Research On The Development And Evolution Of Mud Diapir/Mud Volcano And Their Relationship With Migration And Accumulation Of Petroleum And Natrural Gas-hydrate In Major Basins,northern South China Sea

The development and evolution process of mud diapir /mud volcano and their associated structures have close relationship with the migration and accumulation of oil /gas and other fluid minerals. Mud diapir /mud volcano are not only the windows that reveal the tectonic movement and the fluid activity of the deep earth, but also the important symbol and effective information that indicate the petroleum and other minerals. Moreover, they also control and restrict the distribution and accumulation regularities of fluid and solid minerals in a basin. This study analyzed the development and evolution characteristics of mud diapir /mud volcano and their relationship with the migration and accumulation of petroleum and natural gas hydrate in the main marginal basins of northern South China Sea(the Yinggehai Basin-YGHB,the deep water area of Qiongdongnan Basin-QDNB,deep water area of southern Pearl River Mouth Basin-PRMB,and deep water area of the depression in Taixinan Bsin-TXNB).The highlights of this paper are shown as follows:(1)Based on a large number of geology,seismic and drilling data,the development and evolution features of mud diapirs and mud volcanoes in the main depressions and basins of northern SCS were recognized and confirmed, and the space characteristics and distribution of them were also demonstrated. We found that mud diapirs and mud volcanoes concentrating distributed in four main districts, they are the central mud diapir zone of YGHB, deep water area of southern QDNB, deep water area of southern PRMB(Baiyun sag and Dongsha area),deep water area of the depressions in the southern TXNB(offshore and onshore).The diapirism and hot fluid intruding activities are vigorous in the fore three areas, and the deformed reflection of mud diapirs and gas chimneys are common, but mud volcanoes are rare; However, mud diapirs and mud volcanoes are common and extremely developed in the last area. As a whole, the mud diapir/mud volcano developed in the vicinity of depression centers,around structure transition zones, fault or microcrack developing zones,and the structure week belts.(2)The evolution characteristics and controlling factors of mud diapir and mud volcanoes were analyzed in detail based on the physical, geological and geochemical features of different mud diapir and mud volcanoes in the main basins of northern SCS. The mud diapirs /mud volcanoes and gas chimneys in the main basins of northern SCS were characterized by abnormal reflection with obvious low velocity abnormal on the seismic sections and velocity spectrum, the break off reflection, discontinuous, chaotic, weak and blank reflection, and withdraw speed. The mud diapirs and mud volcanoes formed duo to the flow of plastic mud and shale and fluids, and the vigorous upward piercing and upwarp intruding of mud material with fluids, which resulting the deforming of strata and clear abnormal reflection on seismic profiles. The gas chimney is attributed to the gas charged into sediments, in spite of distinguished gas charging extent, the gas chimney would not change the attitude of strata, and is characterized by fuzzy or blank reflection. The evolution, distribution, influence and controlling factors of mud diapir and mud volcanoes were depended on internal cause and external cause, and none of them can be dispensed with. The plastic flow of rapidly deposited huge undercompacted mud and shale in special region and upward intruding of mud material with hot fluids are the internal causes. The basic formation geological conditions and elements including the structure transform zone, the fault and microcrack active zone and the weak formation are the external causes.(3)Mud diapirs and mud volcanoes have the same genetic mechanism,the similar development and evolution characteristics,and the same controlling factors. The basic formation conditions and elements can be summarized as follows: Rapidly deposited high-plasticity and undercompacted huge lighter density shales, sedimentary dynamics with inverted density and discrepancy gravity. Owing to those inducements, the plastically deforming of mud and shales will strongly intrude and pierce the upper sediments, which resulting in the upheavals and folds of upper surrounding rocks or piercing the upper fractured weak zones. Since the late Neogene, the main marginal basins in northern SCS have developed under rapid subsidence-sediment geo-background, and have deposited huge undercompacted abnormal high pressure shales(source mud material of diaprism)in marine depression period, a large number of mud diapirs and mud volcanoes developed in some weak sedimentary. At the same time, the mud diapirs and mud volcanoes zones are characterized by high heat flow and high geothermal gradient, and the mud source strata accumulated high temperature and high pressure potency because of hydrocarbon generating of organic matter under thermal evolution and the dehydration of clay mineral, which not only facilitated the vertically migrating of large mount of petroleum and other fluids, but also controlled and influenced the tectonic evolution,developing scale and distribution characteristics of mud diapirs, mud volcanoes and associated structures.(4)According to the analysis results of the geochemical data acquired from the exploring activities in the diapirism zones of the main basins of northern SCS,and the geochemical analysis results of mud volcano associated gas, hot spring gas, underground fire gas samples collected in the onshore mud volcanoes of Taiwan, the mud diapir/mud volcano associated gas are dominated by hydrocarbons,with small N2,CO2 and other nonhydrocarbons, while partial regions and segments of strata are rich in CO2.The analysis results of C-isotopes of natural gas and He/Ar isotopes of noble gas had demonstrated that,the hydrocarbon in mud diapirs and mud volcanoes were mature-high mature thermal gas, as well as a small amount of biogenic natural gas, the genetic type of which were the mixed gas of mature-high mature trimellitic-humic type gas, and the hydrocarbon were derived from huge marine depression deposited shales.What’s more, the CO2 associated with mud diapirs/volcanoes were main crust-derived, while some of them were crust –mantle mixed,which were also come from deposited huge marine depression shales(source mud sediments) that controlled by the affection of high temperature and high heat flow of physical and chemical activities.(5)The petroleum geological significance of development and evolution of mud diapirs and mud volcanoes can be summarized as follows: Firstly, mud diapir and mud volcanoes are vary idiomatical seismic-geological abnormal physical in southern SCS, which are characterized by low density, low velocity, and high temperature and high pressure. The process of development and evolution as well as distribution law of mud diapirs and mud volcanoes have intimate time and space coupling relationship with the oil and gas migration and accumulation. Secondly, huge plastic source mud material(undercompacted overpressure shale)are not only the basic material that formed the mud diapir and mud volcano, but also the source rock with great hydrocarbon generation potential that act as favorable mud diapir source kitchens. Thirdly, the associated mud diapir tectonic traps and its uplift tectonic zones that resulted by the up intruding activities of mud material are in the location or near the place of mud diapir source kitchens, and they are the optimum accumulation place and advantageous enrichment region of hydrocarbons. Fourthly, upwarp folding deformation of upper sedimentary caused by upward intruding mud material not only formed a series of associated structures,but also provided good geological setting for developing sand reservoirs and improving the physical properties of reservoirs. Fifthly, the potency of rich fluid with high temperature and high pressure developed in mud material not only facilitate the velocity of hydrocarbon generating of source rocks, but also offer powerful driving force to hydrocarbons that migrate lengthways from deep strata to shallow layers, as a consequence, oil and gas and other fluids migrate continuously from deep source rocks to shallow zones with favorable traps and reservoir-cap assemblages and formed petroleum reservoirs. Last but not least, many superduper hydrocarbon lengthways migrating pathways(mud diapirism associated faults, microcracks and mud intruding conduits)developed during the developing and evolution of mud diapirs and mud volcanoes, which established the “bridge and pathways” that connect deep source kitchens with shallow structure traps and non-structures, that is to say, those conduction system provided deep hydrocarbon large-scale migration and accumulation with advantage conditions and shortcut.(6)Combing with the geological and seismic analysis and interpretation,and based on the petroleum geological conditions,oil and gas exploration and natural hydrate gas prospection achievements of the main basins in northern SCS, we analyzed the distribution characteristics,gas source supply pattern and the main controlling factors of natural gas hydrate in the deep water region of QDNB, Baiyun sag in PRMB, and deep water zone in TXNB.According to the basic principles(source-converge-accumulate) of the theory of petroleum system-“from source rocks to reservoir ”,we firstly researched the genetic type and the hydrocarbon conduction system as well as the supply pattern of natural gas hydrate that acquired by exploration so far, then we depicted and dissected the type of conduction system and its distribution characteristics of natural hydrate gas based on the petroleum geological conditions and the analysis results of seismic-geological interpretation in different basins, furthermore, we summarized and established the major migration and accumulation modes of each basin and demonstrated and illuminated the distribution and its main controlling factors of natural gas hydrate. Although only the “biogenic gas accumulate in place by way of self-generation and self-bearing ”migration and accumulation mode of natural gas hydrate had been discovered in the process of exploration at present, according to the development and evolution of mud diapirs and mud volcanoes,the upward intruding activity characteristics,the growing distribution features and the display information of diapirism in deep water region of northern SCS, we recognized and confirmed that there were two more migration and accumulation modes of gas hydrate, they were “thermal gas accumulate offsite through faults and fractures by way of lower-generation and upper storage”, “thermal gas accumulate offsite through mud diapir and gas chimney by way of lower-generation and upper storage”. In line with the convention reservoirs, the major controlling factors of gas hydrate depended on the preferable time and space couping configuration between enough hydrocarbon supply and certain scale of stable zones with high pressure and low temperature(the accumulation place).(7)Resource potential and exploration prospect of gas hydrate associated with mud diapirs and mud volcanoes in basins of northern SCS were discussed and demonstrated based on the development and evolution characteristics and associated petroleum accumulation regulation. On account of the decades of petroleum exploration and gas hydrate prospection, we confirmed and selected following important fields of oil/gas and gas hydrate as the preferential directions that may get big breakthroughs and exploit new fields:(1)the large mid-deep strata diapirism associated structures in the central mud diapir zone in YGHB,(2)the deep water region in QDNB,especially the large mud diapirism and gas chimney developing area under the central channel which is the favorable prospecting target region to discover the big deep water gas pool and “seepage” gas hydrate with high saturation,(3)The suspected mud diapir and gas chimney zones of deep water region of Baiyun sag in PRMB are the advantageous exploration targets to prospect superposition enrichment fields with deep water petroleum and shallow water gas hydrate,(4)The deep water area of Dongsha in PRMB had get a big gas hydrate breakthrough, and the local regions associated with mud diapirism is the strategy precinct for further exploration,(5)the large mud diapirs and mud volcanoes developing area offshore southern TXNB is the favorable target to prospect deep water petroleum and natural gas hydrate, and the mud volcano associated gas onshore TXNB is also worthy of attention for its exploration and development value.