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Genetic Type And Reservoir Evaluation Of Paleogene Tight Rudstone In The Shulu Sag Of Bohai Bay Basin

Posted on:2017-03-22Degree:DoctorType:Dissertation
Country:ChinaCandidate:L J ZhengFull Text:PDF
GTID:1220330485492212Subject:Mineral prospecting and exploration
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As an important part of unconventional oil and gas, tight oil and gas have broad prospects for exploration. The Shulu Sag is located in the southwestern corner of the Jizhong Depression, Bohai Bay Basin of east China. The lower part of the Shahejie 3 Formation(Es3L) of Paleogene in the middle and southern of the Shulu Sag developed massive conglomerates characterized by low porosity and low permeability with carbonate fragment as its main constituent, which become significant target of tight oil and gas exploration of Huabei Oil field in the recent years. In this thesis, the genetic types and sedimentary characteristics of the Shulu conglomerate are studied in detail based on seismic profiler, logging, cores, petrographic thin-sections and scanning electron microscope(SEM) photos. The types of reservoir space are analyzed and major controlling factors of reservoirs qualities are identified. In addition, a set of reservoir evaluation method is built.Considering the Paleogene intraclasts in lithologic classification scheme, carbonate breccia was divided into five types in the south-central area of Shulu Sag: extraclastic rudstone of grain-supported, mixed-source rudstone of grain-supported, intraclastic rudstone of grain-supported, extraclastic rudstone of matrix-supported and mixed-source rudstone of matrix-supported. The source rocks of extraclast are mainly angular to subangular micritic limestone and dolomite with poor sorting from Ningjin uplift. The Paleogene intraclasts also have obvious boundaries, but are deformed. These intraclasts show deformational characteristics of soft sediment such as elongation or squashed textures, giving lacerated or subrounded morphology. They may be derived from erosion by the gravity flow on the underlying calcilutite substratum.According to the sedimentary structure and distribution characteristics, and many other factors, the carbonate breccia may fall into two categories: one is formed by fan-delta channel sedimentations—the interactions of alluvial fan and lake, the other by earthquake-induced slump fan. Clast-supported rudstone and matrix-supported rudstone are the main lithology of braided river in the fan delta plain and front. A typical characteristic of them is inbricate structures with normal graded bed sequence and reverse grading are aslo developed. A large rudstone bed made of extraclastic rudstone of grain-supported appears in well ST3 over a total thickness of 100 m, which is interpreted as main channel of earthquake-induced slump fan. The rock type of braided channel is more complex, including extraclastic rudstone of grain-supported, mixed-source rudstone of grain-supported, intraclastic rudstone of grain-supported, extraclastic rudstone of matrix-supported and mixed-source rudstone of matrix-supported. These carbonate breccia appear along with typical seismites widely distributed in the sag, which contain soft sediment deformation structures(sedimentary dikes, hydraulic shattering, diapir structures, convolute lamination, load-flame structures, ball-and-pillow structures, loop bedding and subsidence structures) and brittle deformation(synsedimentary faults).The reservoir of carbonate breccia belongs to the type of porosity-fractured reservoir of ultra-low porosity and permeability. The porosity ranges from 0.4% to 6% and the permeability is between <0.04 × 10-3 and 40× 10-3μm2. Permeability is good where fractures were developed. Reservoir space consists of intragranular pore(dissolved pores in grains such as quartz, intercrystalline pores and intercrystalline dissolved pores of crystalline dolostone), intergranular pores, fissures(structural fractures, gravel-edge fractures, fissures created from diagenetic shrinkage and fractures in gravels) and organic matter pores. The physical property of reservoir was mainly affected by gravels compositions, lithofacies, genetic types, diagenesis, tectonism and thermal evolution of organic matter. According to the sedimentary and reservoir characteristics of tight conglomerate, a set of reservoir evaluation method is put forward which takes gravels compositions, lithofacies, genetic types, diagenesis, tectonism and organic matter as evaluating indicators and the favored areas of reservoir are presented.
Keywords/Search Tags:rock type, carbonate conglomerate, genetic types, seismites, reservoir characteristics
PDF Full Text Request
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