Origin of a giant fuzzy reflection zone and its implication for natural gas exploration in the southwestern Qiongdongnan Basin of the South China Sea

The southwestern depression of the Qiongdongnan Basin(QDNB)hosts thick Cenozoic sediments and awaits major hydrocarbon discovery.Multichannel seismic(MCS)profile CFT2011 across the southwestern QDNB reveals a~60-km-wide fuzzy reflection zone(FRZ)within the sediments,but its origin and distribution remain unclear.Here ocean bottom seismometer(OBS)data of Line CFT2011 are processed with focus on the velocity structures by traveltime tomography inversion and analyzed together with the coincident and adjacent MCS profiles.The OBS velocity results show that the giant FRZ features lower velocity with difference up to 1.5 km/s and smaller vertical velocity gradient than the surrounding sedimentary sequences at the same depth,likely resulting from enhanced fluid infilling.The MCS profile exhibits that the giant FRZ is about 3-9-km thick and extends from the Paleogene strata rich in organic matters upward to the lower Pleistocene sediments.Within the shallow overlying sediments,multiple bright spots with reverse polarity are imaged and their reflection amplitudes increase with offset,consistent with the features of gas-charged sediments.They are probably shallow gas reservoirs with gases sourced from the deep FRZ.Therefore,the FRZ is proposed to be a giant gas-charged zone,which probably contains lots of hydrocarbon gases migrated vertically from the deep Paleogene source rocks through the boundary faults of the depressions and the minor fractures generated under overpressure.This FRZ is also imaged on the adjacent MCS profiles MCS-L1 and MCS-L2 with the width of about 40 km and 68 km,respectively.It is roughly estimated to cover an area of~1900 km2 and host a volume of~11400 km3 assuming an average thickness of 6 km,implying huge natural gas potential in the sedimentary depression of the southwestern QDNB of the South China Sea.

A three dimensional visualized physical simulation for natural gas charging in the micro-nano pore system

A micro-nano pore three-dimensional visualized real-time physical simulation of natural gas charging, in-situ pore-scale computation, pore network modelling, and apparent permeability evaluation theory were used to investigate laws of gas and water flow and their distribution, and controlling factors during the gas charging process in low-permeability(tight) sandstone reservoir. By describing features of gas-water flow and distribution and their variations in the micro-nano pore system, it is found that the gas charging in the low permeability(tight) sandstone can be divided into two stages, expansion stage and stable stage. In the expansion stage, the gas flows continuously first into large-sized pores then small-sized pores, and first into centers of the pores then edges of pores;pore-throats greater than 20 μm in radius make up the major pathway for gas charging. With the increase of charging pressure, movable water in the edges of large-sized pores and in the centers of small pores is displaced out successively. Pore-throats of 20-50 μm in radius and pore-throats less than 20 μm in radius dominate the expansion of gas charging channels at different stages of charging in turn, leading to reductions in pore-throat radius, throat length and coordination number of the pathway, which is the main increase stage of gas permeability and gas saturation. Among which, pore-throats 30-50 μm in radius control the increase pattern of gas saturation. In the stable stage, gas charging pathways have expanded to the maximum, so the pathways keep stable in pore-throat radius, throat length, and coordination number, and irreducible water remains in the pore system, the gas phase is in concentrated clusters, while the water phase is in the form of dispersed thin film, and the gas saturation and gas permeability tend stable. Connected pore-throats less than 20 μm in radius control the expansion limit of the charging pathways, the formation of stable gas-water distribution, and the maximum gas saturation. The heterogeneity of connected pore-throats affects the dynamic variations of gas phase charging and gas-water distribution. It can be concluded that the pore-throat configuration and heterogeneity of the micro-nanometer pore system control the dynamic variations of the low-permeability(tight) sandstone gas charging process and gas-water distribution features.

Overpressure and gas charging in tight sandstone:Xujiahe Formation,northeastern Sichuan Basin

Overpressure is a key factor for oil and gas charging in tight reservoirs,but it is still a challenge to evaluate the overpressure evolution and its control on oil and gas charging.Taking Xujiahe Formation in the northeastern Sichuan Basin as an example,this paper presented a method for evaluating overpressure and its effect on natural gas charging in tight sandstone in compressional basin.The abnormally high pressure and its causes were analyzed by measured data and logging evaluation.Theoretical calculation and PVT simulation were used to investigate the amounts of overpressure resulted from hydrocarbon generation and tecto nic compression,respectively.Then the source rock-reservoir pressu re differences were calculated and the characteristics of natural gas charging during the natural gas charging periods were analyzed.It was revealed that hydrocarbon generation and tectonic compression were the main causes of the overpressure.The overpressure of both source rocks and reservoir exhibited a gradually increasing trend from Middle Jurassic to Early Cretaceous(J2-K1),then decreased since Later Cretaceous(K2),and some of that preserved to now.The contributions of the hydrocarbon generation and tectonic compression to overpressure were different in different periods.The residual pressure difference between the source rocks and the reservoir is the major driving force for tight sandstone gas charging.The main hydrocarbon generating area of the source rocks and the area of high driving force were major natural gas enrichment areas,and the driving force determined the natural gas charging space in the pore throat system of the reservoir.This research helps evaluate the overpressure and pressure difference between source rocks and reservoir in compressed basin,as well as investigate the effective pore throat space of tight gas charging by the driver of overpressure.

Forming mechanism of hydrocarbon reservoirs in Yingshan Formation of Yuqi block in Akekule arch,Tarim Basin

The reservoir conditions,oil and gas charge history and accumulation phases were studied for Yingshan Formation of Yuqi block,and an oil and gas accumulation model was established by using the techniques of reservoir prediction,fluorescence thin section and fluid inclusion analysis under the guidance of the theories of oil and gas accumulation.The results indicate that the main rock types in Yingshan Formation are micrite and calcarenite.The carbonate reservoirs are of cave,fracture-pore and fracture types,and their physical properties are intermediate;there are at least four oil/gas charges,i.e.late Hercynian,Yanshanian,early Himalayan and middle Himalayan(Cenozoic).The most important charge periods are late Hercynian,early Himalayan and middle Himalayan;the oil and gas accumulation model is self source-lateral expulsion of hydrocarbon-multistage accumulation,or hydrocarbon sourced from and preserved in the same old rocks-long term expulsion of hydrocarbon-multistage accumulation.

Fracture development and hydrocarbon accumulation in tight sandstone reservoirs of the Paleogene Huagang Formation in the central reversal tectonic belt of the Xihu Sag, East China Sea

By using thin section identification, cathodoluminescence, major and trace elements and fluid inclusion tests and authigenic illite dating, based on observation of core cracks, combined with the microscopic characteristics and imaging logging characteristics of fractures, the stages of the fractures in the Huagang Formation of the central reversal tectonic belt of the Xihu Sag in the East China Sea, and the matching relationship between the fracture development stages and the oil and gas charging stages are clarified. There are diagenetic fractures and tectonic fractures in the reservoirs of the Huagang Formation in the study area. The diagenetic fractures developed during the diagenetic stage of the reservoirs and have less effect on oil and gas migration and transport. The tectonic fractures are divided into three stages based on tectonic movements controlling the fractures and their relationships with hydrocarbon charging: The first stage of fractures was generated in the early stage of the Himalayan Movement–Longjing Movement(12–13 Ma ago), when the tectonic stress caused the sutures and shale strips to twist, deform, and break. Tectonic microfractures generated in this period had short extension, narrow width, and poor effectiveness, and had little effect on oil and gas migration and transport. The second stage of fractures came up during the middle-late period of Himalayan Movement–Longjing Movement(9–12 Ma ago), when tectonic movements caused the development of tectonic fractures in the central reversal tectonic belt, these fractures are of large scale, long extension, and good effectiveness, and matched with the first stage of large scale oil and gas charging(9–12 Ma ago), so they play an important role in oil and gas migration, transportation, and accumulation. The third stage of fractures were created from Himalayan Movement–Okinawa Trough movement to the present day(0–3 Ma ago), the fractures are tectonic ones developing successively;matching with the second stage(0–3 Ma ago) of large-scale oil and gas charging, they created conditions for continuous natural gas migration and transportation. All these prove that the development of reservoir fractures in the Huagang Formation of Xihu Sag can provide seepage space and continuous and effective channels for efficient migration and accumulation of oil and gas.

Hydrocarbon accumulation conditions and key exploration and development technologies for PL 19-3 oilfield

The PL 19e3 Oilfield is the only super-large monolithic oilfield with oil and gas reserves up to 1×10^(9) t in the Bohai Bay Basin,and it has been successfully developed.Exploration and development practices have provided abundant data for analyzing formation conditions of this super-large oilfield.On the basis of the exploration and development history,fundamental reservoir features,and with available geological,geophysical and test data,the hydrocarbon accumulation conditions and key exploration&development technologies of the PL 19e3 Oilfield were discussed.The key conditions for forming the super-large Neogene oilfield include four aspects.Firstly,the oilfield is located at the high position of the uplift that contacts the brachy-axis of the multi-ridge slope in the biggest hydrocarbon-rich sag in the Bohai Bay Basin,thus it has sufficient hydrocarbon source and extremely superior hydrocarbon migration condition.Secondly,the large-scale torsional anticlines which formed in the Neogene under the control of the Tanlu strike-slipping movement provide sufficient storage spaces for oil and gas preservation.Thirdly,the“multiple sets of composite reservoir-caprock assemblages”developing in the special shallow-water delta further contributes greatly to the effective storage space for oil and gas preservation.Fourthly,due to the coupling of the uplift and strike slip in the neotectonic period,extensive faulting activities constantly released the pressure while the late period massive hydrocarbon expulsion of the Bozhong took place at the same time,which assures the constant and intense charging of oil and gas.The super-large PL 19e3 Oilfield was controlled by the coupling effects of all those special geologic factors.In view of this oilfield's features(e.g.violently reformation caused by strike slip,and the special sedimentary environment of shallow-water delta),some key practical technologies for exploration and development have been developed.Such technologies include:the special prestack depth migration processing for gas cloud zones,the prediction of thin interbed reservoirs based on high-precision inversion of geologic model,the reservoir description for the shallow-water braided river delta,the quantitative description for remaining oil in the commingled oil reservoirs with wide well spacing and long well interval,and the well pattern adjustment for formations during high water cut period in the complex fluvial-facies oilfields.