A Research Of The Deep Cambrian High-quality Reservoir Characteristics And Formation Mechanism In Tahe Area

In recent years, the focus of China’s oil and gas exploration direction is from eastto west, by land opposite marine, from clastic rocks to carbonate rocks, from the newgeneration to the Paleozoic, from shallow to deep, marine carbonate oil and gasexploration have made significant progress. The thickness of Tahe Cambrian dolomiteis up to2000m, the temperature is greater than170℃, the pressure is greater than80MPa in the depths over8000m buried environment, the dissolution pores, holes,fissure and other reservoir space development reveals it is good dolomite reservoir,anti-order phenomenon that the porosity is still increasing with the increasing of depthwas also found. It fully illustrate that deep dolomite can be quality reservoir. Oil andgas accumulation and adjustment process have occurred in the deep strata of Tahe area.It shows good exploration prospects in this area. Therefore, it is help to evaluatereservoir properly and predict reservoir quality and the law of reservoir spatialdistribution that recognizing the development characteristics of dolomite reservoir,exploring to establish the development mode of dolomite reservoir, understanding theformation and development mechanism of marine carbonate reservoir deeply in Taheregion.Based on the analysis of the research situation of carbonate reservoir, a detailedanalysis of the problems and extensive data collection and digestion, papers utilizes thetheories of carbonate sedimentology, reservoir physics, geology and geochemistry,reservoir geology, diagenesis, logging geology etc as a guide, combines the existingachievement, takes advantage of the existing core data, drilling data and seismic dataetc, by the method of combine the macroscopic means and microcosmic means,combination of geology and logging, with the advanced analytical techniques meanssuch as cathodoluminescence microscope, scanning electron microscopy, isotope analysis, x-ray diffraction analysis, inclusions, fluorescence analysis, electronmicroprobe, trace element analysis etc, conventional means combine with the newtechnologies, new methods, theoretical analysis combines with the experimental means,geology combines with geochemistry, form the research methods and ideas thatmultidisciplinary combines with professional knowledge, seize the two mainfactors(sedimentation and diagenesis) that affect the formation and development ofreservoir, and emphasis on their correlation and dependence. We do a series of studieson the petrography of dolomite and the formation mechanism of dolomite andstructure-hydrothermal dolomitization.Studying on reservoir space types, physical characteristics and development lawof key dolomite layer with the method of Image logging and conventional loggingincluding natural gamma logging, caliper logging, dual laterolog, tri-porosity logging,spectrum Logging etc. Several main results and new knowledge as follow wereobtained by the research.â‘´Cambrian of the Tahe area is a set of sedimentary formation that mainly madeup of proliferative marine carbonate platform, it was developed from wide and gentleslope-the platform margin-the platform facies sedimentary paleoenvironment ofEarly-Middle Cambrian. It distributes proliferative carbonate platformâ†'the platformmarginâ†'wide and gentle slope from west to east, finally transition to the basin areawhich located in the eastern Tarim Basin (Manjiaer). The main areas of Akekule bumpis mainly made up of proliferative carbonate platform, most area of the east Akekulebump is the platform margin which has foreset struct, Platform margin grain beachand algal mound deposition develops in this area. The reservoirs mainly located in thecarbonate rocks which deposited in platform margin area, it is the beneficial area thatprimary porosity and secondary porosity which was controlled by diagenesis, and thisis the main area that dissolution pores reservoir and crack-dissolution pores reservoirexist.⑵Tahe’s Cambrian mainly includes six types of dolomite, they are micro-powder crystal dolomite, powder-medium crystal anhedral dirty dolomite, powder-medium crystal subhedral-idiotopic fog heart light side(zone) dolomite, coarse-grained dolomite, clean dolomite that fill the structural cracks, alien dolomite etc. Wepropose that porphyritic texture, striatus and Layered structure are the three types ofrock structure in this area. Papers adds two types of rock structure-fog heart light sidedolomite and (residual) grain pattern dolomite on the basis of micro crystal dolomite,powder crystal dolomite, cryptomere dolomite, medium crystal dolomite, coarse- grained dolomite, and summarizes the geochemistry of this six types in depth; and then,points out that the main mechanisms of dolomitization are evaporative pumpingdolomitization, seepage reflux dolomitization, shallow burial dolomitization,mid-depth burial hydrothermal dolomitization etc. Evaporative pumping dolomitize-tion, seepage reflux dolomitization are the main mechanism that generate dolomite,mid-depth burial hydrothermal dolomitization has an important influence on thetransformation of dolomite in some areas.⑶The diagenetic evolution of Tahe area’s dolomite is continued gradualtransition, and burial dolomitization is the most important diagenesis. The diagenesistypes of Tahe’s Cambrian dolomite are complex, there is grading law that frompenecontemporaneous dolomiteâ†'shallow burial dolomiteâ†'mid-depth burial dolomiteâ†'hydrothermal dolomite. It reflects the dolomitization environment was changed fromshallow burial environment to mid-depth burial environment, and finally intohydrothermal environment, the average levels of Fe and Mn increases gradually, thedegree of order in dolomite increases too.â‘·Diageneses affect deepreservoir.Duringburial,reservoir experiences all kindsof diageneses. Early compaction, pressure solution and calcite cementation havedevastating impact on primary porosity of carbonate. Secondary porosity which ismainly controlled by dolomitizaton, burial corrosion and hydrothermal dissolutionplays an important role in carbonate reservoir.⑸Through the research about oxygen and carbon isotope value of matrix calcite,holes and seam filling in Tahe, the change of the matrix calcite’s oxygen and carbonisotope value is small, the oxygen and carbon isotope value of matrix calcite is biasnegative, but the carbon isotope value of holes filling vary greatly, it explains thatmatrix calcite and dolomite are product of burial effect. The diagram of oxygen isotopevalue and depth points out that oxygen isotope value is bias negative with the depthincreases; the diagram of carbon isotope value and depth explains that carbon isotopevalue vary greatly in the same depth, and also shows that there was external fluid whichchanged the properties of the original fluid.⑹The87Sr/86Sr ratio of dolomite and calcite that fill the holes and crack is muchhigher than that of matrix dolomite, the strontium isotope value of filling is muchhigher than the max value of Upper Cambrian’s strontium isotope value, it shows thatCambrian stratum had a rich source of strontium. The strontium isotope became heavierwith the depth increase. It indicates that the fluid which was rich in strontium was fromthe bottom. ⑺The formation mechanism of Tahe area’s dolomite reservoir space has, themain formation mechanism of Tahe area’s dolomite is dissolution; after thehydrothermal dissolution occured, the main factors that preserve the reservoir space arestable burial process and lesser tectonic event.The research results can help to explore hydrocarbon of Lower PaleozoicCambrian large buildups traps, deploy exploration, accelerate the pace of explorationand achieve a breakthrough in new areas, it has important theoretical and practicalsignificance, it is not only has important significance on the exploration of oil and gasin northern Tarim Basin’s deep strata, but also has important research value inpetroleum geology and sedimentology field.