Fluid evolution and related fluid–rock interactions of the Oligocene Zhuhai sandstones in the Baiyun Sag, northern margin of the South China Sea

Pore fluids control the diagenetic processes and storage spaces of deep clastic rock reservoirs and have become a major area of interest within the fields of sedimentology and petroleum geology. This paper aims to relate the diagenetic processes of the Oligocene Zhuhai sandstones in the Baiyun Sag to pore fluids varying with burial depth. The types and distribution patterns of authigenic minerals are investigated through analysis of petrographic, mineralogical, and geochemical features to illustrate the origin and flow patterns of pore fluids and their influences on reservoir diagenesis. Strong cementation of eogenetic carbonate cement near the sandstone–mudstone interface was a consequence of material migration from adjacent mudstones on a large scale. The pore fluids were mainly affected by microbial methanogenesis and carbonate mineral dissolution in adjacent mudstones during eogenesis. The pore fluids were diffusively transported in a relatively open geochemical system within a local range. Support for this model is provided by the heavier stable isotopic values present in eogenetic calcite and dolomite. Feldspar dissolution during early mesogenesis was spatially accompanied by the precipitation of authigenic quartz and ferroan carbonate cement. Pore fluids in this period were rich in organic acids and CO2, and their migration mechanism was diffusive transport. The obviously lighter carbon and oxygen isotopic compositions of the ferroan calcite support this inference. During late mesogenesis, the input of deep hydrothermal fluid might have been partly responsible for the precipitation of ankerite, barite and authigenic albite. Oil charging may have inhibited carbonate cementation and compaction, accordingly preserving porosity, and together with authigenic kaolinite, might have promoted the transition of the reservoir from water wet to oil wet to the benefit of oil entrapment. The findings reported here shed new light on the evaluation and prediction of sandstone reservoirs that have experienced multiple periods of fluid flow.


Geological setting
The Baiyun Sag is located on the northern margin of the South China Sea and covers an area of 1.2 × 10 4 km 2 with water depths of 200 m to 2000 m (Fig. 1a).As a Cenozoic extensional basin, the sag can be divided into a synrift stage between the Eocene and early Oligocene (65-32 Ma) and a postrift stage between the late Oligocene and the present (32-0 Ma) (Fig. 2) 27 .The study area, as one of the most hydrocarbon-rich deep-water areas, lies in the southeastern Baiyun Sag (Fig. 1b).
The sag is filled with Cenozoic sediments and, in ascending order, consists of the Palaeogene Shenhu, Wenchang, Enping and Zhuhai Formations; the Neogene Zhujiang, Hanjiang, and Yuehai Formations; and Quaternary groups (Fig. 2).The evolution of the synrift stage was mainly affected by the Zhuqiong movement and the Nanhai movement and can be subdivided into three stages.The studied section, i.e., the Oligocene Zhuhai Formation, was deposited in the late synrift stage and is mainly composed of Palaeo-Pearl River shelf-margin deltaic sandstones 23 .
The tectonic activity was relatively quiescent during the Zhuhai depositional stage.Shelf breaks stably developed in the southern deep-water area of the sag.Fluctuations in sea level and salinity were frequent and rapid.The Baiyun Sag was characterized by a wide distribution of oscillatory continental-marine transition sedimentary environments.Large-scale grey-greyish white sandstones interbedded with brown-dark grey mudstones developed in the sag, forming a laterally extensive Palaeo-Pearl River shelf-margin delta system.The sedimentary facies were dominated by subaqueous distributary channels, mouth bars and sheet sands in the southern deepwater area of the sag and inter fingered with more subaqueous inter distributary bay and pro delta argillaceous deposits.The thick, organic-rich mudstones are important potential hydrocarbon source rocks for the intercalated thin sandstone reservoirs 27 .

Samples and methods
Sixty-five core samples with burial depths of 800-3000 m were collected from five wells (Fig. 1).All samples were obtained from delta deposits with various thicknesses.To analyse the underlying correlation between reservoir quality and distance from the sandstone-mudstone interface, samples were selected from different positions within one bed.A total of 210 core analysis data points of Zhuhai sandstones were taken to evaluate porosity and permeability.
Point counting was performed on 116 thin sections of Zhuhai sandstone in the southeastern Baiyun Sag in this study to estimate the modal composition.Thin sections were impregnated with blue epoxy under vacuum to identify visual porosity and stained with potassium ferricyanide and Alizarin Red S to aid in distinguishing carbonate.A total of 300 points were counted for each thin Sect. 28.For the accurate estimation of cement and pore contents, 20 micrographs per thin section were first obtained under a Zeiss microscope, and the targets per micrograph were then sketched and calculated using the AxioVision software Rel and Image-Pro Plus software.Finally, the average values of each target area in all micrographs were obtained, which can be regarded as the contents of pores and cement.
All 65 samples were prepared for mineralogical X-ray diffraction (XRD) analysis using an Ultima IV X-ray diffractometer.Samples were X-rayed, centrifuged, glycolated, and heated to 550 °C.XRD analysis was based on the procedure used by 29 within an error range of 10%.After detailed petrographic analysis, seven representative samples were observed using cathodoluminescence microscopy (CL) to distinguish multiphase carbonate cement.For the spatial morphology of authigenic mineral identification, 22 gold-coated samples were observed under a ZEISS EVO LS15 scanning electron microscope (SEM) equipped with an energy-dispersive spectroscopy X-ray microanalyzer.
Thirty organic matter-free sandstone samples were prepared based on petrological studies for carbon and oxygen isotope measurements.To target specific carbonate cement generations, microsamples (0.35-0.45 mg) of different types of cement were drilled from thick petrographic sections using a microscope-mounted dental drill.Each sample was reacted with 100% ortho-phosphoric acid at 70 °C for 4 to 8 h.The carbon and oxygen stable isotope values were obtained from the CO 2 liberated from the carbonate cement samples using a Thermo-Finnigan MAT 253 IRMS.The measurement precision was ± 0.014‰ for oxygen and ± 0.02‰ for carbon.The stable isotope data were reported in δ notation relative to the Pee Dee belemnite (V-PDB).δ 18 O VPDB values were converted to δ 18 O VSMOW (Vienna standard mean ocean water) values using the equation δ 18 O VSMOW = 1.03091 × δ 18 O VPDB + 30.91 30 .
Seven core samples were prepared as doubly polished fluid inclusion wafers for microthermometric measurement.Microthermometry was conducted using a calibrated LINKAM THMSG600 stage.The homogenization temperature (Th) was obtained by cycling.Th values were measured using a heating rate of 10 °C/min when the temperature was lower than 80 °C and a rate of 5 °C/min when the temperature exceeded 80 °C.The measured temperature precision for Th was ± 1 °C.All the analyses mentioned above were performed in the Key Laboratory of Petroleum Resources Research, CAS (Lanzhou).
Feldspar dissolution.Partial to complete feldspar grain dissolution is very common in the Zhuhai sandstones, and most dissolved grains are K-feldspar and locally plagioclase.Feldspar grains preferentially dissolved along the cleavage, revealed as irregular dissolution edges and feldspar residue (Fig. 4a,b,d).Some feldspar grains were almost totally dissolved, forming mouldic pores (Fig. 4c).Spatially, feldspar-dissolution pores are accompanied by pore-filling authigenic clays, albite and quartz cements (Fig. 4c and e).The authigenic albite presents as columnar aggregates of euhedral crystals, and the elongated crystals are parallel to the cleavage of the dissolved K-feldspar (Fig. 4f).The content of dissolved feldspar is 0.1-3.1%, with an average of 1.26%.The quantity of dissolution pores marginally increases with burial depth (Fig. 5a); moreover, there is a marked increase in sandstones more than 1 m away from the sandstone-mudstone contact within a bed (Fig. 6a).
The carbonate cement content is commonly observed to be higher in sandstones less than 1.0 m away from the sandstone-mudstone interface (Fig. 6b), forming sandstones tightly cemented by calcite and dolomite filling all the intergranular pores (Fig. 7).
On the basis of the isotopic analysis, the calcite cements have δ 13 C values between − 0.3 and + 2.51‰, and δ 18 O values range between − 11.27 and − 8.28‰.The dolomite cements have a range of δ 13 C values (− 0.76‰ to + 2.12‰) and δ 18 O values (− 11.24‰ to − 8.49‰) that are similar to those of calcite.In contrast, the ferroan calcite cements have significantly lighter δ 13 C values (− 24.42‰ to − 4.19‰) and slightly lighter δ 18 O values (− 16.63‰ to − 13.67‰) (Table 1).For the ankerite cements, the δ 18 O values do not vary much, ranging between − 18.05 and − 12.06‰, but the δ 13 C values are significantly heavier (− 7.72‰ and − 1.02‰; Table 1, Fig. 8) than those of the ferroan calcite.syntaxial overgrowths, small prismatic euhedral crystals, and microfracture-filling quartz cements (Fig. 9).Two stages of quartz overgrowth with thicknesses varying from 2 to 60 μm could be locally identified (Fig. 9a,b,e).The first stage of quartz overgrowth (Qa1) was enclosed or engulfed by ankerite and ferroan calcite cements (Fig. 7d), indicating that ankerite and ferroan calcite cementation occurred after Qa1.The quartz cement content is relatively low and stable, making up less than 1% (Fig. 5b), and has a positive correlation with feldspar dissolution porosity (Fig. 6c).Aqueous inclusions are primarily present in the microfracture-filling quartz cements (Fig. 10a) and partly in the quartz overgrowths (Fig. 10b), with a few in carbonate cements.These aqueous inclusions are commonly two-phase liquid-vapour inclusions with a diameter range of mainly from 3 to 9.5 μm.The Th values of aqueous fluid inclusions range from 77.5 to 125 °C in the quartz overgrowths and from 94.3 to 146 °C in the microfractures (Table 2, Fig. 11).Only two Th values of aqueous fluid inclusions were measured in ankerite, 113.7 °C and 124.3 °C (Fig. 11).According to measured Th values, two stages of quartz overgrowth were confirmed.The mean Th value of aqueous inclusions in Qa1 is 89.8 °C (range 77.5-107.9°C), and that in Qa2 is 114.3 °C (range 104.5-125 °C) (Table 2).
Clay and other minerals.The main authigenic clay mineral types are kaolinite and illite in the studied interval, followed by illite/smectite mixed layer and chlorite.Kaolinite fills primarily primary pores as well as feldspar-dissolution pores and generally occurs in the forms of vermicular pseudohexagonal and euhedral booklet aggregates (Fig. 4b and c).The kaolinite aggregates consist mainly of thin, closely associated platelets with obvious intercrystalline microporosity (Fig. 9e and f).Some kaolinite crystals feature fibrous edges due to illitization (Fig. 9g).The honeycomb-textured illite-smectite mixed layer features schistose crystals with a rolled border approximately 5 to 10 μm in size under SEM.Fibrous and flaky illite can also be observed in primary pores and dissolved pores and locally on grain surfaces (Fig. 9g).Similar to the quartz cement, a positive relationship also exists between the abundance of clay minerals and feldspar-dissolution porosity (Fig. 6d).
Based on XRD analysis of the clay fraction (< 2 μm) of sandstones, kaolinite is the major clay mineral and is dominant at depths above 2000 m, decreasing sharply in abundance below this depth, particularly below approximately 2500 m.Illite is dominant at depths greater than 2000 m (Fig. 12).Additionally, the contents of the illite/smectite mixed layer and chlorite have wide ranges and show marked increases from 1900 to 2100 m and from 2250 to 2350 m, respectively (Fig. 12).
Barite can be identified as a minor diagenetic mineral with small amounts of less than 1%.Based on the SEM-EDS analysis, irregularly shaped barite (BaSO 4 ) commonly fills the gaps among clastic grains in the Zhuhai sandstones (Fig. 9h and i).

Discussion
Burial and thermal history.The Oligocene Zhuhai sequence experienced stable and persistent subsidence after deposition (Fig. 13).The overlying strata of the Zhuhai layer include up to 2200 m of deep-sea gravity flow and muddy sediments, and the average water depth at present is approximately 1450 m 27 .The Baiyun Sag is a typical hot basin, as demonstrated by previous studies, with present-day heat flow values of 24.2 to 121.0 mW/m 233 .The geothermal gradient has a steady rise from northwest to southeast in the sag, which can be up to approximately 55 °C/km for the study area 34 .This discrepancy could be due to the southward transition of the spreading ridge and crustal thinning caused by the Baiyun movement since 23.8 Ma and neotectonics since ca.13.8 Ma, along with continual deep thermal fluid upwelling in the southeastern Baiyun Sag 35 .
The average subsidence depth of the Zhuhai Formation is approximately 3600 m.The present-day maximum burial depth is approximately 3000 m, with a maximum temperature of approximately 140 °C36 .Overpressure Diagenetic sequence.Paragenetic sequence during eogenesis.A temperature of 70 °C is generally regarded as the dividing line between eogenesis and mesogenesis 8 .Compaction is the dominant eogenetic process.The precipitation of calcite and dolomite is common, especially in sandstones less than 1.0 m away from the sandstone-mudstone interface (Fig. 6b).Siderite is also demonstrated to be an eogenetic product in view of the filling relationships of these cements and primary pores between uncompacted framework grains (Fig. 7f).
According to the latitudinal gradient of the δ 18 O value of modern-day global precipitation 38 , the δ 18 O SMOW values of the sedimentary water in the Baiyun sag can be considered − 5‰ (ranging from 0 to − 10‰) for the palaeolatitude of the sag at approximately 20°N.This value represents the δ 18 O SMOW value of the pore fluid from which carbonate cements precipitated during eogenesis.On the basis of the fractionation equations of the oxygen isotopes for calcite-water 39 and dolomite-water 40 , the calculated formation temperatures are between 29 and 45 °C for the calcite cements and between 59 and 80 °C for the dolomite cements (Table 2).Paragenetic sequence during mesogenesis.Further compaction and feldspar dissolution were the main mesogenetic processes in the Zhuhai sandstones.The secondary pores generated from the dissolution of feldspars are observed to be mostly filled with diagenetic byproducts, such as clay and quartz cements (Figs. 4 and 5) 23,41 .The point and linear contact between detrital minerals indicates that minimal pressure dissolution has occurred in the Zhuhai sandstones.Pressure dissolution may have limited significance for quartz overgrowth 42 .The aqueous inclusions in the quartz overgrowths have a continuous Th distribution (70 °C to 130 °C) (Table 1), indicating continuous development of these cements 43 .The formation temperatures are 77.5-107.9°C and 104.5-125°C for Qa1 and Qa2, respectively.It is inferred that feldspar dissolution and clay mineral transformation acted as an internal silica source for quartz cementation.
Petrological evidence, such as ferroan carbonate cements commonly filling feldspar-dissolution pores, suggests that these cements probably postdate the feldspar dissolution.The δ 18 O value of pore fluid becomes heavier with increasing temperature due to isotopic modification by feldspar alternation and other fluid-rock interactions 12,44 .The δ 18 O SMOW values of the pore fluid during mesogenesis were assumed to be − 5‰, − 2‰, and 0‰ 44 .Using a δ 18 O SMOW value of − 2‰ and the fractionation equations of oxygen isotopes for calcite-water 39 and dolomite-water 45 , the precipitation temperatures are calculated to be 79-101 °C for the ferroan calcite and 91-152 °C for the ankerite (Table 2).The measured Th of aqueous fluid inclusions in the ankerite cements (113.7 °C, 124.3 °C, Table 1) are within the results calculated for the ankerite.Therefore, it is reasonable to  assume that the pore fluid had a δ 18 O SMOW value of − 2‰ when the ferroan carbonate cements precipitated during mesogenesis.Hydrocarbon inclusions are pervasively developed and are closely associated with the coeval aqueous inclusions in quartz microfractures in the Zhuhai sandstones (Fig. 10a).The Th of the coeval aqueous inclusion is regarded as the closest equivalent to the trapping temperature of the coexisting hydrocarbon inclusions, and it ranges from 94.3 to 146 °C, as previously stated.This result agrees with the previous detailed work of 46 , which showed that two periods of oil charge exist in the study area, occurring at 13.1-7.3Ma and 5.5-0 Ma 46 .The first period of oil charge postdated the dissolution of feldspar and antedated or was synchronous with the precipitation of mesogenetic ferroan calcite.The second period of oil charge postdated the late ankerite cementation (Fig. 13).
Figure 13 illustrates the paragenetic sequence of the main diagenetic events of the Zhuhai sandstones.However, not all reservoirs experienced the entire paragenetic sequence mentioned above 47 .Parts of reservoirs, especially thin beds or the marginal parts of thick beds (< 1.0 m), were rich in early calcite and dolomite cements during eogenesis, even filling all the intergranular spaces and becoming tight reservoirs, and hardly any other diagenetic alteration occurred (Figs. 13a and 14a).The middle parts of thick beds (> 1.0 m), by contrast, experienced  www.nature.com/scientificreports/complex diagenetic histories, mainly including compaction, weak early carbonate cementation, and relatively strong dissolution of feldspar (Fig. 14b,c).Subsequently, reservoirs without the early oil charge experienced strong cementation of the late carbonate, whereas the charging of late oil slowed late carbonate cementation to a certain extent (Fig. 13(b) and 14(b)).For reservoirs with the early oil charge, the selective early oil charge affected the path of diagenetic evolution; in particular, it significantly hindered late carbonate cementation.This resulted in the alteration of the wettability from water wet to oil wet.This aided the second period of oil charge (Figs.13b and 14b).The eogenetic products record early burial fluids during the precipitation process and the climatic conditions during deposition 8,23 .The rapid subsidence of the studied interval suggests that the pore fluids during eogenesis were not significantly influenced by meteoric water.In addition, meteoric water has a δ 13 C value of − 7‰, which is much lower than the δ 13 C values of the early-formed calcite cements (− 0.76‰ to + 2.51‰), indicating that it contributed little to the precipitation of these cements.The greater precipitation of calcite and dolomite in sandstones less than 1.0 m away from the sandstone-mudstone interface (Fig. 6b) indicates that interbedded mudstones might have been a crucial source of the early-formed carbonate cements in the sandstones.The interbedded shelf mudstones contain a considerable amount of organic matter (TOC values range from 0.66 to 1.47%, average 1.08%) 48 .The bicarbonate species www.nature.com/scientificreports/related to microbial methanogenesis of organic matter (δ 13 C up to + 8‰) might have been an important carbon source for the early-formed calcite and dolomite.This is supported by the relatively positive δ 13 C values (− 0.76‰ to + 2.51‰) of these cements (Fig. 14a) 49 .In addition, the mudstones in the Zhuhai Formation are high in detrital carbonate minerals (over 15%) 50 .These minerals have δ 13 C values that vary from 0 to + 5.9‰ 51 .The dissolution of the carbonate minerals in adjacent mudstones could thus be another carbon source 52 .Additionally, the Ca 2+ and Mg 2+ ions dissolved in the mudstone were probably transported to sandstone as well.
During this stage, pore fluids from the mudstones interbedded with the sandstones were mainly driven by compaction (Fig. 14d).Large-scale mass transfer occurred from the mudstones to adjacent sandstones, implying that a relatively open diagenetic environment occurred within a local range (e.g., several metres).The rapid subsidence and extensive mechanical compaction of the mudstones during eogenesis may have expelled fluids into adjacent sandstone reservoirs 53 .However, much of the research to date has demonstrated that the rate of fluid flow from mudstones to sandstones is rather low, only several millimetres per year 4,54 .Thus, advection and convection likely made an insignificant contribution to mass transport.With the dissolution of carbonate minerals, microbial alteration of organic matter in mudstones alters the ion concentration of pore fluids, creating steep diffusion gradients.The diffusive transport of dissolved ions from mudstones to sandstones may have had a great impact on the distribution pattern of the early carbonate cements.Similar conclusions have been reached in other sedimentary basins 8,54,55 .
Fluid flow and related fluid-rock interactions during mesogenesis.Due to thermal maturation of kerogen (temperature > 70 °C), a chemical gradient was formed between the source rock and adjacent sandstones.Organic CO 2 and acids were transported via diffusion.This resulted in a certain amount of feldspar dissolution 56,57 .However, the dissolution of feldspar barely occurred near the edge but rather occurred in the middle part of the sandbodies (Fig. 6).The most likely cause is strong carbonate cementation near the sandstone-mudstone interface during diagenesis, resulting in tight layers forming along the sandbody edges, which control the transport of pore fluids rich in organic CO 2 and acids, crossing the sandbody edge and reaching the porous zone in the centre of the sandbody (Fig. 14b).
The solubility of silica is extremely low 42,54 .Together with the relatively limited flux of formation water, which is difficult over long distances and in large quantities, the distribution patterns of quartz cement indicate little or no external silica involvement (Fig. 6) 54 .The intimate association between diagenetic mineral assemblages (Fig. 6) suggests that the silica source of quartz cement is closely related to feldspar dissolution.This is also supported by the precipitation temperature zone of Qa1 (77.5 °C to 107.9 °C, average 89.8 °C).Thus, the dissolution of feldspar may have served as an important silica source for Qa1 34,42,58 .As mentioned before, kaolinite is mainly stable above 2000 m (temperature < 100 °C), and the rapid and mass illitization of kaolinite occurs in the sandstone below 2000 m, especially between 2200 and 2500 m (with temperatures from 100 to 120 °C).The formation temperature of Qa2 ranges from 104.5 to 125 °C, with an average of 114.3 °C, coinciding with the optimal temperature zone of the illitization of kaolinite.It seems highly likely that the illitization of kaolinite was a major source of silica for Qa2 in the study area.
The relatively negative δ 13 C values (− 24‰ to − 1‰) of the ferroan calcite suggest an organic source from adjacent mudstones.
(δ 13 C VPDB from − 25 to − 10‰) (Table 2) 49 .In addition, the precipitation of ferroan carbonate replacing nonferroan carbonate can be caused by abundant organic CO 2 that can dissolve some early-formed carbonate.Thus, the ferroan carbonate precipitation during early mesogenesis was dominated by a mixture of carbon sources from the decarboxylation of organic matter and dissolution of early-formed carbonate cements.The illitization of smectite (60-100 °C) in the mudstones is consistent with the temperature range of the ferroan carbonate cements 25 , which can release Fe 2+ that is transported to sandstones via diffusion, and this process may have served as the main source of the ferroan carbonate cements in the sandstone 49,59,60 , as evidenced by the distribution pattern of carbonate cements proximal and distal to the sandstone-mudstone interface.It is inferred that there was a relatively open system for organic CO 2 and Fe 2+ ion transfer on the local scale during early mesogenesis, and the predominant dissolved material was driven by diffusive transport.
The ankerite cements have higher δ 13 C values than the ferroan calcite and formed at significantly higher temperatures (91-152 °C), suggesting another origin from CO 2 -containing deep hydrothermal fluid (Fig. 14d).The hydrothermal fluid from depth had δ 13 C values of approximately − 5 ± 2‰ and generally positive δ 18 O values ranging from + 5.0 to + 7.0‰ 61 .The increasing δ 13 C and δ 18 O values of the ankerite with increasing temperature may be related to the ascent and intrusion of hydrothermal fluid from depth to the sandstones.This conclusion is supported by the precipitation of barite and albite, which are typical hydrothermal minerals, as noted by others 28,29,62 .It is likely that diffusion was predominant in a locally open system with migration of hydrothermal fluid during late mesogenesis.Further study with more focus on this hydrothermal fluid is suggested.
Oil charge may inhibit the compaction and late carbonate cementation of reservoirs, consequently acting to preserve porosity 63,64 .In the studied interval, the oil saturation of the reservoirs steadily increases as the content of carbonate cements decreases, which corresponds to a gradual increase in porosity (Fig. 15).There are numerous differences between the two periods of oil charge in terms of the impact on reservoir quality 64 .The first period of oil charging effectively inhibited the precipitation of ferroan calcite and ankerite prior to mesogenetic cementation (Fig. 14b,c).The first period of oil charge was synchronous with or subsequent to the transformation of feldspar to kaolinite.Both of the events may have altered the reservoir wettability to oil wet, which helped the subsequent oil charging and entrapment [64][65][66] .Overall, high-quality reservoirs might be present in the middle part of the thickly bedded delta sandstones, and these reservoirs experienced both periods of oil charging.

Conclusions
This paper presents the evolution of fluids with progressive burial and the complex interactions with sandstone reservoirs, as exemplified by the Oligocene Zhuhai sandstones in the Baiyun Sag.On the basis of detailed analyses of petrographic, mineralogical, and geochemical features in the Zhuhai sandstones, this study has demonstrated the following.
(1) The Oligocene Zhuhai sandstones experienced compaction and calcite and dolomite cementation in the eogenetic stage.Subsequently, they underwent two phases of feldspar dissolution and concomitant quartz overgrowth formation, the cementation of ferroan calcite and ankerite, the conversion of clay minerals, and two periods of oil charging in the mesogenetic stage.The precipitation of typical hydrothermal minerals (such as authigenic albite and barite) was synchronous with ankerite in the late mesogenetic stage.
(2) The pore fluids were influenced by microbial methanogenesis and carbonate mineral dissolution in adjacent mudstones during the eogenetic stage.Large-scale mass transfer occurred along steep diffusion gradients in a relatively open geochemical system and resulted in abundant precipitation of calcite or dolomite within 1.0 m of the sandstone-mudstone interface.This conclusion is supported by the heavier carbon and oxygen isotopic compositions during eogenesis.
(3) Thermal maturation of organic matter significantly controlled the pore fluids in the early mesogenetic stage.Organic acid and CO 2 contributed to the dissolution of feldspar and early cements.Diffusion-migrated dissolved material was mainly reprecipitated in situ or in adjacent pores in the form of authigenic quartz and ferroan calcite.This conclusion is supported by the obviously lighter isotopic compositions of the early mesogenetic ferroan calcite.During late mesogenesis, dissolution might have been partly driven by deep hydrothermal fluid.Evidence that supports this interpretation includes the precipitation of typical hydrothermal minerals and the abnormally high isotopic compositions of the late ankerite.
(4) Oil charging, especially the first period of oil charging, may have inhibited carbonate cementation and compaction, accordingly causing the preservation of porosity.This could have altered the reservoir wettability from water wet to oil wet in combination with the precipitation of authigenic kaolinite caused by feldspar dissolution.The multistage pore-fluid activity and related fluid-rock interaction lead to the potential for highquality reservoirs developed in the middle part of the thickly bedded delta sandstones that experienced both periods of oil charge.

Figure 3 .
Figure 3. Triangular diagram of the rock composition of the Zhuhai sandstones in the Baiyun Sag.All data come from optical point counting of thin sections.

Figure 5 .Figure 6 .
Figure 5. Vertical distribution characteristics of the main diagenetic products in the Zhuhai sandstone.Pointcount data of thin sections are used.

Figure 8 .
Figure 8. Crossplot of δ 13 C and δ 18 O compositions of carbonate cements in the Zhuhai sandstones.Standard isotopic zones for carbon sources of variable carbonate cements are according to 32 .

Figure 10 .
Figure 10.Photomicrographs of aqueous inclusions in annealed microfractures in quartz grains (a) and quartz overgrowths (b) under transmitted light observed in Zhuhai sandstones of the Baiyun Sag.

Figure 11 .
Figure 11.Histograms of Th for aqueous inclusions in annealed microfractures, quartz overgrowths and ankerite in Zhuhai sandstone reservoirs.

Figure 12 .
Figure 12.The content of clay minerals versus burial depth in the Zhuhai sandstones.X-ray diffraction data are used.

Figure 13 .
Figure 13.Burial history, diagenetic sequence and porosity evolution of the Zhuhai reservoirs in well LW1-1.

Figure 14 .
Figure 14.Schematic diagram showing the fluid flow and diagenetic evolution of the Zhuhai sandstones.

Figure 15 .
Figure 15.Binary diagram of porosity versus carbonate cement content in sandstones with different oil contents.

Table 1 .
Isotopic composition and calculated formation temperature of carbonate cements in the Zhuhai sandstones in the study area.

Table 2 .
Th of aqueous fluid inclusions in the Zhuhai sandstones of the Baiyun Sag.MF = Microfracture.