A Research Of The Deep Cambrian Reservoir Formation Mechanism In Tarim Area

The Cambrian carbonate rocks are distributed extensively in the Tarim Basin. With the further exploration and several wells drilled across these sediments, more and more materials show the great exploration potential in the Cambrian succession. Understanding the characteristics and the formation mechanism of the reservoirs is, therefore of important significance for both reservoir assessment and petroleum exploration in the deep-burial Cambrian carbonate rocks.Based on the sedimentology, petrophysics, geochemistry, reservoir geology and latest theories about the carbonate diagenesis and well logging, this paper focuses on the features and origin of the Cambrian dolomites reservoirs in Tarim Basin. 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 following is the main results in this study:(1) Three types of matrix dolomite in Cambrian sediments can be identified based on the rock textures and fabrics:micrite dolomite, relict mimetic dolomite and crystalline dolomite. The micrite dolomite is the most widely distributed in the basin. The relict mimetic dolomite occurs in the carbonate platform margin (build-up) facies in the Tahe area. The crystalline dolomite can be divided into very fine to fine-crystalline dolomite and medium to coarse-crystalline dolomite. Most of the Cambrian carbonate reservoirs are developed in the crystalline dolomite in Tarim Basin.(2) According to the integrated analyses of the sedimentology, diagenesis and geochemistry, three dolomitization models have been established:①penecontemporaneous dolomitization is responsible for the micrite dolomite and relict mimetic dolomite. The dolomitization models between Tahe and Tazhong-Bamai area are different due to the distinctive depositional environments, dolomite types and hydrodynamic forces of dolomitizing fluids. The penecontemporaneous dolomitization in Tahe area is interpreted to the seepage-reflux of Cambrian evaporative seawater as the development of the platform margin. However, the dolomite in Tazhong-Bamai area may be associated with the evaporative pumping dolomitization;②burial dolomitization:the dolomites which are related to this process include various of crystalline dolomites with different crystalline size and shape. The formation of burial dolomite can be subdivided into latent reflux dolomitization during shallow-burial diagenetic stage and geothermal convection dolomitization during the deep-burial diagenetic stage.③hydrothermal dolomitization developed extensively in Cambrian dolomite in Tarim Basin. The hydrothermal activities are closely associated with the stretching tectonic movements and volcanism in the late stage of Early Permian. Meanwhile, different kinds of the hydrothermal fluids, which were first identified in this study, influence the development of the Cambrian dolomite reservoirs.(3) The complicated diagenesis of the Cambrian dolomite plays an important role in the formation and distribution of the reservoirs in study area. The diagenesis which improved the porosity includes the karstification, dolomitization, facturing and hydrothermal dissolution. On the other hand, compaction and pressure-solution, over-dolomitization, authigenic minerals decrease the reservoir space in the Cambrian dolomite. Moreover, the dissolution of organic acid fluid in the dolomite reservoirs can be ruled out due to the detailed research on the mechanism of the organic acid reaction and the comparison with the development of secondary porosity in sandstone.(4) The pore types of the Cambrian dolomite reservoir are composed of intercrystalline, vuggy, facture (channel) and cavity pores. The intercrytalline pores are only discovered in the very fine-crystalline dolomite. In contrast, the vuggy and facture pores are considered as the most important pore types in Cambrian strata. In addition, the main types of the reservoirs also are related to the facture type, vuggy type and complex type reservoirs.(5) The origin of the Cambrian dolomite reservoirs are closely associated with the dissolution, including the karstification during the epidiagentic stages and the hydrothermal dissolution cause by the tectonic activities during burial. The dolomitization is the one of the factors improving the porosity development, but the other dissolutions are more important for the formation of the reservoir space. The karstification mainly occurred in the Tianshannan area. The influence range of the erosion was controlled by the features of paleo-geomorphology and faults in this region. On the other hand, the hydrothermal dissolution was subjected to the sources and degree of saturation of the hot water as well as the attributes and the extent of the faults in study area.The creative points in this doctoral dissertation are summarized as follows:(1) Setting up the mechanism of the hydrothermal dissolution and consanguineous precipitation of the hydrothermal dolomite.Based on the analyses of the trace elements, O/C isotopes, strontium isotopes and homogenization temperature, the detailed formation mechanism of the hydrothermal dolomite has been investigated. Because the hydrothermal fluid was not able to result in the dolomite replacement of limestone, the material source of the hydrothermal dolomitization in Tabei area probably came from the dissolution of the precursor dolomite in the Cambrian successions, in turns the previously dissolved dolomite re-precipitated from the hydrothermal fluid in the vuggy and/or facture pores. Meanwhile, it also could be proved by the similar tendencies of the isotopes and trace elements between the matrix dolomite and saddle dolomite.(2) The sources of the two kinds of hydrothermal fluids have been determined.Two distinctive kinds of hydrothermal fluids has been identified through the specifically petrographic and geochemical study. One is the crustal-source hydrothermal with the different C/O, Sr isotopes and trace elements to the matrix dolomite, and highly undersaturated with the dolomite. This fluid was not only responsible for the origin of saddle dolomite but also for the development of the dissolved pores. It is pretty important for the modification of the deep-burial dolomite reservoirs. The other hydrothermal fluid is associated with the formation water. The geochemical features are similar to the matrix dolomite except for the homogenization temperature in the fluid inclusion. This phenomenon can be interpreted as the thermal transmission during the volcanism, without the influx of hydrothermal fluids. Therefore, the dissolution ability of this hydrothermal fluid was weak and unable to form new porosity in the Cambrian dolomite.(3) The advantageous controlling factors and the new exploration targets of the Cambrian reservoirs have been pointed out.There are three exploration targets of the Cambrian dolomite reservoirs in Tarim Basin:①the reservoirs which are associated with the karstification, especially are influenced by both karst and hydrothermal dissolution in Tianshannan area, are the most vital target of the Cambrian reservoirs in the entire Tarim Basin.②the reservoirs which are developed with the fault-controlled hydrothermal dissolution can be considered as the second beneficial objective for the Cambrian exploration. Because the reservoirs are obviously influenced by the faults, especially the strike-slip and wrench faults, the TS1well region, the area between the Tazhong No.1fault belt and No.2fault belt and the western portion of the Bachu Uplift show greater importance than other areas.③the third target is the dolomite reservoirs located in the Bachu area, because they are near to the Cambrian source rock and the evaporative rocks, which can form the good cap, develop extensively in this region.