A Research Of The Cambrian Dolomite Reservoir Characteristics And Formation Mechamism In Tadong Area

In this paper, the basic characteristics, geochemical characteristics and formationmechanism and pattern of the Cambrian dolomite reservoir are studied throughdrilling core, logging and test data. The main results obtained in this study are asfollows:1. In Cambrian dolomite reservoir of east Tarim Basin, penniform authigenicillite precipitation was discovered through thin section observation and electron probemicroanalysis, it enriched mineralogy marks of hydrothermal process.2. Based on thin section observation, core observation, cathodoluminescenceanalysis, carbon and oxygen isotope analysis, trace and rare earth elements analysisand X-ray diffraction order degree analysis,three phases dissolution for Cambriandolomite reservoir of study area were proposed, that is, first hydrothermal dissolution,organic acid dissolution and second hydrothermal dissolution. And first hydrothermaldissolution occured on early diagenesis, correspond with early-midcaledonian; secondhydrothermal dissolution occured on mid-late diagenesis, correspond with latehercynian.3. Five phases fracture diagenesis for Cambrian of study area were proposedthrough core and thin section observation and cathodoluminescence characteristicsanalysis.That are:①Early-mid caledonian structure-hydrothermal fracturediagenesis,correspond with first hydrothermal process.The main evidences are Saddledolomite within tectonic fractures don’t shine or shine dark rosy incathodoluminescence,and it can be remoulded by stylolites；②Late caledonianstructure fracture diagenesis,structure fractures were filled by calcite fully,it don’tshine in cathodoluminescence,and cut stylolites；③Early hercynian structure fracturediagenesis,structure fractures were filled by calcite fully, it shine dark orange incathodoluminescence,and it can be cut by the later structure fracture what shine brightorange in cathodoluminescence or be replaceed by second hydrothermal quartz；④Late hercynian structure-hydrothermal fracture diagenesis,correspond with secondhydrothermal process.It be filled by authigenic quartz, Saddle dolomite what shine bright jacinth and calcite what shine bright orange;⑤Indosinian structure fracturediagenesis,structure fractures were filled by calcite fully and shine dark orange,andcut Saddle dolomite what shine bright jacinth and calcite what shine bright orange inearlier structure fracture.4. Through the core and casting thin section, detect the main reservoir space ofdolomite reservoir of Cambrian dolomite reservoir are intercrystalline pore,intracrystalline solution pore, solution pore or cave and dissolved pore distributionalong the crack; and based on logging and test analysis, point out Cambrian dolomiteof study area has two types reservoir, that are solution pore reservoir and solutionpore-cave reservoir. On the logging curves, the Characteristics solution pore reservoirare as follows: the value of GR is between11API and41API; the value ofresistivity is between110and2000Ω.m; the value of AC is between45.5and49.3μs/ft;the value ofNis between0.56and0.82; the value of DEN is between2.68-2.8g/cm3；the Characteristics solution pore-cave reservoir are as follows: thevalue of GR is between10API and20API; the value of resistivity is between10and1000Ω.m; the value of AC is between49.9and53.5μs/ft; the value ofNisbetween0.8and2.9; the value of DEN is between2.60-2.72g/cm3. On developmentand distribution, the two types reservoir distribute in upper Cambrian (gross thicknessis513.52m, proportion is96.6%), and solution pore-cave reservoir has43singlelayers, gross thickness is332.14m, proportion is62.48%; solution pore reservoir has35single layers, gross thickness is199.43m, proportion is37.52%; Longitudinally,the two types reservoir have characteristic of anisotropism, in the same pure dolomitelayer, some can form reservoir, but some cannot.5. Based on measured porosity, permeability, well logging interpretation porosity,mercury injection data analysis and the carbonate reservoir classification criteria,point out that the solution pore-cave reservoir is Ⅳ-Ⅲ type reservoir and the solutionpore reservoir is Ⅳ type reservoir.The correlation of porosity-permeability of thistwo type reservoir is bad, the pore throat is slender, the connectivity is bad,anisotropism is obvious.6. Based on core and thin section observation, point out that the types of dolomiteof the Cambrian in study area include:①powder-sized to micro crystalline dolomite;②powder-sized to medium crystalline xenomorphic dirty dolomite;③powder-sizedto medium idiomorphic crystalline dolomite;④medium-coarse crystalline dirtydolomite;⑤medium-coarse crystalline clean dolomite and Saddle dolomite; base ongeochemical analysis, point out that formation Mechanism of the dolomite include evaporative pumping, reflux dolomitization, shallow burial dolomitization andhydrothermal dolomitization, and the same type of dolomite experienced the stackingof four dolomitization models.