Oil and gas breakthroughs in ultra-deep Ordovician carbonate formations in Shuntuoguole uplift, Tarim Basin

Transcription

1 CHINA PETROLEUM EXPLORATION Volume 21, Issue 3, May 2016 Oil and gas breakthroughs in ultra-deep Ordovician carbonate formations in Shuntuoguole uplift, Tarim Basin Qi Lixin Sinopec Northwest Oilfield Company Abstract: The Shuntuoguole uplift in the Tarim Basin was formed in the middle stage of the Caledonian epoch. In the early stage of the Early Cambrian epoch, the Yuertusi Formation slope-shelf facies source rocks were formed. During the Early Cambrian-Middle Ordovician epoch, carbonate sedimentary formations with a thickness of up to 3000 m were developed, and multiphase sea level fluctuations contributed to the formation of exposed karst reservoirs. Meanwhile, during the Caledonian-Hercynian epoch, multiphase faulting activities occurred that encouraged fracture development and fluid reformation and caused the formation of multiple types of reservoirs. These very thick mudstones, deposited during the Late Ordovician epoch, can act as regional cap rocks. In the Shuntuoguole uplift, a complete source-reservoir-cap rock assemblage exists in the Lower Paleozoic, suggesting that it has prospects of hydrocarbon accumulation. The oil and gas exploration at the earlier stage focused on the Shaya and Katake uplifts while the deeply-buried Shuntuoguole uplift was not thoroughly understood. Further studies revealed that the Shuntuoguole uplift has been in the low geothermal environment for a long time. During the Himalayan epoch, the Lower Cambrian source rocks were generating oil-natural gas condensate, implying that favorable conditions for the formation of large oil and gas fields with late-stage accumulation existed during that time. The Cambrian Middle-Lower Ordovician carbonate rocks are the most important targets. Based on the seismic technology R&D for ultra-deep zones in desert areas, favorable targets were selected for well drilling. Great breakthroughs have been made at these targets. A giant oil and gas field in the Shuntuoguole uplift is taking shape. The realization of a continuous oil and gas pattern in the central and northern Tarim Basin is promising. Key words: Tarim Basin, Shuntuoguole uplift, Ordovician, carbonate rock, ultra-deep formation, oil and gas breakthrough The Tarim Basin is China s largest inland petroliferous basin, and it has abundant oil & gas resources [1 4]. Exploring these resources, however, remains difficult due to the extremely complex surrounding geological conditions. To date, a total of 37 oil and gas fields have been discovered in two domains, i.e., carbonate and clastic rocks, with 3P reserves reaching tons of oil equivalent (toe), percentage of proven oil reaching 19.11%, and percentage of proven gas reaching 11.1%, all of which imply a large exploration potential and extensive exploration prospects. Previous exploration efforts confirmed the Ordovician carbonate rock as the primary target layer, the Cambrian basinal-slope-shelf facies mudstone (which is characterized by multiphase hydrocarbon generation, multiphase hydrocarbon accumulation and multiphase tectonic movement) as the major source rock, and the Katake and Shaya uplifts as favorable sites for hydrocarbon accumulation. So far, some large-scale oil and gas fields (e.g., Tahe and Central Tarim) [5 6] have been discovered in the Shaya and Katake uplifts. The Shuntuoguole uplift, which lies within the transition zone between these two uplifts, is less explored and studied because of its deep burial depth. Integrated studies in recent years have led to considerations that due to its good hydrocarbon accumulation conditions, well-developed Yuertusi Formation source rock, and thick regional cap rock, the Shuntuoguole uplift has favorable conditions for the formation of large-scale oil and gas fields [7]. Some major breakthroughs and discoveries obtained as a result of this shift in guiding exploration principles brought by deepened understanding and the continual exploration efforts make the Shuntuoguole uplift a new resource province for oil & gas exploration in the Tarim Basin. This paper analyzes the process of creating breakthroughs in oil & gas exploration in the Shuntuoguole uplift, and it also aims to provide a reference for exploration in other areas. 1. Basic geological features The Shuntuoguole uplift is located within a desert area in the central Tarim Basin and is in close proximity to the Katake uplift, the Guchengxu uplift, the Awati depression, and the Manjiaer depression (Fig.1). Received date: 10 Mar. 2016; Revised date: 20 Arp Corresponding author. qilx.xbsj@sinopec.com Foundation item: China National Key Science & Technology Major Project "Formation Rule and Exploration Evaluation of Large- and Medium-Scale Oil & Gas Fields in Marine Carbonate Formations, Tarim and Ordos Basins" (No.: 2011ZX ) and "Enrichment Rule and Exploration Direction of Large-Scale Oil and Gas Fields in Clastic Formations, Tarim Basin" (No.: 2011ZXC ). Copyright 2016, Petroleum Industry Press, PetroChina. All rights reserved.

2 2 CHINA PETROLEUM EXPLORATION Vol. 21, No. 3, 2016 Fig. 1 Structural location of the Shuntuoguole uplift in the Tarim Basin As a result of the Keping movement at the end of the Sinian epoch, denuded zones in the Central Tarim and Southwestern Tarim areas expanded to form a structural unconformity between the Sinian and the overlying Cambrian strata. This unconformity is angular in the Katake uplift and the central-eastern part of the Shuntuoguole uplift, but parallel in the central-western part of the Central Tarim area. During the Early Cambrian epoch, rapid extensional rifting around the Tarim plate brought about a rapid sea level rise, allowing for widespread deposition in the Yuertusi Formation, which holds the first section of major source rock in the platform basin area: the Tarim Basin. The Shuntuoguole uplift is located within the slope-shelf facies zone, where slope facies source rocks were developed. Since the Ordovician epoch, the Shuntuoguole uplift and its adjoining areas have undergone carbonate platform development, intra-platform uplift-slope formation, reformation by NW- and NE-trending faults, magmatic activity, regional tilted disturbances, and early-stage fault reactivation. During the Early to Middle Ordovician, the carbonate platform was developed broadly across the Central Tarim area. During the Late Ordovician, as the platform was separated from the basin in the Central Tarim area, the Katake uplift was uplifted continuously, the Shuntuoguole uplift was situated on a structural slope and subsided steadily, and the NW-trending thrust fault was active. During the Silurian-Early to Middle Devonian, the Katake uplift took form, the Shuntuoguole uplift continued to steadily subside, and the NE-trending strike-slip fault began to be active. During the Carboniferous-Permian, the Shuntuoguole uplift was buried and then once again uplifted at its east, early-stage faults were reactivated, and intense magmatic activity occurred. During the Meso-Cenozoic, the Shuntuoguole uplift subsided entirely, and multiple regional tilting events and reformations by locally activated faults occurred (Fig.2) Caledonian A carbonate platform was developed in the Central Tarim and Shuntuoguole areas during the Early Caledonian epoch. During the Cambrian-Early Ordovician, the Tarim Basin was within the extensional basin stage, with a marine carbonate platform being steadily developed in the Central Tarim area to connect with the Shuntuoguole area [7]. A stable cratonic carbonate formation laid a good material foundation for the formation of multilayered, widespread karst fracture-cave-type carbonate reservoirs. Stratigraphic studies based on field outcrop and carbon and oxygen isotopes indicate that a multistage sea level decrease during the Ordovician exposed the carbonate platform and caused it to be dissolved [8 9], which in turn allowed for the formation of an unconformity related to sedimentary hiatus and the occurrence of karstification via meteoric water. As a result, karst fracture-cave-type reservoirs were formed in the Penglaiba Formation and at the top of the lower member of the Yingshan Formation.

3 Qi Lixin, Oil and gas breakthroughs in ultra-deep Ordovician carbonate formations in Shuntuoguole uplift, Tarim Basin 3 Fig. 2 Nearly E-W structural evolution section of the Shuntuoguole uplift (with location shown in Fig.1) The Katake and Shuntuoguole uplifts were formed during the Middle Caledonian epoch. During the Middle Caledonian Episode I (end of the Middle Ordovician), the Katake uplift was raised, resulting in a significant change to the tectonic framework. The Central Tarim I fault, the Central Tarim South Margin fault, and

4 4 CHINA PETROLEUM EXPLORATION Vol. 21, No. 3, 2016 the Central Tarim II fault jointly controlled the formation of a large-scale composite anticlinal pop up structure in the Katake uplift. In the footwall of the Central Tarim I fault, the Shuntuoguole low uplift was formed in the northwestern part of the Shuntuoguole uplift as a result of segmentation and the uplift s transition into the Manjiaer depression, which was filled with deep-sea trough deposits that later led to a "high in northwest and low in southeast" framework. The Yijianfang Formation in the main part of the Katake uplift was entirely denuded, and the top of the Yingshan Formation was partially denuded by the karstification during the Middle Caledonian Episode I. In the Shuntuoguole uplift, the Yijianfang Formation might be exposed around the Central Tarim I fault. Additionally, in areas far from that fault, the Yijianfang formation primarily contains dissolved pores and caves and fractures that are semi- to fully-filled and whose formation was controlled by pene-contemporaneous exposure and dissolution. Meanwhile, the NW, NE and NEE faulting has also facilitated the development of karst reservoirs. During the Late Ordovician, the North Kunlun Ocean (at the southern margin of the Tarim Basin) closed, and the southern margin morphed into a continuously shrinking compressive structural environment [10]. At the end of the Late Ordovician, the southern part of the Katake uplift was strongly thrusted and compressed [11]. During the Middle Caledonian Episode II (deposition of the Lianglitage Formation), the platform was separated from the basin in the Central Tarim area by the Central Tarim I fault in the north and the South Margin fault in the south. As a result, the Katake uplift became an isolated carbonate platform, the Shuntuoguole uplift subsided, and a very thick basinal facies flysch formation was deposited. At the end of the Ordovician, the carbonate platform was submerged, and it eventually transitioned into the neritic mixed shelf. During the Late Ordovician, a very thick mudstone of the Queerqueke Formation was deposited onto the Shuntuoguole uplift, and this mudstone has the potential to act as a high-quality regional cap rock The Guchengxu uplift was formed during the Late Caledonian The South Tianshan oceanic crust began to subduct towards the Central Tianshan uplift after the Silurian epoch. This continuous subduction and collision exerted an extrusion stress environment on the Tarim Basin. The Ordovician, Silurian, and Devonian strata overlying the Guchengxu uplift were strongly eroded, and the Central Tarim area showed a nose uplift plunging towards the north and west, forming a "high in east and low in east" framework. During the Late Silurian, continual collision between the Early Paleozoic Central Kunlun island arc and the Central Kunlun terrain caused (1) the foreland folding and thrusting to strengthen significantly and march towards the hinterland; and (2) the foreland fold-thrust belt to expand towards the craton basin until the pre-existing foreland basin was transformed into a large-scale thrust belt in a way similar to the transformation of the Early Mesozoic Kuqa foreland basin to a foreland thrust belt during the Neogene period [12-13]. During the Silurian-Devonian, strata were uplifted as thrusting became stronger, which eventually allowed for the formation of the Guchengxu uplift Hercynian During the Middle Devonian, the Aerjin fault uplift was strongly uplifted and the eastern part of the Shuntuoguole uplift was uplifted and exposed to erosion. The inherent movement of the large-scale strike-slip fault in the Shuntuoguole uplift was diffused and extended upward to the Middle-Upper Devonian, inducing a strong structural deformation as the NW-trending thrusting was transferred into the interior uplift. As a result of this effect, wide fracture belts were created, deformations became complex, and a series of echelon and plumose structures that exhibited different deformational features from the Silurian were formed along the main fracture belt. During the late period of Devonian, there was a strong inclined collision event between the northern margin of the Tarim plate and the Yili-Central Tianshan landmass in response to the east to west scissor-type closing of the South Tianshan Ocean. The Shuntuoguole uplift was further uplifted by the Early Hercynian movement, which lead to the absence of the Devonian strata at its axis. A strong Late Devonian collision between the eastern margin of the Tarim plate and the Central Tianshan landmass caused the Shuntuoguole uplift to be strongly compressed from east to west and, as a result, caused the uplift to shrink and narrow, allowing for the shifting of the uplift s top towards the west. A cross section trending east-west shows that the Devonian strata were denuded and thinned progressively from the Awati depression in the west and the Manjiaer depression in the east towards the Shuntuoguole uplift until they pinched out. The Shuntuoguole uplift, as indicated by the distribution of the residual thickness of strata exposed at its axis and the Silurian strata, was not as uplifted as the adjacent Katake and Shaya uplifts, although it was greatly uplifted by the Early Hercynian movement. In response to the transition from transpressional to transtensional regional stress in the Shuntuoguole uplift region, a series of inherited transtensional strike-slip faults were developed on the foundation of some NE-trending transpressional strike-slip faults formed

5 Qi Lixin, Oil and gas breakthroughs in ultra-deep Ordovician carbonate formations in Shuntuoguole uplift, Tarim Basin 5 during the Middle Caledonian, with the presence of a series of associated NW-trending en echelon normal faults displaying a negative flower structure. These transtensional strike-slip faults, together with their associated fault systems, facilitated the development of micro-fractures in carbonate rocks and also provided pathways for reformation of deep hydrothermal fluids, as confirmed by drilling Wells SN4 and ST1. During the Middle-Late Hercynian orogeny (Carboniferous-Permian), compressional or transpressional faults were formed in the Shuntuoguole uplift due to the transition from an extensional setting to a compressional stress setting. As a result, it exhibited a structural framework similar to the one it showed during the Early Hercynian. In the Carboniferous, the inherited local compressional faults were developed. After the deposition of the Carboniferous, the Central Tarim area was integrally subsided, a weak inherited compressive movement occurred at the central fault horst belt at the end of the Carboniferous, and the Xiaohaizi Formation (at the top of the Carboniferous) was denuded. During this period, faulting was small in scale and affected a limited range, i.e., the central main horst belt and the Well CT 5 fault zone [14]. During the Early Permian, volcanic rocks were widespread across the central-western part of the Central Tarim area. These rocks, according to seismic profiles, mostly extended along strike-slip fault belts and thrust faults that cut into the ground, with the exception of those isolated along point-like spots. Small-scale normal faults were present around and at the top of localized volcanic rock development zones (e.g., Well ST1 area). Some of them could cut into the base of Triassic. Other faults include the reactivated pre-existing strike-slip faults or reformed pre-existing faults, which were predominately vertical strike-slip faults and small in scale Indosinian-Yanshanian The Tarim Basin became the southern margin of the Eurasian plate during the Triassic and was bordered to the south by the Paleo-Tethys Ocean and the Qiangtang block. This basin was surrounded by continental marginal uplifts, with the Kuqa foreland depression present at its northern margin and para-foreland depressions formed in its central region [16] where the Manjiaer depression was present. The Shuntuoguole uplift at this time had remained unchanged in its shape since the Carbonaceous-Permian and was a southward extension of the Yingmaili uplift. Together, they comprised of a large-scale nose-like uplift rising towards the north and plunging towards the south. At the end of the Triassic, the Indosinian movement occurred with a wide effect across the Tarim Basin due to the strong collision between the Qiangtang block and the Tarim plate. This resulted in a broad denudation of the Triassic strata. A giant paleo-uplift, namely, the Western Tarim uplift [12, 17], was formed in the western region of the basin, causing the shape of the Shuntuoguole uplift to vanish Himalayan The Indian plate collided with the Eurasian plate at the end of Eocene epoch and had continuously exerted a strong compression since the Neogen period. In the northern part of the Tarim Basin, the northward collision of the Indian plate reactivated the ancient Tianshan orogenic belt [18]. The Neogene period marked the commencement of a strong thrusting-napping of the Kuqa foreland thrust belt, which caused the Kuqa-Awati region to strongly subside during the Neogene, the Kuqa-Awati depression to form, and the subsidence center to continually shift towards the south and east during the Neogene. As a result, a large regional slope trending northwest was formed at the Shuntuoguole and Tabei uplifts, which is the foreland slope of the Kuqa foreland depression. In the Late Himalayan, the present-day Shuntuoguole uplift was formed. During the Cenozoic, particularly the Neogene, the Tarim region further subsided as the surrounding mountains (e.g., the South Tianshan Mountain, the Kunlun Mountain, and the Aerjin Mountain) arose rapidly, and the secondary structural units within the basin took their form. The Paleogene and Neogene strata were well-developed within the basin, and the Central Tarim area eventually calmed down. 2. Exploration history 2.1. Focus on the Katake uplift zone and keep exploring for oil & gas discoveries The first oil & gas discovery was made in 2003 at the Yingshan Formation and the karst fracture-cave-type reservoir of the Middle Caledonian Episode II. It was encountered by drilling Well Z1 in the Ka 1 block, which is located within the Katake uplift zone in the Central Tarim area of the Tarim Basin. Subsequently, a batch of wells, including Z11, Z15, Z19, Shun2, Shun6, Shun3 and Shun4, were drilled successively targeting the Yingshan Formation in the uplift zone and the Lianglitage Formation in the platform margin zone. No significant oil or gas breakthrough was made in these wells, although varying degrees of oil & gas traces were recorded. Analysis reveals the development degree of the reservoir as the primary reason for these failures in drilling exploration. With renewed consideration of this factor, four wells (i.e., Z12CX, Shun7, SX1 and SX101)

6 6 CHINA PETROLEUM EXPLORATION Vol. 21, No. 3, 2016 were drilled that yielded oil & gas discoveries, but two other wells (i.e., Z121 and Z122) drilled afterwards failed and only produced water. While focusing on the Katake uplift, the presence of oil and gas was recorded but no significant breakthrough was made. Analyzing the failure has great significance to the overall evaluation of the Central Tarim area and provided positive feedback for changing our exploration guidelines. These wells (1) prove that the Ordovician carbonate rocks in the Central Tarim area have preferable conditions for the formation of large-scale oil & gas pools, particularly in the north slope of the Katake uplift (i.e., the Shuntuoguole uplift), which has better hydrocarbon source formation conditions than elsewhere in the region and favorable geological conditions for forming large-scale oil & gas fields; (2) confirm the presence of the well-developed Middle Caledonian karst fracture-cave-type reservoir in the Katake uplift, further strengthen the structural evolution study and the sedimentary reservoir study, and allow for additional understanding of the fracture-cave-type reservoir in the Paleozoic carbonate rocks in the Katake uplift and the Shuntuoguole uplift; and (3) enable us to better understand the effects of the NE-trending faults on oil & gas reservoirs and reserves, promote fault interpretation and identification, and reveal the presence of multiple NE-trending faults in the Shuntuoguole uplift Deepen the understanding and lock onto favorable breakthrough plays in the Shunguotuole uplift In order to carry out a basic geological study and provide scientific basis for mock evaluation and exploration domain and target selection, some key issues that limit the exploration ability of the Lower Paleozoic carbonate rocks, such as the main factors that control formation of the source rock, paleo-structural evolution, reservoirs, and hydrocarbons, have been restudied since Study results indicate that the Shuntuoguole uplift has good conditions for hydrocarbon accumulation and good geological conditions for forming large-scale oil & gas fields. Through many years of exploration and continuous study, the Lower Cambrian Yuertusi Formation source rock is determined to be one of the primary source rocks in the Tarim Basin. Well XH1, drilled in the western part of the Shaya uplift, reveals that the Yuertusi Formation argillaceous rock is a good source rock, with an apparent thickness of 31 m, a TOC of 9.43%, a chloroform bitumen "A" averaging μg/g, and a total hydrocarbon content of 295 μg/g. After mapping the seismic facies of Well XH1, the Cambrian platform evolution and sedimentary facies indicate that the Shuntuoguole uplift holds the Lower Cambrian Yuertusi Formation source rock, has lateral proximity to the Cambrian-Middle to Lower Ordovician slope-basinal facies high-quality source rock in the Manjiaer depression, and, with later-stage deep and large faults acting as pathways, possesses the best conditions for hydrocarbon accumulation. With a good reservoir-cap rock assemblage, the Ordovician carbonate rock in the Shuntuoguole uplift has excellent petroleum geological conditions. A structural evolution study indicates that carbonate rocks in the Shuntuoguole uplift were exposed across a large area during the Caledonian Fig. 3 Distribution of the Lower Cambrian Yuertusi Formation source rocks in the Tarim Basin

7 Qi Lixin, Oil and gas breakthroughs in ultra-deep Ordovician carbonate formations in Shuntuoguole uplift, Tarim Basin 7 due to the influences of tectonic movement and a sea level eustacy, which allowed for the development of the Ordovician karst reservoirs. During the Middle Caledonian-Hercynian, NE-trending strike-slip faults were formed in the Shuntuoguole uplift and further reformed the reservoir by acting as pathways for meteoric water and other deep fluids. The present-day Ordovician carbonate fracture-cave-type reservoir was formed as a result of these conditions and has constituted a good reservoir-cap rock assemblage together with the thick-bedded Upper Ordovician Queerqueke Formation mudstone that is present across this area. The Shuntuoguole uplift has had an internal and lateral availability of high-quality source rocks and remained in the directional zone of hydrocarbon migration since the Middle Caledonian. The significant thickness of the overlying Queerqueke Formation regional mudstone cap rock enables a superior regional "reservoir-cap rock" condition. Multiple NE-trending strike-slip fault belts developed in this area and cut down through the Cambrian strata; hence, they could act as favorable pathways for hydrocarbon migration and reservoir reformation by fluids. Fluid inclusion analysis and other methods confirm the Himalayan as the main period for natural gas accumulation, during which the Ordovician carbonate fracture-cave-type traps were available, had good matching conditions, could trap the intense oil & gas over a broad area, and possessed favorable conditions for hydrocarbon accumulation and great geological conditions for the formation of large-scale oil & gas fields. Thus, these favorable conditions contributed to the prosperity of the Shuntuoguole uplift Make significant oil & gas breakthroughs in the Shuntuoguole uplift and open up new exploration domains As a result of updated geological understandings involving the Shuntuoguole uplift, the Lower Paleozoic carbonate rock is defined as the main exploration domain, the Middle-Lower Ordovician multi-origin reservoir is the main exploration formation, and the development zone of favorable reservoirs in the proximity of fault belt have favorable prospects. Guided by this updated information, the drilling of Well SN1 in 2011 recorded oil & gas discoveries from the Yijianfang Formation, and Wells SN4 and SN5, drilled in 2013, produced high-yield gas flows from the Yingshan Formation. Well SN4 is an exploration well drilled on the NE-trending fault belt in the southern part of the Shuntuoguole uplift. When encountering the upper member of the Ordovician Yingshan Formation, a lost circulation occurred along with a drilling break of 5.48 m. After drilling was completed, this well was tested and was found to produce m 3 per day of gas with a 6 7 mm choke without producing any liquid or water. This is a significant natural gas discovery obtained from a new formation in the footwall of the Central Tarim I fault belt. Well SN5 is an exploration well drilled in the position between the main fault belts in the southern part of the Shuntuoguole uplift, which recorded good oil & gas levels from the Yijianfang and Yingshan formations. The gas logging total hydrocarbon value rose from 1.586% to 99.99% when encountering the Yijianfang Formation interval with the best oil & gas levels, and from % to % when encountering the lower member of the Yingshan Formation. Overflow occurred when drilling reached a depth of m, and the flame height reached up to 15 m while circulating with the choke. The open flow of natural gas is estimated to be m 3 per day, which indicates a significant oil & gas breakthrough from the lower member of the Yingshan Formation. As a significant breakthrough was made in the southern part of the Shuotuoguole uplift, an exploration well (ST1) was drilled in the northern part of the uplift in 2015 to test the development features and the oil & gas potential of the Ordovician reservoirs within the NE-trending fault belt and to seek strategic successive domains for the large oil & gas province located on the north slope in the Central Tarim area. This well recorded high-yield condensate oil & gas discoveries from the Yijianfang Formation at the top section of the Yingshan Formation. Good hydrocarbon yield results was recorded when encountering the Ordovician strata, as the maximum gas logging total hydrocarbon value was 99.99%, and the flame height reached up to m while downhole circulating with the choke. The maximum natural gas production during the initial blowout stage was estimated to be m 3 per day, and during the initial blowout through the Christmas tree, the natural gas production was estimated to be m 3 per day (including condensate oil). This discovery is deemed to be a significant breakthrough in oil & gas exploration for the Shuntuoguole uplift because it indicates that the Ordovician strata in the Shuntuoguole uplift is integrally hydrocarbon-bearing, holds multiple hydrocarbon-rich layers, and depicts the outline of a large-scale oil & gas field in the Shuntuoguole uplift. 3. Hydrocarbon accumulation features of the Ordovician 3.1. Reservoir and reservoir-cap rock assemblage Large-scale reservoir bodies are present in multiple sections of the Ordovician strata in the Shuntuoguole uplift,

8 8 CHINA PETROLEUM EXPLORATION Vol. 21, No. 3, 2016 and their reservoir spaces and types vary in different areas and sections. These large-scale reservoirs are spatially superimposed and distributed mainly in the Yijianfang Formation at the top section of the Yingshan Formation, the lower member of the Yingshan Formation, and the Penglaiba Formation. There are three main underlying mechanisms of reservoir development. Fault-fracture systems are a key factor that run through the development of reservoirs in this area and enable a special "layer-controlled" & "fault-controlled" stereoscopic reservoir development mode peculiar to this area. Reservoirs present in the Yingjianfang Formation at the top section of the Yingshan Formation, as indicated by Wells SN7 and ST1, exhibit different types of reservoir spaces, dominated by semi-filled high-angle faults, residual dissolved pores-caves, and a variety of pores and structural micro-fractures. Lithofacies are considered the main factor that influences the degree of development and the type of dissolved pores. Additionally, intra-platform bioherm beach facies and microbial karstified pore-type reservoirs are well-developed. For example, the interval with the best physical properties in Well SN7 is composed of sparry calcarenite and algal boundstone, which contain well-developed intergranular/intragranular dissolved pores and have a porosity ranging from 3% to 4% and a permeability ranging from 1 to 10 md. Additionally, 19 full-diameter core samples obtained from Well SN7 reveal the low porosity and low permeability of reservoir. The sample with the best physical properties shows a lithology dominated by sparry algal limestone and also exhibits a porosity ranging from 3% to 5% and a permeability ranging from 1 to 10 md. Moreover, a statistic correlation of the relationship between micro-fractures and reservoir lithology and physical properties indicate that the development degree of micro-fractures is closely related to permeability, has a limited effect on porosity, and shows no clear relationship to lithology selectivity. For example, the interval containing well-developed micro-fractures encountered by Well SN7 exhibits low porosity and relatively high permeability (i.e., porosity of 0.9% to 1.3% and permeability of 0.3 to 10 md) and shows no selectivity towards lithology. Reservoirs present in the lower member of the Yingshan Formation, as indicated by Wells SN5 and SN7, contain reservoir spaces dominated by fractures, pores-caves and a small amount of dolostone pores. Image logs depict the presence of well-developed dissolved pores-caves and fractures. These reservoirs are seismically represented by "beads" distributed along layers. These beads, as revealed by drilling wells, are commonly present where lost circulation occurs. Large-scale reservoirs are speculated to be dominated by the fracture-cave-type, since multiple wells were tested with industrial gas. A typical natural gas reservoir with "ultra-high temperature, ultra-high pressure and high gas cut" is present in the lower member of the Yingshan Formation. No core sample was recovered from the interval where the reservoir was present in due to the complex well condition. A total of 125 full-diameter core samples recovered from the tight interval reveal a porosity of 0.2% 3% that averages out to about 1.15%, and a permeability of md that averages out to about 1.38 md. The Penglaiba Formation reservoir was encountered by a well drilled on the Guchengxu uplift in the east. This reservoir was formed in a dolostone section, and it displays selectivity in regards to lithology, shows layered distribution, and varies quickly in the lateral direction. Its reservoir spaces are provided primarily by dissolved pore-caves and dolostone intercrystalline pores. Image logs show well-developed dissolved pores-caves and fractures. NMR shows that the porosity of the sidewall core obtained from the Penglaiba Formation dolostone in Well GC7 ranges from 1% to 3%. Diagenesis and its main control factor varies across the layers of the Ordovician in the Shuntuoguole uplift. There are three main root mechanisms (Fig.4). (1) Dissolved pore-type (pore-cave-type) reservoirs controlled by (pene-) contemporaneous deposition-diagenetic environment. Lithofacies and sedimentary hiatuses control the development of reservoirs. Multiple wells encountering both the Yijianfang Formation at the top section of the Yingshan Formation and the top section of the lower member of the Yingshan Formation reveal the presence of permeable structures related to short-term exposure and leaching, residual pores from meteoric water leaching, pores-caves, and moldic pores and intragranular pores formed by selective dissolution. (2) Fracture-cave-type reservoirs formed by fault-fracture systems. NE-trending strike-slip fault systems were developed in this region and remained active during multiple stages. Some secondary (blind) faults developed between main faults and fault belts significantly enhanced the physical properties of the reservoir. Drilling activities confirm that the intensity and segmentation of the NE-trending main fault had a direct impact on the development of a large-scale reservoir. Reservoir analysis indicates that the beads in the lower member of the Yingshan Formation are mostly present where the NE-trending main fault intersects with NEE-trending main fault, and their location also matches well with the secondary faults.

9 Qi Lixin, Oil and gas breakthroughs in ultra-deep Ordovician carbonate formations in Shuntuoguole uplift, Tarim Basin 9 Fig. 4 Development mode of the Ordovician carbonate reservoirs in the Shuntuoguo uplift (3) Fracture-cave-type reservoir controlled by hydrothermal dissolution-replacement-cementation. A study of hydrothermal reformation mechanisms based on multiple wells drilled in this region suggests two kinds of hydrothermal reservoir reformation mechanisms. Hydrothermal reformation represented by Well ST1 is possibly related to the widespread volcanic intrusion body. The intrusive rocks cause a high temperature alteration of the surrounding rocks, which in turn allows for the formation of honeycomb-shaped pores-caves and the occurrence of intense matrix recrystallization. Thermal activity represented by Well SN4 is closely related to silica-rich hydrothermal dissolution-replacement-cementation, and allows fracturecave-type reservoirs to form along strike-slip fault by process of dissolution. The strong reformation of carbonate rocks by silica-rich fluids enables a transition from broad dissolution-replacement by early-stage unsaturated hydrothermal fluids to precipitation of quartz, calcite and dolostone from oversaturated fluids. Wells drilled in the Shuntuoguole uplift to test the Ordovician strata confirm the presence of three major reservoir-cap rock assemblages. (1) The first one is comprised of of a thick-bedded Upper Ordovician mudstone as a regional cap rock and a fracture-pore-cave-type reservoir with laminar distribution in the Yijiangfang Formation at the top section of the Yingshan Formation. This is represented by the discovery made by Well SN7. (2) The second one consists of the steadily deposited tight limestone in the upper member of the Yingshan Formation as a cap rock and a "bead-like" fracture-cave-type reservoir with laminar distribution in the lower member of the Yinghshan Formation, and this is represented by what was discovered by Well SN5. (3) The third one is composed of the steadily distributed tight carbonate rock at the basal lower member of the Yingshan Formation as a cap rock and the Penglaiba Formation dolostone reservoir, and this is represented by what was discovered by Well GC9. In addition, a reservoir-cap rock assemblage peculiar to this area, consisting of tight carbonate rock as a cap rock and fracture-type and fracture-cave-type reservoirs controlled by fault belts, was formed within the NE-trending main strike-slip fault belt Oil & gas reservoir features Crude oil characteristics To date, only two wells (SN1 and ST1) have produced condensate oil from the Ordovician strata in the Shuntuoguole uplift. With a gas-oil ratio exceeding m 3 /m 3, a low density of condensate, a good quality crude oil density of g/cm 3, a viscosity of 2.67 m Pa. s, a paraffin content ranging from 3.73% to 7.78%, a sulfur content of 0.03%, a low solidification point, and a noticeable absence of bitumen or gum, the oil produced from Well ST1 is classified as condensate oil with low viscosity, low sulfur, and high paraffin (Table 1). Geochemically, it shows a high saturated hydrocarbon content (93.5%), a high saturated to aromatic hydrocarbon ratio (46.8), a low to moderate content of nonhydrocarbons including asphaltene (4.5%), a unimodal distribution of prepeak on whole oil chromatography, nc 14 as the prominent peak carbon, and a wholely retained light hydrocarbon composition. Crude oil produced from Well ST1 exhibits a Pr/Ph value of 1.17, but the availability of biomarker compounds for testing is very limited due to the impact caused by the crude oil s high level of

10 10 CHINA PETROLEUM EXPLORATION Vol. 21, No. 3, 2016 maturity. Well ST1 shares a similar crude oil composition and carbon isotope with Well SN1 and Well Shun9. Crude oil produced from these three wells is preliminarily considered to have come from the Cambrian-Middle to Lower Ordovician basinal-slope facies source rock [19] Natural gas characteristics Natural gas produced from Well ST1 in the Shuntuoguole uplift is predominately hydrocarbon gas, with a composition of 94.71% methane, 0.82% ethane, 0.56% propane +, 3.8% CO 2, and 0.11% N 2 along with a hydrogen sulfide count of mg/m 3 H 2 S and a high dry coefficient (98.6%) (Table 2). It shows a δ 13 C 1 of 39.2 and δ 13 C 2 of Well ST1 shares a similar gas composition and carbon isotope with the southern part of the Shuntuoguole uplift. The gas is classified as the typical oil-derived gas generated by marine sapropelic kerogen during the over-mature phase, and mainly comes from the Cambrian-Middle to Lower Ordovician basinal-slope facies source rock [19]. Table 1 Well name Physical properties of crude oil in the Shuntuoguole uplift Horizon Crude oil density/(g. cm 3 ) Kinematic viscosity/(mpa. s) (20 C) Solidification point/ C Sulfur content/% Paraffin content/% ST1 O 2 yj+o 1-2 y SN1 O 2 yj+o 1-2 y < Crude oil property Condensate oil with low viscosity, low sulfur and high paraffin Condensate oil with low viscosity, low sulfur and high paraffin Table 2 Characteristic data of nautrla gas composition in the Suntuoguole uplift Well name Horizon Relative density Methane/% Ethane/% Propane +/% N 2 /% CO 2 /% H 2 S/(mg. m 3 ) Dry coefficient/% Reservoir type ST1 O 2 yj+o 1-2 y Condensate gas reservoir SN1 O 2 yj+o 1-2 y Condensate gas reservoir SN5 O 1-2 y Dry gas reservoir Distribution features of oil & gas reservoirs The Ordovician oil & gas reservoirs in the Shuntuogule uplift can be classified as the carbonate karst fracture-cave type, which is controlled by the degree of thermal evolution affecting the source rock in different areas, the hydrocarbon bearing system, and the heterogeneity of the fracture-cave-type reservoir, with a certain regular change in fluid properties. Regionally, a SW to NE transition from dry gas reservoirs to condensate gas reservoirs to volatile oil reservoirs is evident; and vertically, condensate gas reservoirs are present locally in the upper part of the Ordovician while a dry gas reservoir is present in the lower part (e.g., Well ST1 and Well SN6). Differences in oil & gas properties are related to the degree of thermal evolution of the source rock in different areas, long-term hydrocarbon generation, and multi-stage hydrocarbon accumulation. Oil reservoirs that formed earlier would be reformed through charging with natural gas at later-stages. The Guchengxu uplift that has close proximity to the source kitchen located within the Manjiaer depression was reformed greatly and became dominated by dry gas reservoirs; the Katake uplift in the south, which is far from the source kitchen, was reformed weakly and became dominated by condensate gas reservoirs; and the western part of the Shuntuoguole uplift, which is also far from the hydrocarbon-bearing system in the Manjiaer depression, holds the oil & gas sourced primarily from the Lower Cambrian Yuertusi Formation source rock and is still dominated by condensate gas reservoirs and light oil reservoirs because this region has a low geothermal gradient, and the source rock is less thermally mature so that it remains in the stage of condensate gas and light oil generation. In the Shuotuoguole uplift where oil & gas reservoirs are thick and the elevation difference between the eastern and western parts is significant, no well has yet produced formation water from the Ordovician strata. The depth of oil & gas reservoirs varies in different areas due to the effects of the Queerqueke Formation regional mudstone cap rock, carbonate tight interlayers, and reservoir intervals. For example, Well SN501 still encountered a good reservoir and recorded the presence of gas when entering the Ordovician carbonate rock at a depth 879 m, and when tested, it produced a high productivity gas flow without producing formation water. Well GC9 produced a high productivity gas flow via acid-fracturing treatment from the m interval in the eastern part, and Well ST1 produced a high productivity gas flow from the Yingshan Formation at a depth of m in the western part, indicating an 1870 m difference in gas pay depth (Fig.6) Temperature and pressure system The formation pressure of the Ordovician oil & gas reservoirs in the Shuntuoguole Uplift varies greatly. This has been proved by Well SN4 ( ) and Well SN7 (1.48). As for other wells, although formation pressure was not measured, overflow was recorded while drilling into the lower member of the Yingshan Formation even though the

11 Qi Lixin, Oil and gas breakthroughs in ultra-deep Ordovician carbonate formations in Shuntuoguole uplift, Tarim Basin 11 Fig. 5 Plane distribution of the Ordovician reservoirs in the Shuntuoguole uplift Fig. 6 Section of oil and gas reservoirs in the Shuntuoguole uplift (location shown in Fig.1) mud was at a weight of 1.80 g/cm 3, which implies the presence of an overpressured gas reservoir, which in turn is interpreted to be within a normal pressure to overpressure system. The average temperature gradient varies greatly from east to east. It is averages 2.8 C/100 m in the southern part of the Shuntuoguole uplift, an estimated 2.38 C/100 m in Well ST1 to the west, and ranges from 2.1 C/100 m to 2.2 C/100 m in the western part of the Shunguoguole uplift, thus indicating a low to high temperature system Oil & gas reservoir type The upper member of the Yingshan Formation in Well SN4 and the lower member of the Yingshan Formation in Well SN7 were tested for p-v-t. Both of them are classified as dry gas reservoirs as per their phase state characteristics (Fig.7). By analyzing the oil & gas producing layers and testing productivity results, the Ordovician strata in the Shuntuoguole uplift is believed to hold superimposed, continuous hydrocarbon-bearing reservoirs with hydrocarbons being enriched locally. Multiple fracture-cave units in the carbonate fracture-cave-type reservoirs are superimposed to form a complex reservoir under the control of multi-origin fracture-cave systems. These reservoirs may hold relatively independent pressure systems and constitute a large-scale fracture-cave-type gas reservoir group. This hydrocarbon-

12 12 CHINA PETROLEUM EXPLORATION Vol. 21, No. 3, 2016 that came from the Lower Cambrian Yuertusi Formation source rock. 4. Significances and inspirations 4.1. Significances of oil & gas breakthrough Fig. 7 Hydrocarbon fluid types of gas reservoirs in Wells SN7 and SN4 bearing group, in which hydrocarbon enrichment is controlled by the degree of development of the reservoir, is superimposed and continuous, has a huge gas column height, but has no uniform pressure system (Fig.6) Hydrocarbon accumulation modes Oil & gas trapped in the Shuntuoguole uplift mainly came from the Cambrian-Middle to Lower Ordovician basinal-slope facies source rock in the Manjiaer depression and the Lower Cambrian Yuertusi Formation upper to lower gentle slope facies source rock in the Shunntuoguole uplift. An integrated analysis on the basis of fluid inclusions and oil & gas geochemical features indicated that oil & gas reservoirs in the Shuntuoguole uplift were formed during the Late Hercynian and Himalayan. During the Late Hercynian, the Cambrian-Middle to Lower Ordovician source rock in the Manjiaer depression entered into its oil-generating and wet gas-generating stage while the Lower Cambrian Yuertusi Formation source rock was well within its oil-generating stage. The oil & gas generated by them migrated partially upward into the Silurian strata to form light oil reservoirs and then partially reached the Shuntuoguole uplift and the Katake uplift in the south through a conduit system formed by unconformities, faults, and fractures. This migrating oil and gas eventually formed light oil reservoirs and/or oil reservoirs containing light oil and wet gas. During the Himalayan, the southern part of the Shuntuoguole uplift was charged extensively with the over-mature natural gas sourced from the Manjiaer depression and, as a result, the present-day dry gas-dominated reservoirs were formed in this region; and in the western part of the Shuntuoguole uplift, which is geographically distant from the petroleum system in the Manjiaer depression, the present-day condensate gas reservoir-light oil reservoirs were formed by oil & gas (1) Condensate oil and natural gas were recorded at a depth of over 7800 m in the Tarim platform-basin area, and as a result, present a good exploration prospect at ultra-deep layers and offer up a favorable location that can be expected to hold a hundred million tons of reserves for future exploration activities. In the Tarim Basin, substantial amounts of oil were generated during the Caledonian-Early Hercynian, condensate oil-natural gas was generated along with oil during the Late Hercynian, and natural gas was predominately generated during the Himalayan. In the Shuntuoguole uplift, source rocks were still generating condensate oil and natural gas during the Himalayan because of a relatively low geotemperature field setting and a different structural evolution. Therefore, light oil reservoirs and condensate gas reservoirs should still be present in ultra-deep layers (depth>7800 m), a hypothesis that was confirmed by drilling Well ST1. A deepened study of the origin and type of the Lower Paleozoic carbonate reservoirs points out the joint control of unconformity, sequence interface, and fault and hydrothermal dissolution on dissolved fracture-cave-type reservoirs that are widespread in the Cambrian-Middle to Lower Ordovician carbonate rocks. Based on the theory that the "source-cap rock" controls the reservoir, NE-trending strike-slip fault systems formed during the Middle Caledonian-Hercynian, regional cap rocks, and carbonate rock interior tight cap rocks were studied to propose the idea that hydrocarbon accumulation occurred as a result of its migration along deep faults,the idea that the Himalayan epoch was a time of hydrocarbon accumulation,and a new idea for exploration: that is, "focus on in-situ source rocks to search for later-stage primary large-scale oil & gas reservoirs along deep and large fault belts and surrounding paleo-uplifts and paleo-slopes". This idea has guided the discovery of oil & gas in the Shuntuoguole uplift. An integrated evaluation considers the Shuntuoguole uplift as an important oil- & gas-rich area and preliminarily defines a favorable area that can be expected to hold a hundred million tons of reserves. (2) By discovering multi-type, multi-origin and multiscale dissolved fracture-cave-type reservoir bodies, our understanding with regards to origin and type of carbonate reservoirs in the Tarim Basin is further increased and the exploration domain in that basin is expanded. Exploration practices around the Tahe oilfield and in the

13 Qi Lixin, Oil and gas breakthroughs in ultra-deep Ordovician carbonate formations in Shuntuoguole uplift, Tarim Basin 13 Shuntuoguole uplift at the Tarim Basin, indicate that in areas where the Upper Ordovician is present, the Ordovician reservoirs are dominated by fracture-cave-types, and fault belts exert significant control on reservoir development. Amplitude attributes of 3D seismic data acquired from the southern part of the Tahe oilfield reveals that the "beads" are extended primarily along fault belts. Exploration and development practices also suggest that high productivity wells are distributed along the "X"-shaped strike slip fault belts. For this reason, it is believed that the development of large-scale reservoir bodies is controlled mainly by fault belts. In addition to the presence of fracture belts, core data obtained from the southern part and deep layer of the Tahe oilfield confirms the occurrence of meteoric water dissolution related to interior conformity and weak hydrothermal reformation. The Ordovician carbonate surface reservoir is poorly developed in the Shuntuoguole uplift. Wells SN4 and SN5 reveal that the development of interior reservoir bodies remains controlled by main fault belts and secondary faults. Core data confirms that these reservoir bodies were widely reformed by hydrothermal activity. Based on our understandings with regard to the evolution of structures and strike-slip faults and the difference of reservoir origins in the Shuntuoguole uplift at the Tarim Basin, a karst reservoir development mode is proposed; that is, carbonate reservoirs are controlled by faults in areas where the Upper Paleozoic is present. This mode provides important guidance to oil & gas exploration activities over this region, as it allows for a deepened and increased understanding regarding the origin and type of carbonate reservoirs in the Tarim Basin while also expanding the oil & gas exploration domain. (3) Effectiveness of 3D seismic exploration technology for ultra-deep zones in desert areas is verified, which provides a reference for oil & gas exploration in complex areas. Carbonate reservoirs in the Shuntuoguole uplift are deeply buried and highly heterogeneous. During the exploration process, greatly undulated sand dunes and significant attenuation of seismic signals due to the absorption of desert areas result in poor precision imaging of carbonate fracture-cave reservoir bodies and small faults and also make reservoir prediction and trap determination more difficult [20]. Therefore, an integrated exploration technology incorporating 3D seismic acquisition, objective processing, and integrated interpretation was carried out during the practice of exploring ultra-deep carbonate rocks in desert areas, and a series of more targeted technological methods were formed. Given the fact that seismic data acquired from desert areas has low SNR (signal to noise ratio) at the target layer, low dominant frequency, a narrow frequency band, a strong interference wave, and static correction challenges posed by giant sand dunes, a targeted test is conducted using a survey system with a small bin, more folds, and a wider azimuth in combination with the selection of designed shooting parameters on the basis of fine surface investigations and the optimization of graphs and buried depths of detector arrays [20 24], all in order to enhance the quality of acquired seismic data. Given the fact that seismic signals are attenuated greatly by undulated sand dunes and generally exhibit low SNR and poor quality [25], a workflow for processing the seismic data acquired from desert areas was built, and some technologies, such as the pre-stack high-fidelity denoising technology that enhances the SNR of seismic data acquired from low SNR areas on a "step-by-step, domain-by-domain, frequency-byfrequency, window-by-time, and zone-by-zone" basis, the surface-consistent processing technology makes multiple amplitude compensations step-by-step while also ensuring that the fine velocity modeling and the pre-stack depth (RTM) migration imaging technology [26 28] are formed without sacrificing amplitude or fidelity, which further enhances the precision of detecting fracture-cave reservoir bodies in carbonate rocks. Aiming at different exploration layers, different exploration targets and seismic reflection structural characteristics in the Shuntuoguole uplift, we established the seismic recognition modes for reservoirs in different formations and members, analyzed sensitive attributes of the reservoirs, and formed a series of reservoir predictions and hydrocarbon detection technologies [29 32], such as amplitude analysis, waveform clustering analysis, wavelet decomposition, dip attribute analysis, azimuth attribute analysis, curvature class attribute analysis, discontinuity detection, and frequency class attribute analysis for the interior fracture-cave-type reservoirs, the Yijianfang Formation reservoirs, and the fracture-type reservoirs. On the foundation of studying the geological characteristics of oil & gas reservoirs in the Shuntuoguole uplift and selecting the proper reservoir prediction technology, we established the trap determination & characterization technology that "distinguishes traps from faults and defines interfaces by attributes" and the objective optimized selection & evaluation technology involving "reservoir type, reservoir controlling factors, reservoir model, prediction technology, and well location determination through an integrated study" [20]. Significant oil & gas breakthroughs were made in multiple wells drilled on the Shuntuoguole uplift at the Tarim Basin, which proved the effectiveness and applicability of 3D seismic exploration technology to ultra-deep carbonate res-