A Case Study of the Fuyu Tight Oil Reservoir in the Sanzhao Depression, Songliao Basin

GeoScienceWorld Lithosphere Volume 2021, Article ID 2609923, 14 pages https://doi.org/10.2113/2021/2609923

Research Article Densification and Fracture Type of Desert in Tight Oil Reservoirs: A Case Study of the Fuyu Tight Oil Reservoir in the Sanzhao Depression, Songliao Basin

Binchi Zhang ,1,2 Dejiang Kang ,3 Shizhong Ma ,4 Wenze Duan ,1 and Yue Zhang 5,6

1School of Earth Sciences, Northeast Petroleum University, Daqing 163318, China 2Heilongjiang Oil and Gas Reservoir Forming Mechanism and Resource Evaluation Key Laboratory, Northeast Petroleum University, Daqing 163318, China 3Exploration and Development Research Institute, Daqing Oil Field Co., Daqing 163712, China 4Bohai-Rim Energy Research Institute, Northeast Petroleum University, Qinhuangdao 066004, China 5College of Geosciences, China University of Petroleum, Beijing 102249, China 6State Key Laboratory of Petroleum Resource and Prospecting, China University of Petroleum, Beijing 102249, China

Correspondence should be addressed to Dejiang Kang; [email protected]

Received 18 April 2021; Accepted 17 June 2021; Published 19 July 2021

Academic Editor: Hao Liu

The exploration of unconventional oil and gas, especially the exploration process of tight oil, is closely related to the evolution of tight reservoirs and the accumulation process. In order to investigate the densification and accumulation process of the Fuyu tight oil reservoir in the Sanzhao depression, Songliao Basin, through the new understanding of reservoir petrological characteristics, diagenesis and diagenetic sequence are combined with a large number of inclusions: temperature measurement, spectral energy measurement, and single-well burial history analysis, and then contrastive analysis with current reservoir conditions. The results prove that diagenesis is dominated by compaction and cementation, and the restoration of paleoporosity shows that its porosity reduction rate reached 67% and the densification process started in the early Nenjiang Formation and was finalized at the end of the Nenjiang Formation. The accumulation of the Fuyu oil layer generally has the characteristics of two stages and multiple episodes, and the main accumulation period is the end of the Mingshui Formation. The end of the Nenjiang Formation, where the main body of the reservoir is densified, is just a prelude to the massive expulsion of hydrocarbons in the Songliao Basin, which makes the Fuyu oil layer have the characteristics of first compacting and then accumulating. Through the above analysis, it can be seen that the accumulation of oil and gas in the Fuyu oil layer, Sanzhao depression, is more dependent on the fault-dominated transport system. In addition, it is believed that tight oil accumulation should have the characteristics of short-distance oil enrichment around the fault, and the development area of fracture deserts near the fault sand body should be the key area for further exploration.

1. Introduction resource abundance, locally developed sweet spots, and incomplete reservoir distribution controlled by traps [1–7] The Fuyu oil layer in the Sanzhao depression is located in the but also has the characteristics such as the reservoir pressure northern Songliao Basin, which is close to the source rock of diﬀerent from the characteristics of in-source tight oil [8]. the Qingshankou Formation, and its own reservoir is rela- The Fuyu oil layer has experienced many years of continuous tively tight. Therefore, it has become typical tight oil under exploration and practice, and predecessors have done a lot of source in China. The Fuyu oil layer not only has some typical research on its source rock conditions, sedimentary environ- characteristics of tight oil within the source, such as poor ment, preservation conditions, and other petroleum geologi- reservoir physical properties, large distribution area, low cal conditions [9–12], which has been clear that it has a

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0 1020304050km Crataceous Formation Period Bed thickness (m) Source bed Reservoir Cover coat Stratigraphic section Seismic reﬂection interface Heiyupao depression Oil layer name Second Wuyuer depression 0–381

Mingshui Chronostratigraphy K2m Mingshui terrace First 0–243 Suiling anticline Taikang upli belt 0–413 Sifangtai K2s T03 Fih 0–355 T04 Fourth 0–290 Heidimiao Nengjiang Suihua depression K2n H ird 50–117 T06 Second

e overlap belt of the west 80–253 T 86.68±0.47 Ma First 27–222 07 Changyuan anticline Hulan upli belt T S Saertu Second 50–150 1 Yaojia K2y Sanzhao depression First 10–80 T 1 P Putaohua ird 1 88.0±0.3 Ma Chaoxanggou terrace 53–552 Qingshankou G Gaotaizi K2qn Second 90.4±0.44 Ma Longhupaodaan terrace Upper cretaceous Upper First 25–164 T 91.3±0.48 Ma Qijiagulong depression Wangfu depression 2 Fourth 0–128 F Fuyu

ird 0–692 Yangdachengzi Primary Y sructural belt 1 Changchunling anticline Secondary T2 sructural belt Quantou K2q Proven oil reserves Second 0–479 Proven gas reserves

Forecast oil reserves Qingshankou anticline Controlled First 0–855 Changling depression Fuxin upli belt oil reserves T3 (a) (b)

Figure 1: Exploration situation map and stratigraphic comprehensive column of the northern Songliao Basin.

potential scale of tight oil resources of nearly two billion tons, when the Fuyang oil layer was developed, the Sanzhao but there is still a lack of in-depth discussion and research on depression has the sedimentary characteristics of multiple the relationship between the evolution of the reservoir and sources and multiple water catchment centers. As a whole, the process of accumulation. From the perspective of tight it was dominated by delta front and plain deposits. The river oil, the relative sequence between the densification time of is small in scale and changeable in direction, with the average the reservoir and the main accumulation period directly width being generally less than 800 meters, the thickness of affects the oil and gas abundance and enrichment zones of the sand body is thin, and the average thickness of a single the Fuyu oil layer. If the reservoir is tight first and then accu- sand body is generally less than 4 meters. The overlying mulates, then for the already densified reservoir, the reservoir Qing 1st member source rock has a relatively high abun- has basically lost its ability to transport oil and gas, and under dance of organic matter, mainly type I kerogen, with an the control of the fault-dominated transport system, explora- average organic carbon of 3.14% and the average of chloro- tion should be given priority to high-quality sweet spots near form bitumen A of 0.62%. The average of total hydrocar- faults. On the contrary, exploration can be carried out in bons is up to 4290 × 10−6 ppm, and the average value of areas rich in sand bodies with far faults. Therefore, research the sum of S1 and S2 is as high as 29.27 mg/g. Under the on the reservoir densification and accumulation process is influence of compression and overturning of multiple tec- of great practical significance for releasing tight oil resources tonic stresses, the top surface of the Quantou Formation in the Songliao Basin and further realizing the potential of has a complex structure with alternating uplifts and depres- tight oil. sions and developed nearly 4000 dense faults; the average length is less than 2 km, the vertical fault distance is distrib- 2. Geological Background uted in 20 to 60 meter, and the direction is mainly north- north-west and north-north-east. Exploration practice has The Songliao Basin is a large-scale Mesozoic-Cenozoic com- confirmed that the Fuyang oil layer has relatively poor posite sedimentary basin with a faulted depression double physical properties, with porosity generally ranging from structure, which has developed the Sanzhao depression, 8% to 12% and permeability ranging from 0.2 to 1.2 mD, Qijia-Gulong depression, Heiyupao depression, Changyuan which was dominated by fault-lithologic reservoirs. In the anticline, Mingshui terrace, Chaoyanggou anticline, and past three years, nearly 150 million tons of tight oil reserves other multiple tectonic units. The Sanzhao depression is have been submitted, demonstrating good exploration located in the middle-eastern part of the Songliao Basin, potential. and the middle-shallow exploration strata above 2000 meters are developed with the Fuyang oil layer (FY), Gaotaizi oil layer 3. Lithological Features of the Reservoir (G), Putaohua oil layer (P), Saertu oil layer (S), and other sets of oil-bearing layers from bottom to top, and the main source 3.1. Rock Types and Composition Characteristics. The lithol- rock is the high-quality dark rock of the Qingshankou For- ogy of the Fuyu oil layer mainly includes fine sandstone, fi fi mation (K2qn). The structural diagram of the Songliao Basin siltstone, silt-bearing ne sandstone, rare medium- ne sand- is shown in Figure 1. During the Quantou Formation period stone, and medium sandstone. The rock types are mainly

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200 �m 100 �m

(a) (b)

200 �m 200 �m

Figure 2: Particle contact relation of the compact reservoir in the Fuyu Formation: (a) Shuang 51, the casting thin sections, 1724.09 m, plane- polarized light, ×100; (b) Fang 51, the casting thin sections, 1888.57 m, orthogonal light, ×200; (c) Fang 50, the casting thin sections, 1810.22 m, orthogonal light, ×100; (d) Zhao 26, the casting thin sections, 1886.86 m, orthogonal light, ×100.

feldspar lithic sandstone and lithic feldspar sandstone. Quartz 4. Reservoir Diagenesis and content distribution is in the range of 16.06% to 64.29%, with Diagenetic Sequence an average of 28.2%, and feldspar content ranges from 16.28% to 53.45%, with an average of 34.22%. The content of lithic 4.1. Diagenesis. Microscopic observations of rock thin slices fragment is from 3.45% to 57.89%, with an average of and cast thin slices, scanning electron microscopy, and cath- 37.59%, mainly igneous rock cutting, followed by metamor- ode luminescence [13, 14] show that the diagenesis experi- phic rock cuttings, containing a small amount of sedimentary enced by the tight sandstone reservoirs in the research area rock cuttings. The type of cement is mainly carbonate, clay mainly includes compaction, cementation, metasomatism, mineral, and siliceous, and the carbonate is mainly calcite, and dissolution. The diagenesis has an important inﬂuence with the content distribution range of 1.0% to 55.0%, with on the pore development of the reservoir [15]. the average value of 9.99%. The clay mineral content in descending order is illite with an average of 3.35%, kaolinite with an average of 0.14%, and siliceous cement ranging from 4.1.1. Compaction. During the diagenesis process of the Fuyu 1.0% to 5.0%, with an average of 2.3%. oil layer, it has experienced medium-strength compaction, common line contact, and point-line contact under the microscope, as well as a small amount of concavo-convex 3.2. Rock Fabric. The Fuyu oil layer is mostly ﬁne sandstone contact (Figure 2). Brittle minerals, such as feldspar and with an average content of 52.61%, followed by siltstone with quartz, are crushed, while plastic minerals, such as lithic an average content of 46.26%. The maturity of the sandstone mica, are commonly deformed. The particle contact relation- structure is moderate, and the particles experienced mostly ship indicates that after the Fuyu oil layer experienced uplift point-line contact, followed by line contact, accompanied at the end of the Mingshui Formation deposition, the later by a small amount of bump contact and suture line contact. subsidence did not reach the maximum burial depth stage, The main types of cementation are porous type, contact type, and the sandstone particles did not reach the maximum mosaic type, and base type (Figure 2). degree of compaction.

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500 �m 100 �m

Q Q C C F Q C C

(a) (b)

100 �m 100 �m

R Q

Q Q Q

Figure 3: Typical characteristics of cement of the Fuyu Formation in the Sanzhao depression: (a) Xing 76, 1642.94 m, the fourth member of the Quantou Formation, calcite continuous crystal cementation, base cementation, particles are ﬂoating, ×50; (b) Shuang 51, 1719.88 m, calcite continuous crystal cementation, plaque cementation, orthogonal light, ×200; (c) Shuang 51, conventional thin section, 1723.0 m, the secondary enlargement of quartz, orthogonal light, ×200; (d) Shuang 51, 1723.0 m, the fourth member of the Quantou Formation, the secondary enlargement of quartz (two stages), the secondary enlargement of feldspar, calcite is ﬁlled with other shapes, ×200.

4.1.2. Cementation. The results of casting thin sections, is more common, but the total amount is relatively low. The scanning electron microscopy, and cathode luminescence identified content under the microscope ranges from 1.0% to observations show that the cementation mainly includes cal- 5.0%, most of which is 2.0% to 3.0%. careous cementation, siliceous cementation, and clay mineral cementation. Calcareous cementation is dominated by calcite 4.1.3. Metasomatism. The metasomatism in the study area is cementation, while siliceous cementation is mainly mani- mainly the calcite of feldspar. After the feldspar is replaced by fested in the secondary enlargement of quartz and feldspar calcite, it forms metasomatism phantom and residual struc- particles. Calcite cementation is the most common, mainly ture, and the edge of feldspar presents irregular shape such including the early-formed continuous crystal calcite and as tooth shape (Figure 4). the late-formed splendid calcite, which is orange under cathode luminescence, and the morphological characteristics are 4.1.4. Dissolution. Dissolution refers to the process of dissolv- mainly porous, continuous crystal cementation, blocking ing part or all of the soluble parts in the surrounding rock the pores and reducing the physical properties of the reser- when the acid fluid in the reservoir migrates in the channel, voir. In the samples with strong local cementation, even basal which is the main reason for the formation of secondary cementation is formed, and the particles are floating and pores in the reservoir rock and increases the pore space and form a dense layer (Figures 3(a) and 3(b)). Siliceous cements is an important diagenesis to improve the physical properties are widely distributed in sandstone in the study area, mainly of sandstone reservoirs [16]. The main acidic medium for the appearing on the surface of detrital quartz particles in the dissolution in the study area is the organic acid produced by form of secondary quartz enlarged edges (Figures 3(c) and the decarboxylation of the organic matter in the mud during 3(d)). The secondary increase in quartz in the Fuyu oil layer the thermal evolution of the organic matter, the carbonic acid

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200 �m 200 �m

R Q C Q F Q F C

R C

(a) (b)

Figure 4: Typical metasomatism of the Fuyu Formation in the Sanzhao depression: (a) Shuang 51, 1724.09 m, the fourth member of the Quantou Formation, calcite metasomatic feldspar, rock cutting and quartz, ×100; (b) Zhou 182, 1828.89 m, the fourth member of the Quantou Formation, calcite continuous crystal cementation, metasomatic feldspar, feldspar metasomatism residual, local metasomatism quartz, ×100.

100 �m 200 �m

C q1 Q q2

q2 C q1 Q

(a) (b)

Figure 5: Secondary enlargement of quartz and calcite cement: (a) Fang 162, 1923.04 m, the fourth member of the Quantou Formation, quartz secondary growth in two stages, calcite is later than the second phase of quartz, ×200; (b) Pu 53, 1681.83 m, the fourth member of the Quantou Formation, calcite continuous crystal cementation, calcite metasomatic feldspar, ×100.

formed by the transformation of CO2 and smectite, and the Two stages of quartz enlargement can be seen in the second- acidic water formed by the transformation of clay minerals. ary enlargement. Calcite is developed outside the quartz The main corroded particles are acidic unstable aluminosili- enlargement, and the calcite ﬁlls the intergranular pores cate mineral feldspar, soluble rock cuttings, and carbonate and the metasomatism quartz grains and their enlargement cements. Various types of secondary pores formed by disso- edges (Figure 5). Therefore, the occurrence time of quartz lution in the study area are the main types of the reservoir secondary growth is generally earlier than that of late calcite. space in this area. 4.2.2. Quartz Secondary Growth Occurred Earlier Than or at 4.2. Diagenetic Sequences. Through the analysis of the rela- the Same Time as Dissolution. Silica is one of the main prod- tionship between the following ﬁve minerals under a micro- ucts of feldspar dissolution. Smectite can expel Si4+ in the scope, the diagenetic sequence of the Fuyu oil layer in the process of transforming into clay minerals, forming siliceous Sanzhao area is determined. cement in sandstone. Therefore, the quartz level I increase can occur in the early diagenetic stage, which is earlier than 4.2.1. Quartz Secondary Growth Occurred Earlier Than Late the occurrence time of the dissolution of feldspar minerals. Calcite Cementation. The content of quartz secondary aug- In addition, the early hydration of feldspar also provided a mentation is obviously lower than that of carbonate cements. material source for the early secondary growth of quartz. In

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100 �m 200 �m

Dissolution pores

q2 q1 Q Quartz secondary enlargement Quartz secondary enlargement

(a) (b)

Figure 6: Quartz secondary enlargement and dissolution pore: (a) Fang 162, 1925.28 m, the fourth member of the Quantou Formation, quartz secondary enlargement is earlier than dissolution; (b) Xing 76, 1628.53 m, the fourth member of the Quantou Formation, quartz secondary growth in multistages, quartz secondary enlargement is earlier than dissolution.

this case, the time of secondary growth of quartz is earlier lar pores after calcite filling, which means that the hydrocar- than that of the corrosion of organic acid (Figure 6). Micro- bon charging time is later than the calcite cement formation scopic observation shows that dissolution pores are usually time (Figure 7). developed outside the quartz secondary enlargement edge, Through the statistics of the relationship between the and dissolution remnants of feldspar can be seen in the disso- content of calcite cement and the oil-bearing grade, it can be lution pores. The analysis shows that the quartz secondary seen that the content of calcite cement and the oil-bearing enlargement occurs before the dissolution, and the original grade are negatively correlated; that is, the content of calcite feldspar particles occupy part of the pore space, inhibiting cement in the oil-free samples is the highest, up to 60%. Oil the process of quartz secondary enlargement. immersion—the oil trace level is the second—reduced to about 40%, and the content of calcite cement in the oil- 4.2.3. Late Calcite Was Formed Later Than Dissolution or saturated to oil-rich samples is the lowest, generally below Occurred at the Same Time as Dissolution. Except for the 25%. There is no obvious difference between the oil-bearing early continuous crystal calcite, most of them are splendid grades of quartz secondary enlargement content, which indi- calcite filling the intergranular pores and underwent the phe- cates that the influence of hydrocarbon charging on the nomenon of replacing feldspar or even quartz (Figure 5). The quartz secondary enlargement is not obvious. phenomenon of splendid calcite associated with dissolution Through synthesizing the above analysis, the reservoir pores is very common, and the calcite beside the dissolution compaction of the Fuyu oil layer runs through the entire dia- pores has no obvious dissolution phenomenon, indicating genesis process, and compaction is the strongest in the early that the formation time of calcite is later than the time when diagenetic period. In the early diagenesis stage, the feldspar the dissolution occurs, or the acidic fluid can only dissolve experienced hydration in the shallow burial stage, which pro- the feldspar but not enough to dissolve the calcite, and the vided a material source for the early secondary growth of products formed by the dissolution of feldspar also provide quartz. In the middle diagenetic stage, with the increase in a material source for calcite. the thermal evolution degree of organic matter, organic matter enters the low-mature stage, generating a large amount of 4.2.4. Late Calcite Cementation Occurred Earlier Than Oil organic acids, dissolving soluble minerals such as feldspar, Charging. It is generally believed that hydrocarbon charging and forming secondary pores, which to a certain extent pro- can change the original fluid environment in the pores, vide space and material sources for the cementation of late thereby affecting the process of diagenesis. Observation calcite. At the same time, the dissolution of feldspar con- under a thin-slice microscope and scanning electron micro- sumed organic acids, which also created suitable alkaline scope observation show that the hydrocarbon residual calcite conditions and material sources for calcite and illite precipi- crystal form was intact in the dissolution pores on the outside tation. Organic matter enters the oil generation threshold, a of the calcite cement, indicating that it did not suffer from the large amount of hydrocarbons are expelled from the source dissolution transformation caused by the change of the fluid layer, and hydrocarbons enter the pores of the reservoir, environment owing to the oil charging, and its formation which changes the original fluid environment and inhibits time should be earlier than that of oil charging. In addition cementation to a certain extent [17, 18]. Cementation may to the above phenomena, the symbiosis of calcite and hydro- continue in areas not affected by hydrocarbon charging. carbon residual asphaltenes can also be seen, indicating that Compaction, calcite cementation, siliceous cementation, the hydrocarbon residual asphaltenes prevent the continued and illite filling are the main causes of reservoir deterioration growth of calcite or are located in the remaining intergranu- (Figure 8).

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Q1 D C2

SEM HV:15.0kV WD:5.05 mm MAIA3 TESCAN SEM HV:15.0kV WD:5.00 mm MAIA3 TESCAN View Field:241�m SEM MAG:1.05kx 100 �m View Field:19.5 �m SEM MAG:13.0kx 5 �m Det:BSE, In-Beam SE Date (m/d/y):03/31/16 Perform In Nanospace Det In-Beam SE Date (m/d/y):03/31/16 Perform In Nanospace

1.00k Si K� Si K� � 3.69k 3.78k 0.90k Ca K 3.28k 0.80k 3.36k 0.70k 2.87k 2.94k 2.46k 2.52k 0.60k O K�1 0.50k 2.05k 2.10k 0.40k 1.64k 1.68k � 0.30k C K 1 1.23k O K�1 1.26k O K�1 0.20k � 0.82k � 0.84k Si K� Ca K 1 Nb L 0.10k � Calcite C1 0.41k Quartz Q1 0.42k Quartz Q2 Ca L Nb L� 0.00k 0.00k 0.00k 0.00 1.00 2.00 3.00 4.00 5.00 6.00 7.00 8.00 9.00 0.00 1.00 2.00 3.00 4.00 5.00 6.00 7.00 8.00 9.00 0.00 1.00 2.00 3.00 4.00 5.00 6.00 7.00 8.00 9.00 Si K� 1.10k Si K� 3.60k 0.99k Ca K� 3.69k 3.20k 0.88k 3.28k 2.80k 0.77k 2.87k 2.40k 0.66k 2.46k 2.00k 0.55k O K�1 2.05k 1.60k 0.44k 1.64k � O K�1 1.20k O K 1 0.33k C K�1 1.23k 0.80k 0.22k Si K� Ca K�1 0.82k 0.40k Quartz Q3 0.11k Ca L� Calcite C2 0.41k Quartz Q4 0.00k 0.00k 0.00k 0.00 1.00 2.00 3.00 4.00 5.00 6.00 7.00 8.00 9.00 0.00 1.00 2.00 3.00 4.00 5.00 6.00 7.00 8.00 9.00 0.00 1.00 2.00 3.00 4.00 5.00 6.00 7.00 8.00 9.00

Figure 7: The coexistence of asphaltene and calcite and the mineral energy spectrum in the corresponding region: Fang 162, 1925.28 m, the fourth member of the Quantou Formation, the intergranular pores are ﬁlled with calcite, hydrocarbon charging inhibits calcite growth.

5. Reservoir Pore Evolution and the porosity evolution curve that the burial depth of the same Compaction History formation in the same period is not consistent; that is, the subsidence rate and the amount of subsidence are different In the process of subsidence or uplift, the reservoir has in the same period, indicating that the burial time actually undergone various effects of diagenesis. Although the sec- has an effect on the porosity. The comprehensive equation ondary pores can be formed under the influence of dissolu- of the measured porosity data and logging calculation data tion, it is inevitable that the pores become smaller and the of 5 wells, using paleoburial depth (H) and geological time porosity decreases. The study of pore evolution is the key (t) as independent variables and paleoporosity (Φ) as depen- problem to determine the tight time of the reservoir [19, 20]. dent variables, to establish the binary function of porosity, burial depth, and geological time is as follows: 5.1. Reservoir Paleoporosity Restoration. In the process of stratum subsidence, the porosity continues to decrease under Φ =41:7494 − 0:00984H − 0:3222 × 10−4Ht − 0:06928t: the influence of compaction and cementation. Athy pro- posed the classic Athy model for mudstone and believed that ð1Þ porosity and burial depth are exponentially related. Nowa- days, when analyzing the degree of compaction of mudstone Since the ancient porosity cannot be accurately mea- and sandstone, this exponential relationship is used to sured, in order to verify the reliability of the porosity recovery express the quantitative relationship of porosity and burial model, the measured porosity of exploratory wells in the area depth of clastic rocks [21–24]. not involved in the model establishment was selected as the The paleoporosity restoration method adopted in this calibration and compared with the porosity calculated by this study is as follows: based on the measured sandstone poros- model (Table 1). The analysis results show that the error ity, the sandstone porosity is calculated by using the time dif- between the calculated porosity of this model and the mea- ference of acoustic waves, and the measured porosity is sured porosity is in the range from -0.7 to 1.4. The reasons corrected to calculate the calculation formula, so as to obtain for the error are the following three points. First of all, the the logging interpreted porosity that is consistent with the porosity calculated by the model mainly depends on the fit- measured porosity and then obtain the variation curve of ting between the porosity and the logging curve, and this fit- the average porosity with depth. This curve is the variation ting relationship usually has some fitting accuracy error. curve of present-day porosity with depth, and the variation Secondly, the measured porosity is artificially sampled for curve of paleoporosity with depth can be approximated by data measurement, which invisibly includes whether the considering the denudation thickness. It can be seen from sampling position can replace the physical properties of the

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Compaction Dissolution Mainly occurs in the interval that has not experienced oil charging Calcite Mainly occurs in the interval that has not experienced oil charging Quartz enlargement Kaolinite Mainly occurs in the interval that has not experienced oil charging IIlite Chlorite

I/S formation Main types of diagenesis Smectite >70 50–70 35–50 15–35 I/S formation ratio A B A1 A2 Subage period

Early diagenetic stage Mesogenetic stage stage

Period Diagenesis 80 – Strata upli, cooling stage Paleotemperature (°C) <80 100 100–150 Oil charging period

Immature Semi-mature Low mature Mature Stage 435 <430 – 440–460 T (°C) 430–435 440 max 0.5 <0.35 0.35–0.5 – 0.7–1.0 R (%) 0.7 o Internal Low Pco Carbonate buﬀer Low Pco2 2 External buﬀer system

Organic acid CO 2 High

Organic acid CO2

Oil generation window concentration Organic matter evolution process change Low

Figure 8: Fuyu Formation diagenetic sequence map in the Sanzhao depression.

Table 1: Comparison between the calculated porosity model and present measured porosity of the Fuyu oil ﬁeld.

Present porosity Computed Present porosity Computed Well no. Absolute error Well no. Absolute error measured (%) porosity (%) measured (%) porosity (%) Z1 9.8 10.3 -0.5 Z16 7.4 7.9 -0.5 Z2 10.2 10.1 0.1 Z17 8.2 8.9 -0.7 Z3 10.0 9.6 0.4 Z18 11.4 11.0 0.4 Z4 11.4 11.9 -0.5 Z19 9.6 9.5 0.1 Z5 12.0 10.6 1.4 Z20 10.5 10.9 -0.4 Z6 9.0 9.6 -0.6 Z21 11.3 11.0 0.3 Z7 8.6 8.5 0.1 Z22 12.1 12.5 -0.4 Z8 13.4 12.2 1.2 Z23 8.8 8.4 0.4 Z9 10.6 10.3 0.3 Z24 9.6 10.1 -0.5 Z10 9.7 9.2 0.5 Z25 7.8 7.6 0.2 Z11 7.2 7.8 -0.6 Z26 11.2 11.5 -0.3 Z12 10.8 10.4 0.4 Z27 10.4 10.0 0.4 Z13 11.4 11.2 0.2 Z28 8.8 9.0 -0.2 Z14 9.8 9.3 0.5 Z29 9.2 8.5 0.7 Z15 11.0 10.5 0.5 Z30 11.4 10.0 1.4

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Porosity (%) Porosity (%) 0 10 20 30 40 50 0 10 20 30 40 50 0 0

100 Ma 100 Ma

500 500 88.5 Ma 88.5 Ma 1000 84 Ma 84 Ma 1000

1500

2000 73 Ma 73 Ma 67.7 Ma Ancient burried depth (m) burried depth Ancient Ancient burried depth (m) burried depth Ancient 67.7 Ma 2000 2500 65 Ma

65 Ma Mingshui periodMingshui period Nengjiang period Yaojia period Qinshankou

Mingshui periodMingshui period Nengjiang period Yaojia 2500 period Qingshankou 3000

3500 3000 (a) (b)

Figure 9: Paleoporosity recovery curve. (a) Well Xushen 7 and (b) Well Chang 102.

sand layer and the systematic error caused by the instrument obtain the paleoburial depth data of the end of the Nenjiang measurement. Finally, the porosity parameters calculated by Formation and the end of the Mingshui Formation, and the model are mainly burial depth and burial time. There is compile the paleoporosity plan of the top of the fourth mem- a certain measurement error between the depth calculated ber of the two periods. In general, at the end of the Nenjiang by the model and the actual burial depth of the sand body, Formation, the main body of the Sanzhao depression entered and the burial time error is relatively large. The burial time the compact stage, and the porosity in most areas was below calculated this time is based on the burial time of the top sur- 14%, which was close to the compact stage. At the end of the face of Qingshankou, and the burial time of a specific sand Mingshui Formation, the range of densification further group cannot be accurately given. Combined with the expanded, further extending westward to the Changyuan above-mentioned reservoir classification standards, the error area and gradually southward, including Chaoyanggou ter- of this range should be able to meet the application of reser- races and other higher structures (Figure 10). voir classification of the Fuyu oil layer group. According to the actual calculation of the recovery 5.2. Duration of Reservoir Tightness. According to the analy- model, firstly analyze the corresponding relationship sis of porosity evolution and burial history of the Fuyu oil between the porosity recovery of a single well and the burial layer in the Sanzhao area, this area has gradually entered depth and geological time. Taking Xushen 7 and Chang 102 the tight stage with porosity lower than 12.0% at the end of as an example, the reservoir of Xushen 7 gradually entered the Nenjiang River (Figure 11). In the early burial stage of the densification stage when the burial depth of Chang 102 the Fuyu oil layer, the average initial porosity was about was 1700 m and the burial depth of Chang 102 was 1200 m, 36.0%. Before the Nenjiang Formation, the Fuyu oil layer which corresponds to the early and middle stages of the experienced an early diagenetic stage, and the porosity Nenjiang Formation, while the burial depth of large-scale dropped from 36.0% to 25.0%. The diagenesis experienced densification was 2100 m and 1700 m, respectively, and the at this stage mainly includes compaction, secondary enlarge- geological period was the late stage of the Nenjiang Forma- ment of quartz, and chlorite membrane. The total amount of tion (Figure 9). quartz and chlorite membranes is about 2.7%, the porosity Based on the current stratum thickness, combined with reduction rate is about 7.5%, the compaction reduction is compaction correction and denudation thickness recovery, 8.3%, and the porosity reduction rate is about 23.0%.

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Suihua depression Suihua depression

Daqing 14 18 12 20 16 18 14 Daqing 10 8

8 Sanzhao depression 10 8 Sanzhao depression 16

12 Chaoyanggou terrace 14 12 Chaoyanggou terrace

18 22 16 18 26 28 Changchunling anticline Changchunling anticline

(a) (b)

Figure 10: Upper Quantou Formation paleoporosity plan of the Sanzhao depression. (a) The end of the Nenjiang formation and (b) The end of the Mingshui formation.

Upper cretaceous Paleogene period Paleogene period K qn K1q4 2 K2y K n K n 0 2 1+2 2 3+4+5 N2t

K2m

K2s 500

K2n3+4+5 1000

K2n1+2

Depth (m) 1500 K2y2+3 K2y1

K2qn2+3 2000 K qn � = 12% K2q 1 R 0.5 0.7 1.0 1.3 1 4 o (%)

2500 Intermediate diagenesis A2 stage Early diagenesis Intermediate diagenesis A1 stage

e ﬁrst stage of accumulation e second stage of accumulation

100 50 0 Time (Ma)

Figure 11: Comprehensive model of pore evolution, diagenetic sequence, and burial history of the Fuyu oil layer in the Sanzhao depression.

In the middle of the Nenjiang Formation, the Fuyu oil tion, and hydrocarbon charging. Hydrocarbon charging layer entered the middle diagenetic A2 stage, and by the late inhibited the growth of calcite and illite. Through calculating Nenjiang Formation, the porosity decreased from 21% to the pore-reducing eﬀects of illite cementation and compac- 12%. This stage has undergone compaction, illite cementation, respectively, the pore reduction amount of illite at this

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8 4

7 First scene Second scene First scene Second scene ird scene 6 3 5 4 2

3 Frequency Frequency 2 1 1 0 0 70 80 90 100 110 120 130 140 150 160 170 70 80 90 100 110 120 130 140 150 160 170 Homogenization temperature (°C) Homogenization temperature (°C) 1839.69 m sample 1842.39 m sample

Figure 12: Histogram of homogenization temperature of ﬂuid inclusions in silty sandstone in well Zhou 165.

° stage was about 4.05%, and the porosity reduction rate was inclusions in the second scene is 110.1~129.8 C, with an ° about 11.25%. The amount of pore reduction by compaction average of 117.8 C, and the homogenization temperature of ° is about 4.95%, and the reduction rate is about 13.75%. oil inclusions is 91.5~110.7 C, with an average value of ° In the early diagenesis period, the pore reduction is dom- 102.7 C. The homogenization temperature of salt water ° inated by compaction, followed by quartz secondary growth inclusions in the third scene is 135.2~153.9 C, and the aver- ° and chlorite membrane. In the middiagenesis stage A , the age value is 145.6 C; the homogenization temperature of oil 1 ° pore reduction is dominated by the late calcite cementation, inclusions is 129.1~135.1 C, and the average value is ° followed by compaction, and the dissolution pore increase 132.8 C (Figure 12). is offset by the first two effects. In the middiagenesis stage In this research, the uniform temperature of brine A2, the pore reduction is dominated by compaction, followed obtained in the same period of oil inclusions in the Sanzhao by illite cementation. Throughout the evolution process, the area was used to project on the single-well burial history compaction porosity reduction rate is about 46.22%, the map to determine the period of oil and gas accumulation cementation porosity reduction rate is about 31.97%, the dis- (Figure 13). It can be seen that the Fuyu oil layer has two solution porosity enhancement rate is about 11.58%, and the stages of accumulation. The first stage occurred during the total porosity reduction rate is about 67%. In summary, it is 75~73 Ma of the late period of the Mingshui Formation concluded that the densification degree of tight sandstone deposition, which corresponds to the first scene of charging. of the Fuyu oil layer should have been maximized in the late The second stage occurred during the 67~65 Ma of the late Nenjiang Formation, and the tight reservoirs have basically Mingshui Formation, which corresponds to second and third been formed. scene. In other words, the late period of the Mingshui Forma- tion deposition should be the main accumulation period of 6. Analysis of the Main Accumulation Period of the Fuyu oil layer, while the late period of the Nenjiang the Fuyu Oil Layer Formation is only a small-scale oil and gas discharge, which is the prelude to large-scale hydrocarbon migration and The Songliao Basin has experienced multiperiod tectonic accumulation. movement and formed a multicycle basin. The source rocks of the first member of the Qingyi formation expelled hydro- 7. Relationship between Reservoir Densification carbon for many times in the basin, resulting in a single-stage and Accumulation Period and Its Significance or multistage upflow to the Gaotaizi tight reservoir and down to the Fuyu tight reservoir. In order to determine the history The above research shows that the study of accumulation his- of hydrocarbon accumulation in the Fuyu oil layer, this study tory indicates that the study area has experienced two stages is mainly based on a large amount of microscopic hydrocar- of accumulation, the late Nenjiang Formation (77~73 Ma) bon migration and accumulation information obtained from and the late Mingshui Formation (67~65 Ma), of which the the systematic analysis of fluid inclusion samples and ana- late Mingshui Formation is the main accumulation period. lyzes the accumulation period of tight reservoirs. Taking According to the analysis of the corresponding relationship Zhou 165 as an example, blue-white and blue-green fluores- between the maturity of the regional source rock and the cent oil inclusions were detected in two siltstone samples paleoporosity plane, at the end of the Nenjiang Formation, from the depth of 1839.69 m and 1842.37 m in the Fuyu oil the range of oil and gas maturity and accumulation is roughly layer, indicating at least two episodes of oil charging. The consistent with the tight range at the end of the Mingshui results of microscopic temperature measurement show that Formation deposition. At the end of the Mingshui Forma- the uniform temperature of the salt water inclusions in the tion, the Sanzhao area has entered the source rocks to gener- ° first scene is 95.1~109.4 C, with an average value of ate a large amount of mature hydrocarbons, which is ° 102.5 C. The homogenization temperature of the salt water basically consistent with the distribution range of tight

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Cretaceous period Paleogene Neogene period 0 Qingshankou Yaojia Nenjiang Sifangtai Mingshui Qingan Daan Fengqing Q N2d-E2y K2s K2m K1n5 0.5 50°C K1n3+4

1 70°C K1n2 K1n1 90°C K1y2+3 K1y1 1.5 K1qn2+3

Depth (km) 110°C K1qn1

2 130°C K1q

Homogenization temperature:

Homogenization temperature: 111.2 °C Charging time: 75 Ma 130.3-132.4 °C Charging time: Fang 27 37-66 Ma 80 60 40 20 0 Age (Ma)

Figure 13: Single-well burial history in the Sanzhao depression.

Daqing Suihua depression Daqing Suihua depression 14 18 12 0.7 20 16 18 14 10

8 0.7 0.9

8 8 0.9 10 Sanzhao depression 12

16 0.9

1.1 16 Sanzhao depression

12 14 12 14 0.9 18 20 22 Chaoyanggou terrace 16 Chaoyanggou terrace 0.7 18 26 28 Changchunling anticline Changchunling anticline

(a) (b)

Structural unit line Paleoporosity contours R o contours Well site

Figure 14: The ancient porosity and Ro contour superposition map of the end of the Fuyu Formation and Mingshui Formation in the Sanzhao depression.

reservoirs at the end of the Mingshui Formation (Figure 14). The purpose and significance of studying the relationship Therefore, the Fuyu oil layer has the characteristics of first between reservoir tightness history and accumulation period being tight and then accumulating. are to analyze the differences in the process of oil and gas

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Yuan 141 Zhou 165Zhao 26 Zhou 183 Zhou 181 Zhao 113 Zhao 111 Fang 30 Fang 331 Fang 241 Wei 8

–1500 –1550 –1600 –1650 –1700

–1750 Qing I member –1800 Fuyu oil layer –1850 –1900 Yangdachengzi oil layer –1950 –2000 0 3 6 km N –2050

Oil layer Fault Water layer Source rock Dry layer Migration path

Figure 15: Reservoir proﬁle of the Fuyu Formation in the Sanzhao depression.

accumulation, so as to more accurately find favorable oil and practice, it can often be found that the physical properties gas enrichment zones. In the tight reservoir of the Fuyu oil of the sand bodies dispersed vertically in the Fuyu oil layer layer, if the accumulation period is earlier than the time when are quite different, as well as the phenomenon of oil-dry the reservoir is densified, then the method of comparing the interbeds. The densification degree is also reduced in the high past with the present is adopted. In the accumulation stage, part of the structure or the relatively shallow burial part, the reservoir is still a conventional reservoir, and the oil which forms the sweet sand body of the tight reservoir in and gas can migrate downward to the Fuyu oil layer through the common sense. Under the communication of faults, such the intensive faults at the top of the Quantou Formation. sand body can still form the lateral migration and accumula- With the cooperation of lateral transport of the sand body, tion of a certain distance laterally. Therefore, tight oil reser- the oil and gas can be transported to a relatively far trap for voirs often have the characteristics of strong heterogeneity, accumulation, and such oil and gas area will form a large area no uniform oil-water interface, and relatively high-quality of a continuous distribution pattern. However, in fact, the sand bodies around faults that are relatively enriched in oil densification time of the Fuyu oil layer was earlier than the and gas. Hence, according to the actual characteristics of main accumulation period. A large amount of oil and gas dis- the Fuyu oil layer in the Sanzhao area, which is tight first charged from the overlying Qingshankou source rock was and then accumulates, the sweet spot development area of displaced downward through the fault. In tight reservoirs the sand body near the fault should be a hydrocarbon enrich- with underground porosity less than 12% and permeability ment area and should become the main exploration target. generally less than 0.1 mD, long-distance migration and accumulation cannot be as smooth as conventional reservoir sand bodies. Past exploration practices have also confirmed 8. Conclusions that the types of traps discovered are mainly fault traps and fault-lithological traps. The oil reservoirs are characterized (1) The Fuyu oil layer in the Sanzhao depression is dom- by multilayer superposition on the plane and section, form- inated by fine sandstone and siltstone. The diagenesis ing the characteristics of contiguous distribution after super- is dominated by mechanical compaction and cemen- position. These oil reservoirs are communicated by widely tation, and the dissolution and pore-enhancing effect developed fault facies and exhibit the characteristics of quasi- are not obvious. The paleoporosity recovery show continuous distribution in space, which are jointly controlled that the total porosity reduction rate can reach 67%, by the fault and its adjacent high-quality sand bodies the porosity reduction rate of mechanical compac- (Figure 15). tion is about 46.22%, and that of cementation is about What needs to be clarified and emphasized here is that 31.97%, and layer densification has been basically the so-called reservoir densification does not mean that con- completed at the end of the Nenjiang Formation ventional high-quality reservoirs are transformed into very (2) The Fuyu oil layer expulsion began at the end of the tight sand bodies with extremely low porosity without stor- Nenjiang Formation and reached the peak at the age capacity but refers to the process of reservoir densifica- end of the Mingshui Formation, each stage has 2-3 tion, and this process is also significantly different for episodes of hydrocarbon expulsion history, and the different sand bodies, and the result of densification is gener- most important accumulation period should be at ally maintained at porosity above 8.0%. Therefore, the free the end of the Mingshui Formation fluid field still exists, but it is weaker and more selective than conventional reservoirs. Under the influence of various dia- (3) Combined with the analysis of reservoir tight history genesis, the vertical sand bodies are becoming denser. In and accumulation history, it shows that the Fuyu oil

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