| By conducting extensive literature research and integrating data from field outcrops,2D and 3D seismic,drilling/logging,and cores,a multi-level source-to-sink system analysis was carried out on the Liushagang Formation in the Weixinan Sag of the Beibuwan Basin.Based on the establishment of a first-level source-to-sink system(Beibuwan Basin),a second-level source-to-sink system(Weixinan Sag),and a third-level source-to-sink system(North Steep Belt),the main provenance direction-sedimentary area,dominant sedimentary system,and favorable sand body-diagenetic facies types in the Weixinan Sag in the Beibuwan Basin were summarized.On the basis of the establishment of the multi-level source-to-sink system and combined with various analytical methods(thin sections,cathodoluminescence,scanning electron microscopy,X-ray diffraction,carbon-oxygen isotope analysis),high-quality reservoirs in the North Slope Belt of the third member of the Liushagang Formation were predicted,and a prediction model for high-quality reservoirs was ultimately established based on the firstlevel source-to-sink system controlling material basis,the second-level source-to-sink system controlling morphological characteristics,and the third-level source-to-sink system controlling diagenetic types.The entire study is divided into several parts,including:(1)This study clarified the coupling characteristics of the first level source-to-sink system within and outside of the Beibuwan Basin.The research focus of the first level source-to-sink system mainly includes the distribution characteristics of the peripheral drainage systems and the evolution characteristics of the provenance,as well as the coupling between the basement within and outside the basin.The northern part of South China accepted the influences of the Pearl River system,the Red River system,the Yunkai Block,and the Shiwandashan,while the southern part of the region was affected by the drainage system in the northern part of Hainan Island.During the early stage of the Liushagang Formation,the material supply to the Weixinan Sag and the Wushi Sag mainly came from the Shiwandashan and the Yunkai Block,while the material supply to the Fushan Sag in the south came from Hainan Island,showing the characteristics of nearsource supply.During the late stage of the Liushagang Formation,the material supply to the Weixinan Sag in the north mainly came from the Red River,the western tributaries of the Pearl River,the middle tributaries of the Pearl River,and the Yunkai Block,while the material supply to the Fushan Sag in the south came from Hainan Island,showing the characteristics of slightly far-source supply.Based on the basement lithological characteristics,structural styles,and tectonic features of the Yunkai Uplift and the Qinzhou-Fangcheng trough,it was discovered that the South China Block developed from northwest to southeast with the Shiwandashan foreland basin,the Qinzhou-Fangcheng trough in the S-P fold-thrust belt,the Cambrian-Ordovician tectonic mixing zone,the Yunkai Block,and the Wuchuan-Sihui shear zone.The basement within the basin mainly includes the pre-Sinian crystalline basement,the Sinian-early Paleozoic basement,the Upper Paleozoic basement,and the Mesozoic basement.By comparing the lithological characteristics within and outside the basin,it is believed that the basement of the Beibuwan Basin is an extension of the South China Block basement and Hainan Island towards the sea,consisting mainly of the Jiangnan orogenic belt on the edge of the Yangtze Block,the late Paleozoic Qinzhou-Fangcheng trough,the Indosinian Yanshan fold-thrust belt and foreland basin,and the Precambrian Yunkai Block.(2)Analyzed the parent rock types,sediment transport pathways,and sedimentary systems characteristics of second-level source-to-sink system in the Wenxinan Sag.In terms of parent rock types,three types are mainly developed in the Wenxinan Sag,including the Lower Paleozoic metamorphic rocks,the Mesozoic granites,and the Mesozoic carbonates,which collectively provide sedimentary material for the entire Liushagang Formation.In terms of paleogeography and sediment transport pathways,the Wenxinan Sag can be divided into eight source-to-sink units,including the circular system of the Wanshan Uplift and the belt-shaped systems of the Wenxinan Low Uplift and the Qixi Uplift.Meanwhile,three types of sediment transport pathways can be identified in the Wenxinan Sag,including the long-axis convergent type,the short-axis steep system,and the short-axis slope system.In terms of sedimentary systems,the sublacustrine fan,fan delta,braided river delta,and beach-bar deposits are developed in the Liushagang Formation of the Weixinan Sag.During the Liushagang Formation of sedimentation,the sedimentary combination was "alluvial plain-fan delta-shore-shallow lake" in the third member,"fan delta-barrier lake-lake" in the second member,and "axial source deltalacustrine fan-lake" in the first member.Three source-to-sink models were summarized,including the short-axis steep proximal fan delta model(with coarse grain size,proximal deposition,strong hydrodynamic force,and obvious controlled by fault),the short-axis slope fan delta model(with moderate grain size,moderate deposition,and moderate hydrodynamic force),and the long-axis oblique slope braided river delta model(with fine grain size,distal deposition,and weak hydrodynamic force).(3)The sand bodies,diagenetic facies types and distribution characteristics of the third-level source-to-sink system in the North Steep Belt of the third member of the Liushagang Formation were dissected.The sand layer group of the third member of the Liushagang Formation consists of four sand layer groups from bottom to top.The key segment in the Wenzhou 10-3 area is the psL32-1 sand layer group,the key segment in the Wenzhou 11-6 area is the psL32-2 and psL32-1 sand layer groups,and the key segment in the Wenzhou 6-3 area is the psL32-1 sand layer group.Based on the basic reservoir characteristics,controlling factors,and corresponding logging identification,we believe that there are five types of diagenetic facies in the study area: weak compacted and cemented diagenetic facies,medium compaction and dissolution diagenetic facies,strong compaction and medium dissolution diagenetic facies,compaction and argillaceous filling diagenetic facies,and dense compaction diagenetic facies.The dominant diagenetic facies distribution occurred mainly during the psL31-2 and psL32-1sand layer group.(4)The control of multi-level source-to-sink systems on reservoirs was summarized,and high-quality reservoir prediction was conducted.Due to differences in material supply and structural influences,the Wenzhou 10-3 area has shallow burial depth,coarse grain size,and developed argillaceous cementation,the Wenzhou 11-6 area has medium burial depth,medium grain size,and developed carbonate cementation,and the Wenzhou6-3 area has a large burial depth,fine grain size,and developed argillaceous and carbonate cementation.Due to the influence of sedimentary microfacies,the Wenzhou 10-3 area developed thick-layered underwater distributary channel microfacies,with good reservoir properties.The Wenzhou 11-6 area developed estuary barrier microfacies,with moderate reservoir properties.The Wenzhou 6-3 area developed underwater distributary interchannel microfacies,with poor reservoir properties.Due to the influence of sand body superposition-lithofacies,the rich sand superposition type is mainly composed of weak compacted and cemented diagenetic facies and medium compaction and dissolution diagenetic facies,mainly developing type I reservoirs.The sand-mud intercalation type is mainly composed of strong compaction and medium dissolution diagenetic facies,mainly developing type II reservoirs.The poor sand superposition type is mainly composed of compaction and argillaceous filling diagenetic facies and dense compaction diagenetic facies,mainly developing type II to III reservoirs. |
