Diagenetic Differences And Genetic Mechanisms Of Triassic Baikouquan Formation Reservoirs From Circum-Mahu Depression

In this thesis,the reservoirs of Triassic Baikouquan formation in Huangyangquan fan delta and the Xiazijie fan delta are selected as the research objects.By means of microscope,cathodoluminiscence analysis,scanning microscope,eletron probe,fluid inclusions analysis etc,the components,diagenesis types,diagenetic stage and sequence of reservoir are systematically studied and compared,and the controlling factors and forming mechanism of diagenesis difference are discussed in detail.The main conclusions are as follows:Conglomerate and coarse sandstone are the main rock types in the study area.The gravel compositions include tuff,granite,feldspar,basalt,metamorphic rock and a small amount of sedimentary rock.The sandstone types include lithic sandstone and feldspathic sandstone.The composition maturity in the Xiazijie fan delta is higher than that in the Huangyangquan fan delta.According to the rock compositions,structure and structural characteristics,11 types of lithofacies and 7 types of lithofacies are identified.The sandy conglomerate reservoirs of Baikouquan formation have experienced a variety of diagenesis.The diagenesis types and strength of Xiazijie fan delta and Huangyangquan fan delta are characterized with obvious difference.The compaction intensity of Xiazijie fan delta is weaker than that of Huangyangquan fan delta.In Xiazijie fan delta,a variety of carbonate cements,mainly calcite and ferroan calcite,are developed,and the clay minerals including chlorite,illite and authigenic kaolinite are of relatively low content.Analcite cements can be seen locally.Dissolution components are mainly composed of feldspar,rock fragments and analcime.The Huangyangquan fan delta is mainly filled with clay mineral cementats.It can be seen that a small amount of calcite and quartz overgrowth.Dissolution components are the intergranular tuffaceous matrix.The comprehensive analysis shows that the Baikouquan formation reservoir is in the middle diagenetic stage A,and the two fan deltas lithofacies have different diagenetic evolution process characteristics under the different diagenesis.The main factors causing this difference include sedimentary process,fault characteristics and deep gas reservoir,among which sedimentary process is the fundamental cause of diagenetic difference.The composition and content of the original materials of the rocks determine the types and contents of cemented materials.In Xiazijie fan delta,high component maturity and low matrix content are beneficial to the precipitation of carbonate cements.However,in Huangyangquan fan delta,the high content tuffaceous matrix makes low residual intergranular pore content,which is not conducive to carbonate cementation but beneficial to the precipitation of illite,chlorite and biotite.At the same time,a large amount of intermediate-acid volcanic rocks,tuff,albite and other acidic feldspar are contained in the lithic fractions of Xiazijie fan delta,which provide the necessary sodium ions for the formation of analcime.However,the content of potassium in the Huangyangquan fan delta grain is high,which provides the necessary potassium ion for the formation of illite.Faults and deep gas reservoirs are also two essential factors for diagenetic differences.Carbonate cements is developed in both fan delta,but the mineral assemblages and geochemical characteristics of carbonate cement are quietly different in different well areas.Calcite and ferroan calcite are developed in Xia-9 fault well area of Xiazijie fan delta.Calcite distributes near the interface of sand and mud.With the increasing of distance from the interface,the content of carbonate cement decreases,and theδ¹³C value of carbonate cement increases.Theδ¹³C value reflects an organically sourced carbon by thermal decarboxylation of organic matter in the middle diagenetic stage.The formation time of calcite is at the same time as the second stage of oil and gas filling.Ferroan calcite is distributed in the middle of lithofacies sequence and precipitates in the relatively pure lacustrine formation water at the early diagenetic stage.In the Ma-5 well area,siderite,ferroan calcite,ferroan dolomite and dawsonite assemblage are developed.Siderite is formed in the early diagenetic stage and is related to CO₂ carried by atmospheric fresh water.Ferroan calcite and dawsonite are influenced by inorganic mantle-derived CO₂ fluids.In Jurassic,mantle melting material resulted by magmatic activity erupts along the large strike-slip fault upwelling,and the mantle derived magma CO₂ hot fluid was transported upward by deep faults.In Huangyangquan fan delta,only calcite cements is observed.Theδ¹³C values of calcite cements range from-54.99‰to-27.04‰,showing a strongly negative values.The main source of carbon is methane gas which migrates from the gas reservoir of Carboniferous formation along the Ellik Lake fault.The methane gas is oxidized to CO₂ by the high valence metal ions.The CO₂ dissolved in formation water and formed HCO₃^-,which eventually resulted in calcite precipitation.