Scholarly article on topic 'Characteristics and significance of the penecontemporaneous karst in lacustrine carbonate, Da'anzhai member, lower Jurassic, Beibei area, eastern Sichuan basin'

Characteristics and significance of the penecontemporaneous karst in lacustrine carbonate, Da'anzhai member, lower Jurassic, Beibei area, eastern Sichuan basin Academic research paper on "Earth and related environmental sciences"

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{"Sichuan Basin" / "Da'anzhai Member" / "Lacustrine carbonate" / "Penecontemporaneous karst" / "Tight reservoir"}

Abstract of research paper on Earth and related environmental sciences, author of scientific article — Chengpeng Su, Xiucheng Tan, Kailan Shi, Juan Zou, Feifan Lu, et al.

Abstract There are few reports about the penecontemporaneous karst of the lacustrine carbonate of Da'anzhai Member which developed on the top of Lower Jurassic Ziliujing Formation, eastern Sichuan Basin. Based on the analysis of the macroscopic and microscopic characteristics of Heping Reservoir section in Beibei area, It is discovered that the penecontemporaneous karst in the lacustrine carbonate, Da'anzhai Member, and its characters as follows: (1) The dissolved fractures and karrens are common, which have been filled with vadose silts, bioclasts, terrigenous quartzes, crystal and intergrown calcites; (2) The penecontemporaneous karstification mainly occurs in the micrites and shell limestones of the middle and upper part of the shallowing-upward sequence, and the desiccation cracks occur in some shallowing-upward sequences; (3) Three meteoric-water diagenetic lenses are identified in this section. By further study we also find the factures, stylolite and burial dissolution pores occur along the karst system (tubes, conduits and caves of the karst). And the asphalt not only fills the stylolites and fractures, but also is mixed with the vadose silt in the karst system, although the pore space formed by penecontemporaneous karstification is filled with vadose material and cements. Therefore it is concluded that the development and modification of the tight lacustrine carbonate reservoir of Da'anzhai Member, to some extent, are relevant with the penecontemporaneous karstification, and the penecontemporaneous karst reservoir may develop in the buried Da'anzhai carbonate of the basin.

Academic research paper on topic "Characteristics and significance of the penecontemporaneous karst in lacustrine carbonate, Da'anzhai member, lower Jurassic, Beibei area, eastern Sichuan basin"

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Characteristics and significance of the penecontemporaneous karst in lacustrine carbonate, Da'anzhai member, lower Jurassic, Beibei area, eastern Sichuan basin

Chengpeng Su, Xiucheng Tan, Kailan Shi, Juan Zou, Feifan Lu, Sicong Luo, Hao Tang, Xinyu Zhang

PII: S2405-6561(16)30214-0

DOI: 10.1016/j.petlm.2016.12.006

Reference: PETLM 130

To appear in: Petroleum

Received Date: 26 October 2016 Revised Date: 15 December 2016 Accepted Date: 29 December 2016

Please cite this article as: C. Su, X. Tan, K. Shi, J. Zou, F. Lu, S. Luo, H. Tang, X. Zhang, Characteristics and significance of the penecontemporaneous karst in lacustrine carbonate, Da'anzhai member, lower Jurassic, Beibei area, eastern Sichuan basin, Petroleum (2017), doi: 10.1016/ j.petlm.2016.12.006.

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Characteristics and significance of the penecontemporaneous karst in lacustrine carbonate, Da'anzhai Member, Lower Jurassic, Beibei area,

Eastern Sichuan Basin

Chengpeng Su a,c, Xiucheng Tan a,b,i, Kailan Shi c, Juan Zou d, Feifan Lu c, Sicong Luo c, Hao Tang a,b, Xinyu Zhang c

a State Key Laboratory of Oil and Gas Reservoir Geology and Exploitation, Southwest Petroleum University, Chengdu, 610500, China b The sedimentary and accumulation department of key laboratory of carbonate reservoirs, PetroChina, Southwest Petroleum

University, Chengdu, 610500, China c School of Geoscience and Technology, Southwest Petroleum University, Chengdu, 610500, China

d Exploration and Development Research Institute of PetroChina Southwest Oil & Gas Field Company, Chengdu, Sichuan 610041, China

Abstract: There are few reports about the penecontemporaneous karst of the lacustrine carbonate of Da'anzhai Member which developed on the top of Lower Jurassic Ziliujing Formation, eastern Sichuan Basin. Based on the analysis of the macroscopic and microscopic characteristics of Heping Reservoir section in Beibei area, It is discovered that the penecontemporaneous karst in the lacustrine carbonate, Da'anzhai Member, and its characters as follows: (1) The dissolved fracture and karren are common, which have been filled with vadose silts, bioclasts, terrigenous quartzes, crystal and intergrown calcites; (2) The penecontemporaneous karstification mainly occurs in the micrites and shell limestones of the middle and upper part of the shallowing-upward sequence, and the desiccation cracks occur in some shallowing-upward sequences; (3) Three meteoric-water diagenetic lenses are identified in this section. By further study we also find the factures, stylolite and burial dissolution pores occur along the karst system (tubes, conduits and caves of the karst). And the asphalt not only fills the stylolites and fractures, but also is mixed with the vadose silt in the karst system, although the pore space formed by penecontemporaneous karstification is filled with vadose material and cements. Therefore it is concluded that the development and modification of the tight lacustrine carbonate reservoir of Da'anzhai Member, to some extent, are relevant with the penecontemporaneous karstification, and the penecontemporaneous karst reservoir may develop in the buried Da'anzhai carbonate of the basin.

Key words: Sichuan Basin; Da'anzhai Member; lacustrine carbonate; penecontemporaneous karst; tight reservoir

1 Introduction

The tight lacustrine carbonate reservoir of Da'anzhai Member, Ziliujing Formation in Lower Jurassic is one of the main producing pays in Sichuan Basin [1-3]. According to former studies, the reservoir is controlled by sedimentary facies and mainly formed in the high-energy shell (bioclastic) beach developed area. The major of rock types are bioclastic limestone, shell limestone and mud-bearing shell limestone. Meanwhile, the reservoir space type is pore-fracture pattern, and fractures are the prime reservoir space and vadose channel [4-8]. There is a popular belief that the primary porosity of Da'anzhai Member's limestone is barely vanished due to the intense compaction and cementation during inchoate diagenesis. But fractures formed by later-structural rupture and moniliform corrosion pores formed by the corrosion along fractures improve the property of reservoir in rocks tremendously [4-5, 9]. The corroded pores are formed by organic material dissolved acid waters or compacted water in shale during burial period generally [6, 10]. Zheng et al. [10] once briefly introduced the sources of different stages' corroded hydrothermal fluid in Da'anzhai Member, north Sichuan. The limestone of regionally successively-sedimentary Da'anzhai part has some sedimentary hiatus in some area, according to research of first sub-member of Da'anzhai in south Langzhong area made by Liu at al. [11]. Meanwhile, the limestone are cropped out and early pores and holes are formed by the corrosion of surface water. They thought that the leading cause of the reservoir formation in the first sub-member of Da'anzhai is the shallow water of later sedimentation, which result in the effortless exposure to the surface and corrosion [11-12]. Nevertheless, this viewpoint does not get much attention in recent ten years and merely several authors mention it without deeper research [13-14]. In view of this, the paper set the Da'anzhai member of Heping Reservoir outcrop in Beibei area, east Sichuan as an example. Based on the macroscopic and microscopic characteristics, the paper studies the identification mark of penecontemporaneous karstification and its impact on the reservoir. Moreover, the paper provides referential

* Corresponding author. #8 Xindu Road, Xindu District, Chengdu, 610500, China. E-mail address: suchengpeng90@163.com (C. Su) , tanxiucheng70@163.com (X. Tan)

information to the exploration of tight oil reservoir of lacustrine-carbonate in Da'anzhai member. 2 Geological Background

The Long'men Mountain in west Sichuan keeps rising due to the influence of later Indosinian Movement, vast regions of Sichuan Basin turn into inland lake basin. The Xujiahe Formation of upper Triassic, Ziliujing Formation, Shaximiao Formation, Suining Formation and Penglaizhen Formation of Jurassic are deposited in the basin from the bottom up. Meanwhile, the area of lake surface in Da'anzhai member, Ziliujing Formation, lower Jurassic is the widest among them. The area of the lake surface is more than 10*104 square kilometers and the asymmetric vast lake basin is steep in the north and gently in the south [15-16]. Furthermore, Da'anzhai deposit period has an integrated cycle: lake transgression, maximum lake transgression and lake regression. The third sub-member of Da'anzhai is corresponding to the initial stage of lake transgression and the main lithology is shell limestone (bioclastic limestone) mixed with some shale. The second sub-member of Da'anzhai corresponds to the deepwater stage of maximum lake transgression; sediments are mainly dark-color shale mix with some micrites and little shell limestone. Meanwhile, the first sub-member of Da'anzhai corresponding to the lake regression stage is the main reservoir-developing Formation for Da'anzhai [17-19], which generate abundant shell limestone (bioclastic limestone) mixed with a little dark shale. On the plane (Fig. 1), the depocenter of lake basin during Da'anzhai stage, locates over Yilong and Yinshan. Semi-deep lake, shallow lake to semi-deep lake, shore-shallow lake, shore lake and river facies are developed from the center to circum [20-21]. The shell beach (bioclastic beach) is primarily composed by shell limestone (bioclastic limestone), which is intensely distributed in upper slopes of shallow lake, shore-shallow lake outward-surrounded the depocenter [22-23].

Fig. 1. Geographical location map of study section and palaeogeogrphy of Da'anzhai Member, lower Jurassic in Sichuan Basin

(modified from Zheng et al., 1997) The research section is belonging to high fold belt of eastern Sichuan basin, which locates in the Heping reservoir, Dabagou, Tianfu town, Beibei area, Chongqing (Fig. 1). Structure activity of eastern Sichuan basin and Sichuan basin's periphery are relatively stable in Da'anzhai stage, and there are no obvious lifting and settling [12, 17]. Furthermore, Shaximiao Formation, Suining Formation and Penglaizhen Formation are all conformable contact in Beibei area. Eastern Sichuan basin area experiences intense folding due to the effect of Yanshan Movement and Himalayan Movement, which generate nowadays high fold belt in eastern Sichuan basin after the extruding lifting and transformation of the Jurassic and strata under it [24].

The research area locates in shore-shallow lake facies of south lake basin. Rock types are multiple, which are

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mainly amaranthine calcareous mudstone in shore lake and dark-gray micrite in shallow lake. Meanwhile, shell limestone and bioclastic limestone which constitute shell beach and bioclastic beach are partly developed and the thickness of one single layer is less than 1.5 meter. Besides, micritic dolomite with exposed mark (mudcrack or penecontemporaneous karst) is developed at the top of the sedimentary sequence (Fig. 2).

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Fig.2. Comprehensive stratigraphic column of Da'anzhai Member of Heping Reservoir section in Beibei area, eastern Sichuan Basin

3 Material and Methods

The present study is based on a detailed description of sedimentary facies (Fig. 2) of one major outcrop section (the Heping Reservoir section) of the Ziliujing Formation Da'anzhai Member in eastern Sichuan Basin, China (Fig. 1). The length of the Heping Reservoir section is about 26.64 m. Lithology section was made in the field and laboratory to portray it. Sampled specimens were cut and polished for slabs and thin sections for microscopic observation of lithofacies. Besides, the core photos of Well Mo030-H31 are used to illustrate that penecontemporaneous karst also occurred in nearby area.

4 Characteristics and identification mark of penecontemporaneous karstification

4.1 Identification mark of lacustrine carbonate's penecontemporaneous karst

Penecontemporaneous karstification occurs in penecontemporaneous atmospheric diagenetic environment and it is controlled by secondary sedimentary cycle and eustatic change (or lake level change). In marine regression (or lake regression) sedimentary sequence, granule beach facies with high sedimentary rate and limestone and dolomite in very shallow water are crop out of the water or located in fresh water lenticle along with the temporarily relatively-descending of sea level (or lake level). Moreover, they are leached by the meteoric freshwater contained with massive carbon dioxide and then all size, shape and corroded pores, holes, fractures and fillings are generated in carbonatite [25 - 26]. Meanwhile, the karst host rock is eogenetic rock with high porosity-permeability during the penecontemporaneous exposure period. And that type rock mainly uses intergranular pores for karst water transferring, and accordingly the so-called "piebald" karst system develops [27 -28].

Studies about penecontemporaneous karst of lacustrine carbonate are plentiful in existing research achievements and a number of consensuses are achieved [25 -28]. In contrast, research about penecontemporaneous karstification developed in lacustrine carbonate is rare and special reports of its identification mark are not found either. Consequently, the paper takes the identification mark for penecontemporaneous karst of marine carbonate as a reference and combines the geological background of Da'anzhai member, lower Jurassic, Sichuan basin with characteristics of lacustrine carbonate's meteoric freshwater leaching. Finally, the paper summarizes the principal identification mark of lacustrine carbonate's penecontemporaneous karst in the section.

(1) Nonselective dissolved-fractures and karrens are developed and they are filled by vadose silt, terrigenous quartz granule and biological fragments in varying degrees, which is the typical marker of vadose zone of meteoric water within diagenetic environment [29]. While, only nonselective dissolved-fractures and karrens are found and no selective-corroded pores and holes are visible in the Da'anzhai member of the section. Dissolved-fractures and karrens are reticular and unidirectional (Fig. 3a-c). The width of single karren (or dissolved-fracture) varies from several centimeters to less than 0.1 millimeter. The karst fillings of them are majorly carbonatite silt similar to the layers, a small amount of terrestrial quartz granules and fragments of fish, ostracodes and bivalves (Fig. 3d-f). Moreover, most of fillings accompany asphalt. The 1third stratum's micrites limestone which develops desiccation crack structure develops dissolved-fractures and karrens filled with crystal grain calcite or intergrown calcite (Fig. 3g, h). The phenomenon occurs due to the reason that dissolved-fractures and karrens formed by penecontemporaneous karstification are not filled at the early stage and they are filled by crystal grain calcite and intergrown calcite afterwards.

Besides, through the core observation of Well Mo030-H31, we found that the carbonate rock of Da'anzhai Member develops typical exposure markers, such as unidirection karrens, karst breccias and "piebald" karst system (Fig. 3j, k), which can illustrate that penecontemporaneous karst also occurred in nearby area.

(2) Dissolved-fractures and karrens are developed at the middle and the top of upward-shallowing sequence. Seen from the point of sedimentary sequence, dissolved-fractures and karrens formed by penecontemporaneous karstification show up with high frequency vertically. However, controlled by sedimentary sequence, they are generated at the middle and the top of upward-shallowing sedimentary sequence. The vertical influence depth of

single karst system is less than 1.5 meter generally (Fig.2). Moreover, the top of upward-shallowing sedimentary sequence, which generates in the karst possess representatively exposure mark: desiccation crack structure (Fig. 3i). It symbolizes that lacustrine carbonate crops out to the land along with relatively decline of lake level shortly after its deposition.

Fig.3. Discriminating markers of penecontemporaneous karst in Da'anzhai Member of Heping storage section and core photos of Well

Mo030-H31

a- reticulate dissolved fracture, karren, asphalt infect, polarized light(PPL), the top of layer 3; b- unidirection karren, PPL, the lower-middle of layer 3; c- unidirection karren, polished hand specimen, the middle of layer 3; d- carbonate silt d filling karren, crossed-polarized light(XPL), the upper of layer 22; e- bioclast filling dissolved fracture, PPL, the middle of layer 3; f-terrigenous detrital filling karren, XPL, the lower-middle of layer 3; g-crystalline calcite filling karren, PPL, the middle of layer 12; h- crystal dolomite filling karren, dissolved fracture, PPL, the upper of layer 13; i-mudcrack, the top of layer 13; j- unidirection karren and karst breccia, 1430.47-1430.65m, Da'anzhai Member, Well Mo030-H31; k- "piebald" karst system,

1430.65-1430.84m, Da'anzhai Member, Well Mo030-H31

4.2 Meteoric water diagenesis lens of lacustrine carbonate

Meteoric water diagenesis lens sets the water table as the interface. They are generally divided into two parts: vertical vadose zone at the top and horizontal phreatic zone at the bottom. Vadose zone above the water table is an open system; atmospheric water and air exist in the pore space at the same time. Meanwhile, they are unsaturated state to the carbonate and vertical corrosion system is formed when they are migrating downwards due to the

gravity. Phreatic zone beneath the water table is open to the atmosphere via the vadose zone between water table and land surface. Groundwater migrates horizontally along the primary pore and sedimentary interface and horizontal corrosion system is formed ultimately [30].

In accordance with observation of field section and analysis of 45th thin sections, stratum karst developed and stratum compactum are observed alternately. Three meteoric water diagenesis lenss are identified roughly (Fig. 2). Their positions from the bottom up are: ® the crystal shell limestone in shell beach facies of the third layer; © micritic dolomite and micrites in shore-shallow lake facies of 12th -13th layer; © bioclastic limestone and micrites in shallow lake facies of 2first-2second layer at the top of the section. The thicknesses of them are 1.34 meter, 1.15 meter and 1.07 meter. The penecontemporaneous karst of crystal shell limestone in shell beach facies of the third layer is the most well-developed and representative. Thus, the paper takes it as an example to introduce the vertical growth characteristics of meteoric freshwater lenticle in Da'anzhai member (Fig. 4).

Dissolved-fractures and karrens formed by penecontemporaneous karst at the top of the third layer in shell beach are net-like and intensive. What's more, bed rocks are broken and brecciform (Fig. 3a). Crystal shell limestone in the middle mainly develops unidirectional dissolved-fractures and karrens and they are narrower and less intensive than the top. Moreover, horizontally unidirectional dissolved-fractures and karrens, a small amount of vertically dissolved-fractures and karrens that connected horizontal fractures and karrens are at the bottom. The width of horizontally corrosion system is stable crosswise, but the amount is small. The developmental feature of dissolved-fractures and karrens in different parts of shell beach reflect the fact that penecontemporaneous karstification has disparate mechanism in different parts of shell beach. Finally, combined with the hydrological characteristics of marine meteoric water diagenesis lens and the growth characteristics of penecontemporaneous karst in the third layer of shell beach, the paper summarizes the growth characteristics of meteoric water diagenesis lens in lacustrine carbonate.

thickness karst profile karst zone description of karst characteristic

25 nrf-ßjz. meteoric freshwatei vadosezone of subaerial environment a large number of karren distributed in all directions; part of the bedrock was brecciform

55 TP-' m meteoric freshwater vadose zone karren and dissolved fracture development zone; given priority to with vertical karren and dissolved fracture, local developed smaller horizontal karren

30 meteoric freshwater phreatic zone horizontal karren development zone; the two level by vertical karren and dissolved fracture connected

35 —1—j-L 9 9 slow flowing belt is not affected by meteoric fresh water; did not see karren and dissolved fracture

fro | ..,.-/ | - 1 | 1 j 1 | 11 i J |

§ karren dissolve fracture d karst calcilutite biomicrite crystal shell fillings limestone

Fig.4. Column showing the characteristics of penecontemporaneous karstification in Da' anzhai carbonate, Layer 3, Heping Reservoir

section

At depositional stage, the shell beach deposits shell limestone with the foundation of granular micrites at the top of partial underwater palaeohigh. Meanwhile, its deposition rate is higher than circumjacent sediments (Fig. 5-1). When the construction is relatively stable and the deposition is continuous, the lake level drops comparatively and shell beach crops out of water and its growth terminates (Fig. 5-2). The thickness of shell beach cropped out is small during the early stage. Occasional atmospheric precipitation can merely form miniature meteoric freshwater

lenticle. Meanwhile, massive carbon dioxide contained in atmospheric precipitation promotes the corrosion of calcite precipitated from brackish lake and small-scale and unconspicuous zoning karst system comes into being (Fig. 5-3). The outcrop of shell beach expands along with the drop of lake level, which results in enlarging the distance between beach-top and water table. Then, the thickness of meteoric freshwater vadose zone is growing and vertically dissolved-fractures and karrens grow before karstic water reaches the water table. Net-like and high-density dissolved-fractures and karrens are formed at the top of shell beach due to the reason that it crops out and goes through the process of meteoric freshwater. Moreover, it belongs to the subaerial environment at the top of meteoric freshwater diagenetic vadose zone. The middle of shell beach is mainly vertical corrosion and it is vadose zone of meteoric water diagenesis lens. Meanwhile, lower-middle part of shell beach is primarily horizontally dissolved-fractures and karrens. Horizontal denudation takes an important role and it belongs to the phreatic zone

Fig.5. Sketch showing the depositional evolution and the characteristics of meteoric-water diagenetic lens in shell bank, Da'anzhai

Member, Beibei and neighboring areas

5 Discussions

5.1 The reason whypenecontemporaneous karst in the research section is lack of fabric selective corrosion

Although a large amount of biological particles like bivalve, gastropods and fish fossils are found in penecontemporaneous karstification developed layers of Da'anzhai member in Heping Reservoir section, intragranular dissolved pores, mold pore and intergranular dissolved pores corroded by typical fabric selective corrosion of penecontemporaneous karstification are not found. Moreover, only dissolved-fractures and karrens formed by nonselective denudation are developed. In reaction to the phenomenon, the paper does some research on original mineral constituent and diagenetic evolution of shell granules. The primary structures of fish fossils in Da'anzhai member are well preserved (Fig. 6a). A small part of bivalve shells' primary fabric is preferably preserved and foliated calcite can be identified in the slice (Fig. 6b, c). On the contrary, most of bivalve and gastropod shells are composed by crystalline calcite, which primitive structure is unable to identify (Fig. 6d, e).

Original mineral components of fish's skeleton are majorly calcium phosphate mineral, which are difficult to be corroded and transferred. Therefore, primitive interior structure of them is well preserved. The original mineral component of some well-preserved bivalve fossils is highly possible calcite. Thus, they are not selectively corroded in the process of sedimentation and diagenesis and well preserved [31]. The vast majority of bivalve and gastropod in crystalline grain structure are speculated to be aragonite that are original mineral components, but it turns out to be crystalline calcite under microscope nowadays.

The change of biological shell mineral component might be filled by crystalline calcite cement at latter diagenetic process after massive mold pores formed at the early stage of selective corrosion. Another reason is that shell mineral experience fast neomorphism before the penecontemporaneous karstification and aragonite translates into calcite [31]. What is more, the recrystalization of calcite results in the blurry boundary between some shells and cements (Fig. 6f). Further study shows that crystalline shell fossils are mainly developed at the third layer of the section and the content of shell can reach more than 80%. If rocks suffer selective corrosion and then develop plenty of mold pores, they will be short of supporting structure and collapsed breccias are formed under compaction. Although interior structure of the third layer's shells in the research section disappears, the shape of them can still be identified. Hence, it is concluded that the crystalline shells in the research section are not formed by corroded first and filled afterwards, but formed by shells' (their primitive components are aragonite) fast neomorphism and recrystalization. The stability of unstable aragonite or calcite with high content of magnesium in biological grains after fast neomorphism is similar to the fresh water (or brackish water °) calcite right deposited in the lake. Therefore, fluids contained with massive carbon dioxide react with calcite in the layers (CO2+ CaCO3===Ca(HCO3)2) at meteoric freshwater leaching process and soluble Ca(HCO3)2 are generated. Eventually, nonselective dissolved-fractures and pores are formed in strata.

Fig.6. Characteristics of inner structure of biont grains of Da'anzhai Member in Heping storage section , polarized light a- fish fossils, the middle of layer 3; b- the foliaceous structure of bivalve shell, the middle of layer 3; c-double layer structure of bivalve shell, the lower of layer 21; d- the crystal structure of bivalve shell, the middle of layer 22; e- the crystal structure of gastropod shell, the middle of layer 3;

f-blurry boundaries between bivalve shell and cement, the middle of layer 3.

In summary, the primitive mineral component of well-preserved shells in research section's Da'anzhai member is calcite, which is not selectively corroded in meteoric freshwater. The original mineral component of crystalline biological shells is aragonite, but it transfers to stable calcite after fast neomorphism at the early stage. Therefore, only fabric nonselective-corroded dissolved-fractures and karrens are found in the

° According the research of Wang et al (2006), the lakes in Zigong area of Lower Jurassic, Sichuan Basin are open freshwater lakes, but lakes in Hechuan area is brackish-water lakes with certain closure. The research section locates in the southeast side of Hechuan area and it might be brackish-water environment.

penecontemporaneous karst of Da'anzhai member in the research section and no fabric selective-corroded intragranular pores and mold pores.

5.2 Penecontemporaneous karst of lacustrine carbonatite's meaning to the accumulation of oil & gas

Penecontemporaneous karstification occurs in penecontemporaneous period when the sediments are not completely concreted. Well-connected reservoir system is formed after the leaching of meteoric freshwater due to its excellent porosity and permeability [32-33]. Meanwhile, new dissolved pores and intrinsic pore space are easily filled by vadose silts and cements for the same reason that penecontemporaneous karstification occurs in penecontemporaneous period. Nevertheless, residual porosity can be used as reservoir for oil & gas [34-35]. Fringe system of marginal platform facies in Lianglitage formation, upper Ordovician, middle Tarim area and Ordovician carbonatite in Yingmaili-Halahalang area of Tarim basin develop carbonatite reservoir formed by penecontemporaneous karstification [25, 35].

After the study of Da'anzhai member, Lower Jurassic, Sichuan Basin, we find that drilling tool relief and mud loss take place in some drills of northern Sichuan Basin when they are drilling Da'anzhai member: 2916 meter and 2917 meter of Well Chuan43 in Shilongchang; 2830.6 meter to 2832.4 meter of Well SL2 and 2843 meter to 2847 meter of Well SL11 in southern Langzhong; 2952 meter to 2952.2 meter of Well Chuan48. Meanwhile, the Da'anzhai member of Well Chuan43 in Shilongchang develops dissolved pores, caves and fractures. Mold pores formed by early exposure and corrosion develop in Chuan48 well [4, 7, 12]. In addition, mold pores formed in shells and ichthyodont are striking in the first sub-member of Da'anzhai, Well Chuan47 and the third sub-member of Da'anzhai, Well Chuan44 [4]. Those pore spaces formed by early exposure and corrosion provide the emigration and accumulation of oil & gas with much advantages.

Reservoir system formed by penecontemporaneous karstification in Da'anzhai member of the research section is almost filled by vadose silts and precious little residual porosity is left. But pore space increases markedly after the remold of later diagenesis, tectonism and buried denudation.

( 1 ) Superimposed transformation of diagenesis. At diagenetic stage, rocks can generate fractures because of the pressure from overlaying strata and syneresis, desiccation crack and recrystalization. These fractures are called diagenetic fractures. Moreover, filled by unconsolidated and water-rich vadose silts, dissolved-fractures and karrens generated by penecontemporaneous karstification form karst-related irregular tension fissure after the syneresis at diagenetic process (Fig. 7a).

( 2 ) Superimposed transformation of tectonism. Analyzed from mechanical property, the position where karst develops is the discontinuity point in mechanical property and the anti-pressure ability of rocks decreases [11, 36]. Moreover, it is the same place where tectonic disruption takes place under equivalent pressure [11]. In the research section, fractures superimposed transformed above early penecontemporaneous karst system by tectonism are extremely representative and they are well-developed at the top of the third layer shell beach (Fig. 7b). Meanwhile, some isolated-distributed tectonic fractures are found in the limestone of Da'anzhai member except from tectonic fractures developed along with karst system. In addition, parts of tectonic fractures are filled by crystalline calcite (Fig.7c) and only a small amount of them are not filled (Fig.7d).

( 3 ) Superimposed transformation of buried denudation. Stylolite is developed in limestone after intense compaction and pressolution and its porosity is lower than one percent. Nevertheless, some strata develop buried corroded pores in such tight limestone and the shell limestone in beach facies of 14th and 15 th layers are the most typical. In the research section, dissolved-fractures and karrens formed in buried corrosion along with penecontemporaneous karstification are re-corroded and moniliform dissolved-pores and cavern system with obvious intention of developing along early karst system are formed (Fig. 7e). This phenomenon happens for the main reason that primitive pores in limestone are basically vanished after intense compaction and later corroded fluids cannot emigrate via compact bed rock. Meanwhile, although previous karst system is backfilled by vadose

silts, differential compaction caused by the discrepancy in material composition makes well-connected karst system is still capable of physical property to some extent even after such intense compaction (Fig. 7f). Therefore, latter dissolved fluids re-corrode along the penecontemporaneous karst system and well-connected pore and cavity system is formed [34].

Fig.7. Characteristics of superimposed transformation on the penecontemporaneous karst system, photomicrograph a-diagenetic fracture, the middle of layer 12; b-tectonic fractures associated with karst, filled by asphalt and vadose silt, the upper of layer 3; c-an unfilled tectonic fracture, the lower-middle layer 3; d-tectonic fracture filled by crystalline calcite, the lower of layer 13; e-graniphyric dissolved pore, cave, the middle of layer 3; f-vadose silt infected by asphalt in penecontemporaneous karst system, the lower-middle layer 3.

In conclusion, penecontemporaneous karstification plays positive effect on the reformation of reservoir. Not only fractures and karrens system formed by karstification, but also secondary corroded pores and tectonic fractures formed by component and tectonic stress variation in penecontemporaneous karst can be used as advantageous space for the emigration and accumulation of oil & gas. Therefore, the developed of penecontemporaneous karst in Da'anzhai member, Heping Reservoir section, Beibei area provides the exploration of tight oil in lacustrine carbonate Da'anzhai member, Ziliujing Formation, lower Jurassic, Sichuan basin with new train of thought. Eastern and northern Sichuan basin is similar to the condition, which is located in shore-shallow lake during the Da'anzhai period. In that condition, penecontemporaneous karst may exist in places where shell beaches (or bioclastic beach) are well-developed and much attention should be paid in future exploration. 6 Conclusions

(1) Types of pore spaces formed by penecontemporaneous karstification in lacustrine carbonate, Da'anzhai member, Heping Reservoir section, Beibei area are non-selectively dissolved fractures and karrens, which can be divided into net-like, vertical, horizontal dissolved-fractures and karrens according to their occurrence. There is no selectively dissolved pore space.

(2) Biological particles (original mineral composition is aragonite) experience fast neomorphism in the early stage, which might be the primary reason why penecontemporaneous karst of Da'anzhai member in the research section lack of representatively intragranular dissolved-pores and mold pores formed by fabric selective corrosion.

(3) Penecontemporaneous karsts in Da'anzhai member are majorly developed in micrites and shell limestone at the middle and the top of upward-shallowing sequence. Most of pore spaces formed in penecontemporaneous karsts are filled by vadose silts, biological particles, quartz grains and crystalline calcite, which are usually infected by asphalt. Preferable-connected and unfilled new fractures and pore spaces are formed after the

superimposed transformation of tectonism, diagenesis and buried dissolution, which offer spaces to the storage of oil & gas.

(4) The developed and transformation of tight reservoir in lacustrine carbonate in Da'anzhai member might have some relationship with the existence of penecontemporaneous karsts. Moreover, penecontemporaneous karst reservoirs are likely preserved in subsurface of the basin. Therefore, the discovery of penecontemporaneous karsts in Da'anzhai member of the section provides the exploration of tight oil in lacustrine carbonate of Da'anzhai member, Ziliujing Formation, lower Jurassic, Sichuan basin with new train of thought. Acknowledgement

Professor Jingshan Chen of Southwest Petroleum University and Dr. Jiangang Wang of Institute of Geology and Geophysics, CAS offer me valuable guidance and help in the research. I would like to give my sincere gratitude to them! This sudy was supported by Unconventional Oil & Gas, Scientific Research Innovation Team Building Project, Universities Affiliated to Sichuan Province (Provincial Teaching Designed 127 Education Office of Sichuan Province).

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