Activity Of Carbon Dioxide And Reformation Of Oil Reservoir In The Baiyun – Liwan Deep Water Area,Pearl River Mouth Basin,Northern South China Sea

The Baiyun-Liwan deep water area is located in the ocean-land conversion zone of the northern continental margin of the South China Sea.It has the characteristics of fluid in high temperature background,three sets of source rocks,and a large amount of inorganic carbon dioxide(CO₂)transported and accumulated into the reservoir due to multi-stage magmatic activities.The special lithosphere structure and its deformation process determine that its hydrocarbon accumulation law has its particularity.This article combing the multiperiod activity by igneous rock and geochemical analysis of source rock geochemical tracer system analysis,on the basis of its geological background,and summarizes the source of thermal fluid and time of fluid inclusions,and focus on the gas washing and high temperature inorganic CO₂ reformation of reservoir.Combining with the regional tectonic movement,this article summarizes the main controlling factors of hydrocarbon accumulation and establish the hydrocarbon accumulation model.The main research results are as follows:(1)A large number of oil inclusions were detected in the Baiyun-Liwan deep water area,and the fluorescence colors ranged from yellow,yellow-green,green to blue-green.The main peak wavelength of the fluorescence spectrum was mainly distributed between490 and 545 nm.The lowest maturity oil inclusions are distributed on the uplift far from the depression,and the higher maturity oil inclusions are distributed near the depression.Abundant natural gas inclusions were detected,generally with a large volume(5?10um),which are mainly found in quartz particle cracks,quartz secondary augments and calcite cements.Systematic analysis of burial history and hydrocarbon fluid inclusions in the study area shows that there are three periods of oil filling and two periods of natural gas filling.The first period of oil filling is 20.5?14Ma,the second period is 14?6.5Ma,and the third period is 5?0m A.The two periods of natural gas filling correspond to the second and third period of oil filling.(2)Through the isotopes data of noble gas,confirmed that the origin of inorganic CO₂ is magmatic.Base on homogenization temperature of inclusion and burial history and isotope of 40Ar/36Ar,the age of mantle-derived CO₂ is between 18.9?0Ma.There are three accumulation periods:the first period is 18.9?10.5Ma,the second period is10.5?5.2Ma,and the third period is 5.2?0Ma.(3)Analysis of fluid inclusion characteristics,asphalt characteristics,and normal alkane distribution characteristics can determine whether gas washing has occurred in oil reservoirs,and calculation of normal alkane distribution characteristics and loss amount can quantitatively determine the size of gas washing in oil reservoirs.(4)The influence of mantle source inorganic CO₂ filling the reservoir is mainly in the following four aspects:(a)the high temperature reverses the order degree of clay minerals;(b)with the increase of temperature,the dissolution of sodium feldspar,calcite and iron calcite was obvious,and the dissolution intensity increased with the increase of temperature,accompanied by the appearance of dissolution pores in feldspar grains,dissolution pores between grains and the formation of autogenic kaolinite;(c)resulting in the formation of skeleton grain feldspar minerals and late carbonate cements.(d)under the condition of high CO₂ partial pressure,dawsonite was formed under the condition of alkaline-weak acid fluid at 25?100?.These four characteristics can be used to trace the migration path of mantle source inorganic CO₂.(5)There are mainly four types of oil and gas accumulation in the study area:(a)without the mantle-derived CO₂,oil injection in the early stage,after that gas washing to form condensate gas reservoirs,such as W1 well;(b)mantle-derived CO₂ is charged at the same time as oil and hydrocarbon gas,but the CO₂ volume is small,which fails to displace a large amount of oil and gas.Therefore,condensate gas reservoirs with low CO₂content are formed,such as gas reservoirs in PY34-1 wells;(c)mantle-derived CO₂ is charged at the same time as oil and hydrocarbon gas,and large volume of CO₂ displace a lot of oil and gas,so high CO₂ gas reservoirs with low hydrocarbon gas content are formed,such as gas reservoirs in W14 well;(d)there is no oil and gas filling,but there is mantle-derived CO₂ lateral migration into the trap,so high CO₂ content gas reservoirs formed.(6)The oil accumulation model of Baiyun-Liwan deep water area can be summarized as the accumulation model of"transmission system+fluid potential control".The NWW trending faults and the NWW-NE trending faults control the migration and accumulation of oil and gas.Near the hydrocarbon sag,there is a high potential area for oil and gas.The mature source rocks in the deep continue to supply hydrocarbon to the dominant migration area,making it easier for oil and gas to be filled.The faults which relating to the mantle-derived CO₂ controls the vertical migration and accumulation of inorganic CO₂.Whether and to what extent mantle-derived CO₂ is filled in the oil and gas reservoirs depends on the balance between CO₂ momentum and oil momentum.(7)On the basis of combination of drilling in CO₂ and Cenozoic igneous rock distribution and fracture system,the favorable exploration area in this region is a series of large structural trap group on southwest fault terrace zone,deep water fan in the center of the Baiyun Sag,a series of composite trap on the uplift structure in Baiyun sag.A series of trap in south side of Liwan Sag in deepwater areas have high CO₂risks.In this article,oil and gas migration and accumulation rules deeply analyzed in order to avoid high CO₂ risk and explore oil and gas reservoirs in the Baiyun-Liwan deepwater area,Pearl River Mouth Basin,northern South China Sea.