Analysis of the Factors that Influence Diagenesis in the Terminal Fan Reservoir of Fuyu Oil Layer in the Southern Songliao Basin, Northeast China

1.1 Background

Little controversy exists regarding the sedimentary system that produced the fourth member of the Quantou formation in the Songliao basin; most researchers consider this formation to be a shallow water delta, which is a lake delta model formed in a dry environment, the main types of sedimentary facies are channel, channel flood, sand bar, levee, crevasse splay and sand sheet [1, 2, 3, 4, 5, 6]. A few researchers, such as Peng and Gong, hold other views, defining it as a common delta front [7, 8]. Sun thought it was a meandering river delta, because of the control of the sedimentary system of the meandering river delta in the northwest provenance, the southern part of Songliao Basin mainly developed a large area of thin sand bodies distributed in large areas [9]. Wang thought that the low delta sedimentary facies was developed in the southern Songliao basin. During this period, the uplift rate of the provenance decreased, and the water system was shrinking to the land, the lake basin expanded, and the sediment source came from the surrounding mountainous area [10].However, the latest research showed that the sedimentary characteristics of this region are unique, unlike a common river delta. These features are reflected in aspects such as the lithology, mudstone color, grain size, sand-body extension law and so on.

Therefore, a new terminal fan sedimentary system has been proposed. Aterminal fan is a special fluvial- delta sedimentary system that forms in a dry to semi-dry environment, where the flow rate is reduced at the end of the river. With the gradual decline in terrain slope, water spreads out and the flow rate drops, allowing a large amount of clastic material to be deposited to form the fan-shaped accumulation. The terminal fan is different from the alluvial fan, diluvial fan and turbidite fan, they are not the same type. The rocks deposited by terminal fan are usually finer in grain size, better in sorting, and have higher component maturity and textural maturity. The terminal fan is similar to the river delta, the difference between them is whether there is a stable catchment area at the front of the fan body. River deltas are generally pushed into stable water bodies, forming sediments interacted by rivers and lakes (oceans). However, the front end of the terminal fan usually does not have a stable catchment area, or its size changes rapidly with the season, so, the terminal fan would disappear before entering the water at the end [11]. The microfacies of the terminal fan is defined as four different types in this paper: distributary channel, near channel overflow, far channel overflow and mud flats, and this view is different from that of previous studies.

Mukerji first proposed the concept of a sedimentary fan based on a study of the Sutlej-Yamuna plain [12]. Later, Friend studied the Devonian-age red sandstone and proposed a model for terminal fan deposition from many present-day river systems that have been studied. This literature was the first that studied terminal fan sedimentary deposits but proposed a dry environment for the terminal fan sedimentary model, and its specific sedimentary pattern was not described [13]. Some objections were also raised, North argues that modern rivers in arid regions cannot bifurcate in the lower reaches of the river, and that the so-called branching rivers are caused by continuous channel changes, which can also be seen in some large alluvial fans [14]. Zhang found a widely developed red mudstone layer with shallow water and oxygen-rich lacustrine facies, naming it flood rock. This research laid the foundation for the application of the terminal fan model in a continental lake basin in China [15]. Kelly et al. divided terminal fans into three zones, namely, the recharge channel, distributary channel and remote basin, according to a study of the Devonian formation in England. This view is similar to that in this article, but the degree of division is not sufficiently fine [16]. Newell studied the sedimentary characteristics of Permian strata in the southern Ural River in Russia to better understand each terminal fan facies and divided the facies into various micro depositional environments [17]. This research provided an important theoretical basis to deepen the study of terminal fan sedimentary systems. Parkash classified the terminal fan system of the Makanda River in India into three secondary sedimentary environments, namely, channel, natural levee and flood plain, and identified nine lithofacies. This is a detailed anatomical study of the end fan system [18]. Sanchez constructed a model for a terminal fan sedimentary system from the lower Cretaceous Candeleros formation in the Neuquen basin, dividing the sedimentary body into three components from bottom to top, but this classification method was not conducive to the fine-scale evaluation of reservoirs [19]. Chatmas conducted a series of experiments with controlling variables including mobile substrate thickness, sediment supply rate, and basin slope, in order to reveal the sedimentary origin and influence factors of the terminal fan. The results indicated that a higher sediment discharge rate on a substrate resulted in faster fan progradation coupled with relatively less subsidence and more sediment transport to terminal channels [20].

With the deepening study of reservoir characteristics, increasing numbers of researchers support that sedimentary facies can greatly influence the types of rock diagenesis [21]. Klunk revealed that diagenetic reactions, which are characterized by the dissolution and precipitation of minerals at low temperatures, control the quality of sedimentary rocks as hydrocarbon reservoirs [22]. Mansurbeg analyzed the mineralogy, petrography and geochemistry of siliciclastics to decipher and discuss the diagenetic alteration and subsequent evolution of reservoir quality and determined that the carbonate cements were of eogenetic and mesogenetic origin [23]. Gahtani studied the influence of diagenetic alterations on porosity in the Triassic Narrabeen Group, he studied the rock type and pore structure of the reservoir with a large number of analytical and laboratory data (casting thin section, scanning electron microscope, X-ray and so on), and divided the diagenesis stage in detail, and analyzed the relationship between oil-bearing property and pore structure of the reservoir [24]. Yuan studied the diagenesis of sandstone by using microscopic analysis techniques and suggested that leached feldspars were the internal source of authigenic quartz [25]. Mork discussed the diagenesis and quartz cement distribution of low-permeability sandstones, he believes that the main factor contributing to the low permeability of rocks is the large amount of silica cementation, which is derived from the dissolution of adjacent feldspars and unstable particles, and also from clay-rich microcolumn zeolites [26]. Jia studied the diagenesis and quality evolution of the low permeability reservoir of Suderte Oilfield, established four types of diagenetic facies, it is used to evaluate the reservoir synthetically [27]. Zhang established the diagenesis and physical property evolution sequence of the turbidite reservoir in the Dongying sag, and clearly defined the reservoir characteristics of "the first compact and after the accumulation" [28]. Baiyegunhi discussed the diagenesis of the Permian Ecca Group Sandstones in the Eastern Cape Province, he divided different diagenetic stages according to different reservoir properties, and analyzed the difference of the influence of each diagenetic stage on reservoir properties. Among them, the non-uniform location of the fracture-dominated reservoir caused by the later dissolution resulted in the enhanced heterogeneity of the reservoir, which affected the development effect of the oilfield [29]. Wang discussed the influence of diagenesis on reservoir properties of the lower Eocene red-bed sandstone reservoirs in the Dongying Depression, this research has put forward the viewpoint that diagenesis type, strength and other parameters are closely related to sedimentary environment, In alkaline diagenetic environment, calcite and gypsum dominate cementation, while in acidic environment, the dissolution of feldspar and carbonate cements results in the simultaneous appearance of primary intergranular pores and acidic dissolution pores [30]. Compared with conventional sandstone reservoirs, the diagenesis and physical evolution of ultra-low permeability reservoirs are obviously different, mainly in terms of their strength and action types. In this paper, the diagenesis and physical evolution process of reservoir in the four member of the Daqingzi oilfield are studied by means of various analysis and testing methods, and the matching relationship between reservoir physical property evolution and sedimentary and lithology is analyzed.

1.2 Key methods in this paper

Terminal fan reservoirs are different from conventional fluvial-delta systems, so we must apply specific technical methods to determine their sedimentary characteristics and physical properties [31]. In light of these differences, this study follows a new research approach. First, the sequence and sedimentary characteristics of the K₁q₄ are determined, and the reservoir is divided into different sedimentary microfacies belts. Then, the microscopic and macroscopic characteristics of the sandstone are clarified based on the sedimentary model and analysis of physical experimental data, and the reservoir quality is evaluated. Finally, the diagenetic stages of the sandstone are divided into different periods, and the degree of influence on the reservoir pore structure is determined to clarify why the reservoir had become tight sandstone.

1.3 The novelty of this paper

There are two main innovations in this paper. One is to establish a new sedimentary facies model of the terminal fan, which is different from the previous research. Previously, most researchers believed that the sedimentary facies here was a shallow-water delta, however our study found that the front of the sedimentary body did not have a stable catchment area at the end of the fan, but a seasonal catchment area sometimes absent in arid environments. This does not match the delta model; but the terminal fan can well explain these sedimentary facies problems. Another innovation is the detailed evaluation of the micro-characteristics of Fuyu oil layer in Haituozi area by means of various real experimental data. The influence of diagenesis on reservoir quality is analyzed emphatically, and the diagenetic stages are divided. The main controlling factors affecting reservoir physical properties are defined, and the influence degree is quantitatively described. However, these comprehensive and meticulous work has not been done before in Fuyu oil layer of Haituozi area.

3.1 Rock type

The sandstone types of K₁q₄ are lithic-feldspar sandstone and feldspar-lithic sandstone, with a small amount of lithic sandstone [34] (Fig. 3). The quartz percentage of the debris ranges from 18% to 42%, with an average of 32%. The feldspar percentage ranges from 15% to 50%, with an average of 34%, and mainly consists of potassium feldspar. The lithic percentage is generally distributed from 20% to 50%, with an average of 34%, and includes igneous rocks, metamorphic rocks, internal detritus, volcanic debris, and mica cuttings. Igneous rock and mica lithic fragments are the most common, which indicates that there are a lot of igneous rocks in the provenance area.

Figure 3

Lithological characteristics of K₁q₄

3.2 Mudstone color and the sedimentary environment

The mudstone has four colors: dark gray, grayish green, purple and purplish red. The lower area is mainly purple and lacks dark mudstone sediments, but gradually transitions to grayish green at the top. This pattern indicates that the climatic conditions during the early period were hot and arid or semi-arid and that the conditions during the later period were slightly hot and humid (Fig. 4).

Figure 4

Mudstone color distribution of K₁q₄

3.3 Sedimentary structure

The sedimentary structure mainly reflects the sedimentary characteristics of the drainage, the types and characteristics of which change from the periphery of the study area to the sedimentary center. Near the source area (D56, Q184), the main sedimentary structures are channel erosion structures, such as trough cross-bedding, tabular bedding and parallel bedding; the bottom of the channel contains directionally arranged muddy gravel. However, near the middle of the catchment basin (H7), the sedimentary structure gradually changes to flat bedding and ripple cross-lamination that is dominated by channels and waves (Fig. 5).

Figure 5

Characteristics of sedimentary structures of K₁q₄ in Haituozi area

3.4 Single-well facies characteristics

Fig. 6 shows the facies of Hu3 well (from 2092.0 m to 2110.2 m) in the Haituozi area. The lithology of the sandstone in the core section is mainly siltstone and fine sandstone. Four different types of sedimentary microfacies exist: distributary channels, near-channel overflow, far-channel overflow and mud flats. The thickness of a single distributary channel is approximately 5 m, and the shape of the spontaneous potential curve for the channel sandstone is box-shaped. Near-channel overflow deposits mainly consist of fine sandstone with tabular cross-bedding and ripple cross-lamination; their thicknesses are approximately 2 m, and their spontaneous potential curve is bell-shaped. Mud flat deposits usually contain brown, gray-green mudstone and siltstone, with deformation and bioturbation structures, the shape of the spontaneous potential curve is mostly fingerlike (Fig. 6).

Figure 6

Sedimentary facies analysis section of Hu3 well

3.5 Sedimentary models

Previous research of this paper revealed that the water depth was rather shallow during the deposition of K₁q₄, the climatic conditions during the early and middle stages of deposition were hot and dry or semi-arid and then transformed during the later stage into a hot-humid environment. During the early and middle stages of deposition, most of the sedimentation far from the source area occurred above the water surface where oxidation was strong, and the color of the mudstone is usually red. During the later stage of deposition, the water surface rose and the oxidation of the sediments in this zone gradually decreased. At the same time, the mudstone color usually became green or gray as the reducing conditions further strengthened [12]. Thus, the sedimentary facies of the K₁q₄ formation is a terminal fan which is dominated by the central sub-facies (it contains proximal, midrange and distal sub-facies) in the Haituozi area. The proximal sub-facies is mainly distributed in the northeast and south, and the distal sub-facies is mainly distributed in the middle of the study area. The sandstone gradually thins from the area of provenance to the sedimentary center, and the shale content gradually increases (Fig. 7).

Figure 7

Terminal fan sedimentary models of the K₁q₄

3.6 Sand body and microfacies distribution laws

In the early stage K₁q₄, a fluvial overflow lake began to form in the low-lying areas of the paleo-topography, and sand bodies were thick and widely distributed. Later, a catchment center formed with the gradual expansion of the lake, and the size of the sand bodies gradually decreased. Three provenance directions are exhibited: northeast, southwest, and southeast [35].

The major microfacies of the K₁q₄ in the Haituozi area include distributary channel, near channel overflow, far channel overflow and mud flats. Distributary channel sand bodies are widely distributed and gradually become thinner along the provenance direction, with their particle size gradually becoming finer [36]. The sand body’s thickness decreases from the channel center to either side. The type of connectivity between sand bodies is mainly shoulder-arm or side-lap connectivity, and isolated sand is reduced (Fig. 8).

Figure 8

Sand thickness and microfacies map of 5 small layer of the third sand group

4.1 Pore type

The reservoir pore types of K₁q₄ in the Haituozi area contain intergranular pores, intragranular pores, corrosion pores, micro-fractures and clay matrix pores. The dominant pore types are remnant intergranular pores and partial dissolution granule porosity after adjustment for diagenetic compaction, with minor corrosion pores and micro-fractures (Fig. 9).

Figure 9

Pore type and size of the K₁q₄ in Haituozi area

A casting thin-section analysis of K₁q₄ revealed additional fine microscopic reservoir characteristics: the reservoir particles are medium-sized, tightly packed, and tightly cemented. The number and size of primary intergranular pores are small, and the pores are often filled with hetero-matrix and cements [37] (Fig. 10a). The intragranular pores were mainly produced by the dissolution of feldspars and lithic particles, which are large in number but small in size (Fig. 10b). Corrosion pores formed throughout the dissolution of the feldspar skeleton particles (Fig. 10c). In addition, few micro-fractures were observed in the sandstones (Fig. 10d), which play an active role in improving reservoir properties because they facilitate oil and gas migration and acidic-formation water flow [38, 39].

Figure 10

K₁q₄ in Haituozi area (a) S51, 2085.8 m (b) S51, 2083.55 m (c) S51, 2083.55 m (d) S21, 1793.86 m

Scanning electron microscopy revealed that the sandstone is medium-grained, closely packed, and the degree of cementation is very dense. Few remaining available intergranular pores were present, but these pores are small in size (Fig. 11a). In addition, the intergranular pores are often filled with hybrids and cements, while the bridging growth of illite and other minerals block the pore-throat connections, significantly reducing the reservoir permeability (Fig. 11b). Dissolved feldspars are mostly bay-like or debris, creating intragranular pores and even forming mold pores (Fig. 11c). The cuttings are often dissolved into honeycombs and form large intragranular pores. In addition, some fractures in K₁q₄ have formed in response to rock stress, which create good oil and gas storage spaces (Fig. 11d).

Figure 11

Pore development characteristics by scanning electron microscope a F3, 2098.82 m b S115, 2136.87 m c Q193, 2147.15 m d Q184, 2289.56 m

4.2 Throat characteristics

The connectivity of sandstone pore throats can be determined from the results of a mercury injection test. The sandstone capillary pressure curves for K₁q₄ were similar in that they had no or very short curve platforms. The pore-throat sorting was poor, and the displacement pressure was high, with most values above 0.4 MPa. Thus, sandstones with a mean pore throat radius greater than 0.4 μm, permeability greater than 0.5 md, porosity greater than 12% and drainage pressure less than 0.5 MPa formed reservoirs with relatively good permeability in this area (Fig. 12).

Nuclear magnetic resonance (NMR) experiments have been successfully applied to study the percolation characteristics of sandstone and have achieved good results [40, 41]. NMR data for K₁q₄ was used to quantitatively characterize the mobile and bound fluids in the sandstone pores. The T₂ spectra for most samples were bimodal, but the value of the left T₂ peak was generally greater than the right peak, indicating that the pore throat structure was mainly small, and the binding ability of the particle skeleton to the fluid was greater than the free flow capacity of the fluid. Because the porosity and permeability of sandstone are so small, the T₂ spectra for some samples exhibited single peaks, or the right peak values were too low to be recognized (Fig. 13). Overall, the T₂ spectra peak values for most samples were low and the distribution range was narrow. Thus, the pore size of the sandstone was small, with fine pore-throat structure but good pore separation. Most of the fluid in the pore was bound by the particle framework, and the remaining free flow was small, comprising approximately 50%. Therefore, the productivity of such reservoirs would not be very large.

Figure 12

Constant speed mercury injection capillary pressure curve of sandstone

Figure 13

Reservoir NMR spectrum distribution curve of the K₁q₄ in Haituozi area

4.3 Classification and evaluation of reservoir pore structure

The reservoir space of sandstone in K₁q₄ reservoir is mainly primary intergranular pore and secondary dissolution pore, and a small amount of intergranular micropore and micro-fracture of authigenic clay mineral, which belongs to porous tight sandstone reservoir. With the increase of reservoir burial depth, the relative content of primary pores decreases, while the relative content of secondary dissolved pores increases. In addition, the pores of K₁q₄ reservoir sandstone are mainly micron-sized, and the throats are mainly nano-sized, belonging to micro-nanopore throat system. The connectivity of pore throat is poor, and the proportion of effective pore throat system is small. The poorer the physical property of the reservoir is, the higher the pore throat content is. A small amount of larger pore throat can significantly improve the permeability of tight sandstone reservoirs.

In order to evaluate the low permeability reservoir better, the pore-throat structure of K₁q₄ reservoir in Haituozi area was classified into I, II, III and IV grades by using the experimental parameters. The reservoir pore structure grade in the study area is mainly grade II. Table 1 is the division standard of pore-throat structure.

5.1 Compaction effect

According to scanning electron microscope data that were obtained from a casting thin slice, plastic deformation, structural fractures and detrital contacts were the main compaction phenomena in the sandstone reservoir.

1. The compaction deformation of plastic particles was mainly caused by the bending and elongation of mica, mudstone and debris. Some hard minerals were pressed into plastic particles and formed concave/convex contact lines at their edges because of differences in strength (Fig. 14a).

   

   Figure 14

   Characteristics of reservoir sandstone compaction electron microscopy of K₁q₄. (a) The compaction deformation of plastic particles, Q193, 2213.62 m. (b) A rigid particle, such as quartz or feldspar, S130, 2179.41 m. (c) The generation of tectonic fracture, Q184, 2289.75 m. (d) The contact relation between clastic particles changed, S21, 1804.46 m.

2. When the overburden pressure exceeds the particle compressive strength, the particles break along the weak surface: feldspar breaks along its cleavage surface. Cleavage and fractures in quartz and feldspar crystals may be filled or remain open during diagenesis. Open fractures can become oil and gas passages and be expanded by dissolution (Fig. 14b).

3. When the sandstone is semi-consolidated or consolidated, structural fractures can occur under compaction. Most of these fractures have been filled with chemically precipitated minerals, and few are fully open as oil and gas passages. The structural fractures are connected with the pores in the rock, which favors the seepage of oil and gas (Fig. 14c).

4. Particles become increasingly compact as the burial depth of the rock increases. The contact geometry between the debris particles gradually transitions from point contacts to line contacts and then to concave/convex contacts, gradually reducing the intergranular pores. In the sample from K₁q₄, quartz particles formed locally dense cemented masses by intense pressure dissolution (Fig. 14d).

5.2 Cementation

Cementation plays an important role in converting sediments into sedimentary rocks and is one of the major reasons for the reduction of porosity and permeability. Cementation is a destructive diagenetic process that can occur during various stages of diagenesis. The scanning electron microscope analysis of casting thin sections revealed that the clastic rock cementation types in K₁q₄ in the Haituozi area included clay-mineral cementation (kaolinite, illite and palygorskite), quartz cementation, carbonate cementation, dawsonite cementation and feldspar cementation (Fig. 15).

Figure 15

Statistical chart of reservoir sandstone cementation types of K₁q₄

1. Carbonate mineral cementation

Carbonate cements, including calcite and dolomite, mainly formed during the late portion of the early diagenetic stage. SEM observations showed that the carbonate cements were mostly rhombohedral or cubic and that calcite may have had less crystallinity, but the dolomite may have had better crystalline form [42]. Whole-rock X-ray diffraction analysis results showed that the calcite content ranged from 0.6% to 10%, with an average of 3.54%, and the dolomite content ranged from 0.9% to 4.3%, with an average of 2.54%. Samples with high textural maturity (good sorting and well-rounded grains) and low matrix content had high carbonate content during the early stage of diagenesis andwere usually crystalline in form. Carbonates precipitated in the pores as the main cement type, and this dense cementation impeded mechanical compaction (Fig. 16a). Samples with poor sorting and high matrix content had low calcite content, which was mostly granular and dispersed in the pores. The surfaces of these calcite crystals were relatively dirty and produced metasomatism with the surrounding particles (Fig. 16b). Carbonate minerals in the K₁q₄ sandstone that formed during the middle diagenetic period were mainly calcite and dolomite, with small amounts of iron carbonate.

Figure 16

Characteristics of carbonate cementation scanning electron microscope of K₁q₄ (a)Calcite filling in the rock with high textural maturity and low matrix content, H14, 2128.91 m, (b) Calcite filling in the rock with low content of separation and high content of matrix, H115, 2132.09 m

1. Quartz cementation

The siliceous cement in the K₁q₄ sandstone in the Haituozi area displays two main types, namely, quartz overgrowth and authigenic pore quartz. Siliceous cementation is very common, and the continuous overgrowth of quartz occurs in almost every sample [43]. Because of the existence of clay rings on the surface of quartz particles, the quartz overgrowth becomes discontinuous and the widths differ (Fig. 17a). The width of the area of quartz overgrowth is less than 50 μmin most samples, and its degree approaches level II. Most of the samples exhibit multiphase overgrowth (Fig. 17b).

Figure 17

Characteristics of quartz enlarged thin section of K₁q₄ in Haituozi area (single polarized light), (a) Discontinuous quartz secondary enlargement, S130, 2179.41 m. (b) Multistage secondary enlargement of quartz, S115, 2134.35 m.

The quartz overgrowth results from the growth of silicon that is dissolved in formation water along the surface of quartz particles. This particular mode of growth allows the optical orientation of the secondary silica and quartz particles to be consistent.

Under SEM, quartz overgrowth appeared as complete crystal surfaces, and growth surfaces and defect cavities can be seen on some crystal faces (Fig. 18a). In the Haituozi area, the direction of quartz overgrowth is generally towards the pore interior and then fills the pore, which may have caused the larger intergranular pores to become intergranular pores. The regenerative cementation of quartz often occurred when multiple quartz particles grew simultaneously (Fig. 18b). The silica in the formation water also crystallized to form fine-grained quartz, which showed good hexagonal prismatic crystals under an electron microscope (Fig. 18c), with vertical pore wall growth. In the Haituozi area, authigenic quartz and illite clay minerals often coexist, indicating the transformation of clay minerals, especially the transformation of smectite to illite. At the same time, secondary quartz precipitated around the feldspar particles, which indicates that the dissolution of feldspar particles provided important source material for the formation of quartz cement (Fig. 18d).

Figure 18

Characteristics of quartz secondary enlargement edge of K₁q₄ in Haituozi area (a) The quartz crystal shows a more complete crystal surface, S14, 2128 m. (b) The direction and growth characteristics of secondary enlargement of quartz, S14, 2128.91 m. (c) Siliceous from the formation water can also be crystallized to form small quartz crystals, C20, 2069.79 m. (d) Quartz is often associated with illite and other clay minerals, H152, 1734.51 m.

1. Clay cementation

Clay minerals, such as illite, kaolinite, chlorite, mixed illite-smectite (I/S) layers, palygorskite and halloysite, commonly occur in the pores of the K1q₄ sandstone in the Haituozi area. Among these minerals, the mixed I/S layers and illite are the most important, followed by chlorite and kaolinite (Fig. 19).

Figure 19

Histogram of clay minerals of K₁q₄ in Haituozi area

Illite is a clay mineral that is commonly found in the Haituozi area, with an average content of 15.8%. Illite forms gaskets in pores and then fills the pores (Fig. 20a). Kaolinite is also a common clay mineral (8.47%) and has a fake hexagonal crystal that appears as a plate [44] and is generally 3 to 10 μmwide. Its polymers are usually vermicular and page-shaped and have a dispersed flaky distribution (Fig. 20b). The average chlorite content in the clay minerals is 12.15%. Under SEM, monocrystals are nearly hexagonal, with smooth surfaces, uniform size, and diameters from 2 to 5 μm (Fig. 20c). Mixed I/S layers are the most abundant clay mineral in K₁q₄; the morphology ranges between montmorillonite and illite and mainly formed from flocculation (Fig. 20d).

Figure 20

Characteristics of reservoir clay minerals of K₁q₄ in Haituozi area (a) S14, 2128 m (b) S21, 1799.85 m (c) Q184, 2282.86 m (d) Q193, 2143.05 m

Palygorskite is a very slender fibrous crystal with clear outlines; the diameter of the fiber is generally approximately 0.01 μm (Fig. 21a). Palygorskite is a chain magnesium silicate mineral that is rich in magnesium and easily forms in alkaline environments, often in intergrowths with calcite. In the Haituozi area, rock palygorskite is often wrapped with mica, from which its formation is inferred to be related to alkaline fluids and volcanic rock debris [45].

Figure 21

Characteristics of reservoir clay mineral of K1q4 by scanning electron microscope (a) Palygorskite cementation arranged in bundles, Q184, 2289.75 m. (b) Halloysite cementation arranged in short tubular, H152, 1704.23 m. (c) Feldspar cementation arranged in a tabular shape, Q184, 2262.34 m. (d) Dawsonite cementatio, H152, 1704.23 m.

Halloysite is a rare clay mineral that is typically tubular in shape (sometimes long, sometimes short) (Fig. 21b). Halloysite can form from the transformation of allophane micelles. This form of halloysite often exhibits a globular shape with some surface cracks, grooves, and fine hairs [46]. In the Haituozi area, halloysite evolved from illite and exhibits a short tubular shape.

In the Haituozi area, most of the feldspar cementation is a combination of small euhedral crystals, with minor detrital feldspar overgrowths (Fig. 21c). Feldspar cementation was related to the abundance of SiO₂, Al₂O₃, Na⁺ and K⁺ ions in the pore fluid.

Dawsonite is a rare carbonate mineral that belongs to the orthorhombic system. The most common aggregates are columnar, radial and fan-shaped (Fig. 21d). According to previous research, the formation of dawsonite in the Haituozi area was associated with inorganic CO₂ from deep magma that ascended through fractures to the target layer and then released CO₂ [47].

5.3 Dissolution

Dissolution was very common in the Haituozi area, mostly resulting from feldspar dissolution. Debris dissolution was relatively rare, and matrix dissolution was not obvious. The dissolution of detrital grains, such as feldspars and debris, was mainly influenced by organic acids that were generated during the thermal evolution of organic matter.

The dissolved feldspars have edges with many indentations or dissolution along the cleavage that forms jagged edges. Strongly dissolved feldspar can be debris-shaped, forming an intragranular dissolution hole and even a mold hole. Electron microscopy could often resolve this feldspar dissolution into honeycomb shapes, which were pane- and debris-like and formed mold holes, clad holes and rib-shaped holes (Fig. 22).

Figure 22

Scanning electron microscopic characteristics of reservoir dissolution of K₁q₄

6.1 Effect of compression on the rock’s physical properties

The formation pressure coefficient in Haituozi area is generally 0.95-1.02, the buried depth of Fuyu reservoir is generally 1800-2200‘ m, the average formation pressure is 20.9 MPa, and the original bottom layer temperature is 88.2 ^∘C. It belongs to normal temperature and pressure system.

Because of the nature of the rock itself, including the compressive strength of mineral grains, the type of interstitial material and the type of fluid in the pores, the porosity and permeability of the rock decreases as the burial depth increases. Thus, the effect of compaction on K₁q₄ significantly reduced the porosity and permeability. The rate of reduction was approximately 1% per 100 m of compaction. Compaction affected different microfacies differently: channel overflow facies had many muddy components, which were strongly affected by the compaction, but the effect of compaction on the distributary channel facies was comparatively weak (Fig. 23).

Figure 23

Changes of reservoir physical properties with depth of Haituozi area

6.2 Effects of clay cements on the rock’s physical property

Pressure and temperature also increase with increasing burial depth. The release of water between layers and the removal of cations can cause the recrystallization and transformation of clay minerals [48]. Under shallow burial conditions, clay minerals may have kaolinite and montmorillonite; meanwhile, these minerals disappear and turn into illite or chlorite under deep burial conditions. The relationship between clay minerals and changes in depth in the Haituozi area is complicated because it is affected by the source of volcanic pyroclastic debris (Fig. 24). Under shallow burial conditions, illite and chlorite reached a relatively mature stage of evolution, and their content rapidly increased. This result was mainly related to the volcanic rock, which provided a sufficient source of iron for the transformation of montmorillonite to illite and thus accelerated the formation of illite and chlorite. The content of cuttings in the rocks in the study area is very high, with an average of 34%, which leads to the low maturity of sandstone composition, with an average of 0.44%. The composition of the debris is mainly acidic volcanic rock debris, accounting for more than 60%, and contains a small amount of intermediate basic volcanic rock debris (andesite, basalt). The main types of volcanic debris are rhyolite debris, tuff debris, andesite debris and basalt debris. These provide favorable conditions for the conversion of clay minerals, and increase the content of illite and chlorite in shallow burial conditions.

Figure 24

Changes of reservoir clay minerals with depth of Haituozi area

The influence of clay minerals on reservoir quality is mainly manifested in two aspects:

1. Making reservoir physical properties worse

Clay minerals adhere to the surface of particles or fill in pores, which reduces the original pore space of rock. In addition, clay matrix has strong plasticity and is easy to flow, so it is easy to fill pore throat and reduce reservoir permeability. The clay mineral content of Quan4 reservoir is between 10% and 20%, which is inversely proportional to porosity and permeability. With the increase of clay mineral content, porosity and permeability both show a downward trend, and the downward trend of permeability is more obvious (Fig. 25).

Figure 25

Relationship between reservoir physical property and clay content

1. Making reservoir physical properties better

The intergranular micropores formed by authigenic clay minerals in Quan4 reservoir are widespread, mainly including kaolinite intergranular pores, illite intergranular pores and other types. However, the pores between these crystals are small in size, mainly in nanoscale pores, and the connectivity between pores is very poor (Fig. 20).

6.3 Effects of carbonate cement on rock physical property

Carbonate cementation is one of the main factors to destroy reservoir conditions in this area. According to their precipitation order and composition changes, they can be divided into two phases: early and late. Early carbonate cements were mainly formed in the early stage of middle diagenesis, including calcite and dolomite, mostly rhombohedral or cubic. Late carbonate minerals are mainly iron-bearing calcite and dolomite, mostly occurring after quartz enlargement, mainly formed in the late middle diagenesis. The content of iron-bearing carbonate minerals in the sandstone of Fuyu reservoir in Haituozi area are less, so most carbonate minerals were formed in early stage.

Because the formation thickness of Fuyu reservoir is relatively small, and most of the experimental samples are located between 2100 to 2300 m, so only from the relationship between carbonate minerals and buried depth, the two are not closely related. With the further development of diagenesis, early carbonate minerals will be dissolved or metasomatized, so with the increase of depth, carbonate minerals have a certain decreasing trend, but the trend is not obvious (Fig. 26).

Figure 26

Relationship between carbonate minerals and burial depth

The carbonate cementation levels were high, and carbonate dissolution was not strong. Carbonate cementation destroyed porosity, as seen from the correlation between the carbonate content and reservoir physical properties: the higher the carbonate content, the worse the physical properties of the reservoir became (Fig. 27).

Figure 27

Changes of physical properties with content of carbonate cement in Haituozi area

6.4 Influence of diagenesis on the quantitative evaluation of the rock’s physical properties

Reservoirs in the study area have undergone intensive diagenesis with various types, including compaction, cementation and authigenic mineral precipitation, metasomatism and dissolution. Sandstone has high degree of consolidation, and the contact mode between particles is point-line contact; vitrinite reflectance Ro is less than 1.2%. In addition, there are many kinds of authigenic minerals in sandstone, such as illite, chlorite, kaolinite, quartz authigenic overgrowth, feldspar authigenic overgrowth, plagioclase albitization and so on. The degree of quartz overgrowth is grade II. A small amount of crystalline ankerite can be seen. Dissolution is mainly caused by feldspar, cuttings and interstitial materials. The comprehensive analysis shows that the diagenesis stage of K₁q4 reservoir is the A period of the middle diagenetic stage (Table 2).

According to the relationship between the structure and original properties (Sneider chart), the original porosity of the K₁q₄ reservoir in the Haituozi area can be estimated to be approximately 40% [49, 50] (Fig. 28). Compaction was a destructive diagenetic process that reduced the primary porosity. Early quartz enlargement and calcite cementation also reduced the primary porosity, but the extent of this effect was less than that of compaction. The dissolution of particles and interstitial substances was a positive diagenetic process and was the main mechanism that formed the secondary porosity. Kaolinite, chlorite, illite, albite and other mineral fillings also had different effects on the pore characteristics of the reservoirs. Different microfacies had slightly different effects, and the sand mat was strongly affected by compaction, followed by the front sand bar and braided channel.

Figure 28

Effect of diagenesis on reservoir porosity