Dolomitization And Origin Of The Cambrian-Ordovician Dolomite Reservoirs In The Central Uplift, Tarim Basin

As the oil&gas exploration of Tarim Basin towars the strategies of stereoscopic exploration, overall evalaution and various areas for breakthrough, it is necessary to prepare the geologic research of new stratum and fields in advance. Cambrain-Ordovian dolomite reservoir which is considered as the most realistic potential repalcement field for next exploration shows great prospect in study area. Understanding the dolomitization and the formation mechanism of the reservoirs is, therefore of important significance for both reservoir assessment and prediction in the Central Uplift. Based on the sedimentology, petrophysics, geochemistry, reservoir geology and latest theories about the carbonate diagenesis and well logging, this paper focuses on the dolomitization and origin of the reservoirs in study area. A number of advanced methods have been used for this study, including photomicrography of thin-section, cathodoluminescence, SEM and BSE, analyses of stable (oxygen/carbon) isotopes and radioactive (strontium) isotopes and trace elements, tests of homogenization temperature (Th) and final ice melting temperature (Tm) of fluid inclusion and physical properties.The study of the dolomite texture, as the foundation of dolomitization analysis, not only indicates the origin of the dolomites but controls the dolomite reservoir quality significantly. The new classification of dolomite textures are propoed in this study. The results show that there are two types of replacement dolomite:One is the dolomite with well-preserved precursor lithologic fabric that can be subdivided to micritic dolomite and (relict) dolo-grainstone; The other type is the crystalline dolomite, including fine crystalline, planar-e(euhedral) dolomite, fine crystalline, planar-s(subhedral) dolomite and medium to coarse crystalline, nonplanar-a(anhedral) dolomite. Additional, two types of void-filling dolomite cement have been observed that are very fine to fine crystalline, planar-e(s) dolomite void-filling and medium to coarse crystalline, saddle dolomite void-filling, respectively. Different texture types of the dolomite indicate that the diagenetic environments and formation processes of the dolomite are various. For study area, the dolomite with well-preserved precursor fabric is associated with the mimetic dolomitization during the penecontemporaneous period. Abundance of high-supersaturation dolomitizing fluid is beneficial to preserve precursor fabric. The planar-e and planar-s dolomite probably formed from reflux of penesaline seawater during early diagenetic stage by the low-temperature dolomitization. Overdolomitization may play a vital role for planar-e crystals transferring to planar-s crystals with decreasing of pore space. The nonplanar-a dolomite is interpreted as the product of high-temperature/hydrothermal dolomitization and recrystallization during the medium or deep burial, the high temperature is the key to cause dolomite crystal boundary transferring to nonplanar-anhedral.Based on the study of dolomite volume, texture, pore types and formation environments of differernt kinds of dolomites, this paper summarized the relationship between dolomitization and pore development in matrix dolomites. The result shows that only shallow burial dolomitization is associated with pore development, which produced the intercrystalline porosity by inherited the porosity of precursor carbonate rather than through the way of molar replacement. Other types of dolomitization styles are not conducive to the formation of porosityDissolution vugs accounting for a large proportion in the dolomite reservoir shows that the corrosion plays an important role in the formation of the dolomite reservoir in the study area. All kinds of dissolution of dolomite reservoir in the study area, therefore has been researeched including (high-frequency) sequence boundary controlled meteoric water dissolution, karstification and hydrothermal dissolution and their reservoir characteristics, fluid geochemical properties and formation mechanisms. Dissolution related to the sequence boundary occurs widely, especially in the Upper Cambrian and Lower Ordovician. This reservoir which is composed of intercrystalle vugs and needle vugs, displays as thin layers in vertical and can be comparative in lateral. Middle Caledonian stage I karstification reservoirs, mainly occur in the west of Tazhong area, are maede of middle and small size solution pores and fracture, and have the feature of layered distribution. Middle and Late Caledonian-Early Hercynian karstification, mainly concentrated in the thrust belts in esetern Maigaiti slope and SE Tazhong area, are featured by the development of large cave reservoirs. Hydrothermal dsissolution produced vugs and freature with pore-filling such as saddle dolomite, fluorite, barite, authigenic quartz, et, al. The reservoir developed without horizon selection, but is closely associated with faults. In addition, compound origin of reservoirs are common in study area, especially the composition of dolomitization, karstification and hydrothermal dissolution.Finally, the favorable exploration direction was pointed out based on the in-depth study of dolomitization and origin of dolomite reservoirs as well as the conditions of pretroluem geology in the Central Uplift, Tarim Basin.