Sedimentary, Diagenetic and Accumulation Characteristics of an Offshore Oil-gas Field—a Case Study of Huizhou Depression, Pearl River Mouth Basin, South China Sea

In the Z21 oil-gas eld, a total of six depositional lithofacies and two depositional elements were identied based on core observation. Three main diagenetic processes, namely mechanical compaction, cementation, and dissolution of Miocene Zhujiang Formation sandstones were identied according to thin section and scanning electron microscope (SEM) of core samples. Cementations mainly contain silica cementation, carbonate cementation, clay minerals and pyrite. A total of three main pore types, residual primary intergranular pores, secondary dissolution pores and micropores, were identied. Sand sheet deposited in low-energy environment and is characterized by relatively low porosity and permeability values. Lager grain-sized sandstones are of higher quality compared to smaller-sized sandstones. Mechanical compaction, calcite cementation and clay mineral cementation play a key role in reducing porosity and permeability, whereas dissolution of feldspar and debris contribute signicantly to improving the reservoir quality. The gas charge occurs prior to oil charge, forming a gas cap in the structural high and an oil ring in the lower formation. Irreducible water stored in the lenticular sandstone of low-porosity and permeability reservoir may convert to movable water as the drill and production perform. sequence 8 , depositional system 9–11 and other aspects. Huizhou an important area rich in oil and gas with a total reserve of 20.87×10 8 m 3 and a proved of 6.03×10 8 m 3 12,13


Introduction
Ocean energy resources play a signi cant role in national interest and people's livelihood. Development of ocean energy resources not only guarantee our country's energy security, but also re ect the national sustainable capacity 1 . Hydrocarbon resource, as an essential type of ocean energy resources, has been drawing more and more attention in respect of perspective, exploration, exploitation and other respects. According to the results of the national oil and gas resources evaluation, geological reserves of China's offshore oil and gas are 1.074×1010t and 8.1×1012m3, respectively 2 . As with the Bohai Bay Basin 3 and the East China Sea Shelf Basin 4 , the Pearl River Mouth Basin (PRMB) in the South China Sea ( Figure. 1a) is an agate treasure basin characterized by abundant petroleum resources and other natural resources 5,6 . Huizhou depression, one of the many potential depressions in the PRMB ( Figure.1b), has been studied by many geologists in terms of source rock 7 , stratigraphic sequence 8 , depositional system [9][10][11] and other aspects. Huizhou depression is an important area rich in oil and gas accumulation with a total geological reserve of 20.87×10 8 m 3 and a proved reserve of 6.03×10 8 m 3 12,13 . The offshore Z21 gas eld ( Figure. 1b and 1c), discovered in 1990, and is the only one gas eld in the Huizhou depression. So far, there are a total of 17 development wells and 3 exploraty wells in the eld.
The reservoir connectivity and reservoir quality are two concerned issues for the gas eld. The reservoir connectivity of Z21 oil-gas eld was discussed in a qualitative sequence-related method by Ding et al. 10 and in a quantitative method by Liu et al. 14 . For the sake of future development and production of the eld, it is critical to study the controlling factors on the reservoir quality. This paper aims at studying the sedimentary features and diagenesis processes of Miocene Zhujiang Formation in Z21 oil-gas eld and tries to quantitatively analysis the controlling factors on reservoir quality. Meanwhile, petroleum accumulation characteristics were qualitatively analyzed based on reservoir quality study.
The Pearl River Mouth Basin is an underwater extension of the South China continent with a length of about 800km, a width of 100-300 km and an area of 27 km. Its depth varies from dozens of meters to more than 3000 m 15,16 (Figure. 1a and 1b). Huizhou depression lies in the mid-eastern region of PRMB, limited by the North Fault Terrace in the north and Dongsha uplift in the south. Its east and west are Huilu Low uplift and Xi-jiang depression respectively ( Figure. 1b). The studied area Z21 oil-gas eld lies in the south of Huizhou depression, adjacent to the Dongsha uplift ( Figure. 1b). In a structural sense, structural highs are developed in the east and structural lows are in north-west, southwest and northeast ( Figure. 1c). Fence diagram from three-demensional structural model provides a visual perspective to understand the structural feature ( Figure. 1c). The target reservoir, K22 layer, lies in the middle of Zhujiang Formation ( Figure. 2), is the main production interval of Z21 oil-gas eld.
The deposits of ZH Formation, characterized by a thickness range of 500-1100 m, vary from continental delta to marine deltic 8, 19 and also consists of sediments dominated by tides 20 . In terms of stratigraphic sequence, the short-term, middle-term and long-term cycles were studied by scholars such as Cheng et al. 21 and Wei et al. 8 . Based on seismic technology such as root mean square amplitudes, with the assistance of sedimentology and sequence stratigraphy, favorable areas for prospecting reserves in Huizhou depression were predicted 16,21,22 .
It is a common knowledge that the WC Formation and EP Formation played an essential role in the hydrocarbon accumulation of the Huizhou depression 7,23 . EP Formation are characterized by two thin sets of sandstones in the lower formation and a thick set of darkcolored mudstone in the upper formation ( Figure. 2a), which is viewed as the potential source rocks 14,24 . The underlying WC Formation has an unconformable con-tact relationship with EP Formation and its lower strata is dominated by a thick set of semi-deep to deep lacustrine dark gray mudstones ( Figure. 2a), which were considered essential source rocks in Huizhou depression 19,22 . In short summary, the dark-colored mudstones of the WC and EP Formations are important source rocks for hydrocarbon ac-cumulations in the Huizhou depression 14 .

Materials And Methods
Depositional system including depositional lithofacies and elements were analyzed based on sedimentary structure, texture, grain size, rock color, seismic data and strati-graphic characteristics and other preexisting geologic knowledge. A total of 40 representative core samples in the Miocene reservoirs were collected from 5 cored wells in the Huizhou depression. To determine pore types and diagenetic minerals, 20 thin sections were made after each core sample was vacuum impregnated with blue epoxy resin. 20 core samples were used for scanning electron microscope (SEM) to identify diagenetic minerals and pore types, using a HITACHI H600 equipment equipped with a LinKQX-200 energy dispersive spectrometer operating at an accelerating voltage of 20 kV and an emission current of 9000 or 9400 nA. The point-counting method (200-300 points per thin section) was applied to analysis rock constituents such as quartz, feldspar, rock fragment and cements. The XRD were conducted using a Rigaku D/MAX-2400 X-ray diffractometer at the Rock-Mineral Preparation and Analysis Lab of the Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing, China. The porosity and permeability values are collected from Shenzhen Branch of CNOOC (China National Offshore Oil Corporation).

Results
Mineral composition and texture. The detrital mineralogy of the Miocene Zhujiang sandstones is dominated by quartz (range 60.5-85.9 vol%, average 75.7 vol%), followed by feldspar (range 4.7-24.1 vol%, average 14.5 vol%), and rock fragments (range 4.6-18.6 vol%, average 9.9 vol%) ( Table. 1). Therefore, the reservoir sandstones of the Z21 oil-gas eld are mainly classi ed as subarkose and lithic arkose, in the average framework composition of Q 76 F 14 R 10 based on the classi cation by Folk 25 (Figure. 3).
1. Massive ne-to medium-grained sandstone (Sm). This facies is generally ne-to medium-grained, 1 to 3 meters in thickness, light gray in color (Figure. Depositional elements. Two primary depositional elements characterize the K22 set interval: nearshore sand bar (SB) and sand sheet (SS). Identi cation of these two elements is mainly based on litho-facies, sedimentary structures, and sequences. Description and interpretation of the two elements are brie y discussed below.
1. Depositional element SB. As shown in Figure. 4, element SB varies from 1.5 5m in thickness with multiple facies association. The sandstone: mudstone ratio is high, and the continuity of SB is generally interrupted by thin units of facies Fl or Sl. Both the upper and lower contact with facies Fl and Sl are gradual or/and sharp. In many places, especially within K22A layer, massive sandstone of SB is well developed (Figure. 5). It's common that isolated iron concretions, ellipse, sphere, bamboo or nger-shaped, are developed within this element. In places, severely rip-up muddy chips are dispersedly distributed within the silty facies Sm. Bioturbation is occasionally found within facies Sw, Sl or Fl in this succession.
Element SB represent the deposits of delta front bedform, which is modi ed by waves and currents 11 . In this paper, recognition of SB in the plane with the assistance of seis-mic data ( Figure. 4) is accordant with predecessors' works 8, 10,31 . As shown in Figure. 3, the west and southwest area of the study area, marked by three zones, namely I, II and III, have been widely acknowledged as delta front depositional system, whereas the main zone of Z21 eld is involved in the sand ridges which are distributed far away from the delta lobe 10,32 . As the delta front was thrust toward sea basin, the preexisting coarse-grained deposits are continuously transported away from the prodelta zone by the force of wave and then resettle down. If the sediment supply is su cient, the resettled sandy deposits will be transported again for a certain distance and settle down until the wave force vanishes 14  SS is interpreted as the result of transferring and reconstructing of the large delta front bedform by wave and coastal current 11,33 .
According to the amplitude slice of K22 set, amplitude of the SS is relatively low, and its geometry is sheet-like ( Figure.  Pore classi cation and physical properties. According to the classi cation by Schmidt and Mcdonald 33 , based on the study of thin section and SEM images, a total of three types of pores, namely residual primary intergranular pores ( Figure.6a and 6b), secondary dissolution pores ( Figure.6a and 6b), and micropores ( Figure.6c and 6d), are distinguished in the Miocene reservoir sandstones.
Results of point counting of the thin sections showing residual primary intergranular pores accounting for 82.5% of the total pores, indicating that the pores of the Miocene sandstones in the ZJ Formation are dominated by this pore type. Through observation, the residual primary intergranular pores mainly occur in the condition that there are rare or no cement or matrix blocking the space between detrital grains ( Figure.6a and 6b). The secondary dissolution pores are further classi ed into intragranular pores ( Figure. 6a) and intergranular dissolution pores ( Figure.6b, moldic pores). The former generally were produced by incomplete dissolution of unstable grains such as feldspar and debris, whereas the latter were mainly generated from dissolution of the edge of detrital grains. The moldic pores were results of complete dissolution of detrital grains (Figure. 6b). There are two kinds of micropores developed within ZJ Formation: micro-fractures ( Figure. 6c) and intercrystalline micropores (Figure. 6d). The micro-fractures are mostly observed within the intervals within directionally arranged grains ( Figure. 6c), whereas the intercrystalline micropores are mainly developed within intervals of illitic clays (Figure. 6d).
The porosity values of target formation vary from 10.1-20.1%, with a mean value of 13.9%. The minimum permeability value is 0.2mD, the maximum value is 230mD and the average value is 35.8mD. The porosity and permeability values are characterized by positive linear relationship ( Figure. 7a). The Lorenz plot of permeability in Figure. 7b aims to describe the reservoir heterogeneity. For a homogeneous reservoir, the Lorenz plot is characterized by a straight-line AC, unlike a heterogeneous reservoir 35,36 . The Lorenz plot of permeability for the Z21 oil-gas eld is an arc between a 0-75.21% of cumulative probability of rock sample range, indicating strong reservoir heterogeneity.

Diagenesis And Diagenetic Mineralogy
Compaction. Sandstone of K22 layer underwent mechanical compaction progress, as indicated by oriented-arrangement grains ( Figure. 8a) and deformed mica ( Figure. 8b). Indications for chemical compaction such as sutured contacts are rare in this study interval.
Cementation. In K22 sandstone, cementation minerals mainly include siliceous cements, clay minerals and carbonate cements. Siliceous cements are dominated by authigenic quartz, which are commonly found in the intergranular pores, adjacent to unstable grains like feldspar ( Figure.8c). In some cases, quartz overgrowth is observed. The

Discussion
Diagenetic history. Diagenetic history of the Miocene ZJ Formation sandstones is analyzed according to the types of diagenetic processes, cements, pore types and physical properties and other aspects 37,38 . Based on thin section and SEM observation, the diagenetic processes of studied interval consist of mechanical compaction; quartz; carbonate and clay mineral cementation; pyrite and dissolution. According to Morad et al. 38 , the diagenetic processes can be divided into eodiagenesis and mesodiagenesis. Eodiagenesis in this area is characterized by sediments underwent a paleogeotemperture of approximately 70°C, generally occurs at a depth less than 2 km. As the burying goes on, the diagenetic process comes to mesodiagenesis 38 . Based on reconstruction of burial-thermal history of well H1 in Z21 oil-gas eld by Wu 39 , it is found the target ZJ Formation sandstones (mainly 2-3 km) has a corresponding formation temperature of 85-120°C ( Figure. 9), illustrating that mesodiagenetic processes occurred.
Mechanical compaction is the bulk volume reduction result from lithostatic stress, characterized by reorientation of framework grains ( Figure.  However, there is minimal pressure dissolution of detrital quartz in the ZJ Formation sandstones, proved by no observation of detrital quartz dissolution in the view of thin section and SEM. the typical temperature of chlorite formation is approximately 60-70°C 42 , which refers to the end of the eodiagenetic stage. The photomicrographs of the SEM show that quartz overgrowth coated by authigenic chlorites indicate that authigenic chlorites occur after quartz overgrowth. The terminal of quartz overgrowth is restricted by the occurrence of pyrite ( Figure. 8i), indicating that pyrite occurs prior to quartz overgrowth. Feldspar dissolution is considered as important material resources for quartz, kaolinite and illite precipitation 43 , therefore, it is inferred that quartz, kaolinite and illite cementations occur penecontemporaneously or kaolinite occur a little bit earlier than quartz cementations. In acidic conditions, the extensive illitization is associated with a temperature of 140°C 44 , indicating that illite occurs from the eodiagenetic stage, but is mainly formed in the mesodiagenetic stage.
The early calcite completely lls the intergranular pores, and the irregular shape of the calcite indicates it is formed prior or contemporary with severe mechanical compaction ( Figure. 8g). In some cases, calcite together with other clay minerals such as kaolinite, usually partially ll the interparticle pores ( Figure.

Diagenetic Controls On Reservoir Quality
Sedimentary facies controls on reservoir quality. As mentioned above, deposits of H21 gas eld were located far away from the delta front which is characterized by complex hydrodynamic conditions 10 , re ected by variable sedimentary structures ( Figure. 5). Linking the heterogeneous porosity and permeability values ( Figure. 7) to the varying lithofacies types, it is preliminarily inferred that reservoir quality of H21 gas eld was signi cantly affected by depositional settings. As shown in Figure. 10a, statistics of different depositional elements show that both porosity and permeability values of SB are generally higher than those of SS. The SS depositional element is interpreted as sandstones deposited in low-energy environment, poorly sorted and with high matrix content 51 .
Grain size controls on reservoir quality. Another factor affecting the reservoir quality is grain size. The grain size which re ects the primary texture of sandstones, may control the extent of the subsequent diagenetic events 52 . Statistics show that different grainsized sandstones, namely -ne-grained, medium-grained, and coarse-grained sandstone have different porosity and permeability distribution centers ( Figure. 10b). As the gain sizes increase, the porosity and permeability values generally become bigger ( Figure. 10b). Compared to the smaller-sized sandstones, the larger-sized sandstones are usually well sorted with less matrix grains; meanwhile, rigid framework grains such as quartz are less in uenced from complex compaction processes if they are larger-sized 53 .
Diagenetic controls on reservoir quality.
1. Mechanical compaction. Mechanical compaction, intergranular pressure solution, cementation, framework grain dissolution, and cement dissolution have all been documented as playing signi cant roles in modifying porosity of various sandstones 54 . In the Miocene ZJ Formation sandstones, mechanical compaction is characterized by directional arrangement of grains, concave-conves contacts between the grains and plastic deformation of ductile grains. Upon burial, sediments will compact mechanically when the effective stress due to over-burden is increased, so that the porosity and the total rock volume are reduced 55 . As a result of increasing effective stress from the overlying strata during burial, the effect of mechanical compaction increases with the increase of burial depth in eodiagenesis 56 .
As shown in Figure. Figure. 11, in general, both porosity and permeability values decrease as the increasing of calcite content, showing a remarkable negative relationship with R 2 =0.7449. However, it is noticed that when the content of calcite is less than 9%, there is no remarkable negative correlation between porosity and calcite ( Figure. 12a). The sample that has a relatively higher calcite content of 18% with a permeability value of 48 mD ( Figure. 12b) and a porosity value of 4% ( Figure. 12a), is interpreted by the development of micro-fracture ( Figure. 6c).
Typically, authigenic kaolinite, illite or other clay mineral may be found in nearby primary or secondary pores 57 . Precipitation of kaolinite can only occur when the K + /H + ratio and silica concentration in the pore water are below certain values and such low K + /H + ratios are normally only found in fresh or brackish water 58 . That means in intervals where CO 2 concentration is high and dissolution of feldspar and debris is severe, the relative content of kaolinite precipitation could reach as high as 82% (Table. 2). However, the intercrystalline micropores within kaolinite aggregates is poorly developed. Therefore, the more kaolinites are, the more pore spaces were lled, showing a negative correlation between kaolinite and reservoir physical properties ( Figure. 13a and 13b). As for illite, intercrystalline micropores within illite aggregates are generally well developed ( Figure. 5d), the porosity and permeability values increase as the content of illites increase ( Figure.13c and 13d). However, the convert from illite to the mixed layers of illite and smectite reduces some of pore spaces ( Figure. 8e). At another hand, the pore lling illite aggregates may occupy some of pore spaces and result in a decrease in porosity and permeability ( Figure.13c and 13d).
Grain-coating chlorites are generally considered as the porosity-preserving components in the sandstones 42,51,58 . Within ZJ Formation, as shown in Figure. 13e, the porosity increases and then decreases slightly, implying the chlorites coatings may retard quartz overgrowth within a limited content range ( Figure. 8f). The relationship between chlorites and permeability is like that between chlorites and porosity ( Figure. 13f). As the volume of chlorites accumulate, the porosity and permeability values decrease slightly due to the plugging of porelling chlorite aggregates.
In this studied area, although a single type of clay mineral may enhance the reservoir physical properties and another may weaken the congeneric properties, the porosity and permeability still display a decreasing trend with the total clay minerals, with low R 2 values (0.25 and 0.5206, respectively) ( Figure. 13g and 13h), indicating that clay mineral cementation is an important control factor of reservoir quality in the area. 3. Dissolution. Dissolution is generally considered as a constructive factor that enhance reservoir quality 51,57,58 . The secondary dissolution pores are dominated by dissolution of feldspar and debris, and therefore, it is meaningful to ascertain whether dissolution is responsible for the deeper sandstones but with higher porosity and permeability values. The types of pores re ected by thin sections were analyzed by point counting and it is found that the deeper sandstones with higher porosity and permeability values usually have a greater proportion in secondary dissolution pores, whereas the shallower sandstones are dominated by residual primary intergraunar pores, which are limited in bulk volume ( Figure. 14). Although in uence of dissolution on reservoir quality studied in this way is not that rigorous, to some extent, dissolution enhancing the porosity and permeability is still proved in a qualitative way.
Distribution pattern of water, oil and gas in Z21 structure. Sources of oil and gas in the Z21 structure in Huizhou Depression has been studied by Zhu et al. 16,50 . The Z21 oil-gas eld are characterized by multiple sources. Both the condensate gas in gas reservoir and solution gas in oil reservoir in the upper ZJ Formation are from the same source rocks of EP and WC Formation in HZ21 subsag, which are de-posited in shallow lacustrine to swamp, whereas the black oil in the lower ZJ Formation is from semi-deep to deep lacustrine source rocks with signi cant terrigenous parent organic matters in HZ26 subsag 16 (Figure. 15). The gas reservoir formed earlier than that of oil reservoir in Z21 structure 16 . In 2014, modular formation dynamics test (MDT) was conducted in K22 set of well H1DSa and pure oil samples were collected; however, the subsequent drill stem test (DST) detected water show (not water from mud ltrate caused by drilling engineering) in the same depth of well H1DSa (location see Figure.  In this study, a possible schematic pattern for water, oil and gas is proposed based on the sedimentary structures of cores, diagenesis, and physical properties ( Figure. 16). As it has been proved that there is a low-permeability belt connecting main zone and the west zone ( Figure. 16a) 14 . This is important foundation of the proposed water-oil-gas distribution pattern in this study. The original formation of K22 set is characterized by high water saturation ( Figure.16a). As the gas begins to charge, a mass of effective pore spaces was lled with gas; meanwhile, the original formation water was forced to move to the low-pressure area by pore pressure, which is higher than the hydrostatic pressure ( Figure.16b and 16c). Local sandstones (e.g., Figure. 5-C and 5-D) with low porosity and permeability were uncharged by gas and the original formation water was left behind, regarding as irreducible water ( Figure.16b and 16c). Subsequently, oil begins to charge when the gas accumulation was nished 16 and stops until both oil-water and oil-gas inter-faces were formed ( Figure. 16d). When wells were drilled in the main zone, the formation pressure of west zone decreases, breaking the preexisting balance state of oil and water, partial irreducible water convert to movable water ( Figure. 16e). As the production of the main zone goes on and the drill of well H1D, H1DSa and H18, formation of the west zone decreases severely, resulting in more irreducible water turning into movable water ( Figure. 16f).

Conclusions
A total of 6 depositional lithofacies, Sp, Sm, Sr, Sw, Sl, Fl, and two depositional elements, SB and SS, were identi ed. The SB was deposited in relatively high-energy hydrodynamic environment, forming better-quality reservoir than SS. The Miocene reservoir in Z21 oilgas eld is dominated by medium-grained sandstones and generally the coarser-grained sandstones are characterized by better reservoir quality than smaller-sized sandstones.
Three main pore types, namely residual primary intergranular pores, secondary dis-solution pores and micropores are observed. The deeper sandstones may have higher porosity and permeability values than the shallower sandstones because the former have a greater proportion in secondary dissolution pores.
Mechanical compaction, cementation of calcite and clay mineral cementation are re-sponsible for the porosity and permeability reduction.
Gas and oil in the Z21 structure are characterized by multiple-source supply and the charge of gas occurs prior to oil. The Z21 structure could be regarded as a pool which has a gas cap and oil ring; within the oil ring there exists local irreducible water stored in poor-quality sandstones like lenticular sandstones. This kind of formation water convert to movable water when the formation pressure decreases caused by drill and production activities. K22A, K22B and K22C are three subdivided layers from K22 layer. The red arrows refer to the structural high, the black arrow refers to the structural low.     Burial-thermal history of Well H1 and the paragenetic sequence and types of diagenesis in the Miocene Zhujiang Formation sandstones39, 50. Q=Quaternary.

Figure 10
Statistics of porosity and permeability of different depositional elements (a) and different grain-sized sandstones (b).

Figure 11
Plot of the depth versus porosity (a) and permeability (b) for the Miocene ZJ Formation sandstones of Z21 gas eld in Huizhou Depression.

Figure 12
Plot of the calcite versus porosity (a) and permeability (b) for the Miocene ZJ Formation sandstones of Z21 oil-gas eld eld in Huizhou Depression.  Tendency chart between pore proportion and porosity/permeability values of certain samples.

Figure 16
Distribution pattern of water, oil and gas of K22 set in Z21 structure.