| The Amu Darya Right Bank Block stretches across northeastern of the Amu-Darya Basin, and serves as the major gas source of Central Asian Natural Gas Pipeline and West-East Gas Pipeline, where the reef-bank reservoirs developed in sub-salt Callovian and Oxfordian of middle and upper Jurassic, and natural gas resources is abundant. The Amu Darya Right Bank Area experienced the sedimentary evolution from gentle slope of Callovian to rimmed carbonate platform of Oxfordian, which is characterized by rich sedimentary microfacies and diversified reservoir types. Different from the main exploration and development targets of platform margin reef-bank, intra-platform dolomite bank and weathering crust karst reservoirs in China, the major exploration and development targets in the Amu Darya Right Bank Area concentrated in intraplatform and platform margin slopes facies, however, there was not a clear recognition of the types and distribution regularity of large scale reservoirs in platform and platform margin slopes, and the reservoirs prediction was difficult, as a result, the exploration and development process were severely restricted in this area, therefore, the research on sedimentary reservoirs has significant importance for effective natural gas exploration and development.Taking the Callovian and Oxfordian of middle and upper Jurassic in the Amu Darya Right Bank Area as the research object, and guided by the theories of sedimentology and sequence stratigraphy, through full use of outcrops, drilling cores, thin sections, mud logging, logging, seismic data and analytical test data, the development features, genetic types, major controlling factors and geologic models of reef-bank reservoirs in platform and platform margin slopes are studied in-depth based on high-resolution sequence stratigraphy and sedimentary microfacies research. The achievements are as follows:1. Establishing sequence stratigraphic framework of Callovian and Oxfordian, clarifying the relationship of lithostratigraphic units, electro-stratigraphic units and sequence stratigraphic units, and developing the correlation schemes of multiple strata in different carbonate sedimentary facies belts.Predecessors classified the Callovian-Oxfordian at margin and intraplatform in the west of the Amu Darya Right Bank Area into 8 lithologic units, and 6 ones at platform margin slope and basin in central and east area, the isochronous correlation relationships of the lithologic units in different facies belts were uncertain and thus influenced the recognitions on geologic law. Through in-depth study on petrologic characteristics and lithologic assemblage law of field outcrops, the lithostratigraphic units of outcrops were classified, then through the comparison among the outcrops, conventional logging and image logging, the detailed analysis on the interface and the internal logging response characteristics of all lithologic sections was made, and the fine classification scheme of lithostratigraphic units at Callovian and Oxfordian in the Amu Darya Right Bank Area was defined.Through sequence stratigraphic analysis, different orders sequence stratigraphic boundaries were identified, and the Callovian and Oxfordian were divided into two 3rd-order sequences. While single sequence is composed of the transgressive system tract, the early-stage high stand system tract and the late-stage high stand system tract, and the single system tract can be further divided into two parasequence sets and multiple parasequences. Based on the establishment of regional high-resolution sequence stratigraphic frameworks, the correlation relationships of lithostratigraphic units, electro-stratigraphic units and sequence stratigraphic units are identified, and the correlation schemes for multiple strata in different facies belts of carbonate platform sedimentary system were developed, and lithostratigraphic units classification and correlation of Callovian and Oxfordian in more than 60 wells were completed, the schemes and programs have been accepted by oilfield companies and applied into production, which solves the isochronous strata correlations problem of different lithostratigraphic units plagued exploration for a long time. On this basis, the evolution process from slope-type carbonate platform of Callovian to rimmed carbonate platform of Oxfordian was restored, and the geological foundation of different lithostratigraphic units and rock assemblage features were revealed, thus laying solid basement for sedimentary microfacies and reservoir research in intraplatform and platform margin slope.2. Building 2 standard sedimentary microfacies sequence evolution profiles through drilling cores and logging facies analysis, creating the quantitative interpretation charts of multi-resolution clustering logging facies of different sedimentary microfacies in platform and platform margin slope, fulfilling continuous sedimentary microfacies interpretation of Callovian-Oxfordian, and revealing the spatial distribution law of sedimentary microfacies in platform and platform margin slope.A complete carbonate platform sedimentary system developed in Callovian and Oxfordian in the Amu Darya Right Bank Area, while the sedimentary microfacies and rock types are abundant and diversified in platform and platform margin slope. The logging identification charts in sedimentary microfacies classification based on coring intervals are helpful in realizing continuous sedimentary microfacies interpretation and uncovering the spatial distribution law of sedimentary microfacies in platform and platform margin slope.Based on petrologic analysis of drilling cores, different microfacies of evaporative platform, restricted platform, open platform, platform margin, platform margin slope and basin were recognized. Through continuous coring data analysis, two standard profiles of sedimentary microfacies sequence evolution in platform and platform margin slope were built. The logging response styles of different sedimentary microfacies were set up, then different facies models of the conventional logging, the clustering logging facies, and the imaging logging were formed, the quantitative multi-resolution clustering logging facies interpretation charts of sedimentary microfacies in platform and platform margin slope were created, and continuous sedimentary microfacies were interpreted. Through single well microfacies interpretation and seismic facies analysis, the correlation and planar maps of sedimentary microfacies were made, revealing the spatial distribution law of different sedimentary microfacies.3. Conducting reef-bank reservoir features study and quantitative evaluation in platform and platform margin slope, defining reservoir space types and physical properties characteristics, diagenetic types and evolution stages, establishing the quantitative interpretation charts of conventional logging and imaging logging, and determining the lower limit of reservoir and identification criteria of fluid types.The carbonate reservoir space types of platform and platform margin slope of Callovian and Oxfordian are dominated by secondary intragranular and intergranular dissolved pores, some primary pores are kept in high energy grain bank bodies of platform, and fractures are developed of platform margin slope in middle and east of the Amu Darya Right Bank Area. Relevant studies show that compaction and cementation are main destructive diagenesis; while rupturing and burial dissolutions such as thermochemical sulfate reduction(TSR) are main constructive diagenesis. By analyzing paleotemperature, paleosalinity, trace element, carbon and oxygen isotope and strontium isotope, the diagenesis and porosity evolution models are separately established in platform and platform margin slope, laying the foundation for reservoir genetic analysis. Based on the establishment of mineral models for different areas and layers, the mathematic models appropriate for logging interpretation of carbonate in platform and platform margin slope are selected, and the quantitative interpretation charts for conventional logging of carbonate reservoirs in platform and platform margin slope are set through parameters optimization by means of logging data statistics and Laboratory analytical results; according to the identification of fracture types, quantitative fractures and vugs are interpreted from image logging. Through multi-parameters logging crossplots and oil testing result, the conservative lower limit of the carbonate reservoir with porosity of 4% is confirmed. Based on multi-means fluid property identification research, the criteria of fluid types identification is defined as follows: gas layer(SW≤50%), gas-water layer(50%<SW<70%), water layer(SW≥70%), which lay basis for gas-water interface identification, reservoir types study, reserves calculation, selection of oil testing layers and perforating intervals.4. Classifying carbonate reservoir types in platform and platform margin slope, establishing the comprehensive identification charts for various types of reservoirs, creating the quantitative model for identifying reservoir types with conventional logging data as variables, analyzing major controlling factors in reservoir forming, clarifying the formation condition and distribution law of large scale reservoirs, and establishing the distribution model of reservoirs in platform and platform margin slope.The rich varieties of carbonate reservoir spaces and assemblage modes in platform and platform margin slope reveal the complesity of reservoir types and genesis, defining their horizontal and vertical distribution law is of great significance in efficient exploration and development. Based on core data, physical properties analytical data and logging data, four categories of carbonate reservoirs in platform and platform margin slope are classfied as porous reservoir, cavern reservoir, fractured-porous reservoir and fractured-cavern reservoir, then the comprehensive identification charts of various reservoirs are set up. Through statistics analysis to logging response value of different reservoirs, the quantitative logging identification model with the ratio of deep resistivity and shallow resistivity(RD/RS), total density-neutron porosity and acoustic porosity difference(Φt-Φs) as variables in reservoir types identification is established, and the quantitative reservoir types classification methods on the basis of conventional logging data is developed, which solves the problem in unclear understanding of reservoir distribution law.Analysis of major controlling factors of various reservoirs development indicates that porous and cavern reservoirs are mainly controlled by sedimentary microfacies, sequence interface and related dissolutions, the fractured-porous reservoir is mainly controlled by sedimentary microfacies, fractures and dissolutions, while the fractured-cavern reservoir is mainly controlled by fractures and related burial dissolutions. In combination with reservoirs correlation within sequence stratigraphic frameworks, the forming conditions and distribution law of large scale reservoirs are specified and the reservoir types distribution mode in platform and platform margin slope is set.5. Revealing that Samantepe is the thick superposed intraplatform bank bodies on concealed paleouplift, breaking through traditional recognition that platform margin reef as the main productive layer in Samantepe gas field, clarifying the sedimentary geological features, the controlling factors and the distribution law of large scale intraplatform bank bodies, establishing large scale intraplatform bank bodies development mode, thus providing guidance for discoveries of multiple large scale intraplatform bank bodies in the west of the Amu Darya Right Bank Area.Predecessors believed that the Samantepe in the west of of the Amu Darya Right Bank Area was a platform margin reef gas field. The integration analysis of lithologic microfacies and seismic facies reveal that the Oxfordian production layer in Samantepe gas field is thick superposed intraplatform bank on concealed paleouplift, which breaks through the previous geological understanding that the main platform margin barrier reef production layer in the gas field.Combining paleogeomorphology with sea level fluctuation analysis, the planar distribution law, development horizons, bank types, assemblage characteristics, large scale intraplatform bank reservoirs development condition are specified, and the geological mode of intraplatform bank development is established. While the monolayer thickness of intraplatform bank is larger, multistage superposed vertically and continuous distributed laterally in the high land of paleouplift, where can form large scale superposed intraplatform bank reservoirs; in contrast, both the monolayer and total thickness are smaller in the depressed areas and the scale of intraplatform bank reservoirs is limited, while the intraplatform bank reservoirs are mainly developed in the middle and upper parts of parasequence sets.Based on the geological cognition about the controlling effects of paleogeomorphology on intraplatform bank development, the layer flattening technique, the residual thickness method and the compensation thickness method are applied in identifying two large scale basement paleouplifts and multiple secondary highlands in the west area. On this basis, multiple development areas of large scale superposed intraplatform bank bodies were forecasted and eventually proved by drilling wells, which guided the discoveries of multiple large scale intraplatform bank bodies in the west of the Amu Darya Right Bank Area. |
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