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Sequence Stratigraphy And Hydrocarbon Reservoirs Of The Upper Triassic Yanchang Formation In The Fuxian Prospect Area Of The Ordos Basin

Posted on:2003-08-16Degree:DoctorType:Dissertation
Country:ChinaCandidate:Y XieFull Text:PDF
GTID:1100360062986596Subject:Paleontology and stratigraphy
Abstract/Summary:PDF Full Text Request
Much of the oil and gas resources reside in lake basins in the world including China. The approaches of the geology, especially sequence stratigraphy of lake basins are interpreted to be most important and much debated. Exemplified by the Ordos Basin, the sedimentary facies, sequence stratigraphy and hydrocarbon reservoirs in the Fuxian prospect area are examined in detail in the present paper on the basis of reservoir sedimentology, continental sequence stratigraphy, reservoir geology and trace element geochemistry.The Yanchang Formation is divided, for the first time, into six third-order sequences, each with an average time duration of about 4.5 Ma. The single sequence is composed of deltaic and lacustrine sandstones , siltstones and mudstones. The subaerial and subaqueous distributary channel and channel-mouth bar sandstone reservoirs are well developed mainly in the lowstand and highland systems tracts. The formation and evolution of the sequence stratigraphy of the Yanchang Formation are governed by slow tectonic subsidence, warm humid climates, sediment supply and lake-level fluctuations.The integrated study of heavy mineral, palaeocurrent direction, and sedimentary facies distribution has disclosed that the detritus are derived chiefly from the southeast and east-northeast during the deposition of the Yanchang Formation in the Late Triassic. The variations in the palaeocurrent directions have consequences for the development and distribution of the sequence stratigraphy and sedimentary facies of the Yanchang Formation. The major and trace element analyses of the mudstones from the Yanchang Formation also show that the lake waters would be freshened to brackish. The variations in trace element contents and ratios may hold up a mirror to the lake-level fluctuations and climatic changes, which, in turn, have exerted a direct and important effect on the sequence stratigraphic development.Collectively, the reservoir rocks from the Yanchang Formation consist dominantly of the delta front subfacies subaqueous distributary channel and channel-mouth bar microfacies and delta plain subfacies subaerial distributary channel microfacies fine-grained feldspar sandstones and siltstones. The reservoir rocks have been subjected to three stages of diagenetic and porosity evolution: (1) the porosity reduction by the mechanical compaction and chemical pressure solution during the early diagenetic stage; (2) the porosity increase by the dissolution during the A substage of the late diagenetic stage, and (3) the porosity reduction by the cementation and filling during the B substage of the late diagenetic stage. In general, they have low porosity and permeability.The porosity types consist of residual intergranular porosity, intergranular solution openings, intragranular solution openings, intercrystal porosity and intercrystal solution openings. The cracks and fissures are commonly less developed. The driving pressures and median pressures are relatively high. The porosity textures include fine- to microporosity and fine-throat to microthroat types.The examination of the controls of the reservoir rocks indicates that the sedimentary microfacies and diagenesis are the principal geological factors controlling the development and distribution of the reservoir rocks in the study area. The rock types comprise the delta front subfacies subaqueous distributary channel, channel-mouth bar and distal bar microfacies, and delta plain subfacies subaerial distributary channel microfacies sandstones and siltstones. As for the reservoir quality, the subaqueous (subaerial) distributary microfacies channel sandstones are the first, the channel-mouth bar microfacies sandstones come second, the distal bar microfacies sandstones is the third, and the last one is the interdistributary bay microfacies sandstones. The intense compaction, pressure solution and late carbonate cementation are responsible for the reduction of the porosity and permeability of the reservoir rocks. Both the later weak organic acidic dissolution and stru...
Keywords/Search Tags:Fuxian prospect area, Yanchang Formation, sedimentary facies, sequence stratigraphy, hydrocarbon reservoirs
PDF Full Text Request
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