Analysis of the Factors That Influence Diagenesis in the Terminal Fan

Open Geosci. 2018; 10:866–881

Research Article Open Access

Jinkai Wang*, Zhang Jinliang, and Jun Xie Analysis of the Factors that Influence Diagenesis in the Terminal Fan Reservoir of Fuyu Oil Layer in the Southern Songliao Basin, Northeast China https://doi.org/10.1515/geo-2018-0068 Keywords: Songliao basin, ultra-low permeability, termi- Received April 26, 2018; accepted November 8, 2018 nal fan, pore type, diagenesis Abstract: The diagenesis mechanism and the physical properties of a terminal fan reservoir are determined by nuclear magnetic resonance (NMR), X-ray diffraction and 1 Introduction scanning electron microscopy. The main provenance directions are NE and SE, and the two oppositely directed 1.1 Background fans converge to form a small catchment basin. The mudstone color is red or purplish red, which accounts for Little controversy exists regarding the sedimentary sys- 60% of the total rock. The sandstones are lithic-feldspar tem that produced the fourth member of the Quantou for- sandstones and feldspar-lithic sandstones, with a smaller mation in the Songliao basin; most researchers consider quartz component relative to the adjacent sandstone for- this formation to be a shallow water delta, which is a mations. The reservoir mainly consists of intergranular lake delta model formed in a dry environment, the main pores (51%), intragranular pores (22%), corrosion pores types of sedimentary facies are channel, channel flood, (20%), micro-fractures (5%) and clay matrix pores (2%). sand bar, levee, crevasse splay and sand sheet [1–6]. A The porosity of the reservoir is only 13%, and the throats few researchers, such as Peng and Gong, hold other views, are fine with high displacement pressure. The diage- defining it as a common delta front [7, 8]. Sun thought it netic processes included compaction, cementation, re- was a meandering river delta, because of the control of placement, and dissolution, and the most influential fac- the sedimentary system of the meandering river delta in tor on the reservoir porosity was compaction. The detri- the northwest provenance, the southern part of Songliao tal rock cement mainly consists of clay minerals (48%), Basin mainly developed a large area of thin sand bodies quartz (23%), carbonate (19%), feldspar (7%) and daw- distributed in large areas [9]. Wang thought that the low sonite (3%). Among them, the mixed I/S layer has the most delta sedimentary facies was developed in the southern content and the most important cementation. In addition, Songliao basin. During this period, the uplift rate of the a small amount of dawsonite is found in the pores of the provenance decreased, and the water system was shrink- sandstone, which is a unique mineral that is related to the ing to the land, the lake basin expanded, and the sed- background of inorganic CO2. The main diagenesis factors iment source came from the surrounding mountainous that affected this sandstone’s porosity were compaction, early quartz overgrowth and calcite cementation, which reduced the porosity from 40% to approximately 8%. Al- *Corresponding Author: Jinkai Wang: College of Earth Science though dissolution and fracture increased the porosity and Engineering, Shandong University of Science and Technology, (from 8% to 26%), clay- and carbonate-mineral cementa- Qingdao, 266590, China tion during the late diagenesis period had a dramatic ef- Laboratory for Marine Mineral Resources, Qingdao National Labora- fect, forming a typical low-porosity and low-permeability tory for Marine Science and Technology, Qingdao 266237, China Jun Xie: reservoir. College of Earth Science and Engineering, Shandong Uni- versity of Science and Technology, Qingdao, 266590, China Laboratory for Marine Mineral Resources, Qingdao National Labora- tory for Marine Science and Technology, Qingdao 266237, China Zhang Jinliang: College of Earth Science and Engineering, Shan- dong University of Science and Technology, Qingdao, 266590, China College of Resources Science and Technology, Beijing Normal Uni- versity, Beijing 100875, China

Open Access. © 2018 Jinkai Wang et al., published by De Gruyter. This work is licensed under the Creative Commons Attribution- NonCommercial-NoDerivs 4.0 License. Analysis of the Factors that Influence Diagenesis in the Terminal Fan Reservoir Ë 867 area [10].However, the latest research showed that the sed- vision is not sufficiently fine [16]. Newell studied the sedi- imentary characteristics of this region are unique, unlike a mentary characteristics of Permian strata in the southern common river delta. These features are reflected in aspects Ural River in Russia to better understand each terminal such as the lithology, mudstone color, grain size, sand- fan facies and divided the facies into various micro depo- body extension law and so on. sitional environments [17]. This research provided an im- Therefore, a new terminal fan sedimentary system has portant theoretical basis to deepen the study of terminal been proposed. A terminal fan is a special fluvial-delta sed- fan sedimentary systems. Parkash classified the terminal imentary system that forms in a dry to semi-dry environ- fan system of the Makanda River in India into three sec- ment, where the flow rate is reduced at the end of the river. ondary sedimentary environments, namely, channel, nat- With the gradual decline in terrain slope, water spreads ural levee and flood plain, and identified nine lithofacies. out and the flow rate drops, allowing a large amount of This is a detailed anatomical study of the end fan sys- clastic material to be deposited to form the fan-shaped ac- tem [18]. Sanchez constructed a model for a terminal fan cumulation. The terminal fan is different from the alluvial sedimentary system from the lower Cretaceous Candeleros fan, diluvial fan and turbidite fan, they are not the same formation in the Neuquen basin, dividing the sedimentary type. The rocks deposited by terminal fan are usually finer body into three components from bottom to top, but this in grain size, better in sorting, and have higher component classification method was not conducive to the fine-scale maturity and textural maturity. The terminal fan is similar evaluation of reservoirs [19]. Chatmas conducted a series to the river delta, the difference between them is whether of experiments with controlling variables including mo- there is a stable catchment area at the front of the fan body. bile substrate thickness, sediment supply rate, and basin River deltas are generally pushed into stable water bodies, slope, in order to reveal the sedimentary origin and in- forming sediments interacted by rivers and lakes (oceans). fluence factors of the terminal fan. The results indicated However, the front end of the terminal fan usually does not that a higher sediment discharge rate on a substrate re- have a stable catchment area, or its size changes rapidly sulted in faster fan progradation coupled with relatively with the season, so, the terminal fan would disappear be- less subsidence and more sediment transport to terminal fore entering the water at the end [11]. The microfacies of channels [20]. the terminal fan is defined as four different types in this pa- With the deepening study of reservoir characteristics, per: distributary channel, near channel overflow, far chan- increasing numbers of researchers support that sedimen- nel overflow and mud flats, and this view is different from tary facies can greatly influence the types of rock diagen- that of previous studies. esis [21]. Klunk revealed that diagenetic reactions, which Mukerji first proposed the concept of a sedimentary are characterized by the dissolution and precipitation of fan based on a study of the Sutlej-Yamuna plain [12]. Later, minerals at low temperatures, control the quality of sedi- Friend studied the Devonian-age red sandstone and pro- mentary rocks as hydrocarbon reservoirs [22]. Mansurbeg posed a model for terminal fan deposition from many analyzed the mineralogy, petrography and geochemistry present-day river systems that have been studied. This lit- of siliciclastics to decipher and discuss the diagenetic al- erature was the first that studied terminal fan sedimen- teration and subsequent evolution of reservoir quality and tary deposits but proposed a dry environment for the ter- determined that the carbonate cements were of eogenetic minal fan sedimentary model, and its specific sedimen- and mesogenetic origin [23]. Gahtani studied the influ- tary pattern was not described [13]. Some objections were ence of diagenetic alterations on porosity in the Triassic also raised, North argues that modern rivers in arid re- Narrabeen Group, he studied the rock type and pore struc- gions cannot bifurcate in the lower reaches of the river, ture of the reservoir with a large number of analytical and that the so-called branching rivers are caused by con- and laboratory data (casting thin section, scanning elec- tinuous channel changes, which can also be seen in some tron microscope, X-ray and so on), and divided the dia- large alluvial fans [14]. Zhang found a widely developed genesis stage in detail, and analyzed the relationship be- red mudstone layer with shallow water and oxygen-rich la- tween oil-bearing property and pore structure of the reser- custrine facies, naming it flood rock. This research laid the voir [24]. Yuan studied the diagenesis of sandstone by us- foundation for the application of the terminal fan model ing microscopic analysis techniques and suggested that in a continental lake basin in China [15]. Kelly et al. di- leached feldspars were the internal source of authigenic vided terminal fans into three zones, namely, the recharge quartz [25]. Mork discussed the diagenesis and quartz ce- channel, distributary channel and remote basin, accord- ment distribution of low-permeability sandstones, he being to a study of the Devonian formation in England. This lieves that the main factor contributing to the low perme- view is similar to that in this article, but the degree of di- ability of rocks is the large amount of silica cementation, 868 Ë Jinkai Wang et al. which is derived from the dissolution of adjacent feldspars based on the sedimentary model and analysis of physical and unstable particles, and also from clay-rich microcol- experimental data, and the reservoir quality is evaluated. umn zeolites [26]. Jia studied the diagenesis and quality Finally, the diagenetic stages of the sandstone are divided evolution of the low permeability reservoir of Suderte Oil- into different periods, and the degree of influence onthe field, established four types of diagenetic facies, it isused reservoir pore structure is determined to clarify why the to evaluate the reservoir synthetically [27]. Zhang estab- reservoir had become tight sandstone. lished the diagenesis and physical property evolution sequence of the turbidite reservoir in the Dongying sag, and clearly defined the reservoir characteristics of "the first 1.3 The novelty of this paper compact and after the accumulation" [28]. Baiyegunhi discussed the diagenesis of the Permian Ecca Group Sand- There are two main innovations in this paper. One is to stones in the Eastern Cape Province, he divided different establish a new sedimentary facies model of the terminal diagenetic stages according to different reservoir proper- fan, which is different from the previous research. Previ- ties, and analyzed the difference of the influence of each ously, most researchers believed that the sedimentary fa- diagenetic stage on reservoir properties. Among them, the cies here was a shallow-water delta, however our study non-uniform location of the fracture-dominated reservoir found that the front of the sedimentary body did not have caused by the later dissolution resulted in the enhanced a stable catchment area at the end of the fan, but a sea- heterogeneity of the reservoir, which affected the devel- sonal catchment area sometimes absent in arid environ- opment effect of the oilfield [29]. Wang discussed thein- ments. This does not match the delta model; but the ter- fluence of diagenesis on reservoir properties of the lower minal fan can well explain these sedimentary facies prob- Eocene red-bed sandstone reservoirs in the Dongying De- lems. Another innovation is the detailed evaluation of the pression, this research has put forward the viewpoint that micro-characteristics of Fuyu oil layer in Haituozi area by diagenesis type, strength and other parameters are closely means of various real experimental data. The influence of related to sedimentary environment, In alkaline diage- diagenesis on reservoir quality is analyzed emphatically, netic environment, calcite and gypsum dominate cemen- and the diagenetic stages are divided. The main control- tation, while in acidic environment, the dissolution of ling factors affecting reservoir physical properties are de- feldspar and carbonate cements results in the simultane- fined, and the influence degree is quantitatively described. ous appearance of primary intergranular pores and acidic However, these comprehensive and meticulous work has dissolution pores [30]. Compared with conventional sand- not been done before in Fuyu oil layer of Haituozi area. stone 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 2 Regional geology of the research paper, the diagenesis and physical evolution process of reservoir in the four member of the Daqingzi oilfield are area studied by means of various analysis and testing methods, and the matching relationship between reservoir physical The research area is located in Jilin Province in northeast- property evolution and sedimentary and lithology is ana- ern China. Tectonically, this area is located in the middle lyzed. depression area to the south of the Da’an-Honggang Ter- race in the southern Songliao basin, its structure is a continuous eastward slope [32]. The main oil-bearing series 1.2 Key methods in this paper is the Fuyu oil layer (K1q4), which has a thickness of approximately 100 m and consists of fine siltstone and mud- Terminal fan reservoirs are different from conventional stone. The middle and lower of Fuyu oil layer is dominated fluvial-delta systems, so we must apply specific techni- by purple-red mudstone and fine sandstone, and the up- cal methods to determine their sedimentary characteris- per area mainly consists of dark mudstone and siltstone tics and physical properties [31]. In light of these differ- (Fig. 1). ences, this study follows a new research approach. First, Based on a high-resolution sequence stratigraphic the sequence and sedimentary characteristics of the K1q4 framework, the K1q4 formation can be divided into three are determined, and the reservoir is divided into different base-level cycles or sequence levels: short-term, mid-term sedimentary microfacies belts. Then, the microscopic and and long-term [33]. This formation has 4 mid-term base- macroscopic characteristics of the sandstone are clarified level cycles, which are the superposition of short-term cy- Analysis of the Factors that Influence Diagenesis in the Terminal Fan Reservoir Ë 869

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 1: Location map of research are cles that consist of 3-5 progressive or regressive structures. Only one long-term base-level cycle exists, consisting of two asymmetric half-cycles. The K1q4 formation consists of 12 small layers, the small layers 1-4 belong to sand group Figure 3: I, the small layers 5-7 belong to sand group II, the small Lithological characteristics of K1q4 layers 8-10 belong to sand group III, and the last two small layers belong to sand group IV (Fig. 2).

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 2: Sequence stratigraphic framework of K1q4 3.3 Sedimentary structure

The sedimentary structure mainly reflects the sedimen- 3 Sedimentary microfacies tary characteristics of the drainage, the types and characteristics of which change from the periphery of the study distributions area to the sedimentary center. Near the source area (D56, Q184), the main sedimentary structures are channel ero- 3.1 Rock type sion structures, such as trough cross-bedding, tabular bedding and parallel bedding; the bottom of the channel con- The sandstone types of K1q4 are lithic-feldspar sandstone tains directionally arranged muddy gravel. However, near and feldspar-lithic sandstone, with a small amount of the middle of the catchment basin (H7), the sedimentary lithic sandstone [34] (Fig. 3). The quartz percentage of structure gradually changes to flat bedding and ripple the debris ranges from 18% to 42%, with an average of cross-lamination that is dominated by channels and waves 32%. The feldspar percentage ranges from 15% to 50%, (Fig. 5). 870 Ë Jinkai Wang et al.

Figure 6: Sedimentary facies analysis section of Hu3 well

3.5 Sedimentary models

Previous research of this paper revealed that the water

Figure 4: Mudstone color distribution of K1q4 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. Dur- ing 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 Figure 5: Characteristics of sedimentary structures of K1q4 in Haituozi area formation is a terminal fan which is dominated by the central sub-facies (it contains proximal, midrange and distal 3.4 Single-well facies characteristics 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 Fig. 6 shows the facies of Hu3 well (from 2092.0 m to study area. The sandstone gradually thins from the area of 2110.2 m) in the Haituozi area. The lithology of the sand- provenance to the sedimentary center, and the shale con- stone in the core section is mainly siltstone and fine sand- tent gradually increases (Fig. 7). stone. Four different types of sedimentary microfacies exist: distributary channels, near-channel overflow, far- channel overflow and mud flats. The thickness ofasin- gle 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 7: Terminal fan sedimentary models of the K1q4 Analysis of the Factors that Influence Diagenesis in the Terminal Fan Reservoir Ë 871

3.6 Sand body and microfacies distribution partial dissolution granule porosity after adjustment for laws diagenetic compaction, with minor corrosion pores and micro-fractures (Fig. 9).

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 Figure 9: Pore type and size of the K1q4 in Haituozi area area include distributary channel, near channel overflow, far channel overflow and mud flats. Distributary channel sand bodies are widely distributed and gradually become A casting thin-section analysis of K1q4 revealed ad- thinner along the provenance direction, with their parti- ditional fine microscopic reservoir characteristics: the cle size gradually becoming finer [36]. The sand body’s reservoir particles are medium-sized, tightly packed, and thickness decreases from the channel center to either side. tightly cemented. The number and size of primary inter- The type of connectivity between sand bodies is mainly granular pores are small, and the pores are often filled shoulder-arm or side-lap connectivity, and isolated sand with hetero-matrix and cements [37] (Fig. 10a). The intra- is reduced (Fig. 8). granular 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 ob- served in the sandstones (Fig. 10d), which play an ac- tive role in improving reservoir properties because they fa- cilitate oil and gas migration and acidic-formation water flow [38, 39].

Figure 8: Sand thickness and microfacies map of 5 small layer of the third sand group

4 Pore structure features Figure 10: K1q4 in Haituozi area (a) S51, 2085.8 m (b) S51, 4.1 Pore type 2083.55 m (c) S51, 2083.55 m (d) S21, 1793.86 m

The reservoir pore types of K1q4 in the Haituozi area con- Scanning electron microscopy revealed that the sand- tain intergranular pores, intragranular pores, corrosion stone is medium-grained, closely packed, and the degree pores, micro-fractures and clay matrix pores. The dom- of cementation is very dense. Few remaining available in- inant pore types are remnant intergranular pores and tergranular pores were present, but these pores are small 872 Ë Jinkai Wang et al. 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 K q have formed in response to 1 4 Figure 12: Constant speed mercury injection capillary pressure rock stress, which create good oil and gas storage spaces curve of sandstone (Fig. 11d).

teristics of sandstone and have achieved good results [40, 41]. NMR data for K1q4 was used to quantitatively charac- terize 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, com- prising approximately 50%. Therefore, the productivity of 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 such reservoirs would not be very large. 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 Figure 13: Reservoir NMR spectrum distribution curve of the K1q4 in Haituozi area 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 charac- Analysis of the Factors that Influence Diagenesis in the Terminal Fan Reservoir Ë 873

4.3 Classification and evaluation of lines at their edges because of differences in strength reservoir pore structure (Fig. 14a). (2) When the overburden pressure exceeds the particle

The reservoir space of sandstone in K1q4 reservoir is compressive strength, the particles break along the weak mainly primary intergranular pore and secondary disso- surface: feldspar breaks along its cleavage surface. Cleav- lution pore, and a small amount of intergranular micro- age and fractures in quartz and feldspar crystals may be pore and micro-fracture of authigenic clay mineral, which filled or remain open during diagenesis. Open fractures belongs to porous tight sandstone reservoir. With the in- can become oil and gas passages and be expanded by dis- crease of reservoir burial depth, the relative content of pri- solution (Fig. 14b). mary pores decreases, while the relative content of sec- (3) When the sandstone is semi-consolidated or consoli- ondary dissolved pores increases. In addition, the pores of dated, structural fractures can occur under compaction. K1q4 reservoir sandstone are mainly micron-sized, and the Most of these fractures have been filled with chemically throats are mainly nano-sized, belonging to micro-nano- precipitated minerals, and few are fully open as oil and gas pore throat system. The connectivity of pore throat is poor, passages. The structural fractures are connected with the and the proportion of effective pore throat system is small. pores in the rock, which favors the seepage of oil and gas The poorer the physical property of the reservoir is, the (Fig. 14c). higher the pore throat content is. A small amount of larger (4) Particles become increasingly compact as the burial pore throat can significantly improve the permeability of depth of the rock increases. The contact geometry between tight sandstone reservoirs. the debris particles gradually transitions from point con- In order to evaluate the low permeability reservoir bet- tacts to line contacts and then to concave/convex contacts, ter, the pore-throat structure of K1q4 reservoir in Haituozi gradually reducing the intergranular pores. In the sample area was classified into I, II, III and IV grades by using from K1q4, quartz particles formed locally dense cemented the experimental parameters. The reservoir pore structure masses by intense pressure dissolution (Fig. 14d). grade in the study area is mainly grade II. Table 1 is the division standard of pore-throat structure.

5 Sandstone Diagenetic types

The sandstone in K1q4 in the Haituozi area was mainly affected by three diagenetic types: compaction, cementation and dissolution. The compaction effect was the main factor that influenced diagenesis. The cement types included quartz, illite, kaolinite, chlorite and carbonate. Dissolu- tion was mainly caused by the dissolution of feldspar and volcanic rock cuttings, and the physical properties of the reservoirs were the result of a common remodeling of these diageneses. Figure 14: Characteristics of reservoir sandstone compaction electron microscopy of K1q4. (a) The compaction deformation of plastic 5.1 Compaction effect 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 According to scanning electron microscope data that were changed, S21, 1804.46 m. 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 874 Ë Jinkai Wang et al.

Table 1: The division standard of pore-throat structure and the four grades

Evaluation para Pore throat structure I II III IV Porosity (%) >12 9~12 6~9 <6 Permeability(10−3µm2) >1 0.5~1 0.1~0.5 <0.1 Median value of pore radius (µm) >0.5 0.3~0.5 0.1~0.3 <0.1 Mean value of pore throat radius (µm) >0.4 0.2~0.4 0.2~0.1 <0.1 Displacement pressure(0.1 MPa) <0.5 0.5~0.7 0.7~1.1 >1.1 Relative evaluation results Optimal Good Middle Bad

of diagenesis and were usually crystalline in form. Carbon- ates 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 Figure 15: Statistical chart of reservoir sandstone cementation types small amounts of iron carbonate. of K1q4

5.2 Cementation

Cementation plays an important role in converting sediments into sedimentary rocks and is one of the major rea- sons for the reduction of porosity and permeability. Ce- mentation is a destructive diagenetic process that can occur during various stages of diagenesis. The scanning electron microscope analysis of casting thin sections revealed Figure 16: Characteristics of carbonate cementation scanning elec- that the clastic rock cementation types in K1q4 in the tron microscope of K1q4 (a)Calcite filling in the rock with high tex- Haituozi area included clay-mineral cementation (kaolin- tural maturity and low matrix content, H14, 2128.91 m, (b) Calcite ite, illite and palygorskite), quartz cementation, carbonate filling in the rock with low content of separation and high contentof cementation, dawsonite cementation and feldspar cemen- matrix, H115, 2132.09 m tation (Fig. 15). (1) Carbonate mineral cementation Carbonate cements, including calcite and dolomite, (2) Quartz cementation mainly formed during the late portion of the early dia- The siliceous cement in the K1q4 sandstone in the genetic stage. SEM observations showed that the carbon- Haituozi area displays two main types, namely, quartz ate cements were mostly rhombohedral or cubic and that overgrowth and authigenic pore quartz. Siliceous cemen- calcite may have had less crystallinity, but the dolomite tation is very common, and the continuous overgrowth may have had better crystalline form [42]. Whole-rock X- of quartz occurs in almost every sample [43]. Because of ray diffraction analysis results showed that the calcite con- the existence of clay rings on the surface of quartz par- tent ranged from 0.6% to 10%, with an average of 3.54%, ticles, the quartz overgrowth becomes discontinuous and and the dolomite content ranged from 0.9% to 4.3%, with the widths differ (Fig. 17a). The width of the area of quartz an average of 2.54%. Samples with high textural maturity overgrowth is less than 50 µm in most samples, and its de- (good sorting and well-rounded grains) and low matrix gree approaches level II. Most of the samples exhibit multi- content had high carbonate content during the early stage phase overgrowth (Fig. 17b). Analysis of the Factors that Influence Diagenesis in the Terminal Fan Reservoir Ë 875

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 sili- con 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. Figure 18: Under SEM, quartz overgrowth appeared as complete Characteristics of quartz secondary enlargement edge of K q in Haituozi area (a) The quartz crystal shows a more complete crystal surfaces, and growth surfaces and defect cavities 1 4 crystal surface, S14, 2128 m. (b) The direction and growth charac- can be seen on some crystal faces (Fig. 18a). In the Haituozi teristics of secondary enlargement of quartz, S14, 2128.91 m. (c) area, the direction of quartz overgrowth is generally to- Siliceous from the formation water can also be crystallized to form wards the pore interior and then fills the pore, which may small quartz crystals, C20, 2069.79 m. (d) Quartz is often associ- have caused the larger intergranular pores to become in- ated with illite and other clay minerals, H152, 1734.51 m. tergranular pores. The regenerative cementation of quartz often occurred when multiple quartz particles grew simul- taneously (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 Figure 19: Histogram of clay minerals of K1q4 in Haituozi area feldspar particles, which indicates that the dissolution of feldspar particles provided important source material for the formation of quartz cement (Fig. 18d). minerals is 12.15%. Under SEM, monocrystals are nearly (3) Clay cementation hexagonal, with smooth surfaces, uniform size, and diam- Clay minerals, such as illite, kaolinite, chlorite, mixed eters from 2 to 5 µm (Fig. 20c). Mixed I/S layers are the most illite-smectite (I/S) layers, palygorskite and halloysite, abundant clay mineral in K1q4; the morphology ranges be- commonly occur in the pores of the K1q4 sandstone in the tween montmorillonite and illite and mainly formed from Haituozi area. Among these minerals, the mixed I/S layers flocculation (Fig. 20d). and illite are the most important, followed by chlorite and In addition, other types of mineral cements are kaolinite (Fig. 19). present in the Haituozi area, such as palygorskite, hal- Illite is a clay mineral that is commonly found in the loysite, feldspar and dawsonite, with a total content of ap- Haituozi area, with an average content of 15.8%. Illite proximately 5%. forms gaskets in pores and then fills the pores (Fig. 20a). Palygorskite is a very slender fibrous crystal with clear Kaolinite is also a common clay mineral (8.47%) and has a outlines; the diameter of the fiber is generally approxi- fake hexagonal crystal that appears as a plate [44] and is mately 0.01 µm (Fig. 21a). Palygorskite is a chain magne- generally 3 to 10 µm wide. Its polymers are usually vermic- sium silicate mineral that is rich in magnesium and eas- ular and page-shaped and have a dispersed flaky distri- ily forms in alkaline environments, often in intergrowths bution (Fig. 20b). The average chlorite content in the clay with calcite. In the Haituozi area, rock palygorskite is often 876 Ë Jinkai Wang et al.

Figure 20: Characteristics of reservoir clay minerals of K q 1 4 Figure 21: Characteristics of reservoir clay mineral of K1q4 by scan- in Haituozi area (a) S14, 2128 m (b) S21, 1799.85 m (c) Q184, ning electron microscope (a) Palygorskite cementation arranged in 2282.86 m (d) Q193, 2143.05 m 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, wrapped with mica, from which its formation is inferred to H152, 1704.23 m. be related to alkaline fluids and volcanic rock debris [45]. Halloysite is a rare clay mineral that is typically tubular in shape (sometimes long, sometimes short) (Fig. 21b). The dissolved feldspars have edges with many indenta- Halloysite can form from the transformation of allophane tions or dissolution along the cleavage that forms jagged micelles. This form of halloysite often exhibits a glob- edges. Strongly dissolved feldspar can be debris-shaped, ular shape with some surface cracks, grooves, and fine forming an intragranular dissolution hole and even a mold hairs [46]. In the Haituozi area, halloysite evolved from il- hole. Electron microscopy could often resolve this feldspar lite and exhibits a short tubular shape. dissolution into honeycomb shapes, which were pane- and In the Haituozi area, most of the feldspar cementation debris-like and formed mold holes, clad holes and rib- is a combination of small euhedral crystals, with minor shaped holes (Fig. 22). detrital feldspar overgrowths (Fig. 21c). Feldspar cementa- + tion 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). Accord- ing 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]. Figure 22: Scanning electron microscopic characteristics of reservoir dissolution of K1q4 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. 6 Discussion The dissolution of detrital grains, such as feldspars and debris, was mainly influenced by organic acids that were Diagenesis was the main factor that controlled changes in generated during the thermal evolution of organic matter. the petrophysical properties, with different diagenetic pro- Analysis of the Factors that Influence Diagenesis in the Terminal Fan Reservoir Ë 877 cesses having different effects. Some diagenetic processes, 6.2 Effects of clay cements on the rock’s such as compression and cementation, destroyed porosity physical property and made the rock denser. However, some diagenetic processes, such as dissolution and metasomatism, increased Pressure and temperature also increase with increasing the porosity and permeability of the rock. In the Haituozi burial depth. The release of water between layers and area, the reservoirs have fairly abundant pore types and the removal of cations can cause the recrystallization and complex diagenetic histories, with differences in the de- transformation of clay minerals [48]. Under shallow burial gree of influence of certain diagenetic processes and the conditions, clay minerals may have kaolinite and montmo- stages of diagenesis. This study reveals the degree of influ- rillonite; meanwhile, these minerals disappear and turn ence of diagenesis on the reservoir porosity. 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 6.1 Effect of compression on the rock’s by the source of volcanic pyroclastic debris (Fig. 24). Un- physical properties der shallow burial conditions, illite and chlorite reached a relatively mature stage of evolution, and their content The formation pressure coefficient in Haituozi area is gen- rapidly increased. This result was mainly related to the vol- erally 0.95-1.02, the buried depth of Fuyu reservoir is gen- canic rock, which provided a sufficient source of iron for erally 1800-2200‘ m, the average formation pressure is the transformation of montmorillonite to illite and thus 20.9 MPa, and the original bottom layer temperature is accelerated the formation of illite and chlorite. The con- ∘ 88.2 C. It belongs to normal temperature and pressure sys- tent of cuttings in the rocks in the study area is very high, tem. with an average of 34%, which leads to the low matu- Because of the nature of the rock itself, including the rity of sandstone composition, with an average of 0.44%. compressive strength of mineral grains, the type of inter- The composition of the debris is mainly acidic volcanic stitial material and the type of fluid in the pores, the poros- rock debris, accounting for more than 60%, and contains ity and permeability of the rock decreases as the burial a small amount of intermediate basic volcanic rock debris depth increases. Thus, the effect of compaction onK1q4 (andesite, basalt). The main types of volcanic debris are significantly reduced the porosity and permeability. The rhyolite debris, tuff debris, andesite debris and basalt de- rate of reduction was approximately 1% per 100 m of com- bris. These provide favorable conditions for the conversion paction. Compaction affected different microfacies differ- of clay minerals, and increase the content of illite and chlo- ently: channel overflow facies had many muddy compo- rite in shallow burial conditions. nents, 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 Figure 24: Changes of reservoir clay minerals with depth of Haituozi Haituozi area area

The influence of clay minerals on reservoir quality is mainly manifested in two aspects: 878 Ë Jinkai Wang et al.

(1) Making reservoir physical properties worse two are not closely related. With the further development Clay minerals adhere to the surface of particles or fill of diagenesis, early carbonate minerals will be dissolved in pores, which reduces the original pore space of rock. In or metasomatized, so with the increase of depth, carbon- addition, clay matrix has strong plasticity and is easy to ate minerals have a certain decreasing trend, but the trend flow, so it is easy to fill pore throat and reduce reservoir is not obvious (Fig. 26). 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

Figure 26: Relationship between carbonate minerals and burial (2) Making reservoir physical properties better depth The intergranular micropores formed by authigenic clay minerals in Quan4 reservoir are widespread, mainly including kaolinite intergranular pores, illite intergranu- The carbonate cementation levels were high, and car- lar pores and other types. However, the pores between bonate dissolution was not strong. Carbonate cementation these crystals are small in size, mainly in nanoscale pores, destroyed porosity, as seen from the correlation between and the connectivity between pores is very poor (Fig. 20). the carbonate content and reservoir physical properties: the higher the carbonate content, the worse the physical properties of the reservoir became (Fig. 27). 6.3 Effects of carbonate cement on rock physical property

Carbonate cementation is one of the main factors to de- stroy 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 rhombo- Figure 27: Changes of physical properties with content of carbonate hedral or cubic. Late carbonate minerals are mainly iron- cement in Haituozi area 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 Analysis of the Factors that Influence Diagenesis in the Terminal Fan Reservoir Ë 879

6.4 Influence of diagenesis on the fects on the pore characteristics of the reservoirs. Different quantitative evaluation of the rock’s microfacies had slightly different effects, and the sand mat physical properties was strongly affected by compaction, followed by the front sand bar and braided channel. 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, plagio- clase 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).

Table 2: Parameter table for diagenetic stage division Figure 28: Effect of diagenesis on reservoir porosity

Division index of diagenetic stage Diagenetic stage Paleotemperature( ) 85-105

Ro(%) ℃ 0.65-1.18 Smectite proportion in I/S(%) 15-24 I/S zonation Orderly Sandstone consolidation degree High Compaction degree Strong I/S (%) 20-75,average 52.89 7 Conclusions kaolinite (%) 1-25,average 6.47 Illite (%) 5-41,average 15.8 Authigenic Chlorite (%) 3-28,average 12.15 A period of the (1) The sedimentary facies of K1q4 in the Haituozi area is minerals in Calcite (%) 0.6-10,average 3.54 middle diagenetic a terminal fan, with a distributary channel, near-channel sandstone Dolomite (%) 0.9-4.3,average 2.54 stage Ankerite (%) A small amount of crystal overflow, far-channel overflow, and mud flats. The prove- Quartz overgrowth Grade II nance mainly originated from the northeast and the south. Feldspar overgrowth Little, low grade Dissolution of feldspar Moderate degree The sandstone thickness gradually decreases from the Dissolution and debris Carbonates Moderate degree source area to the center of the catchment. Contact types of particles Point to linear contact (2) The sandstone lithology of K1q4 in the Haituozi area Primary pore + secondary Pore type pore consists of lithic-feldspar sandstone and feldspar-lithic sandstone. The pore types include intergranular pores, intragranular pores, mold pores, fractures and clay ma- According to the relationship between the structure trix pores. The main diagenetic types for K1q4 were com- and original properties (Sneider chart), the original poros- paction, cementation and dissolution, with compaction ity of the K1q4 reservoir in the Haituozi area can be esti- being the most influential factor. mated to be approximately 40% [49, 50] (Fig. 28). Com- (3) The main diagenetic processes that affected the reser- paction was a destructive diagenetic process that reduced voir physical properties were compaction and cementa- the primary porosity. Early quartz enlargement and calcite tion; these processes also explain why the sandstones be- cementation also reduced the primary porosity, but the ex- come denser. Although dissolution and fractures have in- tent of this effect was less than that of compaction. The creased the porosity, clay- and carbonate-mineral cemen- dissolution of particles and interstitial substances was a tation during the late stages of diagenesis reversed this re- positive diagenetic process and was the main mechanism sult, forming a typical low-porosity and low-permeability that formed the secondary porosity. Kaolinite, chlorite, il- reservoir. lite, albite and other mineral fillings also had different ef- 880 Ë Jinkai Wang et al.

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