Scholarly article on topic 'Natural gas exploration prospect in the Upper Paleozoic strata, NW Sichuan Basin'

Natural gas exploration prospect in the Upper Paleozoic strata, NW Sichuan Basin Academic research paper on "Earth and related environmental sciences"

CC BY-NC-ND
0
0
Share paper
Academic journal
Natural Gas Industry B
Keywords
{"Sichuan Basin" / Northwest / "Late Paleozoic" / "Natural gas" / "Combination trap" / "Platform reef" / "Ancient landform" / "Favorable exploration zone"}

Abstract of research paper on Earth and related environmental sciences, author of scientific article — Hao Shen, Hua Wang, Long Wen, Hualing Ma, Yi Li, et al.

Abstract Progress of natural gas exploration has recently been made in the Upper Paleozoic marine-facies reservoirs in the NW Sichuan Basin. It has been proved that the Middle Permian Qixia Fm pore-type dolomite reservoirs may be distributed continuously in a large area, besides, for the first time favorable gas-bearing Devonian dolomite reservoirs in great thickness were discovered in the downhole there. Recent research achievements indicate that: (1) The Lower Cambrian quality source rocks in huge thickness were found in the study sedimentary area with advantageous hydrocarbon source conditions. (2) Influenced by the Caledonian movement, the whole NW Sichuan Basin was lifted up to form a massive box-like NS-trending plunging uplift low steep in the west but high sluggish in the east, laying a robust foundation for the inheritance of Devonian, Carboniferous, and Permian platform margin reef large-scale sedimentation and dolomitization. (3) In the Upper Paleozoic strata, there were multiple planes of unconformity and multi-stage supergene karst occurred, improving the preservation and seeping capacity of reservoirs. (4) Tectonic deformations resulted from the nappe structure in the Longmenshan Mountain provided good hydrocarbon source fractures and sealing conditions for oil and gas reservoirs, which is conductive to the formation of a giant gas pool. It is concluded that the back-thrust anticline belt east to Hidden fault I in this study area has favorable conditions for large-scale structural–stratigraphic and structural–lithologic combination traps. As suggested in this study, relatively strong dolomitization was found in the Qixia Fm in the Shejianhe River–Zhongba higher palaeogeomorphic deposit area, so the most favorable exploration target of the Qixia Fm will be around the central axis of Caledonian paleo-uplift to the east ridgeline, and the most favorable prospect of the Upper Palaeozoic will be the central axis to the west of Hidden fault I where three gas bearing reservoirs will be possibly explored at the same time.

Academic research paper on topic "Natural gas exploration prospect in the Upper Paleozoic strata, NW Sichuan Basin"

Available online at www.sciencedirect.com

ScienceDirect

Natural Gas Industry B 3 (2016) 526-536

Research Article

www.elsevier.com/locate/ngib

Natural gas exploration prospect in the Upper Paleozoic strata, NW Sichuan

Shen Haoa, Wang Huaa*, Wen Longa, Ma Hualing0, Li Yia, Zhang Benjian0

1 Exploration and Development Research Institute of PetroChina Southwest Oil & Gas Field Company, Chengdu, Sichuan 610041, China b Northwest Division of PetroChina Southwest Oil & Gas Field Company, Jiangyou, Sichuan 627000, China

Received 16 August 2016; accepted 25 August 2016 Available online 5 July 2017

Abstract

Progress of natural gas exploration has recently been made in the Upper Paleozoic marine-facies reservoirs in the NW Sichuan Basin. It has been proved that the Middle Permian Qixia Fm pore-type dolomite reservoirs may be distributed continuously in a large area, besides, for the first time favorable gas-bearing Devonian dolomite reservoirs in great thickness were discovered in the downhole there. Recent research achievements indicate that: (1) The Lower Cambrian quality source rocks in huge thickness were found in the study sedimentary area with advantageous hydrocarbon source conditions. (2) Influenced by the Caledonian movement, the whole NW Sichuan Basin was lifted up to form a massive box-like NS-trending plunging uplift low steep in the west but high sluggish in the east, laying a robust foundation for the inheritance of Devonian, Carboniferous, and Permian platform margin reef large-scale sedimentation and dolomitization. (3) In the Upper Paleozoic strata, there were multiple planes of unconformity and multi-stage supergene karst occurred, improving the preservation and seeping capacity of reservoirs. (4) Tectonic deformations resulted from the nappe structure in the Longmenshan Mountain provided good hydrocarbon source fractures and sealing conditions for oil and gas reservoirs, which is conductive to the formation of a giant gas pool. It is concluded that the back-thrust anticline belt east to Hidden fault I in this study area has favorable conditions for large-scale structural—stratigraphic and structural—lithologic combination traps. As suggested in this study, relatively strong dolomitization was found in the Qixia Fm in the Shejianhe River—Zhongba higher palaeogeomorphic deposit area, so the most favorable exploration target of the Qixia Fm will be around the central axis of Caledonian paleo-uplift to the east ridgeline, and the most favorable prospect of the Upper Palaeozoic will be the central axis to the west of Hidden fault I where three gas bearing reservoirs will be possibly explored at the same time.

© 2017 Sichuan Petroleum Administration. Production and hosting by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).

Keywords: Sichuan Basin; Northwest; Late Paleozoic; Natural gas; Combination trap; Platform reef; Ancient landform; Favorable exploration zone

The northwestern part of the Sichuan Basin (i.e. NW Sichuan Basin) with relatively complete Upper Paleozoic Fms is a major prospect for petroleum exploration. Since the 20th century, persistent researches have been made on the pervasive

* Supported by the National Science and Technology Major Project "Permian-Middle Triassic natural gas accumulations and evaluation in the Sichuan Basin" (No.: 2016ZX05007-004) and PetroChina Science and Technology Major Project "Deep marine carbonate exploration & development field experiments in the Sichuan Basin" (No.: 2014E-3208(GF)).

* Corresponding author.

E-mail address: wanghua1230@petrochina.com.cn (Wang H.).

Peer review under responsibility of Sichuan Petroleum Administration.

Cambrian, Devonian and Permian oil and gas seepages and bituminous veins as well as the Upper Paleozoic hydrocarbon accumulations in Longmenshan thrust nappe belt [1—3], where Wells K1, K2 and K3 were drilled from 2003 to 2005. Well K2 was drilled with heavy bedded porous dolomite in the Middle Permian Qixia Fm; only fresh water was recovered because of poor preservation conditions. Other two wells were drilled into a complex structural zone with no reservoir rocks detected. But the discovery of thick porous dolomite in Well K2 is still a promising sign [4]. From 2005 to 2007, Wells L16, L17 and WJ1 were drilled at the front land of the Micangshan Mountains. Some fractured-vuggy gas reservoirs with ultra-

http://dx.doi.org/10.1016/j.ngib.2017.05.002

2352-8540/© 2017 Sichuan Petroleum Administration. Production and hosting by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).

high pressure were discovered in the Middle Permian, but no porous reservoirs with good properties were drilled. From 2014 to 2016, Wells ST1, ST2 and ST3 were deployed for the exploration of the Middle Permian Qixia Fm in the uplifted zone at the footwall of Longmenshan thrust nappe belt. All the wells were drilled with the sparry bioclastic bank of 60 m thick in the Qixia Fm. The bank drilled in Wells ST1 and ST3 was dolomitized to generate massive dolomitic reservoir rocks of 15.3 and 22.8 m thick, respectively, which were interpreted to be gas zones on log data. Well ST1 yielded commercial gas flow of 87.6 x 104 m3/d in the well completing test. In accordance with (1) the geologic features and hydrocarbon accumulations on 11 field geologic sections across Nianziba and Hejialiang in the Longmenshan region, western Sichuan Basin, (2) the drilling results of Wells HS1, K2 and K3, and (3) some basic information about the Permian hydrocarbon accumulations in Well ST3, Well ST3 was deepened into the Carboniferous and Devonian systems, so that hydrocarbon accumulations were detected in the Upper Paleozoic in the NW Sichuan Basin. As a result, dolomite of 13.5 m thick was drilled in the Lower Carboniferous Zongchanggou Fm and massive dolomite of 59.0 m thick was drilled in the Middle Devonian Guanwushan Fm. The cored interval in the Guan-wushan Fm was observed to have dolomitic reservoir rocks of 14.7 m thick with dissolved pores and vugs. Significant oil and gas shows occurred frequently in the drilling process. This well is the first well drilled with the Devonian dolomitic gas reservoirs in the Sichuan Basin and witnessed the first step of the Devonian gas exploration in the basin. In this paper, the conditions for hydrocarbon accumulation are discussed in the Devonian and Carboniferous systems and the Middle Permian Qixia Fm in the NW Sichuan Basin.

1. Upper Paleozoic stratigraphic, sedimentary and reservoir distribution features

1.1. Multi-phase tectonic movements dominate sedimentary landforms and stratigraphic distribution

The Upper Paleozoic system in the NW Sichuan Basin comprises relatively complete Devonian, Carboniferous and Permian Fms (Fig. 1). As per the stratigraphic subdivision proposed previously [5], the Devonian system is further divided into 7 formations in three series, i.e. Pingyipu Fm and Ganxi Fm of Lower Devonian, Yangmaba Fm, Jinbaoshi Fm and Guanwushan Fm of Middle Devonian, and Shawozi Fm and Maoba Fm of Upper Devonian. The Carboniferous system is divided into 3 formations in 3 series, i.e. Zongchanggou Fm of Lower Carboniferous, Huanglong Fm of Middle Carboniferous, and Chuanshan Fm of Upper Carboniferous. The Middle Permian is divided into 3 formations, i.e. Liangshan Fm, Qixia Fm and Maokou Fm. Outcrops and drilling data indicate that the Middle Devonian Jinbaoshi Fm and Guan-wushan Fm and the Lower Carboniferous Zongchanggou Fm and Middle Carboniferous Huanglong Fm remain in the region on the north of Jiangyou. During the Upper Paleozoic deposition, the NW Sichuan Basin experienced multi-phase

tectonic movements, including the Caledonian orogeny, the Liujiang—Yunnan movement, and the Dongwu movement, which played a crucial role in the Upper Paleozoic sedimentation and reservoir rock distribution.

The Leshan—Longniisi palaeouplift, generated by the Caledonian orogeny in central and western Sichuan Basin at the end of the Silurian, greatly dominated the Upper Paleozoic deposition. Different from the dome-shaped palaeouplift in central Sichuan Basin, the Caledonian palaeouplift in the Guangyuan—Jiange—Mianyang area in the NW Sichuan Basin is a box-like uplift slowly plunging toward the NNE and ascending toward the SSW. In the NE direction, the steep western flank is structurally low and gentle eastern flank is structurally high. The palaeouplift witnessed the gradual transition from the Cambrian in the gentle central part to the Ordovician, Lower Silurian, Middle Silurian and Upper Silurian at both flanks (Fig. 2).

The eastern and western flanks of the box-like uplift constitute the east and west crestal lines. The eastward transgression in the Devonian was baffled by the east crestal line and reached a peak in the Middle Devonian Guanwushan Fm period (Fig. 3-c). The Jinbaoshi Fm is 0—253 m thick and is composed of thick grey to greyish white quartz sandstone. The Guanwushan Fm is 0—160 m thick and is composed of shallow water shelf deposits, which mainly spread in the innermost part and on the west of the Caledonian palaeouplift as per well drilling, geologic and seismic data. The shelf break at the second structural high close to the west crestal line provided the space for the deposition of marginal platform reefs and banks during marine transgression, while the gentle innermost part of the Caledonian palaeouplift was covered with post-reef bank, shelf and tidal flat deposits (Fig. 4-a). Vertically, the bottom interval of the Guanwushan Fm is 5—10 m thick and consists of fine to moderate stratified dark grey micritic argillaceous limestone; the lower—middle interval is 25—90 m thick and consists of thick massive variegated, mainly light purple mixed with grey and dark grey, brecciated dolomite sandwiched with moderate to fine stratified dark grey to grey fine crystalline bioclastic dolomite; the upper—middle interval is 15—20 m thick and consists of abundant organic reefs composed of grey coral limestone and stromatoporoid limestone sandwiched with moderate to heavy bedded fine to moderate crystalline dolomite; the top interval is 15—30 m thick and consists of moderate to heavy bedded light grey to greyish brown fine to moderate crystalline bioclastic dolomite and oolitic dolomite. The Guanwushan Fm is in dis-conformable contact with the overlying Carboniferous system due to stratigraphic uplifting and denudation caused by the Liujiang movement at the late stage of Guanwushan deposition.

Marine transgression in the Carboniferous reached a peak in the Early and Middle Carboniferous Zongchanggou Fm and Huanglong Fm periods, when the sedimentary environment was similar to that in the preceding Devonian Guanwushan Fm period (Fig. 3-b). The Carboniferous Zongchanggou and Huanglong Fms in the NW Sichuan Basin are 0—80 m thick and spread in a similar area to the range of the Devonian

Strata Thickness Lithologie section Lithologie description

System Series Formation / m

,, . 120-320 — 1 — Daik grey to greyish black argillaceous limestone with organic

MaoKou 1 1 - i

\ matter, black mudstone strips and eyed structures at the bottom

/ / Middle and upper intervals: greyish white to

e <5S tu cj 100 / 9 light grey sparry and micriticbioclastic lime-

s 13 X 9 / 9 / stone and thick massive light grey to grey

<U Oc O / fine to moderate crystalline bioclastic dolo-

120 / mite; Lower interval: fine to moderate stratified dark grey to black grey micritic bioclastic limestone andbiolithite

9 / 9

1 /Thin light grey aluminous mudstone sandwiched with greyish black shale

3 =3 -- 1 Light grey sparry bioclastic limestone sand-

g ï 1 1

■2 a g -S .S a g g DG N 0-80 • '/ • / wiched with thick massive greyish brown to

1 u brown grey line to moderate crystalline do-

1 1 lomite

...... Top interval: moderate to heavy bedded

/ ^ / light grey to greyish brown fine to moderate crystalline bioclastic dolomite and oolitic dolomite; Upper-middle interval: abundant organic reefs composed of grey coral limestone and stromatoporoid limestone sandwiched with moderate to heavy bedded fine to moderate crystalline dolomite; Lower-middle interval: thick massive variegated, mainly light purple mixed with grey, brecciated dolomite sandwiched with moderate to fine strati-

Guanwushan 60 / ^/

/ ; A

160 A 0 / A

/ 9 /, A /

a <3 CU A/ A

« o -a -o A » / 9 fied dark grey to grey fine crystalline bioclastic dolomite; Bottom interval: Fine to

<u Q A / A / moderate stratified dark grey micriticar-gillaceous limestone

o ce -Q Moderate to heavy bedded grey to grey-

120 ish green medium quartz sandstone sandwiched with ooliticquartz sandstone and thin pelitic siltstone

as 0 & Grey to greyish yellow pebbledlithic coarsc sandstone and sandstone

& -o 0-300 & A 0

a 2 0 A

<u 3 Upper interval: Grey to greyish green alternating beds of thick sandstone, calcareous siltstone and shale; Middle and lower intervals: thick to

S oo 00 c 2

s o . 1

cs3 CJ J >1 200 ËÉzil

oo moderately thick greyish green shale sandwiched with silty mudstone

Fig. 1. The Middle Permian—Cambrian stratigraphie column for some areas in NW Sichuan Basin.

system. Similar to the Guanwushan Fm, these two formations are mainly composed of shoal and tidal flat microfacies. The upper interval consists of moderate to heavy bedded greyish white, ivory white to light grey sparry to micritic bioclastic limestone; the lower interval consists of light grey sparry bioclastic limestone sandwiched with thick massive greyish

brown to brown grey fine to moderate crystalline dolomite. The Zongchanggou and Huanglong Fms are in dis-conformable contact with the overlying Middle Permian Liangshan Fm due to stratigraphic uplifting and denudation caused by the Yunnan movement at the late stage of Carboniferous deposition.

Fig. 2. Palaeogeologic map before the Devonian deposition, NW Sichuan Basin.

Hejialiang ST3 ST1

West crestal line of East crestal line of

the Caledonian palaeoupiift the Caledonian palaeouplift

ra s Em m EZ3 □ EZ3

Limestone Argilliferous Organic reef Bank-fecies Marginal plat- Tida| flat Intra-plat-limestone dolomite form bank form bank

Fig. 3. Upper Paleozoic depositional model in NW Sichuan Basin.

a. Guanwushan Fm

Fig. 4. Guanwushan and Qixia Fms sedimentary facies in NW Sichuan Basin.

Marine transgression in the Middle Permian Qixia Fm period spread over a larger area than that in the Middle Devonian and the Early and Middle Carboniferous epochs. The whole Upper Yangtze region was covered with trans-gressive sediments. Due to inherited deposition in the Middle Devonian and Middle Carboniferous as well as stratigraphic uplifting and denudation caused by the Liujiang—Yunnan movements, a structurally high zone with small relief occurred in the central part of the Caledonian palaeouplift between the east and west crestal lines in the NW Sichuan Basin, where inherited marginal platform banks were deposited on a large scale in the Qixia period (Fig. 3-a).

The Qixia Fm is a major pay zone discovered in this prospect, as is indicated by the drilling results of Wells ST1 and ST3. It is 100—120 m thick and comprises deposits of marginal platform banks and inter-bank sea microfacies (Fig. 4-b). Its upper interval is 60—120 m thick and consists of heavy bedded greyish white, light grey to light brown grey sparry bioclastic limestone and fine to moderate crystalline bioclastic dolomite. Its lower interval is 0—60 m thick and consists of fine to moderate stratified grey, dark grey to black grey micritic bioclastic limestone, biolithite and argilliferous limestone. The Qixia Fm is in conformable contact with the underlying Liangshan Fm and the overlying Maokou Fm.

1.2. Reservoir properties

The Upper Paleozoic high-graded porous dolomite reservoirs in marginal platform banks in the NW Sichuan Basin are the products of constructive diageneses, including admixing water dolomitization, burial dolomitization, hydrothermal dolomitization, hypergenic karstification, burial corrosion, and tectonic disruption. Field outcrops and exploratory drilling results indicate that dolomite reservoirs occur widely in the Dovenian, Carboniferous and Middle Permian Qixia Fm. Reservoir space is composed of dolomite intercrystalline pores, intercrystalline dissolved pores, visceral pores, and

dissolved caverns as well as structural fractures and dissolved fractures (Fig. 5).

The Middle Permian Qixia Fm is the most important reservoir unit in the Upper Paleozoic in the NW Sichuan Basin. The Qixia Fm dolomites drilled by Wells ST1 and ST3 have similar tectonic location, thickness and internal structure; thus it is inferred that the Middle Permian Qixia Fm dolomite reservoir rocks spread pervasively in the NW Sichuan Basin. This package of reservoir rocks has consistent thickness and abundant pores and vugs. On the geologic sections across Nianziba and Hejialiang in Qingchuan, the Qixia Fm dolomite with dissolved pores exceeds 100 m; but the rocks drilled by Wells K2, ST1 and ST3 are only 42, 15.3 and 22.8 m, respectively. It is believed that the Qixia Fm dolomite reservoir rocks may be much thicker in the area close to the east crestal line of the original Caledonian palaeouplift. As per the analyses of 223 field and drilling core samples, the porosity of the Qixia Fm dolomite is 3.45%; 79.82% samples have the porosity above 2%. The average porosity is 4.0%, which indicates good reservoir properties.

The Devonian and Carboniferous systems are another two important reservoir units with large area and consistent thickness in the Upper Paleozoic in the NW Sichuan Basin. They were filled with a large amount of liquid hydrocarbon at the early stage due to the small distance to the Cambrian source rocks. Owing to increased burial depth at the late stage, liquid hydrocarbon in fossil oil reservoirs was cracked and gasified, and many pores, vugs and fractures were filled with residual bitumen, but a mass of pore space was still preserved. Reservoir rocks in the Jinbaoshi Fm are mainly massive greyish white quartz sandstone with intergranular dissolved pores totally or partially filled by bitumen and oil. On field geologic sections, cracks and intergranular pores were usually observed to be filled with asphaltene with an apparent oily smell. As per petrophysical analyses of 27 field samples, the porosity of the Jinbaoshi sands ranges from 1.88% to 17.51% with an average of 8.1%; the average permeability is 0.95 mD;

Fig. 5. Reservoir space types in the NW Sichuan Basin.

about 70% samples have the porosity above 8%. Thus, the Jinbaoshi sands are a potential package of reservoir rocks. The Devonian Guanwushan Fm reservoir rocks consist of moderate to heavy bedded grey to light grey fine crystalline dolomite, reef-facies dolomite, and residual dolarenite with subhedral to automorphic dolomite crystal sandsome areneillusions through microscopic observation. Pore space is mainly composed of intercrystalline dissolved pores, growth framework dissolved pores and visceral dissolved pores. As per the analyses of 132 outcrop and drilling samples, the porosity ranges from 0.5% to 8.0%. The Carboniferous Zongchanggou Fm reservoir rocks are mainly fine crystal dolomite with intercrystalline dissolved pores. The porosity ranges from 2.04% to 5.14%.

The Devonian and Carboniferous systems in the NW Sichuan Basin were uplifted and denuded as a whole due to the impact of the Liujiang and Yunnan movements. Two weathering and erosional surfaces occurred between the east and west crestal lines of the original Caledonian palaeouplift. One lies between the Middle Devonian Guanwushan Fm and overlying Lower Carboniferous Zongchanggou Fm, and the other between the Lower and Middle Carboniferous Zong-changgou Fm and Huanglong Fm and overlying Middle Permian Liangshan Fm. Karstic breccia, vadose silt, corroded gullies, dissolved caverns, dissolved fractures and dissolved pores can be observed everywhere on outcrop and drilling cores (Fig. 6). Similar to the Carboniferous Huanglong Fm

reservoir rocks in eastern Sichuan Basin, intense karstification induced more pore space as well as corroded gullies, dissolved fractures and fractures, which greatly enhanced the reservoir permeability.

2. Control of sedimentary and tectonic evolution over hydrocarbon accumulation

2.1. Good Upper Paleozoic hydrocarbon accumulation conditions generated by multi-phase tectonic movements

2.1.1. Source conditions generated by the Tongwan movement, Khanka taphrogeny and Caledonian orogeny

The Tongwan movement mainly occurred in the Dengying Fm period and featured large-scale uplifting. A giant erosional valley in near NS direction was generated by the Tongwan movement at the late stage around Guangyuan, Mianyang, Yibin and Huidong in the Sichuan Basin. The succedent Khanka taphrogeny occurring in the Early Cambrian featured fault subsidence; consequently, massive high-graded Lower Cambrian source rocks were deposited in the valley. In the Gaoshiti—Moxi area in central Sichuan Basin, high-graded source rocks reside aside the Sinian Dengying reservoir rocks and below the Cambrian Longwangmiao reservoir rocks; thus, the Sinian Dengying Fm gas reservoirs and Cambrian Longwangmiao Fm gas reservoirs formed two giant gas fields,

a. Qingchuan Hejialiang section, D,g, dolomite with karstic rubbles

d. Well ST3, Djg, 7581.92-7582.01 m, karstic brecciated dolomite

b. Qingchuan Hejialiang sectioa Djg, brecciated dolomite with dissolved pores and vugs filled with bitumen

e. Well ST3, D;x, 7586.33 m, fine to moderate crystalline dolomite with dissolved fractures, 2.5*10(-)

c. Well ST3, D2g, 7583.73-7583.95 m, karstic brecciated dolomite

f. Qingchuan Nianziba sectioa D,g, karstic brecciated dolomite with dissolved ftactures and intragranular dissolved pores, 25:- Hj -j

Fig. 6. Devonian karstification in NW Sichuan Basin.

i.e. Anyue and Moxi. In the region around Guangyuan and Mianyang in the NW Sichuan Basin, high-graded Lower Cambrian source rocks exceeding 500 m thick generated massive hydrocarbon for the Upper Paleozoic reservoir rocks. Oil seepages and bituminous veins, most of which were considered to be originated from the Cambrian source rocks, were found to pervasively exist in the Middle Permian Qixia Fm and Carboniferous, Devonian and Cambrian systems in the NW Sichuan Basin [1,6—8].

Due to the effect of the Caledonian palaeouplift, most Devonian Fms directly overlie the Cambrian system; thus the Cambrian argillaceous source rocks became a major hydrocarbon supplier to the Upper Paleozoic reservoir rocks in the NW Sichuan Basin. The Qiongzhusi Fm mudstone contains sapropelic organic matter with high abundance. The average basin-wide organic matter content is 3%. As indicated by the hydrocarbon generating intensity map (Fig. 7-a) [9], the basin-wide Qiongzhusi Fm gas generating center is reconciled with the palaeo-aulacogen around Deyang and Anyue [10]. The aulacogen extends in N—S direction in central Sichuan Basin and its boundary could be clearly observed in western Sichuan

Basin. In the NW Sichuan Basin, the hydrocarbon generating intensity of Qiongzhusi Fm source rocks is (40—80) x 108 m3/ km2.

The Silurian source rocks at both flanks of the Caledonian palaeouplift and its northern pitching end are another package of Upper Paleozoic source rocks in the NW Sichuan Basin. The Middle Permian source rocks include argillaceous limestone and argillite. The former contains mixed organic matter with more humic content; the average basin-wide TOC content is 1.4%. The latter with small thickness contains humic organic matter; the average basin-wide TOC content is 2.24%. As indicated by the hydrocarbon generating intensity map (Fig. 7-b) [11], the NW Sichuan Basin is among the Middle Permian gas-generating centers in the basin with a large intensity of (18—40) x 108 m3/km2.

2.1.2. Extensive hydrocarbon accumulations in fossil reservoirs due to the Indosinian movement

The west crest and western central part of the Caledonian palaeouplift were extruded and uplifted by the Longmenshan Mountains to experience folding, deformation and denudation

Fig. 7. Lower Cambrian and Middle Permian hydrocarbon-generating intensity.

in the Indosinian. This region is the northwest boundary of the basin nowadays.

The region on the east of the innermost Caledonian palaeouplift lay at the footwall of the early fault. Due to the effect of compressive stress, the region was uplifted to form the Tianjingshan palaeouplift [12,13] extending in NNE direction in the Indosinian. The central part occurred around Guangyuan and Mianyang. The formations older than the third to first members of the Middle Triassic Leikoupo Fm have been denuded. The area is 7680 km2 ( ). The Leikoupo Fm is in disconformable contact with the overlying Upper Triassic Xujiahe Fm, which is overlain with complete series of strata younger than the Upper Triassic.

The Lower Cambrian source rocks became mature enough to generate mass oil in the Early Triassic and generate mass gas in the Middle Triassic, while the Tianjingshan palaeouplift formed in the Indosinian. Such a coincidence was favorable for hydrocarbon accumulation in the Upper Paleozoic. Residual Triassic Jialingjiang Fm and Leikoupo Fm gypsum rocks may exceed 300 m thick and were not penetrated by early faults in the basin; thus early hydrocarbon accumulations in the Upper Paleozoic may be well preserved within the Tianjingshan palaeouplift forming in the Indosinian.

2.1.3. Hydrocarbon scale enrichment dominated by the Yanshan—Himalayan orogeny

Owing to the stress induced by Longmenshan Mountains napping in the Yanshanian and Himalayan, 3 nappe tectonic units, i.e. Qingchuan, Beichuan and Majiaoba thrust nappe belts [14—16] (Fig. 9), occurred from west to east in the region with high relief on the west of the basin boundary formed in the Indosinian. A number of large- to medium-sized faults and large inversion structures grew in the thrust nappe belts, which gave rise to an extremely complicated tectonic framework. Napping-induced compressive stress has been fully released.

The region on the east of the basin boundary formed in the Indosinian underwent relatively weak compressive stress. Only a hidden fault, named Hidden fault I on seismic data, extends in a long distance at the west flank of the Tianjingshan palaeouplift formed in the Indosinian. The growth of Hidden fault I led to further tectonic stress relief; thus the back thrust anticlinal belt at the footwall in the east is an inherited uplifted high zone developed against the tectonic setting of the Tian-jingshan palaeouplift formed in the Indosinian.

In the Himalayan, what accumulated in the Upper Paleozoic was gaseous hydrocarbon. As is indicated by seismic and drilling data, early faults generated by the Indosinian movement further grew in the process of the Yanshan—Himalayan orogeny; but the Lower and Middle Triassic gypsum rocks were not penetrated by these faults yet (Fig. 9). In addition, rock column pressure on fault surfaces increased with the bural depth, which indicates better sealing performance of faults for gas accumulation. Due to the impact of napping and compression by the Longmenshan Mountains on the west and Micangshan Mountains on the north, the back thrust anticlinal belt at the footwall of Hidden fault I grew on the Tianjingshan palaeouplift formed in the Indosinian and became a large anticlinorium high zone. Within this tectonic unit, the Upper Paleozoic constituted a large closed system bounded by stratigraphic pinch-outs, lithologic interfaces and hidden faults with a good sealing performance. Natural gas re-accumulated in each series of strata to form gas reservoirs.

2.2. Promising Upper Paleozoic gas accumulations in the NW Sichuan Basin

The wells drilled in the region on the east of the footwall of the Longmenshan thrust nappe belt in the NW Sichuan Basin had good oil and gas shows in the Upper Paleozoic with no water yield and water shows. Well K3 yielded gas flow of

Fig. 9. Faults and structural features in NW Sichuan Basin.

0.12 x 104 m3/d from the Carboniferous system in the production test after well completion and acid treatment. Well HS1 drilled at the Hewanchang structure had gas invasion and well kick shows in the Devonian Jinbaoshi Fm; collected gas was burned. Well H6 yielded a slight amount of gas from the Devonian system in the open-hole oil testing. Well ST1 yielded gas flow of 87.6 x 104 m3/d and 126.77 x 104 m3/d from the Qixia Fm and Maokou Fm, respectively, in the production test. Well ST2 yielded gas flow of 0.79 x 104 m3/d from the Qixia Fm in the production test. Well ST3 drilled recently had frequent gas invasion and gas logging anomalies in the Upper Paleozoic. Gas zones of 21.8 m thick in the Qixia Fm were interpreted on log data with a weighted average gas saturation of 88%; gas zones of 18.7 m thick in the Devonian Guan-wushan Fm were interpreted on log data with a weighted average gas saturation of 85%. No water zones were interpreted. As per well drilling results, the region on the east of the footwall of the thrust nappe belt in the NW Sichuan Basin may have pervasive gas accumulations in the Upper Paleozoic, whereas the thrust nappe belt may have poor preservation conditions due to its complicated structures and shallow burial of the Upper Paleozoic.

The formation pressure at the middle of Qixia Fm pay zones in Well ST1 was 95.43 MPa and the pressure coefficient was 1.35. As per well testing data of this well, the Qixia Fm contains quasi-homogeneous layered porous reservoir rocks. As was mentioned above, the Qixia Fm dolomite drilled by Wells ST3 and ST1 has similar tectonic location, thickness and internal structure; thus it is inferred that the Middle Permian Qixia dolomite reservoir rocks spread pervasively with high gas saturation in the NW Sichuan Basin.

Vertically, there are no tight barriers between the Upper Paleozoic Devonian, Carboniferous and Middle Permian Qixia Fm reservoir beds with a spacing interval of 50—70 m. The Upper Paleozoic may belong to one petroleum system due to the existence of abundant faults and fractures. As per the studies of Qixia Fm hydrocarbon accumulation in northwestern Sichuan Basin, laterally there may be a large combination trap system with its west boundary on the east of Hidden fault I at the footwall of the Longmenshan thrust

nappe belt. This west-inclined fault disappeared downward in the Cambrian system and upward in Lower and Middle Triassic gypsum rocks. Due to its good sealing performance, this fault may act as the west boundary of the trap system. Owing to the existence of the Caledonian palaeouplift, marginal platform banks in the Qixia Fm experienced weak dolomitization around Hewanchang, Shejianhe and ST2 well field in the north with low relief and intense dolomitization in the south with high relief; thus, a lithologic interface may occur between porous dolomite and tight limestone. The Devonian and Carboniferous Fms pinched out eastward to form a stratigraphic barrier. Inside the Qixia Fm, there may also be a lithologic interface between porous dolomite and tight limestone, because the sedimentary facies changed from marginal platform bank to intra-platform bank and interbank sea. The system with complicated structural—stratigraphic and structural—lithologic traps covers an area of 3300 km2 inside the Tianjingshan palaeouplift formed in the Indosinian. Source, reservoir, migration and preservation conditions make pervasive gas accumulation possible.

The Lower Cambrian source rocks were mature enough to generate liquid hydrocarbon at the end of the Silurian. At the peak of hydrocarbon generation in the Early Triassic, liquid hydrocarbon moved along the early faults and fractures induced by the Liujiang—Yunnan movements and Dongwu movement and quickly accumulated in the Upper Paleozoic. Mass gaseous hydrocarbon was expelled from the Cambrian system in the Middle Triassic, while the Tianjingshan palaeouplift formed in the Indosinian. Such a coincidence was favorable for early hydrocarbon accumulation in the Upper Paleozoic. In the Himalayan, the Upper Paleozoic gas re-accumulated in the back thrust anticlinal belt at the footwall of the hidden fault. Thick gypsum rocks deposited in the Middle Triassic functioned as the overburden of Upper Paleozoic gas reservoirs. A mass of oil and gas seepages and bituminous veins have been observed at the hanging wall of the thrust nappe belt, whereas no evidences of oil and gas release have been discovered by well drilling in the large anticlinorium high zone at the footwall of the thrust nappe belt, which indicates a good sealing performance for pervasive gas accumulation.

Fig. 10. Upper Paleozoic prospects in NW Sichuan Basin.

3. Exploration direction and favorable targets

Over a decade of efforts on the Middle Permian Qixia Fm porous dolomite reservoirs since 2003, Wells ST1 and ST3 have been drilled with the Qixia Fm reservoirs and Devonian Guanwushan Fm porous dolomite reservoirs for the first time. This was the beginning of the Upper Paleozoic exploration in the NW Sichuan Basin. Marine hydrocarbon exploration became possible in western Sichuan Basin. After the discoveries of Sinian Dengying Fm and Cambrian Longwangmiao Fm gas reservoirs in the central Sichuan palaeouplift, the Upper Paleozoic in western Sichuan Basin may become another successor for gas exploration and development in the Sichuan Basin.

The Upper Paleozoic exploration in the thrust nappe belt in the NW Sichuan Basin yielded no results. In recent years, exploration activities focused on the large complex structural— stratigraphic and structural—lithologic traps in the back thrust anticlinal belt at the footwall of the thrust nappe belt (Fig. 10), where the top elevation of the Qixia Fm ranges from —6000 m to —7300 m and the closure of the complex trap system is 1300 m. The area on north Mianyang is 3300 km2 and the Upper Paleozoic gas resources are estimated to be 1.55 x 1012m3.

Due to the impact of the Caledonian palaeouplift plunging northward, the region around Shejianhe and Zhongba was structurally high during deposition and thus experienced more intense dolomitization in the Qixia Fm. The Qixia Fm dolomite reservoirs may be thick from the central axis of the Caledonian palaeouplift to the east crestal line, which may be a promising prospect for Qixia Fm gas exploration. The Devonian and Carboniferous Fms pinched out toward the east.

In the region around Shejianhe and Zhongba, the Guanwushan Fm and Carboniferous reefs and banks distribute widely from the central axis of the Caledonian palaeouplift to Hidden fault I to the west. Superposed karstification improved reservoir properties in a large area. This region has three Upper Paleozoic series of strata for exploration, which include Qixia Fm and Devonian Jinbaoshi reservoirs, and thus is a major promising prospect in recent exploration.

The back thrust anticlinal belt at the footwall of the thrust nappe belt is a major prospect for Upper Paleozoic exploration in the NW Sichuan Basin now. If there is sufficient knowledge about gas accumulations in the back thrust anticlinal belt, future exploration may deal with the Upper Paleozoic gas accumulations in the thrust nappe belt to expand the area of exploration.

4. Conclusions

1) The Upper Paleozoic in the NW Sichuan Basin has good gas accumulation conditions, including thick high-graded Cambrian source rocks, porous dolomite reservoirs of reef-bank facies, faults connected with source rocks, and overburdens with a good sealing performance. Natural gas may accumulate in the large complex trap system dominated by stratigraphic pinch-outs, lithologic interfaces and hidden faults with good sealing performance.

2) The quartz sandstone in the Upper Paleozoic Devonian Jinbaoshi Fm is a potential package of reservoir rocks. Several packages of dolomite reservoir rocks rich in pores and vugs distribute steadily in the Devonian Guanwushan Fm, Carboniferous and Middle Permian Qixia Fm. These packages of reservoir rocks with a

spacing interval of 50—70 m have not been separated by tight rocks and may belong to the same petroleum system due to similar source, reservoir, migration, preservation and accumulation conditions.

3) Large structural—stratigraphic and structural—lithologic gas reservoirs may occur in the back thrust anticlinal belt on the east of Hidden fault I. The region from the central axis of the Caledonian palaeouplift to the east crestal line may be the most promising prospect for Qixia Fm gas exploration, while the region from the central axis to Hidden fault I may be the most promising prospect for concurrent exploration of 3 Upper Paleozoic series of strata.

4) After the discoveries of Sinian Dengying Fm and Cambrian Longwangmiao Fm gas reservoirs in the central Sichuan palaeouplift, the Upper Paleozoic in western Sichuan Basin may become another successor for gas exploration and development in the Sichuan Basin.

References

[1] Zhou Wen, Deng Hucheng, Qiu Dongzhou, Xie Runcheng. The discovery and significance of the Devonian paleo-reservoir in Tianjingshan structure of the northwest Sichuan, China. J Chengdu Univ Technol Sci Technol Ed 2007;34(4):413—7.

[2] Deng Hucheng, Zhou Wen, Qiu Dongzhou, Xie Runcheng. Oil sand-forming conditions and evaluation on resource of oil sand in Tianjing-shan structure in northwest part of Sichuan Basin. J Jilin Univ Earth Sci Ed 2008;38(1):69—75.

[3] Sun Xiaomeng, Xu Qiangwei, Wang Yingde, Tian Jingxiong, Wang Shuqin, Du Jiyu. Reservoir forming characteristics and main controlling factors of oil sandstones in the northern Longmen Mountain Thrust Zone of the Northwest of Sichuan. J Jilin Univ Earth Sci Ed 2010;40(4):886—96.

[4] Yang Guang, Wang Hua, Shen Hao, Yang Yuran, Jia Song, Chen Wen, et al. Characteristics and exploration prospects of middle Permian reservoirs in the Sichuan Basin. Nat Gas Ind 2015;35(7):10—6.

[5] Pang Yanjun, Zhang Benjian, Feng Renwei, Wang Yingrong. Evolution of Devonian depositional environment in northern Longmenshan tectonic belt. Glob Geol 2010;29(4):561—8.

[6] Xie Banghua, Wang Lansheng, Zhang Jian, Chen Shengji. Vertical distribution and geochemical behaviors of the hydrocarbon source rocks in the north section of Longmen Mountains. Nat Gas Ind 2003;23(5):21—3.

[7] Huang Difan, Wang Lansheng. Geochemical characteristics of bituminous dike in Kuangshanliang area of the northwestern Sichuan Basin and its significance. Acta Pet Sin 2008;29(1):23—8.

[8] Liu Chun, Zhang Huiliang, Shen Anjiang, Qiao Zhanfeng, Ni Xinfeng, Zhao Xueqin. Geochemistry characteristics and origin of the Devonian oil-sandstone in the northwest of Sichuan Basin. Acta Pet Sin 2010;31(2):253—8.

[9] Luo Bing, Xia Maolong, Zhang Hongying. Report on the new incremental proved natural gas reserves in the 4th member of the Sinian Dengying Fm from the well Gaoshi 1 in the Gaoshiti block, Anyue gas field, Sichuan Basin. Chengdu: Exploration and Development Research Institute of PetroChina Southwest Oil & Gasfield Company; 2015.

[10] Xu Chunchun, Shen Ping, Yang Yueming, Luo Bing, Huang Jianzhang, Jiang Xingfu, et al. Accumulation conditions and enrichment patterns of natural gas in the lower Cambrian Longwangmiao Fm reservoirs of the Leshan-Longnusi Paleohigh, Sichuan Basin. Nat Gas Ind 2014;34(3):1—7.

[11] Xie Zengye, Dong Caiyuan, Zhu Hua. Research on source rocks and resource potential related to the Permian gas reservoirs in the Sichuan Basin. Chengdu: Exploration and Development Research Institute of PetroChina Southwest Oil & Gasfield Company; 2015.

[12] Chen Zongqing. On five crustal movements and petroleum exploration in Lower Paleozoic, Sichuan Basin. China Pet Explor 2013;18(5):15—23.

[13] Hu Shouquan, Guo Wenping, Tong Chongguang. Extra late period bruchfaltung and hydrocarbon potential in north of the Longmenshan tectonic belt. J Southwest Petroleum Inst 2001;23(2):5—8.

[14] Luo Xiaoquan, Li Shubing. Discussion on forming evolution of Long-menshan nappe zone and its oil/gas correlation. Nat Gas Technol 2009;3(1):16—8.

[15] Wu Xuefeng, Han Song. Exploration potential of Xujiahe Fm gas reservoir in thrust belt, Sichuan Foreland Basin. Nat Gas Explor Dev 2014;37(1):10—3.

[16] Wang Lansheng, Han Keyou, Xie Banghua, Zhang Jian, Du Min, Wan Maoxia, et al. Reservoiring conditions of the oil and gas fields in the north section of Longmen Mountain nappe structural belts. Nat Gas Ind 2005;25(S1):1—6.