Genesis of Miocene litho-stratigraphic trap and hydrocarbon accumulation in the Qiongdongnan Basin, northern South China Sea
© The Author(s) 2018
Received: 3 September 2017
Accepted: 23 April 2018
Published: 30 April 2018
In recent years, several large gas fields have been discovered in western Qiongdongnan Basin. It is important and necessary to illustrate their sedimentary characteristics and hydrocarbon migration so that more gas fields could be discovered in the future. Previous regional tectonic-sedimentary researchers show that large-scale source rock of the Yacheng Formation developed in the Ledong and Lingshui sags due to the Red River Fault pull-apart strike slip in early Oligocene. The main targets for hydrocarbon exploration in this area are the Miocene deep water reservoirs. In late Miocene, the Huangliu Formation reservoirs are composed of the early channels which were sourced by river systems in Hainan uplift and the consequent channels were sourced by Qiupen River in Kunsong uplift. Both axial channels exhibit unique spatial distribution patterns and geometries. The other kind of reservoir developed in the middle Miocene Meishan Formation, which compose of slope break-controlled submarine fan. They can be further classified into three types—slope channelized fan, basin floor fan, and bottom current reworked fan. The various fans have different reservoir quality. These two kinds of reservoirs contribute to four types of litho-stratigraphic traps under the actions of sedimentation and subsidence. The overpressure caused by hydrocarbon generation can fracture deeper strata and result in regional fractured network for hydrocarbon migration. Therefore, free gas driven by overpressure and buoyancy force can be migrated into Miocene litho-stratigraphic traps to accumulate. The revealed genesis of Miocene lithologic trap and hydrocarbon accumulation in the Qiongdongnan Basin would greatly contribute to the further hydrocarbon exploration in northern South China Sea and can be helpful for other deep water areas around the world.
In recent several years, large-scale high-quality 3D seismic acquisition and application, including the widespread application of pre-stack depth migration in slope break belt, make it possible to identify and evaluate Paleogene the Yacheng Formation as source rock, which is also favorable for Miocene seismic sedimentology, paleotopography, and sedimentation researches. With the help of high-resolution electric image logging technology, sedimentary textures associated with reservoir formation environment can be clearly identified. Moreover, this study conducted the pressure–accumulation relation research and description of fracture net which can be served as vertical migration paths. Research results illustrate reservoir-forming mechanism using new 3D data and advanced imaging logging technology, which is helpful for gas exploration and leads to the recent breakthrough of gas exploration in this area, such as LS25 gas field and LS13 gas field.
The Ledong and Lingshui Sags are located in southwestern Qiongdongnan Basin, northern South China Sea. The Ledong sag is bounded by Zhongjian uplift, like an irregular square narrows eastward, while Lingshui sag is similar to a narrow irregular trapezoid (Fig. 1). The extension amount in the Ledong sag can reach up to 138 km, comparing to 20 km that of the Lingshui sag. The Ledong sag is a graben controlled by northern Fault No. 2 and southern Fault No. 13. As contrasted to larger fault throw of Fault No. 13, Fault No. 2 exhibits remarkable segmented activity and fault throw is generally small. The western Lingshui sag features a graben, while its eastern part is a half-graben with gentle-slope and fault terrace which faults in the south and overlaps in the north (Zhao et al. 2010; Lei et al. 2011). It is the variation of tectonic stress from east to west that resulted in the different extension amount between two sags. Early Oligocene was the critical period for the development of source rock, when the Ledong sag was controlled by Red River Fault (Fault No. 1), which had a large component of extension amount owing to pull-apart strike-slip processes at that time (Chen et al. 1996; Tapponnier et al. 1990). So it can be inferred that source rock in the Ledong sag would be thicker and larger than that in the Lingshui sag in the view of tectonic–stratigraphic evolution.
At present, although there is no well drilled into the Oligocene in the Ledong and Lingshui sags, carbon isotope characteristics of gas in the Huangliu Formation from the Ledong and Lingshui sags is similar to those from the Yacheng 13-1 gas field, whose gas source has been proven to be from coal measures in the Yacheng Formation (Xie and Tong 2011). Recent acquired 3d seismic data allow us to better understanding the sequence stratigraphy in the area. Seismic stratigraphic correlation shows the super-thick early Oligocene (the Yacheng Formation) developed in the Ledong and Lingshui sags, while the Eocene is only restricted to subsidence centers, and its range of distribution gradually decreases from north to south. On the contrary, Oligocene Yacheng Formation becomes thicker northward, which is coincident with the extensional pattern in western Qiongdongnan Basin. These findings differ from previous researches, which assumed that Eocene and early Oligocene in the Ledong and Lingshui sags were mainly neritic environment (Zhu et al. 2008; Gong and Li 2004).
It can be inferred that in the Yacheng Formation the terrain sloped gently in the Ledong and Lingshui sags from the fact that the thickness of the Yacheng Formation is relatively even in western basin and depocenters are not obvious. This paleogeomorphology is similar to the nearby Yannan sag, which is not only favorable for the development of marine-terrestrial transitional sediment-coal measures, but also for the concentration of high organic matter due to semi-closed neritic environment that spread widely like a ribbon in the central Ledong and Lingshui sags (Mi et al. 2010; Liu et al. 2011; Li et al. 2011). The distribution of the Yacheng Formation with 2000 m thick is over 5300 km3 in the Ledong sag, while that with the similar thickness in the Lingshui sag is about 3900 km2. The mean TOC (total organic carbon) of the Yacheng coal layers in the Yanan sag is approximately 20%. The carbonaceous mudstone has the average TOC of 5%, and coal layers developed as a form of thin interbeds. Kerogen type of the Yacheng source rock is humic (type III) and gas generation is predominant. Huge total thickness and continuous hydrocarbon-generating ability of the source rock make up for the deficiency of thin coal layers.
Reservoir and litho-stratigraphic trap characteristics
The hydrocarbon explorations in this area have documented two kinds of reservoirs: one is axial channels developed in the Late Miocene Huangliu Formation and the other is the Middle Miocene submarine fans in the Meishan Formation.
Provenance of axial channels in Huangliu Formation
The axial channels in Huangliu Formation are distributed in the central part of the whole basin. The early axial channels developed mainly in northern Ledong sag, while the later channels spread from eastern Ledong sag to Lingshui sag and further east.
During late Miocene, the Yinggehai Basin and Qiongdongnan Basin began to combine into one sedimentary assemblage, developing a great number of channels along the long axis of basin (Lin et al. 2011a, b). There are three kinds of perspectives on the provenance system of these channels: (1) red river system from the north of the Yinggehai Basin (Wang et al. 2011; Yuan et al. 2010a, b), (2) river system from Indo-China peninsula (east coast of Vietnam) (Yao et al. 2008), and (3) river system originated from eastern the Yinggehai Basin (Wu and Qin 2009; Wu et al. 2011). There are two arguments on the origin of channels. One is gravity flow origin, which claims that the channels are caused by the incision of turbidity current and mass transported flow due to slumped delta front or foreset beds of slope (Yao et al. 2008; Xie et al. 2011); the other is tractive current origin which claims that the hydrodynamic force is derived from bottom current or internal tidal action (Zhu et al. 2007, 2008; Shao et al. 2010).
Sedimentary characteristics of Huangliu channel reservoirs
The spatial distribution of early channels is unstable with high curvature and frequent incision. The early channels mainly developed at the bottom of Huangliu Formation. The slope gradient was gentle so that the gravity fluid flow is absent, and thus their lateral switches are frequent. Consequently, all of the early channels converged to the east. The later channels had larger scale with stable spatial distribution, and eventually converged to one central canyon in the Qiongdongnan Basin. Their straight geometries indicate that the terrain of the Qiongdongnan Basin was low in the west with large slope gradient. Furthermore, the terrigenous clastic sediments carried by the Qiupen river flowed down from the Kon Tum Uplift were accumulated in the Ledong Sag, and then moved eastward, so the abundant sediment supply from the west increased the topographic gradient of the eastern Qiongdongnan Basin, accelerating effect of gravity fluid flow which led to the formation of deep straight restrictive canyon. The restrictive terrain caused greater canyon erosion, and the scale of canyon increased greatly with decreased sinuosity. The extensive canyon erosion process was continuous till the sea level rose to a certain point, and then the canyon started to accept depositions. Large-scale canyon in negative landform facilitated multistage depositions of sandstone. The stacking thickness of channel sandstones could be more than 400 m.
Sedimentary characteristics of submarine fan reservoirs in the Meishan Formation
Different from the Huangliu Formation axial channel sandstone, drilling results indicate that the heavy mineral assemblages of the Meishan Formation in the Ledong and Lingshui sags display the magmatic components characteristics. It means that the provenance of the Meishan Formation sediments was mainly from the Ningyuan River and Lingshui River, which originated from the southwestern Wuzhi Mountain in the Hainan uplift. Because of regional sea level falling sharply in 13.8 Ma (Pang et al. 2005), terrigenous clastic sediments carried by these rivers were transported from the northern slope break of the Ledong and Lingshui sags, resulting in submarine fan deposits (Xie et al. 2016). In addition, the southwestern slope break of the Ledong sag might be another source area of the Meishan submarine fan (Zuo et al. 2015), although there is not enough seismic data to confirm it.
The second type of submarine fan is basin floor fan. Due to differential subsidence, there was no typical Miocene abyssal plain developed in the Qingdongnan Basin. Conversely, many wide low-lying areas existed from slope toe to the center of the Ledong and Lingshui sags, making it easy to form stable superposed gravity fluid flow deposits. The basin floor fans have classical fan shapes with several sedimentary units, such as inner fan feeding channel, branch channel, overbank-natural, bank front lobe, and so on. Their feeder channels locate in the slope toe along NW–SE direction. There are two short-straight feeder channels developing successively, which extend about 10 km long and 1 km wide. The ends of channels converge and overlap each other, indicating a stable environment in their depositional center. The seismic profiles present the features of medium-strong amplitude reflection and continuous seismic events, and the V-shaped incisions can be seen on underlying stratums. In addition, a few natural levees are developed on the margin of channels. Drilling results reveal that the fillings in the channels are moderate-coarse sandstone with poorer sorting and box-shaped logs.
The third type of the Meishan submarine fans is the fan reworked by bottom currents, which are distributed in the central low-lying area in the Ledong and Lingshui sags. Sand-body develops along the axis of the basin. Moreover, the sand-body thickness becomes thinner abruptly, comparing to the adjacent basin floor fan, and the seismic profiles show low-amplitude incised characteristics in sandstone bottom interface. All the features above illustrate that the effect of bottom current paralleling with the slope existed in the basin center. It is speculated that the bottom current occur when channelizing processes of submarine fan began. The sandstone reworked by bottom currents has better sorting, homogeneity, and permeability than those of the other two types of fans. Drilling results prove that the submarine fans reworked by bottom currents have box-shaped characteristics of gamma ray curve. The sedimentary features mostly include massive bedding and its lithology is well-sorted thick fine sandstone. The channels have weak erosion with medium-strong amplitude reflection shown in seismic profiles.
Origin and types of traps
The trap formation for Meishan submarine fans is mainly caused by the deposition pinching out (Fig. 8d). As the entire basin subsided continuously in Neocene, the later differential regional tectonic subsidence affected the structural height. In Pliocene and Pleistocene, the subsidence centers transferred to the Ledong sag gradually, which led to terrain reversal, so the pinch out position of fans reworked by bottom currents became the highpoint of trap formation. Overall, deposition pinching out is the main controlled factor of submarine fan traps.
Hydrocarbon migration under fractured system induced by overpressure
According to the relationship between gas maturity and methane δ13C1, the gas maturity of entirely channel sandstone presents a tendency that the maturity increases from the east to the west, so it can be inferred that gas migrated from west to east. However, the maturity tendency of LS17 gas field in the east is reversed, which means that the strip-like channel traps across the entire Ledong and Lingshui sags were charged by gas with different maturities.
Gas migration under fractured system induced by overpressure
Except for the reactivation of Fault No. 2 in the north slope of the Lingshui sag, all other fault activities were absent in the Ledong and Lingshui sags in Neocene, and thus oil and gas mainly migrated vertically through the fractures induced by overpressure.
Controlled by early Oligocene pull-apart strike slip of the Red River Fault in, the Yacheng Formation source rock in the Ledong and Lingshui sags were developed in a large scale with enormous resource potential. Middle-late Miocene abundant terrestrial sediments contributed to the formation of reservoirs, including the Huangliu axial channel sandstone and the Meishan submarine fan sandstone, which resulted in four types of lithology-stratigraphic traps under the actions of deposition and differential subsidence. Regional fractured network in deeper formation below these traps were induced by hydrothermal and hydrocarbon-generating overpressure, and connected source rock to reservoirs; therefore, gas could migrate to lithology-stratigraphic traps driven by buoyance and overpressure to be accumulated finally. The genesis of Miocene lithologic trap and hydrocarbon accumulation in the Qiongdongnan Basin have been revealed by several recent discoveries, and it will contribute to further hydrocarbon exploration in northern South China Sea.
CF designed the research. TJ and KL drew the relevant figures with JT. All authors discuss the obtained results and drafted. All authors read and approved the final manuscript.
The authors greatly appreciate Mr. Song Peng and Mr. Zhou Fan for their sincere helps. The authors are grateful also to Western South China Sea Laboratory for providing some experimental datum.
The authors declare that they have no competing interests.
Availability of data and materials
The experimental data referred in the paper are available from the corresponding author on reasonable request.
Ethics approval and consent to participate
This study was supported by the China National Science and Technology Major Project (2016ZX05024-005).
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