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Volume 32 Issue 4
Aug.  2021
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Yong'an Xue, Meng Zhao, Xiaojian Liu. Reservoir Characteristics and Controlling Factors of the Metamorphic Buried Hill of Bozhong Sag, Bohai Bay Basin. Journal of Earth Science, 2021, 32(4): 919-926. doi: 10.1007/s12583-021-1415-1
Citation: Yong'an Xue, Meng Zhao, Xiaojian Liu. Reservoir Characteristics and Controlling Factors of the Metamorphic Buried Hill of Bozhong Sag, Bohai Bay Basin. Journal of Earth Science, 2021, 32(4): 919-926. doi: 10.1007/s12583-021-1415-1

Reservoir Characteristics and Controlling Factors of the Metamorphic Buried Hill of Bozhong Sag, Bohai Bay Basin

doi: 10.1007/s12583-021-1415-1
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  • The burial depth of the metamorphic buried hill of the Bozhong sag is more than 4 500 m, however, the controlling factors of the reservoirs are not clear. Based on cores and sidewall cores obtained from 15 wells, this paper describes the reservoir characteristics and discussed their controlling factors. The metamorphic basement of the Bozhong sag consists of metamorphic granite, migmatitic granite and gneiss. These felsic rocks are more likely to develop fractures, thereby improving the reservoir properties. The Indosinian, Yanshanian and Himalayan tectonic events greatly reformed the Bozhong 19-6 metamorphic buried hill, forming a large scale fracture system. Weathering and deep thermal fluid contributed to the development of dissolved pores of the reservoirs. In general, controlled by lithology, tectonics, weathering and deep thermal fluid, the reservoir pattern of the metamorphic buried hill of the Bozhong 19-6 structure was established.
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Reservoir Characteristics and Controlling Factors of the Metamorphic Buried Hill of Bozhong Sag, Bohai Bay Basin

doi: 10.1007/s12583-021-1415-1

Abstract: The burial depth of the metamorphic buried hill of the Bozhong sag is more than 4 500 m, however, the controlling factors of the reservoirs are not clear. Based on cores and sidewall cores obtained from 15 wells, this paper describes the reservoir characteristics and discussed their controlling factors. The metamorphic basement of the Bozhong sag consists of metamorphic granite, migmatitic granite and gneiss. These felsic rocks are more likely to develop fractures, thereby improving the reservoir properties. The Indosinian, Yanshanian and Himalayan tectonic events greatly reformed the Bozhong 19-6 metamorphic buried hill, forming a large scale fracture system. Weathering and deep thermal fluid contributed to the development of dissolved pores of the reservoirs. In general, controlled by lithology, tectonics, weathering and deep thermal fluid, the reservoir pattern of the metamorphic buried hill of the Bozhong 19-6 structure was established.

Yong'an Xue, Meng Zhao, Xiaojian Liu. Reservoir Characteristics and Controlling Factors of the Metamorphic Buried Hill of Bozhong Sag, Bohai Bay Basin. Journal of Earth Science, 2021, 32(4): 919-926. doi: 10.1007/s12583-021-1415-1
Citation: Yong'an Xue, Meng Zhao, Xiaojian Liu. Reservoir Characteristics and Controlling Factors of the Metamorphic Buried Hill of Bozhong Sag, Bohai Bay Basin. Journal of Earth Science, 2021, 32(4): 919-926. doi: 10.1007/s12583-021-1415-1
  • The concept of buried hill, which was first proposed by Sidney Powers, refers to topographic highs on old land surfaces which are reflected in the superposed sedimentary deposits (Powers, 1926). The Cantarell buried hill oilfield in the Gulf of Mexico is one of the largest oilfields in the world (Powers and Clapp, 1932). Other buried hill oilfields worldwide include the Bach Ho Oilfield in Vietnam (Cuong and Warren, 2009), the Jatib Arang oil and gas field in the Jawa Basin in Indonesia (Petford and McCaffrey, 2003) and the granitic buried hill in Bongor Basin in Chad (Dou et al., 2015). Buried hill oilfields also have been found in China, e.g., the Tahe Oilfield in the Tarim Basin (Zhao et al., 2012), the Xushen Oilfield (Xu et al., 2006) in the Songliao Basin, the Renqiu Oilfield in Jizhong depression (He et al, 2017), the Xinglongtai buried hill of the Liaohe Oilfield (Xie et al., 2012) among others. In the Bohai Sea area, petroleum exploration is successful in all kinds of buried hills including the igneous buried hill, the sedimentary buried hill and the metamorphic buried hill, discovering a series of oilfields, e.g., the Bozhong 28-1 Oilfield (Zhao et al., 2017), the Penglai 9-1 Oilfield (Hu et al., 2017; Zhou et al., 2015; Li et al., 2014; Xia et al., 2013), the Jizhou 25-1S Oilfield (Deng, 2015) and the Bozhong 19-6 Oilfield (Hou et al., 2019; Wang et al., 2019; Xu et al., 2019; Xue and Li, 2018).

    The metamorphic buried hill is the most promising exploration target in the Bohai Sea area. Most of the Archean basement traps are buried 4 500 m underground and contact with the hydrocarbon source rocks directly. Their hydrocarbon accumulation depends on the internal structure of the buried hill. In 2006, the China National Offshore Oil Corporation drilled the BZ19-6-1 well in the Bozhong 19-6 structure in Bohai Sea and succeeded in the gas exploration of Archean metamorphic buried hill at the depth of 4 500–4 600 m underground (Hou et al., 2019; Wang et al., 2019; Xu et al., 2019; Xue and Li, 2018). The discovery of the 1 000×108 m3 scale Bozhong 19-6 gas field has been a great breakthrough in gas exploration in the Bohai Sea. However, the major controlling factors of the reservoir are unclear. In this paper, based on 15 wells of the Bozhong 19-6 structure, we described the reservoir characteristics, discussed the controlling factors of the reservoir, and finally established the reservoir pattern of the metamorphic buried hill of the Bozhong 19-6 structure.

  • The Bohai Bay Basin, a continental rift basin in the eastern North China Craton, is the most hydrocarbon-rich basin in eastern China (Zhang et al., 2020; Liu et al., 2016; Zuo et al., 2011; Ren et al., 2002; Allen et al., 1997). The area of Bohai Bay Basin is about 200×103 km2. The Bozhong sag, located in the central Bohai Sea area, is the biggest sag and the Cenozoic depocenter of the Bohai Bay Basin (Zhu et al., 2020; Song et al., 2019; Li et al., 2012; Qi and Yang, 2010; Wu et al., 2006). The Bozhong 19-6 structure is in the southwestern Bozhong sag (Fig. 1a). The basement of the Bozhong 19-6 structure is 20×103 km2 and consists of Archean metamorphic rocks which are buried 4 500 m underground, and partly covered by Paleozoic strata in the eastern part of the Bozhong 19-6 structure. In general, the strata of Bozhong 19-6 structure consist of Archean metamorphic basement, Paleozoic carbonate, Mesozoic igneous rocks and sedi-mentary rocks and the Cenozoic sedimentary rocks (Fig. 1b). The Paleogene Shahejie Formation and the Dongying Formation are characterized by thick mudstone > 1 000 m, which serve as the cap rock. The Neogene Guantao Formation and the Minghuazhen Formation are dominated by sandstones and mudstones (Hou et al., 2019; Sun et al., 2019; Xu et al., 2019).

    Figure 1.  (a) Location of the Bozhong 19-6 structure and the sampled wells; (b) the geological profile of the Bozhong 19-6 structure.

  • Fifteen wells of the Bozhong 19-6 structure were sampled (Fig. 1). In total, 23 m cores and 355 sidewall cores were collected for core observation, thin section analysis, casting thin section analysis, fluorescence thin sections, XRD (X-ray diffraction) whole rock analysis, SEM (scanning electron microscope), and EDS (energy dispersive spectrometer) analysis. All the samples were prepared and analyzed at the Central Laboratory Bohai Branch of China National Offshore Oil Corporation.

  • The thin sections and XRD analysis of 355 sidewall cores have revealed that 90% of the samples are felsic rocks with few dark minerals. The average content of quartz is 46%, the average content of feldspar is 42%, while the average content of dark minerals, e.g., hornblende and mica, is between 5%–15%, representing metamorphic granite (Figs. 2a, 2b) and migmatitic granite (Figs. 2c, 2d). The other 10% of the samples are mafic rocks with abundant dark minerals. The average content of quartz is 35%, the average content of feldspar is 41%, while the average content of dark minerals, e.g., hornblende and mica is between 20%–30%, representing biotite plagioclase gneiss (Figs. 2e, 2f). Besides, the intrusive rocks in the Archean basement are diorite porphyrite (Figs. 2g, 2h).

    Figure 2.  Petrology of Bozhong 19-6 buried hill. (a) BZ19-6-7, 5 254 m, metamorphic granite, sidewall core; (b) BZ19-6-7, 5 254 m, metamorphic granite, perpendicular polarized light; (c) BZ19-6-12, 5 263 m, migmatitic granite, sidewall core; (d) BZ19-6-12, 5 263 m, migmatitic granite, plane polarized light; (e) BZ19-6-13, 4 872 m, biotite plagioclase gneiss, sidewall core; (f) BZ19-6-13, 4 872 m, biotite plagioclase gneiss, plane polarized light; (g) BZ19-6-2, 4 076 m, diorite porphyrite, sidewall core; (h) BZ19-6-2, 4 076 m, diorite porphyrite, plane polarized light.

  • The porosity and permeability analysis of the 355 sidewall cores has revealed that the porosity of the Bozhong 19-6 Archean reservoir is 0.8%–15.8%, about 6.4% on average, while the permeability is 0.02×10-3–508.61×10-3 μm2, about 8.9×10-3 μm2 on average, representing heterogeneous reservoir. Ninety-three percent of the sidewall cores are characterized by low porosity (< 10%), while 95% of the samples are characterized by low permeability (< 1.0×10-3 μm2).

    Based on core observation and thin section analysis, the metamorphic buried hill reservoirs are divided into fracture-type and pore-type. The fractures are the dominant storage space of reservoirs. The fractures, which are related to multiple tectonic movements, cut through each other, bearing organic fluorescence (Figs. 3a, 3b).

    Figure 3.  Storage space of Bozhong 19-6 buried hill. (a) BZ19-6-11, 5 127.61– 5 128.3 m, fractures in the core, white light; (b) BZ19-6-11, 5 127.61–5 128.3 m, fractures in the core, fluorescence; (c) BZ19-6-7, 4 544.5 m, fractures cutting through each other, perpendicular polarized light; (d) BZ19-6-7, 4 544.5 m, fractures cutting through each other, fluorescence; (e) BZ19-6-17, 4 815 m, dissolved pores in feldspar, plane polarized light; (f) BZ13-2-2, 4 736 m, enlarged dissolved pores along the fractures, plane polarized light; (g) BZ19-6-4, 4 468 m, dissolved feldspar, SEM; (h) BZ19-6-4, 4 459 m, illite filling in the fractures, SEM.

    The horizontal and low-angle fractures forming at early stage are almost completely filled, providing little storage space. The high-angle fractures forming at late stage are partly filled with calcite, which could provide effective storage space for oil and gas and are filled with organic materials (Figs. 3c, 3d).

    The pore-type reservoirs are dominated by dissolved pores between the minerals (Fig. 3e). Besides, enlarged dissolved pores along the fractures (Fig. 3f) also have been observed. The dissolved mineral was feldspar (Fig. 3g), with the filling minerals consisting of pyrite, illite, and illite-smectite mixed layers (Fig. 3h).

  • The weathering crust of metamorphic rocks was studied and divided into several sections where outcrops were available (Wyns et al., 1999; Thomas, 1997). However, such studies are rare in Bohai Sea due to the limited drillings. The Bozhong 19-6 structure has undergone complicated geological history. Based on the 15 wells, we have studied the vertical zoning of the metamorphic basement of the Bozhong 19-6 structure (Fig. 4). In the northern gentle terrain of Bozhong 19-6 structure, the vertical zoning of the metamorphic basement has revealed a complete sequence, including the upper weathered glutenite zone, the weathered fracture zone, the internal fracture zone and the bedrock zone. In the southern cliffy terrain of Bozhong 19-6 structure, the vertical zoning of the metamorphic basement includes the weathered fracture zone at the top, the internal fracture zone in the middle section and the bedrock zone at the bottom. But in the southern cliffy terrain of Bozhong 19-6 structure, the weathered glutenite zone is not preserved.

    Figure 4.  The vertical zoning of the buried hill.

    The weathered glutenite zone at the top of the buried hill is generally 8–40 m thick, and is preserved in the structural low area, yet denuded in the structural high area. The weathered glutenite consist of metamorphic rocks, weathered lithics and clay. The porosity of the weathered glutenite is 5%–7%, about 6% on average due to deep burial depth (4 500 m). The permeability of the weathered glutenite is 0.9×10-3–1.67×10-3 μm2, about 1.3×10-3 μm2 on average. The storage space consists mainly fractures and pores. Intergranular pores and fractures have been observed (Fig. 3).

    The weathered fracture zone is 60–316 m thick, developing below the weathered glunenite zone. This part of the buried hill is characterized by better reservoir properties. The porosity is 1.2%–12.8%, about 4.9% on average, while the permeability is 0.05×10-3–5.4×10-3 μm2, about 0.9×10-3 μm2 on average. Within the storage space, network of fractures and dissolved pores in feldspar have been observed (Fig. 3), with fewer fractures as the depth increases. The weathered fracture zone is the favorable reservoir of the metamorphic buried hill.

    The internal fracture zone is 38–740 m thick, developing below the weathered fracture zone. Controlled by tectonic movement, this zone consists of cataclasite, cataporphyry, tectonobreccia and mylonite. The porosity is 1.5%–8.8%, about 4.1% on average, while the permeability is 0.5×10-3–0.9×10-3 μm2, about 0.5×10-3 μm2 on average. Fractures predominate in this zone with dissolved pores in feldspar (Fig. 3). The internal fractures are related to the internal faults of the buried hill. A 50–300 m thick relatively compact zone would develop where there are few internal faults.

    The bedrock zone, which has not been influenced by tectonic movement or weathering, consists mainly of metamorphic basement rocks with fewer fractures, thus, unable to provide effective storage space.

  • According to our studies, the metamorphic buried hill reservoirs of the Bozhong 19-6 structure are controlled by the rock type, the nature and the intensity of tectonic stress, and the fluid.

  • The tectonic movements control the development of fractures. The Archean metamorphic buried hill in the Bozhong sag experienced multiple tectonic movements. On one hand, tectonic uplifting exposed the buried hill to weathering, forming favorable weathered-crust reservoir at the top of the buried hill. On the other hand, multiple tectonic movements greatly reformed the basement rock, resulting in lots of fractures, forming fracture-type reservoir.

    The Caledonian orogeny in the Early Paleozoic and the Hercynian orogeny in the Late Paleozoic caused the uplifting of the North China Craton, which resulted in the missing of the Upper Ordovician–the Lower Carboniferous (Wu et al., 2007; Qi et al., 2003), yet did not greatly influenced the metamorphic buried hills. The Indosinian collision between the North China Craton and the Yangtze Craton controlled the uplift of the Bozhong 19-6 structure, resulting in the denudation of the Lower Paleozoic and the exposure of the Archean basement. Lots of EW thrust faults developed during this period and formed lots of EW internal fractures within the metamorphic buried hill. The Yanshanian subduction of the Pacific Plate towards the East Asia (Liu et al., 2017; Liang et al., 2016; Ren et al., 2002) formed the EN strike-slip faults of the Bozhong sag, which were related to the EN fractures within the metamorphic buried hill. The Himalayan right lateral strike-slip of the Tanlu fault activated the fractures, which was essential for the development of effective fractures (Fig. 5). The Indosinian, Yanshanian and Himalayan movements greatly reformed the metamorphic buried hill, forming a large scale fracture system in the buried hill of the Bozhong 19-6 structure, which provided the storage space for petroleum.

    Figure 5.  The tectonic evolution of the Bozhong 19-6 structure.

  • The content of dark minerals in metamorphic rocks is related to the development of favorable reservoirs. Fractures are easily developed in metamorphic rocks with fewer dark minerals (Wang et al., 2019; Song et al., 2011). About 90% of the Archean metamorphic basement of the Bozhong 19-6 structure is felsic rocks with low content of dark minerals, e.g., metamorphic granite, gneiss and migmatite. The other 10% constitutes mafic rocks with higher content of dark minerals, e.g., plagioclase amphibolite, biotite plagiogneiss and biotite granulite. The felsic rocks are more likely to develop fractures (Figs. 3a, 3b), which contribute to the large scale fracture system in the buried hill of the Bozhong 19-6 structure.

    In Well BZ19-6-13, the drilled buried hill is divided into three sections (Fig. 6). The top section is 150 m thick biotite plagiogneiss, and the bottom section is 100 m thick amphibolite plagiogneiss, both featuring abundant dark minerals, e.g., biotite, which reveal chloritization. The average content of biotite is 23% according to XRD. The porosity ranges between 0.7%–2.5%, while the average permeability is 1.6%, which is not interpreted as reservoir. The middle section of the buried hill is monzonite gneiss with more felsic minerals and fewer biotite, 4%–10% according to XRD. The porosity ranges between 1.2%–12.8%, while the average permeability is 4.7%, which is interpreted as gas reservoir with 99.8 m thick gas. Under the same tectonic stress, compared with the top and the bottom section, the middle section with more felsic minerals and fewer dark minerals is more likely to develop fractures which serves as gas reservoir.

    Figure 6.  The metamorphic basement drilled in BZ19-6-13.

  • Both weathering and deep thermal fluid could greatly change the reservoir properties. In the process of weathering, the dissolved CO2 in fresh water entered into the metamorphic buried hill through fractures and joints, and reacted with the felsic minerals, forming enlarged dissolved pores along the fractures and dissolved pores in feldspar, which greatly improved the reservoir properties. Therefore, the 300 m thick weathered glutenite zone and weathered fracture zone (Fig. 4) influenced by fluid corrosion during weathering are favorable reservoirs.

    The deep thermal fluid includes the thermal fluid related to magmatic events and the organic acid discharged by hydrocarbon source rocks. The Bozhong 19-6 metamorphic buried hill experienced multiple stages of magmatic events from Mesozoic to Cenozoic. The thermal fluid related to these magmatic events reformed the reservoir in two separate ways. In the negative way, the fluid with siliceous minerals and ferruginous minerals formed quartz vein, flint vein, pyrite and siderite, which were hard to dissolve. They filled the pores and fractures, eventually damaging the reservoir. In the positive way, the thermal fluid and organic acid dissolved some feldspar, creating dissolved pores in feldspar and enlarged dissolved pores along the fractures. The thermal fluid entered the buried hill through fractures, thus only influenced the fracture zone (Fig. 7).

    Figure 7.  The fluid influence of the metamorphic buried hill of the Bozhong 19-6 structure.

  • The reservoir pattern of the metamorphic buried hill of the Bozhong 19-6 structure is controlled by tectonics mainly. The vertical zoning of weathered glutenite zone, weathered fracture zone, internal fracture zone and bedrock zone are controlled by paleomorphology, lithology and the development of fractures. The top of the buried hill at low gentle terrain is a suit of weathered glutenite, which serves as layered reservoir controlled by paleomorphology. Due to great burial depth of 4 000 m, this weathered layer is very dense, featuring fracture-pore storage space. The weathered fracture zone of the middle section is controlled by tectonics and weathering, characterized by lots of fractures, usually developing within the top 300 m of the buried hill. Below this section is the internal fracture zone, which is con-trolled by faults. The fracture zone is favorable reservoir with deeper distribution to more than 1 000 m.

    In general, the reservoir of the Bozhong 19-6 structure is characterized by great thickness and deep burial depth. A reservoir pattern of the metamorphic buried hill of the Bozhong 19-6 structure has been established, revealing its large scale fracture system and the vertical zoning (Fig. 8).

    Figure 8.  The reservoir pattern of the metamorphic buried hill of the Bozhong 19-6 structure.

  • (1) The metamorphic basement of the Bozhong 19-6 structure consists of metamorphic granite, gneiss, migmatitic granite and biotite plagiogneiss. The storage space of reservoir includes fractures and pores. The dominant storage space is fractures.

    (2) The vertical zoning of the metamorphic buried hill of the Bozhong 19-6 structure is divided into the upper weathered glutenite zone, the weathered fracture zone, the internal fracture zone and the bedrock zone from top to bottom. Among them, the weathered fracture zone and the internal fracture zone are favorable reservoirs due to the developed fractures and faults.

    (3) The metamorphic basement reservoir of the Bozhong 19-6 structure has been controlled by tectonics, lithology and fluid. The development of fractures has been controlled by tectonics and lithology, while dissolution related to fluids has influenced the reservoir properties.

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