seepage system of oil gas and its exploration in yinggehai basin located at northwest of south china sea

Both the indications of large-scale oil-gas outflow at Yingdong Slope, which have been booming for a hundred years; and the occurrence of pockmarks at the central mud diapir belt, along

Original research paper

Seepage system of oil-gas and its exploration in Yinggehai Basin located at

northwest of South China Sea

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 oil-gas 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

1 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

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 Mediterra-nean [11,12], the Black Sea[13], and the Barbados Ridge in the Atlantic [14] The presence of seepage on the seabed in-dicates 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

* 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.

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

ScienceDirect

Journal of Natural Gas Geoscience xx (2017) 1e13

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2468-256X/Copyright © 2017, Lanzhou Literature and Information Center, Chinese Academy of Sciences AND Langfang Branch of Research Institute of Petroleum Exploration and Development, Petro-China 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/ ).

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

appli-cation 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

abnor-mality 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

out-flows, 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 genera-tion, gathering and dispersal of oil-gas; and are direct in-structions 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

2 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 1 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 chim-neys 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 seismic-profile and shallow layer seismic 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 1 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 1 faultage, Yingdong faultage, multi-regional unconformities and sand bodies are originated here; it further forms a high speed channel for oil gas

migration and accumulation, which leads to abnormal

devel-opment 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]

3 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 1 and No 2 shallow boreholes In addition, some samples in the Yazhou Bay were acquired directly from

Fig 1 The distribution characteristics of major tectonic units and the stratigraphic system in Yinggehai Basin (Modified from Refs [21,23] ).

Fig 2 The seismic profile across Yinggehai Basin (The location of seismic lines seen Fig 1 ).

3

J He et al / Journal of Natural Gas Geoscience xx (2017) 1e13

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

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]

4 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 1 fault, to

the northwest of No 5 fault and between the downthrown wall

of No 1 fault, the central concave field and the central diapir

zone It is 1.5
104km2in 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, reconnais-sance survey of petroleum and investigation of outflow of oil-gas had been done by the former Ministry of Petroleum In-dustry 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 1 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 ver-tical 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 oil-gas for this area are mainly concentrated on southwest

Fig 3 Two leakage systems and migration and accumulation characteristics of oil and gas in diapir zone and slope, Yinggehai Basin.

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

inten-sive 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

genera-tion is larger than amount of dispersaldmaintaining the

dy-namic 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

condi-tions of oil-gas; under good trap condicondi-tions 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 Quater-nary 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 4 The distribution of oil-gas outflows and gas chimneys in shallow water, Yinggehai Basin.

5

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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

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

un-conformities 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

over-mature 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

characteris-tics There are two forms of natural gas [21,34,35,39]:

methane-based hydrocarbon; and carbon-dioxide-based

non-hydrocarbon 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

hydro-carbon 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

be-tween Miocene calcareous sandstone and mudstone and

intrusion activities of hot fluid of diapir[40,41] Accumulation

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 1 and Well Yingchong No 2 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 2 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 high-density, 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 under18.5C, and total distillate quantity is at 70%e 90% (fraction of kerosene-diesel at temp 260e360C 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 char-acteristics 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 6andTable 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 be-longs 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

Fig 5 Random blur reflection of gas chimney characteristics on upper part of

cap rock in seismic profile in central diaper belt.

Table 1

Comparation on physical features between shallow crude oil in the oil and gas seedlings zone of the Yingdong Slope and the condensate oil in the central mud diapir and the crude oil (condensate) in adjacent areas.

position

Density (20C, g/cm3)

Viscidity (50C, mm2/s)

Freezing point/C

Wax content/%

Sulfur content/%

Pectin þ Asphaltine/%

Initial boiling point/C

quality/%

100C 200C 260C 300C 360C Yinggehai

Basin

Yingdong Slope

Yingchong

No 1

Yingchong

No 2

Central Diapir zone

Qiongdongnan

Basin

of super shallow well of Yingdong Slope is mainly

accumu-lated in diapir zone of central concave area of the basin

Gas storage of Lingtou 1-1 is explored at Yingdong Slope

(next to central diapir zone) and a little accompanying

condensed oil (extracted from water) was found in the 1990's,

but physical characteristics can't be analyzed due to its low

amount, and it comes to a result that the saturated hydrocarbon

content is high, the CPI factor is high, maturity parameters of

sterane and terpanes is low, and marks of advanced plant of

terrestrial sources-oleanane is low This indicates that maturity

parameter of the condensate oil is low and is identical with the

accompanying low-maturity natural gas storage (discussed

below), so the parent material of hydrocarbon feeding area

belongs to humic type of parent material in deep water area,

and source of it is probably the same with hydrocarbon of

super shallow well, which is from humic marine facies organic

matter of concave of central basin that is accumulated by

cause of deep-water environment

4.4 Geochemical characteristics and cause analysis of

gas outflow and natural gas

Comparison of geochemical characteristics between

outflow of gas and natural on the Yingdong Slope, natural gas

of Well Lingtou 1-1-1, natural gas of Well Lingtou 35-1-1 to

shallow storage natural gas of central diapir zone of basin is

shown inTable 2 It's obvious that composition of gas outflow

for shallow water of Yingdong Slope is based on CH4, only a

small amount of gas is high in CO2content, and N2, and O2is

at high percentage (air is probably mixing in) The most

prominent feature of the gas here is the high aridity

coeffi-cient, with C1/P

Cn being above 0.98 The aridity coefficient

is low only in shallow well (Well Sea No 1) and natural gas of

Well Lingtou 1-1-1, with C1/P

Cnbeing 0.94e0.95; Methane carbon isotope of outflow of gas is sort of heavier, d13C1is mainly between 38.34‰ and 32.14‰, and it's usually above36.13%, which indicates that maturity of the natural gas is on the high side, and reaches process of mature-high maturity It should be pointed out that there are quite a lot

of differences between the compositions of natural gas of Well Lingtou 1-1-1 and Well Lingtou 35-1-1; the former is on CH4 base, while the latter rich in CO2, and great differences also exist in aridity coefficient, Methane carbon isotope, and light hydrocarbon maturity parameter For the former, its C1/P

Cnis 0.95, its d13C1 is 47.73‰, paraffin index and heptane numbers are very small, so it falls in category of light mature natural gas according to hydrocarbon maturity diagram; while for the latter, its aridity coefficient is large, C1/P

Cnis 0.97, methane carbon isotope is on the heavy side,d13C1is as heavy

as32.14‰, which obviously falls into category of gas at a similar mature-high maturity of gas at diapir zone (Table 2) In general, content of gas and natural gas of shallow water at Yingdong Slope basin is mainly hydrocarbon with a little non-hydrocarbon which is rich in CO2, and there are two types of maturity of gas outflow and natural gas: maturity-high matu-rity degree, which indicates that there are gas storages in different evolutionary processes of this gas area, composed of various types of parent materials which all went through different types of geological conditions for hydrocarbon evo-lution Yingdong Slope is one kind of marginal slope zone; cenozoic sedimentary and organic matter is poor, and the geothermal gradient is low, so the conditions for hydrocarbon accumulation are not available Therefore, gas source for outflow of oil-gas is basically from hydrocarbon source rock in different maturities and hydrocarbon types in the nearby diapir

of deep part of central concave of basin and in the deep inner

Fig 6 The contrast of waxy and sulfur content among crude in the shallow well of Yingdong slope, condensate oil in central diapir and crude in adjacent areas.

Table 2

Comparation on natural gas composition and geochemistry between gas seeps, natural gas in Yingdong Slope and shallow gas in central mud diapir zone.

position

coefficient

for natural gas

Cn d13C 1 /‰ dD/‰

Area of outflow of

gas for Yingdong Slope

Gas outflow of village

of Yinggehai

Gas outflow of Nanshan Mountain foot

part of lower part of No 1 faultage In general, there are no

geological conditions available for accumulation in Yingdong

Slope However, for oil gas accumulation area of Yingdong

Slope, especially the shallow water area, is quite active so far,

and the hydrocarbon source of it is mainly from hydrocarbon

at central deep diapir of concave of basin (Fig 3), and the

geological model of mechanism for distribution and

accumu-lation and storage for oil-gas outflow are shown inFigs 3 and

4 It is obvious to know that Yingdong Slope is on direction of

lateral migration for oil-gas at deep part of basin and is the

main area of accumulation and dispersing for oil-gas As long

as conditions for trap and seal-preservation are available in

this area, it is possible to form accumulation and storage of

oil-gas So, seal condition is key factor for target exploration,

which is also very important for breakthrough and success

ratio for exploration

4.5 Cause and gas source of gas chimneys and

pockmarks for micro-seepage

Gas chimneys and pockmarks of micro-seepage systems are

very common at the central concave diapir of Yinggehai

Basin, and gas chimneys of the nearby Yingdong Slope, which

is next to the diapir, develops well [23e25] These gas

chimneys and pockmarks are usually recognized by many

seismic blur random reflections and pockmarks [42,43] and

are confirmed on seismic profile nowadays Their

character-istics are found in regular 2D seismic-profile or in shallow

seismic profile (sonar profile) of engineering geological survey

with small faultage and a lot of crannies on them, and together

with faultage and crannies, they form the random blur seismic

reflection geologic body in different forms (Fig 7), which are

similar with “chimneys”, and so it's called gas chimney

Accumulation and storage of natural gas is a relatively

balanced process, and dispersing or seepage of natural gas in gas field is perennial, absolute and perpetual, so even when the quality of trap and cap rock is very good, it's not strictly sealed

up There is micro-seepage for natural gas more or less The process of such micro-seepage of natural gas finally results in

a kind of microorganism which lives on hydrocarbon The microorganism and other geochemistry index are used as principles and methods to predict the inner structures, trap oil-gas possibility and oil-gas storage[44] So, essential cause for gas chimneys and pockmarks is that the accumulated natural gas overcomes and stops gas loss during process of accumulation and storage, and a relatively dynamic balance is achieved, or there is micro-seepage which invades into cap rock (soil layer) which leads to abnormal seismic speed and deformed reflec-tion wave Storage for natural gas is result of dynamic balance

of generation, migration, gathering and dispersing for oil-gas There will be dynamic balance for gas storage only when the feeding amount of gas is more than the loss amount in gas dispersing But we have to emphasize that accumulation and storage for natural gas is relative and temporary, but its loss and dispersing are a long-term process, everlasting So, the two process will be going on, especially the dispersing There

is a typical area[45,46]of central diapir of Yinggehai Basin in which oil-gas is active, so there must be a geologic phenom-enon of gas chimneys and pockmarks in shallow gas bearing structures all the way to the deep part of sea, and the gas source is coming from hydrocarbon at the diapir of the basin and the extremely active gas system (Fig 3)

4.6 Relationships between oil-gas seepage system and migration and accumulation of oil-gas

Phenomena of the main oil-gas accumulation basin, oil seepage in Southeast Asia, and distribution of areas with rich oil-gas, have indicated [1,47] that: there is a definite rela-tionship between seepage of oil-gas and outflow of oil-gas and hydrocarbon accumulation area; and this relationship is especially strong in the area of intensive activity in the diapir zone, fault development zone, basin uplift and slope of basin margin It must be pointed out that oil-gas outflow covering directly an oil-gas field is very rare, and reason for such phenomenon is that: the essential cause for the existence of macro-seepage of oil-gas is the destroyed oil-gas storage, or suboptimal trap for oil-gas storage So there is usually no macro-seepage system and huge oil-gas outflow distributed in cap rock directly covering an oil-gas field (there is micro-seepage system existing) But we know for sure that places

of intensive activities, diapir and basin developed from fault-age, where signal of oil and gas seepage and great amount of outflow of oil-gas have told us that there are oil-gas accu-mulations, migration, gathering and dispersing for oil-gas, extremely intensive oil-gas and hydrocarbon at high effi-ciency in the deep part of these areas [5e7] So a certain relationship is found between the outflow of oil-gas, and its accumulation area; at the same time, it indicates that there must be basic conditions of geological petroleum for oil-gas accumulation and storage There should be a bright future

Fig 7 The columnar blur random seismic reflection geologic body of oil-gas

outflow in different morphological in Yinggehai Basin.