diagenetic evolution and formation mechanisms of high...

1 Introduction Diagenesis is a series of responses that changes the

physical and chemical properties of rocks or sediments within a certain burial environment, which could be defined as the sum of the physical and chemical conditions during

the diagenetic processes experienced by the rocks (Li Zhong et al., 2006). The understanding of the diagenetic environment is the key to the study of reservoir diagenesis. The formation temperature, pressure, fluid and closeness of the environment are theessential factors of the diagenetic environment (Bjørlykke, 2014). In a reservoir, differences in the evolution of the diagenetic environment lead to various diagenetic transformations that control the

Diagenetic Evolution and Formation Mechanisms of High-Quality Reservoirs under Multiple Diagenetic Environmental Constraints: An Example from the Paleogene Beach-Bar Sandstone Reservoirs in the

Dongying Depression, Bohai Bay Basin

WANG Jian1, *, CAO Yingchang1, SONG Guoqi2 and LIU Huimin3

1 School of Geosciences, China University of Petroleum (East China), Qingdao 266580, Shandong, China 2 Sinopec Shengli Oil Field Company, Dongying 257000, Shandong, China 3 Institute of Geology, Sinopec Shengli Oil Field Company, Dongying 257015, Shandong, China Abstract: The diagenetic environment, diagenetic responses, diagenetic transformation model and formation mechanisms of high-quality reservoirs (beach-bar sandstones of the Paleogene fourth member) in the Dongying depression were studied through the analysis of fluid inclusions, thin section and burial evolution history. The diagenetic fluids of the beach-bar sandstone reservoirs evolved from early high salinity and weak alkalinity to low salinity and strong acidity, late high salinity and strong alkalinity and late low salinity and acidity, which were accompanied by two stages of oil and gas filling. The fluids at the margins of the sandbodies were continuously highly saline and strongly alkaline. The western (eastern) reservoirs experienced early open (closed), middle open, and late closed diagenetic environments during their burial history. The flow pattern was characterized by upwelling during the majority of the diagenesis (in the east, a non-circulating pattern transitioned into an upwelling current). Due to the evolution of the diagenetic fluids, the diagenetic sequence of the beach-bar reservoirs was as follows: early weak carbonate cementation; feldspar and carbonate cement dissolution and authigenic quartz cementation; late carbonate and anhydrite cementation, authigenic feldspar cementation, and late quartz dissolution; and late carbonate cementation, feldspar dissolution, and authigenic quartz cementation. The diagenetic strength during these stages varied or was absent altogether in different parts of the reservoirs. Due to the closeness of the diagenetic environment and the flow pattern of the diagenetic fluids, the diagenetic products are variably distributed in the sandstones interbedded with mudstones and in the fault blocks. The evolution of multiple alternating alkaline and acidic diagenetic environments controlled the distribution patterns of the reservoir diagenesis and reservoir space, and the reservoir quality index, RQI, increased gradually from the margins to the centers of the sandstones. The closeness of the diagenetic environment and the flow patterns of the diagenetic fluids controlled the differences in the reservoir properties among the fault blocks. With increasing distance from the oil-source faults, the RQI values in the west gradually decreased and in the east initially increased and then decreased. Key words: diagenetic environment, diagenesis, diagenetic fluids, beach-bar, high quality reservoir, Dongying depression

Vol. 91 No. 1 pp.232–248 ACTA GEOLOGICA SINICA (English Edition) Feb. 2017

* Corresponding author. E-mail: [email protected]

© 2017 Geological Society of China

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formation mechanisms and development of high-quality reservoirs. Previous studies of reservoir diagenetic environments have focused on single factors, particularly the transformations of acidic and alkaline formation fluids (Surdam et al., 1989; Hartmann et al., 2000; Qiu Longwei et al., 2002). However, all aspects of the diagenetic environment contribute to the environment. During the burial of clastic sediments, the reservoir diagenetic environment changes constantly through the influence of such factors as the formation temperature, formation pressure, sources of formation fluids and water-rock interactions. These types of changes lead to the presence of different diagenetic environments in the same area over different periods of time or in different areas during the same time period, which result in multiple diagenetic characteristics (Rossi et al., 2001; Zhou Yaoqi et al., 2011; Wang Jian et al., 2013) and affect the development of high-quality reservoirs (Alaa et al., 2000; Chi et al., 2003).

The Es4 Formation was deposited in the Dongying depression during a transition from initial rifting to extensional rifting. Relatively gentle paleotopography developed on the southern gentle slope belt of the Dongying depression after this early phase of sedimentary filling. The supply of terrigenous clastic material from around the basin was sufficient to develop large-scale shallow-shore lacustrine beach-bar sandbodies along the gentle slope belt of the Dongying depression (Jiang Zaixing et al., 2011). Beginning in 2006, many oil fields containing huge oil and gas reserves have been found in these beach-bar sandbodies. In recent years, the proven reserves in the beach-bar sandbodies (the important oil-bearing reservoir type in the Dongying depression) increased by an average of 30 million tons annually. Studies of the diagenesis of beach-bar sandstone reservoirs of the gentle slope belt of the Dongying depression have focused on the diagenetic processes and their effects on the reservoir petro-physical properties. The diagenetic evolution of the reservoir has not been intensively studied, which limits the understanding of the formation mechanism involved in the development of high-quality reservoirs in these beach-bar sandstones. The diagenetic environment, diagenetic responses, diagenetic transformation model and formation mechanisms of the high-quality reservoirs in the beach-bar sandstone of the Paleogene fourth member in the Dongying depression were studied through the analysis of fluid inclusions, thin section data and burial evolution history.

2 Geological Setting

The Dongying depression is a secondary negative

tectonic unit in the Jiyang Subbasin (Fig. 1a). This

depression is a Mesozoic-Cenozoic rift-depression basin within the North China platform (Lampe et al., 2012; Wang Jian et al., 2016). The southern gentle slope belt is an important hydrocarbon accumulation belt of the Dongying depression and constitutes approximately 1/3 of the depression (Fig. 1b). The deposition of the Es4 Formation occurred during the transition from initial rifting stage to extensional rifting stage of the Dongying depression. During this stage, the paleotopography was relatively gentle and the terrigenous clastic material supplied from around the basin was sufficient for the development of large-scale lacustrine beach-bar sandbodies (Fig. 1c). These thin beach-bar sandbodies are commonly interbedded with lacustrine mudstone. The high-quality hydrocarbon source rocks of the Es4

Formation not only provide sufficient oil and gas but are also good cap rocks for the beach-bar sandstone reservoirs. Due to the presence of oil-source faults, the beach-bar sandbodies within the fault blocks display very favorable oil and gas accumulation conditions. The beach-bar sandbodies are important hydrocarbon exploration targets in the gentle slope belt of the Dongying depression. 3 Materials and Methods

Sixty-four samples were collected from 21 wells in the Paleogene Es4s red-bed interval on the gentle slope belt in the Dongying depression (Fig. 1). Samples were impregnated with blue or red resin before thin sectioning in order to highlight the pores. Thin sections were partly stained with Alizarin Red S and K-ferricyanide for carbonate mineral differentiation. Compositional modal analyses of thin sections were performed by counting 200 points per thin section. Samples were collected for mineralogical analyses using X-ray diffraction (XRD). A D8 DISCOVER was used for XRD analysis with Cu-Ka radiation, a voltage of 40 kV, and a current of 25 mA. Prior to analysis each sample was oven-dried at 40 °C for 2 days and ground to <40 μm using an agate mortar to thoroughly disperse the minerals. No chemical pre-treatment was employed. Samples were scanned from 3 to 70 with a step size of 0.02. Quantitative analysis of the diffractograms provided identification and semi-quantitative analysis of the relative abundance (in weight percent) of the various clay mineral phases.

Eight samples were selected for fluid inclusion microthermometry. These samples were prepared as doubly polished fluid inclusion wafers. Microthermometry was carried out using a Linkam THMGS-600 heating- freezing stage. Calibration of the stage was performed following the method used by MacDonald and Spooner (1981). In addition, synthetic fluid inclusion standards

234 Vol. 91 No. 1 Feb. 2017 ACTA GEOLOGICA SINICA (English Edition) http://www.geojournals.cn/dzxben/ch/index.aspx http://mc.manuscriptcentral.com/ags

were used (pure CO2 and water). The measurement precision is ±0.2°C at −56.5°C and ± 2°C at 300°C. The last ice melting temperatures (Tmice) and homogenization temperatures (Th) were observed at a heating rate of no more than 5 °C/min.

4 Evolution of the Diagenetic Environment

4.1 Diagenetic product records of the evolution of the diagenetic environment

The diagenetic features in the beach-bar sandstone

Fig. 1. Tectonic setting, structural mapand plain distribution of beach-bar sandbodies of Es4s of Dongying depression in the south-ern Jiyang subbasin.


reservoirs of the Es4 Formation in the Dongying depression, mostly reflect an alkaline diagenetic fluid environment. In thin sections, quartz and its overgrowth appear to have been dissolved, forming embayments or nibbled shapes (Figs. 2a and 2b). Calcite and dolomite (formed in the early stage) co-exist with ferro-calcite and ankerite (formed in the late stage), and the former were surrounded and replaced by the latter (Fig. 2c). There is also evidence of feldspar overgrowths (Fig. 2d). These diagenetic features indicate that the beach-bar sandstone reservoirs experienced a relatively strongly alkaline fluid environment (Goldstein and Rossi, 2002; Zhang Siting and Liu Yun, 2009; Ji Youliang et al., 2014). Furthermore, multiple stages of carbonate cements indicate that the reservoirs experienced multiple stages of alkaline fluid environments (Qiu Longwei et al., 2002; Howari et al., 2010; Zhou Yaoqi et al., 2011).

There are also considerable diagenetic phenomena indicative of an acdic diagenetic fluid environment in the beach-bar sandstone reservoirs. In thin sections, the feldspar and carbonate cements appear to have been dissolved, forming embayments or nibbled shapes and

irregularly shaped dissolution pores (Figs. 3a, 3b, 3c and3d). Multiple stages of quartz overgrowth with straight edges are common (Figs. 2a, 2b and Figs. 3a, 3c, 3d). The dissolution of feldspar, carbonate and ferro-carbonate cements and the multiple stages of quartz overgrowth cementation indicate that the reservoirs experienced multiple stages of acidic diagenetic environments (Tan Xianfeng et al., 2010).

4.2 Fluid inclusion records of the diagenetic environment

Fluid inclusions that formed during diagenesis record detailed information regarding the diagenetic environment, including information on the diagenetic fluid and formation temperature, providing the most direct evidence regarding the evolution of the diagenetic fluids (Eadington and Hamilton, 1991; Rossi et al., 2002; Azmy and Blamey, 2013). Fluid inclusions in the beach-bar sandstone reservoirs primarily include primary liquid-rich two-phase aqueous inclusions hosted in quartz overgrowths and carbonate cements and secondary liquid-rich two-phase aqueous inclusions and hydrocarbon inclusions hosted in

Fig. 2. Evidence of alkaline fluid in beach-bar sandstone reservoirs of Es4s in the Dongying depression. (a), Lai110-2883m, dissolution of quartz overgrowth (-); (b), W58-3021.5m, dissolution of quartz overgrowth, 310×; (c), G89-2999m, calcite, ferro-calcite and ankerite cements (-); (d), Lai110-2883m, feldspar overgrowth and calcite (+).


annealed microfractures (AMFs) in quartz grains. The distributions of the salinities and the homogenization temperatures of the aqueous inclusions are shown in Fig. 4.

The salinities and the homogenization temperatures of the primary fluid inclusions in the quartz overgrowths and secondary fluid inclusions in the AMFs are in the same range and have similar distributions, which indicate a consistent formation environment and formation timing of the two types of fluid inclusions. The salinities and homogenization temperatures of the inclusions in the carbonate cements are very different from those of the inclusions in the quartz overgrowths and AMFs, which indicates that the fluid inclusions in the quartz overgrowths, AMFs and carbonate cements formed in different diagenetic environments.

The plots of salinity vs. homogenization temperature indicate that the aqueous fluid inclusions in the quartz overgrowths and AMFs may be classified into two groups: QTZ1 and QTZ2 (Fig. 5). The homogenization temperature of the QTZ1 group is lower than that of the QTZ2 group, and there is little difference in salinity between the two groups. The plots also indicate that the aqueous fluid

inclusions in the carbonate cements define a single group (CCT1). The homogenization temperature of CCT1 is higher than that of QTZ1 but is similar to or higher than that of QTZ2. The salinities of the fluid inclusions in the carbonate cements are clearly higher than those of the quartz overgrowth and AMF inclusions (Fig. 5). Therefore, the aqueous fluid inclusions in the quartz overgrowths, AMFs and carbonate cements record three stages of diagenetic fluids with different properties in the beach-bar sandstone reservoirs during burial.

Hydrocarbon fluid inclusions in the beach-bar sandstone reservoirs are primarily hosted in the AMFs and are synchronous with the aqueous fluid inclusions. The hydrocarbon fluid inclusions are classified into two groups based on their fluorescent characteristics and homogenization temperatures. The first group of fluid inclusions generally fluoresce yellow or white, whereas the second group generally fluoresces bluish white. The fluorescent characteristics and homogenization temperatures indicate that the beach-bar sandstone reservoirs experienced two stages of oil and gas filling (Table 1).

Fig. 3. Evidence of acid fluid in beach-bar sandstone reservoirs of Es4s in the Dongying depression. (a), L902-2160.96m, dissolution of feldspar and quartz overgrowth (-); (b), WX583-3467.75m, dissolution of feldspar (-); (c), G890-2598.2m, disso-lution of feldspar, quartz overgrowth and authigenic kaolinite and chlorite, 1000×; (d), W580-3170.25m, dissolution of carbonate cement and quartz overgrowth (-).


4.3 Evolution of diagenetic fluids Hanor (1980) indicated that the methane content was

fixed at 0.2 moles CH4 per kg H2O, which corresponds to 3200 ppm CH4 and was an estimate for subsurface waters in sedimentary basins. The aqueous fluid inclusions formed during diagenesis generally contain trace amounts of CH4. During heating, due to the partial pressure of methane, the pressure of the bubble point line in a P-T phase diagram of methane-bearing aqueous inclusions is clearly higher than that of pure aqueous inclusions (Hanor, 1980; Goldstein and Reynolds, 1994). Based on experimental data and the homogenization temperatures of

inclusions from the Dongying depression, the pressure-correction value of methane-bearing inclusions is usually less than 15°C (Zhou Yaoqi et al., 2011). Here, the homogenization temperature could be interpreted as the trapping temperatures of the aqueous fluid inclusions after the addition of 8–15°C. The trapping temperatures of hydrocarbon fluid inclusions are similar to those of the synchronous aqueous fluid inclusions.

The analysis of the trapping temperatures, the corresponding burial history and the consequent sequence of authigenic diagenetic minerals in the beach-bar sandstone reservoirs indicates that the trapping times of

Fig. 4. Histograms showing salinities and homogenization temperatures for liquid-rich two-phase aqueous inclusions in beach-bar sandstone reservoirs of Es4s in the Dongying depression.


the aqueous fluid inclusions in groups QTZ1, QTZ2 and CCT1 were 34.8–24.9 Ma, 8.9–0 Ma and 24.9–21.1 Ma, respectively, and that the trapping times of the two groups of hydrocarbon fluid inclusions were 28.1–25.7 Ma and 6.2–0 Ma, respectively. The order of the formation of the three groups of aqueous fluid inclusions was QTZ1, CCT1 and QTZ2. The formation time of the two groups of hydrocarbon fluid inclusions corresponded to the formation time of groups QTZ1 and QTZ2, respectively. Hence, the multiple stages of aqueous fluid inclusions and hydrocarbon fluid inclusions recorded the evolution of acidic and alkaline diagenetic fluids and the filling of the beach-bar sandstone reservoirs with oil and gas.

The deposition of the beach-bar sandstones of the Es4 Formation began at 45 Ma. There is no fluid inclusion record of the formation fluid from the initial burial stage until 34.8 Ma. The evolution of the pore water medium and the diagenesis of the clastic reservoirs were influenced by the properties of the syn-depositional water, which was the dominant factor controlling the early diagenetic

environment (Shaw et al., 1990; Xian Benzhong et al., 2004; Carvalho et al., 2014; Jiang Tao et al., 2015). Lacustrine carbonate rocks formed at the same time as the beach-bar sandstones, which provides information about the characteristics of the lake’s primary depositional water. The isotopic δ18O values reflect the paleo-salinity of the lake. The calculated paleo-salinity of the lake during the deposition of Es4 was 10.63–12.24 eq.wt% NaCl (Shen Ji et al., 2001) (Table 2). The calculated pH of the paleo-lake water was less than 8.72 based on the content of organic carbon. This value indicates a weakly alkaline environment.

The beach-bar sandstone reservoir diagenetic fluids of the early burial stage were also affected by the transformation of clay minerals in the mudstones. The clay minerals of the mudstones interbedded with beach-bar sandstones were classified into three zones of transformation (Fig. 6) based on the depth of burial. When the burial depth was less than 1,750 m, the reflectance of vitrinite was less than 0.3%, the formation temperature was less than 80°C, the content of the illite/smectite mixed

Fig. 5. Salinity vs. Homogenization temperature plots for the studied fluid inclusions in beach-bar sandstone reservoirs of Es4s in the Dongying depression.

Table 1 Homogenization temperatures for hydrocarbon inclusions in beach–bar sandstone reservoirs of Es4s in the Dongying depression

Well Depth (m) Number Fluorescence TH (°C) Average TH (°C) TH of synchronous aqueous FIs (°C) 5 Yellow 79.6–90.6 86.1 105.1–117 G891 2808.5 2 Blue white 100.8–104.2 102.5 124–129.8

G89 2997.1 3 Yellow 83.7–98.7 91.4 96.7–110.6 L105 3159.02 4 Blue white 87.2–97.9 93.4 120.2

4 White 86.9–93.9 89.9 116.0 C107 2883.5 9 Blue white 96.1–104.6 99.5 112.7–123.5


layer (I-S) was generally more than 60%, and the S:I ratio was generally more than 50%. During this stage, the diagenesis of the mudstones primarily consisted of compaction and weak transformation of smectite to illite, which released a large quantity of adsorbed primary depositional water. When the burial depth was between 1,750 and 2,650 m, the reflectance of vitrinite was between 0.3% and 0.5%, the formation temperature was between 80°C and 120°C, the content of kaolinite reached 70%, the content of illite and chlorite markedly increased, the content of I-S markedly decreased, and the S: Iratio was generally between 20% and 50%. During this stage, the smectite rapidly transformed to illite and released considerable amounts of metal cations, such as Ca2+, Fe2+, Na+and Mg2+, which increased the salinity of the fluid along the contacts with the sandstone (Zhong Dakang et al., 2004; Merriman, 2005). When the burial depth was greater than 2,650 m, the reflectance of vitrinite was more than 0.5%, the formation temperature was more than 120°C, the content of kaolinite was less than 50%, the content

of illite was up to 60%, the content of chlorite was up to 50%, the content of I-S was less than 50%, and the S:I ratio rapidly decreased to 20%. During this stage, the transformation from smectite to illite gradually ceased, but the kaolinite continued to rapidly transform to illite and chlorite, releasing considerable amounts of metal cations (Zhong Dakang et al., 2004; Merriman, 2005). Therefore, there was weak transformation of clay minerals in the mudstones during the early burial stage of the beach-bar sandstone reservoirs. The small amount of released water had little effect on the reservoir. The early diagenetic fluid was dominated by primary depositional water.

The analysis of organic acid in the thermal evolution experiment of the source rock of Es4s in the Dongying depression indicates that the concentration of organic acid was a maximum when the temperature was between 100°C and 140°C (Table 3; Zeng Jianhui et al., 2007), which corresponds to Surdam’s result (1989). With the increase in burial depth, the hydrocarbon source rocks were within the temperature range favorable for generating organic

Fig. 6. Distribution of reflectance of vitrinite and clay minerals in interbedded mudstones of Es4s in the Dongying depression.


acid from 35.1 to 26.9 Ma (Surdam et al., 1989; Zhu Xiaomin et al., 2009; Jiang Youlu et al., 2016), which is consistent with the timing of 31.3–26.4 Ma recorded by the fluid inclusions in the quartz overgrowths and AMFs. The organic acid neutralized the early weakly alkaline fluid, which transformed the diagenetic fluid from weakly alkaline to acidic. The salinity of the diagenetic fluid was (1–6) eq.wt% NaCl, and the calculated pH was between 5.37 and 5.88, based on the Liu Bin (2011) method (Fig. 7). Hydrocarbon fluid inclusions with yellow and white fluorescence indicate that this stage of diagenetic fluid was accompanied by filling of low-maturity oil. The rapid transformation of smectite to illite led to an alkaline fluid with high salinity at the margins of the sandstones.

With the continuous increase in burial depth,

considerable organic acid decarboxylation began to occur when the formation temperature exceeded 120°C (Surdam et al., 1989; Zeng Jianhui et al., 2007). Simultaneously, the gypsum-salt rocks released considerable amounts of alkaline fluid with metal cations and altered the acidic diagenetic environment to an alkaline environment. Fluid inclusions in ankerite record this diagenetic fluid stage, which spanned from 24.9 to 21.1 Ma. The salinity of this diagenetic fluid stage was more than 15 eq.wt% NaCl. The second rapid transformation of clay minerals in the mudstones increased the salinity of the diagenetic fluid. This diagenetic fluid stage resulted in the late ferro-calcite and ankerite precipitation in the beach-bar sandstone reservoirs, indicating that the formation water was primarily the carbonate type. Based on the relationship of

Fig. 7. Evolution of diagenetic fluids in beach-bar sandstone reservoirs of Es4s in the Dongying depression.

Table 3 Total contents of organic acid in the thermal evolution experimentfor source rock of Es4s in the Dongying depression (after ZengJianhui et al., 2007)

Total concentration of organic acid (mg/L) No. Well Depth (m) Lithology Content of organic carbon (%) 60°C 100°C 140°C 180°C 220°C 300°C

1 W35 2143.5 Gray brown mudstone 2.11 61.5 75.4 93 74.8 50.4 35.4 2 W31 2573 Gray brown mudstone 3.7 94.8 91.5 95 87.8 92.8 63.6 3 W18 1628.5 Gray brown mudstone 0.18 73.7 76.3 96.9 77.5 71.2 51.5


pH to salinity and the hydrochemical lake types on the Qinghai–Tibet Plateau (Zheng Mianping and Liu Xifang, 2009), the pH of this diagenetic fluid stage was greater than 9 (Fig. 7). Due to the thermal evolution of the hydrocarbon source rocks, the filling with low-maturity oil may have accompanied this fluid stage.

The hydrocarbon source rocks again entered the temperature range for the generation of organic acids due to tectonic uplift at the end of deposition of the Dongying Formation (24.6 Ma). This transition transformed the formation fluids from alkaline to acidic. After 10 Ma, the formation temperatures again exceeded 120° C, the hydrocarbon source rocks began to generate large quantities of high-maturity oil and gas, and this period became the primary stage of oil filling of the beach-bar sandstone reservoirs. With the increase in temperature, the content of organic acid in the formation fluid decreased, and the partial pressure of CO2 gradually increased, thereby forming a late weakly acidic environment (Fig. 7). The activity of the acidic diagenetic fluid was recorded by the group QTZ2 fluid inclusions after 8.9 Ma; the diagenetic fluid activity during this stage occurred after 21.1 Ma, based on a basin simulation. The salinity of this fluid stage was (2–8) eq.wt% NaCl, and the pH was 5.64–6.08 (Fig. 7).

4.4 Evolution of closeness of the diagenetic environment

Formation pressure is an important factor that controls the closeness of diagenetic environments (Zhang Shanwen

et al., 2012; Bjørlykke, 2014). The trapping pressures of the fluid inclusions were calculated via the method outlined in Zhang and Frantz (1987). The evolution of the formation pressure and the closeness of the diagenetic environment during burial werereconstructed based on the pressure calculations and previous research (Fig. 8).

The beach-bar sandstone reservoirs experienced two apparent cycles of pressure increase during burial. The first pressure increase cycle occurred before 24.6 Ma, and the second occurred after 13 Ma. The characteristics of the pressure evolution of the western and eastern parts of the Dongying depression were clearly different. During the first pressure increase cycle, the pressure coefficient of the western part was primarily between 0.9 and 1.2, the formation pressure was dominated by normal pressure or weak overpressure, the diagenetic environment was open, and the formation fluid was primarily affected by an upwelling current controlled by compaction. In contrast, the pressure coefficient of the eastern part was generally more than 1.2 (as high as 1.5), the formation was moderately to strongly overpressured, the diagenetic environment was closed, and it was difficult for the formation fluid to exchange material with the external environment. During the second pressure increase cycle, the pressure coefficients of the western and eastern parts were generally more than 1.1 (as high as 1.5; the maximum pressure coefficient of the modern formation pressure is greater than 1.6), the formation was primarily

Fig. 8. Evolution of formation pressure in beach-bar sandstone reservoirs of Es4s in the Dongying depression.


moderately to strongly overpressured, and the western and eastern parts were both closed diagenetic environments. At the end of deposition of the Dongying Formation, the formation pressure experienced leakage due to tectonic uplifting. Consequently, the formation pressure transitioned from medium-strong pressure to weak-normal pressure, the diagenetic environment transitioned from closed to open, and the formation fluid formed anupwelling current. Therefore, the diagenetic environment of the western part of the reservoirs evolved from early and mid-term open conditions to late closed conditions, whereas the eastern part of the reservoirs evolved from early closed conditions to mid-term open and late closed conditions. The pressure evolution curves from wells Fs1 and N107 in the Es4s Formationin a basin numerical simulation analysis correspond well with the pressure evolution curves derived from the analysis of the fluid inclusions (Li Shanpeng et al., 2004), which further confirms the differences in the reservoir pressures and closeness of the diagenetic environment between the western and eastern parts of the Dongying depression. 5 Discussions 5.1 Diagenetic sequence and distribution of diagenetic products

The diagenetic fluids of the beach-bar sandstone reservoirs evolved from early high salinity and weak alkalinity to low salinity and strong acidity, late high salinity and strong alkalinity, and late low salinity and acidity. The sandstone margins developed an alkaline diagenetic environment that was continuously affected by the diagenesis of the interbedded mudstones. Due to the evolution of the diagenetic fluids, the diagenetic sequence of beach-bar reservoirs was as follows: early weak carbonate cementation; feldspar and carbonate cement dissolution and authigenic quartz cementation; late carbonate and anhydrite cementation, authigenic feldspar cementation, and late quartz dissolution; and late carbonate cementation, feldspar dissolution, and authigenic quartz cementation.

The diagenetic strength was different or absent in various parts of the reservoirs during different diagenetic stages as a result of the reservoir development position, fluid flow pattern and filling with oil and gas (Xu Fa et al., 2015). For example, at the margins of the sandstone bodies, only carbonate cementation occurred, whereas multiple types of diagenesis occurred in the centers of the sandstone bodies.

The properties of the diagenetic fluids, the closeness of the diagenetic environment and the fluid flow pattern controlled the distribution of diagenetic products in the

reservoir (Liu Mingjie et al., 2016; Meng Yuanlin et al., 2016). Due to the diagenesis of the interbedded mudstones, the cement content is markedly higher within 1 m of the edge of the sandbodies. Beyond 1 m, the cement content slowly declined and became stable (Fig. 9a). Generally, base-type calcite cementation developed at the margins of the sandstones, which caused an increase in porosity from the boundary to the center of the sandstones (Fig. 9b and Figs. 10a, 10b). The late formation fluid was primarily concentrated in the centers of the thick sandstones with high porosity due to the strong cementation on their margins. Therefore, the relative content of ferro-carbonate (which formed during the late stage) and the proportion of dissolution pores of feldspar, carbonate cement, and quartz gradually increased from the margins to the centers of the sandstones (Figs. 9a, 9c and 9d). The relative content of dissolution pores decreased rapidly and tended to be stable from the margins to the centers of the sandstones. Heavy cementation caused large losses of primary pores and the presence of only a small number of dissolution pores at the margins of the sandstones, which comprised the majority of the reservoir space. The cementation was weak in the centers of the sandstones, and a large number of primary pores were retained. The content of dissolution pores was high, but the primary pores dominated the reservoir space at the centers of the sandstones (Fig. 10b).

The beach-bar sandstone reservoirs were separated within many downdropped fault blocks that were offset along oil-source faults. Oil faults in the downdip direction of the fault blocks effectively connected the formation fluids and allowed fluids to enter the reservoirs. The open early and middle stages of the diagenetic environments corresponded to the early acidic and late alkaline diagenetic environment, respectively, which were the primary diagenetic stages of the beach-bar sandstone reservoirs.

The diagenetic environment of the reservoirs in the western part was open, the dissolution ability of organic acids and alkaline fluids decreased gradually and dissolution products were transported to the upper part of the fault blocks during the updip migration via upwelling. The dissolution porosity of feldspar and carbonate cement and the dissolution porosity of quartz decreased gradually with increasing distance from the oil fault (Figs. 11a and 11b), whereas the content of authigenic quartz and carbonate cement gradually increased with increasing distance from the oil fault (Figs. 11c and 11d).

The early closed diagenetic environment made it difficult for the acidic fluid to flow into the fault blocks. The acidic diagenetic fluid was primarily distributed in the lower parts of the fault blocks, and the upper parts of the


fault blocks accepted less acidic fluid. The dissolution by organic acid decreased gradually updip within the fault blocks, which caused a gradual decrease in the dissolution porosity with increasing distance in the fault blocks (Fig. 11). Because the formation fluid circulation was slow, the feldspar dissolution products, such as SiO2 and kaolinite, were not transported out of the dissolution area and were deposited in situ, forming cements. The authigenic quartz content in the reservoirs decreased gradually with increasing distance from the oil-source faults (Fig. 11c).

There are abundant examples of kaolinite filling of reservoir pores in the lower parts of fault blocks (Taylor et al., 2010; Bjørlykke and Jahren, 2012; Bjørlykke, 2014; Li Mei, 2016). During the late alkaline environment stage, the diagenetic environment transitioned from closed to open. The content of the dissolution pores of quartz gradually decreased upward within the fault blocks due to the upwelling (Fig. 11b), whereas the amount of carbonate cement gradually increased with increasing distance from the oil-source faults (Fig. 11d).

Fig. 9. Relationship between content of diagenetic productsand porosity of beach-bar sandstone reservoirs and distance to inter-face of sandstone and mudstone of Es4s in the Dongying depression.

Fig. 10. Characteristics of porosity, cementation and dissolution in different positions of beach-bar sandstones interbedded with mudstone of Es4s in the Dongying depression.


5.2 Reservoir diagenetic evolution Between the time of deposition and 34.8 Ma, the burial

depth of the beach-bar sandstone reservoirs was shallow, the formation temperatures were low, and the diagenetic fluid was controlled by the primary depositional water. A small amount of early calcite cement developed along the margins of the sandstones due to the discharge of primary depositional water from the mudstones into the interbedded sandstones (Fig. 12).

During the stage from 34.8 Ma to 24.9 Ma, the diagenetic environment changed from weakly alkaline to acidic. Due to the first rapid transformation of the clay minerals, the carbonate cementation was strong along the margins of the sandstones, forming a cementation shell of a certain thickness (Fig. 12). Acidic formation fluids caused heavy feldspar dissolution in the centers of the thick sandstones, forming a large number of dissolution pores. Due to the upwelling in the open diagenetic environment in the western part of the Dongying depression, a large quantity of authigenic quartz developed, and the reservoir porosity of the lower parts of the fault blocks noticeably increased. Due to the closed diagenetic environment in the eastern part of the Dongying depression, considerable authigenic quartz developed in the dissolution zones in the lower parts of the fault blocks, and the dissolution was insufficient to effectively increase the reservoir porosity (Fig. 12). Late in this stage, the beach-bar sandstone reservoir experienced the first stage of oil filling, which promoted dissolution and restrained

the cementation of authigenic quartz toa certain extent. During the stage of 24.9 Ma to 21.1 Ma, the diagenetic

environment changed from acidic to alkaline. Due to the second rapid clay mineral transformation, the carbonate cementation along the margins of the sandstones increased further. The alkaline formation fluid caused dissolution of quartz and its overgrowths in the centers of the thick sandstones, forming alkaline dissolution pores. Due to the tectonic uplift, the diagenetic environments of the western and eastern parts became open, and the upwelling that created dissolution pores in the quartz decreased gradually upward within the fault blocks. In contrast, the amount of carbonate cement gradually increased upward within the fault blocks (Fig. 12).

After 21.1 Ma, the diagenetic environment changed from alkaline to acidic. The hydrocarbon generation from the thermal evolution of the hydrocarbon source rocks increased the pressure from normal to moderately and strongly overpressure. The diagenetic environments of the western and eastern parts shifted from open to close during this stage, which included the filling of the reservoirs with overpressured oil and gas. Organic acids and carbonic acid created a small amount of feldspar and carbonate cement dissolution pores in the lower parts of the fault blocks (Fig. 12), which led to the modern porosity and diagenetic characteristics of the beach-bar sandstones.

5.3 Formation mechanism of high-quality reservoirs

The characterization of diagenesis in the reservoirs and

Fig. 11. Relationship between content of diagenetic products in beach-bar sandstone reservoirs and distance to oil source faults of Es4s in the Dongying depression.


the analysis of diagenetic evolution indicate that the evolution of multiple alternating alkaline and acidic diagenetic environments and changes in the closeness of the diagenetic environments were the controlling factors in the development of the beach-bar sandstone reservoirs.

The evolution of multiple alternating alkaline and acidic diagenetic environments controlled the diagenesis of the reservoirs and the distribution pattern of the reservoir space (Lu Zhengxiang et al., 2015; Xi Kelai et al., 2015). The cementation shell along the margins of the sandstones (formed by the diagenesis of mudstones duringthe early-middle diagenetic evolution process)controlled the late-stage distribution of acidic and alkaline formation fluids in the reservoirs, which is reflected in the current distribution of diagenetic features and reservoir space (Wang et al., 2016). The diagenesis along the margins and in the pinchout zone of the sandbodies is of a single type and is primarily the product of an alkaline environment. In contrast, the central portions of the thick sandbodies exhibit a multi-phase superposition of alkaline dissolution/

cementation and acidic dissolution/cementation. Therefore, due to the evolution of multiple alternating alkaline and acidic diagenetic environments, the reservoir space is primarily located in the centers of the thick sandbodies and is characterized by the coexistence of primary intergranular pores and acidic and alkaline dissolution pores. Due to the differences in the diagenetic evolution between the margins and the cores of the sandstone bodies, the reservoir quality index RQI increases gradually from the margins to the centers of the sandstones (Fig. 13a).

The closeness of the diagenetic environments and the flow patterns of formation fluids controlled the variations in the physical properties of the cores of the thick sandbodies inside the fault blocks. The upwelling in the early-middle open diagenetic environment of the western part transported the diagenetic products out of the dissolution zone, which caused a net increase in the reservoir porosity and permeability in the centers of the thick sandstones in the lower parts of the fault blocks. The

Fig. 12. Diagenetic evolution and formation mechanism of beach-bar sandstone reservoirs of Es4s in the Dongying depression.


diagenetic products were concentrated in the upper parts of fault blocks, causing very low reservoir porosity and permeability and even tight layers. The RQI value gradually decreased with increasing distance from the oil-source fault in the fault blocks (Fig. 13b). The reservoir quality in the lower-central portion was better than that in the upper parts of the fault blocks. Due to the early closed diagenetic environment in the eastern part, dissolution led to extensive feldspar dissolution pores, although the dissolution products were not transported out of the dissolution zone. Therefore, the dissolution did not effectively increase the reservoir porosity. In addition, due to the filling of primary intergranular pores with authigenic quartz and kaolinite and a decrease in the sizes of pore-throat structures, the dissolution actually decreased the reservoir permeability to a certain degree in the lower parts of the fault blocks. Late alkaline diagenetic fluid flowed updip through the fault blocks during the middle stage of the open diagenetic environment. Late carbonate cements primarily precipitated in the upper parts of the fault blocks and reduced the reservoirs’ petro-physical properties. The RQI values increase and then decrease with increasing distance from the oil-source faults in the fault blocks (Fig. 13b), which indicates that the reservoir pore-throat structures were well developed in the centers of the fault blocks and became progressively narrower farther outward. The reservoir pore-throat structures in the lower parts of the fault blocks were better than those in the upper parts.

6 Conclusions

(1) The diagenetic fluids of the beach-bar sandstone

reservoirs of the Es4 Formation in the Dongying depression evolved from early high salinity and weak alkalinity to low salinity and strong acidity, late high salinity and strong alkalinity and late low salinity and acidity. This evolution was accompanied by two stages of oil and gas migration. The properties of the fluids at the

margins of the sandbodies were continuously highly saline and strongly alkaline. The western (eastern) reservoirs experienced early open (closed), middle open, and late closed diagenetic environments during burial. The flow pattern was characterized by upwelling during the majority of the diagenesis (in the east, a non-circulating pattern transitioned into an upwelling current).

(2) Due to the evolution of the diagenetic fluids, the diagenetic sequence of the beach-bar reservoirs was as follows: early weak carbonate cementation; feldspar and carbonate cement dissolution and authigenic quartz cementation; late carbonate and anhydrite cementation, authigenic feldspar cementation, and late quartz dissolution; and late carbonate cementation, feldspar dissolution, and authigenic quartz cementation. The diagenetic strength varied or was absent in various parts of the reservoirs during certain diagenetic stages. Due to the closeness of the diagenetic environments and the flow patterns of the diagenetic fluids, the amount of carbonate cement decreased from the margins to the centers of the sandstones, the relative amount of ferro-carbonate increased, the numbers of acidic and alkaline dissolution pores increased, the reservoir porosity increased, and the relative number of dissolution pores decreased. The number of acid and alkaline dissolution pores decreased upward within the fault blocks, the content of carbonate cement increased, and the content of authigenic quartz in the western (eastern) part increased (decreased).

(3) The evolution of multiple alternating alkaline and acid diagenetic environments and the closeness of the diagenetic environments controlled the diagenetic transformation of the beach-bar sandstone reservoirs and further controlled the formation of the high-quality reservoirs. The evolution of the multiple alternating alkaline and acidic diagenetic environments controlled the distribution of reservoir diagenesis and reservoir space. The margins and pinchout zones of the sandstones developed a cementation shell, filling the primary intergranular pores. Acidic and alkaline dissolution pores

Fig. 13. Distribution of reservoir quality index of beach-bar sandstone reservoirs in fault blocks of Es4s in the Dongying depression.


co-existed in the cores of the thick sandstones, and the reservoir quality index RQI gradually increased from the margins to the centers of the sandstones. The closeness of the diagenetic environment and the flow pattern of the diagenetic fluids controlled the variations in the reservoir properties of the fault blocks. The RQI values in the western part gradually decreased and first increased and then decreased in the eastern part with increasing distance from the oil-source faults. Acknowledgments

This research work was jointly funded by the National

Nature Science Foundation of China (grants No. 41402095 and U1262203), the Fundamental Research Funds for the Central Universities (grants No. 16CX02027A and 15CX08001A), and the Scientific and Technological Innovation Project Financially Supported by the Qingdao National Laboratory for Marine Science and Technology (grant No. 2015ASKJ01). We thank the Geosciences Institute of the Shengli oilfield, SINOPEC, for permission to access their in-house database, providing background geologic data and permission to publish the results.

Manuscript received Nov. 8, 2014 accepted Oct. 9, 2015

edited by Hao Qingqing

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About the first author WANG Jian, male, born in the Heze, Shandong province, China,

in December 1985. Earned the doctorate of Geology in the China University of Petroleum (East China) in June 2013. Now he is an associate professor in the University, engaged in sedimentology, sequence stratigraphy and reservoir geology.Add: 66 West Changjiang Rd., Economic and Technological Development Zone, Qingdao 266580, Shandong Province. E-mail: [email protected]; [email protected].

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