Causes of Oil Accumulation of Isolated Bars in Lacustrine Delta

energies

Article Causes of Oil Accumulation of Isolated Bars in Lacustrine Delta

Panpan Chen 1, Cunlei Li 2,*, Jinliang Zhang 3, Shengrong Li 1, Guiyang Ma 2 and Yang Zhao 2

1 School of Earth Sciences and Resources, China University of Geosciences, Beijing 100083, China; [email protected] (P.C.); [email protected] (S.L.) 2 College of Petroleum Engineering, Liaoning Shihua University, Fushun 113001, China; [email protected] (G.M.); [email protected] (Y.Z.) 3 College of Resources Science and Technology, Beijing Normal University, Beijing 100875, China; [email protected] * Correspondence: [email protected]

Received: 26 January 2020; Accepted: 19 March 2020; Published: 21 March 2020

Abstract: After over 20 years of development in Daqingzijiang Oilfield, isolated sand bodies under the delta front and pro-delta in the Qingshankou Formation(qn)have become important oil and gas reservoir bodies. However, the cause for large amounts of isolated bar sand bodies in the lake-basin delta sediment system has not been reported in China. This article, through core observation, reveals plenty of evidence that there storm waves once existed. Combined with paleogeography and hydrodynamic force analysis, it describes the transformation effect of waves on delta sand bodies and on the formation mechanism of bar sand bodies. Based on a study on paleogeomorphology and the statistics of sand body distribution, we consider ‘Storm waves conveying sand’ and ‘landform controlling sand’ as the cause and distribution model of the delta’s isolated bar sand body formation. We also think that the superposition of multiple bar sand bodies is the direct cause of the strong anisotropy in reservoirs and the complex relationship between oil and water in reservoirs. Most of these sand bodies have formed into lenticular lithologic hydrocarbon accumulations. On the basis of this integrated study on hydrocarbon accumulation, we set up an accumulation model of lenticular hydrocarbon accumulation involving the variables ‘Surrounded by source rocks to generate hydrocarbon’, ‘Driven by pressure difference’, ‘Migration through multi pathways’ and ‘Accumulation by filtering water’.

Keywords: Daqingzijing Oilﬁeld; delta; isolated sand bodies; sand-control mechanism; hydrocarbon accumulation

1. Introduction At present, the exploration and development of most oilfields in China is shifting from the easily-developing conventional reservoirs to subtle lithologic reservoirs. However, subtle reservoir exploration is different from conventional reservoirs due to having increased difficulties in exploration and development. Reservoirs with a tectonic background in Daqingzijing Oilfield in Jilin Province have reduced well yield when the region’s subtle reservoirs is low, and while it is hard construct these reservoirs productively, which are problems that are also being faced by most Chinese oilfields. As further exploration and development of lithologic reservoirs (which are trapped by lithologic changes in the reservoir) produced by reservoir lithology changes and traps in Daqingzijing Oilfield have been conducted, it is becoming more and more obvious that the sand body distribution characteristics under the guidance of the theory of the sedimentary model of finger-like bar distribution does not match with productive practices, which seriously restricts exploration and development [1–3].

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Energiescharacteristics2020, 13, 1489 under the guidance of the theory of the sedimentary model of finger-like2 ofbar 13 distribution does not match with productive practices, which seriously restricts exploration and development [1–3]. With core observation and description of over 60 wells in the study area, and on With core observation and description of over 60 wells in the study area, and on the basis of the the basis of the comprehensive study on sand body distribution, this study suggests that the comprehensive study on sand body distribution, this study suggests that the Qingshankou Formation Qingshankou Formation in the Daqingzijing area has mainly developed into a highly destructive in the Daqingzijing area has mainly developed into a highly destructive delta. Under the guidance of delta. Under the guidance of the model, we established distribution laws for isolated bars and the model, we established distribution laws for isolated bars and discussed the cause of sand bodies in discussed the cause of sand bodies in isolated bars. On the basis of a study on reservoir characteristics, isolated bars. On the basis of a study on reservoir characteristics, we considered the fluid distribution we considered the fluid distribution characteristics of various reservoirs and established the characteristics of various reservoirs and established the lenticular reservoir accumulation model having lenticular reservoir accumulation model having the variables ‘Surrounded by source rocks to the variables ‘Surrounded by source rocks to generate hydrocarbon’, ‘driven by pressure differences’, generate hydrocarbon’, ‘driven by pressure differences’, ‘Migration through multi pathways’ and ‘Migration through multi pathways’ and ‘Accumulation by filtering water’. ‘Accumulation by filtering water’. 2. Depositional Setting 2. Depositional Setting The Qingshankou Formation was formed from the first overall sinking and expansion of the SongliaoThe Qingshankou Basin, and with Formation obvious was ‘rushed formed beginning, from the slow first ending’overall sinking characteristics. and expansion Daqingzijing of the OilfieldSongliao liesBasin, in theand center with ofobvious Changling. ‘rushedThe beginnin centralg,depression slow ending’ area characteristics. is in the south Daqingzijing of Songliao Basin.Oilfield liesThe in Changling the centerDepression of Changling. consists The central of two depression secondary area depressions. is in the south One of Songliao is the Qian’an Basin. SecondaryThe Changling Depression Depression in the consists north of and two the secondary other is the depressions. Heidimiao One Secondary is the Qian’an Depression Secondary in the south.DepressionThe in Daqingzijing the north andStructure the other is on is thethe middleHeidimiao of the Secondary two secondary Depression depressions, in the south. clamped The byDaqingzijing Huazijing Structure Terrace and is on Da’an-Honggang the middle of the Terrace two secondary in the east depressions, and west. Daqingzijing clamped by OilfieldHuazijing is interlinkedTerrace and withDa’an-Honggang Qian’an Oilfield Terrace in the in the northeast east and and west. interlinked Daqingzijing with Oilfield the Qian is 133interlinked well area with of HuazijingQian’an Oilfield Terrace. in Inthe the northeast south of and the oilfieldinterlinked there with is the the south Qian area 133 ofwell Heidimiao area of Huazijing and in the Terrace. northwest In therethe south is the of Haituozi the oilfield Oilfield there of is Da’an-Honggang the south area of TerraceHeidimiao (Figure and1 ).in Allthe thesenorthwest factors there make is Daqingzijing the Haituozi OilfieldOilfield anof Da’an-Honggang ideal regional tectonic Terrace location (Figure for 1). rich All oil these and factors gas sources. make Daqingzijing Oilfield Oilfield isan also ideal a grandregional lithologic tectonic reservoir location groupfor rich of oil the and Jilin gas Oilfield sources. with Daqingzijing over 100 million Oilfield tons is of also oil anda grand gas equivalent lithologic reservesreservoir [ 4group,5]. of the Jilin Oilfield with over 100 million tons of oil and gas equivalent reserves [4,5].

Figure 1. The regional tectonic location of Daqingzijing Oilfield [4,5]. Figure 1. The regional tectonic location of Daqingzijing Oilfield [4,5]. During the sedimentary period of Qingshankou Formation sediment, the Songliao Lake Basin had a highDuring rate of the subsidence, sedimentary the lake period basin of expanded, Qingshankou and theFormation hot and drysediment, paleoclimate the Songli becameao Lake warm Basin and humid.had a high Sedimentary rate of subsidence, material provided the lake bybasin the Tongyu-Baokangexpanded, and the river hot system and dry from paleoclimate the southwest became was formedwarm and into humid. delta sediment Sedimentary in the Daqingzijingmaterial provided area. Underby the transgressive Tongyu-Baokang conditions, river wavessystem in from the lakethe basinsouthwest played was an formed increasing into role delta in thesediment transformation in the Daqingzijing of the existing area. delta Under system, transgressive which destroyed conditions, the sand bodies depositing before like diffluent pathways in the delta front, after which more uniform sand Energies 2020, 13, x FOR PEER REVIEW 3 of 13 Energies 2020, 13, 1489 3 of 13 waves in the lake basin played an increasing role in the transformation of the existing delta system, which destroyed the sand bodies depositing before like diffluent pathways in the delta front, after bodies in bars with the extensive reverse rhythm was formed. Only a few sand bodies with normal which more uniform sand bodies in bars with the extensive reverse rhythm was formed. Only a few rhythms in the pathways remained. The sand bodies at the edge of river-mouth bars were transformed sand bodies with normal rhythms in the pathways remained. The sand bodies at the edge of river- into ones with reverse rhythms separated from the main parts of bars, namely front bars. At the same mouth bars were transformed into ones with reverse rhythms separated from the main parts of bars, time, sediment was re-deposited, which was re-carried by waves to form offshore bar sediment on namely front bars. At the same time, sediment was re-deposited, which was re-carried by waves to offshore slopes and was connected with the delta front, forming highly destructive transgressive delta form offshore bar sediment on offshore slopes and was connected with the delta front, forming highly sediment transformed by (stormy) waves that are unique to the Daqingzijing area [6,7]. Controlled by destructive transgressive delta sediment transformed by (stormy) waves that are unique to the sharp transgressive conditions, the delta sub-facies sediment in the transgressive delta system was Daqingzijing area [6,7]. Controlled by sharp transgressive conditions, the delta sub-facies sediment narrow. When rivers converged into the lake, the delta front and pro-delta sediment formed. So to in the transgressive delta system was narrow. When rivers converged into the lake, the delta front speak, the rivers did not form deltaic plain sediment in the lake shore before they were controlled and pro-delta sediment formed. So to speak, the rivers did not form deltaic plain sediment in the lake by transgressive conditions and transformed into underwater reduction sedimentary environment, shore before they were controlled by transgressive conditions and transformed into underwater and thus they could not form typical delta plain facies tracts. During the mid- and later-periods of reduction sedimentary environment, and thus they could not form typical delta plain facies tracts. Qingshankou Formation sediment, the water body decreased gradually and delta plain sub-facies During the mid- and later-periods of Qingshankou Formation sediment, the water body decreased appeared (Figure2). gradually and delta plain sub-facies appeared (Figure 2).

Figure 2.2. Sediment sequences and models of Daqingzijing Oilfield.Oilfield. ( aa)) Single well facies analysisanalysis profileprofile of Hei 102, showingshowing the superposition sedimentssediments in a near provenance channel and bar sand bodies; (b) Single well facies analysis profileprofile of HeiHei 84,84, showingshowing thethe superpositionsuperposition sediments of bar sand bodies; (c) Single well facies analysis profile of Qing 3, showing the sand body sediments in argillite; (d)Highly destructive delta sediment model.

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3. Distribution Characteristics of Isolated Sand Bodies By analyzing the statistics from 34 modern deltas and using several different parameters to describe the characteristics of basins, valleys, delta plains, and transgressive basins, Coleman and Wright established six separate delta models. Each of these models had a specific background and had unique sand body distribution patterns [8,9]. Among them, deltas of the VI type developed into bar sand bodies which are separate from delta sand bodies. Appalachicola Delta and Brazos Delta, which have mediate wave energy, a small tidal range, gentle offshore slopes, and a low supply of sediments, are typical examples of this type. Few examples of highly destructive deltas that developed in lake basins have been reported around the world and the genetic mechanism is quite complicated. In the broad environment of lake basins, the cause of isolated bar sand body formation is similar to that of the sediment of beach bars. Feng et al. (2011) proposed that storm sediments played an important part in the formation of beach sand bodies [10]. Wang et al. (2012) put forward a genetic mechanism for beach bars in paleogeomorphology, paleohydropower, and paleobasal-level under the high-energy circumstances of shallow-shore lakes [11]. Starting with the study on the distribution pattern of sand bodies and combined with other studies on wave energy and basin bottom shapes, this article analyzed the cause of formation of encouraging bar sand bodies and referred to ‘Storm waves conveying sand, landform controlling sand’ as the formation cause of deltaic isolated bar sand bodies and their distribution models.

3.1. Profile Characteristics and Horizontal Distribution Characteristics of Sand Bodies The core sequence directly presents the superposition relationship and characteristics of the sand bodies in isolated bars. The core shows that the sandstone lithology is mainly fine sandstone and siltstone, part of which is oil-spotted sandstone or oil-soaked sandstone. The bars are in gradual contact with mudstone at the bottom and in sharp contact at the top with a characteristic of obvious reverse rhythms from bottom to top. The bottom has mainly developed into wavy and lenticular beddings and is distributed as lacerate mud chips, deformation structures, nodules, and politic strips. The medium and upper parts are mainly made up of siltstone with wave-ripple beddings and water-ripple beddings, fine sandstone with parallel beddings, and mid-fine sandstone with low-angle cross beddings. Lamina of charcoal appears in some of sandstone members with occasional nodule developments. The thickness of the sand layers is between 2.5 m and 3m, with the thickest one being up to 8.7 m (Figure3). Mudstone is dark grey and ash dark, and the thickness varies with various mudstone layers, which are mainly effective interlayers. Energies 2020, 13, 1489 5 of 13 Energies 2020, 13, x FOR PEER REVIEW 5 of 13

2 Figure 3. Distribution characteristics of the isolated bodies in Thin Layer 1 of Sand Bed I in qn member Figure 3. Distribution characteristics of the isolated bodies in Thin Layer 1 of Sand Bed I in qn2 member of Daqingzijing Oilfield. (The figure uses data from 1000 wells, and the producing wells are not of Daqingzijing Oilfield. (The figure uses data from 1000 wells, and the producing wells are not be be shown). shown). 3.2. Genetic Explanation of Isolated Sand Bodies 3.2. Genetic Explanation of Isolated Sand Bodies 3.2.1. The Discovery of Storm Deposition Shows That There Is Higher Wave Energy in the Study Area 3.2.1. The Discovery of Storm Deposition Shows That There Is Higher Wave Energy in the Study The storm etching structures are various etching filling structures which are formed because of Area storm flows scouring and sapping sediments (Figure4a). Scouring-filling structures remaining at the bottomThe of storm sediments etching are structures important are signs various to help etching identify filling tempestites structures [12 which–15]. Generally,are formed hummocky because of stormcross beddings flows scouring are sedimentary and sapping structures sediments formed (Figure due 4a). to stormScouring-filling waves’ impact structures (Figure remaining4b–d), which at the is bottoman outstanding of sediments feature are of important storm sediments signs to help [10]. identify Since a stormtempestites flow is[12–15]. a density Generally, current, hummocky numerous crosssmall beddings burdens andare sedimentary ball-and-pillow structures deformation formed structures due to storm are waves’ founded impact in storm (Figure sediments 4b–d), which in the isDaqingzijing an outstanding area feature (Figure 4ofe). storm They sediments formed very [10]. thin Since deformation a storm flow structural is a density strata andcurrent, were numerous clamped smallin tempestites. burdens Rippleand ball-and-pillow marks on the topdeformation surface of stru lakectures tempestites are founded are mostly in storm symmetric sediments and slightly in the Daqingzijingasymmetric. Thearea appearance (Figure 4e). of They wave-ripple formed very beddings thin deformation is a sign ofwave structural activities, strata and and ripple were marksclamped in indeep tempestites. water are Ripple the evidence marks foron the top impact surface of storms of lake (Figure tempestites4f). are mostly symmetric and slightly asymmetric. The appearance of wave-ripple beddings is a sign of wave activities, and ripple marks in deep water are the evidence for the impact of storms (Figure 4f).

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FigureFigure 4. 4.Photographs Photographs of of cores cores ofof stormstorm sediments.sediments. ( (aa)) Qing3-3, Qing3-3, 2417.35m, 2417.35m, scouring scouring filling ﬁlling structure; structure; (b)(b H169,2341.3m,) H169,2341.3m, hummocky hummocky cross cross bedding;bedding; (c) H169,2350.1m, hummocky hummocky cross cross bedding; bedding; (d) ( dH169,) H169, 2342.5m,2342.5m, hummocky hummocky cross cross bedding; bedding; ((ee)) H84,H84, 2457.8m, slot slot mold mold structure; structure; (f ()f H102,) H102, 2419.1m, 2419.1m, wave wave rippleripple cross cross bedding. bedding.

InIn the the Qingshankou Qingshankou Formation Formation period period of of Daqingzijing Daqingzijing Oilfield, Oilfield, there there were were storm storm sediments sediments in in the wellsthe ofwells Hei of 83, Hei Hei 83, 84, Hei Hei 84,109, Hei Hei109, 102,Hei 102, and and Qing Qing 3-3. 3-3. There There were were complete complete storm storm sequences sequences in in the wellsthe ofwells Hei of 83, Hei Hei 83, 84, Hei and 84, Heiand 109,Hei and109, and there there were were only only thin thin stagnant stagnant strata strata caused caused by by storm storm in in Hei 102Hei and 102 Qing and 3-3. Qing The 3-3. appearance The appearance of storm of storm sediments sediments show show that thethat water the water body body of the of lakethe lake basin basin in the studyin the area study was area once was deep once in deep the in period the period of Qingshankou of Qingshankou Formation Formation and and was was greatly greatly influenced influenced by stormby storm waves waves (Figure (Figure5). From 5). aFrom side, a this side, reflects this reflects that waves that playedwaves anplayed important an important role in the role formation in the offormation the sediment of the system sediment of Qingshankou system of Qingshankou Formation. Formation.

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Figure 5. 5. CharacteristicsCharacteristics of storm of sediment storm sediment sequence sequenceof Qingshankou of Qingshankou Formation in Formation Daqingzijing in Oilfield.Daqingzijing Oilfield. 3.2.2. Broad and Gentle Offshore Slope Landform Is Conducive to the Formation of Isolated Bar 3.2.2. Broad and Gentle Offshore Slope Landform Is Conducive to the Formation of Isolated Bar In the depression phase of the Qingshankou Formation sediment period, the slopes of lake basins In the depression phase of the Qingshankou Formation sediment period, the slopes of lake in Daqingzijing area were gentle, the terrain sloped gently, and the water body was rather deep. As the basins in Daqingzijing area were gentle, the terrain sloped gently, and the water body was rather surface of the lake basins expanded, the generating area extended and the fetch reached farther. Waves deep. As the surface of the lake basins expanded, the generating area extended and the fetch reached on the lake surface arose rapidly from a storm, so the delta sand bodies formed in previous periods farther. Waves on the lake surface arose rapidly from a storm, so the delta sand bodies formed in were discarded under a transgressive background and were sharply transformed by lake waves. As a previous periods were discarded under a transgressive background and were sharply transformed result, sediments in delta plains did not grow or were in deficiency. Part of the front of bar sand bodies by lake waves. As a result, sediments in delta plains did not grow or were in deficiency. Part of the at the river mouth was transformed into isolated sand bodies or sand bodies interlinked with river front of bar sand bodies at the river mouth was transformed into isolated sand bodies or sand bodies mouth bars, namely front bar sand bodies, which lies in the front of bar sand bodies at the river mouth. interlinked with river mouth bars, namely front bar sand bodies, which lies in the front of bar sand Meanwhile, fragmentary materials from the delta front were re-carried by strong storm waves and bodies at the river mouth. Meanwhile, fragmentary materials from the delta front were re-carried by formed dispersed offshore bar sediments on the broad and gentle offshore slopes. strong storm waves and formed dispersed offshore bar sediments on the broad and gentle offshore The analysis on the mudstone color values show that mudstone colors turned from a partially slopes. reduced color into a partial oxidation color in the early and later periods of Qing2 member, which The analysis on the mudstone color values show that mudstone colors turned from a partially appears to be an obvious regressive process for a half-deep lake into a shallow lake. We chose 28 wells reduced color into a partial oxidation color in the early and later periods of Qing2 member, which in Sand Bed V from the near to the distant on the plane according to the offshore line to make a diagram appears to be an obvious regressive process for a half-deep lake into a shallow lake. We chose 28 of mudstone color values so as to develop an environmental explanation and found that the water body wells in Sand Bed V from the near to the distant on the plane according to the offshore line to make between the shore line and the well area of Hei 89 is deeper and more stable, while the water body a diagram of mudstone color values so as to develop an environmental explanation and found that between the well areas of Hei 50 and Qian 139 is shallow with an oxidation-color-based shallow lake. the water body between the shore line and the well area of Hei 89 is deeper and more stable, while The center of the lake basin is the sedimentary environment of the deep lacustrine facies. Seen from the water body between the well areas of Hei 50 and Qian 139 is shallow with an oxidation-color- the plane, the shallow area with higher color values is oval-shaped or long and narrow, whose major based shallow lake. The center of the lake basin is the sedimentary environment of the deep lacustrine axis is parallel with the shoreline in most parts (Figure6). The comprehensive analysis reveals that in facies. Seen from the plane, the shallow area with higher color values is oval-shaped or long and the front of the deeper area on the offshore side, the landform presents the characteristic similar to the narrow, whose major axis is parallel with the shoreline in most parts (Figure 6). offshore underwater paleohigh. The comprehensive analysis reveals that in the front of the deeper area on the offshore side, the landform presents the characteristic similar to the offshore underwater paleohigh.

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Figure 6. MudstoneMudstone Color Values of Sand Group V in Qiang-2 Member. (a) A diagram for water depth interpretation basedbased onon thethe analysisanalysis of of mudstone mudstone color color values; values; (b ()b A) A well well position position map map (the (the asterisk asterisk is foris for the the well well in in Figure Figure6a). 6a).

4. Characteristics of Reservoir Accumulation in Isolated Bars Many great discoveries and 80% of the newly verifiedverified reserves in Jilin OilfieldOilfield these years are achieved in the delta front’s thinthin interbeddings,interbeddings, which are in aa relativerelative largelarge area.area. It is of great importance for exploration and development to conduct studies on the accumulation laws of the reservoirs in Daqingzijing Oilfield’sOilfield’s lithology and strata. In In this kind of reservoir, thethe developmentdevelopment of the single sandsand bodybody isis relativelyrelatively stablestable [[16,17].16,17]. Due to the changes in lithology and physical properties, lithologiclithologic reservoirs reservoirs were were formed formed and and the impactthe impact of gravitational of gravitational differentiation differentiation on complex on low-ultralowcomplex low-ultralow permeability permeability reservoirs reservoirs in the oil-water in the oil-water system, influencedsystem, influenced by lithology by lithology and physical and properties,physical properties, is not obvious. is not obvious. Reservoirs Reservoirs are mainly are mainly single oil-bearingsingle oil-bearing series, andseries, faults and andfaults minor and structuresminor structures just act just as anact enrichment as an enrichment source. Accumulationsource. Accumulation is controlled is controlled by lithology by lithology and physical and properties.physical properties. Based on Based the outcrop on the observation outcrop observat over reservoirs,ion over thisreservoirs, article sets this up article the accumulation sets up the modelaccumulation of lenticular model reservoirsof lenticular as reservoirs using the as variables using the of variables ‘Surrounded of ‘Surrounded by source by rocks source to generate rocks to hydrocarbon’,generate hydrocarbon’, ‘Driven by ‘Driven pressure by di ffpressureerences’, diffe Migrationrences’, through Migration multi through pathways’ multi and pathways’ ‘Accumulation and by‘Accumulation filtering water’. by filtering water’. 4.1. Surrounded with Source Rocks to Generate Hydrocarbon 4.1. Surrounded with Source Rocks to Generate Hydrocarbon The southern Songliao Basin experienced two major lake flooding events. The large number The southern Songliao Basin experienced two major lake flooding events. The large number of of aquatic organisms in the basin offered source materials for the formation of kerogen, and thus aquatic organisms in the basin offered source materials for the formation of kerogen, and thus maintain rich organic materials for the dark mudstone of half-deep lake and deep lake facies. The dark maintain rich organic materials for the dark mudstone of half-deep lake and deep lake facies. The mudstone in Qingshankou Formation and Nenjiang Formation indicates the two sets of regional dark mudstone in Qingshankou Formation and Nenjiang Formation indicates the two sets of regional high-quality source rock strata formed in the processes of the two lake water expansions. The dark high-quality source rock strata formed in the processes of the two lake1 water2 expansions. The dark mudstone in first and second member of Qingshankou Formation (qn and qn ) is widely distributed mudstone in first and second member of Qingshankou Formation (qn1 and qn2) is widely distributed with thicknesses ranging between 50 m and 150 m. with thicknesses ranging between 50 m and 1501 m. 2 Organic materials in dark mudstone in qn and qn in Daqingzijing Oilfield are mostly made up of Organic materials in dark mudstone in qn1 and qn2 in Daqingzijing Oilfield are mostly made up maceral composition of sapropelinite and exinite contents in a few samples are very high. Based on the of maceral composition of sapropelinite and exinite contents in a few samples1 are very high. Based analysis of organic material elements in kerogen, the organic materials in qn member in the study area on the analysis of organic material elements in kerogen, the organic materials in qn1 member in the mainly belong to the I-type and II1-type with a few of them belonging to the II2-type, while organic study area mainly belong to the I-type2 and II1-type with a few of them belonging to the II2-type, materials in mudstone kerogen in qn mainly belong to the I-type and II2-type, which have a better while organic materials in mudstone kerogen in qn2 mainly belong to the I-type and II2-type, which capacity for oil generation. With rock pyrolysis, organic materials in source rocks can also be classified have a better capacity for oil generation.1 With 2rock pyrolysis, organic materials in source rocks can and evaluated—organic materials in qn and qn member mainly belong to the I-type and II1-type [18]. also be classified and evaluated—organic materials in qn1 and qn2member mainly belong to the I-type By using comparison fingerprint- chromatograms of crude oil samples and mudstone samples, and II1-type [18]. as Figure7 shows, the m /z 217 and m/z 191 fingerprints are very similar between crude oil and By using comparison fingerprint- chromatograms of crude oil samples and mudstone samples, mudstone in the Qingshankou Formation. as Figure 7 shows, the m/z 217 and m/z 191 fingerprints are very similar between crude oil and mudstone in the Qingshankou Formation.

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FigureFigure 7. 7. TheThe m/z m217/z and 217 m/z and 191 m /fingerprintsz 191 fingerprints between betweencrude oil and crude mudstone oil and in mudstone the Qingshankou in the QingshankouFormation. Formation. 4.2. Driven by Pressure Difference 4.2. Driven by Pressure Difference Oil and gas generated from source rocks enter into reservoirs after primary migration. Unlike Oil and gas generated from source rocks enter into reservoirs after primary migration. Unlike in in conventional reservoirs where buoyancy is the main source of migration power, the porosity conventional reservoirs where buoyancy is the main source of migration power, the porosity and and permeability in deep reservoirs are much lower than mid-level and shallow reservoirs [19,20]. permeability in deep reservoirs are much lower than mid-level and shallow reservoirs [19,20]. This This makes the interface tension between oil and gas and water-bearing rocks much larger than the makes the interface tension between oil and gas and water-bearing rocks much larger than the buoyancy, so it is hard for oil and gas to float separately when driven by buoyancy. The abnormal high buoyancy, so it is hard for oil and gas to float separately when driven by buoyancy. The abnormal pressure in source rocks and the pressure difference (potential energy difference) between caprocks and high pressure in source rocks and the pressure difference (potential energy difference) between reservoirs are necessary dynamic conditions for the formation of deep and native lithologic reservoirs. caprocks and reservoirs are necessary dynamic conditions for the formation of deep and native Since the hydrocarbon migration in sandstone lens wrapped by mudstone can be seen as a special lithologic reservoirs. form of primary migration, this means there is a transfer of oil and gas from source rocks with particles Since the hydrocarbon migration in sandstone lens wrapped by mudstone can be seen as a of a low porosity and permeability to carrier beds and reservoirs with a relatively high porosity and special form of primary migration, this means there is a transfer of oil and gas from source rocks with permeability. The maximum migration distance can be up to 1 km. Therefore, so to speak, as long as particles of a low porosity and permeability to carrier beds and reservoirs with a relatively high sandstone bodies are wrapped by or in contact with effective source rocks, the accumulation of oil and porosity and permeability. The maximum migration distance can be up to 1 km. Therefore, so to gas is sufficient. speak, as long as sandstone bodies are wrapped by or in contact with effective source rocks, the The formation pressure measured data and calculating data prove pressure of mudstone accumulation of oil and gas is sufficient. reaches up to 22 MPa, and the sandstone pressure is close to normal hydrostatic pressure The formation pressure measured data and calculating data prove pressure of mudstone reaches (Figure8). Xiao et al. (1999) suggested, with their mudstone stress rupture experiments, that up to 22 MPa, and the sandstone pressure is close to normal hydrostatic pressure (Figure1 8). Xiao et when the overpressure is larger than the mudstone ambient pressure (10 MPa) in qn member, al. (1999) suggested, with their mudstone stress rupture experiments, that when the overpressure is abnormally high pressure (the overpressure value can arrive at over 10 MPa) flow will be released [21]. larger than the mudstone ambient pressure (10 MPa) in qn1 member, abnormally high pressure (the Hou et al. (2006) calculated that the pressure gradient at present increases gradually from Yaojia overpressure value can1 arrive at over 10 MPa) flow will be released [21]. Hou 1et al. (2006) calculated Formation down to qn member and decreases sharply from the bottom of qn member and Fuyang that the pressure gradient at present increases gradually from Yaojia Formation down to qn1member reservoir [5]. Based on this, they inferred that the abnormal pressure, which was generated from and decreases sharply from the bottom of qn1member and Fuyang reservoir [5]. Based on this, they the end of Nenjiang Formation sediment period and Mingshui Formation sediment period to the inferred that the abnormal pressure, which was generated from the end of Nenjiang Formation peak period of oil generation and expulsion in the Neogene, was larger than mudstone fracture sediment period and Mingshui Formation sediment period to the peak period of oil generation and pressure. The overpressure value depends on hydrocarbon enrichment and the height of containing expulsion in the Neogene, was larger than mudstone fracture pressure. The overpressure value hydrocarbon. The overpressure’s plane distribution range of source rocks in q 1 and q 2member depends on hydrocarbon enrichment and the height of containing hydrocarbon. nThe overpressure’sn controls the hydrocarbon’s plane distribution in the Gaotaizi reservoir. However, while the areas at plane distribution range of source rocks in qn1 and qn2member controls the hydrocarbon’s plane the edge of the depression with low hydrocarbon-generating strength and overpressure distribute distribution in the Gaotaizi reservoir. However, while the areas at the1 edge of2 the depression with some structural changes because the thickness of dark mudstone in qn and qn members is very thin, low hydrocarbon-generating strength and overpressure1 2 distribute some structural changes because the rest of the areas distribute changes in qn and qn members [22]. the thickness of dark mudstone in qn1 and qn2members is very thin, the rest of the areas distribute changes in qn1 and qn2 members [22]. In lithologic reservoirs, the main factors controlling pressure difference are lithology-faults and fissures. The communication system between faults and fissures lies in the areas with relative low pressure and the argillite area is a high-pressure area. From mudstone to sandstone to fissure-fault system, a complete pressure gradient decreasing system is constructed.

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Figure 8. Formation pressure characteristics of the Xin171 well. Figure 8. Formation pressure characteristics of the Xin171 well. In lithologic reservoirs, the main factors controlling pressure difference are lithology-faults and 4.3. Migration Through Multi Pathways fissures. The communication system between faults and fissures lies in the areas with relative low pressureFor a and lenticular the argillite reservoir area iswith a high-pressure low permeability, area. Fromthe faulting mudstone system to sandstone is a main to pathway fissure-fault for hydrocarbonsystem, a complete migration pressure [20]. gradientThe development decreasing of systema faulting is constructed.system determines the reservoir type and plane as well as its vertical distribution characteristics. On the west slopes and the central fault belt of4.3. Daqingzijing Migration Through Oilfield, Multi the Pathwaysfaulting system has developed well. In the reservoir developing zone, the hydrocarbonFor a lenticular migration reservoir amount with is low sufficient permeability, and larger the faultingcapillary systemresistance isa can main be pathwayovercomefor in thehydrocarbon process of migration hydrocarbon [20]. migration. The development In addition, of a faulting oil and system gas expulsion determines is relatively the reservoir complete. type and In planethis case, aswell hydrocarbon as its vertical accumulation distribution is a characteristics. lot more demanding On the for west traps, slopes which and shows the central that reservoirs fault belt inof Daqingzijingthe west slopes Oilfield, were mainly the faulting formed system in fault has structures, developed while well. on In the the east reservoir slopes developing of Daqingzijing zone, Oilfield,the hydrocarbon the faultingmigration system amount is relatively is su ffiundevelopedcient and larger and capillarythe hydrocarbon resistance migration can be overcome amount inis not the processsufficient, of hydrocarbonwhich provides migration. a much lower In addition, capacity oil andfor oil gas to expulsion expulse water. is relatively In the complete.preconditionIn this of some case, certainhydrocarbon capillary accumulation resistance, isoil a lotis not more able demanding to expulse for water traps, effectively which shows in a that certain reservoirs range. in This the westdetermines slopes werethat the mainly reservoir formed type in faulton east structures, slopes transforms while on the gradually east slopes from of Daqingzijing lithologic structural Oilfield, reservoirthe faulting to systemfault lithologic is relatively reservoir, undeveloped eventually and to the lithologic hydrocarbon reservoir. migration The oil-water amount isrelationship not sufficient, in whicheach set provides of oil-bearing a much series lower is capacitycomplicated for oil(Figure to expulse 9a). water. In the precondition of some certain capillary resistance, oil is not able to expulse water effectively in a certain range. This determines that the reservoir type on east slopes transforms gradually from lithologic structural reservoir to fault lithologic reservoir, eventually to lithologic reservoir. The oil-water relationship in each set of oil-bearing series is complicated (Figure9a).

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Figure 9. ReservoirsReservoirs distribution distribution and and th thee accumulation accumulation characteristics characteristics of delta of delta front front sand sand bodies. bodies. (a) (Reservoira) Reservoir Types Types and and reservoir reservoir sector sector in Daqingzijing in Daqingzijing Oilfield; Oilﬁeld; (b) Diagram (b) Diagram of the of Mechanism the Mechanism of filter- of ﬁlter-stylestyle accumulation; accumulation; (c) Sketch (c) Sketch of hydrocarbon of hydrocarbon migration migration direction. direction.

4.4. Accumulation by Filtering Water By studying the migration paths for oil, gas, and water in low-ultralow permeability reservoir, reservoir, dynamic conditions, and non-Darcy seepage states [23], [23], we suggest that the retention effect effect is crucial for oil to accumulate accumulate in in lenticles. lenticles. Study Study results results show show that that when when hydrocarbon hydrocarbon migration migration happens happens in inlow-ultralow low-ultralow permeability permeability reservoir, reservoir, gas, gas, and and water water migrates migrates in the in manner the manner of single of single molecule molecule or a orfew a fewmolecules molecules combined combined and and they they can can pass pass the the bottleneck bottleneck freely freely [24]. [24]. However, However, the minimum diameter of an oil droplet is larger than the diameterdiameter of the bottleneck, so it needs to deform to get through it. InIn this this way, way, oil oil and and water water can can be be migrated migrated in in priority priority and and the the migration migration of ofoil oil droplets droplets is isnot not allowed. allowed. ThenThen accumulation accumulation taketakess place. place. Pressure Pressure differences differences in fluids in fluids and buoyancy and buoyancy are the are main the mainfactors factors affecting affecting oil migration, oil migration, while while capillary capillary resistance resistance is the is main the main resistance resistance factor factor affecting affecting oil oilmigration. migration. When When the the power power is islarger larger than than the the resi resistance,stance, oil oil strands strands to to form form accumulation. In the syncline area, buoyancy inside reservoirs is smaller than capillary resistance, so normal gravitational didifferentiationfferentiation doesdoes notnot happenhappen betweenbetween oiloil andand water.water. Indoor experimentsexperiments and and field field production production data data show show that reservoirthat reservoir wettability wettability has a great has influence a great oninfluence hydrocarbon on hydrocarbon migration. migration. In the porosity In the of porosity reservoir of rocks reservoir where rocks wettability where iswettability hydrophilic, is thehydrophilic, injected oil the and injected water, whichoil and are water, wrapped which in waterare wrapped films formed in water on the films particle formed surface, on the move particle with manysurface, twists move and with turns. many Most twists void and water turns. is Most expulsed void and water continuous is expulsed hydrocarbon and continuous phases hydrocarbon are formed. Atphases this moment,are formed. buoyancy At this doesmoment, not function buoyancy at all.does Oil not droplets function can at formall. Oil in thedroplets middle can of form larger in drill the waysmiddle and of oillarger droplets drill areways subjected and oil to droplets buoyancy, are capillary subjected resistance, to buoyancy, and pressure. capillary resistance, and pressure.Based on the analyses above, we put forward the mechanism of filter-style accumulation. The core contentsBased include: on the analyses above, we put forward the mechanism of filter-style accumulation. The (1)core contentsThe reservoir include: is a lenticular and low permeability reservoir. (1) The reservoir is a lenticular and low permeability reservoir.

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(2) From mudstone to sandstone to a ﬁssure-fault system, a set of complete pressure decreasing system is constructed (Figure9b,c). (3) The retention eﬀect is the main mechanism for water expulsion and oil retention in sandstone.

5. Conclusions and Discussions (1) Based on the overview about the two decades of development experience in Daqingzijing 1 Oilfield, this paper suggests that the isolated river-mouth bar sand bodies in the qn member of Daqingzijing Oilfield with a background of delta sediments is a type of important oil and gas reservoir. However, the cause of the formation of numerous isolated river-mouth bar in the lake basin delta sediment system is a puzzle which vexes geographical researchers. To solve this problem, the authors obtained lots of evidence of storm waves’ existence by using core observation. Combined with analyses of paleogeography and hydrodynamics, this paper gives a detailed discussion on the transformation of delta sand bodies by using waves and the genetic mechanism of an isolated river-mouth bar. In addition, on the basis of paleogeomorphology research and the sand body distribution statistics, we consider ‘Storm waves conveying sand, landform controlling sand’ as the cause and distribution model of the delta’s isolated bar sand body formation. We also think the superposition of multiple bar sand bodies is the direct cause of the strong anisotropy in reservoir and the complex relationship between oil and water in reservoir. This conclusion mainly draws lessons from the sedimentary environment and sand body distribution of Apalachicola delta and assumes that the Qingshankou formation sedimentary period has the same characteristics, including modest wave energy, little tidal range, and a gentle offshore slope. This assumption does not rebel against the principles of sedimentology, and the conclusion is more closely aligned with the exploration and development and more reasonable in explaining hydrocarbon accumulation. Unfortunately, this kind of delta has not been found in the present lacustrine basin. (2) Isolated sand bodies in the delta front mostly form lenticular lithologic reservoirs. Based on the outcrop observation over reservoirs, we think that the direct contact between source rocks and lenticular sand bodies is a precondition of the formation and existence of lenticular and lithologic reservoirs. From mudstone to sandstone to a fissure-fault system, a complete pressure gradient decreasing system is constructed. After water-oil mixing facies enter into sand bodies, water passes through sand bodies first and oil remains in sand bodies due to their different qualities.

Author Contributions: Conceptualization was conducted by P.C.and C.L., rock analysis by J.Z., formation pressure calculation by G.M., and sedimentary models research by Y.Z.; S.L. directed the revision of the manuscripts; P.C., C.L. and J.Z. contributed to the writing of the manuscript. The authors thank China National Petroleum Co., LTD, and Jilin Oilfield Corporation for supplying researching data and allowing us to observe the cores. All authors have read and agreed to the published version of the manuscript. Funding: This research was funded by [Liaoning Revitalization Talents Program] grant number [XLYC1807129], [Liaoning Province Nature Fund Support Program] grant number [2019-MS-220] and [Talent Scientific Research Fund of LSHU] grant number: [XJJ2016-026]. Conflicts of Interest: The authors declare no conflict of interest.

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