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Petroleum 2 (2016) 334e343

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Original Article

Reservoir formation mechanism analysis and deep high-qualityreservoir prediction in Yingcheng Formation in Longfengshanarea of Songliao Basin, China

Yongmei Zhang a, Xiaoqi Ding b, *, Peng Yang a, c, Yan Liu b, Qi Jiang b, Shaonan Zhang a

a School of Geoscience and Technology, Southwest Petroleum University, Sichuan 610500, Chinab State Key Laboratory of Oil and Gas Reservoir Geology and Exploitation, Chengdu University of Technology, Sichuan 610059, Chinac Northwest Petroleum Bureau of Sinopec, Xinjiang 830011, China

a r t i c l e i n f o

Article history:Received 6 June 2016Received in revised form9 September 2016Accepted 12 September 2016

Keywords:DiagenesisLaumontite cementHigh-quality reservoirSecondary poreThe outside subaqueous fan-delta

* Corresponding author.E-mail address: xiaoqiding@qq.com (X. Ding).Peer review under responsibility of Southwest Pe

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a b s t r a c t

Although commercial gas flow was produced in several wells with recent years' exploration ofLongfengshan area in Changling fault sag, the formation mechanism and controlling factors forhigh-quality reservoirs still remained undefined. Here, the Yingcheng tight gas reservoirs ofLongfengshan area are used as an example to characterize high-quality reservoir formationmechanism and distribution rules. Based on the thin section, SEM, X-ray diffraction, computedtomography (CT) scanning, burial history, constant-rate mercury penetration and physical prop-erties testing, formation mechanism and controlling factors for high-quality reservoirs wereanalyzed. Results show the following characteristics. First, the reservoir is dominated by chloriteand laumontite cements, and compaction is the most important factor to control reservoir physicalproperties. According to this, the reservoir can be divided into compacted tight sandstones,chlorite-cemented sandstones and laumontite-cemented sandstones. Second, the high-qualityreservoirs are formed due to early extensive laumontite precipitation and the later dissolution oflaumontite by organic acid. Meanwhile, it is found that the distribution of cementation anddissolution exhibits some regulations in sedimentary facies, and the distribution is mainly effectedand controlled by the lake water and charging of fresh water. Besides, the distribution model ofvarious types of sandstones was established. Studies over diagenesis and sedimentary facies revealthat the high-quality laumontite-cemented sandstones exist in the outside subaqueous fan-delta ofthe deep sag in Longfengshan area. These findings have been validated by recent exploration wellswhich obtained high industrial gas flow.

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1. Introduction

Accurate prediction of reservoir quality plays an importantrole in studying natural gas's accumulation, especially in deepsiliciclastic reservoirs with burial depth exceeding 3000m and

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burial temperatures higher than 100 �C [1,2]. In general, it isbelieved that physical properties of the reservoir, such asporosity and permeability, decrease as its burial depth increases[3]. However, exceptions are found in the Norphlet sandstones ofGulf of Mexico and Garn sandstones of Kristin field, Norway,where the reservoir quality is preserved with high burial depths.For instance, the Norphlet sandstone has a burial depth deeperthan 6000m and temperature up to 215 �C with porosity greaterthan 20% [4], whereas the Garn sandstone has a burial depthbetween 4500m and 5000 m with porosity values ranging from18% to 20% [5]. Similar situation is also found in YingchengMember of Changling fault sag, Songliao Basin. The explorationconfirms that high-quality reservoir develops with average

ing by Elsevier B.V. on behalf of KeAi Communications Co., Ltd. This is an openy-nc-nd/4.0/).

Y. Zhang et al. / Petroleum 2 (2016) 334e343 335

porosity 9.1% and average permeability 3.1mD and industrial gasflows are acquired in the Yingcheng Member with buried depthgreater than 3000m (Fig. 1). In the studying area, sandstoneswith permeability of more than 0.5mD and porosity of more than8% are defined as high-quality sandstones. Meanwhile, the con-trolling factors for deep high-quality reservoir are also the hot-spot in nowadays petroleum geology [6e10].

Based on the observation of polished thin sections, CT, SEM,X-ray diffraction, physical properties testing, constant-ratemercury penetration and production test, this paper studiesthe deep reservoir characteristics of Yingcheng Member inLongfengshan sub-sag, analyzes the formation mechanism andcontrolling factors for high-quality reservoirs, defines the high-quality distribution characteristics and predicts the sweet spot.The research can provide a theoretical guidance for the deep gasexploration in Longfengshan area.

Changling fault sag locates in the south of central downwarpzone in Songliao Basin with an area of 1.3 � 104 km2 (Fig. 2). Theinternal fracture of Changling fault sag influenced by the EWcrustal detachment from Jurassic to early Cretaceous [11,12].Longfengshan sub-sag (also known as south Changling sub-sag)locates in the south Changling fault sag. Controlled by the NEboundary fault of Beizhengzhen in west, it formed a dustpan-shaped fault sag with faulting in the northwest and overlyingin the southeast.

From the bottom to the top, the Mesozoic is composed of theJurassic Huoshiling Formation, the lower Cretaceous ShaheziFormation, Yingcheng Formation, Denglouku Formation, Quan-tou Formation and upper Cretaceous (Fig. 3). Depending on thelithology, the Yingcheng Formation is divided into seven mem-bers, but the Ying I and Ying II intervals are usually denuded.Three episodes of deformation, which are related to regionaltectonics and associated with stratigraphic signatures, arerecognized in Longfengshan sub-sag: fault sag, depression and

Fig. 1. Prosity versus depth of sandstones for each group of Yingcheng Formation.

structure inversion. The fault sag layers include the HuoshilingFormation of upper Jurassic, the Shahezi Formation, the Ying-cheng Formation and the Denglouku Formation which representthe different stage's sediments of early Cretaceous. ShaheziFormation is deposited in the braided delta and lacustrinesediment environment in the early Cretaceous period. Due to thehigh depositional rate of fault sag's settling and the humidclimate, the uppermost coal-bearing source rocks were formedin the area of Shahezi Formation [14]. The depocenter of thesource rock is located near the northern part of the well B22, andthe thickness ranges from 100 to 400 m (Fig. 2). Subsequently,the paleoclimate changed from humid to semi-arid, the sag fieldenlarged to the north in Longfengshan area, the sediment supplywas ample and the settling became extremely active. All the el-ements lead to the formation of fan-delta sedimentary system inYingcheng Formation, which develops pebble to gravel granular,glutenite, and fine-to coarse-grained sandstone, siltstone andmudstone. The Yingcheng and the Shahezi Formation are themain gas-producing Members. Besides, geologists have paidgreat attention on delta because of their substantial oil and gasreserves and high reservoir quality [15].

2. Methods

To predict the reservoir quality, 142 thin sections and 70polished thin sections are made. These counts per section areselected for composition analysis, while 100 counts per section isutilized for grain size and sorting examination. More than 100thin sections and 400 scanning electron microscope (SEM)photographs are used for paragenetic sequence of cementationand pores characteristics analysis. A total of 30 reservoir sand-stone samples are analyzed for whole-rock (bulk) and clayfraction mineralogy using XRD. The pore throat characteristicsare analyzed by computed tomography (CT) scanning data andconstant-rate mercury penetration 5 samples. We also use thephysical properties testing of net overburden stress from 25samples to reflect the differences of porosity and permeabilityunderground.

3. Reservoir characteristics

The reservoir lithology of the Yingcheng Formation includesgranular, glutenite, fine-to coarse-grained sandstone. The sedi-ments are characterized with low compositional maturity andtextural maturity, the poor grain sorting and roundness (sub-angular to sub-rounded), due to the near-source deposits. Inaccordance with the thin section examination results of 142samples, the detrital components of the sandstones in YingchengFormation are high in rock fragment and low in feldspar andquartz. Moreover, the major rock type is litharenite, and feld-spathic litharenite takes the second place (Fig. 4). The content ofvolcanic rock fragment is normally higher than 55%, and themaximum content is 79.2%. Meanwhile, the content of meta-morphic rock fragments is less than 10% in the east part of thestudying area, which can be increased to 20.8% in the west.Volcanic rock fragments contain a large amount of andesite andbasalt, while metamorphic rock fragments are mainly mica-quartzite schist, phyllite and quartz fragments.

4. Diagenesis and diagenetic sequence

4.1. Compaction

The compaction of sandstones is moderate to fierce in Ying-cheng Formation of Longfengshan area, which is showed by the

Fig. 2. Overlaying map of structure map of the top interval III in Yingcheng Formation and hydrocarbon distribution of source rock in Longfengshan Sub-sag of Songliao basin.

Fig. 3. Stratigraphic column showing the Mesozoic stratigraphy, lithology and depo-sitional environments in Changling Fault sag, Songliao Basin, Northeast China [13].

Y. Zhang et al. / Petroleum 2 (2016) 334e343336

following phenomenon: (1) The major grain are long andconcavo-convex contact. (2) Deformation of ductile rock frag-ments reduced intergranular volume-obviously.

4.2. Cementation

Four types of the most common diagenetic minerals areidentified in the Yingcheng Formation sandstones. (1) Authigenicclay cements: chlorite are formed early in the diagenetic historyas indicated by the occurrence of clay rims on detrital grains(Fig. 5A). Smectite is altered from volcanic materials thentransform to I/S mixed layer during diagenesis (Fig. 5B). As evi-denced from the occurrence of thin illite plates on the surfaces ofdetrital grains (Fig. 5C), it is possible that montmorillonite mighthave been altered to illite during progressive burial. (2) Siliceouscements: the siliceous cements in sandstones of the Yingcheng

Fig. 4. The triangular plot of Yingcheng Member in Longfengshan area.

Fig. 5. Typical microphotograph of sandstones of the Yingcheng Member. A. chlorite rims fill intergranular pores, 3113.35m, B202; B. I/S mixed layers block intergranularpores, 2995.12m, B202; C. Illite, 3255.7m, B201-1; D. Crystallite quartz appears in the leaves of chlorite rims, 3101.49m, B202; E. Ferroan calcite forms after the crystallitequartz, 3410.98m, B211; F. Laumontite cement, 3222.96m, B201-1; G. Dissolved feldspars result in intragranular pores, 2807m, B201-8; H. Laumontite were dissolved alongthe columnar cleavage, 3222.96m, B201-1; I. Volcanic debris were dissolved and formed intragranular pores, 3112.5m, B201-13.

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Formation presents in two forms: crystallite quartz (Fig. 5D) andchalcedony, while the chalcedony is rarely seen. (3) Laumontitecements: Laumontite cements are mainly formed in earlydiagenetic period, and exhibits coaxial laumontite cement or apatchy distribution (Fig. 5E). (4) Carbonate cements: Carbonatecements are evident in the sandstones of Yingcheng Formationbecause of the presence of ferroan calcite and a small amount ofcalcite. Ferroan calcite exhibits poikilotopic or coaxial distribu-tion (Fig. 5F). Thin section study reveals that ferroan calcite isgenerated after the chlorite rims and siliceous cements, and isnot coexist with laumontite.

4.3. Dissolution

Two stages of the dissolution are recognized in YingchengFormation The first stage of dissolution is caused by the chargingof atmospheric fluid. During the early diagenesis, the interval ofsubaerial fan-delta with shallow burial is influenced by atmo-spheric fluid which results detrital fragments and feldspar grainswere partially dissolved. But the scale of this stage of dissolutionis small, and most dissolved pores are either compacted or filledby cements. The second stage of dissolution occurred just beforethe intense gas generation stage. Organic acid generatedwith theevolution of source rock results partial to complete dissolution ofauthigenic laumontite minerals, detrital fragments and feldsparsgrains (Fig. 5G, H, I). The second stage of dissolution is the key forthe formation of high-quality reservoir.

4.4. Diagenetic sequence

As the cements are mixed layer clay, chlorite, laumontite,authigenic quartz and ferrocalcite. Nevertheless, the content ofcement varies from one area to another. As a consequence, res-ervoirs can be divided into compacted tight sandstone, chlorite-cemented sandstone and laumontite-cemented sandstone.Diagenetic and porosity evolution of these 3 types sandstones inYingcheng Formation can be concluded, based on the diageneticminerals paragenetic sequence.

Compacted tight sandstones formed by strong compactionhave low cement content, and ductile grains deformation makesthe detrital particles exhibit long, concavo-convex contact, dueto the low maturity and textural maturity. The compaction cannot only result in rotation and rearrangement of grains, but alsodeformation of ductile rock fragments, reducing intergranularvolume while weakening dissolution. From the perspective oflow cement content and burial history, densification of com-pacted tight sandstones occurred early, and was completedbefore gas charging. For compacted tight sandstones, thecompaction caused a 29% loss of porosity. Therefore, the com-pacted tight sandstones are non-reservoir in the research area.

Chlorite-cemented sandstones are common in coarse-grainedsandstones and conglomerates. Based on the overall relation-ships of the different cements identified in the chlorite-cemented sandstones, a paragenetic sequence has been devel-oped (Fig. 6). The typical characteristic is chlorite coating. Theaverage content of chlorite is 3.1%, but the thickness of chlorite

Fig. 6. The burial history, hydrocarbon generation history and reservoir porosity evolution of Yingcheng Member.

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rims varies enormously in different areas. Normal thickness isabout 5 mmand the thickest one can reach 10 mm. In spite of thickchlorite rims, chlorite-cemented sandstones can still offerpassageway for the injection of later acidic water. On this ac-count, some dissolved pores are formed and the physical prop-erty of reservoir is enhanced as a consequence. The averagesurface porosity of chlorite-cement sandstones under thin-section is 6.4%.

The principal diagenetic minerals in order of abundance inthe laumontite-cemented sandstones are chlorite coating andlaumontite. The loss of primary porosity is mainly due to lau-montite cements. Laumontite-cemented sandstone containshigh laumontite content, which can be more than 8%, and themaximum content can be up to 32%. They are mainly distributedin well-sorted and less-matrix sandstones with the form of

coaxial laumontite cement. But when the content is low, it ispatch-like in pore space. Then the laumontite cements weredissolved and formed enormous secondary pores. The averagesurface porosity under thin-section is 9%.

5. Reservoir characteristics

5.1. Physical characteristics of reservoir

The porosity of Laumontite-cemented sandstone is 8%e13%,and permeability of it is 0.4mD to 10mD (the majority are greaterthan 1mD). Under 24 MPa overburden pressure, permeabilitydecreases to approximately 30%e40% of the ground perme-ability, showing an overburden pressure permeability of 0.1mDto 1mD (Fig. 7). The physical characteristic of Chlorite-cemented

Fig. 7. Plot of porosity and permeability of different sandstones at varying levels ofnet overburden stress illustrating the overburden stress on porosity and perme-ability. For each core sample, two levels of net overburden are shown: ambientcondition 5.5 MPa and 24 MPa.

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sandstone is relatively poor and the porosity is 6%e10%, andpermeability is 0.005mD to 0.1mD. Under 24 MPa overburdenpressure, the permeability is approximately 7%e20% that of theground permeability and the underground permeability is0.0005mD to 0.002mD. The physical characteristics of thesandstones mainly formed by compaction are the worst, and thestress sensitivity of compacted tight sandstone is very strong.Under 24 MPa overburden pressure, the porosity and perme-ability are nearly zero. The porosity of laumontite-cementedsandstone and chlorite-cemented sandstone does not changemuch in the presence of the overburden pressure, which sug-gests that the volume of throat occupies a small proportion of thetotal pore volume, and reflects a strong cementation effect.

5.2. Pore throat characteristics

The contact types of laumontite-cemented sandstones be-tween grains are point-contact, due to the reason that earlierformed laumontite cement increases anti-compaction degree.But the intergranular pores are not developed in laumontite-cemented sandstones for the cause that extensive laumontitecements occupy intergranular pores. A good deal of laumontitesolution pores and solution fissures are developed because of thesolution of laumontite later (Fig. 8A). These solution pores areconnected mutually (Fig. 8B), which can improve the physicalproperty of the reservoir enormously. Constant-rate mercurypenetration results indicate that the pore radius is 120e200 mm(Fig. 9A). The average pore radius is 145.8 mm and the radius forthroat distribute mainly between 0.837 mm and 3.638 mm.Meanwhile, the average radius of throats is 2.142 mm (Fig. 9B)and the throats which radius is larger than 2 mm are 66.73%.

The chlorite rims usually form at early diagenetic stage andplay an important role in the preservation of primary inter-granular pores on the basis of former studies [16e18]. On thecontrary, chlorite coating in some part of the studying area aretoo thick, which result in filling and cutting the intergranularpore (Fig. 8C), blocking the throat, decreasing the connectivitybetween pore and throat (Fig. 8D) and finally influencing thephysical property of reservoir (especially the permeability). Forchlorite-cemented sandstone, constant-rate mercury penetra-tion results indicate that the pore radius is 120e200 mm. Theaverage pore radius is 151.5 mm (Fig. 9C) and the radius for throatdistribute mainly between 0.2 mm and 1.0 mm. Meanwhile, the

average radius of throats is 0.846 mm (Fig. 9D) and the throatswhich radius is larger than 2 mm are 6.6%.

According to the analysis of the reservoir's micro-porestructure, the connectivity of dissolution pores in laumontite-cemented sandstones is fine and the proportion of greaterthroat is significant high. Hence the throats under overburdenpressure are barely non-enclosed and the decrease of thepermeability is tiny. Although there are only very small differ-ences in the distribution of pore radius between chlorite-cemented sandstones and laumontite-cemented sandstones,throats of chlorite-cemented sandstones are mostly closed andpermeability declined quickly under overburden pressure due tothe reason that tiny throats take an exorbitant percentage.

6. The formation mechanism and distribution predictionof high-quality reservoir

In accordance with the study on the characteristics ofdifferent types of sandstone reservoirs, laumontite-cementedsandstones are the high-quality reservoir in the Yingcheng For-mation due to its extensive dissolution pores. Therefore, theanalysis of the formation of laumontite-cemented sandstones,dissolution mechanism and the prediction of the high-qualityreservoirs' distribution are the key point of further exploration.

6.1. The formation-dissolution mechanism of laumontite-cemented sandstone

Laumontite (CaAl22Si2O8$4H2O) is a typical diageneticauthigenic-mineral and the most unique cement of the deepreservoir in Songliao Basin [19]. Combined the actual situation ofthe research area and predecessors' studies [20e22], laumontiteis mainly due to alteration and dehydration of the volcanicsubstances. Volcanic substances separate out Ca2þ, Mg2þ, Fe2þ

after dehydrated alteration. Then Mg2þ, Fe2þ react with silicateto produce chlorite sediment. Furthermore, the pH of fluids inthe pore increase while the volcanic materials are alternatedcontinuously and the laumontite formed eventually.

Laumontite is unstable in the acidic fluids [23,24]. The ho-mogeneous temperature in the inclusions of Yingcheng Forma-tion reservoir is mainly distributed in 120e150 �C (Fig.10), whichreflects the hydrocarbon begin to charge from the early QuantouFormation with the burial depth about 2000m in Longfengshanarea. The charging of hydrocarbon continues since then. Beforethe intense generation of hydrocarbon, the source rocks produceorganic acid and CO2 which can form acidic fluids [25]. Massiveorganic acids dissolve the laumontite cements and secondarypores are formed [26] (1, 2). Meanwhile, the whole process isalways accompanied with the production of kaolinite and silicondioxide [27]:

CaAl2Si4O12$4H2O (laumontite) þ 2Hþ ¼ Al2Si2O5(OH)4(kaolinite) þ 2SiO2 þ 3H2O þ Ca2þ (1)

3CaAl2Si4O12$4H2O (laumontite)þ CO2¼ CaCO3þ Al2Si2O5(OH)4(kaolinite) þ 2SiO2 þ 2H2O (2)

6.2. Prediction of high-quality reservoir's distribution

The generation and conservation of laumontite require someconditions like relatively high pH (9.1e9.9) [28,29], low partialpressure of carbon dioxide and so on. Chlorites are normallygenerated in the alkalescent condition and they can hardly form

Fig. 8. Thin-section, CT and SEM microphotographs of reservoir in Yingcheng Member. A. Secondary pores developed by the dissolution of laumontite, connectivity betweenpores and throats is good, 3254.7m, B209. B. CT scanning microphotographs reveals great connectivity between pores and throats (the color blue to red means the volume ofpores increases), 3222.96m, B201-1. C. the throat is cutted by chlorite, 3022.54m, B202; D. CT scanning microphotographs reveals poor connectivity of pores and throats,3128.9m, B202.

Fig. 9. Pore and throat distributions for different types of sandstone in Yingcheng Member. A: The pore radius distribution in laumontite-cemented sandstones, B209,3256.96m; B: The throat radius distribution in laumontite-cemented sandstones, B209, 3256.96m; C: The pore radius distribution in chlorite-cemented sandstones, B202,3126.65m; D: The throat radius distribution in chlorite-cemented sandstones, B202, 3126.65m.

Y. Zhang et al. / Petroleum 2 (2016) 334e343340

Fig. 10. The distribution histogram of the homogeneous temperature in the in-clusions in Yingcheng Member of Changling fault sag, Songliao Basin.

Table 1The distribution of chlorite rims, content and the laumontite content of differentwells.

Sedimentary facies Subaerialfan-delta

Innersubaqueousfan-delta

Outsidesubaqueousfan-delta

Well name B202 B201 B201-4 B201-1 B201-13 B209

Thickness of Chlorite rims (mm) 10 3.5 2 2 0 0Chlorite content (%) 10 3.63 1.2 1.29 0 0Laumontite content (%) 0 0 8 15.6 18.3 32

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in strong-alkaline condition [30]. The well B202 is close to thesource area, and the sediments are granular and coarse-grainedsandstones, and the color is pale. They are primary debris flowand channel deposits in subaerial fan-delta. The well B209 is faraway from the source, the sediments are fine- and medium-grained sandstones and glutenite, They are distributary chan-nel deposits of outside subaqueous fan-delta. The well location,grain size of deposits and the color of the sands of the well B201-1 are between the well B202 and the well B209. It is the inner

Fig. 11. Schematic model showing high-quality reserv

subaqueous fan-delta deposits [31]. According to the observationof thin section, X-Ray diffraction analysis, the thickness andcontent of chlorite coating decrease from the source area to thecenter of lake basin and the content of laumontite cementgradually increases on the contrary (Table 1). Therefore we candefine the forming environment of laumontite and chlorite ac-cording to the variation of their content, and forecast where thehigh-quality reservoir development zones are generated even-tually. The thickness of chlorite coating ranges from 10 mm to2 mm, and the chlorite absolute content reduces from 10% to 1.2%from subaerial fan-delta to inner subaqueous fan-delta. Thechlorite coating are invisible in outside subaqueous fan-delta. Onthe contrary, the content of laumontite content increases from8% to 32% from inner subaqueous fan-delta to outside subaque-ous fan-delta, while laumontite cement is invisible in subaerialfan-delta.

Thin sections show that the contact types among grainscharacterized by point to line contact in chlorite-cement sand-stones and laumontite-cement sandstones. So it can be deter-mined that chlorite and laumontite precipitationwere formed inearly diagenetic stage before the severe tight process caused bymechanical compaction. The formation mechanism of thechange in content of chlorite and laumontite can be interpretedas follow. The subaerial fan-delta depositional district is majorlycontrol by freshwater. The charging of freshwater including at-mospheric freshwater and river decreases the pH of formationwater and makes it alkalescent. As a result, it provides the for-mation of chlorite with essential water condition. Moreover, thealteration of volcanic fragments offers abundantmaterial sourcesto the generation of chlorite. Thick chlorite coatings weredeveloped in delta plain and formed chlorite cement. During theinitial stage of the volcanic debris' alteration, the inner sub-aqueous fan-delta is controlled by both atmospheric freshwaterand lake water. The formation water is alkalescent. The Mg2þ,Fe2þ (generated by the alteration of volcanic rock fragments)react with silicate to produce chlorite sediments under thosediagenetic conditions. More alkaline ions are generated and thepH of formation water is increased with the enhancement of thevolcanic fragments' alteration. Finally, water conditions for the

oir of Yingcheng Member in Longfengshan area.

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generation of laumontite are formed. Massive laumontite areproduced within sufficient Ca2þ. The outside subaqueous fan-delta is mainly controlled by lake water and barely influencedby freshwater. The climate in Yingcheng period became semi-humid to semi-dry, and the lacustrine basin evoluted intobrackish water environment which led to the alkaline aqueousmedium in sedimentary period [32]. Meanwhile, the pH of for-mation water is increased due to the reason that the salinizedlacustrine water get command of the formation water's propertyand plenty of metal cations are released during the alteration ofvolcanic fragments. As a result, massive laumontite cements areformed. To summarize, laumontite-cemented sandstones aremajorly developed in the outside subaqueous fan-deltacontrolled by lake water. Combined the phenomenon aboveand the actual situation of the research area, we can divide thearea from the center of lake basin to the lakeshore into threeparts: lake water control zone, mixing water control zone andfreshwater control zone (Fig. 11).

Before the maturity of source rock (Ro > 0.5%), the charging oforganic acids dissolved laumontite. As for the well B209, thelaumontite is dissolved fiercely. But the laumontite in well B201-1 is dissolved gently, coaxial laumontite cementations are stillvisible yet. The depocenter of source rock in the studying arealies in the north of the well B209. Acid fluids produced by sourcerock dissolves laumontite cements nearby firstly and secondarypores are generated. The dissolution gradually diminishes in thelake shore. Hence force, it is concluded that high quality reser-voirs still exists in the north deep-sag of Yingcheng Formation inLongfengshan area.

Other cements are not formed after the dissolution of lau-montite. Gas & oil enter into the reservoir after the organic acidsdissolve the laumontite, and the formation of secondary poresmatch well with the filling of them. The products generated bythe dissolution of laumontite are taken out of the reaction sys-tem with formation water. The fluids in the pores of deep partbring the calcium and silicon produced by the dissolution oflaumontite to the updip area. Meanwhile, cements are formeddue to the decline of temperature and pressure. The convectionpushes the dissolved matters emigrate continuously, whichresult in the formation of partly secondary pores developmentzone. In addition, those laumontite cements which are not dis-solved by acid can also restrain compaction, enhancedcompressive strength of rock, and profit the conservation ofsecondary pores. Therefore, the outside subaqueous fan-delta isthe development area for laumontite cements, which is also thedevelopment zone for later dissolution. The area is the sweetspot of the further exploration. Moreover, later exploration doesindicate the deep region in the north develops high qualityreservoir and favorable potential for oil & gas exploration showsup indeed.

7. Conclusions

(1) The physical property of sandstones in Yingcheng For-mation is controlled by diagenesis. According to thecharacteristics of cements and diagenetic evolution,sandstones in Yingcheng Formation can be classified ascompacted tight sandstones, chlorite-cemented sand-stones and laumontite-cemented sandstones.

(2) The formation of secondary pores match well with thecharging of hydrocarbon, and the secondary pores withgreat connectivity between pores and throats are pre-served well in laumontite-cement sandstones. So thelaumontite-cemented sandstones are the high qualityreservoirs in the research area.

(3) The research area can be divided into three parts from thecenter of lake basin to the lakeshore: lake water control-ling zone, mixing water controlling zone and freshwatercontrolling zone. The outside subaqueous fan-delta isunder the control of lake water and laumontite cement isformed by the alteration of volcanic fragments. The lau-montite is dissolved lately and secondary pores areformed. Therefore, the deep region of Longfengshan area issupposed to be the key point of next exploration anddevelopment.

Acknowledgments

The authors gratefully acknowledge the financial support ofthe National Nature and Science Fund project (41302115) andPost-doctoral Science Fund (2012M511941).

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