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Accumulation mechanism and controlling factors of the continuous hydrocarbon plays in the Lower Triassic Baikouquan Formation of the Mahu Sag, Junggar Basin, China
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ScienceDirect Journal of Natural Gas Geoscience 1 (2016) 309e318 http://www.keaipublishing.com/jnggs
Original research paper
Accumulation mechanism and controlling factors of the continuous hydrocarbon plays in the Lower Triassic Baikouquan Formation of the Mahu Sag, Junggar Basin, China* Ablimit Imin a,*, Yong Tang a, Jian Cao b, Gangqiang Chen a, Jing Chen a, Keyu Tao b a
Research Institute of Exploration and Development, PetroChina Xinjiang Oilfield Company, Karamay, 834000, China b School of Earth Sciences and Engineering, Nanjing University, Nanjing, Jiangsu, 210023, China Received 24 July 2016; revised 22 August 2016 Available online 19 October 2016
Abstract Hydrocarbon exploration in the Mahu Sag of the northwestern Junggar Basin has recently resulted in a significant discovery that defined the new “continuous oil plays” in the Lower Triassic Baikouquan Formation. This formation hosts glutenite reservoirs with oil reserves of more than hundreds of millions of tons. This resource is an example of an extensive hydrocarbon accumulation that's far from source rocks. Three conditions are necessary for this type of hydrocarbon accumulation: large hydrocarbon source, good reservoirs and effective seals, and favorable timing of hydrocarbon charge and migration. In the Mahu Sag, high-quality lacustrine source rocks are prolific hydrocarbon source, large-scale strike-slip faults enabled and facilitated long-distance hydrocarbon migration, and fan-delta glutenites are excellent reservoirs for hydrocarbon accumulation. The hydrocarbon accumulation is controlled by three factors: pro-delta sedimentary facies, structural highs, and faults. Abnormal reservoir over pressuring and fracturing has had an important impact on enhancing hydrocarbon production. The consideration of all these factors should govern the selection of future exploration targets. Copyright © 2016, Lanzhou Literature and Information Center, Chinese Academy of Sciences AND Langfang Branch of Research Institute of Petroleum Exploration and Development, PetroChina. Publishing services by Elsevier B.V. on behalf of KeAi Communications Co. Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). Keywords: Continuous oil and gas plays; Fan-delta; Lacustrine high-quality source rocks; Highly mature light oil and gas; Distal hydrocarbon accumulation; Baikouquan Formation; Mahu Sag
1. Introduction The Junggar Basin is a typical large petroliferous basin in the western China. Recent exploration was conducted in this basin by PetroChina [1,2], they discovered new and unusual continuous light oil (and gas) plays that are extremely distal *
This is English translational work of an article originally published in Natural Gas Geoscience (in Chinese). The original article can be found at: 10. 11764/j.issn.1672-1926.2016.02.0241. * Corresponding author. E-mail address: [email protected] (A. Imin). Peer review under responsibility of Editorial Office of Journal of Natural Gas Geoscience.
from source sequences in the Lower Triassic Baikouquan Formation. This formation contains glutenite reservoir rocks with proven oil reserves of hundreds of millions of tons. The Mahu Sag lies on the downthrown block of the “hundreds of miles oil region” fault zone. This is the first discovery within the Junggar Basin possessing an “old source and new reservoir” type of accumulation, which covers a large area within a sag structure. Compared to local and international examples of hydrocarbon reservoirs, this “old source and new reservoir” type that's associated with glutenites is unusual [3e6]. In order to better understand this hydrocarbon reservoir, this paper investigates the accumulation mechanism and controlling factors of this hydrocarbon resource.
http://dx.doi.org/10.1016/j.jnggs.2016.10.003 2468-256X/Copyright © 2016, Lanzhou Literature and Information Center, Chinese Academy of Sciences AND Langfang Branch of Research Institute of Petroleum Exploration and Development, PetroChina. Publishing services by Elsevier B.V. on behalf of KeAi Communications Co. Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).
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2. Basic characteristics of the hydrocarbon plays Oil and gas exploration has proven to be successful in the Baikouquan Formation in the Mahu Sag of the Junggar Basin (Fig. 1). Exploration wells, namely, Ma 131, Ma 15, and Xia 72 in the Xiazijie fan group of the western part of the basin have reserves of hundreds of millions of tons. The Well Mahu1 in the Karamay fan group and Well Ma 18 in the Huangyangquan fan group could produce oil as much as 60 t/d. In the Fengnan fan in the western part of the basin, large reserves have also been discovered. Moreover, there are some other exploration wells, such as Well Yanbei 1 and Well Yanbei 2 in the Madong-Xiayan fan group, which also have oil flows. These wells show that the hydrocarbon-producing zone is extensive and highly productive and such wells will be an important area for continued exploration of the Xinjiang Oil Field Company, or even PetroChina. Through Well Ma 131, the Baikouquan hydrocarbon resources were first discovered, and some insights about the characteristics of the Baikouquan Formation provided were determined. According to lithological, electrical resistivity, oil, and sedimentary facies' data, the target layer (Triassic Baikouquan Formation) can be divided into three sections: T1b1,
T1b2, and T1b3. T1b2.; the said layers can be subdivided into T1 b2 1 and T1 b2 2 , with T1 b2 1 being the main oil reservoir. T1 b2 1 rocks are mostly comprised of gray and brownegray conglomerate, along with pebbly argillaceous siltstone and argillaceous siltstone (Fig. 2). The distribution of oil and gas in these rocks is related to the location of the faults, for example, in Well Ma 131, oil is sealed by a fault from the Well Ma 13. The reservoir pressure data shows that a single fault block is cut into three sub-fault blocks by the east fault of the Well Ma 131 and the east fault of the Well Ma 15. Hydrocarbons from the Baikouquan Formation of the Mahu Sag are mainly crude oils with gas. Oil densities range 0.78e0.86 g/cm3, and are predominantly 0.82e0.86 g/cm3. The methane content is 59.7%e92.8% (average 76.8%), whereas the ethane content is estimated to be 2.5%e11.9% (average 7.4%). These gas ratios are indicative of wet gas, but deep exploration has recently discovered dry gas. Therefore, according to these basic oil and gas characteristics, this hydrocarbon resource is the product of mature to highly mature source rocks. In general, such oil and gas systems have characteristics that include oil (and gas) distributed throughout the whole
Fig. 1. Geological sketch map showing hydrocarbon exploration results and predicted hydrocarbon occurrences in the Lower Triassic Baikouquan Formation of the Mahu Sag.
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Fig. 2. Cross-section of the Lower Triassic Baikouquan Formation in the Mahu Sag showing oil accumulation zones.
reservoir belts, localized areas of enrichment, and high productivity. This corresponds to typical “continuous oil (and gas) play” characteristics [3,7,8]. Hydrocarbon-source correlation studies have shown that the hydrocarbons are mainly derived from deep lower Permian Fengcheng lacustrine source rocks [9,10] (see Section 3.3), which is different from the typical “continuous oil (and gas) plays” that are generally sourcereservoir integrated [8,11,12]. Therefore, this petroleum resource is termed an extensive “continuous hydrocarbon play” that is distal from its source rocks and represents a global model for the study of the formation and nature of such oil (and gas) reservoirs. 3. Hydrocarbon accumulation mechanism The average depth of the Baikouquan Formation in the Mahu Sag is 3.5 km. The Baikouquan reservoir is separated from the underlying Permian source rocks by 1e2 km. The various lines of evidence suggest that three main factors led to the formation of the large continuous hydrocarbon plays, which are discussed as follows. 3.1. Hydrocarbon source rocks The deep CarboniferousePermian rocks in the Mahu Sag include four types of high-quality source rocks (Fig. 3). Mudstone samples with a mediumehigh abundance of organic matter are found in all the four types of source rocks. The Lower Permian Fengcheng lacustrine source rocks are of the highest quality, although they do not have the highest organic carbon content, these rocks have the greatest hydrocarbon generation potential. This reflects the fact that the organic
matter type in the Fengcheng lacustrine source rocks is optimal for hydrocarbon generation as proved by the carbon isotope (relatively light) and hydrogen index (relatively high) data. The Lower Permian Fengcheng lacustrine source rocks were deposited in a saline, alkaline, and reducing environment [9,13]. Previous studies have shown that alkaline lacustrine sediments with abundant microbial organic matter have a hydrocarbon generation capacity that is often several times than that of non-alkaline lacustrine source rocks. Thus, the generated crude oil has a lower density and often times readily migrates [14,15]. Faults cutting through the source rock sequence (Fig. 4) constitute favorable conditions for hydrocarbon migration [16]. During the early Carboniferous to Permian times (foreland uplift period) the Junggar Basin was affected by compressive stress, as a result, it formed a series of large-scale reverse faults that amended the source rock sequence. Reverse faults continued to develop until the EarlyeMiddle Triassic periods. These faults cut through the Baikouquan Formation, causing the Fengcheng source rocks to link with the Baikouquan reservoir, forming an effective conduit for hydrocarbon migration (Fig. 4) [17,18]. In the Zhongguai-Wuba and Manan slope areas, large strike-slip fault systems have developed, where the reservoir is directly linked to the source rocks. The Baikouquan Formation forms a topographic and lithologic oil reservoir. In the Mabei slope area, the Xiazijie structural belt has formed a nose structure that abuts against a reverse fault. The fault breaks off to the bottom of T2k, thereby linking the lower hydrocarbon generation layers with the reservoir. In the Madong slope area, a series of reverse faults have formed along the nose structure due to the frontal uplift belts. These
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Fig. 3. Geochemical profiles of CarboniferousePermian hydrocarbon source rocks in the Mahu Sag.
faults break off into T2k, yet again linking the hydrocarbon generation layers with the reservoir. The study area is located in the central part of a hydrocarbon-rich depression with faults connecting source rocks with reservoirs. Thus, high maturity source rocks for oil and gas are abundant, and it forms the basis for extensive hydrocarbon accumulation distal from these source sequences, which ultimately led to a high production area. 3.2. Reservoir nature and conditions The Baikouquan Formation in the Mahu Sag has the reservoir qualities to form a “continuous hydrocarbon play”. It specifically includes widespread conglomerate reservoirs that
have excellent permeability and porosity that aid in the accumulation of large amounts of oil and gas. Previous studies have shown that the conglomerate was deposited in a fan-delta sedimentary system [19e21]. There are four distinct genesis regions to the depositional systems around the Mahu Sag and the six fan bodies that are associated. The six fan bodies are Xiazijie, Huangyangquan, Karamay, Zhongguai, Yanbei, and Xiayan. The six fan bodies provided abundant terrigenous detrital material during the deposition of the Baikouquan Formation. The slope of the sedimentary system was gentle during deposition, thus, resulting in a well-developed pro-delta sedimentary facies with sand bodies extending into the center of the basin (i.e., T1b1 and T1b2). The thick layers of glutenite are distributed
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Fig. 4. Seismic section showing the structural highs in the Baikouquan Formation, Mahu Sag.
widely throughout the basin and have good physical properties for a hydrocarbon reservoir. The frontal facies of each fan are distributed over a large area, with each facies covering several hundreds of square kilometers. The fan bodies developed in response to the paleo-geomorphological controls, with mountain passes and valleys defining the main groove and plains facies. Large strike-slip faults extended the fan deposits. In addition, mudstone units were formed adjacent to the paleomountains (Fig. 5). Oil and gas migrated along the faults of the deep CarboniferousePermian source rocks to the Triassic pro-delta reservoirs, which are sealed at the top, bottom, and sides. Ever since the Late Cretaceous the regional tectonic activity has been minor, and the faults have become less active and sealed, thereby preventing re-migration or escape of oil and gas. All of these conditions are favorable for oil and gas accumulation, it also explains the wide distribution of oil and gas [22]. The Middle Triassic Karamay mudstone overlies the Baikouquan Formation. At the bottom of the Baikouquan Formation, dense glutenites overly the middle Permian lower Wuerhe mudstone (Fig. 6). The sides of the reservoir are the near-provenance (structural highs),
these are comprised of fan-delta-plain facies overlying the dense glutenites; these and the mudstones above and below the Baikouquan Formation provide effective reservoir seals. The fan-delta-plain facies sandstone and conglomerate are clay-rich but poorly sorted. They have poor reservoir physical properties with porosities being >5% and permeability mostly <0.1 mD, for example, such rocks in the Well Xia 72 have a porosity that is 2.8%, whereas in the Well Xia 9, the porosity is 3.2%. On the contrary, the main Baikouquan sealed reservoir is predominantly fan and pro-delta front facies gray sandstone and conglomerate. These reservoir rocks are well sorted but poor in clay. They have porosities ranging 6%e14% that averaged ~10%, and permeability that is generally >1 mD. An example is the rocks in the Well Ma 18 that possesses an average porosity and permeability of 10.5% and 5.66 mD, respectively, whereas, in the Well Ma 2, the average porosity and permeability are 9.4% and 1.41 mD, respectively. Therefore, these reservoir properties such as overlying and underlying seals, and absence of bottom water all contribute to making the formation favorable for hydrocarbon charging and accumulation [22,23].
Fig. 5. Fan-delta sedimentary system model, where the Lower Triassic Baikouquan Formation of the Mahu Sag was deposited.
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Fig. 6. Lithological correlation and conditions of overlying, underlying, and side preservation of the Lower Triassic Baikouquan Formation in the Mahu Sag.
3.3. Hydrocarbon charging and evolution There are several types of high-quality source rocks in the Baikouquan Formation (Fig. 2) with the best being the Fengcheng rocks (Fig. 3). The Fengcheng source rocks are most likely the ideal source rock for hydrocarbon generation in the study area as validated by the hydrocarbon geochemical characteristics. Carbon isotope data for all crude oils in the study area have ad13C less than 28‰, indicating a typical oilekerogen crude oil. Pr/Ph values are less than 1.3, this reflects the reducing sedimentary in the environment in which the source rocks were deposited (Fig. 7a). In addition, the gammacerane indices of all the crude oils are greater than 0.2, indicating that the oil is from saline (alkaline) lacustrine source rocks. Tricyclic terpanes C20, C21, and C23 are mainly in increasing proportions. All these characteristics are indicative of the Fengcheng source rocks in the study area [9,10]. Isotopic analyses show that the natural gasses are mainly oiltype gas (Fig. 7b), corresponding to Fengcheng source rocks [9,10,24]. Therefore, crude oil and natural gas of the Baikouquan reservoir in the Mahu Sag are mainly derived from the Fengcheng source rocks. The Fengcheng source rocks had two hydrocarbon generation peaks: Late Triassic and Early Cretaceous (Fig. 8). The
two-stage oil and gas charging process affected the Baikouquan frontal facies. This hydrocarbon charging and evolution, as well as three-dimensional sealing brought by the dense glutenites and mudstones, good overlying and underlying preservation, and ideal storage conditions (Fig. 6) all led to the enrichment and accumulation of hydrocarbons. As shown in Fig. 8, the two hydrocarbon expulsion stages were in the Triassic to the Early Jurassic and the Early Cretaceous [9,10]. The first stage was mainly continuous mature oil charging, with bitumen being involved leaving yellow fluorescent-free hydrocarbon inclusions in the reservoirs. In contrast, the second stage was mainly charging of highly mature oil, represented by blue and white fluorescent hydrocarbon inclusions. Combined with studies of the reservoir evolution, it can be inferred that the Baikouquan Formation was buried in a shallow area (0.5e1.0 km) during the first oil charging stage (Triassic to Early Jurassic). This resulted in a weak compaction and preservation of good reservoir properties (porosity of 20%). The Baikouquan reservoir was then buried deeply (3e4 km) during the second oil charging stage (Early Cretaceous). The compaction then became more striking with a resultant deterioration of reservoir properties (porosity of 10%), therefore, the oil and gas accumulated in the favorable frontal facies reservoir. These essential reservoir physical
Fig. 7. Hydrocarbon geochemistry of the Lower Triassic Baikouquan Formation, Mahu Sag.
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Fig. 8. Two-stage hydrocarbon generation and charging in the Lower Triassic Baikouquan Formation, Mahu Sag.
properties resulted in highly mature oil accumulating in a large area of the slope belt. The threshold porosity for charging of intermediate and light oils into the Baikouquan Formation is 10.7% and 7.7%, respectively. Therefore, the oil and gas (mature and highly mature) accumulated in two stages while being in favorable reservoirs. In summary, the Baikouquan “continuous hydrocarbon plays” in the Mahu Sag was formed due to three factors: highquality and abundant source rocks, good reservoir quality and seals, and well-matched hydrocarbon charging and evolution. 4. Regularities of hydrocarbon accumulation and factors influencing high hydrocarbon production 4.1. Regularities of hydrocarbon accumulation Through comparison with similar hydrocarbon plays in China and internationally, there are three favorable characteristics that can be identified for the development of “continuous hydrocarbon plays” distal from source rocks in the Mahu Sag of the Junggar Basin. Firstly, high-quality lacustrine source rocks provide a good hydrocarbon source. Secondly, large-scale strike-slip faults connect the source and reservoir rocks allowing long-distance hydrocarbon migration. Thirdly, the fan-delta sedimentary system provides good conditions for reservoir formation and continuous hydrocarbon accumulation. The Mahu Sag has the greatest hydrocarbon generation potential in the Junggar Basin (Fig. 9). Its high-quality source rocks are mainly Permian (as well as Carboniferous) in age. Permian sedimentary rocks have characteristics typical of deposition in a foreland basin, including the Lower Permian Jiamuhe and Fengcheng Formations and middle Permian Wuerhe Formation. These three sets of Permian source rocks, along with the other Carboniferous age, are thick and widely distributed. They also generated high amounts of oil and gas, providing the basis for hydrocarbon accumulation. The Fengcheng lacustrine source rocks are the most significant of these four sets of source rocks
(Fig. 3). After the oil and gas expulsion from the source rocks, they migrate upwards along the NW to NWW-trending strikeslip faults. These faults are steeply dipping and typically unconformable, they're overlain by Triassic rocks or have only experienced small amounts of later activity (Fig. 4) [17,25]. The faults extend down to the high-quality source rocks (Fengcheng) and extend upwards to the Triassic unconformity. The faults are sealed by thick Triassic mudstones, which form a three-dimensional oil and gas migration and trapping system (Fig. 4). Given that these faults intersect, the relatively highquality Baikouquan fan-delta reservoir rocks have been favorable sites for hydrocarbon accumulation. The Baikouquan glutenite reservoir has a thickness ranging 50 and 100 m, its porosity is between 5% and 12%, and the oil zone thickness is between 10 and 50 m. The main types of reservoir space are primary grain and secondary solution porosity (feldspar), with minor clay shrinkage porosity and micro-cracks. Organic-rich acidic waters produced in the underlying Permian hydrocarbon source rocks resulted in upward dissolution along the cracks and unconformities that was the main factor in generating secondary porosity. The secondary porosity constitutes a complex reservoir space in addition to the primary porosity and cracks. The primary porosity features of the reservoir are controlled by the six fan bodies around the sag during its formation, which supplied abundant terrigenous detrital material along a gentle sediment slope allowing a sand body to extend into the lake center. These provided favorable conditions for the formation of this oil and gas “continuous” plays. 4.2. Factors influencing hydrocarbon accumulation and high production Hydrocarbon accumulation is controlled by three factors: a favorable sedimentary facies, structural highs, and faults. Abnormal reservoir over pressuring and fracturing has had an important influence on hydrocarbon production. These three factors are discussed in detail below:
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Fig. 9. Geological cross-section showing petroleum migration and accumulation in the Lower Triassic Baikouquan Formation, Mahu Sag.
(1) Sedimentary facies: In the exploration area, the most productive reservoir area is the Xiazijie fan group, which can be further subdivided into two fan bodies (Xiazijie and Fengnan fan bodies). Oil and gas are also found in other fan bodies, including the Huangyangquan, Karamay, and MadongeXiayan fan groups. The frontal sub-facies of each fan body further control the degree of oil and gas enrichment. The Baikouquan reservoirs were formed in the pro-delta and plain sedimentary facies environments. The fan delta and plain facies rocks are brown glutenite and conglomerate, and for the pro-delta underwater channels are gray glutenite and conglomerate. The fan delta and plain facies rocks are poorly sorted with mixed accumulations of sand, mud, and gravel including an argillaceous matrix, which is responsible for its relatively poor reservoir properties. In contrast, the pro-delta underwater channel rocks formed in a more active hydrodynamic setting, thus, these rocks have low clay contents; they are also well sorted and are relatively good reservoir material. Therefore, the best reservoirs are the gray glutenites and conglomerates, an example would be the subfacies of the deposition which controlled the quality of oil and gas accumulation. (2) The accumulation of oil and gas was also controlled by the distribution of structural highs and faults. In general, oil and gas are located in the noses of the structures in the
study area. These noses are connected by faults that provide favorable locations for the accumulation of migrated hydrocarbons [26]. Typically, structural highs are associated with faults, which are important for hydrocarbon accumulation and enrichment, especially considering the long-distance migration that has taken place in this region. The faults are of great importance as they facilitate an upward migration of hydrocarbon generated by deep source rocks to the reservoirs (Fig. 4). The northeastern main faults controlled the distribution of the fan body and dense series of channels, and the east-west-striking subfault on the flank controlled the distribution of slope breaks laterally. Thus, these faults controlled the distribution of pro-delta sand bodies and the subsequent development oil-bearing layers (Fig. 9). (3) Reservoir over pressuring and fracturing also affect the production of oil and gas. In general, it is considered that these processes “open up” the reservoir, thus, allowing high production which is important for the tight continuous reservoirs [27e30]. The Baikouquan Formation in the Mahu Sag is over pressured (pressure coefficient >1.3) and is favorable for high oil and gas flow rates. Core observations show that the reservoir is highly fractured, particularly in brittle lithologies. These fractures also improve the reservoir properties and enhance production rates.
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In summary, hydrocarbon accumulation and enrichment in the Baikouquan Formation of the Mahu Sag is controlled by favorable sedimentary facies (pro-delta), structural highs, and faults. High hydrocarbon production is further enhanced by reservoir over pressuring and fracturing. The heavy consideration of these factors should be used to guide future exploration of oil and gas in this region (Fig. 1). The favorable areas for exploration were predicted (Fig. 1). 5. Conclusions (1) Hydrocarbon exploration in the Mahu Sag of the northwestern Junggar Basin, China for the first time identified a “continuous hydrocarbon play” distant from source rocks. The plays are extensive in area and they have a largely proven capability to produce hydrocarbons. (2) There are three main conditions that have contributed to this hydrocarbon accumulation, namely, large hydrocarbon source, good reservoir and seals, and balanced hydrocarbon charging and evolution. High-quality lacustrine source rocks provide an abundant source of hydrocarbons. Largescale strike-slip faults link the source and reservoir rocks, thus, facilitating long-distance hydrocarbon migration. The fanedelta sedimentary system deposited sediments are excellent reservoirs for hydrocarbon accumulation. (3) Hydrocarbon accumulation is controlled by three factors: pro-delta sedimentary facies, structural highs, and faults. These factors should all be considered when selecting future exploration targets. In addition, reservoir over pressuring and fracturing enhance hydrocarbon production.
Foundation item Supported by Projects of National Science and Technology Major Project of China (2016ZX05001-005) and PetroChina Science and Technology Major Project “Xinjiang's Daqing” (2012E-34-01). Conflict of interest The authors declare no conflict of interest. Acknowledgments We thank editors and anonymous reviewers for their valuable reviews and comments, which improved the article. References [1] L.C. Kuang, Y. Tang, D.W. Lei, T. Wu, J.H. Qu, Exploration of fancontrolled large-area lithologic oil reservoirs of Triassic Baikouquan Formation in slope zone of Mahu depression in Junggar Basin, China Pet. Explor. 19 (6) (2014) 14e23. [2] D.W. Lei, I. Abulimit, Y. Tang, J. Chen, J. Cao, Controlling factors and occurrence prediction of high oil-gas production zones in Lower Triassic Baikouquan Formation of Mahu Sag in Juggar Basin, Xinjiang Pet. Geol. 35 (5) (2014) 495e499.
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[28] F. Hao, S.T. Li, Y.C. Sun, Q.M. Zhang, Characteristics and origin of the gas and condensate in the Yinggehai Basin, offshore South China Sea: evidence for effects of overpressure on petroleum generation and migration, Org. Geochem. 24 (3) (1996) 363e375. [29] Z. Yang, J.H. Wang, S.H. Lin, S.T. Wu, Y. Liu, Q.Y. Li, P.Z. Zhang, Hydrocarbon accumulation mechanism near top overpressured surface in central Junggar Basin, J. China Univ. Pet. Ed. Nat. Sci. 35 (3) (2011) 19e25. [30] G. Fu, Y.C. Xue, Role of abnormal high pressure in the formation and preservation of oil(gas) reservoirs, Xinjiang Pet. Geol. 20 (5) (1999) 379e382.
ScienceDirect Journal of Natural Gas Geoscience 1 (2016) 309e318 http://www.keaipublishing.com/jnggs
Original research paper
Accumulation mechanism and controlling factors of the continuous hydrocarbon plays in the Lower Triassic Baikouquan Formation of the Mahu Sag, Junggar Basin, China* Ablimit Imin a,*, Yong Tang a, Jian Cao b, Gangqiang Chen a, Jing Chen a, Keyu Tao b a
Research Institute of Exploration and Development, PetroChina Xinjiang Oilfield Company, Karamay, 834000, China b School of Earth Sciences and Engineering, Nanjing University, Nanjing, Jiangsu, 210023, China Received 24 July 2016; revised 22 August 2016 Available online 19 October 2016
Abstract Hydrocarbon exploration in the Mahu Sag of the northwestern Junggar Basin has recently resulted in a significant discovery that defined the new “continuous oil plays” in the Lower Triassic Baikouquan Formation. This formation hosts glutenite reservoirs with oil reserves of more than hundreds of millions of tons. This resource is an example of an extensive hydrocarbon accumulation that's far from source rocks. Three conditions are necessary for this type of hydrocarbon accumulation: large hydrocarbon source, good reservoirs and effective seals, and favorable timing of hydrocarbon charge and migration. In the Mahu Sag, high-quality lacustrine source rocks are prolific hydrocarbon source, large-scale strike-slip faults enabled and facilitated long-distance hydrocarbon migration, and fan-delta glutenites are excellent reservoirs for hydrocarbon accumulation. The hydrocarbon accumulation is controlled by three factors: pro-delta sedimentary facies, structural highs, and faults. Abnormal reservoir over pressuring and fracturing has had an important impact on enhancing hydrocarbon production. The consideration of all these factors should govern the selection of future exploration targets. Copyright © 2016, Lanzhou Literature and Information Center, Chinese Academy of Sciences AND Langfang Branch of Research Institute of Petroleum Exploration and Development, PetroChina. Publishing services by Elsevier B.V. on behalf of KeAi Communications Co. Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). Keywords: Continuous oil and gas plays; Fan-delta; Lacustrine high-quality source rocks; Highly mature light oil and gas; Distal hydrocarbon accumulation; Baikouquan Formation; Mahu Sag
1. Introduction The Junggar Basin is a typical large petroliferous basin in the western China. Recent exploration was conducted in this basin by PetroChina [1,2], they discovered new and unusual continuous light oil (and gas) plays that are extremely distal *
This is English translational work of an article originally published in Natural Gas Geoscience (in Chinese). The original article can be found at: 10. 11764/j.issn.1672-1926.2016.02.0241. * Corresponding author. E-mail address: [email protected] (A. Imin). Peer review under responsibility of Editorial Office of Journal of Natural Gas Geoscience.
from source sequences in the Lower Triassic Baikouquan Formation. This formation contains glutenite reservoir rocks with proven oil reserves of hundreds of millions of tons. The Mahu Sag lies on the downthrown block of the “hundreds of miles oil region” fault zone. This is the first discovery within the Junggar Basin possessing an “old source and new reservoir” type of accumulation, which covers a large area within a sag structure. Compared to local and international examples of hydrocarbon reservoirs, this “old source and new reservoir” type that's associated with glutenites is unusual [3e6]. In order to better understand this hydrocarbon reservoir, this paper investigates the accumulation mechanism and controlling factors of this hydrocarbon resource.
http://dx.doi.org/10.1016/j.jnggs.2016.10.003 2468-256X/Copyright © 2016, Lanzhou Literature and Information Center, Chinese Academy of Sciences AND Langfang Branch of Research Institute of Petroleum Exploration and Development, PetroChina. Publishing services by Elsevier B.V. on behalf of KeAi Communications Co. Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).
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2. Basic characteristics of the hydrocarbon plays Oil and gas exploration has proven to be successful in the Baikouquan Formation in the Mahu Sag of the Junggar Basin (Fig. 1). Exploration wells, namely, Ma 131, Ma 15, and Xia 72 in the Xiazijie fan group of the western part of the basin have reserves of hundreds of millions of tons. The Well Mahu1 in the Karamay fan group and Well Ma 18 in the Huangyangquan fan group could produce oil as much as 60 t/d. In the Fengnan fan in the western part of the basin, large reserves have also been discovered. Moreover, there are some other exploration wells, such as Well Yanbei 1 and Well Yanbei 2 in the Madong-Xiayan fan group, which also have oil flows. These wells show that the hydrocarbon-producing zone is extensive and highly productive and such wells will be an important area for continued exploration of the Xinjiang Oil Field Company, or even PetroChina. Through Well Ma 131, the Baikouquan hydrocarbon resources were first discovered, and some insights about the characteristics of the Baikouquan Formation provided were determined. According to lithological, electrical resistivity, oil, and sedimentary facies' data, the target layer (Triassic Baikouquan Formation) can be divided into three sections: T1b1,
T1b2, and T1b3. T1b2.; the said layers can be subdivided into T1 b2 1 and T1 b2 2 , with T1 b2 1 being the main oil reservoir. T1 b2 1 rocks are mostly comprised of gray and brownegray conglomerate, along with pebbly argillaceous siltstone and argillaceous siltstone (Fig. 2). The distribution of oil and gas in these rocks is related to the location of the faults, for example, in Well Ma 131, oil is sealed by a fault from the Well Ma 13. The reservoir pressure data shows that a single fault block is cut into three sub-fault blocks by the east fault of the Well Ma 131 and the east fault of the Well Ma 15. Hydrocarbons from the Baikouquan Formation of the Mahu Sag are mainly crude oils with gas. Oil densities range 0.78e0.86 g/cm3, and are predominantly 0.82e0.86 g/cm3. The methane content is 59.7%e92.8% (average 76.8%), whereas the ethane content is estimated to be 2.5%e11.9% (average 7.4%). These gas ratios are indicative of wet gas, but deep exploration has recently discovered dry gas. Therefore, according to these basic oil and gas characteristics, this hydrocarbon resource is the product of mature to highly mature source rocks. In general, such oil and gas systems have characteristics that include oil (and gas) distributed throughout the whole
Fig. 1. Geological sketch map showing hydrocarbon exploration results and predicted hydrocarbon occurrences in the Lower Triassic Baikouquan Formation of the Mahu Sag.
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Fig. 2. Cross-section of the Lower Triassic Baikouquan Formation in the Mahu Sag showing oil accumulation zones.
reservoir belts, localized areas of enrichment, and high productivity. This corresponds to typical “continuous oil (and gas) play” characteristics [3,7,8]. Hydrocarbon-source correlation studies have shown that the hydrocarbons are mainly derived from deep lower Permian Fengcheng lacustrine source rocks [9,10] (see Section 3.3), which is different from the typical “continuous oil (and gas) plays” that are generally sourcereservoir integrated [8,11,12]. Therefore, this petroleum resource is termed an extensive “continuous hydrocarbon play” that is distal from its source rocks and represents a global model for the study of the formation and nature of such oil (and gas) reservoirs. 3. Hydrocarbon accumulation mechanism The average depth of the Baikouquan Formation in the Mahu Sag is 3.5 km. The Baikouquan reservoir is separated from the underlying Permian source rocks by 1e2 km. The various lines of evidence suggest that three main factors led to the formation of the large continuous hydrocarbon plays, which are discussed as follows. 3.1. Hydrocarbon source rocks The deep CarboniferousePermian rocks in the Mahu Sag include four types of high-quality source rocks (Fig. 3). Mudstone samples with a mediumehigh abundance of organic matter are found in all the four types of source rocks. The Lower Permian Fengcheng lacustrine source rocks are of the highest quality, although they do not have the highest organic carbon content, these rocks have the greatest hydrocarbon generation potential. This reflects the fact that the organic
matter type in the Fengcheng lacustrine source rocks is optimal for hydrocarbon generation as proved by the carbon isotope (relatively light) and hydrogen index (relatively high) data. The Lower Permian Fengcheng lacustrine source rocks were deposited in a saline, alkaline, and reducing environment [9,13]. Previous studies have shown that alkaline lacustrine sediments with abundant microbial organic matter have a hydrocarbon generation capacity that is often several times than that of non-alkaline lacustrine source rocks. Thus, the generated crude oil has a lower density and often times readily migrates [14,15]. Faults cutting through the source rock sequence (Fig. 4) constitute favorable conditions for hydrocarbon migration [16]. During the early Carboniferous to Permian times (foreland uplift period) the Junggar Basin was affected by compressive stress, as a result, it formed a series of large-scale reverse faults that amended the source rock sequence. Reverse faults continued to develop until the EarlyeMiddle Triassic periods. These faults cut through the Baikouquan Formation, causing the Fengcheng source rocks to link with the Baikouquan reservoir, forming an effective conduit for hydrocarbon migration (Fig. 4) [17,18]. In the Zhongguai-Wuba and Manan slope areas, large strike-slip fault systems have developed, where the reservoir is directly linked to the source rocks. The Baikouquan Formation forms a topographic and lithologic oil reservoir. In the Mabei slope area, the Xiazijie structural belt has formed a nose structure that abuts against a reverse fault. The fault breaks off to the bottom of T2k, thereby linking the lower hydrocarbon generation layers with the reservoir. In the Madong slope area, a series of reverse faults have formed along the nose structure due to the frontal uplift belts. These
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Fig. 3. Geochemical profiles of CarboniferousePermian hydrocarbon source rocks in the Mahu Sag.
faults break off into T2k, yet again linking the hydrocarbon generation layers with the reservoir. The study area is located in the central part of a hydrocarbon-rich depression with faults connecting source rocks with reservoirs. Thus, high maturity source rocks for oil and gas are abundant, and it forms the basis for extensive hydrocarbon accumulation distal from these source sequences, which ultimately led to a high production area. 3.2. Reservoir nature and conditions The Baikouquan Formation in the Mahu Sag has the reservoir qualities to form a “continuous hydrocarbon play”. It specifically includes widespread conglomerate reservoirs that
have excellent permeability and porosity that aid in the accumulation of large amounts of oil and gas. Previous studies have shown that the conglomerate was deposited in a fan-delta sedimentary system [19e21]. There are four distinct genesis regions to the depositional systems around the Mahu Sag and the six fan bodies that are associated. The six fan bodies are Xiazijie, Huangyangquan, Karamay, Zhongguai, Yanbei, and Xiayan. The six fan bodies provided abundant terrigenous detrital material during the deposition of the Baikouquan Formation. The slope of the sedimentary system was gentle during deposition, thus, resulting in a well-developed pro-delta sedimentary facies with sand bodies extending into the center of the basin (i.e., T1b1 and T1b2). The thick layers of glutenite are distributed
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Fig. 4. Seismic section showing the structural highs in the Baikouquan Formation, Mahu Sag.
widely throughout the basin and have good physical properties for a hydrocarbon reservoir. The frontal facies of each fan are distributed over a large area, with each facies covering several hundreds of square kilometers. The fan bodies developed in response to the paleo-geomorphological controls, with mountain passes and valleys defining the main groove and plains facies. Large strike-slip faults extended the fan deposits. In addition, mudstone units were formed adjacent to the paleomountains (Fig. 5). Oil and gas migrated along the faults of the deep CarboniferousePermian source rocks to the Triassic pro-delta reservoirs, which are sealed at the top, bottom, and sides. Ever since the Late Cretaceous the regional tectonic activity has been minor, and the faults have become less active and sealed, thereby preventing re-migration or escape of oil and gas. All of these conditions are favorable for oil and gas accumulation, it also explains the wide distribution of oil and gas [22]. The Middle Triassic Karamay mudstone overlies the Baikouquan Formation. At the bottom of the Baikouquan Formation, dense glutenites overly the middle Permian lower Wuerhe mudstone (Fig. 6). The sides of the reservoir are the near-provenance (structural highs),
these are comprised of fan-delta-plain facies overlying the dense glutenites; these and the mudstones above and below the Baikouquan Formation provide effective reservoir seals. The fan-delta-plain facies sandstone and conglomerate are clay-rich but poorly sorted. They have poor reservoir physical properties with porosities being >5% and permeability mostly <0.1 mD, for example, such rocks in the Well Xia 72 have a porosity that is 2.8%, whereas in the Well Xia 9, the porosity is 3.2%. On the contrary, the main Baikouquan sealed reservoir is predominantly fan and pro-delta front facies gray sandstone and conglomerate. These reservoir rocks are well sorted but poor in clay. They have porosities ranging 6%e14% that averaged ~10%, and permeability that is generally >1 mD. An example is the rocks in the Well Ma 18 that possesses an average porosity and permeability of 10.5% and 5.66 mD, respectively, whereas, in the Well Ma 2, the average porosity and permeability are 9.4% and 1.41 mD, respectively. Therefore, these reservoir properties such as overlying and underlying seals, and absence of bottom water all contribute to making the formation favorable for hydrocarbon charging and accumulation [22,23].
Fig. 5. Fan-delta sedimentary system model, where the Lower Triassic Baikouquan Formation of the Mahu Sag was deposited.
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Fig. 6. Lithological correlation and conditions of overlying, underlying, and side preservation of the Lower Triassic Baikouquan Formation in the Mahu Sag.
3.3. Hydrocarbon charging and evolution There are several types of high-quality source rocks in the Baikouquan Formation (Fig. 2) with the best being the Fengcheng rocks (Fig. 3). The Fengcheng source rocks are most likely the ideal source rock for hydrocarbon generation in the study area as validated by the hydrocarbon geochemical characteristics. Carbon isotope data for all crude oils in the study area have ad13C less than 28‰, indicating a typical oilekerogen crude oil. Pr/Ph values are less than 1.3, this reflects the reducing sedimentary in the environment in which the source rocks were deposited (Fig. 7a). In addition, the gammacerane indices of all the crude oils are greater than 0.2, indicating that the oil is from saline (alkaline) lacustrine source rocks. Tricyclic terpanes C20, C21, and C23 are mainly in increasing proportions. All these characteristics are indicative of the Fengcheng source rocks in the study area [9,10]. Isotopic analyses show that the natural gasses are mainly oiltype gas (Fig. 7b), corresponding to Fengcheng source rocks [9,10,24]. Therefore, crude oil and natural gas of the Baikouquan reservoir in the Mahu Sag are mainly derived from the Fengcheng source rocks. The Fengcheng source rocks had two hydrocarbon generation peaks: Late Triassic and Early Cretaceous (Fig. 8). The
two-stage oil and gas charging process affected the Baikouquan frontal facies. This hydrocarbon charging and evolution, as well as three-dimensional sealing brought by the dense glutenites and mudstones, good overlying and underlying preservation, and ideal storage conditions (Fig. 6) all led to the enrichment and accumulation of hydrocarbons. As shown in Fig. 8, the two hydrocarbon expulsion stages were in the Triassic to the Early Jurassic and the Early Cretaceous [9,10]. The first stage was mainly continuous mature oil charging, with bitumen being involved leaving yellow fluorescent-free hydrocarbon inclusions in the reservoirs. In contrast, the second stage was mainly charging of highly mature oil, represented by blue and white fluorescent hydrocarbon inclusions. Combined with studies of the reservoir evolution, it can be inferred that the Baikouquan Formation was buried in a shallow area (0.5e1.0 km) during the first oil charging stage (Triassic to Early Jurassic). This resulted in a weak compaction and preservation of good reservoir properties (porosity of 20%). The Baikouquan reservoir was then buried deeply (3e4 km) during the second oil charging stage (Early Cretaceous). The compaction then became more striking with a resultant deterioration of reservoir properties (porosity of 10%), therefore, the oil and gas accumulated in the favorable frontal facies reservoir. These essential reservoir physical
Fig. 7. Hydrocarbon geochemistry of the Lower Triassic Baikouquan Formation, Mahu Sag.
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Fig. 8. Two-stage hydrocarbon generation and charging in the Lower Triassic Baikouquan Formation, Mahu Sag.
properties resulted in highly mature oil accumulating in a large area of the slope belt. The threshold porosity for charging of intermediate and light oils into the Baikouquan Formation is 10.7% and 7.7%, respectively. Therefore, the oil and gas (mature and highly mature) accumulated in two stages while being in favorable reservoirs. In summary, the Baikouquan “continuous hydrocarbon plays” in the Mahu Sag was formed due to three factors: highquality and abundant source rocks, good reservoir quality and seals, and well-matched hydrocarbon charging and evolution. 4. Regularities of hydrocarbon accumulation and factors influencing high hydrocarbon production 4.1. Regularities of hydrocarbon accumulation Through comparison with similar hydrocarbon plays in China and internationally, there are three favorable characteristics that can be identified for the development of “continuous hydrocarbon plays” distal from source rocks in the Mahu Sag of the Junggar Basin. Firstly, high-quality lacustrine source rocks provide a good hydrocarbon source. Secondly, large-scale strike-slip faults connect the source and reservoir rocks allowing long-distance hydrocarbon migration. Thirdly, the fan-delta sedimentary system provides good conditions for reservoir formation and continuous hydrocarbon accumulation. The Mahu Sag has the greatest hydrocarbon generation potential in the Junggar Basin (Fig. 9). Its high-quality source rocks are mainly Permian (as well as Carboniferous) in age. Permian sedimentary rocks have characteristics typical of deposition in a foreland basin, including the Lower Permian Jiamuhe and Fengcheng Formations and middle Permian Wuerhe Formation. These three sets of Permian source rocks, along with the other Carboniferous age, are thick and widely distributed. They also generated high amounts of oil and gas, providing the basis for hydrocarbon accumulation. The Fengcheng lacustrine source rocks are the most significant of these four sets of source rocks
(Fig. 3). After the oil and gas expulsion from the source rocks, they migrate upwards along the NW to NWW-trending strikeslip faults. These faults are steeply dipping and typically unconformable, they're overlain by Triassic rocks or have only experienced small amounts of later activity (Fig. 4) [17,25]. The faults extend down to the high-quality source rocks (Fengcheng) and extend upwards to the Triassic unconformity. The faults are sealed by thick Triassic mudstones, which form a three-dimensional oil and gas migration and trapping system (Fig. 4). Given that these faults intersect, the relatively highquality Baikouquan fan-delta reservoir rocks have been favorable sites for hydrocarbon accumulation. The Baikouquan glutenite reservoir has a thickness ranging 50 and 100 m, its porosity is between 5% and 12%, and the oil zone thickness is between 10 and 50 m. The main types of reservoir space are primary grain and secondary solution porosity (feldspar), with minor clay shrinkage porosity and micro-cracks. Organic-rich acidic waters produced in the underlying Permian hydrocarbon source rocks resulted in upward dissolution along the cracks and unconformities that was the main factor in generating secondary porosity. The secondary porosity constitutes a complex reservoir space in addition to the primary porosity and cracks. The primary porosity features of the reservoir are controlled by the six fan bodies around the sag during its formation, which supplied abundant terrigenous detrital material along a gentle sediment slope allowing a sand body to extend into the lake center. These provided favorable conditions for the formation of this oil and gas “continuous” plays. 4.2. Factors influencing hydrocarbon accumulation and high production Hydrocarbon accumulation is controlled by three factors: a favorable sedimentary facies, structural highs, and faults. Abnormal reservoir over pressuring and fracturing has had an important influence on hydrocarbon production. These three factors are discussed in detail below:
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Fig. 9. Geological cross-section showing petroleum migration and accumulation in the Lower Triassic Baikouquan Formation, Mahu Sag.
(1) Sedimentary facies: In the exploration area, the most productive reservoir area is the Xiazijie fan group, which can be further subdivided into two fan bodies (Xiazijie and Fengnan fan bodies). Oil and gas are also found in other fan bodies, including the Huangyangquan, Karamay, and MadongeXiayan fan groups. The frontal sub-facies of each fan body further control the degree of oil and gas enrichment. The Baikouquan reservoirs were formed in the pro-delta and plain sedimentary facies environments. The fan delta and plain facies rocks are brown glutenite and conglomerate, and for the pro-delta underwater channels are gray glutenite and conglomerate. The fan delta and plain facies rocks are poorly sorted with mixed accumulations of sand, mud, and gravel including an argillaceous matrix, which is responsible for its relatively poor reservoir properties. In contrast, the pro-delta underwater channel rocks formed in a more active hydrodynamic setting, thus, these rocks have low clay contents; they are also well sorted and are relatively good reservoir material. Therefore, the best reservoirs are the gray glutenites and conglomerates, an example would be the subfacies of the deposition which controlled the quality of oil and gas accumulation. (2) The accumulation of oil and gas was also controlled by the distribution of structural highs and faults. In general, oil and gas are located in the noses of the structures in the
study area. These noses are connected by faults that provide favorable locations for the accumulation of migrated hydrocarbons [26]. Typically, structural highs are associated with faults, which are important for hydrocarbon accumulation and enrichment, especially considering the long-distance migration that has taken place in this region. The faults are of great importance as they facilitate an upward migration of hydrocarbon generated by deep source rocks to the reservoirs (Fig. 4). The northeastern main faults controlled the distribution of the fan body and dense series of channels, and the east-west-striking subfault on the flank controlled the distribution of slope breaks laterally. Thus, these faults controlled the distribution of pro-delta sand bodies and the subsequent development oil-bearing layers (Fig. 9). (3) Reservoir over pressuring and fracturing also affect the production of oil and gas. In general, it is considered that these processes “open up” the reservoir, thus, allowing high production which is important for the tight continuous reservoirs [27e30]. The Baikouquan Formation in the Mahu Sag is over pressured (pressure coefficient >1.3) and is favorable for high oil and gas flow rates. Core observations show that the reservoir is highly fractured, particularly in brittle lithologies. These fractures also improve the reservoir properties and enhance production rates.
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In summary, hydrocarbon accumulation and enrichment in the Baikouquan Formation of the Mahu Sag is controlled by favorable sedimentary facies (pro-delta), structural highs, and faults. High hydrocarbon production is further enhanced by reservoir over pressuring and fracturing. The heavy consideration of these factors should be used to guide future exploration of oil and gas in this region (Fig. 1). The favorable areas for exploration were predicted (Fig. 1). 5. Conclusions (1) Hydrocarbon exploration in the Mahu Sag of the northwestern Junggar Basin, China for the first time identified a “continuous hydrocarbon play” distant from source rocks. The plays are extensive in area and they have a largely proven capability to produce hydrocarbons. (2) There are three main conditions that have contributed to this hydrocarbon accumulation, namely, large hydrocarbon source, good reservoir and seals, and balanced hydrocarbon charging and evolution. High-quality lacustrine source rocks provide an abundant source of hydrocarbons. Largescale strike-slip faults link the source and reservoir rocks, thus, facilitating long-distance hydrocarbon migration. The fanedelta sedimentary system deposited sediments are excellent reservoirs for hydrocarbon accumulation. (3) Hydrocarbon accumulation is controlled by three factors: pro-delta sedimentary facies, structural highs, and faults. These factors should all be considered when selecting future exploration targets. In addition, reservoir over pressuring and fracturing enhance hydrocarbon production.
Foundation item Supported by Projects of National Science and Technology Major Project of China (2016ZX05001-005) and PetroChina Science and Technology Major Project “Xinjiang's Daqing” (2012E-34-01). Conflict of interest The authors declare no conflict of interest. Acknowledgments We thank editors and anonymous reviewers for their valuable reviews and comments, which improved the article. References [1] L.C. Kuang, Y. Tang, D.W. Lei, T. Wu, J.H. Qu, Exploration of fancontrolled large-area lithologic oil reservoirs of Triassic Baikouquan Formation in slope zone of Mahu depression in Junggar Basin, China Pet. Explor. 19 (6) (2014) 14e23. [2] D.W. Lei, I. Abulimit, Y. Tang, J. Chen, J. Cao, Controlling factors and occurrence prediction of high oil-gas production zones in Lower Triassic Baikouquan Formation of Mahu Sag in Juggar Basin, Xinjiang Pet. Geol. 35 (5) (2014) 495e499.
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