Characteristics and formation mechanism of Changxing Formation-Feixianguan Formation reef-shoal reservoirs in Yuanba Gasfield

Petroleum Research (2016) 2,123-134

Characteristics and formation mechanism of Changxing Formation-Feixianguan Formation reefshoal reservoirs in Yuanba Gasfield Yongsheng Ma*, Xunyu Cai and Peirong Zhao China Petroleum & Chemical Corporation, Beijing 100728, China Received May 12, 2016; Accepted September 1, 2016

Abstract: Taking advantage of the successful experience in exploring and discovering the Puguang Gasfield, and targeting the Changxing Formation-Feixianguan Formation organic reef and shoal lithological trap, SINOPEC drilled Well Yuanba 1 in the Bazhong area in Northeast Sichuan in 2006, and discovered the Yuanba Gasfield with a high-production commercial gas flow of 503×103 m3/d. As a normal-pressured lithological gas reservoir with high H2S content, Yuanba Gasfield is characterized by weak tectonic deformation and deep burial, with an average depth of 6,600 m in the central part of the gas reservoir, and is the deepest marine gas field in the Sichuan Basin. Yuanba Gasfield is dominated by large-scale reef-shoal reservoirs of Changxing Formation. The formation of the reservoirs was primarily controlled by early meteoric freshwater dissolution and dolomitization, and less affected by deep-burial dissolution and tectonic movement. A comparative analysis was made on the characteristics of deep reef-shoal reservoirs in the Yuanba and Puguang gas fields so as to explore their formation mechanisms. It is concluded that the reservoir size and early pore development was controlled by early depositional-diagenetic environment. Fracture formation and dissolution were controlled by structure–fluid coupling, pore reworking and preservation is determined by fluid–rock interaction.

Key words: Yuanba Gasfield; Puguang Gasfield; Upper Permian; Lower Triassic; reef-shoal; ultra-deep formation; carbonate reservoir

1 Introduction Taking advantage of the experience in exploring Puguang Gasfield, Yuanba Gasfield was discovered on the platform margin in the west of Kaijiang-Liangping continental shelf. Natural gas production horizon of Yuanba Gasfield is 1500 m deeper than that of Puguang Gasfield, and Yuanba Gasfield is currently the deepest marine gas field in China, which further prove the fact that high quality pore-type reservoirs exist in deep and ultra-deep formations. Reservoirs of both

Yuanba and Puguang gas fields are Permian and Triassic organic reef–shoal dolomite. The formation and development of high quality reservoirs in both gas fields are similar to some extent, but still differ greatly. To guide the exploration of deep carbonate hydrocarbon reservoirs, this paper expounds the reservoir characteristics of Yuanba Gasfield, compares the main controlling factors of formation of high quality reservoirs in both gas fields, and further discusses the development and preservation mechanism for high quality carbonate reservoirs in deep and ultra-deep formations.

* Corresponding author. Email: [email protected]

© 2017 Chinese Petroleum Society. Publishing Services by Elsevier B.V. on behalf of KeAi. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). 123

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2D seismic survey lines were deployed and implemented, totaling 408.2 km. The seismic interpretation showed that Changxing Formation-Feixianguan Formation in the Yuanba area had similar reef-shoal seismic reflection characteristic to that of Puguang Gasfield. Based on the understanding, previous exploration idea of prospecting tectonic traps was changed, instead a new exploration idea was proposed, i.e., “trying to explore Changxing Formation-Feixianguan Formation reefshoal dolomite structure-lithological complex trap”. In March 2006, the 2D seismic data were used to deploy Well Yuanba 1, mainly to explore the Feixianguan Formation-Changxing Formation organic reef-shoal lithological trap. Drilling of this well was initiated on May 30, 2006 and completed in May 2007. Well testing achieved a low-production gas flow of 3000 m3/ d. Interpretation of newly collected 3D seismic data suggested that body of organic reef trap was not encountered in drilling. Afterwards, the southwestward sidetrack drilling was carried out on Jul. 18, 2007 and completed at a depth of 7427.23m on Sep. 14. Testing on Nov. 19 achieved a high-production commercial gas flow of 503×103 m3/d, and Yuanba Gasfield was thus discovered (Fig. 1).

2 Discovery of Yuanba Gasfield Exploration work in Yuanba area in the Northeast Sichuan, began from near-surface petroleum geological survey in 1950s. Afterwards, the Second Geophysical Prospecting Brigade of the former Southwest Petroleum Bureau, Ministry of Geology and Mineral Resources carried out seismic reconnaissance survey and general survey during 1967-1983, when terrestrial shallow wells such as Chuanhua 52, Chuanfu 69, Chuanfu 56, and Chuantang 70 were drilled. Hydrocarbon indication was observed in Daanzhai member of Upper Triassic Xujiahe Formation, but no commercial gas flow was obtained in well testing. After 1990s, the exploration has been at a standstill. After gaining the exploration right of Bazhong area in 2002, Southern Exploration and Development Company, SINOPEC had enhanced regional geological researches by taking advantage of successful experiences of discovering Puguang Gasfield. It was confirmed that Yuanba area was located on the platform margin in the west of Kaijiang–Liangping continental shelf during the late Permian-early Triassic and had a regional geological setting of forming organic reef–shoal deposits. In 2003, eight 0

50km

Guangyuan IV IV 1

Tongjiang

IV 2

Nanchong I

III Ya’an

Dazhou

I2

Dayi Chengdu

II 3

IV 3

Langzhong

Mianyang

III 1

N

Nanjiang

Liangping

Suining

Wanxian

II 1 Shizhu

II

I1

III 2

Fuling Leshan

Weiyuan

Yibin

Chongqing

Luzhou

I3 II 2

Study area

Primary structural belt

Secondary structural belt

Fault

Gasfield

Oilfield

Character I represents Central Sichuan gentle fold area, I1 represents Weiyuan–Longnvsi massive uplift structural belt, I 2 represents Central Sichuan ladder low flat structural belt, I3 represents Zigong low and medium fold structural belt; Character II represents East Sichuan high steep fold area, II1 represents East Sichuan high steep structural belt, II2 represents East Sichuan high and medium complex structural belt, II3 represents Northeast Sichuan imbricated complex structural belt; Character III represents West Sichuan nappe fold area, III1 represents Guanxian-Mingshan high and medium structural belts, III2 represents Longquanshan-Xiongpo nappe belt; Character IV represents North Sichuan low flat fold area, IV1 represents Longmenshan piedmont nappe belt, IV2 represents Zitong gentle structural belt, IV3 represents Tongjiang low flat structural belt

Fig. 1  Structural location of Yuanba Gasfield

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Based on phase 1 and phase 2 three-dimensional seismic data, nine exploratory wells were drilled according to a crisscrossshaped large-profile deployment during 2007-2008. Highproduction commercial gas flows were achieved in platform margin reef–shoal reservoirs of Changxing Formation at such wells as Yuanba 2, Yuanba 11, Yuanba 12, Yuanba 101, and Yuanba 102, generally controlling the distribution of platform margin reef-shoal facies belt of Changxing Formation.

than Puguang Changxing Formation gas reservoir, the Yuanba Changxing Formation gas reservoir is the deepest gas reservoir ever found in the Sichuan Basin. According to the drilling revelation, in current high tectonic setting, the lithological gas reservoir has a high gas fullness degree and no water has been found. At a low tectonic site, the lithological gas reservoir has bottom water and edge water characteristics. Burial depth of gas-water contact varies different gas reservoirs.

In May 2009, in accordance with an overall idea of “integrated deployment, stepwise implementation, and explorationdevelopment integration”, well drilling was deployed and implemented in two rounds, and verified overall reserve of Changxing Formation gas reservoirs. By the end of 2012, accumulated proven reserve of natural gas in marine formation of Yuanba Gasfield was up to 219.5×109m3, among which reefshoal gas reservoir of Changxing Formation had a proven reserve of 183.42×109 m3, while the oolitic shoal gas reservoir of Feixianguan Formation had a proven reserve of 25.27×109 m3 and a tertiary reserve up to 515×109 m3.

According to the estimated relationship among formation temperature, pressure and burial depth, the gas reservoir has a geothermal gradient of 19.4-20.5°C/km and a pressure coefficient of 1.01-1.12, suggesting a normal-pressure lowgeothermal-gradient system. Hydrocarbon constituents of the natural gas are primarily methane, while non-hydrocarbon constituents are mainly H 2S and CO2 . The average methane content, H2S content and CO2 content are 88.35%, 5.22%, 6.43%, respectively. Stable per-well production is (200-2400)×103 m3/d.

3 Geological characteristics of Yuanba Gasfield 3.1 Yuanba Gasfield is a lithological gas reservoir with high H2S content in ultra-deep formation The Yuanba Gasfield is tectonically located at the junction between North Sichuan low flat fold area and Central Sichuan gentle fold area in Sichuan Basin (Fig. 1), showing gentle formation attitude, weak tectonic deformation and undeveloped faults. T he gas reser voi r is composed of a number of independent organic reefs and bioclastic shoals. Gas reservoir enrichment is controlled by lithology, characterized by a distribution pattern of “one reef, one shoal, one trap and one gas reservoir”, though gas reservoir units differ slightly in gas thickness, gas-water relation, and fluid property. 3.1.1 Changxing Formation reef-shoal lithological gas reservoir is a normal-pressure medium to high-production gas reservoir Yuanba Changxing Formation gas reservoir is a stratoid lithological gas reservoir that develops from platform-margin organic reefs and shoals which stacked into patches in a paleostructural high setting. The gas reservoir is controlled by distribution of platform-margin reef-shoal reservoirs and backreef bioclastic shoal reservoirs. Yuanba Changxing Formation gas reservoir is characterized by shallow buried depth in the west and south and deep buried depth in the east and north. The buried depth of the central part is 6327.9-6897.5 m, averaging 6600m. The maximum depth is 7013.7 m and the height is 48.7-481.8 m. 1000-1500 m deeper

In summary, Yuanba Changxing Formation gas reservoir is a fracture-pore type large-sized organic reef-shoal lithological gas reservoir, characterized by high H2S content, medium CO2 content, ultra-deep formation, medium-to-high production, dominant elastic gas drive and partial edge water or bottom water drive. 3.1.2 Feixianguan Formation oolitic shoal gas reservoir is a high-pressure low-production gas reservoir In high paleostructural setting, Yuanba Feixianguan Formation is a lithological gas reservoir formed by open platform-platform margin oolitic shoals which stacked into patches. Gas reservoir is mainly controlled by the distribution of oolitic limestone reservoir. Feixianguan Formation gas reservoir is 6317.0-6387.5 m deep in the central part, and the height of the gas reservoir is 284-425 m. Neither present drilling results nor logging interpretations shows any water layers, The gas-drive mechanism is dominated by elastic gas drive. The gas reservoir has a pressure coefficient of 1.95-1.96 and an average geothermal gradient of 21.1°C/km, suggesting a high-pressure low-geothermal-gradient system. Average methane content, H2S content and CO2 content of the natural gas are 94.21%, 1.72% and 3.35% respectively. Moreover, per-well production is (36-100) ×103 m3/d. The gas reservoir in Member 2 of Feixianguan Formation is a fracture-pore type oolitic shoal lithological gas reservoir, characterized by ultra-deep formation, high pressure and low geothermal gradient, elastic gas drive, medium H 2S and CO2 content.

3.2 Characteristics of Upper Permian Changxing Formation organic reef-shoal reservoir The gas in Yuanba Gasfield is mainly distributed within Upper Permian Changxing Formation organic reef-shoal reservoir.

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Lithology of the reservoir is predominantly relict bioclast crystalline dolomite, organic reef dolomite and sparry bioclastic limestone. Reservoir spaces are dominated by intercrystalline dissolution pores and intercrystalline pores, followed by interparticle dissolution pores, intraparticle dissolution pores, intracoelom dissolution pores and dissolution pores among organic frameworks (Fig. 2), vugs and fractures are relatively less. 2mm

1mm

(a)

(b)

0.5mm

1mm

(c)

According to seismic and drilling revelation, Yuanba Gasfield has characteristics of fore-reef and back-reef shoals on the horizontal level. The organic reef belt is composed of multiple organic reefs with variable sizes, and bioclastic shoals with variable thicknesses deposited between and behind organic reefs. The reefs or shoals are connected by inter-reef, inter-shoal and tidal gullies as well as tidal channels. Stratoid Changxing Formation reef-shoal gas reservoir was formed through continuous superposition of shoals exposed on the top of organic reef group and back-reef bioclastic shoals.

3.3 Characteristics of Lower Triassic Feixianguan Formation oolitic shoal reservoir

(d)

(a) coral coelom pores in sparry organic reef dolomite, Well Yuanba 102 at a depth of 6724.55 m; (b) dissolution pores and dissolution-enlarged fractures in fine-to-medium-grained dolomite, Well Yuanba 123 at a depth of 6973.68 m; (c) intercrystalline dissolution pores in sparry bioclast dolomite, Well Yuanba 27 at a depth of 6303.27 m; (d) intercrystalline dissolution pores and organic dissolution pores in silty dolomite, Well Yuanba 271 at a depth of 6324.4 m

Fig. 2  Characteristics of main reservoiring spaces in Changxing Formation reservoir, Yuanba Gasfield

According to the statistic of 733 core samples from Changxing Formation, reservoir has a maximum porosity of 23.59% and a minimum value of 0.74%, averaging 5.18%; a maximum permeability of 2385.4826 mD and a minimum permeability of 0.0026 mD, averaging 0.3800 mD (Fig. 3), thus belonging to a low-porosity medium-to-low-permeability and medium-porosity medium-to-high permeability reservoir.

Lithology of Feixianguan Formation reservoir is primarily sparry oolitic limestone and sparry oolitic limestone containing dissolution pores, together with pisolite (compound ooid) oolitic limestone with dissolution pores, and ooid pisolite (compound ooid) limestone with dissolution pores, among which sparry oolitic limestone with dissolution pores has the best physical property, though with thin thickness and Limited distribution. Reservoiring spaces are primarily ooid moldic pores and dissolution pores, with limited intercrystalline pores (calcite intercrystalline pores and intercrystalline micropores) and fractures (Fig. 4). The reservoir has weak dolomitization, only regionally distributed in Well Yuanba 9 and etc. The reservoir has a porosity of 0.99%-16.27%, averaging 3.48%; and a permeability of 0.0021-1340.2840 mD, averaging 0.0223 mD (Fig. 5). It has low-porosity low-permeability and mediumporosity low-permeability characteristics. It is primarily a fracture-pore type reservoir, in which microfractures are well connected. As a platform-margin ooid shoal deposit in high-energy

500

500

400

400 Frequency

Frequency

The porosity-permeability correlation analysis of core samples indicates a good positive correlation between porosity and permeability. However, when porosity is less than 5%, porosity is poorly correlated with permeability, indicating fracture development in part of the samples and suggesting a pore-type and fracture-pore-type reservoir.

300 200 100 0

300 200 100

<2

<5-10 2-5 Porosity/%

≥10

0

0.002-0.25

0.25-1 Permeability / mD

≥1

Fig. 3  Porosity and permeability of Changxing Formation reservoir, Yuanba area

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1mm

1mm

(a)

4 Comparison of reservoir characteristics with adjacent reef-shoal gas fields

(b)

1mm

1mm

(c)

On both sides of Kaijiang-Liangping continental shelf, largesized and ultra-large-sized gas fields have been discovered successively, such as Luojiazhai, Puguang, Longgang, and Yuanba. Compared with adjacent gas fields, the reef–shoal reservoir of Yuanba Gasfield has the following characteristics.

(d)

(a) intraparticle dissolution pores and interparticle dissolution pores, Well Yuanba 204; (b) ooid moldic pores, Well Yuanba 22; (c) ooid intraparticle dissolution pores and fractures, Well Yuanba 224; (d) ooid moldic pores and pressolution fractures, Well Yuanba-27

Fig. 4  Characteristics of reservoir spaces in Feixianguan Formation of Yuanba Gasfield

4.1 Organic reef-shoal reservoir of Changxing Formation has large size, deep burial and good accumulation property The organic reef-shoal reservoir in Changxing Formation of Yuanba Gasfield exhibits an NW-trending distribution along a high-energy facies belt at the platform margin. It consists of seven organic reef bodies (group) in four rows varying in size, and six interreef and backreef bioclastic shoals varying in thickness (Fig. 6).

300

300

200

200

Frequency

Frequency

environment, Feixianguan Formation reservoir is dominated by oolitic limestone with low mud content. Ooid shoals are characterized by superimposed development, large distribution scope and small thickness.

100

0

<2

2-5

<5-10 Porosity/%

100

0

≥10

0.002-0.25

0.25-1 Permeability/mD

≥1

Fig. 5  Porosity and permeability of Member 2 reservoir of Feixianguan Formation, Yuanba area

Organic reefs, shoals exposed on reef crests, and backreef bioclastic shoals constitute a number of stratoid reef-shoal lithological traps in Changxing Formation. A single organic reef lithological trap is 7.2-16.15 km long and 2.22-6.31 km wide. The single reef covers an area of 12.57-52.76 km2, and the average thickness of single reef reservoir is 10-90 m. A single bioclastic shoal is 7.84-16.91 km long and 4.33-7.69km wide. The single shoal covers an area of 28.3081.72 km2 and averages 20-40 m in thickness. Located at the east of Kaijiang-Liangping continental shelf, Changxing Formation organic reef belt has a narrower distribution in Puguang area, exhibiting approximately NW-SE-trending banded distribution, mainly distributed near wells Puguang 5, Puguang 6, Puguang 8, and Puguang 9. Organic reef traps have a cumulative area of 32 km2, and the reef bodies are about 50 m thick. The thickness of bioclast dolomite of the exposed-shallow facies on reef crest is 70-80 m. No bioclastic shoals develop behind reef bodies (Ma et al., 2005a; Ma et al., 2005b; Ma, 2007).

0

N Yuanba 27 Yuanba 29 Yuanba 2

5km

Yuanba 101 Yuanba 1

Yuanba 21 Yuanba 102

Yuanba 5

Yuanba 10 Yuanba 3

Yuanba 11 Yuanba 16 Yuanba 224

Reef

Yuanba 12

Yuanba 9

Bioclastic Tidal Intershoal Platform Continental Well shoal channel shelf location margin

Fig. 6  Distribution of organic reef-shoal in Changxing Formation, Yuanba Gasfield

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In Luojiazhai-Gunziping area at the east of the continental shelf, organic reef body generally has an extension of 5-8 km, averaging 6.9 km (Su et al., 2004; Ran et al., 2005; Zhao et al., 2011). In Longgang area adjacent to Yuanba area, reef and shoal bodies extend in NW direction. They are less than 10 km long (averaging 5.6 km), and the transverse extensions, e.g., wells Longgang 1, Longgang 001-3 and Longgang 2 are less than 5 km apart (Zhao et al., 2011; Zhu et al., 2013); the reef and shoal bodies show thin-interbeded distribution vertically. The buried depth of central part of organic reef-shoal reservoir in Yuanba Changxing Formation, is 700-1500 m deeper than the

adjacent Longgang Gasfield, 800-1500 m deeper than Puguang Gasfield, 2600 m deeper than Wubaiti Gasfield, and 3200-3700 m deeper than Tieshan Gasfield. Therefore, it is the deepest reservoir ever discovered in the Sichuan Basin. Porosity of Yuanba Changxing Formation reservoir is higher than that of Wubaiti Gasfield and Tieshan Gasfield (Han, 2005; Wen et al., 2010; Yu et al., 2012; Tang et al., 2013), comparable to Longgang Gasfield (Zhao et al., 2011), and lower than that of Puguang Gasfield (Tang et al., 2013) and Luojiazhai Gasfield (Ran et al., 2005). The reservoir has a low permeability, the average of which is the lowest among the gas fields described above (Table 1).

Table 1  Parameters of Changxing Formation-Feixianguan Formation reef-shoal reservoirs in Yuanba Gasfield and its peripheral gas fields Reservoir Parameter

Feixianguan Formation

Yuanba

Longgang

Dissolution-pore Sparry oolitic limestone residual ooid dolomite, and dissolution-poredissolution-pore fineLithology bearing sparry oolitic to-medium-grained limestone residual ooid dolomite Platform-margin ooid Platform-margin ooid Microfacies shoal shoal Buried depth 6317-6388 4800-6000 /m

Puguang Residual ooid crystalline dolomite

Luojiazhai (Wubaiti)

Tieshan

Dissolution-pore Oolitic dissolution residual ooid pore dolomite, silty dolomite, silty–finedolomite grained dolomite

Platform-margin ooid shoal

Platform-margin ooid shoal

Platform-margin ooid shoal

4776-6008

3215-4570

2775-2925

Interparticle pores, Interparticle pores, Interparticle Intraparticle pores, intraparticle pores, intraparticle pores dissolution pores, Reservoiring Intraparticle pores and interparticle pores and intercrystalline and intercrystalline moldic pores and space interparticle pores intercrystalline pores pores, vugs and pores intercrystalline pores fractures Porosity 0.99-16.27/3.48 2-20/8 0.94-28.86/8.17 0.23-26.80/7.00 0.24-23.85/3.27 /% Permeability 0.0021-1340/0.0223 0.0004-1036/20.78 0.0112-3354.7/94.42 0.01-1160/20 0.08-423/7.03 /mD Fracture-pore type Pore type and Reservoir Fracture-pore type Fracture-pore type Fracture-pore type and pore-fracture type fracture-pore type type Crystalline Reef dolomite, Residual bioclast Reef dolomite, silty dolomite, grain dolomite Residual bioclast dolomite, relict bioclast dolomite and finedolorudite, and Lithology and crystalline crystalline dolomite medium-to-fine-grained grained dolomite sponge reef dolomite dolomite dolomite Microfacies

Changxing Formation

Gas fields

Buried depth /m

Reef cap and highReef core and highenergy bioclastic shoal energy bioclastic shoal 6328-6898

Reservoiring Intercrystalline pores space Porosity /% Permeability /mD Reservoir type

4900-6200

Reef cap

Reef flat shoal, framereef and bindreef

Bafflereef

5259-6141

3740-4220

3089-3194

Interparticle pores, Moldic pores, Interparticle pores, and intraparticle intercrystalline Interparticle pores, and and intercrystalline pores, moldic pores, pores, interparticle intercrystalline pores pores and intercrystalline pores, and organic pores framework pores

0.14-23.59/5.18

2.0-20.5/6.4

1.11-23.05/7.66

0.01-20.26/2.65

1.50-7.62/1.87

0.0026-2385.5/0.38

0.0003-414/7.346

0.01161391.2/16.6226

0.01-1313.02/8

0.01-67.28/1.91

Fracture-pore type

Fracture-pore type

Pore type and fracture-pore type

Fracture-pore type, and pore-fracture type

Numbers following “/” are average values, reservoir of Luojiazhai Gasfield is Feixianguan Formation, and that reservoir of Wubaiti Gasfield is Changxing Formation

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4. 2 Oolitic shoal reser voir of Feixianguan Formation is thin and weakly dolomitized, with poor accumulation property Compared with the gas fields such as Puguang and Longgang, the Feixianguan Formation Member 2 reservoir of Yuanba Gasfield has the same depositional environment, i.e., platformmargin ooid shoal deposit. However, it shows differences in lithology and thickness: (1) Oolitic shoal deposits are characterized by wide planar distribution, small reservoir thickness, and horizontal-lateral migration. Based on seismic inversion data, the ooid shoal lithological trap of Member 2 of Feixianguan Formation has an area of 115.1 km2, maximum extension of 16.02 km in north or south direction and maximum width of 12.26 km in east or west direction. The reservoir thickness is about 26.7 m. In Puguang Gasfield, the oolitic shoal body covers a small area, but the reservoir thickness is large and up to 117.4-204.6 m. The oolitic shoal has a characteristic of vertical aggradation. In Longgang Gasfield, the thickness of Feixianguan Formation oolitic shoal reservoir is 20-80 m, ranking between Yuanba Gasfield and Puguang Gasfield, and the shoal body has a characteristic of early vertical aggradation and late lateral migration (Zhu et al., 2013). (2) Reservoir in Member 2 of Feixianguan Formation in Yuanba Gasfield is weakly dolomitized. The reservoir rock is primarily sparry oolitic limestone with or without dissolution pore. Dolomite was only encountered in drilling of the wells Yuanba 9 and Yuanba 10 in the eastern area. In Puguang and Longgang area, Feixianguan Formation reservoirs are both composed of ooid dolomites, showing obvious dolomitization. (3) Compared with Puguang, Longgang and Luojiazhai gas fields (Su et al., 2004; Ran et al., 2005; Ma et al., 2010; Hu et al., 2011; Zhao et al., 2011), the reservoir of Yuanba Gasfield has worse physical property and lower per-well production capacity, lower average porosity and lower average permeability (Table 1).

Tectonic disruption

4.3 Changxing Formation high-quality reservoir controlled by early meteoric freshwater dissolution and dolomitization with relatively weaker deepburial dissolution and tectonism During formation process, Changxing Formation high-quality reservoir in Yuanba Gasfield underwent early meteoric freshwater dissolution, dolomitization, deep burial dissolution by organic acids and thermochemical sulfate reduction (TSR), and transformation by multiphase tectonic fracturing action. Based on core observation, rock thin sections, geochemical data analysis and statistic (Fig. 7), main reservoir spaces of high quality reservoir in Yuanba Gasfield are intercrystalline pores and intercrystalline dissolution pores due to dolomitization, and intracoelom pores and intraparticle pores formed due to selective dissolution by early meteoric freshwater (Fig. 2). Besides, there are low percentages of fractures and nonselective pores formed due to late deep-burial dissolution. The formation of high quality reservoir was primarily controlled by early meteoric freshwater dissolution and dolomitization. As to high quality reservoir in Puguang area, non-selective dissolution pores and fractures were formed due to deep burial dissolution and take a large share of reservoir space. The formation of high quality reservoir is jointly controlled by early meteoric freshwater dissolution, dolomitization, deep burial dissolution, and tectonism (Ma et al., 2010).

5 Causes of reservoir differences between Yuanba Gasfield and Puguang Gasfield The above differences between ultra-deep high-quality reservoir in Yuanba Gasfield and high quality reservoir in Puguang

Meteoric freshwater dissolution

n tio

ld

Bu

42 samples

ria

to

ni

cd

( a ) Puguang Gasfield

Te c

Meteoric freshwater dissolution

Dolomitization

iss

isr

Burial dissolution

olu

up

tio

n

Dolomitization

Per-well testing capacity of Yuanba Gasfield is (35-100) ×103 m3/ d, which is much lower than the stable production of Puguang Gasfield of (155.2-1281.5)×103 m 3/d, and also lower than the stable production of Longgang Gasfield of (61.8-1038.8)×103 m3/ d (Zhao et al., 2011).

80 samples (b)Yuanba Gasfield

Fig. 7  Comparison of the diagenesis contribution of various factors between Puguang and Yuanba reef-shoal reservoirs

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Gasfield have three major causes.

5.1 Distribution and size of reef-shoal reservoirs controlled by early tectonic deposition setting Comparison of the reservoir characteristics between Yuanba Gasfield and Puguang Gasfield suggests that Yuanba Gasfield is dominated by Changxing Formation organic reef-shoal reservoir, Yuanba 2

Yuanba Gasfield Yuanba 1 Yuanba 5

while Puguang Gasfield is dominated by Feixianguan Formation oolitic shoal reservoir. This is closely related to difference in depositional setting between both areas (Fig. 8). As revealed by drilling and regional seismic data, Yuanba area was located on the platform margins of steep gentle-slopes with a gradient of 8°-10° in the west of Kaijiang-Liangping continental shelf during Changxing Formation deposition period, Puguang Gasfield Puguang 84 Puguang 5 Puguang 2 Puguang 4

Yuanba 4

Sequence

Sequence SQ3

SQ2

HST

HST

TST

TST

HST

HST SQ2 TST HST SQ1 TST

TST SQ1 HST TST Open platform Restricted platform Intraplatform shoal Platform-margin shoal

Platform-margin reef

Platform-margin slope

SQ3

Continental shelf

Fig. 8  Depositional mode of Changxing Formation–Feixianguan Formation in Puguang and Yuanba areas

while Puguang area was on the platform margins of steep slopes with a gradient of 15°. In steep gentle-slope environment, seawater travelles a long distance before arriving at platform margins, so the seawater dynamics is weak, resulting in slow growth of organic reefs in Yuanba area. Moreover, this area is dominated by bafflereefs and bindreefs; individual reef bodies has a small thickness, averaging 3.57 m. Reef body deposition is controlled by vertical aggradation and lateral migration, forming a planar framework of multiphase reef bodies stacked within a large scope. Since syn-depositional organic reefs have small thickness and developed interreef channels, reef body barrier action is weak and backreef hydrodynamic condition remains very strong, resulting in widely distributed backreef shoal deposits. In Yuanba area, a gentle-slope platform-margin reef-shoal depositional mode characterized by lateral migrating forereef and backreef shoals was developed. Multiphase and vertically-stacking deposits were formed. In a steep slope environment and strong seawater dynamics, Puguang area is dominated by framereefs and bafflereefs. Individual reef bodies have a large thickness, generally ranging from 4.68 m to 8.63 m. Organic reef bodies were controlled by vertical aggradation, resulting in a quasi-NW-SE-trending banded barrier reef belt. The backreef hydrodynamics was weak, and backreef shoals were undeveloped. Sediments are mainly micritic limestone which deposited within a low-energy environment. During depositional period of Feixianguan Formation, Puguang area was still in a steep-slope depositional environment, ooid shoal deposits were developed in platform margin. Vertical aggradation of shoal bodies went on, forming ooid shoal

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deposits with a thickness over one hundred meters. During this period, Yuanba area was subjected to compression and tilting of Motianling Paleocontinent in the northwest, which caused the dipping of ramps in Yuanba area. Thus a progradational downlap body of Member 1 and Member 2 of Feixianguan Formation was formed, and composed of muddy limestone and micritic limestone, where terrigenous clast content was very high, and the ooid shoal was located at the top of the progradation body (near the wave base). The progradation body advanced constantly towards the continental shelf. Accordingly, the shoal body continuously kept horizontal lateral migration and formed a large area of oolitic limestone (Fig. 8). Tectonic difference between both sides of the continental shelf led to change in depositional environment, and caused great difference in the scale and configuration of ooid shoal bodies in Yuanba and Puguang areas.

5. 2 Ea rly pore development cont rolled by deposition-diagenesis environment 5.2.1 Reservoir physical property is controlled by early depositional environment According to the relationsh ips between reser voi r a nd sedimentary facies in Yuanba Gasfield, Puguang Gasfield (Ma et al., 2010), and Longgang Gasfield (Zhao et al., 2011; Zhu et al., 2013), formation of high quality reservoirs were apparently controlled by sedimentary facies and it distributed over platform-margin shoals and in platform-margin reef-shoal depositional environment with strong hydrodynamic conditions (Table 2). For organic reef, longitudinal favorable reservoirs

Y.Ma et al./Petroleum Research (2016) 2,123-134

were mainly developed in reef caps, while horizontal favorable reservoirs were distributed in reef crests and back reefs (Table 3). Shoal facies can be divided into high-energy shoals and lowenergy shoals. Favorable reservoirs were primarily developed in high-energy shoal cores, followed by high-energy shoal margins and low-energy shoals, intershoal reservoirs were relatively poor (Fig. 9). High quality reservoirs were mainly developed in an exposure-dissolution-prone depositional environment with strong hydrodynamic.

5.2.2 Early dissolution and dolomitization controlled by paleogeomorphy Meteoric freshwater of early shallow buried environment and supergene environment is an important factor for formation are high quality reservoirs. Among reservoir space, a significant proportion of selective dissolution pores were induced by meteoric freshwater. Paleotopography and high-quality reservoir distribution are positively correlated. High quality reef-shoal

Table 2  Petrophysical properties of different sedimentary facies in Changxing Formation, Yuanba area Physical property

Platform-margin reef-shoal facies

Platform-margin shoal facies

Open platform facies

Restricted platform facies

Slope-continental shelf facies

Porosity/%

0.59-23.59/ 5.24 (329)

0.99-20.51/ 4.87(365)

0.62-19.98/ 2.71(70)

0.95-3.59/ 1.53(43)

1.02-3.01/ 1.56(21)

Permeability/mD

0.003-1720.719/ 40.165(329)

0.003-2385.483/ 38.053(365)

0.002-224.757/ 4.300(70)

0.003-253.504/ 15.590(43)

0.004-945.480/ 60.698(21)

Numbers following “/” are average value, and those in bracket are numbers of samples

Table 3  Thickness and porosity of Changxing Formation organic reef reservoirs in different microfacies, Yuanba area Vertical position of reef body

Reservoir thickness /m

Planar position of reef body

Average

Type I and type II of reservoir

Total thickness

Type I of reservoir

Type II of reservoir

Type III of reservoir

Reef cap

40.0

2.5

18.2

19.3

5.2

reef front

38.00

10.60

Reef core

14.8

0

2.0

12.8

3.2

reef crest

86.80

43.02

Reef base

0.6

0

0.1

0.5

0.5

backreef

88.55

25.20

reservoirs are mainly developed in reef caps, back reefs and shoal cores, such microfacies are paleotopographic highs deposited at that time. Several exposures took place due to multiple sea level changes, favorable for dissolution by meteoric freshwater. 80

Shoal core 6 9.0

Reservoir type

60 Thickness/m

Reservoir thickness /m

Average porosity /%

Type I

25.8

0

Type III

38.5

40 20

Type II

10.4 Well Yuanba 12

Shoal margin Shoal margin 6.3 6.6 0

13.2 0.9

Shoal margin 0

0 1.8

Well Yuanba 16 Well Yuanba 124 Well Yuanba 122

Fig. 9  Comparison of thickness of different types of gas reservoirs in different bioclastic shoal microfacies of Yuanba area

Dolomitization is the other important factor to form high quality reservoirs in Puguang and Yuanba areas. According to relationship between high quality reservoir and lithology, high

quality reservoirs in both Puguang Gasfield and Yuanba Gasfield are dominated by dolomite, because dolomite has higher accumulation property than limestone. For instance, in the gas reservoir of Member 2 of Feixianguan Formation in Yuanba Gasfield, limestone is the main rock type. Reservoir property and per-well production are obviously worse than dolomite of Changxing Formation gas reservoir. Quantitative statistic of pore evolution in reef-shoal dolomite and limestone indicates that the early primary porosity is 30%. Dolomite porosity is 7% higher than the final porosity of limestone due to dolomitization. Dolomite distribution of Yuanba area is obviously different from that of Puguang area. Dolomitization of Changxing Formation in Yuanba area has clear stratification as dolomite-limestone interbeds in bioclastic shoals. Dolomite was mainly developed in the local areas of organic reefs, such as back reefs, reef caps and reef cores. Based on C, O and Sr isotope analyses of dolomite, it indicates that dolomite in Yuanba area is predominantly dominated by low-temperature dolomite, and the temperature of dolomitized fluid is generally at 30.99-53.69°C. During early diagenesis, there were primarily three types of dolomitization as follows: (1) evaporative mixed-water dolomitization, mainly

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occurring in reef caps and paleotopographic highs of bioclastic shoals during exposure due to fall of relative sea level and meteoric freshwater dissolution. (2) Brine reflux dolomitization, mainly occurring on back reefs, bioclastic shoals and reef cores during the rise of relative sea level. (3) Burial dolomitization distribution had no obvious regularity, mainly manifesting as isotope redistribution and recrystallization. In the ooid shoal reservoir of Member 2 of Feixianguan Formation in Yuanba Gasfield, dolomitization was less frequent and mainly occurred in Yuanba 9 well block with high paleotopography, dominated by evaporative dolomitization (Guo et al., 2010). In other areas, dolomitization did not occur due to low topography, slightly deeper water environment, weak evaporation and normal seawater salinity.

convection in shallow burial environment was proposed as the main formation mechanism for extremely thick-bedded crystalline dolomite in Feixianguan Formation of Puguang area (Fig. 10). Puguang area is situated on platform steep slope, which is an important factor causing thermal convection between seawater and intraplatform shallow burial environment, resulting in extremely thick crystalline dolomite in Changxing Formation and Feixianguan Formation (Huang et al., 2011; Huang et al., 2012). Eodiagenetic dolomitization has certain constructive effect on pore formation of high quality reservoirs of Puguang and Yuanba areas. Reservoir space of the original high-quality reservoirs are preserved or converted through dolomitization. Moreover, compared with limestone, dolomite is more resistant to compaction and pressolution, more favorable to pore preservation and fracture formation, this can further improve permeability and lay a good foundation for retention of reservoir

In contrast, crests of Feixianguan Formation and Changxing Formation in Puguang area show overall dolomitization and large dolomite thickness. A model of intraplatform thermal

Sabkha environment

High salinity brine of Member 4 of Feixianguan Fm.

Active-latent reflux

Outward convection

Outward convection

Δt V

Δt L

Carbonate platform

Δt V

Δt L

Geothermal flow

Oolite of Member 1 and Member 2 of Feixianguan Fm.

Deep Sr-enriched fluid

Permeable fault

Intraplatform seawater

Δt L

Seawater surrounding the platform after deposit of Member 1and Member 2of Feixianguan Fm.

87 Sr-enriched noncoeval seawater

Lateral temperature difference between oceanic water and platform water

Δt V

Vertical temperature difference of intraplatform fluid

Fig. 10  A model of intraplatform thermal-convection dolomitization in shallow burial environment (Huang et al., 2011)

accumulation property under deep burial and ultra-deep burial conditions.

5. 3 L ate diagenetic pore preser vation and transformation influenced by different tectonic activities

As revealed by seismic data, Yanshanian-Himalayan tectonic activity differs significantly between Puguang and Yuanba areas. Puguang Gasfield is located in a tectonic area featuring double superimposition of the northeastern segment of East

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Sichuan fault-fold structural belt and Dabashan thrust fold belt. It underwent Yanshanian, early and late Himalayan tectonic deformation, and formerd NNE-trending and NWtrending structures, such as intense folds and developed faults. Yuanba Gasfield is located in a junction between North Sichuan depression and Central Sichuan low gentle structural belt. It is located at a joining point of three large-sized positive structures and blocked by these three structures. In this area, formation attitude is gentle, tectonic deformation is weak, and faults are undeveloped.

Y.Ma et al./Petroleum Research (2016) 2,123-134

Difference in tectonic setting led to contrasting difference in fracture development between Puguang and Yuanba areas. In Puguang Gasfield, fractures were mainly developed in Member 3, Member 2, Member 1 of Feixianguan Formation, and Changxing Formation. Fracture density is greater than 50 per meter, and fractures of well core are of net-like distribution locally (Fig. 11). Fracture development intervals are generally coincided with pore development intervals. Fractures are primarily by mediumto-low angle. Fractures with a dip angle less than 70° accounts for over 90% of total fractures. Totally five phases of fracture development occurred in Puguang area (Ma et al., 2005a; Ma et al., 2007; Ma et al., 2010), among which the second and third phases of fractures were formed during liquid hydrocarbon stage of middle diagenesis, while the fourth and fifth phases of fractures were formed during gaseous hydrocarbon stage of late diagenesis. Fracture density/ per meter

100 80

Puguang Yuanba

60 40 20 0

Member 4 of Member 3 of Member 2 of Member 1 of Changxing Fm. Feixianguan Fm. Feixianguan Fm. Feixianguan Fm. Feixianguan Fm.

Fig. 11  Fracture density in Feixianguan Formation and Changxing Formation of Puguang and Yuanba areas

In Yuanba Gasfield, fractures are mainly developed in the platform-margin reef dolomite of Changxing Formation, and fracture density is 10-20 per meter. Density of platformmargin reef limestone fractures is generally less than 10 per meter. Medium to high angle fractures are predominant with a dip angle of 40°-90° (Guo et al., 2011). Fracture density here is lower than that in Puguang Gasfield, same as Luojiazhai Gasfield (21.8 per meter) and higher than that in Longgang area (0.2-9 per meter) (Ma et al., 2005b). Fracture development in Yuanba area has three phases: Phase 1 occurred during the early diagenesis, with weak fracturing and fewer fractures; Phase 2 occurred in liquid hydrocarbon during early telodiagenesis, with weak fracturing activity and few fractures; Phase 3 occurred in gaseous hydrocarbon during late telodiagenesis, with strong fracturing activity and abundant almost unfilled fractures that dissect all fractures, thus improved accumulation property greatly. Fractures contributed to formation of high quality reservoirs in term of two major aspects: (1) direct contribution, i.e., fractures served as reservoir spaces. They played an important role in the formation of ultra-deep high-quality reservoirs in Puguang, Yuanba, and Longgang gas fields. (2) Indirect contribution, i.e.,

fractures connected reservoir internal space, contributing to flow and circulation of diagenetic fluids, particularly unsaturated fluids such as organic acids and H 2 S during hydrocarbon generation. Consequentially burial dissolution occurred in the reservoir. Puguang area has multiphase fracture activities, and has higher fracture density and higher probability of late dissolution than Yuanba area. This is an important cause leading to higher percentage of telodiagenetic deep-burial dissolution pores in Puguang area than that in Yuanba area. Uplift magnitudes in these two areas were different. According to the burial history of wells Yuanba 2 and Puguang 2, both Puguang area and Yuanba area had the deepest burial depth up to 8500 m during the Cretaceous. These two areas began to uplift due to late Yanshanian–Himalayan tectonism. continuous lifting led to constant decline of hot fluid temperature. Fluids were in unsaturated state, and TSR-derived H 2 S enhanced its capability of dissolving carbonate minerals and resulted in carbonate dissolution. Uplift magnitude of Puguang area amounts to 2500-3700 m, which is 1000-1500 m more than that of Yuanba area, implying that the extent of magnitude of hot fluid cooling in Puguang area is greater than that in Yuanba area. Solubility in the hot fluid in Puguang area is stronger than that in Yuanba area, which is another important reason why deep burial under TSR in Puguang area was stronger than that in Yuanba area.

6 Conclusions (1) Depositional-diagenesis environment controlled reservoir scale and early pore development. Different ramp topographic environments led variation of depositional models, and thus controlled stacking and distribution of organic reef-shoal bodies. Vertically stacking reef-shoal deposits often formed on the steep slope of platform margin, while lateral migration and aggradation were stronger in the gentle slope environment. Organic reef-shoal bodies with strong hydrodynam ics and abundant organisms provided favorable depositional environment for development of high-porosity carbonate. Meanwhile, high-energy organic shoal-reef environment was prone to exposure. This provided favorable conditions for further dissolution by meteoric water. Development of primary pores was an important condition for dolomite formation by dolomitized fluid-rock interaction. Early dolomitization allowed early pores to be replaced. Under deep burial condition, dolomite preserves pores more easily than limestone and had better compaction resistance, laying a foundation for preservation of high-porosity dolomite under deep burial settings. (2) Structure-fluid coupling controlled fracturing and dissolution. Tectonism-derived burial and uplifting influenced on changes in temperature and pressure of diagenetic fluid, as well as closure and opening of diagenetic environment, thus controlling fluid

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dissolution. Tectonism was a key controlling factor for rock fracture formation and enlargement, as well as development of fracture reservoiring spaces. Fractures connected internal pores, creating a condition for interaction between rock and acidic fluids. Fracture phases and intensities influenced the fluid-rock reaction degree. (3) Fluid-rock interaction controlled pore preservation and transformation. Dissolution and precipitation occurred during different diagenesis stages at the presence of eodiagenetic dolomitized fluids, CO2 in meteoric freshwater, organic acids arising from organic matter decomposition during the middle and late diagenesis, and H 2S formed by the reaction between hydrocarbons and sulfates in rocks. Pore preservation and transformation were determined by changes in temperature and pressure of fluid–rock interaction and the availability of open or closed environments. The presence of favorable deposition-diagenesis environment, post-tectonic reworking and fluid-rock interaction is essentially a hierarchical coupling process. During the formation of high quality reef-shoal ultra-deep reservoirs, the depositionaldiagenesis environment is the base, the structure-fluid coupling is the prerequisite, and the rock-fluid interaction is the key. Differences in depositional paleotopography, diagenetic process, tectonic setting and fluid physical and chemical conditions led to different reservoir characteristics between Yuanba and Puguang gas fields.

Acknowledgments This study was supported by China Geological Survey (Zi [2012] 02-029-006).

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