According to the observation of thin sections and the whole-bulk mineral compositions through X-ray diffraction analysis of 193 samples in the study area, significant differences were found in detrital components of the Wenchang Formation in different depressions (Fig. 2). The sandstone reservoirs in the Wenchang Formation of Huizhou Sag are mainly influenced by Mesozoic granites^[13,21], and are dominated by litharenite and feldspathic litharenite with a low content of quartz and high content of lithic and feldspar (Fig. 2a). The debris is composed mainly of granite and quartz lithic, and the feldspar is mainly potassium feldspar. The sandstone reservoir in Wenchang Formation of Lufeng Sag is influenced mainly by Mesozoic sediment provenance ^[22-23], and is composed mainly of subarkose, lithic arkose, sublitharenite, and litharenite with high content of quartz and low content of feldspar and lithic (Fig. 2a). The debris types are mainly quartzite and extrusive rock debris, and the feldspar is mainly potassium feldspar.

According to the data gathered on thin sections and X-ray diffraction of clay minerals, the HZ25 subsag is dominated by a tuffaceous matrix averaging 11.19%, with an average content of clay matrix of 3.88%. The cementation is dominated by authigenic kaolinite and carbonate cements, with a small amount of authigenic quartz (Fig. 2b). The content of tuffaceous matrix in the HZ26 subsag is relatively low, with an average of 6.13%, with a clay matrix of 3.24%. The cementation is dominated by laumontite, with a small amount of authigenic quartz and carbonate cements (Fig. 2b). The average content of tuffaceous matrix and clay matrix in LF13 subsag is 8.33% and 3.27%, respectively (Fig. 2b). The content of tuffaceous matrix in Well LF-13-A is relatively high, and authigenic kaolinite, apatite and authigenic quartz are developed, while in Well LF-13-B the content of interstitial fillings in Well LF-13-B is generally low, mainly composed of authigenic quartz. The LF15 subsag is dominated by tuffaceous matrix with an average content of 6.79%, and the average amount of clay matrix is 2.88%. Authentic quartz is relatively well developed, with low content of the other types of cements (Fig. 2b).

Compared with the clay matrix, the tuffaceous matrix has finer particle size and is often in the form of irregular clumps, without luminance under orthogonal light (Fig. 3a, 3b). The tuffaceous materials without obvious dissolution in the study area show intensive compaction, and only micropores with pore size less than 1 μm could be found (Fig. 3c). In the HZ25 subsag, the unaltered tuffaceous materials are glassy, and part of the tuffaceous materials show the characteristics of alteration caused by diagenetic fluid (Fig. 3a, 3b). It is brown-yellow under single polarized light, and has the characteristics of alteration products under orthogonal light (Fig. 3b). The unaltered tuffaceous materials in the HZ26 subsag are also glassy, but accompanied by the precipitation of iron minerals (Fig. 3d). The tuffaceous materials in LF15 and LF13 subsags have similar characteristics and are prone to be dissolved along the edge of the dissolution pore (Fig. 3e to 3h). It is difficult to classify the types of tuffaceous materials accurately based on observation of microscopic characteristics only. Therefore, the tuffaceous materials without obvious dissolution or alteration in each subsag were selected to compare their elementary compositions; the TAS discrimination diagram and the inactive trace element ratios (Zr/TiO₂ and Nb/Y) were used to classify the types of tuffaceous material ^{[24⇓⇓-27]} (Fig. 4). A total of 54 major-element data and 33 trace-element data were obtained (Table 1).

The tuffaceous materials in the HZ25 subsag are mainly distributed in the rhyolite-rhyodacite area, which indicates the source of acidic magma ^[25] (Fig. 4). They are characterized by a high content of silica and low content of alkalinity ^[25], with an average SiO₂ content of 67.93% and (Na₂O+K₂O) content of 3.38%, relatively rich in potassium (Table 1). The tuff in the HZ26 depression is mainly distributed in the trachyandesite-trachyte area, which indicates the source of basic magma ^[25] (Fig. 4). It is characterized by low content of silica and high content of alkalinity, with an average SiO₂ content of 48.29% and (Na₂O+K₂O) content of 6.47%, relatively rich in sodium (Table 1). Tuffaceous materials in the LF15 and LF13 depressions are mainly distributed in the Andesite-Basaltic Andesite area, which indicates the intermediate magma source (Fig. 4), with an average SiO₂ content of 59.02% and (Na₂O+K₂O) content of 3.76%, relatively rich in potassium (Table 1). However, some tuff samples from well LF-13-A in the LF13 depression are distributed in partial alkalinity areas such as the trachyandesite and basaltic trachyandesite areas (Fig. 4), with an average SiO₂ content of 57.16% and (Na₂O+K₂O) content of 8.39%, relatively rich in sodium. The relatively high CaO and P₂O₅ contents, which average 3.25% and 8.13%, respectively (Table 1), also indicate the alkaline magma source.

The sandstone reservoirs in the Wenchang Formation of Huizhou Sag and Lufeng Sag are mainly in the middle diagenetic stage A1 to middle diagenetic stage A2, stages that were dominated by the dissolution of acidic fluids ^[9,13,28]. Previous studies have shown that the Eocene in Zhu I Depression experienced different degrees of uplift and denudation during the early sedimentary stage, which was the main period of meteoric water dissolution ^[13-14,18]. Meanwhile, the concentration of atmospheric carbon dioxide was relatively high owing to intense volcanic eruptions ^[29], which provided stable acidic fluid for the dissolution of tuffaceous material in sandstone reservoirs of the Wenchang Formation. Later, in the deep burial process of Eocene, the infiltration depth of acidic meteoric water was limited, so it no longer acted on the reservoir to cause dissolution ^[13]. With an increase of burial depth and of temperature, organic acids generated by kerogen thermal evolution together with CO₂ generated by decarboxylation of organic acids entered the reservoir, providing the source of acidic fluid dissolution of tuffaceous materials ^[13,30]. Microscopic characteristics show that there were relatively few dissolution pores developed in the acid tuffaceous materials in the HZ25 subsag, which were dominated by dissolution expansion along the edges of particles or tuffaceous micro-fractures (Fig. 5a). Also, it was hard to generate large-scale and effectively connected intergranular dissolution pores for the acid tuffaceous materials, and strong alteration often occurred near the dissolution pores (Fig. 5b). The dissolution of basic tuffaceous materials in the HZ26 subsag was relatively strong and the intergranular tuff dissolution pores were relatively developed with less residual tuffaceous materials (Fig. 5c, 5d). However, the dissolution pores in intermediate tuff in the LF15 subsag and Well LF-13-B were filled with a large amount of residual tuff (Fig. 5e, 5f). The alkaline tuffaceous materials in Well LF-13-A could be strongly dissolved to form intergranular dissolution pores, often accompanied by the precipitation of rod-like apatite or columnar apatite (Fig. 5g, 5h).

In order to quantitatively characterize the relationship between dissolution of tuffaceous materials and alteration products of the Wenchang Formation in the study area, image analysis software was used to calculate the content of tuffaceous dissolution, the areal porosity of dissolution pores, and the contents of authigenic kaolinite, quartz, laumontite, and apatite for different types of tuffaceous material through thin sections (Fig. 11). Chlorite and illite are not discussed in this study because, due to their formation, they are difficult to calculate. With an increase of acid tuff dissolution, the content of authigenic kaolinite increases monotonically. The resulting areal porosity of the tuffaceous dissolution pores were generally less than 2%, and those contents were relatively high only in the zone with relatively low content of kaolinite (Fig. 11a). When the dissolution content of basic tuff was less than 4%, the authigenic mineral contents were relatively low, and intergranular dissolution pores developed. With an increase of the tuffaceous dissolution amount of basic tuff, the content of laumontite was positively correlated with the dissolution amount of basic tuff. However, due to the dissolution of laumontite, the contents of laumontite in some areas with high tuff dissolution content decreased (Fig. 11b). The intermediate tuff dissolution products were mainly authigenic quartz, which did not completely fill the dissolution pores, so the areal porosity of tuffaceous dissolution pores showed a positive correlation with the tuffaceous dissolution amount (Fig. 11c). There was also a positive correlation between the contents of apatite and the contents of alkaline tuff dissolution. Some apatite precipitated along the edge of the tuffaceous dissolution pores, leading to a significant increase of areal porosity of tuffaceous dissolution pores and the apatite contents in areas with high tuffaceous dissolution content (Fig. 11d).

The intensity of tuffaceous dissolution and the differentiation of alteration products were controlled by the type and chemical properties of tuffaceous material ^[5,9]. Combined with the diagenetic evolution of the reservoir in the study area, the evolution pathways of different types of tuffaceous dissolution were established (Fig. 12). Among them, weak dissolution showed itself in the acid tuff, mainly along the microfracture of tuff and along the rims of grains (Figs. 3a and 5a), and the tuffaceous dissolution pores were relatively isolated (Fig. 5a). In the process of dissolution, the tuff could be altered into kaolinite ^[5] (Figs. 6a, 6b, and 12) and precipitated microcrystalline quartz. With an increase of burial depth, the residual tuffaceous materials could change into illite (Figs. 6d and 12). The basic tuff was more likely to form tuffaceous dissolution pores ^[5,9,25] (Fig. 5c, 5d). With the dissolution of tuff, the residual tuff was changed into chlorite, with the precipitation of authigenic microcrystalline quartz (Figs. 6c and 12). In addition, plate-like or strip-like laumontite filled the dissolution pores of tuff (Fig. 6i), or appeared in the form of porous tight cementation (Figs. 6j and 12). With an increase of burial depth and of temperature, unstable minerals such as laumontite dissolved to form dissolution pores ^[4,34] (Figs. 6k and 12), and the remnants of chlorite and intergranular tuffaceous dissolution were also changed into illite (Fig. 6e). The dissolution evolution pathways of intermediate tuff are similar to that of acidic tuff (Fig. 12), but the dissolution was stronger, the contents of alteration products were less, and the tuffaceous dissolution pores were relatively developed (Fig. 5e, 5f). Alkaline tuff was dissolved into pores by acidic fluid ^[24,37] (Fig. 5g, 5h), and released active elements like Ca and P, resulting in the precipitation of early stages of granulated apatite aggregates and authigenic quartz along the edge of tuffaceous dissolution pores (Figs. 6h, 6m, 6o, and 12). With the continuous reaction of the fluid, the dissolution expansion of tuffaceous dissolution pores and the early stage of apatite occurred ^[37], and the needle rod-like late apatite were precipitated along the dissolution pores (Figs. 6m and 12). The alkaline tuff in some layers dissolved, and the tuffaceous dissolution residue underwent strong alteration with the precipitation of late-stage short columnar apatite crystals (Fig. 6p).

The evolution pathways of dissolution of tuffaceous materials can be divided into three types, which have different effects on the physical properties of reservoirs.

(1) Type I is the evolution pathway with strong tuffaceous alteration, and weak dissolution of the alteration products. Taking acid tuff in the HZ25 subsag as an example: It does not readily dissolve or form effectively connected dissolution pores ^[5] (Fig. 5a, 5b). With increasing amounts of dissolution of tuffaceous material in the reservoir, abundant kaolinite was precipitated nearby (Fig. 11a). The tuffaceous dissolution pores were transformed into intercrystalline pores of kaolinite, and were compacted during burial evolution (Fig. 6a, 6b), which not only failed to improve the reservoir properties, but were also not conducive to the subsequent dissolution of acidic fluids ^[5,31]. In addition, the kaolinite caused by alteration did not readily dissolve again, but plugged the pore throat because it would alter into illite ^[31] (Fig. 6d). Overall, in reservoirs bearing acid tuffaceous matters, the dissolution process of this material is dominated by strong alteration, and the precipitation of alteration products is not conducive to the improvement of reservoir physical properties.

(2) Type II follows an evolutionary pathway with strong dissolution of tuffaceous materials, and strong alteration of the residual. Taking intermediate tuff in Well LF-13-B and Well LF-15-A as examples, under the influence of acidic fluid, the intermediate tuffaceous materials dissolved and formed dissolution pores ^[5] (Figs. 3e and 5e). The dissolution byproducts in the reservoirs were mainly authigenic quartz in relatively small amounts (Figs. 6g and 11c); a large number of tuffaceous dissolution pores were still retained. However, due to the incomplete dissolution of tuffaceous material, the residual remained in the dissolution pores and was altered into illite (Figs. 5f, 6e and 6g), which led to the obstruction of pore-throat connectivity. The alkaline tuffaceous materials were also strongly dissolved in Well LF-13-A, where short columnar apatite crystals along the dissolution pores were precipitated (Figs. 5h and 6p) or, in the form of needle rod-like apatite crystals, filled the dissolution pores. Those apatite crystals braced each other like a frame (Figs. 5g and 6o), and could retain some tuffaceous dissolution pores, but it was not conducive to an increase of reservoir permeability. Therefore, although the tuffaceous dissolution caused by intermediate and alkaline tuff could effectively increase porosity, it was difficult to increase reservoir permeability.

(3) Type III is the evolution pathway with strong dissolution and alteration of tuffaceous material, and strong dissolution of the alteration products. In the reservoirs bearing basic tuffaceous materials in HZ26 subsag, the dissolution pores of tuffaceous materials were formed easily (Figs. 5c, 5d and 11b), and precipitated chlorite and laumontite (Fig. 6c, 6i-k). The precipitation of laumontite decreased the compaction in the burial process to some extent, and with the increase of burial depth, acidic fluid could dissolve laumontite again to form different scales of intergranular dissolution pores ^[4,34] (Fig. 9k). Therefore, the dissolution of the basic tuffaceous material could not only form a large number of intergranular dissolution pores, but could also dissolve the alteration products again, which significantly improved the quality of the reservoir.

Combined with the physical characteristics of sandstone reservoirs in the Wenchang Formation of the study area, the tuffaceous dissolution pathway of Type III gives the most obvious improvement of reservoir physical properties. Observed mainly in the sandstone reservoirs in the HZ26 subsag with the enrichment of basic tuffaceous material, the average porosity is 14.88% and the average permeability is 24.63×10^-3 μm² (Fig. 13 and Table 2). The dissolution of tuff and laumontite in the reservoirs provided fine pore throat connectivity.

The second best is the tuff dissolution pathway of Type II, which developed mainly in the LF13 subsag and LF15 subsag with the enrichment of intermediate or alkaline tuff in the reservoirs. The average porosity is 13.85%, and the average permeability is 2.69×10^-3 μm² (Fig. 13 and Table 2). The intergranular tuff dissolution pores developed and were not fully filled by the authigenic minerals. Therefore, the dissolution of tuff could effectively connect the intergranular dissolution pores and primary pores to improve the quality of reservoirs. However, the permeability of the reservoir that underwent this type of tuff dissolution pathway is relatively low due to the blockage of pore throats by the residual of tuff dissolution, authigenic quartz, and apatite. Moreover, a large amount of authigenic kaolinite was deposited in the reservoir during burial evolution due to the tuff dissolution pathway of Type I. The alteration byproducts did not readily redissolve and the intergranular pores could be destroyed by the late compaction during the deep burial process. The reservoirs that underwent this kind of tuff dissolution could not significantly improve the reservoir physical properties; the average porosity is 11.37% and the average permeability is 4.13×10^-3 μm² (Fig. 13 and Table 2).