PETROLEUM EXPLORATION AND DEVELOPMENT, 2020, 47(5): 977-989 doi: 10.1016/S1876-3804(20)60110-0

Reservoirs properties of slump-type sub-lacustrine fans and their main control factors in first member of Paleogene Shahejie Formation in Binhai area, Bohai Bay Basin, China

PU Xiugang,1,*, ZHAO Xianzheng1, WANG Jiahao2, WU Jiapeng1, HAN Wenzhong1, WANG Hua2, SHI Zhannan1, JIANG Wenya1, ZHANG Wei1

1. Dagang Oilfield Company, PetroChina, Tianjin 300280, China

2. Key Laboratory of Tectonics and Petroleum Resources of Ministry of Education, China University of Geosciences, Wuhan 430074, China

Corresponding authors: *E-mail:puxgang@petrochina.com.cn

Received: 2019-08-24   Revised: 2020-09-7   Online: 2020-10-15

Fund supported: CNPC Science and Technology Major Project2018E-11

Abstract

High-yielding oil wells were recently found in the first member of Paleogene Shahejie Formation, the Binhai area of Qikou Sag, providing an example of medium- and deep-buried high-quality reservoirs in the central part of a faulted lacustrine basin. By using data of cores, cast thin sections, scanning electron microscope and physical property tests, the sedimentary facies, physical properties and main control factors of the high-quality reservoirs were analyzed. The reservoirs are identified as deposits of slump-type sub-lacustrine fans, which are marked by muddy fragments, slump deformation structure and Bouma sequences in sandstones. They present mostly medium porosity and low permeability, and slightly medium porosity and high permeability. They have primary intergranular pores, intergranular and intragranular dissolution pores in feldspar and detritus grains, and structural microcracks as storage space. The main factors controlling the high quality reservoirs are as follows: (1) Favorable sedimentary microfacies of main and proximal distributary gravity flow channels. The microfacies with coarse sediment were dominated by transportation and deposition of sandy debris flow, and the effect of deposition on reservoir properties decreases with the increase of depth. (2) Medium texture maturity. It is shown by medium-sorted sandstones that were formed by beach bar sediment collapsing and redepositing, and was good for the formation of the primary intergranular pores. (3) High content of intermediate-acid volcanic rock detritus. The reservoir sandstone has high content of detritus of various components, especially intermediate-acid volcanic rock detritus, which is good for the formation of dissolution pores. (4) Organic acid corrosion. It was attributed to hydrocarbon maturity during mesodiagenetic A substage. (5) Early-forming and long lasting overpressure. A large-scale overpressure compartment was caused by under-compaction and hydrocarbon generation pressurization related to thick deep-lacustrine mudstone, and is responsible for the preservation of abundant primary pores. (6) Regional transtensional tectonic action. It resulted in the structural microcracks.

Keywords: faulted lacustrine basin ; slump-type sub-lacustrine fan ; reservoir ; Paleogene Shahejie Formation ; porosity-permeability structure ; Qikou Sag ; Bohai Bay Basin

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PU Xiugang, ZHAO Xianzheng, WANG Jiahao, WU Jiapeng, HAN Wenzhong, WANG Hua, SHI Zhannan, JIANG Wenya, ZHANG Wei. Reservoirs properties of slump-type sub-lacustrine fans and their main control factors in first member of Paleogene Shahejie Formation in Binhai area, Bohai Bay Basin, China. [J], 2020, 47(5): 977-989 doi:10.1016/S1876-3804(20)60110-0

Introduction

As exploration continues in China’s continental rift lacustrine basins, the expansion to the central part of basin and the medium-deep layers has become a general trend of oil and gas exploration. The reservoir sandbodies in the central part of rift lacustrine basins are generally gravity flow deposits, which are characterized by large burial depth and poorer physical properties. It is urgent to research the distribution and controlling factors of high-quality reservoirs in gravity flow deposit with medium-deep burial depth.

Previous understandings on the formation mechanism and controlling factors of deep high-quality reservoirs provide good references for this study[1]. However, there are still no unified classification schemes regarding “deep layers” and “high-quality reservoirs”. “Deep layers” generally refer to layers greater than 3000 m or 3500 m in depth[2,3,4,5]. Zhong Dakang et al.[2] summarized that high-quality reservoirs in China mostly had a porosity greater than 10% and permeability greater than 10×10-3 µm2. Jia Yancong et al.[5] defined that high-quality reservoirs had a porosity of 10%-20% and a permeability of (0.1-1.0)×10-3 μm2. In this study, considering the above definition and the physical properties of reservoirs in the study area, layers greater than 2500 m deep with medium porosity and low permeability or better (porosity greater than 15%, permeability greater than 5×103 μm2) are taken as medium-deep, high quality reservoirs[5]. Generally, as the degree of compaction increases, the primary pores gradually reduce, so the medium-deep formations are generally low in porosity and permeability[6]. However, exploration practices show that there are still high-quality reservoirs with large amounts of primary pores preserved in deep clastic rock formations. The controlling factors behind this phenomenon include strengthened compaction resistance caused by early carbonate cementation and high content of quartz particles, pore preservation by overpressure, dissolution by organic acids, inhibition of quartz secondary overgrowth by chlorite coating, structure of sandstone and mudstone formations, dissolution of laumontite, etc.[2, 3, 5, 7-9]. Wu Hao et al. summarized the favorable control factors mentioned above into two categories: “porosity increase” and “porosity preservation”[10].

Regarding the storage properties of gravity flow deposits, it has been traditionally believed that the rapid transportation and deposition of gravity flow are not conducive to sediment differentiation, resulting in poor sorting, high matrix content, and poor storage properties of sandstone. The study on the gravity flow deposits in the Baiyun Sag of the Pearl River Mouth Basin by Zheng Rongcai et al. pointed out that the sandbodies of sandy debris flow deposits were mostly well-sorted, with a small amount in moderate sorting. The sandstone was mainly sub-angular and sub-circular, and of minorly medium to high texture maturity. The formed reservoirs at medium-deep depth were of medium-high porosity and medium-high permeability[9,10]. The study of Chen Bintao et al. on the Cretaceous Qingshankou Formation in the Yingtai area of ​​the Songliao Basin revealed that this formation had fine sandstone reservoirs belonging to gravity flow deposits. The reservoirs had sub-angular, sub-circular and well-sorted clastic particles, and were particle-supported, with the muddy matrix content as low as 3%, and characterized by low compositional maturity and high texture maturity petrologically. Due to the mass transport mechanism of gravity flow, these sandbodies inherited the original porosity and permeability characteristics of sandstone in deltic front with strong hydrodynamics. They had more primary intergranular pores and a small amount of intragranular dissolved pores preserved, and even had physical properties better than sandbodies in delta front[11].

The coastal area, the study area, is located at the center of the Qikou Sag in the Bohai Bay Basin, and is far from the edge of the Qikou Sag during the development of the first member of Paleogene Shahehie Formation (shortened as Sha 1 Member hereafter). In recent years, some oil wells in this area tapped high yields in the Sha 1 Member, making this area an important replacement block for oil and gas exploration in the Dagang Oilfield. The sandstone reservoirs in the first member of the Shahejie Formation in this area are generally greater than 2500 m in burial depth, and have the characteristics of gravity flow deposit, high-quality reservoirs, and hydrocarbon accumulation in the center of lacustrine basin[12,13]. We take these reservoirs as the research object, analyze data of core, logging, ordinary thin sections, cast thin sections, scanning electron microscope and physical properties to discuss the major controlling factors of the high-quality reservoir from the aspects of sedimentation and diagenesis.

1. Geological background

The Qikou Sag is located in the middle of ​​the Huanghua Depression, the Bohai Bay Basin. It is part of the Meso-Cenozoic rift basin group in the Bohai Bay Basin and has undergone tectonic evolution from the Paleogene rift to the Neogene depression. During the rifting period, controlled by faults striking NNE, NE, and EW, 11 secondary structural units were developed, including one major sag (Qikou Main Sag), five sub-sags (Banqiao, Qibei, Qinan, Beitang and Shanan), and five salients (Beidagang, Nandagang, Tanggu-Xingang, Dashentang and Jiannan), as well as the Chengbei fault step belt. Generally, there is structural pattern of zonation from east and west, and block division from north and south (Fig. 1a)[14].

Fig. 1.

Fig. 1.   Tectonic units (a) and stratigraphic sequence (b) of Qikou Sag.


Supplied by the Cangxian uplift, Chengning uplift, Yanshan Orogenic belt and Shaleitian uplift, huge clastic rocks and a small amount of carbonate rocks deposited in the Qikou Sag during the Paleogene, and are divided into the Shahejie Formation and Dongying Formation from bottom up. The Shahejie Formation is further subdivided into three members, Sha 3 Member (Es3) (including Es33, Es32, and Es31), Sha 2 Member (Es2), and Sha 1 Member (including three submembers of Es1x, Es1z, and Es1s). The Dongying Formation is subdivided into Dong 3 Member, Dong 2 Member, and Dong 2 Member (Fig. 1b). Each package of the Sha 3 Member, Sha 2-Sha 1 members and the Dongying Formation presents a fining-upward and then coarsening-upward sedimentary cycle. Especially, the Sha 3 Member is characterized by the depositional system of fan delta-braided river delta-sublacustrine fan in deep lake setting; the Sha 2 and Sha 1 members feature the depositional system of fan delt-carbonate shoal-sublacustrine fan in shallow-deep lake setting; and the Dongying Formation is characterized by delta deposits under the background of shallow-deep lake. During the developement of the Sha 1 Member, gravity flow deposits were widespread in the Qikou Sag, especially the Qikou Main Sag was mostly covered by gravity flow deposits[12]. The coastal area is located in the west of the Qikou Main Sag, where the gravity flow deposits lead to major discoveries of oil and gas in recent years (Fig. 1a). Pu Xiugang et al.[12] emphasized the gravity flow deposits far from the basin edge, and termed them as off-shore subaqueous fans. Wang Jiahao et al.[15] further confirmed that they were originated by the slump sedimentary process, and named them slump-type sub-lacustrine fan.

2. Sedimentological and petrological characteristics of reservoir

2.1. Sedimentological characteristics

The coastal area was far from the basin edge during the depositional period of the Sha 1 Member, but multiple sandstone reservoirs were revealed in the Sha 1 Member of wells drilled in this area. The observation of cores from more than 20 wells shows that these sandstone reservoirs are mainly composed of silty-fine-grained sandstone and a small amount of pebbly medium- to coarse-grained sandstone, generally containing brown- dark gray gravels and dark gray sand-mudstone fragments, massive to graded bedding, and Bauma Sequence and slump deformation structures locally. In particular, the mud clasts are rounded, with a maximum grain size of 4 cm, and often distributed on the top of sandstone with normal graded bedding. In contrast, the mudstone fragments have rupture signs of sharp edges and corners, and thin interbeds of sand and mud and convolution deformation texture inside (Fig. 2a). The above characteristics indicate that the sublacustrine fan is resulted from slumping gravity flow[15].

Fig. 2.

Fig. 2.   Sedimentary structure (a) and logging curve characteristics (b) of the sublacustrine fan in the Sha1 Member of Well Gangshen 72.


Generally, slumping gravity flow is formed by creeping → rupturing → fragmentation → flow of soft sediment deposited on the landform or sedimentary slope (such as the delta front) when the gravity exceeds the shear strength of the sediment. The mechanisms triggering the gravity flow include increase of slope gradient, increase of sediment thickness, sediment liquefaction caused by increase of pore fluid pressure inside sediment, and earthquake etc. In marine basins, they still include incentives such as tsunamis and storm surge, etc. The Qikou Main Sag is adjacent to the Beidagang salient to the north. There are 3 to 4 steps of en echelon normal faults between them, constituting the fault terrace background. The Beidagang salient was an underwater highland during the development of the Sha 1 Member, and beach and bar deposits formed in a shore to shallow lake environment[16,17]. In addition to clastic beach bars, there also developed a small amount of carbonate bars made up of oolitic limestone and bioclastic limestone. The sandstone in the Sha 1 Member in the coastal area contains a small amount of oolite and oolitic limestone detritus, which is reliable evidence that the source sediments of sublacustrine fan came from the early beach and bar deposits at the Beidagang salient. Meanwhile, the multi- level fault steps provided conditions for multi-level acceleration of gravity flow, and the activity of the syn-depositional normal fault was also a factor triggering slump-type gravity flow. From the flow regime of gravity flow, the slump process generally generates sandy debris flow, which activates in various channels of slumping sublacustrine fan, and then transforms into turbidity currents in the lobe of outer fan[9, 11-12, 18-20]. Pu Xiugang et al. reached a similar understanding on the study area[12].

Based on core observation, logging curves and thickness contour map of sandstone, the vertical evolution and plane distribution of sedimentary microfacies were investigated in this study. The sublacustrine fans appear as large-scale coarsening-upward depositional cycles vertically (Fig. 2b). There are many wells drilled in the Tangjiahe area. The thickness contour map of the lower Es1s sandstone shows that this area is a major site of source sediment input. The sandbody thins from north to south and has three levels of bifurcation, and thus is identified as the sedimentary microfacies of main channel in the inner fan, proximal distributary channel and the distal distributary channel in the middle fan, and the mat lobe in the outer fan, and channel edge (Fig. 3). Among them, the sediment of main channels is coarse in grain size, composed of fine-grained sandstone and pebbly medium- to coarse-grained sandstone, and present high-amplitude cylindrical natural gamma curves. The microfacies of proximal distributary channels are composed of fine sandstone and present bell-shaped natural gamma curves. The microfacies of distal distributary channels are composed of siltstone and are represented by Christmas tree-shaped natural gamma curves. The mat-like lobe microfacies are composed of muddy siltstone and siltstone and takes on serrated funnel-shaped natural gamma curves. The microfacies of channel edge are composed of siltstone and shows Christmas tree-shaped and fine serrate-shaped natural gamma curves. The exploration results show that sedimentary sandstone layers in the main channels and the proximal distributary channels are thick, with an individual thickness of 2.0-5.5 m, and a maximum thickness of 20 m of multiple phases combined, they are high in oil storage too, so are the two sedimentary microfacies that are favorable for the formation of major reservoirs in high-yield oil wells of the study area.

Fig. 3.

Fig. 3.   Planar distribution of sedimentary microfacies of sublacustrine fan in the lower part of Es1s in the Tangjiahe well area.


2.2. Petrological characteristics

Through analyzing cores and grain size, and thin sections of cores taken from Wells Gangshen 59, Gangshen 72, Gangshen 47, Gangshen 46, and Gangshen 48, the Sha 1 Member sandstone layers in the coastal area have quartz contents of 17%-61%, 40.1% on average, feldspar contents of 21.2%- 59.4%, 45.5 % on average, and detritus contents of 8.2%- 43.5%, 14.4% on average, representing feldspar sandstone, lithoclastic feldspar sandstone, and a minor of feldspar lithoclastic sandstone. These sandstone layers feature diverse components of detritus, including andesite, granite, rhyolite, metamorphic rock, flint, oolitic limestone, micrite limestone, and oolitic stone, etc. (Fig. 4)[21].

Fig. 4.

Fig. 4.   Composition and texture characteristics of sandstone detritus particles in the Sha 1 Member in the coastal area. (a) Well Binshen 6, 3570.0 m, Es1Z, more andesite detritus and a small amount of oolitic particles, (+); (b) Well Binhai 1, 3310.0 m, Es1Z, oolitic limestone detritus, (+); (c) Well Binshen 1, 4014.0 m, Es1X, dissolution of limestone detritus, (-); (d) Well Binshen 22, 4180.4 m, Es1X, flint detritus, calcareous cement, (+); (e) Well Binshen 6, 3570.0 m, Es1X, andesite detritus, (+); (f) Well Binshen 22, 4085.0 m, Es1X, early calcite basement cement (dyed red), (+); (g) Well Bin 21, 3419.4 m, Es1S, a large number of primary intergranular pores and a few intragranular dissolved pores, (-); (h) Well Binhai 4, 3778.6 m, Es1S, feldspar cleavage fractures and calcite filling, (+).


In terms of texture, the sandstone layers are poor in sorting and roundness generally, but some sandstone layers are medium sorted. Thirty-one sandstone samples from the Sha 1 Member of the aforementioned wells were analyzed on granularity. The probability cumulative curves are in the form of one-section pattern and two-section arching pattern. The former is the typical pattern of turbidite flow deposit, and the latter is similar to the characteristics of traction flow deposition (Fig. 5). Through the evaluation of particle size parameters, the samples with good sorting, medium sorting, and poor sorting account for 11.3%, 25.8%, and 53.2%, respectively. In general, the reservoir sandstone has the characteristics of low compositional maturity and low-medium textural maturity, and grain size curve similar to traction flow sandstone, which verify the inheritance of characteristics of early sedimentation to a certain extent, which is also an important feature of slumping gravity flow deposition[9, 11].

Fig. 5.

Fig. 5.   Grain-size distribution of sandstone samples from the Sha 1 Member in the coastal area.


3. Physical properties of reservoir

3.1. Reservoir porosity and permeability

A total of 2582 samples were taken from the Sha 1 Member in the coastal area, including fine sandstone, a small number of medium sandstone and siltstone ones. Statistics and cross- plot analysis show positive correlation between the porosity and permeability of these samples. The samples with medium porosity (15%-2%), low porosity (10%-15%) and high porosity (25%-30%) account for 54.61%, 31.84% and 3.29%, respectively. Whereas the samples with ultra-low permeability (less than 5×10-3 μm2), low permeability (5-50)×10-3 μm2, and medium permeability (50-500)×10-3 μm2 account for 59.26 %, 34.15 %, and 5.75 %, respectively. On one hand, the sandstone reservoirs aren’t consistent in the change trends of porosity and permeability, and decline more significantly in permeability, which brings great challenges to oil and gas exploration in the study area; on the other hand, although the target intervals are generally greater than 2500 m in burial depth, and thus content a large proportion of low-porosity and low-permeability reservoirs, there are still multiple medium-porosity low-permeability and a small number of high-porosity and medium-permeability reservoirs, which are the main objects of this study (Fig. 6).

Fig. 6.

Fig. 6.   Variations of porosity (a) and permeability (b) with depth of sandstone samples from the Sha 1 Member in coastal area.


3.2. Types of storage space

Through analysis of casting thin sections and scanning electron microscopy images, the storage space in the sandstone reservoirs of the study area include four types, primary intergranular pores, dissolution pores, feldspar cleavages and microfractures.

(1) Primary intergranular pores. It is generally believed that reservoirs with more than 3000 m burial deep almost completely lose the primary pores due to compaction and cementation, and have mainly secondary pores[6]. However, more and more studies show that well preservation of primary pores in clastic rock reservoirs at 3500-4500 m depth is necessary for abnormally high porosity and permeability reservoirs with medium to deep burial depth [4,5]. The study area is no exception, where sandstone reservoirs generally retain a small amount of primary intergranular pores that even have enlarged due to later dissolution. As shown in Fig. 4g, the sample from the Es1S at 3419.4 m of Well Bin 21 is clastic feldspar fine sandstone, with good sorting and rounding. The particles are sub-angular to subrounded and in point to line contact. The sample has a large amount of primary intergranular pores preserved and a small amount of dissolution pores.

(2) Dissolution pores. They include two types, intergranular dissolution and intragranular dissolution ones. The intergranular dissolution pores present bay-shaped on the granular edge. In contrast, the intragranular dissolution is more prominent. In addition to the commonly seen dissolution of feldspar particles, it also includes the intragranular dissolution of andesite, granite, and limestone detritus particles, and even some particles are hollow partially due to severe dissolution (Figs. 4a, 7a-7c).

(3) Tectonic micro-fractures. Many micro-fractures were identified in thin sections. They extend straightly and directionally, and are in step-like arrangement in local parts. They are divided into two types, bitumen-filled or unfilled, indicating tensile and torsional tectonic origin and multi-phase activity (Fig. 7d-7h). As shown in Fig. 7d, the sandstone sample from 5154.8 m depth of Well Binhai 4 has poor sorting, sub-angular to subrounded particles, a few intergranular pores, intergranular and intragranular dissolved pores, and micro-fractures.

Fig. 7.

Fig. 7.   Types of storage space in sandstone reservoirs of the Sha 1 Member in the coastal area. Well Binshen 1, 3602.0 m, Es1s, enlarged intergranular dissolution pores and intragranular dissolution pores, (-); (b) Well Binhai 4, 5356.8 m, Es1x, intragranular dissolution pores in andesite detritus, (-); (c) Well Binhai 4, 4053.4 m, Es1s, dissolved pores in feldspar grains, (+); (d) Well Binhai 4, 5154.8 m, Es1z, unfilled fractures and dissolved pores (-); (e) Well Binshen 1, 4019.0 m, Es1x, micro-fractures in nearly parallel arrangement, with bitumen filling, (-); (f) Well Binshen 1, 4019.0 m, Es1x, intergranular dissolution pores and bitumen-filled fractures, (-); (g) Well Gangshen 78, 4220.0 m, Es1x, scanning electron microscopy, micro-fractures in quartz particles; (h) Well Binhai 4, 3778.6 m, Es1s, intragranular dissolution and micro-fractures in feldspar particle, secondary overgrowth of quartz particles, (+).


(4) Feldspar cleavages. It is showed that part of the dissolution is developed along the cleavage fractures, or some of the cleavage fractures are filled with carbonate cement (Fig. 7h).

Among the above four types of storage spaces, dissolution pores are the most commonly seen and are the most important type of storage space in the study area. A large number of primary intergranular pores are preserved in some samples, so they are of great significance for high-quality reservoirs. Microfractures function outstandingly in improving permeability, especially under the background of a large decrease in permeability in the study area. The feldspar cleavages are small in number, and are generally subject to dissolution. Pure feldspar cleavage fractures contribute little to the storage space.

4. Major factors controlling high-quality reservoirs

Except in local areas near the Beidagang salient, the Sha 1 Member in the coastal area is generally greater than 2500 m deep, and reaches 5500 m deep in some parts. However, drilling wells revealed many reservoirs with medium porosity and low permeability, and a small number of reservoirs with high porosity and medium permeability were found (Fig. 6). These reservoirs have many primary intergranular pores preserved locally, bringing high oil and gas yield of some wells. To predict the distribution of high-quality reservoirs in the study area, major factors controlling high-quality reservoirs are examined in this work.

4.1. Favorable sedimentary microfacies

Different sedimentary microfacies have differences in lithological composition and transport-deposition mechanism, thereby controlling reservoir physical properties. Many studies have revealed that physical properties are positively correlated with granularity of sandstone. Thick sandstone layers are beneficial for maintaining reservoir properties, and sandy debris flow is more conducive to the formation of high-quality reservoirs than turbidity current[2, 9, 11]. Based on core observations and oil-bearing characteristics, the sediments in the main channels of inner fan of the sublacustrine fan in the study area are coarse-grained and thick, and mainly brought about by sandy debris flow, so the main channels of inner fan is the most favorable sedimentary microfacies for finding high-yield oil and gas wells in the study area. The proximal distributary channels have mainly medium-sized sandy debris flow and some turbidity current deposits with medium oil content. The microfacies of distal distributary channel, channel edge, and mat-like lobe in the outer fan are dominated by fine and thin-layered sandstone of turbidity current with low oil content (Figs. 2-3)[12,13].

A large number of samples were collected from Es1x and Es1s in the study area for physical property analysis. Data of 2300 samples from 29 wells were analyzed and compared to find out the physical properties of the five microfacies, main channel, proximal distributary channel, distal distributary channel, distributary channel, and mat-like lobe. As the burial depth increases towards the center of the depression, the maximum burial depth difference of the samples exceeds 2000 m. In order to eliminate the influence of different burial depth, the relationships between porosity and depth, and permeability and depth were fitted first, the porosity and permeability of the samples were corrected to the burial depth of 3000 m [22], and then the porosity-permeability cross plot was compiled. The results show that the sedimentary microfacies of Es1S and physical properties of the reservoir are evidently correlated. The main channels are the best in physical properties, followed by proximal distributary channels, and the distal distributary channels and the channel edges are slightly worse than proximal distributary channels. The mat-like lobe is the worst (Fig. 8a). In contrast, the Es1x sedimentary facies and physical properties of reservoir have weaker correlation. Except mat-like lobes and channel edges which have poor physical properties, the primary channels, proximal distributary channels, and distal distributary channels have slight differences in physical properties. The intersection points of this layer are more dispersed in distribution than those of Es1S. The corrected permeabilities of the samples are generally higher, indicating that secondary pore development and reservoir physical properties are more affected by diagenesis (Fig. 8b). Therefore, as the burial depth increases, the controlling effect of sedimentation on the physical properties of reservoir weakens, while the controlling effect of diagenesis increases.

Fig. 8.

Fig. 8.   Porosity and permeability cross plot of reservoirs in different sedimentary facies of Sha1 Member.


4.2. Medium textural maturity

In terms of rock texture, the study area has a large amount of medium-good sorting sandstone layers (Fig. 4g), the formation of which is related to the sedimentary characteristics of slump gravity flow. The sandstone cores contain a small amount of gastropod fossils and oolitic and oolitic limestone detritus (Fig. 4a-4b), indicating that the source sediment of the sublacustrine fan mainly came from the beach bar sandstone previously deposited on the Beidagang salient. After slumping, the sediment was transported by sandy debris flow in mass to form the sublacustrine fan, so the sandstone in the sublacustrine inherits the texture of the beach bar sandstone and is well sorted, which is conducive to the development of primary pores and enhancement of compaction resistance[5, 16-17].

4.3. High content of intermediate-acid volcanic rock detritus

Clastic rock particles constitute the pore framework of reservoir, so the composition and texture of clastic particles have an important influence on the physical properties of reservoirs. The outstanding petrological characteristics of reservoirs in the study area are the high content and diverse types of detritus, mostly intermediate-acid magmatic detritus particles. Due to the development of dissolution pores of detritus particles, the low composition maturity has become an advantageous factor for the high-quality reservoirs. As shown in Fig. 7b-7e, a large number of dissolved pores appear in limestone, andesite, granite detritus and feldspar particles, especially andesite and granite detritus particles are often dissolved into a mesh. In particular, in the sandstone sample from 5356.8 m in Well Binhai 4, andesite particles have generally suffered from intragranular dissolution. Medium-deep reservoirs have experienced organic acid dissolution caused by organic meterial maturation during burial diagenesis. Under this premise, the content of soluble minerals is decisive for the development of dissolution pores. Andesite and granite contain hornblende, pyroxene, feldspar and other unstable minerals, which are prone to weathering and leaching during exposure and erosion, and likely to be dissolved during subsequent burial diagenesis[23,24]. In contrast, the high quartz content is conducive to enhancement of the compaction resistance of the rock, but the sandstone in the study area has distinct secondary overgrowth of quartz, which has destructive effect on the reservoir physical properties, indicating that high compositional maturity is not conducive to the development of high-quality reservoirs (Fig. 7h).

4.4. Organic acid dissolution

The extensive development of dissolution pores is a common characteristic of medium-deep high-quality reservoirs[4]. Previous studies have revealed that reservoir sandstones in the Cenozoic rift basins in Eastern China generally entered into the medium diagenesis A stage at the depth of 3000 m. Correspondingly, destructive diagenetic processes such as compaction, calcareous cementation, and quartz secondary overgrowth in this stage resulted in densification of reservoir. This was also a stage when organic matter matured and oil and gas began to be generated in large quantity, which was accompanied by generation of organic acid. Subsequently, the organic acid dissolution occurred extensively[12]. Surrounded by source rock, with suitable burial depth and soluble particles, the reservoirs of gravity flow deposits in the Sha 1 Member of the study area suffered prominent dissolution.

4.5. Overpressure formed early and sustained long

Overpressure has a protective effect on the physical properties of clastic rock reservoirs, which is shown as resisting to compaction, restricting fluid activity, inhibiting cementation, inhibiting the conversion of montmorillonite to illite which is conducive to maintaining rock permeability, increasing solubility of carbonate rock, and slowing down the development of carbonate cement[25]. In the currently reported medium- to deep-buried high-quality reservoirs, overpressure is very common, so it is an indispensable condition for the development of medium-deep high-quality reservoirs[3-5, 10]. Pu Xiugang et al.[25] found that the Shahejie Formation of the Qikou Main Sag had multiple sections of overpressure, which corresponded with the depths of the mudstone sections under the sequence stratigraphic framework, implying the genetic relationship between overpressure and undercompaction and hydrocarbon generation of thick mudstone. The profile of mudstone’s acoustic time difference with depth was plotted in this work. This profile shows that overpressure occur at different depth in wells in the coastal area. Overpressure generally occurs in the Sha1 Member and extends downward for over 2000 m. As a result, a huge overpressure compartment with 3 to 4 pressure gradient changes inside are related to the distribution of thick mudstone layers formed in lacustrine transgression periods (Fig. 9). For example, the middle of Es1z in Well Binhai 8 has a high magnitude of overpressure, which corresponds to the thick mudstone in this section.

Fig. 9.

Fig. 9.   Variation of acoustic wave time difference of mudstone layers in coastal area with depth.


In the coastal area, lacustrine fan sandbodies are embedded in thick deep lacustrine mudstone layers, and the development of overpressure compartment ensures the preservation of a large number of primary pores in the reservoirs. The sample in Fig. 4g is of a burial depth of 3 419.4 m. The thin section shows that the sandstone has a low degree of compaction, point-to-point contact between particles, and a large number of primary intergranular pores preserved, which is the best evidence of “under-compaction”. At the same time, this “undercompaction” also indicates that the overpressure was formed early and belongs to the type of early overpressure, which is different from the late overpressure of “diagenesis first and overpressure later”[4]. In short, the overpressure in the study area is the result of poor fluid drainage and undercompaction overpressure formed earlier that was further strengthened by the subsequent pressurization of hydrocarbon generation under the premise of continuous deposition of thick mudstone in the center of lacustrine basin.

4.6. Transtensional region tectonic movement

Micro-fractures, as the “highways” of fluid seepage activity, are very important for petroleum accumulation. On the premise that the deep reservoirs greatly reduce in permeability, micro-fractures can greatly improve the permeability of reservoirs. In addition, micro-fractures are also important paths for acidic fluids to get in and dissolve reservoirs and outlets for the migration of dissolved substances, and have irreplaceable significance for dissolution. Based on deep seismic coherent slices and seismic section interpretation, the coastal area has two basement strike-slip faults striking NE. They activated in the Paleogene and extended a distance of 70 km on the plane, forming a 4-11 km wide rhombus area (Fig. 10a). These two faults intersect with large normal faults such as the Haihe-Xin’gang Fault, Qizhong Fault, and Zhangdong Fault, and are accompanied by small faults arranged densely and obliquely. These small faults show flower- like structure in local parts (Fig. 10b), indicating transtensional tectonic force in the area, which is consistent with the nature of tension and torsion signed by the stepped microfractures in the thin sections. These two strike-slip faults across most of the coastal area, so are the reason for a large number of micro-fractures in the study area[14].

Fig. 10.

Fig. 10.   Distribution and flower structure of tension-torsional faults in Qikou Sag.


The above understandings have been used to guide production. The primary measures include: compile sedimentary microfacies maps according to the sublacustrine fan model with multi-level distributary channels to search for favorable reservoir sedimentary facies belts (sweet spots) of the main channel and proximal distributary channel; deploy wells according to the distribution of fault belt; and conduct conventional fracturing or acidification in line with the poor reservoir permeability. Hence, good exploration and development results have been achieved. For example, Well Gangshen 33 located in the transtensional structural zone encountered the major distributary channel sandstone at 4114-4183 m of Es1x, with an average porosity of 12.9% and an average permeability of 0.4×10-3 μm2. The well obtained high-yield oil flow of more than 20 t/d after being fractured.

5. Conclusions

Based on core, grain-size distribution, thin section and physical property test data, the sedimentary facies, petrologic and reservoir characteristics, and major controlling factors of the Sha 1 Member in the coastal area were analyzed, and the following understandings have been reached: (1) The reservoir sandstone belongs to slump-type sublacustrine fan deposit, is formed by the slumping and re-deposition of previous beach bar sandbody, and has identification markers of mudstone fragments, slump deformation structure and Bauma Sequence. The reservoirs include inner fan primary channel, middle fan proximal distributary channel-distal distributary channel, outer fan lobe, and channel edge microfacies. (2) The reservoirs are mostly low-porosity and low-permeability, medium-porosity and low-permeability types, and a few high-porosity and medium-permeability type. Pores in them mainly include primary intergranular pore, intergranular and intragranular dissolution pore, microfractures etc. (3) The major controlling factors of high-quality reservoirs are, favorable sedimentary microfacies including inner fan gravity flow primary channel and middle fan proximal distributary channel; medium textural maturity, which is conducive to the development of primary intergranular pores; high content of intermediate-acid volcanic rock detritus, which is highly soluble and thus laid the material basis for the development of dissolution pores; organic acid dissolution; overpressure formed early and sustained long, which enables preservation of large amount of primary intergranular pores; and transtensional tectonic movement, which resulted in the micro-fractures that improve reservoir permeability significantly.

Reference

BLOCH S, LANDER R H, BONNELL L.

Anomalously high porosity and permeability in deeply buried sandstone reservoirs: Origin and predictability

AAPG Bulletin, 2002,86(2):301-328.

[Cited within: 1]

ZHONG Dakang, ZHU Xiaomin, WANG Hongjun.

Analysis on the characteristics and formation mechanism of high quality clastic reservoirs in China

SCIENCE CHINA Earth Sciences, 2008,38(S1):11-18.

[Cited within: 4]

LIU Xixiang, ZHANG Shaonan, YANG Peng, et al.

Formation mechanism of deep high-quality reservoirs of Yingcheng Formation in Longfengshan area, Songliao Basin

Lithologic Reservoirs, 2017,29(2):117-124.

[Cited within: 3]

GUO Jiajia, SUN Guoqiang, MEN Hongjian, et al.

Genetic analysis of anomalously high porosity zones in deeply buried reservoirs in the west part of northern edge of Qaidam Basin, NW China

Acta Sedimentologica Sinica, 2018,36(4):777-786.

[Cited within: 4]

JIA Yanchong, CAO Yingchang, LIN Changsong, et al.

Formation mechanism and distribution of high-quality reservoirs for beach-bar sandstones in upper part of Es4 in Boxing Sag, Dongying Depression

Journal of Jilin University (Earth Science Edition), 2018,48(3):652-664.

[Cited within: 7]

ZHAO Xianzheng, ZHOU Lihong, PU Xiugang, et al.

Geological characteristics of shale rock system and shale oil exploration in a lacustrine basin: A case study from the Paleogene 1st sub-member of Kong 2 Member in Cangdong sag, Bohai Bay Basin, China

Petroleum Exploration and Development, 2018,45(3):361-372.

[Cited within: 2]

AJDUKIEWICZ J M, LARESE R E.

How clay grain coats inhibit quartz cement and preserve porosity in deeply buried sandstones: Observations and experiments

AAPG Bulletin, 2012,96(11):2091-2119.

[Cited within: 1]

CAO Yingchang, YUAN Guanghui, LI Xiaoyan, et al.

Types and characteristics of anomalously high porosity zones in Paleogene mid-deep buried reservoirs in the north slope, Dongying Sag

Acta Petrolei Sinica, 2013,34(4):683-691.

ZHENG Rongcai, LI Yun, DAI Chaocheng, et al.

Depositional features of sandy debris flow of submarine fan in Zhujiang Formation, Baiyun Sag

Journal of Jilin University (Earth Science Edition), 2012,42(6):1581-1589.

URL     [Cited within: 5]

The deep-water Zhujiang Formation in Baiyun sag of Pearl River Mouth basin has been a new area for hydrocarbon exploration and development, but there is still much controversy about the genesis of massive sandstone reservoirs. Based on comprehensive analysis of composition, sedimentary structures, grain size distribution, ancient topography as well as seismic and logging data, the Zhujiang Formation was deposited by submarine fan system, and a wide range of thick massive sandstoneswere products of sandy debris flow filling withininner-middle fan channels. According to detailed features and identification signs, the sandy debris flow was related to gravitational slump and sand collapse of sandbodies from the shelf edge delta front triggered by certainmechanism duringsignificant sea-level falling, and then the “Source-Drainage-Sink” coupling model of sandy debris flow deposits in submarine fan was set up.

WU Hao, JI Youliang, ZHOU Yong, et al.

Origin of the Paleogene deep buried high-quality reservoirs in the southern Nanpu Sag

Journal of China University of Mining & Technology, 2019,48(3):553-569.

[Cited within: 3]

CHEN Bintao, PAN Shuxin, LIANG Sujuan, et al.

Main controlling factors of high-quality deep-water mass transport deposits(MTDs) reservoir in lacustrine basin: An insight of Qingshankou Formation, Yingtao Area, Songliao Basin, Northeast China

Journal of Jilin University (Earth Science Edition), 2015,45(4):1002-1010.

[Cited within: 4]

PU Xiugang, ZHOU Lihong, HAN Wenzhong, et al.

Gravity flow sedimentation and tight oil exploration in lower first member of Shahejie Formation in slope area of Qikou Sag, Bohai Bay Basin

Petroleum Exploration and Development, 2014,41(2):138-149.

[Cited within: 7]

ZHAO Xianzheng, ZHOU Lihong, PU Xiugang, et al.

Development and exploration practice of the concept of hydrocarbon accumulation in rifted-basin troughs: A case study of Paleogene Kongdian Formation in Cangdong Sag, Bohai Bay Basin

Petroleum Exploration and Development, 2018,45(6):1092-1102.

[Cited within: 2]

WANG Jiahao, WANG Hua, REN Jianye, et al.

A great oblique transition zone in the central Huanghua Depression and its significance for petroleum exploration

Acta Petrolei Sinica, 2010,31(3):355-360.

[Cited within: 2]

WANG Jiaha, WANG Hua, XIAO Dunqing, et al.

Differention between hyperpycnal flow deposition and slump-induced gravity flow position in terrestrial rifted lacustrine basin

Acta Petrolei Sinica, 2020,41(4):392-411.

[Cited within: 2]

ZHOU Liqing, SHAO Deyan, FANG Shiyu, et al.

Beach and bar sandbodies of Shahejie Formation in Banqiao Depression

Oil & Gas Geology, 1998,19(4):351-355.

[Cited within: 2]

SHANG Xiaofei, HOU Jiagen, DONG Yaoyue, et al.

Sedimentary mechanism and distribution pattern of beach-bar sandbodies mainly dominated by contemporaneous faults in Banqiao Sag

Journal of China University of Petroleum (Edition of Natural Science), 2014,38(6):32-39.

[Cited within: 2]

LI Xiangbo, LIU Huaqing, WANYAN Rong, et al.

First discovery of the sandy debris flow from the Triassic Yanchang Formation, Ordos Basin

Lithologic Reservoirs, 2009,21(4):19-21.

[Cited within: 1]

SHANMUGANM G.

New perspectives on deep-water sandstones: Implications

Petroleum Exploration and Development, 2013,40(3):294-301.

ZOU C, WANG L, LI Y, et al.

Deep-lacustrine transformation of sandy debrites into turbidites, Upper Triassic, Central China

Sedimentary Geology, 2012,265/266:143-155.

[Cited within: 1]

ZHAO Xianzheng, PU Xiugang, JIANG Wenya, et al.

An exploration breakthrough in Paleozoic petroleum system of Huanghua Depression in Dagang Oilfield and its significance

Petroleum Exploration and Development, 2019,46(4):621-632.

DOI:10.1016/S1876-3804(19)60042-X      URL     [Cited within: 1]

WANG Jiahao, PENG Guangrong, LIU Baojun, et al.

Flattening diagenesis of clastic rocks and quantitative characterization of sedimentary control on reservoir properties: A case study of Baiyun Sag in Pearl River Mouth Basin

Acta Petrolei Sinica, 2019,40(S1):115-123.

[Cited within: 1]

LIU Xiaohong, FENG Mingyou, XI Aihua, et al.

Diagenesis and pore evolution of Carboniferous volcanic reservoirs in Dixi Area, Kelameili Gas Field

Lithologic Reservoirs, 2016,28(1):38-48.

[Cited within: 1]

MA Shangwei, CHEN Chunyong, LUO Jinglan, et al.

Research of major controlling factors on high-quality reservoir of the Carboniferous volcanic rocks in Xiquan Area, Junggar Basin

Geological Journal of China Universities, 2019,25(2):197-205.

[Cited within: 1]

PU Xiugang, ZHOU Lihong, WANG Wen’ge, et al.

Medium- deep clastic reservoirs in the slope area of Qikou Sag, Huanghua Depression, Bohai Bay Basin

Petroleum Exploration and Development, 2013,40(1):36-48.

[Cited within: 2]

/