J. Cent. South Univ. Technol. (2008) 15: 244-250
DOI: 10.1007/s11771-008-0046-3
Forming mechanism of hydrocarbon reservoirs in
Yingshan Formation of Yuqi block in Akekule arch, Tarim Basin
KUANG Li-xiong(旷理雄)1, GUO Jian-hua(郭建华)1, HUANG Tai-zhu(黄太柱)2
(1. School of Geoscience and Environmental Engineering, Central South University, Changsha 410083,China;
2. Institute of Petroleum Exploration and Development of SINOPEC Northwest Branch Company,
Urümqi 830011, China)
Abstract: The reservoir conditions, oil and gas charge history and accumulation phases were studied for Yingshan Formation of Yuqi block, and an oil and gas accumulation model was established by using the techniques of reservoir prediction, fluorescence thin section and fluid inclusion analysis under the guidance of the theories of oil and gas accumulation. The results indicate that the main rock types in Yingshan Formation are micrite and calcarenite. The carbonate reservoirs are of cave, fracture-pore and fracture types, and their physical properties are intermediate; there are at least four oil/gas charges, i.e. late Hercynian, Yanshanian, early Himalayan and middle Himalayan (Cenozoic). The most important charge periods are late Hercynian, early Himalayan and middle Himalayan; the oil and gas accumulation model is self source—lateral expulsion of hydrocarbon—multistage accumulation, or hydrocarbon sourced from and preserved in the same old rocks—long term expulsion of hydrocarbon—multistage accumulation.
Key words: reservoir; oil and gas charge; accumulation model; Yingshan Formation; Akekule arch; northern Tarim Basin
1 Introduction
Yuqi block is an important oil and gas exploration play in the surroundings of Tahe oil province in Akekule arch, Tarim Basin. The exploration degree of Yuqi block is lower. Because of the limitation of information precision and understanding, the study of oil and gas accumulation has been done mostly in Tahe province in the south of Akekule arch, while the study of Yuqi block is very little[1-5]. In 2004-2005, wells were drilled in Yuqi block by using the oil and gas accumulation theory and cognition acquired in Tahe oil province, but the effect is not fairly good. It shows that forming mechanism of hydrocarbon reservoirs in Yuqi block is fairly complex and is different from that of Tahe oil province, the oil and gas accumulation feature of Yuqi block is multistage basin forming, multistage basin reconstruction, multistage hydrocarbon generation, multistage hydrocarbon migration, multistage dissipation and multistage accumulation.
In this work, reservoir conditions, oil and gas charge history and accumulation periods of Yuqi block were studied, and oil and gas accumulation model were established by using the techniques of reservoir prediction, fluorescence thin section and fluid inclusion analysis[6-8].
2 Geological background
Yuqi block is situated in the north of Akekule arch in Shaya uplift, northern Tarim Basin. It is adjacent to Halahatang sag in the west，to Caohu sag in the east, to Yakela fault-arch in the north, and to Tahe oilfield in the south. Its area is 2 348 km2 (Fig.1). Tahe oilfield and a group of favourable oil and gas- bearing structures have been found in the south of Akekule arch, their reserves are more than 1×108 t, and the main pay zone is Middle—Lower Ordovician carbonate rock.
The exploration degree of Yuqi block is lower. In 1985-1995, two-dimensional seismic survey was done, the survey grid was 4 km×4 km-4 km×2 km, and seven wells were drilled. In 2004-2005, three-dimensional seismic survey was done in Yuqi block covering an area of 1 548.94 km2, four wells (Well YQ1, YQ2, YQ3 and YQ4) were drilled, oil and gas was discovered for the first time, and oil and gas shows were found in Triassic and Middle—Lower Ordovician respectively. All these show that Yuqi block is a favorable oil and gas exploration play.
3 Reservoirs characters
According to the results of core observation and
Fig.1 Structural location of Yuqi area in the north of Akekule arch
rock thin section analysis of a number of wells, the rock types of Middle—Lower Ordovician Yingshan Forma- tion carbonate rocks in Yuqi block are micrite, calcarenite, dolomitic limestone, crystalline limestone, calcareous dolomite, dolomite and karast rock(Fig.2). Of all the rock types, micrite and calcarenite are two main rock types in Yingshan Formation, which have the best development and the widest distribution. The occurrence frequencies of them are 43.7% and 28.8%, respectively. The second type is dolomite with occurrence frequency of 12.8%. The dolomite is thicker, but its occurrence is locally only in the well YQ2 and its vicinity. The last one is karast rock with occurrence frequency of 6%. The karast rock is not so thick, but the lithology is special, and its distribution is mostly in the west of Yuqi block. Other rock types do not develop so well.
The carbonate rock reservoirs in Yuqi block are of cave, fracture-pore and fracture type. Cavern reservoirs are the best. Commercial oil and gas flow is often found in cavern reservoirs in the surroundings of Tahe oilfield whose output is higher. Cavern reservoirs develop widely in the west slope of Yuqi block, but cave filling is very compact. There are less exploration wells in the east of Yuqi block, and cavern reservoirs in
Yuqi block have not been found yet.
Fracture-pore reservoirs are better, fracture and pore can be combined to form a lot of complex types. For example, medium-small fractures in core of the well YQ3 develop sufficiently (Fig.3), cavern pore identified by microscope develops very well too, but fracture and cavern pores are filled with bitumen and heavy oil, according to oil traces to oil spots, it is a better kind of reservoir. In the east of Yuqi block, the fractures develop very fully at late Hercynian, and on the seismic sections, there are various disordered reflections that are related to fractures. It is assumed that fracture-pore reservoirs may develop.
Fracture reservoirs are relatively poor. In the Yuqi block, fracture reservoirs are relatively developed. For example, the fractures of local core of the well YQ4 develop better, their density is 14/m.
According to the physical property analysis of 16 whole diameter core samples of five wells in Yuqi block, the average porosity of Yingshan Formation is 2.85%, and the range is 1.1%-8.2%; the average permeability is 5.436×10-3 μm2, and the range is 5.0×10-3-6.24× 10-3 μm2 (Fig.4). The above samples mostly reflect the typical characters of porosity and permeability of carbonate rock reservoir matrix and some small fractures and caves. In the light of the evaluation standards of carbonate rock reservoirs of Tarim Basin, the porosity and permeability of Yingshan Formation are intermediate (Class Ⅱ). There are two kinds of better reservoirs, one is grainstone or calcarenite that is related to dolomitization, the stylolite and small fractures in the reservoir cores develop well, cavern pores in the reservoir identified by microscope develop very well too, and they are fracture-pore reservoirs whose porosity and permeability are relatively high; the other type is closely related to karast cave. The porosity and permeability of the two kinds of reservoirs are improved to a certain extent by the development of cavern fractures.
Fig.2 Rock types distribution of Yingshan Formation
Fig.3 Rock core fracture distribution of Yingshan Formation of well YQ3 (imaging log: 5 843-6 001 m)
Fig.4 Porosity(a) and permeability distribution(b) of Yingshan Formation
4 Characters of oil and gas charge history and accumulation phases
Akekule arch is a long successively developed palaeo arch. Based on the fluid potential energy analysis, the potential energy in Yuqi block and Tahe block is relatively low, and oil source region in the south of Akekule arch is relatively high, which shows that Yuqi block and Tahe area have been the regions where oil and gas from the hydrocarbon source directed to, and the oil and gas generated in different periods continually migrated and accumulated in Yuqi block along carrier beds including unconformity, fracture and crack system and so on. Based on the information of the homo-temperatures of fluid inclusions and the fluorescence thin sections of the reservoirs in the wells YQ1 and YQ3, oil and gas charge history and accumulation periods in Yuqi block are to be analysed as follows.
4.1 Characters of fluorescence bitumen in reservoirs
Observing fluorescence bitumen in reservoirs through fluorescence thin sections is one of the important methods in the study of oil and gas accumulation[9-10]. The sample shapes and their basic fluorescence characters of the wells YQ1 and YQ2 are shown in Table 1. Considering the diagenesis evolvement sequence in Yuqi block, oil and gas charge history is confirmed (Table 1) in the light of fluorescence bitumen colours of the samples, their shapes and correlations.
It is shown in Table 1 that there are at least three oil/gas charges, i.e. early Hercynian, late Hercynian and Indosinian and Yanshanian epoch. In early Hercynian, the regional and important cap rocks (C1b2-3+C1kl1) were not deposited, and their preservation conditions in Yuqi
Table 1 Fluorescence thin section and hydrocarbon charges in Yuqi block
block are poor. Oil and gas reservoirs formed in this period were mostly destroyed, so they do not contribute much to the forming of reservoirs.
In late Hercynian, the highly mature oil and gas from the southern Cambrian source rocks and the oil mostly from Ordovician source rocks largely charged into the cavern fracture-pores and structural fracture reservoirs. In this period, regional cap rocks developed well, and oil and gas preservation conditions are also preferable, the oil/gas charge is favorable to the forming of reservoirs, and is the most important charge period.
At Indosinian and Yanshanian epoches, the gas from Cambrian source rocks and the mature oil and gas from Lower Ordovician source rocks migrated and charged the Yingshan Formation reservoirs. In this period, regional cap rocks developed, and oil and gas preservation conditions are preferable, the oil/gas charge is favorable to the forming of reservoirs, and is also the charge period in Yuqi block.
In Himalayan, the highly to over mature light oil and gas from oil source area migrated and charged the Yingshan Formation reservoirs. In this period, because of the great subsidence resulting from Himalayan movement in Yuqi block, cap rocks develop better, and oil and gas preservation conditions are more preferable, and the oil/gas charge in reservoirs is much more favorable to the forming of reservoirs.
In the light of characters of fluorescence bitumen in reservoirs, it is shown that there are at least three oil/gas charges, i.e. late Hercynian, Indosinian-Yanshanian and Himalayan.
4.2 Characters of fluid inclusion homo-temperature of reservoirs
It has been widely applied that the forming periods of hydrocarbon reservoirs are determined in the light of fluid inclusions homo-temperature of reservoirs[7-8,11]. The homo-temperature of salt water inclusions is homochronous with hydrocarbon inclusions in the reservoir, and represents the temperature when the oil and gas charge the reservoirs. In the light of the temperature, the basined paleogeothermal model and burial history of reservoirs, the buried depth of strata and its geology period are determined, and the forming periods of hydrocarbon reservoirs are also determined[12].
Based on the fluid inclusion homo-temperature of Yingshan Formation reservoirs in Yuqi block, a histogram of homo-temperature of fluid inclusions was made and analyzed. It shows that the range of homo- temperature of fluid inclusions is 89-152 ℃, and there exist three temperature intervals. They are 89-101 ℃, 107-116 ℃ and 122-152 ℃, separately. Therefore, there exist at least three oil/gas charges. The first one is 89-101 ℃, the second is 107-116 ℃, and the third is 122-152 ℃. Projecting these three homo-temperature intervals onto the burial history map with paleogeothermal evolution, their corresponding geologic times are 79.1-63.7 Ma, 60.1-53.5 Ma and 50.2-31.3 Ma respectively. So, there are at least three oil/gas charges of Yingshan Formation in Yuqi block, i.e. late Yanshanian, early Himalayan and middle Himalayan (Fig.5).
In the light of the studies of hydrocarbon generation history of source rocks in northern Tarim Basin[4], Yanshanian-Himalayan is the third peak of hydrocarbon generation and expulsion. The gas from Cambrian source rocks in the south and the mature oil and gas from Lower Ordovician source rocks migrated and charged the reservoirs in late Yanshanian, early Himalayan and middle Himalayan. In this period, cap rocks developed very well, and oil and gas preservation conditions are preferable, being favorable to the forming of reservoirs. So, there exist at least three oil/gas charges of Yingshan Formation in Yuqi block, i.e. late Yanshanian, early Himalayan and medium-term Himalayan.
On the basis of the comprehensive analysis of fluid inclusion homo-temperature of reservoirs and fluorescence thin section information of reservoirs, it is determined that there are at least four oil/gas charges and the corresponding periods in Yuqi block, i.e. late Hercynian, Yanshanian, early Himalayan and middle Himalayan, among them the most important charge and accumulation periods are late Hercynian, early Himalayan and middle Himalayan.
5 Oil and gas accumulation history analysis
5.1 Accumulation period from late Caledonian to early Hercynian
In late Caledonian, the source rocks of Manjiaer depression began to enter the peak of hydrocarbon generation and expulsion. The oil and gas generated by the source rocks of Manjiaer depression migrated to the Akekule arch that had a rudimentary form at time along the T74 unconformity (the base of Upper Ordovician) and T70 unconformity (the bottom of Silurian), and accumulated in a few of traps formed in Yuqi block and its vicinities. But the basic elements of oil and gas system in Yuqi block and its vicinities were intensively destroyed by the movement of early Hercynian that began at the end of Middle Devonian, and the reservoirs formed during late Caledonian to early Hercynian were mostly destroyed, and great quantity of oil and gas of the reservoirs escaped. Now there is only bitumen or carbonaceous bitumen in early pores and fractures (Fig.6).
5.2 Accumulation periods in late Hercynian
The most important oil and gas migration and accumulation period in Yuqi block and its vicinities is late Hercynian. It is an important period when the mature oil and gas generated by the source rocks of Manjiaer depression. And the oil and gas migrated to the Akekule arch largely along the T70 unconformity and carrier beds, and accumulated in cavern fracture-pore reservoirs of Lower Ordovician. At the same time, the Upper Carboniferous and the Permian system in Yuqi block were completely eroded because of the movement of late Hercynian that began at the end of Permian. And the Lower Carboniferous was more or less eroded; in the northern part of Yuqi block it was completely eroded; in the southern part of Yuqi block it was partially eroded; and the residue is generally 250-300 m in thickness. Its sealing system wasn’t badly destroyed, the Ordovician hydrocarbon reservoirs may be preserved. However, because of the atmosphere fresh water which flow from northeast to southwest in southwestern part of Yuqi block and its vicinities (the well Lg9, the well Lg15), oil in the Middle-Lower Ordovician was subjected to water wash, oxidization and biodegradation to some extent, and became heavy oil zone.
5.3 Accumulation periods during Indosinian to early Himalayan
During the period of Indosinian to early Himalayan, abundant mature-highly mature oil and gas from the southern source rocks provided oil and gas for the forming of reservoirs in the period. Along with the deposition of thick Triassic, Jurassic, Cretaceous and Eogene sandstone and mudstone, the regional sealing system in Yuqi block and its vicinities was rebuilt, and the hydrocarbon reservoirs formed in this period and late Hercynian and Indosinian to early Himalayan can be well preserved.
Fig.5 Hydrocarbon charge of Yingshan Formation in Yuqi block (Burial history from Ref.[13])
Fig.6 Oil and gas accumulation model in Yuqi block and its vicinities: (a) At late Himalayan; (b) At early Yanshanian; (c) At late Hercynian; (d) At late Caledonian to early Hercynian
5.4 Accumulation period in late Himalayan
Because of the great thickness of eogene deposits (＞3 km) in late Himalayan, the regional sealing system in Akekule arch is finally fixed, and the previous hydrocarbon reservoirs are well preserved. At the same time, because of the intensive horizon compression in early Himalayan, the upper, middle and lower structural layers in Yuqi block were formed, which are different from Tahe oilfield and have an important influence on oil and gas migration direction and pathways.
1) The lower structural layer. It is of Lower Paleozoic (Cambrian to Ordovician), the main body of Tahe oilfield and the south of Tahe oilfield dip to the south as a whole. Oil and gas from the southern source rocks continue to migrate to Tahe oilfield along the T74 unconformity. But Yuqi block dips to the north as a whole, and the oil and gas volume from the southern source rocks may be less.
2) The middle structural layer (Carboniferous). The main body of Tahe oilfield and the southern base of Tahe oilfield dip to the south, but their upper parts dip to the north, and a large wedge is formed. However, the Yuqi block dips to the north as a whole, and the oil and gas migrated to and trapped in Yuqi block is mostly along the fractures.
3) The upper structural layers (Mesozoic to Cenozoic). The main body of Tahe oilfield, the south of Tahe oilfield and Yuqi block gently dip to the south during Indosinian to Yanshanian, and changing dip to the north regionally in late Himalayan.
Based on the above mentioned analysis of forming mechanism of hydrocarbon reservoirs, the oil and gas accumulation models in Yuqi block are summed up as self source—lateral multistage expulsion of hydrocarbon—multistage accumulation or hydrocarbon sourced from and preserved in the same old rocks—long term expulsion of hydrocarbon—multistage accumulation. That is to say, the oil and gas of Yuqi block come mostly from the southern Cambrian— Ordovician source rocks, and migrate laterally to Yuqi block and accumulated through multistage expulsion of hydrocarbon and along carrier beds such as unconformity (above Cambrian), faults and fracture system.
6 Conclusions
1) The Yingshan Formation carbonate reservoirs in Yuqi block are of cave, fracture-pore and fracture types, the porosity and permeability of reservoirs are intermediate; the big cavern reservoirs and the fracture and pore of core develop widely in the west slope of Yuqi block, and relatively poor in the east slope of Yuqi block, but fracture, pore and cave filling is very compact; the best reservoir is located in the area where the Well YQ3 and the Well YQ4 are located.
2) There are at least four oil/gas charges in this block, i.e. late Hercynian, Yanshanian, early Himalayan and middle Himalayan, among these the most important ones are late Hercynian, early Himalayan and middle Himalayan (Cenozoic).
3) The forming mechanism of hydrocarbon reservoirs in Yuqi block can be summed up as multistage charge and multistage accumulation. From late Caledonian to early Hercynian is the period of construction and destruction of initial accumulation; late Hercynian is the period of main accumulation and local reconstruction of hydrocarbon reservoirs; from Indosinian to early Himalayan is the period of succession and development of hydrocarbon reservoirs; late Himalayan is the period of adjustment and final fixation of hydrocarbon reservoirs. Its oil and gas accumulation models are self source—lateral multistage expulsion of hydrocarbon—multistage accumulation or hydrocarbon sourced from and preserved in the same old rocks— long term expulsion of hydrocarbon—multistage accumulation.
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(Edited by YANG Hua)
Foundation item: Project(P05009) supported by the Item of Science and Technology and Development of SINOPEC Stock Limited Company of China
Received date: 2007-10-10; Accepted date: 2007-11-20
Corresponding author: KUANG Li-xiong, Associate professor, PhD; Tel: +86-13027480788; E-mail: kuanglixiong@163.com
