the blue line marks the top boundary of the Shimentan Formation, and the green line marks its base.

).

Using lithofacies data from individual wells and well-tie seismic profiles, volcanic facies within the Shimentan Formation were correlated with distinct reflection patterns, informed by analogs from other basins. The seismic characteristics of each facies are summarized as follows:

Base surge: exhibits a tabular geometry with semi-continuous to continuous, high-frequency, medium- to strong-amplitude reflections. The top and bottom boundaries are marked by parallel, strong reflections, with internal parallel to subparallel configurations.

Simple lava flow: shows a tabular to lenticular geometry. The top boundary is marked by convex-upward, discontinuous reflections of medium to strong amplitude; the bottom is slightly concave-downward with similar reflection characteristics. Internally, the flow exhibits progradational, semi-continuous reflections with low-frequency and medium to strong amplitude.

Compound lava flow (effusive facies): displays tabular to wedge-shaped geometry with strong, parallel boundary reflections. Internally, it presents low to medium-frequency, semi-continuous to discontinuous, medium to strong reflection.

Lava dome: characterized by mound-shaped geometry with strong, convex-upward reflections at the top. The internal configuration is chaotic, with medium to high frequency and medium to weak (occasionally strong) amplitudes and discontinuous reflections.

Volcanic conduit: defined by a pipe-like geometry. Its internal seismic configuration closely resembles that of lava domes, with chaotic reflections of variable amplitude and frequency.

6. Characteristics of Volcanic Edifices

6.1. Types of Volcanic Edifices

Numerous classification schemes exist for volcanic edifices. In the context of reservoir prediction, classifications based on the relationship between volcanic edifices and reservoirs—particularly those derived from volcanostratigraphic texture—are more applicable. For example, drilling data from the Songliao Basin suggest that volcanic edifices can be grouped into three types based on differences in volcanostratigraphic textures: stratified, pseudostratified, and massive volcanic edifices [32].

In the Xihu Sag, current drilling data reveal the presence of stratified volcanic edifices (SVE), pseudostratified-massive volcanic edifices (PSMVE), and massive volcanic edifices (MVE). Seismic data further indicate the development of pseudostratified volcanic edifices (PSVE). These types are described in detail below.

6.1.1. Stratified Volcanic Edifice (SVE)

SVEs in the Xihu Sag are primarily formed by the superposition of lava flows and base surge deposits, with lithologies dominated by lava and volcaniclastic rocks. These units exhibit predominantly horizontal to subhorizontal bedding and low aspect ratio geometries that are parallel to adjacent strata. Drilling data indicate that SVEs are mainly composed of tuff associated with base surge deposits, with individual layers averaging approximately 30 m in thickness.

A representative example is the SVEs encountered in Well B-2 within the Shimentan Formation, consisting of approximately 20 m thick dacitic and rhyolitic tuff layers. Logging curves exhibit a box-finger with micro-tooth features in GR and RT curves and a linear-finger shape with micro-tooth features in DEN and AC curves.

6.1.2. Pseudostratified-Massive Volcanic Edifice (PSMVE)

PSMVEs are characterized by the superposition of wedge-shaped, tabular, gentle mound-shaped, lenticular, and wavy lithological units, often intruded by dome-shaped volcanic bodies. Lithologies include lava, volcaniclastic rocks, pyroclastic lava, and volcanic debris flow deposits, spanning effusive (lava flow), eruptive (pyroclastic flow, base surge), extrusive (lava dome), and volcano-sedimentary facies.

The basal portions of PSMVEs exhibit horizontal to subhorizontal bedding, transitioning upward into inclined strata. Lava domes associated with feeder conduits often display cross-cutting relationships with adjacent layers. Adjacent sedimentary units typically intersect the edifice boundaries at low angles, resulting in low aspect ratios.

In the Xihu Sag, drilling reveals that these edifices are primarily composed of base surge-related tuff. Seismic facies are typified by chaotic reflection patterns. In Well K-1, a 140 m thick succession of rhyolitic tuff, andesitic tuff, and rhyolite was encountered. Log responses include linear-finger with micro-tooth in GR curves, linear-finger-box with micro-tooth in DEN curves, smooth linear in AC curves, and box-bell-tooth in RT curves.

6.1.3. Massive Volcanic Edifice (MVE)

MVEs are primarily composed of domal to cylindrical, massive lava units, with minor wedge-shaped and tabular volcanic deposits. These edifices result from the accumulation of lava and pyroclastic rocks and can be divided into two lithofacies associations: (1) extrusive facies (lava dome) with minor contributions from effusive (lava flow), eruptive (pyroclastic flow), or volcano-sedimentary facies; and (2) eruptive facies (volcaniclastic diatremes) combined with volcano-sedimentary deposits. Internally, MVEs exhibit homogeneous textures with indistinct layering and ambiguous bedding orientations. They are marked by high aspect ratios and steep flanks. In the Xihu Sag, these edifices are dominated by intermediate lavas derived from lava domes. For instance, Well L-1 intersected an approximately 150 m thick sequence of trachyte and trachytic cryptoexplosive breccia. Log responses include linear-tooth features in GR curves and linear-finger with micro-tooth characteristics in RT, DEN, and AC curves.

6.2. Seismic Facies Characteristics of Volcanic Edifices

Building upon the seismic facies interpretation of volcanic facies discussed in Section 4.2, this section presents the seismic characteristics of volcanic edifices in the Shimentan Formation, Xihu Sag. Integrating slope angle data and comparative interpretations from volcanic edifices in the Songliao Basin [18], the seismic attributes facies of the four identified edifice types are summarized below:

SVE: SVEs exhibit tabular geometries with low aspect ratios. Their boundaries are delineated by parallel, strong-amplitude, semi-continuous to continuous reflections. Internal reflection configurations are parallel to subparallel and continuous, with medium-to-strong amplitudes, high frequencies, and gentle stratigraphic dips.

PSVE: PSVEs show tabular to wedge-shaped geometries with moderate aspect ratios. Boundary reflections are parallel, strong in amplitude, and semi-continuous to continuous. Internally, they display progradational reflection configurations with medium-to-strong amplitude, low-to-medium frequency, and semi-continuous to discontinuous reflections. Stratigraphic dips are moderate. A regional example of a PSVE is observed in the Xiaxiong Formation (Zhejiang Province), where vesicular rhyolite with approximately 10° dips extends laterally for several kilometers. Although undrilled in the study area, these seismic characteristics are consistent with similar features in the Cretaceous volcanic edifices of the Songliao and Bohaiwan Basins.

PSMVE: PSMVEs exhibit tabular to wedge-shaped (locally mound-shaped) geometries with relatively high aspect ratios. Top boundaries show parallel, strong-amplitude, semi-continuous reflections. Internally, reflection patterns transition from progradational to chaotic, with semi-continuous to discontinuous reflections, medium-to-weak amplitudes (locally strong), and a wide range of frequencies (low to high). Stratigraphic dips range from moderate to steep.

MVE: MVEs are characterized by mound-shaped geometries and high aspect ratios. Their convex tops show strong-amplitude, semi-continuous reflections. Internally, chaotic reflection configurations dominate, with weak to medium amplitudes, discontinuous reflections, and a frequency range of medium to high (rarely low). These edifices are distinguished by steep stratigraphic dips (Figure 6).

7. Interpretation of Volcanic Edifices and Volcanic Facies

Integrating drilling data with seismic attribute analysis, this study interprets volcanic conduits, and associated facies within the Hangzhou Slope belt. Key seismic attributes—including coherence cube analysis, dip angle attribution, and waveform classification—were utilized to support the identification and interpretation of volcanic features, ultimately facilitating the prediction of favorable volcanic reservoir zones.

7.1. Interpretation of Volcanic Conduits

The coherence cube is a geophysical technique that utilizes 3D seismic data volumes to quantitatively assess the similarity of local seismic waveforms and phase characteristics. Regions with low coherence values often correspond to geological discontinuities such as faults, stratigraphic boundaries, or lithological interfaces). As such, coherence slices are particularly effective for highlighting structural features and fracture networks, as well as areas with abrupt lithological changes.

In volcanic environments, lithological complexity and fault density vary spatially. In distal facies—where lithology is relatively homogeneous and tectonic fracturing is minimal—high coherence values typically dominate. In contrast, the cone and crater zones, especially the crater, are characterized by complex lithologies and abundant faulting, resulting in low coherence values. Generally, coherence values decrease toward the crater, a trend that also holds true for the internal stratigraphy.

Modern volcanic systems often exhibit annular and radial fracture patterns around conduits and vents (Figure 7a,b). In horizontal slices of the coherence cube, these features manifest as concentric zones of low coherence values. forming annular or radial configurations (Figure 7c,d). In the Shimentan Formation of the Hangzhou Slope Belt, coherence cube horizontal slices reveal widespread annular and radial patterns of low coherence (Figure 8a), indicating a dense and extensive distribution of volcanic conduit systems (Figure 8b).

7.2. Interpretation of Volcanic Edifices

Dip angle is a key parameter for characterizing stratigraphic architecture and provides a basis for the quantitative or semi-quantitative classification of volcanic edifices. As established in Section 5.2, four types of volcanic edifices—SVE, PSVE, PSMVE, and MVE—can be differentiated based on seismic facies and variations in dip angle. This classification was applied to the interpretation of volcanic edifices in the Hangzhou Slope Belt using seismic dip angle attributes.

It is important to note that zones of high dip angles exhibiting banded patterns in horizontal slices are interpreted as structurally controlled fault planes rather than massive volcanic edifices, despite their steep dips. Interpretation results indicate that PSVEs are the most widely distributed edifice type in the study area, while the remaining three types are more spatially restricted. MVEs, in particular, are mainly concentrated in the central region of the Hangzhou Slope Belt (Figure 9).

7.3. Interpretation of Volcanic Facies

Variations in lithofacies systematically influence seismic waveform characteristics—such as amplitude, frequency, and phase—thereby enabling the prediction of volcanic facies through waveform classification. Building upon the spatial distribution of volcanic edifices discussed earlier, distinct volcanic facies associations have been observed for each edifice type. An integrated interpretation that combines volcanic edifice identification with waveform classification allows for refined seismic facies delineation, establishing a robust framework for volcanic facies interpretation.

Based on waveform classification results, the seismic facies in the study area can be grouped into the following categories: Type I seismic facies are characterized by chaotic reflection zones composed of multiple trace models, corresponding to massive lava dome-dominated volcanic facies. Type II seismic facies occur along the peripheries of Type I domains and volcanic conduits, exhibiting banded to tongue-shaped geometries composed of semi-continuous single or dual trace models. These represent simple lava flow-dominated facies surrounding extrusive features. Type III seismic facies are arranged in lobate stacking patterns aligned with dominant flow directions. These consist of single or dual trace models and correspond to compound lava flows forming stratiform stacking sequences with high azimuthal continuity. Type IV seismic facies manifest as linear belts with high trace density and multiple trace models. These are interpreted as tectonically induced faults planes that postdate volcanic emplacement (Figure 10a). Based on the integrated interpretation of volcanic edifices and seismic facies characteristics, the study is subdivided into genetic zones, including lava dome zones, simple lava flow–lava dome associations, simple lava flow zones, simple–compound lava flow associations, and lava flow–base surge associations. Simple and compound lava flows are widely developed across the Hangzhou Slope Belt, while lava domes are primarily concentrated in its south-central region (Figure 10b).

7.4. Analysis of Reservoir Potential

Large-scale Cretaceous volcanic reservoirs in eastern China are predominantly hosted in acidic volcanic rocks, where effusive facies lava flows dominate the lithofacies, and volcanic edifices are primarily represented by PSVE [4,17,18].

For instance, in the Qingshen Gasfield of the Songliao Basin, rhyolite and rhyolitic pyroclastic lava reservoirs are associated with explosive pyroclastic flows and effusive lava flows, forming PSVE structures [33]. In the Changling–Songnan Gasfield of the same basin, rhyolitic reservoirs exhibit porosities as high as 28% [34], with pore systems consisting of primary vesicles, contractional joints, and secondary dissolution pores—corresponding to compound lava flows within laterally extensive PSVEs. In the Bohai Bay Basin, the BZ8-3S structure hosts an approximately 400 m thick rhyolite reservoir with medium porosity (14%–18%), dominated by effusive facies and PSVE features [8]. Similarly, the LK7-1 structure contains intermediate volcanic reservoirs composed of vertically stacked andesitic lava flows that also form PSVE-type edifices [35].

Drawing from comparative analyses of volcanic facies and seismic reflection characteristics in the Songliao and Bohaiwan Basins, widespread development of PSVEs and effusive lava flows (predominantly rhyolitic) has been identified within the Cretaceous Shimentan Formation of the Xihu Sag, East China Sea Basin. Given the extensive distribution of contemporaneous rhyolite in the Zhejiang–Fujian region, it is plausible that analogous rhyolite also occurs in the Hangzhou Slope Belt of the Xihu Sag. Therefore, the Shimentan Formation in this region is interpreted to hold substantial potential for the development of large-scale Mesozoic rhyolitic volcanic reservoirs, indicating promising prospects for future hydrocarbon exploration.

8. Conclusions

Based on previous exploration studies of volcanic reservoirs in other Chinese basins, this study systematically evaluates the hydrocarbon potential of volcanic units in the Shimentan Formation by microscopic thin section identification and major element analysis; furthermore, it integrates drilling data with seismic interpretation techniques. The main findings are as follows:

(1) The volcanic lithology of the Shimentan Formation in the Xihu Sag is composed of three primary rock types: lava (predominantly trachyte and dacite), pyroclastic lava (mainly dacitic tuff lava), and volcaniclastic rocks (primarily intermediate-acidic tuff). Lithofacies are categorized into three main facies and four subfacies: explosive facies, including pyroclastic flow and base surge subfacies; effusive facies, including subaerial lava flow subfacies; and extrusive facies, including subaerial lava dome subfacies.

(2) Drilling and seismic data have revealed three volcanic edifice types within the Shimentan Formation of the Xihu Sag: stratified volcanic edifice (SVE) (Logging curves exhibit a box-finger with micro-tooth features in GR and RT curves and a linear-finger shape with micro-tooth features in DEN and AC curves; internal reflection configurations are parallel to subparallel and continuous, with medium-to-strong amplitudes, high frequencies, and gentle stratigraphic dips), pseudostratified-massive volcanic edifice (PSMVE) (Log responses include linear-finger with micro-tooth in GR curves, linear-finger-box with micro-tooth in DEN curves, smooth linear in AC curves, and box-bell-tooth in RT curves; internal reflection configurations transition from progradational to chaotic, with semi-continuous to discontinuous reflections, medium-to-weak amplitudes (locally strong), a wide range of frequencies (low to high) and stratigraphic dips range from moderate to steep), and massive volcanic edifice (MVE) (Log responses include linear-tooth features in GR curves and linear-finger with micro-tooth characteristics in RT, DEN, and AC curve; internally, chaotic reflection configurations dominate, with weak to medium ampli-tudes, discontinuous reflections, and a frequency range of medium to high (rarely low). These edifices are distinguished by steep stratigraphic dips). Seismic interpretation further identifies pseudostratified volcanic edifice (PSVE), which display progradational reflection configurations with medium-to-strong amplitude, low-to-medium frequency, and semi-continuous to discontinuous reflections. Stratigraphic dips are moderate), supported by analogs of Late Cretaceous rhyolitic PSVEs observed in the Xiaoxiong Basin of Zhejiang Province.

(3) Through integrated analysis of volcanic facies and edifice characteristics from the Songliao and Bohaiwan Basins and employing seismic attributes such as coherence cube slicing for identifying volcanic conduits, dip angle analysis for classifying volcanic edifices, and waveform classification for delineating volcanic lithofacies. This study delineates favorable distributions of PSVEs and effusive facies—including simple and compound lava flows—within the Hangzhou Slope Belt of the Xihu Sag. These results indicate significant potential for the development of large-scale Mesozoic rhyolitic reservoirs in the study area.