Genetic source,migration and accumulation of helium under deep thermal fluid activities:A case study of Ledong diapir area in Yinggehai Basin,South China Sea

Based on the geochemical parameters and analytical data,the heat conservation equation,mass balance law,Rayleigh fractionation model and other methods were used to quantify the in-situ yield and external flux of crust-derived helium,and the initial He concentration and thermal driving mechanism of mantle-derived helium,in the Ledong Diapir area,the Yinggehai Basin,in order to understand the genetic source,migration and accumulation mechanisms of helium under deep thermal fluid activities.The average content of mantle-derived He is only 0.0014%,the ^(3)He/^(4)He value is(0.002–2.190)×10^(−6),and the R/Ra value ranges from 0.01 to 1.52,indicating the contribution of mantle-derived He is 0.09%–19.84%,while the proportion of crust-derived helium can reach over 80%.Quantitative analysis indicates that the crust-derived helium is dominated by external input,followed by in-situ production,in the Ledong diapir area.The crust-derived helium exhibits an in-situ 4 He yield rate of(7.66–7.95)×10^(−13)cm^(3)/(a·g),an in-situ 4 He yield of(4.10–4.25)×10^(−4)cm^(3)/g,and an external 4 He influx of(5.84–9.06)×10^(−2)cm^(3)/g.These results may be related to atmospheric recharge into formation fluid and deep rock-water interactions.The ratio of initial mole volume of 3 He to enthalpy(W)is(0.004–0.018)×10^(−11) cm^(3)/J,and the heat contribution from the deep mantle(X_(M))accounts for 7.63%–36.18%,indicating that deep hot fluid activities drive the migration of mantle-derived 3 He.The primary helium migration depends on advection,while the secondary migration is controlled by hydrothermal degassing and gas-liquid separation.From deep to shallow layers,the CO_(2/3) He value rises from 1.34×10^(9)to 486×10^(9),indicating large amount of CO_(2)has escaped.Under the influence of deep thermal fluid,helium migration and accumulation mechanisms include:deep heat driven diffusion,advection release,vertical hydrothermal degassing,shallow lateral migration,accumulation in traps far from faults,partial pressure balance and sealing capability.

Fluid Charging and Paleo-pressure Evolution in the Ledong Slope Zone of the Yinggehai Basin,South China Sea

Large numbers of gas reservoirs have been discovered in overpressure basins.Fluid charging has a close relationship with paleo-pressure evolution,affecting the migration of gas reservoirs.To study fluid charging and the related pressure system,we analyzed burial histories and fluid inclusion(PVTx)simulations and conducted basin modeling of the Ledong Slope Zone in the Yinggehai Basin as an example.On the basis of fluid-inclusion assemblages(FIAs),homogenization temperature(Th),final melting temperature(Tm,ice)and Raman spectroscopy in fluid inclusions,there are three stages of fluid charging:during the first and second stage,methane-dominated fluid was charged at 2.2–1.7 Ma and 1.7–0.9 Ma,respectively.In the third stage,CO_(2)-rich hydrothermal fluid was charged since 0.9 Ma.It could be concluded from the well-logging data that the disequilibrium compaction in the Yinggehai Fm.,along with the fluid expansion and clay diagenesis in the Huangliu and Meishan formations,resulted in the overpressure in the Ledong slope zone.The evolution of paleo-pressure was affected by the sedimentation rate of the Yinggehai Fm.,as well as the hydrocarbon generation rate.Additionally,the Ledong Slope Zone is less affected by diapir activity than the nearby diapir area.Based on fluid inclusions,paleo-pressure,basin modeling and geological background,the gas migration history of the Ledong Slope Zone can be divided into four stages:in the first stage,excess pressure was formed around 5 Ma;from 2.2 to 1.7 Ma,there was a reduction in the charging of hydrocarbon fluid and steadily increasing excess pressure;during the 1.7–0.9 Ma period a large amount of hydrocarbon was generated,excess pressure increasing significantly and hydraulic fractures forming in mudstones,With gas reservoirs developing in structural highs;since 0.9 Ma,CO_(2)-rich hydrothermal fluid accumulated in reservoirs adjacent to faults and the pressure coefficient remained stable.The research results are helpful in the study of fluid migration and accumulation mechanisms in overpressure basins.

Distribution characteristics of delta reservoirs reshaped by bottom currents:A case study from the second member of the Yinggehai Formation in the DF1-1 gas field,Yinggehai Basin,South China Sea

The Dongfang1-1 gas field(DF1-1)in the Yinggehai Basin is currently the largest offshore self-developed gas field in China and is rich in oil and gas resources.The second member of the Pliocene Yinggehai Formation(YGHF)is the main gas-producing formation and is composed of various sedimentary types;however,a clear understanding of the sedimentary types and development patterns is lacking.Here,typical lithofacies,logging facies and seismic facies types and characteristics of the YGHF are identified based on high-precision 3D seismic data combined with drilling,logging,analysis and testing data.Based on 3D seismic interpretation and attribute analysis,the origin of high-amplitude reflections is clarified,and the main types and evolution characteristics of sedimentary facies are identified.Taking gas formation upper II(IIU)as an example,the plane distribution of the delta front and bottom current channel is determined;finally,a comprehensive sedimentary model of the YGHF second member is established.This second member is a shallowly buried“bright spot”gas reservoir with weak compaction.The velocity of sandstone is slightly lower than that of mudstone,and the reflection has medium amplitude when there is no gas.The velocity of sandstone decreases considerably after gas accumulation,resulting in an increase in the wave impedance difference and high-amplitude(bright spot)reflection between sandstone and mudstone;the range of high amplitudes is consistent with that of gas-bearing traps.The distribution of gas reservoirs is obviously controlled by dome-shaped diapir structural traps,and diapir faults are channels through which natural gas from underlying Miocene source rocks can enter traps.The study area is a delta front deposit developed on a shallow sea shelf.The lithologies of the reservoir are mainly composed of very fine sand and coarse silt,and a variety of sedimentary structural types reflect a shallow sea delta environment;upward thickening funnel type,strong toothed bell type and toothed funnel type logging facies are developed.In total,4 stages of delta front sand bodies(corresponding to progradational reflection seismic facies)derived from the Red River and Blue River in Vietnam have developed in the second member of the YGHF;these sand bodies are dated to 1.5 Ma and correspond to four gas formations.During sedimentation,many bottom current channels(corresponding to channel fill seismic facies)formed,which interacted with the superposed progradational reflections.When the provenance supply was strong in the northwest,the area was dominated by a large set of delta front deposits.In the period of relative sea level rise,surface bottom currents parallel to the coastline were dominant,and undercutting erosion was obvious,forming multistage superimposed erosion troughs.Three large bottom current channels that developed in the late sedimentary period of gas formation IIU are the most typical.

Effects of Overpressured Fluid Flow on Petroleum Accumulation in the Yinggehai Basin

The Yinggehai Basin is a strongly overpressured Cenozoic basin developed in the northern continental shelf of the South China Sea. The flow of overpressured fluids in this basin has given rise to strong effects on petroleum accumulation. (1) The overpressured fluid flow has enhanced the maturation of shallow-buried source rocks, which has caused the source rocks that would have remained immature under the conduction background to be mature for hydrocarbon generation. As a result, the overpressured fluid flow has increased the volume and interval of mature source rocks. (2) The overpressured fluid flow has strong extraction effects on the immature or low-mature source rocks in the shallow parts. This has increased, to some extent, the expulsion efficiency of the source rocks. More importantly, the extraction effects have strongly limited the effectiveness of biomarker parameters from oil and condensate in reflecting the source and maturity of the oil and gas. (3) The flow has caused the sandstones in the shallow parts to get into the late diagenesis stage, and significantly reduced the porosity and permeability of the sandstones. This study confirms that even in sedimentary basins in which no topography-driven groundwater flow systems have ever developed, the cross-formation migration of overpressured fluids and the resultant energy conduction and material exchange can significantly affect the thermal regime, source rock maturation and sandstone diagenesis. As a result, the effects of overpressured fluid flow must be taken into account in analyzing the mechanism of petroleum accumulation.

Characteristics of Overpressure Systems and Their Significance in Hydrocarbon Accumulation in the Yinggehai and Qiongdongnan Basins,China

Overpressure systems are widely developed in the central depression and paleo-uplift in the Yinggehai and Qiongdongnan basins. They can be divided into three types according to the origin of abnormally high formation pressure in the reservoirs, i.e. the autochthonous, vertically-transmitted and laterally-transmitted types. The autochthonous overpressure system results from rapid disequilibrium sediment loading and compaction. In the allochthonous overpressure system, the increase of fluid pressure in sandstone originates from the invasion of overpressured fluid flowing vertically or laterally through the conduit units. The autochthonous overpressure system occurs in the deep-lying strata of Neogene age in the central depression of the Yinggehai and Qiongdongnan basins. The vertically transmitted overpressure system is developed in the shallow strata of Late Miocene and Pliocene ages in the diapiric zone of the central Yinggehai basin, and the laterally transmitted overpressure system occurs in the Oligocene strata of paleo-uplifts, such as the structure of Ya-211 in the Qiongdongnan basin. The results indicate that the autochthonous overpressure system is generally a closed one, which is unfavorable for the migration and accumulation of hydrocarbons. In the allochthonous overpressure system, hydrocarbon accumulation depends on the relationship between the formation of overpressure systems and the spatial location and duration of hydrocarbon migration. The interval overlying the overpressure system is usually a favorable hydrocarbon accumulation zone if the duration of fluid expulsion coincides with that of hydrocarbon accumulation.

Sequence Stratigraphic Modeling of Sedimentary Basin: Conceptual Model and Application to Erlian and Yinggehai Basins, China

A quantitative two-dimensional model of sequence stratigraphy is formulated to simulate the development of sequence architecture in sedimentary basins. The model takes into account sea level change, tectonic subsidence, compaction of sediments, flexural isostatic compensation, erosion and sedimentation. It may be used to test sequence stratigraphic model, to analyze the development of sequence architecture and sequence boundaries, and to predict facies distribution in basins. The computer model, combined with backstripping technique, is cali- brated to model the sedimentary filling of the Early Cretaceous Erlian basin and the Tertiary Yinggehai basin. The study shows that the development of high order sequences in the basins was closely related to the multiple stretching and inversion. The development of the progradational sequence set of the Yinghuang Formation in Yinggehai basin formed in synrift period was mainly controlled by a large amount of sediment input high fluctuation of sea level and gradual subsidence of the basin.

Observation of Catastrophic Degassing from Mantle-Crust in Yinggehai Basin, South China Sea

To lower the CO 2 risk for hydrocarbon exploration in the west continental shelf of Yinggehai basin, South China Sea, we do attempt not only to know the CO 2 origins, but also to make an understanding of the degassing processes from the mantle and crust. Based on the stable carbon isotope ratios of CO 2 alone, the organic and inorganic CO 2 can be successively distinguished, but the formation conditions and mixing processes for inorganic CO 2 are still not clear. The relationships between lg[R(= 3He/ 4He)/R a(=1.386×10 -6)] and CO 2 content (%), CO 2/ 3He and δ 13C CO 2 have been employed, respectively, to obtain that the CO 2 gases in the reservoirs can be classified into three categories: (1) organic CO 2 with very low contents but contaminated by mantle-derived helium; (2) inorganic CO 2 gases with lower to higher contents being mixtures of crustal CO 2 with mantle-derived CO 2, the mantle- contributed percentage being in the range of 0 %-27 %, and (3) mainly crust-derived inorganic CO 2 gases being characterized by high contents (more than 50 %) and indicating the crustal addition by metamorphism of rich-in carbon rocks in basement. Nevertheless, some CO 2/ 3He ratios of organic CO 2 fall into the range 10 8-10 10, which made us inquire whether the CO 2/ 3He=(1-10)×10 9 can be the unique signature of magmatic CO 2 or not. All the observation of plutonic activities, fluid inclusion measurements in gas reservoirs, pre-stack depth/time seismic sections and the satellite infrared remote photography taken from Yinggehai basin, South China Sea, during Chichi earthquake in Taiwan on September 21, 1999, supports that the degassing processes are in a discontinuous mode, which may be triggered by igneous intrusion or extrusion, or earthquakes. In the central diapir zone of the basin, at least 3 to 4 orders of discharge of across-formational thermal fluid flows through fractures can be determined in different scales. The mantle degassing process may have a strong effect on overpressured system forming and outgassing in crust. However, it is very difficult to estimate the transferring rates for a special fractured zone at a specific time interval.

Genesis and Characteristics of Miocene Deep-water Clastic Rocks in Yinggehai and Qiongdongnan Basins,Northern South China Sea

Various deep-water deposits developed in the Yinggehai Basin and Qiongdongnan Basin(Ying-Qiong Basin)in the northern South China Sea,making the two basins significantly hydrocarbon-producing areas and ideal for studying the genetic mechanism and sedimentary characteristics of deep-water clastic rocks.Using cores,image well logging,heavy mineral assemblages,and seismic data,we thoroughly studied the geometry,tectonic background,driving mechanism and source-to-sink process of deep-water deposits in the study area.The results showed that:(1)there were five genetic mechanisms of deep-water clastic rock,i.e.,slides,slumps,debris flows,turbidity currents,and bottom currents.(2)The sliding deposits were distributed from the delta front to the continental slope toe.The slumping deposits were mainly distributed at the continental slope toe or the basin’s central area,far from the delta front The turbidity deposits were widely developed in the deep-water area,but with huge differences in thickness.The bottom currents mainly reworked previous deposits far from the slope.(3)Slip and extension along the preexisting fault zone were the main structural factors that drive the axial channel formation at the slope foot.(4)The sand-rich gravity sediment flows in the Ying-Qiong basin were primarily caused by the direct supply of terrigenous debris into the marine environment over the slope break.

Marine green alga Cladophora aokii Yamada and its epiphytes from Yinggehai, Hainan, China： morphological taxonomy, formation and analysis of its environmental adaption

Based on specimens collected in Yinggehai, Hainan, China from 2013 to 2016, a stable epiphytic taxon is found on the surface of the individual of marine green alga Cladophora aokii Yamada. According to the morphological characteristics, the taxonomy of Cl. aokii and its epiphytes is carried out. There are some epiphytes attached on Cl. aokii Yamada including Cl. fascicularis （Mertens ex C. Agardh） Kfitzing, Chaetomorpha pachynerna （Montagne） Kiitzing, Cerarniurn carnouii Dawson, Licmophora abbreviata Agardh, Lyngbya sp. and Chattonella sp.. The formation of the individual of Cl. aokii is dissected and explained, which can help to analyze the adaption in details among this species, its epiphytes and native marine environment. The results reveal the marine macroepiphytic taxonomy in Ha/nan, China, and preliminarily explain the adaptive relationship between macroalgae and environment.

Development of Diapirs and Accumulation of Natural Gases in Yinggehai Basin

Overpressure developed throughout most of the Yinggehai basin. The burial depth to top overpressure varied from about 1 600 m to 4 500 m, with the shallowest top overpressure occurring in the depocenter. The main cause of the overpressure was disequilibrium compaction resulting from rapid sedimentation of fine grained sediments. The overpressure was strengthened by the retention of fluids including gases due to lack of faults before diapir development. The diapirism in the Yinggehai basin was a combined result of the strong overpressure and the tensile stress field caused by the right lateral slip of the boundary fault. The diapirism, a product of the movement of overpressured fluids and plastic shales, shaped the vertical conduits from source to traps that would be absent without overpressured fluid release. Natural gas accumulation in traps in the diapir structure zones was also intermittent, which can be inferred from the inter reservoir compositional heterogeneity, transient thermal effect of fluid flow and migration fractionation.

Overpressure Identification and Pressure Prediction in Yinggehai Basin

The accurate prediction of overpressure is one of the key issues that restrict the effective development of oil and gas resources in the Yinggehai Basin. In this paper, the formation mechanism of overpressure in Yinggehai Basin is studied. Based on this mechanism, the quantitative prediction model and empirical parameters of overpressure are optimized in Yinggehai Basin and applied in engineering. The results show that the formation mechanism of overpressure in the Yinggehai Basin is complicated, and the causes of overpressure in different blocks of basin are different. The eastern block mainly develops loading-type overpressure, while the Ledong block is dominated by unloading high pressure. Different blocks should employ different abnormal high-pressure prediction models. The East block mainly adopts the Eaton method, and the Ledong block mainly utilizes the Bowers method. The empirical parameters of different models can be determined according to the actual drilling conditions. The practical application demonstrates that the abnormal high-pressure prediction error is within 2%, and it is able to satisfy the requirements of on-site engineering.

Heterogeneity and influencing factors of marine gravity flow tight sandstone under abnormally high pressure:A case study from the Miocene Huangliu Formation reservoirs in LD10 area,Yinggehai Basin,South China Sea

The characteristics of reservoir heterogeneity of the marine gravity flow tight sandstone from the Miocene Huangliu Formation under abnormally high pressure setting at LD10 area in Yinggehai Basin are studied,and the influencing factors on reservoir heterogeneity are discussed,based on modular formation dynamics test,thin sections,XRD analysis of clay minerals,scanning electron microscopy,measurement of pore throat image,porosity and permeability,and high pressure Hg injection,as well as the stimulation of burial thermal history.The aim is to elucidate characteristics of the heterogeneity and the evolution process of heterogeneity of the reservoir,and predict the favorable reservoirs distribution.(1)The heterogeneity of the reservoir is mainly controlled by the cement heterogeneity,pore throat heterogeneity,quality of the reservoir heterogeneity,and the diagenesis under an abnormally high pressure setting.(2)The differences in pore-throat structure caused by diagenetic evolution affected the intergranular material heterogeneity and the pore throat heterogeneity,and finally controlled the heterogeneity of reservoir quality.(3)Compared with the reservoir under normal pressure,abnormally high pressure restrains strength of the compaction and cementation and enhances the dissolution of the reservoir to some extent,and abnormally high pressure thus weakening the heterogeneity of the reservoir to a certain degree.The favorable reservoirs are mainly distributed in the gravity flow sand body under the strong overpressure zone in the middle and lower part of Huangliu Formation.

Episodic Fluid Injections in Overpressured Basins:A Case Study from Yinggehai Basin

The characteristics, origin and injection history of the Dongfang gas field, the largest gas field found in the Yinggehai basin, are studied by integrated geological, geothermal and geochemical data. The gas field shows considerable variation in hydrocarbon gas, nitrogen and carbon dioxide content, aud has been filled by at least four stage fluids. A strong thermal anomaIy caused by hydrothermal fluid flows occurs in the gas field, as evidenced rrom drill-stem test and fluid iuclusion homogenization temperatures, rock-eval tmax,vitrinite reflectance as well as clay-mineral transformation profiles. Such a thermal anomaly suggests focused, rapid flow of deeply-sourced hydrothermal rluids. The inter-reservoir heterogeneities, the strong migotion rractionation related to abrupt changes in pressure and temperature and tbe short-lived, transient nature of the thermal effect of fluid flow are evidences of episodic rluid injections from the overpressured systems into the reservoirs.

Structural Effects of Overpressure Fluid Activities in Yinggehai Basin

The characteristics and distribution of faults in Yinggehai basin discussed in this paper reveal the structural effects of the overpressure fluid expulsion. The rapid subsidence and mud rich intervals of the marine rocks dominate the formation of the overpressure systems and the enormous volumes of the overpressure fluids in the basin. Triggered by some faults, the overpressure fluids were expulsed rapidly from the overpressure compartments to form a series of diapirs in the basin, resulting in the dense fractures or faults and folds in the limbs of diapirs. These fractures and faults provided the migration pathway for the vertical flow of hydrocarbons, so that the gas fields arising from this process might migrate upwards to the sandstone reservoir. Therefore, the hydrocarbon accumulations are usually located in the upper parts of diapiric structures.

Origin and migration model of natural gas in L gas field, eastern slope of Yinggehai Sag, China

Based on the chemical and stable carbon isotopic composition of natural gas and light hydrocarbons, along with regional geological data, the genetic type, origin and migration of natural gases in the L lithologic gas field, the eastern slope of Yinggehai Sag were investigated. The results show that these gases have a considerable variation in chemical composition, with 33.6%–91.5% hydrocarbon, 0.5%-62.2% CO2, and dryness coefficients ranging from 0.94 to 0.99. The alkane gases are characterized by δ13C1 values of -40.71‰--27.40‰,δ13C2 values of –27.27‰– –20.26‰, and the isoparaffin contents accounting for 55%–73% of the total C5–C7 light hydrocarbons. These data indicate that the natural gases belong to the coal-type gas and are mainly derived from the Miocene terrigenous organic-rich source rocks. When the CO2 contents are greater than 10%, the δ13CCO2 values are –9.04‰ to – 0.95‰ and the associated helium has a 3He/4He value of 7.78×10^–8, suggesting that the CO2 here is crustal origin and inorganic and mainly sourced from the thermal decomposition of calcareous mudstone and carbonate in deep strata. The gas migrated in three ways, i.e., migration of gas from the Miocene source rock to the reservoirs nearby;vertical migration of highly mature gas from deeper Meishan and Sanya Formations source rock through concealed faults;and lateral migration along permeable sandbodies. The relatively large pressure difference between the “source” and “reservoir” is the key driving force for the vertical and lateral migration of gas. Short-distance migration and effective “source - reservoir” match control the gas distribution.

Tectonic deformation features and petroleum geological significance in Yinggehai large strike-slip basin, South China Sea

The subtle strike-slip tectonic deformation and its relationship to deposition, overpressure and hydrocarbon migration were studied on the basis of systematic sorting of tectonic data.(1) The local T(tension) fractures derived from sinistral strike-slip process were formed before 10.5 Ma, large in number in the nose structure of the eastern slope, and reactivated episodically under the effect of fluid overpressure in the late stage, they served as dominant vertical hydrocarbon migration paths in the slope area of basin.(2) The dextral strike-slip extension was conducive to the increase of depositional rate and formation of regional under-compacted seal, and induced generation of local T fractures which triggered the development of diapirs; in turn, the development of diapirs made T fractures grow in size further.(3) The sinistral strike-slip process weakened after 10.5 Ma, causing tectonic movement characterized by compression in the north and rotational extension in the south, and the uplift and erosion of strata in Hanoi sag and a surge in clastics supply for south Yinggehai sag. Finally, migrating slope channelized submarine fans and superimposed basin floor fans were developed respectively on the asymmetrical east and west slopes of the Yinggehai sag.

Geological Structure and Dynamics of the Yinggehai Active Rift Basin,South China Sea

The Yinggehai Basin is a unique NNW-trending petroliferous basin in the northwestern South China Sea.This paper mainly utilized stratigraphic,tectonic and seismic data by characterizing the geological structures and conducting the geo-mechanical analysis to study the formation,evolution and dynamics of the Yinggehai Basin.The study indicates that the Ailaoshan-Truong Son extruded terrane is composed of multiple secondary extruded bodies.The Red River fault zone,located within the Qiangtang-Simao-Yinggehai mantle flow channel and basin zone,experienced transform-type sinistral strike-slip motion before the basin forming stage and formed a NW-trending extruded mantle uplift,which activated the Yinggehai basin.After experiencing the rift depression,fault depression,and fault subsidence,the basin eventually formed large-scale,thick sedimentation features with ideal hydrocarbon-forming conditions at the end of the Miocene.Later,the basin dynamically transformed and entered a period of tectonic superposition,reworking,and thermal subsidence.Superposition of the NNW thrust sinistral strike-slip fault zone on the northern Hanoi sub-basin complicated the basin structure.Since the Pliocene,the southern Yinggehai main basin has been transformed into an extensional dextral strike-slip environment that hosted numerous mud diapirs.The thin crust and high geothermal gradient provide favorable conditions for the large-scale accumulation of natural gas.

超压盆地水力破裂与天然气成藏的关系——以莺歌海盆地乐东斜坡区LD-A气田为例

莺歌海盆地乐东斜坡区LD-A气田的发现进一步证实了高温高压岩性圈闭领域的巨大资源潜力。该气田储层压力系数最高可达2.3,流体超压诱发的水力破裂对天然气的运聚起到重要的控制作用,明确超压条件下地层是否发生水力破裂及其演化过程是深入研究莺歌海盆地天然气成藏的关键。为此,在明确天然气垂向输导体系的基础上,以岩石破裂准则为理论依据,利用地应力及岩石抗张强度确定水力破裂模式,厘定了地层水力破裂条件;最终结合超压演化,分析了水力破裂动态演化过程对天然气成藏过程的控制作用。研究结果表明:(1)断裂、亚地震断裂和裂缝共同构成了LD-A气田天然气垂向输导体系,超压诱发的水力破裂控制了输导体系的启闭性和有效性,决定了天然气能否规模富集。(2)LD-A气田的中深层发育张性水力破裂,其中LD-A1气区现今黄流组顶部处于水力封闭状态,梅山组顶部处于水力渗漏状态;LD-A2气区和LD-A3气区上中新统黄流组顶部和中中新统梅山组顶部现今均处于水力封闭状态。(3)LD-A气田的水力破裂演化存在封闭—破裂—封闭型(LD-A1气区与LD-A2气区)和持续封闭型(LD-A3气区)2种类型,不同的水力破裂演化过程导致了天然气在不同层位砂体差异性充注和富集。结论认为,LD-A1气区与LD-A2气区浅层上新统莺歌海组二段和上中新统黄流组优质砂体是天然气有利富集对象,LD-A3气区应在埋藏更深的梅山组砂岩中寻找突破。

致密砂岩储层孔隙结构分形与分级评价——以莺歌海盆地乐东区黄流组为例

[目的]明确致密砂岩储层孔喉微观形态,划分储层孔隙结构级次,多角度半定量评价储层的储集特征和渗流能力,有助于明确具有高效产能潜力的储层类型。[方法]以莺歌海盆乐东区黄流组致密砂岩为例,利用铸体薄片、扫描电镜、高压压汞等实验手段,开展储层孔隙结构分形维数计算与分级评价的研究。[结果]致密砂岩储层质量的影响因素之一是大孔喉孔隙结构;分形维数越接近3,孔隙结构越复杂,连通的大孔喉越少。依据孔喉分级特征、成岩相、孔隙结构复杂程度,建立了孔隙结构分级评价的四类模型:类型Ⅰ为小孔喉优势型储层,强胶结成岩相,大孔喉孔隙度占比小于45%,D_(max)值为2.75~2.90,致密储层;类型Ⅱ为大—小孔喉连续型储层,弱胶结—弱溶蚀成岩相,大孔喉孔隙度占比45%~70%,D_(max)值为2.70~2.85,低渗储层;类型Ⅲ为大孔喉优势型储层,强溶蚀成岩相,大孔喉孔隙度占比大于70%,D_(max)值为2.55~2.65,优质储层;类型Ⅳ为大孔喉单峰型储层,强压实成岩相,大孔喉孔隙度占比50%~60%,D_(max)值为2.65~2.75,低渗储层。通过Fisher判别方法,建立储层类型测井预测模型,对比储层类型与气测响应和储层含气饱和度,发现Ⅲ类储层的产气效能高,具有勘探价值。[结论]研究成果为致密砂岩储层评价与分类提供了新思路,明确了储层孔隙结构对气水分布存在控制作用。

深部热流体活动背景下氦气的成因来源与运聚机制——以莺歌海盆地乐东底辟区为例

基于地球化学参数及分析数据,应用热守恒方程、质量平衡定律及瑞利分馏模型等方法,通过对莺歌海盆地乐东底辟区的壳源氦气原位产率和外部通量、幔源初始氦浓度及热驱动机制等定量分析,探讨深部热流体活动背景下氦气的成因来源及运聚机制。研究结果表明:(1)乐东底辟区氦气的来源以壳源氦为主,幔源为辅。其中,研究区幔源^(3)He/^(4)He值为(0.002~2.190)×10^(-6),R/R_(a)值为0.01~1.52,测算幔源He贡献占比为0.09%~19.84%,占比较小;而壳源贡献占比则高达80%以上。(2)壳源氦的原位^(4)He产量仅为(4.10~4.25)×10^(-4)cm^(3)/g,外部^(4)He通量则显著高值,为(5.84~9.06)×10^(-2)cm^(3)/g,表明壳源氦气以外部输入为主,推测与地层流体受大气补给以及深部岩石-水相互作用有关。(3)底辟区深部热流体活动显著影响地温场,^(3)He初始质量体积与对应焓的比值(W)为(0.004~0.018)×10^(-11) cm^(3)/J,来自深部地幔的热贡献(XM)为7.63%~36.18%,揭示底辟热流体对幔源^(3)He迁移具有一定的热驱动作用。(4)研究区氦气的初次运移方式以平流为主,二次运移受控于水热脱气和气液分离过程;氦气从深部至浅部的运移过程中,CO_(2)/^(3)He值由1.34×10^(9)升至486×10^(9),指示受到壳幔混合和脱气效应影响,存在CO_(2)规模性析出和^(3)He的明显逸散。在深部热流体影响下,氦气运移聚集机制包括:深部热驱动扩散、平流释放、垂向水热脱气,浅部横向迁移、远离断裂的圈闭聚集,分压平衡、完整密闭的盖层封存等方面。