+ MODEL Available online at www.sciencedirect.com ScienceDirect Journal of Natural Gas Geoscience xx (2017) 1e13 http://www.keaipublishing.com/jnggs Original research paper Seepage system of oil-gas and its exploration in Yinggehai Basin located at northwest of South China Sea Jiaxiong He a, Wei Zhang a,b,*, Zhenquan Lu c a Key Laboratory of Marginal Sea Geology, Guangzhou Institute of Geochemistry, Chinese Academy of Sciences, Guangzhou 510640, China b Guangzhou Marine Geological Survey, Guangzhou 510075, China c Oil & Gas Survey, China Geological Survey, Beijing 100029, China Received 26 September 2016; revised 27 December 2016 Available online ▪ ▪ ▪ Abstract Seepage systems of oil-gas in Yinggehai Basin are divided into two types, namely: “micro-seepage”, which is presented by gas chimneys and pockmarks; and “macro-seepage”, which is also called oil-gas outflow; and, in addition, the combination of the two basic types Among the oil seepage systems, the combined seepage system at Yingdong Slope of Yinggehai Basin is the most eye-catching, and gas chimneys and pockmarks micro-leakage systems in mud diapir zones in the central part of the basin are very common Both the indications of large-scale oilgas outflow at Yingdong Slope, which have been booming for a hundred years; and the occurrence of pockmarks at the central mud diapir belt, along with the chaotic seismic reflection of widely-distributed shallow gas chimneysdhave shown that hydrocarbon in this area is sufficient and oil-gas is now in dynamic equilibrium of the processes of accumulation, migration, gathering and dispersing It builds up good conditions for the accumulation, migration, gathering and reserving of oil and gas However, it must be noted that the results of oil-gas exploration at Yingdong Slope didn't turn out to be satisfactory, despite the presence of oil-gas outflow and gas chimney combined seepage systems So, strengthen synthesized analysis and study on oil-gas seepage systems and on the conditions for accumulation, migration, gathering and dispersing; the forecasting and evaluation to the advantageous conditions for enriched oil and gas zones; and trap preservation in accordance with the dynamic balance theories; are of significant importance for purposes of exploration Copyright © 2017, 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: Oil-gas seepage system; Outflow of oil-gas/gas chimney; Macro-seepage/micro-seepage; Trap conditions; Yinggehai Basin; South China Sea Introduction Widely distributed in the marine environment, especially on shelf seabed offshore, seepage systems have been researched throughout the offshore areas of continental margins around the world by many energy companies, universities and research institutions [1e7] Seepage on the seabed is the * Corresponding author Key Laboratory of Marginal Sea Geology, Guangzhou Institute of Geochemistry, Chinese Academy of Sciences, Guangzhou 510640, China E-mail address: zwgmgs@foxmail.com (W Zhang) Peer review under responsibility of Editorial office of Journal of Natural Gas Geoscience process in which shallow (mainly biogenic) or deep (mainly thermogenic) gas sources overflow from the seabed in the buoyancy along the channels (such as deposition layer gap, the fault plane, mud volcanoes) [5,6] Seepage can exist in the range of 10e3000 m in ocean depth in a variety of geological environments, such as passive continental margins in the Gulf of Mexico [8], Monterey Bay transition zone in the USA [9], the Australia's offshore [10], the Sea Ridge of the Mediterranean [11,12], the Black Sea [13], and the Barbados Ridge in the Atlantic [14] The presence of seepage on the seabed indicates that it may be rich in oil and gas resources in seafloor sediments [5] The study of seepage can help us know the progress of generation, migration, accumulation and dispersal http://dx.doi.org/10.1016/j.jnggs.2017.01.001 2468-256X/Copyright © 2017, 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/) Please cite this article in press as: J He, et al., Seepage system of oil-gas and its exploration in Yinggehai Basin located at northwest of South China Sea, Journal of Natural Gas Geoscience (2017), http://dx.doi.org/10.1016/j.jnggs.2017.01.001 + MODEL J He et al / Journal of Natural Gas Geoscience xx (2017) 1e13 of oil-gas, and to predict the oil and gas prospects quickly, economically and effectively [5e7] Internationally, the early studies of oil and gas seepage systems are based on geochemical analysis of bitumen, water geochemistry “sniffing” sampling, synthetic aperture radar, and so on [10,15] In recent years, the comprehensive application of modern acoustic detection methods has improved the reliability of investigation and interpretation of oil and gas seepage For example, in the Yampi shelf area of northwestern Australia [10], using side-scan sonar and 120 kHz single-beam bathymetric reveal, a large number of active oil-gas seepage systems have been discovered, corresponding to the true seepage position China has begun to focus on research in this area in recent years Chen et al [16], and Li [17] summarized the geophysical characteristics and identification methods of seepage on the seabed Li et al [18] pointed out the existence of gas seepage system in the central sag zone of Yinggehai Basin, based on analyzing the date of high-resolution seismic profiles, 3.5 kHz shallow stratigraphic section, side-scan sonar image interpretation Zhao et al [19] used the full-covered Side-Scan Sonar and high-resolution seismic survey method to research hydrocarbon seepage in the north depression of South Yellow Sea Basin However, systematic and targeted exploration work remains to be further research Seepage systems of oil-gas at Yinggehai Basin located northwest of South China Sea develop tremendously, mainly in forms of outflow of oil-gas (described as “macro-seepage”), gas chimneys and pockmarks (described as “micro-seepage”), and the combined type of the two seepages Oil-gas outflows are widely distributed in shallow water in adjacent areas of the Yingdong Slope surrounding the southwest margin of Hainan Island [20e23] It's been over a hundred years since the macro-seepage of oil-gas outflow has shown a strong abnormality and a great amount and large distribution of oil-gas rarely found domestically and abroad While indications of micro-seepage, such as gas chimneys and pockmarks in the seabed, are mainly found in the shallow layer of the central diapir belt along the concave area at the central part of the basin, the Yingdong Slopedespecially the margin part of the basin where it's near the central diapir zonedtogether with indications of oil and gas outflow, form the combined seepage system of oil and gas [23e25] The existence and boom of oil-gas seepages, oil-gas outflows, gas chimneys, and seabed pockmarks; as well as other indications of macro-seepage and micro-seepage; have shown that there are oil and gas reserves and active oil-gas generation systems and hydrocarbon sources in its deep underground, as well as in the adjacent areas [21,23] However, the traditional theories and methods on oil and gas exploration all take these as signs, important grounds, and clues for exploring and searching for oil, gas reserves, and oil fieldsdwhich means that research is done on the basis of seepages in exploring and tracing the major direction of oil and gas accumulation, as well as advantageous locations for their reserves; so as to deploy and implement exploration to locate advantageous areas with rich reserves So, the existence of seepages and the outflow of oil-gas and gas chimneys are signs of the generation, gathering and dispersal of oil-gas; and are direct instructions and traces of oil-gas explorationdwhich fully suggests that it is a very active area with oil-gas storage, accumulation and dispersal; thus, it is of great significance to oil geology and oil gas exploration [5e7,21,23] This paper aims to study the distribution characteristics of macro-seepage system of oil-gas and outflows in Yinggehai Basin, as well as the micro-seepage system of oil-gas and gas chimneys Moreover, the paper will focus on their genesis as well as the characteristics of its hydrocarbon source It will analyze and discuss the geological significance and prospects in oil exploration, in order that the study would help promote oil and gas exploration in areas with active oil-gas outflow and gas chimneys Geological background Yinggehai Basin is a young pull-apart basin, characterized by high temperature and overpressure, which develops on basis of the Red River strike-slip fault zone [26e29] Huge Neocene neritic sediments (over 10 km) deposited in the basin center, provide material for mud diapir and oil-gas generation [25] However, the slope areas have thin sediments The long axis of the basin is oriented along the north-west direction, and there are three primary structural units: the Yingdong Slope, the Yinggehai concave area (central concave) and the Yingxi slope from northeast to southwest (Figs and 2) The tectonic evolution of the basin can be divided into two stages: a Paleogene extensional rifting event and a Neogene post-rift thermal subsidence [27,30,31] Basically two mega sequence, and thick deposits of Cenozoic clastics on Paleozoic and Mesozoic basement rocks, filled the basin (Fig 1); abnormal high temperature and a super pressure system existing under 3200 m [26,32,33] formed an uplift tectonic zone at the central diapir zone at the scale of 20,000 km2 (Fig 1) At present, a number of middle-large gas reservoir groups and gas bearing structures have been found in this shallow layer [34e36] There is a great amount of gas chimneys and pockmarks in the shallow layer of gas fields and gas bearing structures, and even the seabed [23e25], which is an obviously common signal of a natural gas seepage system in the gas field and the gas bearing structure Gas chimneys and pockmarks are valid and very common in regular 2D seismicprofile and shallow layer seismic profile obtained by seafloor engineering and geological survey of exploration wells The slope of areas, such as the Yingdong Slope (Fig 2), has a thinner accumulation compared with the central diapir zone; geothermal field is low with the existence of discordogenic fault released by flow of No faultage and Yingdong faultage So there is no possibility of a stratigraphic system of abnormal high-temperature and super-pressure, and there is no diapir zone development We can say that there are no hydrocarbon conditions [21e37] However, the transforming system of lateral migration made by No faultage, Yingdong faultage, multi-regional unconformities and sand bodies are originated here; it further forms a high speed channel for oil gas Please cite this article in press as: J He, et al., Seepage system of oil-gas and its exploration in Yinggehai Basin located at northwest of South China Sea, Journal of Natural Gas Geoscience (2017), http://dx.doi.org/10.1016/j.jnggs.2017.01.001 + MODEL J He et al / Journal of Natural Gas Geoscience xx (2017) 1e13 Fig The distribution characteristics of major tectonic units and the stratigraphic system in Yinggehai Basin (Modified from Refs [21,23]) Fig The seismic profile across Yinggehai Basin (The location of seismic lines seen Fig 1) migration and accumulation, which leads to abnormal development for oil gas seepage systems, mark-outflow of oil-gas, and gas chimneys [25,28,37] Among them, macro-seepage of oil-gas in middle-south section of the slope is most typical, and the outflow of oil-gas is not only strong but also active and widely distributed nowadays (a large amount of outflow of oil-gas can be seen on sea level) [37] Data and methods The data discussed in this paper are provided by a joint CNOONeBP research project and a CNOOC survey project on the seepage investigation with modern detection equipments and advanced acquisition technologies in the Yinggehai Sea in 1960's and in 1990's [22,38]; 39 outflows of oil-gas have been found and confirmed through the investment of the southwest continental margin of the coastal zone of Hainan Island before 1960's, and 70 new oil-gas outflows were found by CNOOC and BP Oil company after 1991 The gas seepage investigation was made using two side-scan sonars equipped on the surveying ships with frequencies of 3.5 kHz and 12 kHz, respectively [23] Most of the seepage gas samples were collected from the water about 21e49 m below sea level using a funnel, such as Yingchong No and No shallow boreholes In addition, some samples in the Yazhou Bay were acquired directly from Please cite this article in press as: J He, et al., Seepage system of oil-gas and its exploration in Yinggehai Basin located at northwest of South China Sea, Journal of Natural Gas Geoscience (2017), http://dx.doi.org/10.1016/j.jnggs.2017.01.001 + MODEL J He et al / Journal of Natural Gas Geoscience xx (2017) 1e13 the seafloor using a remotely operated vehicle (ROV) [23] A lead bomb tube with a maximum sampling depth of 30 cm was used to collected seafloor sediments In addition, mud snapper samples were also collected at or near the seafloor A few crude oil samples were also taken from Qiongdongnan Basin, which were used for comparing with Yinggehai Basin (the location of drillings are near Yingdong Slope in Yinggehai Basin) Furthermore, at least 30 gas samples are collected from the drillings or reservoirs in the Yingdong Slope and shallow gas in the central mud diapir zone The above collected sediment samples were then extracted in dichloromethane and were analyzed by GC and GC/MS The composition of the obtained seepage gases and reservoir gases were analyzed using a Hewlett Packard 5890 II gas chromatograph Methane, ethane, and CO2 were separated from the gas samples for d13C measurements using a Finnigan-MAT251 mass spectrometer [23] Results and discussion 4.1 Characteristics of the combined seepage system and outflow of oil-gas at Yingdong Slope Yingdong Slope belongs to a secondary tectonic element of the northeast margin of Yinggehai Basin, and is located in the northeast slope of the basin, i.e territorial waters surrounding the southwest continental margin of Hainan Island, which is generally referred to as the area to the north of No fault, to the northwest of No fault and between the downthrown wall of No fault, the central concave field and the central diapir zone It is 1.5 Â 104 km2 in area Outflow of oil-gas is widely distributed in neritic region of Yingdong Slope, and it is developing abnormally in middle-south section of slope zone with a history of over hundred years Since 1957, reconnaissance survey of petroleum and investigation of outflow of oilgas had been done by the former Ministry of Petroleum Industry of China and the fuel department of Guangdong Province at the southwest continental margin of the coastal zone of Hainan Island [21] And 39 outflows of oil-gas have been found and confirmed China Offshore Oil Nanhai West Corporation, together with BP Oil company, resampled this area systematically with modern detection equipments and advanced acquisition technologies in 1990 70 new oil-gas outflows were found So the total amount of the outflow is as many as 115 by the two investigations in 1960's and in 1990's [38] Characteristics of combined seepage system of shallow water at Yingdong Slope (made up of micro-seepage and macro-seepage) and outflow of oil-gas are generally as follows [20e23,37]: migration and accumulation pathways for macro-seepage system of oil-gas are mainly composed of these paths: No faultage and Yingdong faultage which are originated from faultage of the bottom of Cenozoic, and extend to shallow water or seabed; T60 (unconformity of basin Paleogene-Neogene faultage conversion), T40 (unconformity of Middle Miocene), and sand Transforming system of vertical and lateral migration of oil gas composed by major dislocation, region unconformity and sand body helps to dredge oil gas sources lifted by super-pressure hydrocarbon source of the diapir zone at deep concave part of central basin (Fig 3), so macro-seepage of oil-gas is very obvious, and existence of outflow of oil-gas is extremely strong and widely distributed; meanwhile, there are many gas chimneys near the concave diapir zone of central basin; second, outflows of oilgas for this area are mainly concentrated on southwest Fig Two leakage systems and migration and accumulation characteristics of oil and gas in diapir zone and slope, Yinggehai Basin Please cite this article in press as: J He, et al., Seepage system of oil-gas and its exploration in Yinggehai Basin located at northwest of South China Sea, Journal of Natural Gas Geoscience (2017), http://dx.doi.org/10.1016/j.jnggs.2017.01.001 + MODEL J He et al / Journal of Natural Gas Geoscience xx (2017) 1e13 continental margin from Hainan Island-Yinggehai to shallow water of Yacheng, and slightly distributed in the End of Earth to port of Sanya From a regional perspective, outflow intensive area is mainly located in Yingge Chew, Wanglou port, Xigu Island and Nanshanjiao of Pre-Tertiary Basement of middle-south section of Yingdong Slope, and it's partially distributed in other areas The biggest outflow has an area of 800 km2 At the same time, there are many gas chimneys distributed in the south outflow areas near the central diapir (Fig 4) Continuous seepage and activity of outflow of oil-gas have begun 100 years ago; besides, most of the outflow and gas chimneys are still active nowadays, which are found in many shallow waters and seismic profile of engineering and geological surveys, which also include the pockmarks In other words, macro-seepage oil-gas outflows and gas chimneys in this area are developing abnormally and at high frequency, which indicate that the process of accumulation, migration, gathering and dispersing of oil-gas is still going on so far; its root cause is the sufficient hydrocarbon source, which provides pre-conditions so that the amount of generation is larger than amount of dispersaldmaintaining the dynamic balance in the processes of migration, gathering and dispersing, and further leading to huge amounts of outflow of oil-gas and gas chimneys in the area under poor trap conditions of oil-gas; under good trap conditions it will form accumulation and reserves of oil-gas 4.2 Micro-seepage systems of natural gas in central diapirs and characteristics of shallow layer gas reservoirs Shallow layer gas storage and gas bearing structures have been successively found after explorations for almost 20 years in the central diapir of the concave part of Yinggehai Basin [34e36], and its gas reservoirs are in the shallow layer, and are mainly distributed in Marine sandstone reservoirs of Quaternary Yinggehai formation-Pliocene Yinggehai at 350e1300 m, falling into categories of normal temperature and normal pressure gas storage with high output It has been assessed and forecasted according to drilling achievements and geological and geophysical data that the amount of gas storage is on a scale of over a trillion cubic meters According to analysis of shallow layer gas storage and possible source rock geology of geochemical characteristics [21,34,35,39], it is confirmed that the main source of this shallow layer gas storage is typically from the marine shale of the deep part of Miocene and sea bottom part of Pliocene Because the geological age of source rock and reservoir is very young, and accumulation and storage of hydrocarbons is late in time, accumulation efficiency is high while the dissipation loss is low Meanwhile, the geological age of reservoir cap rock is young (Pliocene to Quaternary) and storage is in shallow layer (350e1300 m) so diagenetic consolidation degree is Fig The distribution of oil-gas outflows and gas chimneys in shallow water, Yinggehai Basin Please cite this article in press as: J He, et al., Seepage system of oil-gas and its exploration in Yinggehai Basin located at northwest of South China Sea, Journal of Natural Gas Geoscience (2017), http://dx.doi.org/10.1016/j.jnggs.2017.01.001 + MODEL J He et al / Journal of Natural Gas Geoscience xx (2017) 1e13 poordwhich leads to poor diagenetic consolidation of the shallow layer gas storage, and natural gas is subject to seepage and then disperses This phenomenon is easy to be seen in 2D reflection seismic profile But this type of micro-seepage of shallow layer natural gas is due to poor cap diagenetic nature, or cranny caused by slight rupture of capdand this is one kind of micro-seepage, quite different from macro-seepage of outflow of oil-gas formed for discordogenic faults and unconformities at Yingdong Slope, random blur reflection on upper part of cap rock in seismic profile-blur reflection of characteristics of gas chimneys and micro-crannies (Fig 5) It's obvious that this is the direct sign and reflection of the micro-seepage of natural gas The existence of micro-seepage of natural gas shows that there is a dynamic balanced process of accumulation, storage, gathering and dispersing, but natural gas storage will not exist where the gas source is sufficient, and the amount of gas generation is far larger than loss in this process The central diapir of Yinggehai Basin is a practical example for the dynamic balanced process of accumulation and storage The gas reservoir of the central shallow layer diapir zone is mainly concentrated on associated structural traps and other traps which have a relationship with diapir activities [25,35,36]; gas source is typically produced by mature to overmature hydrocarbon sources of marine gas source rocks of Miocene in deep parts (Fig 3) Gas reservoir in shallow layer is mainly featured by mature to over-mature gas characteristics There are two forms of natural gas [21,34,35,39]: methane-based hydrocarbon; and carbon-dioxide-based nonhydrocarbon gas reservoir The dry coefficient of natural gas is large and there is a little light oil or condensate oil in it Geological and geochemical analyses on natural gas indicate that hydrocarbon comes from mature to over-mature hydrocarbon sources of marine gas source rocks in Miocene [34,35]; non-hydrocarbon such as carbon dioxide is mainly originated from combined result of physical and chemical effects between Miocene calcareous sandstone and mudstone and intrusion activities of hot fluid of diapir [40,41] Accumulation Fig Random blur reflection of gas chimney characteristics on upper part of cap rock in seismic profile in central diaper belt and distribution of hydrocarbon and carbon-dioxide-based non-hydrocarbon both have the enrichment characteristics of partition of sub-blocks and hierarchical sub-band, which lead to complexity in shallow layer gas of the diapir, and so it significantly adds difficulty to the exploration 4.3 Physical and chemistry characteristics and hydrocarbon analysis of shallow well crude oil and condensate oil Super shallow wells: Well Yingchong No and Well Yingchong No were drilled by Dayton drilling on the Yingdong Slope in the 1960's, obtaining 150 L of crude oil in the sand soil layer of Quaternary (depth of the well is about 20 m) which is not in the form of a rock layer After that, Well Sea No was drilled by mat support jack-up rig, obtaining 10.3 L crude oil in Quaternary sand soil layer at 28e49 m These super shallow wells are characterized by highdensity, high-sulfur content, high initial boiling points, high fractions (light fraction) of kerosene-diesel, low-freezing points, and low colloid and asphalt with or without wax, which is typical of marine facies regular High-sulfur low-wax crude; but it is very different from the diapir zone of the basin coal series of terrigenous deposit of Qiongdongnan Basin in neighbor, or humic-type base crude oil and physical and chemical characteristics of condensate oil (Table 1) Sulfur content of crude oil obtained in shallow well at slope of Yingdong is as much as 0.31%e0.65% without separation of wax under À18.5 C, and total distillate quantity is at 70%e 90% (fraction of kerosene-diesel at temp 260e360 C is at large percentage, which is also called light fraction); it has marine facies crude oil characterized by low wax and high sulfur; while sulfur of condensate oil of shallow layer gas storage at Yinggehai Basin is usually about 0.02%e0.09% with maximum 0.11% Meanwhile, its gasoline fraction is high and heavy component is low Although the wax content is low, there is a little wax at percentage range of 0.02%e2.34%, and total fraction is at 70%e98.6% (kerosene distillate is the main part); sulfur content of Qiongdongnan Basin in neighbor is low, usually below 0.09%, and wax content is much higher with maximum percentage being 33.42%, usually above 3% The total fraction quantity is low, usually 29.2%e81.6%, and it is mostly kerosene distillate (Table 1) In view of the characteristics in comparison among crude oil of shallow well at Yingdong Slope, condensate oil storage of shallow layer of central diapir zone and crude oil and condensate oil of Qiongdongnan Basin in neighborhood (Fig and Table 1), it's obvious that crude oil of the super shallow well at Yingdong Slope has the common feature of marine facies oil, which is high sulfur content without wax, and has light distillate of kerosene or diesel, short distillation range, low freezing point and premium quantity So we speculate that hydrocarbon belongs to marine facies parent material, and it is quite different from humic type condensate oil of central diapir zone, coal type crude oil and condensate oil of Qiongdongnan Basin in neighborhood, which are accumulated in terrigenous humic type of hydrocarbon in marine facies, but hydrocarbon source Please cite this article in press as: J He, et al., Seepage system of oil-gas and its exploration in Yinggehai Basin located at northwest of South China Sea, Journal of Natural Gas Geoscience (2017), http://dx.doi.org/10.1016/j.jnggs.2017.01.001 Area Yinggehai Yingdong Basin Slope Well No 26.28 Q 0.9244 10.30