Origin of Dolomite in Lacustrine Organic-Rich Shale: A Case Study in the Shahejie Formation of the Dongying Sag, Bohai Bay Basin

Introduction

Shale oil and gas resources are becoming the major sources of energy in transition to low carbon energy resources (Vengosh et al., 2014; Liu et al., 2019; Wu et al., 2019; Wu et al., 2020; Tian et al., 2021; Zeng et al., 2021a; Li et al., 2022). And dolomitic rocks are the major hosts for sedimentary ore deposits and conventional hydrocarbon reservoirs (Hsu, 1966; Davies, 1979; Braithwaite et al., 2004; Wu et al., 2020; Ma et al., 2021; Zeng et al., 2021b). However, the “dolomite problem” in the sedimentary realm is still a subject of long-standing interest and study (Shukla and Baker, 1988; Land, 1992; Braithwaite et al., 2004; Burns et al., 2010). Dolomite is an important component of most of the lacustrine shale oil reservoirs in China, with various types, abundant occurrences, and wide distributions, including the second member of the Kongdian Formation of the Cangdong sag (Pu et al., 2016), the upper part of the fourth member and the lower part of the third member of the Shahejie Formation of the Dongying sag (Ma et al., 2020), and the Fengcheng member of the Mahu sag (Tang et al., 2021). Although a series of dolomitization models (e.g., evaporative, seepage-reflux, hydrothermal dolomitization) have been established to demonstrate the formation of massive marine dolomites (Morrow, 1982; Warren, 2000; Machel, 2004), there are fewer studies on the formation of dolomite in organic-rich shales. Recently, researchers have found that various microbes have essential implications during dolomitization in natural sedimentary environments (Wright, 1999; Bontognali et al., 2014; Petrash et al., 2017), which are related to the formation of dolomite in organic-rich shales.

Organic material of microbial origin has been proven to catalyze dolomite precipitation in sedimentary systems, especially in shales (Vasconcelos et al., 2006; Bontognali et al., 2008; Bontognali et al., 2010; Diloreto et al., 2020). Organic matter degradation plays a vital role in dolomite dissolution and precipitation in shales (Irwin et al., 1977; Seewald, 2003). Organic matter degradation can be driven by various biotic and abiotic reactions, which significantly influence dolomitization fluids (Irwin et al., 1977; Mazzullo, 2000; Petrash et al., 2017). Previous studies have shown a close relationship between dolomite formation in shales and depositional environments, microbial activities, and progressive burial diagenesis (Chowdhury and Noble, 1996; Braithwaite et al., 2004; Denny et al., 2020). This relationship makes the evaluation of the formation process of dolomite more challenging.

The lower part of the third member and the upper part of the fourth member of the Eocene Shahejie Formation have thick organic-rich shales with substantial vertical and horizontal variations in lithofacies and reservoir characteristics (Zhu et al., 2004; Yuan et al., 2006). Dolomite and calcite are two main minerals and are widely distributed in organic-rich shales in the Dongying sag. Furthermore, the dolomite content in organic-rich shales reaches 95%. A few authors have interpreted the rhombohedral dolomite as authigenic precipitates formed during early diagenesis and burial through mineralogical and petrographic examinations in organic-rich shale in the Dongying sag (Liu et al., 2019). In the current research, the petrology, element composition, and Sr isotopes are integrated to investigate the origin of dolomite in organic shales from the Shahejie Formation of the Dongying Sag during burial diagenesis. The mean objectives of this study are as follows:

1) to identify the generation of the dolomites;

2) to clarify the potential source(s) for the dolomitization process in organic-rich shales;

3) to investigate the major dolomitization mechanism.

Geological Setting

The Bohai Bay Basin (Figure 1), which developed on top of the Paleozoic North China Craton basement in eastern China, is a proliferous petroleum basin regarded as a significant target for shale oil (Ma et al., 2018; Luan et al., 2019). The basin has not experienced significant changes for the past 65 Ma (Jin et al., 2022). The Dongying sag is a Mesozoic-Cenozoic lacustrine half-graben rift-down warped basin lying in the southeastern Bohai Bay Basin (Cao et al., 2014; Ma et al., 2017). It covers an area of 5700 km² and is bounded to the east by the Qingtuozi Salient, to the south by the Luxi Uplift and Guangrao Salient, to the west by the Linfanjia and Gaoqing Salient, and to the north by the Chenjiazhuang-Binxian Salient (Guo et al., 2010; Luan et al., 2022). Due to the influences of the Tanlu fault, three predominant tectonic development stages have been experienced during sedimentary deposition, including an initial rifting stage, a sun-rifting stage and postrifting thermal subsidence (Liang et al., 2018).

FIGURE 1

FIGURE 1. Location, structural map and stratigraphic column of the Dongying Sag, modified after (He et al., 2017).

From bottom to top, stratigraphic successions in the Dongying sag consist of the following formations: 1) the Paleogene Kongdian, Shahejie (Es), and Dongying; 2) the Neogene Guantao and Minghuazhen (Nm); and 3) the Quaternary Pingyuan (Qp). The Shahejie Formation can be divided into four intervals: Es1, Es2, Es3, and Es4. The Eocene Shahejie Formation was deposited with abundant algal organic matter inputs include dinoflagellates of the Defladrea, Senegalinium, Palacostomocysts species, and green algae (Bohaiensis, Pediastrum, and Botryococcus genera) (Geochemistry of petroleum systems in the Niuzhuang South Slope of Bohai Bay Basin—part 1: source rock characterization).

The upper Es4 and lower Es3 are the major layers for shale oil exploration (Feng et al., 2013). Due to the hot and dry paleoclimatic conditions and the broad, subsiding rift basin conditions with deep saltwater, a thick layer of gypsum and a set of organic-rich shales were deposited in Es4s. Previous studies have shown that seawater intrusion may have caused saltwater conditions and organic-rich shale deposition during this period (Liu et al., 2020). The early Es3, which was deposited in a stable lacustrine basin and humid paleo-climatic conditions, resulted in another set of organic-rich shales (Liang et al., 2017).

Sampling and Methodology

A total of 42 samples were collected from the shale intervals of the NY1 and FY1 wells. All the samples were prepared as polished thin sections for detailed petrographic observation by polarizing microscopy and scanning electron microscopy (SEM).

Cathodoluminescence (CL) was performed under a microscope equipped with a CL8200- MK5 CL instrument at conditions of 17 kV and 600 μA.

Spot analyses of 42 major, trace, and rare earth elements in dolomite crystals were conducted by laser ablation–inductively coupled plasma–mass spectrometry (LA–ICP–MS) at Beijing Createch Testing International (Beijing, China) using an ESI NWR 193 nm excimer laser ablation system coupled to an Analytik Jena Plasma Quant mass spectrometer. The spot size and frequency of the laser were set to 35 mm and 10 Hz, respectively. All elements were calibrated against multiple synthetic reference glasses (NIST SRM 610, NIST SRM 612, BHVO-2G, BCR-2G, and BIR-1G). All the rare earth element (REE) values were normalized to the Post-Archean Australian Shale (PAAS) values, and Ce anomalies were used for calculations with the formulae of Bau and Dulski (1996).

The ⁸⁷Sr/⁸⁶Sr values of dolomites were analyzed with a Neptune Plus multicollector (MC)–ICP–MS equipped with a Resolution SE 193 nm excimer ArF laser ablation system at Beijing Createch Testing International (Beijing, China).

The bulk and clay X-ray diffraction (XRD) data of 168 samples and total organic carbon (TOC) contents of 146 samples were collected from Shengli Oilfield Company for basic mineral composition analysis.

Results

Definition of Lithofacies

The shale samples from the Es4s and Es3x intervals in the NY1 and FY1 wells are composed of calcite (1–74%, average of 38.8%), quartz (1–62%, average of 23.9%), feldspar (1–35%, average of 3.6%), clay minerals (9–44%, average of 21.8%), dolomite (1–97%, average of 24.9%), pyrite (0–48%, average of 2.8%), and anhydrite (0–90%, average of 1.4%), with rich organic matter ranging from 0.15 to 12.28% (average of 4.1%). Five lithofacies, including light brown laminated calcareous shale, brown laminated calcareous shale, grey silty shale, greyish green gypsum-bearing shale, and grey massive calcareous shale, are defined according to the colour, structure, mineral composition, and TOC (Figure 2).

FIGURE 2

FIGURE 2. Types and features of main lithofacies.

Dolomite Petrography

Petrographic examinations reveal four main generations of dolomite (Dol-1, Dol-2, Dol-3, and Dol-4). The investigated dolomite is mainly present in light brown laminated calcareous shale and brown laminated calcareous shale.

Dolomite 1 (Dol-1, 4–93%) occurs as scattered and/or laminated dolomicrite in all four lithofacies, showed the most abundant in the lower part of Es4s. It consists of a replacive micritic to near-micritic pervasive dolomite that ranges from 2 to 10 μm, is generally mimetic, and preserves sedimentary fabrics (Figure 3A). In addition, Dol-1 is usually tightly packed (nonporous) and has nonplanar mosaic micritic crystals with irregular intercrystallite boundaries and dull CL. Moreover, euhedral pyrites and pyrite framboids are often observed in the Dol-1-rich samples (Figure 3E).

FIGURE 3

FIGURE 3. Macroscopic and microscopic photographs of petrographic features of the investigated dolomites. (A) Tightly packed Dol-1 in greyish green gypsum-bearing shale, plane-polarized light, Well NY1, 3494.28 m. (B) Scattered Dol-2 in laminated calcareous shale, plane-polarized light, Well NY1, 3326.58 m. (C) Microscopic images of Dol-3, plane-polarized light, Well FY1, 3195.3 m. (D) Microscopic images of Dol-4, Well NY1, 3474.9 m (E) backscattered electron (BSE) image with energy-dispersive X-ray spectroscopy (EDS) spectrum of Dol-1, Well NY1, 3494.28 m (F) BSE image with EDS spectrum of Dol-2, Well FY1, 3211.53 m (G) BSE image with EDS spectrum of Dol-3, Well NY1, 3314.97 m (H) BSE image with EDS spectrum of Dol-1, Well NY1, 3494.28 m (I) BSE image of Dol-2, Well NY1, 3431.05 m (J) BSE image with EDS spectrum of barite, Well FY1, 3405.14 m (K) BSE image of Dol-4, Well NY1, 3474.9 m. Py-pyrite; Ba-barite.

Dolomite 2 (Dol-2, 2–27%) occurs as sub-to euhedral zoned rhombs (cloudy cores with clear rims) with a planar-e texture (Figure 3B), which are attributed to progressive dolomitization during burial and range from 20 to 100 μm (Figure 3E). Dol-2 is the most abundant in the upper part of Es3x and the middle part of Es4s, and it is especially less abundant in greyish green gypsum-bearing shale in the lower part of Es4s (less than ca. 3%). Observations reveal that intercrystallite porosity is developed in Dol-2. Under fluorescence, Dol-2 shows zoned green/yellow autofluorescence (Figure 3H). Furthermore, euhedral pyrites and pyrite framboids show a syngenetic relationship with Dol-2.

Dolomite 3 (Dol-3) is not abundant. By qualitative evaluation, Dol-3 is less than ca. 1% per thin section and is usually scattered in organic-rich laminae in light brown laminated calcareous shale in lower Es3x (Figure 3C). In addition, some Dol-3 has synthetic relationships with barite and coalesced pyrite (Figure 3J). Dol-3 forms medium to coarse planar-s to nonplanar crystals (100–500 μm) with dull luminescence. Dol-3 crystals show undulate (sweeping) extinction similar to saddle dolomite, encased by phosphatic particles. In SEM images, these phosphatic particles are clearly remnantsof ostracods or algae (Figure 3G).

Dolomite 4 (Dol-4) postdates Dol-2 and consists of densely packed undeveloped unzoned crystals with irregular crystal boundaries (up to ca. ∼5% volume) with coarser crystals than Dol-1 and Dol-2 (more than 500 μm in size) that appear relatively transparent under plane-polarized light (Figure 3D). This type of dolomite, which is observed only in a few abnormal pressures in calcareous shale samples, shows a syngenetic relationship with coalesced pyrite (Figure 3K). Under CL, Dol-4 exhibits a dull red luminescence. It shows corroded rims in fractures and has no sign of zoning or earlier fabric. As shown in Figure 3K, Dol-4 postdates the previous dolomite types and is the product of the latest diagenetic stage.

Occurrence of Dolomite in Shales

Observations found that dolomite has a high content in greyish green gypsum-bearing shale (5–98%, Figure 4A) and brown laminated calcareous shale (21–95%, Figures 4B,C). And Dol-1 and Dol-2 are often scattered and have less content (2%–21%, 4%–28%, respectively) in grey massive calcareous shale (Figure 4D) and grey silty shale (Figure 4E), mixed with scattered organic matter, clay minerals, calcite, and terrigenous debris (Figures 4I,M) with Dol-3 and Dol-4 rarely development.

FIGURE 4

FIGURE 4. Macroscopic and microscopic photographs of the investigated dolomites. (A) Greyish green gypsum-bearing shale, Well NY1, 3491.95 m. (B) Type Ⅰ laminated calcareous shale, Well NY1, 3438.97 m. (C) Light brown laminated calcareous shale, Well FY1, 3361.44 m. (D) Grey massive calcareous shale, Well NY1, 3387.75 m. (E) Grey silty shale, Well NY1, 3468.65 m (F) BSE image of Dol-1 and gypsum in greyish green gypsum-bearing shale, Well NY1, 3491.95 m (G) BSE image of Dol-1 in brown laminated calcareous shale, Well NY1, 3438.97 m (H) BSE image of dolomite lamina in light brown laminated calcareous shale, Well FY1, 3361.44 m. (I) Microscopic image of grey massive calcareous shale, Well NY1, 3387.75 m (J) EDS mapping of Dol-1 and gypsum in greyish green gypsum-bearing shale, Well NY1, 3491.95 m (K) EDS mapping of Dol-1 in brown laminated calcareous shale, Well NY1, 3438.97 m (L) EDS mapping of dolomite lamina in light brown laminated calcareous shale, Well FY1, 3361.44 m. (M) Microscopic image of grey silty shale, Well NY1, 3468.65 m. Gyp-gypsum; Tg-terrigenous silty clastic; Py-pyrite.

Brown laminated calcareous shale, which brown dolomite lamina interbedded with dark grey organic-rich lamina (Figures 4G,H), with a single lamina was less than 500 µm. The dolomite lamina primary consisted of Dol-1, which is similar in texture to that found in modern settings where precipitation typically occurs in alkaline and saline waters with high Mg/Ca ratios (Last et al., 2012). Brown laminated calcareous shale was less developed, which was discontinuously developed vertically, with less than 40 cm thickness.

Light brown laminated calcareous shale developed more frequently, forming continuous laminae to discontinuous lenses with alternating greyish and brown layers. The thickness of a single lamina is approximately 200 µm. The greyish layers are mainly Dol-2 and Dol-1 with clay minerals, terrigenous debris, and organic matter, while the brown layers are mainly composed of calcite (Figures 4H,L). Moreover, Dol-3 and Dol-4 are mainly observed in light brown laminated calcareous shale.

The enriched dolomite greyish green gypsum-bearing shale is composed of greyish-green and white layers, and the evaporative minerals are mixed with Dol-1 (Figures 4F,J).

Major and Trace Elements

Only two chemical compositional data points of Dol-1 are obtained due to its small mineral size. The geochemical compositions of the investigated dolomite are summarized in Table 1. Dol-1 has a lower CaO content (Dol-1 = 24.40 ± 2.45%, n = 2), while the other investigated dolomites have comparable CaO contents (Dol-2 = 37.50 ± 6.13%, n = 10; Dol-3 = 42.49 ± 9.72%, n = 6; Dol-4 = 45.59 ± 1.38%, n = 5). Among them, Dol-3 and Dol-4 are similar but are slightly higher than Dol-2. All four types of dolomites are comparable in MgO (Dol-1 = 18.56 ± 0.56%, n = 2; Dol-2 = 20.56 ± 5.08%, n = 10; Dol-3 = 20.56 ± 6.91%, n = 6; Dol-4 = 18.54 ± 0.76%, n = 5). Therefore, dolomites are mainly nonstoichiometric. From Dol-1 to Dol-4, the trace elements show progression, especially in Fe and Mn contents (Figure 5A), despite the general overlap in the elemental compositions. The Fe concentrations increase from 1598 ± 198 ppm in Dol-1 to 16861 ± 7926 ppm in Dol-2 to 18733 ± 8494 ppm to 22853 ± 1968 ppm in Dol-4. The Mn contents vary from 800 ± 66 ppm in Dol-1 to 5276 ± 2716 ppm in Dol-2 to 9098 ± 3800 ppm in Dol-4 to 3581 ± 271 ppm in Dol-4. The Sr concentrations decrease from approximately 1500 ppm in Dol-1 to approximately 500 ppm in Dol-4. Higher Sr contents than dolomite characterize calcite. The Mn content is similar to that of dolomite (Figure 5B).

TABLE 1

TABLE 1. CaO, MgO, Fe, Mn, Sr, REE, statistics of the investigated dolomites.

FIGURE 5

FIGURE 5. Scatter diagrams of Mn vs. (A) Fe and (B) Sr for the investigated dolomites.

Rare Earth Elements

Table 1 summarizes the statistics of the REE concentrations and the calculated Ce and Eu anomalies of the investigated samples from Es4s and Es3x. The mean ∑REE values of dolomites are the lowest in Dol-1 (40.48 ± 1.38 ppm, n = 2) and the highest in Dol-4 (155.91 ± 99.74 ppm, n = 6). Dol-2 and Dol-3 have moderate values (Dol-2 = 70.27 ± 14.92 ppm, n = 5; Dol-4 = 61.25 ± 28.06 ppm, n = 6). The variations in the REE concentrations of the investigated carbonate minerals are also indicated by their shale-normalized patterns (Figure 6). The Dol-1, Dol-2, and Dol-3 patterns are very similar, although that of Dol-4 is different.

FIGURE 6

FIGURE 6. Patterns of mean shale-normalized REE concentrations of the investigated dolomites.

Calculations of the Ce and Eu anomalies (Bau and Dulski, 1996) show that Dol-4 has the lowest mean Ce anomaly (0.93 ± 0.08), whereas Dol-1 has the highest mean Ce anomaly (1.16 ± 0.06). The mean Ce anomalies of Dol-2 and Dol-1 are between the following end-member values: Dol-1 = 1.13 ± 0.09; Dol-2 = 1.01 ± 0.08. The Eu anomalies (Figure 6) range from 1.02 ± 0.11 in Dol-4 to 1.52 ± 0.28 in Dol-3.

Sr Isotopes

The ⁸⁷Sr/⁸⁶Sr ratios largely overlap in Dol-1 to Dol-4, with values of 0.71071–0.71133, 0.71080–0.71145, 0.71085–0.71131, and 0.71071–0.71083, respectively. The ⁸⁷Sr/⁸⁶Sr values of Dol-4 tend to be lower than those of the others (Figure 7).

FIGURE 7

FIGURE 7. ⁸⁷Sr/⁸⁶Sr values of the investigated dolomites.

Discussion

Sedimentary Environmental Reconstruction During Dolomite Formation

The enrichment of Dol-1 in greyish green gypsum-bearing shale indicated that the formation of Dol-1 was continuously under shallow water, reducing evaporative and high-salinity environments in lower part of Es4s, with high ratios of Mg/Ca, which were thought to promote dolomitization.

The enrichment of dolomite in brown laminated calcareous shale was similar in texture to that found in modern settings where precipitation typically occurs in alkaline and saline waters, with high Mg/Ca ratios (Last et al., 2012). Such as in Pellet Lake and Milne Lake, laminated dolomites with small crystal sizes and poorly-defined faces are interpreted to form from lake water by rapid precipitation. And in lakes from Western Victoria, Australia, dolomites had been proved to precipitate as a result of 1) high salinity, 2) high Mg/Ca ratios, and 3) high alkalinity (Deckker and Last, 1988). These dolomite occur as similar to Dol-1. The flat parallel to wavy parallel laminae of dolomite in light brown laminated calcareous shale implies that the environment was stable for a period, and the alternation of greyish and brown layers indicates that the climate changed periodically on a small scale. The continuous regular alternation of greyish and brown layers is interpreted as a steady rhythm from seasonal changes. Previous studies show that algae’s periodic thriving caused much calcite precipitation in the lake, and brown calcite lamina formed. In winter, greyish lamina was formed during intervals of algae productivity with a large number of organic accumulation (Zhao et al., 2019). In winter, algal activity declines, and the lake hydrodynamic conditions weaken. Organic-rich matter and debris tend to accumulate at the lake bottom. The occurrence of framboidal pyrite (Figure 4H) indicated a more anoxic environment. But the sporadical distribution of framboidal pyrite suggested the limited amount of SRB, which proved that the alkalinity of the anoxic environment is not elevated, and carbonate precipitation is inhibited continuously. Under such weak hydrodynamic conditions, the detrital and organic matter are accessibly preserved. Therefore, more minor dolomite precipitated, and organic matter accumulated among the debris. Moreover, framboidal pyrite is distributed sporadically along with the sediment-water interface. So, the greyish layer formed.

Variations in Dolomitization Fluids

Dolomite Petrography

As suggested by the petrographic characteristics of Dol-1, the nonluminescence of Dol-1 dolomite crystals under CL and its high Fe content indicate the widespread occurrence of Fe²⁺ (the main suppressor of luminescence in carbonate minerals) in dolomitizing fluids from which Dol-1 dolomites were formed (Machel, 1985; Savard et al., 1995; Perri and Tucker, 2007). The Dol-1 tends to be enriched in high-salinity, shallow water, reducing, and evaporative environments. That is, the parental fluid of this type of dolomite was most likely derived from the evaporation of water. Moreover, high Mg/Ca conditions favoured precipitation of dolomite. In addition, as gypsum forms, SO₄²⁻ can be consumed and reduce the inhibition of dolomite formation by sulfate ions. However, some Dol-1 has no syngenetic relationship with greyish green gypsum-bearing shale, indicating a lower salinity of dolomitizing fluid than gypsum precipitation (Lu and Meyers, 1998; Jones et al., 2000; Azomani et al., 2013; Rott and Qing, 2013; Olanipekun et al., 2014). Alternatively, Dol-1 could have precipitated from mixed evaporative meteoric waters (Azomani et al., 2013). Previous research has established that bacterial sulfate reduction (BSR) can lower the kinetic barrier that prevents direct dolomite precipitation from magnesium-saturated brines at low temperatures, which would facilitate dolomite formation under such conditions (Vasconcelos et al., 1995; Vasconcelos and McKenzie, 1997; Van Lith et al., 2003; Petrash et al., 2017). Microscopic observations show the widespread occurrence of euhedral pyrites and pyrite framboids in the Dol-1-rich samples, suggesting that BSR may have been operative in reducing diagenetic fluids, which possibly promoted dolomite precipitation from mixed evaporative meteoric waters (Vasconcelos and McKenzie, 1997; Wacey et al., 2007; Bontognali et al., 2014). Additionally, BSR can consume SO₄²⁻ and release Mg²⁺ that provides Mg²⁺ for dolomitization.

Most Dol-2 crystals show cloudy cores with clear rims attributed to progressive dolomitization during burial that resulted in the replacement of precursor carbonate (cloudy cores) followed by later cementation as a clear rim during crystallization (Kırmacı, 2008; Rameil, 2008), with red rim and dull core CL images. The nonluminescence of the clear rims of Dol-2 dolomite crystals indicates the incorporation of Fe²⁺ during their further overgrowth in deeper burial realms (Machel, 1985; Savard et al., 1995). Moreover, the subhedral to euhedral crystals of Dol-2 suggest that they formed at low temperatures, likely lower than the critical roughening temperature for dolomites (50–60°C; Gregg and Sibley, 1984). While there are no clear remnant biological components revealed by SEM examination, the green/yellow autofluorescence of Dol-2 confirms the presence of diffuse organic matter, which points toward a biological association (Dupraz et al., 2004; Dupraz and Visscher, 2005; Perri and Tucker, 2007; Last et al., 2012). Dol-2 tends to accumulate in organic-rich laminae, indicating an organic matter-related origin.

In particular, Dol-3, which is encased by phosphatic particles and the remains of ostracods or algae, occurs with organic matter and is regarded as a potential microbial-related feature. The scattered residual phosphatic particles within Dol-3 indicate Dol-3 replacement of phosphatic particles (Liu et al., 2020). Results from earlier studies demonstrate that bacterial cells can, for example, serve as templates for the nucleation and growth of phosphatic particles in an oversaturated phosphate solution. In the study area, bacterial cells can also provide nucleation sites for dolomite crystals. The occurrence of phosphatic particle grains within organic matter may imply microbial degradation of organic matter, which releases phosphate to pore waters and promotes the reprecipitation of phosphate (Baturin and Kochenov, 2001). Furthermore, in contrast to Dol-1 and Dol-2, the coarser crystal sizes and nonplanar crystal surfaces with undulate (sweeping) extinction of Dol-3 suggest that it precipitated at higher temperatures during progressive burial (Gregg and Sibley, 1984; Gregg and Shelton, 1990), as indicated by the occurrence of enclosed barite and coalesced pyrite. Based on the burial history of the shale in Es4s and ES3x, the diagenetic temperatures of Dol-3 may exceed 100°C, considering the limited influence of hydrothermal activities in the Dongying sag (Song et al., 2009).

Fracture-filling Dol-4 (saddle dolomite) precipitated at high temperatures (over 100°C) during deep burial, as indicated by its undulate extinction (Gregg and Sibley, 1984). There might be some relationship with sulfate reduction, as indicated by the occurrence of enclosed coalesced pyrite in Dol-4 (Radke and Mathis, 1980). Pyrite filled the abnormal pressures related to the thermal evolution of organic matter, indicating that its formation had a close relationship with hydrothermal activities.

Major and Trace Elements

As shown in Table 1, the high Na content in Dol-1 is related to the high Mg/Ca fluid generated by strong syngenetic evaporation and concentration, and this kind of high-salinity fluid from strong evaporation can overcome the dynamic dolomitizing barrier to facilitate dolomite formation by alteration (Yang et al., 2021). The Ca concentrations of dolomite indicate that all types of dolomites are nonstoichiometric, showing that these nonstoichiometric dolomites are related to the formation of the rock-buffered diagenetic system (Warren, 2000) and lack sufficient fluid mobility and/or a relatively inadequate supply of Mg²⁺. Previous studies show that lacustrine dolomites with an organogenic origin are mainly Ca-rich and finely crystalline (Armenteros, 2010). Thus, the high Ca contents of dolomites may be related to the carbonate-precipitating microbial consortium.

The high Sr contents of Dol-1 (Dol-1>550 ppm) support a sabkha or bacterial origin (Sánchez-Román et al., 2011; Azmy et al., 2013; Hou et al., 2016). Conventional distribution coefficients of dolomite (i.e., 0.015–0.06; Banner, 1995; Pedone and Dickson, 2000) and present-day Sr/Ca ratios of lake water (0.016–0.018) can reveal the origin and nature of their parent dolomitizing fluids in a particular diagenetic environment. The Sr/Ca molar ratio of the dolomitizing fluid can be calculated from the following equation:

$D_{Sr}=\left[{\left({}^{\text{m}}Sr/{}^{m}Ca\right)}_{dol}/{\left({}^{\text{m}}Sr/{}^{m}Ca\right)}_{solution}\right]$

Where ^mSr and ^mCa are the molar concentrations of Sr and Ca in dolomites, respectively. The calculated Sr levels of dolomite in the study are approximately 50–150 ppm. Thus, the ∼1000 ppm average Sr concentration in the dolomite suggests that the mineral originated due to the replacement of aragonite. However, recent studies show that laboratory-precipitated microbial dolomite in a sulfate-rich culture demonstrates elevated Sr levels. The Sr concentration obtained from the experimental results is consistent with that of dolomite in the study area, which suggests that the origin of the dolomite in the study area is influenced by microbes.

Iron (Fe) and manganese (Mn) are regarded as reliable proxies of redox conditions (Table 1) because of their enrichment in carbonates in enhanced reducing conditions during progressive burial (Land, 1986), whereas they can also be influenced by the concentrations of these elements in the diagenetic fluid (Azomani et al., 2013; Olanipekun et al., 2014). The data show a clear trend in which the mean values of Fe and Mn increase with progressive burial from Dol-1 to Dol-2 to Dol-3. This trend suggests that the geochemical properties of dolomitization fluids are similar. The Sr isotope results support the former scenario.

Rare Earth Elements

Fifteen elements, ranging in atomic number from 57 (Lanthanum) to 71 (Lutetium) on the periodic table of elements, compose the rare earth element (REE) group, which is subdivided into three groups: 1) light rare earth elements (LREEs), from La to Nd; 2) medium rare earth elements (MREEs), from Sm to Dy; and 3) heavy rare earth elements (HREEs), from Ho to Lu. It has been demonstrated that the diagenesis of carbonates at low water/rock interaction ratios (insignificant recrystallization) does not significantly alter their REE compositions (Banner and Hanson, 1988; Webb and Kamber, 2000; Nothdurft et al., 2004; Webb et al., 2009). Therefore, REEs can provide significant information on chemical changes in diagenetic fluids, equilibrium in water-rock interactions, and even the properties of diagenetic fluids and redox conditions (Azmy et al., 2013; Azomani et al., 2013; Hou et al., 2016). All the REEs are trivalent, except for cerium (Ce) (Ce⁴⁺ in some redox-sensitive environments) and europium (Eu) (Eu²⁺ in hydrothermal waters), which have similar ionic radii. Their comparable distribution coefficients that are incorporated in carbonates provide clues about the geochemistry of the parent fluid (Sholkovitz et al., 1994; Shaojun and Mucci, 1995; Sholkovitz and Shen, 1995). REEs are stable and cannot be altered in diagenetic conditions with low water/rock interactions. Therefore, they can be regarded as good proxies of the nature and origin of diagenetic fluids and redox conditions.

The shale-normalized patterns of Dol-1, Dol-2, and Dol-3 are similar. Still, they exhibit insignificant differences, which strongly supports the suggested scenario of the same dolomitizing fluid in which its elemental composition varied slightly during dolomitization and excluded the involvement of magmatic fluids or fluids of other origins in dolomitization.

The Dol-1, Dol-2, and Dol-3 Ce/Ce* values are almost or approximately 1 (Figure 6), suggesting possible conditions for reduction (anoxic and dysoxic). However, the petrographic characteristics of Dol-1 support an early stage of dolomitization under surface or near-surface conditions. The slight enrichment in Ce indicates that the dolomitizing parent fluids mixed with fluids that progressively circulated with REE-rich, particularly Ce-rich, siliciclastic rocks in the basin, such as the cooccurring shales and sandstones, which changed its geochemical characteristics (Abanda and Hannigan, 2006).

However, the Ce/Ce* values of Dol-4 are different from those of other types of dolomite, which are lower than one. The petrographic characteristics of Dol-4 suggest its close relationship with the thermal evolution of organic matter, which causes this significant difference in Ce/Ce* compared to other types of dolomite. All samples have slight positive Eu/Eu* anomalies. These anomalies are usually associated with the dissolution of plagioclase feldspars in clastic rocks, such as sandstones (Bau et al., 2010). Moreover, the shale-normalized patterns of Dol-4 show a significant difference. The patterns are more similar to a bell-shaped curve compared to those of Dol-1, Dol-2, and Dol-3. This finding supports that the Dol-4 parent dolomitizing fluid had a different composition, possibly due to extensive circulation in the surrounding basinal rocks. This is consistent with the comparable REE mean values (Table 1) and their occurrence.

Sr Isotope

The Sr isotope values indicate a common source of dolomitization of the investigated dolomite. The Sr isotope values higher than those of seawater suggest that the deep burial conditions caused diagenetic fluids to circulate through clastic rocks, such as shale beds within the basin. The dolomitization resulted in a slight shift to more radiogenic values in the host carbonate.

Formation Mechanism of Dolomites in Lacustrine Shale

Dol-1: During the Es4s and Es3x periods, the Dongying sag was a lacustrine basin influenced by sea transgression, with warm, semiarid to arid climatic conditions. A sea transgression brought enough Mg²⁺ and increased the lake salinity. Moreover, this kind of setting was favourable for water evaporation and increased the salinity of water in the study area. The water salinity was elevated under such conditions, and the evaporites began precipitating (e.g., gypsum/anhydride). At that time, many Dol-1 crystals likely formed. In the deep part of the lacustrine basin, mixed evaporative meteoric waters were insufficient for the precipitation of evaporites (Figure 8A). Under both circumstances, BSR helped overcome the kinetic barrier of dolomite and promoted the precipitation of Dol-1 at a low temperature in Earth surface settings (Machel, 1985; Vasconcelos and McKenzie, 1997). Moreover, the initially deposited carbonate minerals provided nucleation sites for primaeval dolomite crystals, which was beneficial for dolomite precipitation. The heavy density of brines caused the brines to percolate to the deeper sediments, leading to extensive dolomitization. During this process, a minor number of Dol-1 crystals likely formed in organic-rich shales. At that time, the brines influenced BSR-mediated dolomite crystals, resetting their geochemical signatures. This process caused the BSR-mediated dolomite to have identical geochemical signatures to dolomite formed from evaporated water.

FIGURE 8

FIGURE 8. Dolomitization mechanism of the investigated dolomite. Py-pyrite; Ba-barite; Cal-calcite; Dol-dolomite; Cl-clay minerals; Q-quartz; OM-organic matter.

Dol 2: As suggested by Dol-1 petrological and geochemical characteristics, Dol-2 resulted from the recrystallization of the precursor Dol-1 during burial diagenesis and was influenced by the thermal evolution of organic matter. At such burial depths, the transformation of clay minerals during diagenesis can release magnesium ions that influence diagenetic fluid properties and the diagenetic microenvironment, thereby facilitating the process of dolomitization. Considering the mature stage of the organic matter, many organic acidic fluids and HS⁻ existed in these strata, which dissolved a small amount of calcite and provided adequate space for dolomitization. Furthermore, abundant Fe²⁺ in reductive lake environments combined with HS⁻ form sulfide and pyrite. The dolomitization process was also facilitated by sulfide dissolution, which explains the syngenetic relationship between Dol-2 and pyrite (Figure 8B).

Dol-3: Petrological and geochemical characteristics suggest that Dol-3 resulted from the intense recrystallization of previously formed dolomite. In winter, algal and Ostracoda died and deposited, forming a calcareous skeleton. At the early stage, bacteria inside calcified ostracods and algae provided nucleation sites for dolomite. The transformation of clay minerals and the dissolution of dissolved calcite and dolomite by organic acids provided enough Ca²⁺ and Mg²⁺ to the inside of calcified biological skeletons for dolomitization. Then, the geochemical characteristics of the former dolomite were resettled by early diagenetic fluid. Moreover, with the thermal evolution of organic matter, the inside temperature of calcified biological skeletons was high, which influenced the precipitation of Dol-3 (Figure 8C).

Dol-4: Several carbonate minerals were dissolved with the thermal evolution of organic matter, thereby supplying Mg²⁺ and Ca²⁺ to pore water. The transformation of clay minerals also provided Mg²⁺ and Ca²⁺ for dolomitization. Because of excessive pressure, a small number of abnormal pressures formed. This process released Mg²⁺-rich pore water into the fractures, and then Dol-4 precipitated (Figure 8D).

Conclusion

Based on detailed field investigations and petrographic and geochemical analyses of the dolomite in the lacustrine organic-rich shale of the Shahejie Formation, Dongying sag, Bohai Bay Basin, the main conclusions are as follows:

1) Four types of dolomites, which represent episodic recrystallization, were classified based on crystal size and shape: (1) micritic dolomite (Dol-1), (2) euhedral dolomite (planar-e) (Dol-2), (3) anhedral dolomite (nonplanar) encased by phosphatic particles (Dol-3), and (4) fracture-filling anhedral dolomite (Dol-4).

2) Dol-1 formed from evaporated water or mixed evaporative meteoric waters with seawater provided by transgression during the early burial stage, possibly with microbial mediation (BSR).

3) Dol-2 resulted from recrystallization of the precursor Dol-1 during burial diagenesis and was influenced by the thermal evolution of organic matter.

4) Dol-3 likely resulted from intense recrystallization and was influenced by the thermal evolution of organic matter and bacterial activity.

5) Dol-4 precipitated from the hydrothermal fluid that resulted from the thermal evolution of organic matter, with extensive circulation in the surrounding basinal rocks.

Data Availability Statement

The original contributions presented in the study are included in the article/Supplementary Material, further inquiries can be directed to the corresponding authors.

Author Contributions

All authors listed have made a substantial, direct and intellectual contribution to the work, and approved it for publication.

Funding

This paper is supported by the National Natural Science Foundation of China (41972142; 42102153), China Postdoctoral Science Foundation (2021M693500), Specialized Fund for Shandong Postdoctoral Innovation Project (202103012), and the Fundamental Research Funds for the Central Universities (20CX06086A).

Conflict of Interest

YH was employed by the company CNOOC Tianjin Company. WL was employed by the company Oil and Gas Survey.

The remaining authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

Publisher’s Note

All claims expressed in this article are solely those of the authors and do not necessarily represent those of their affiliated organizations, or those of the publisher, the editors and the reviewers. Any product that may be evaluated in this article, or claim that may be made by its manufacturer, is not guaranteed or endorsed by the publisher.

Acknowledgments

We are grateful to Paulo Joaquim Nota from China University of Petroleum (East China) who modified the manuscript. We are indebted to Cunfei Ma from China University of Petroleum (East China) who collected and generously provided the shale samples.

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