جستجوی مقالات مرتبط با کلیدواژه « دولومیتی شدن » در نشریات گروه « مهندسی شیمی، نفت و پلیمر »

One of the key and influential factors that affect reservoir quality is the occurrence of effective diagenetic processes, including dolomitization, anhydritization, and dissolution (moldic and vuggy porosities). This research focuses on studying these processes in the Upper Dalan Member and Kangan carbonate formation located in the Central Persian Gulf. Initially, a comprehensive analysis of petrography was conducted to examine the effects of diagenesis at different depths within core samples from key well. Subsequently, well logs such as NDS, Vp/Vs, RHOB, VDL, and SPI were utilized to create electrofacies related to dolomitization, anhydritization, and dissolution using MRGC method. By applying the geostatistical kriging method to the data of electrofacies from twelve wells, three-dimensional models of dolomitization, anhydritic cementation, and dissolution processes were constructed at a field scale. Based on the sedimentary model of ramp carbonate depositional environment in the Permian-Triassic and the dolomitization model of seepage-reflux, it has been determined that in horizontal sections, the major dolomitized and anhydritic portions are predominantly observed in the southern and middle parts of the field. In the vertical sections, based on the minimum sea-level surfaces (middle K4, lower K3, upper K2, middle K2, and upper K1), the major dolomitized portions are observed in Upper K1, Upper K2, and Lower K3. The major anhydritic portions are found in Upper K1, Middle K2, and Lower K3 to the top of the boundary with K4. Considering that dissolution predominantly occurs in areas exposed to water and that most moldic and vuggy porosities are formed as a result of dissolution, dissolution is more prevalent in the southern to middle parts of horizontal sections. Consequently, there is a higher concentration of moldic and vuggy porosities in these southern to middle parts of horizontal sections. In vertical sections as well, moldic and vuggy porosities align with those seen in upper K1, upper K2, and lower K3.

The aim of the present study is to investigate the permeability changes and the ratio of horizontal to vertical permeability in Kangan and Upper Dalan carbonate reservoirs. The effect of geological parameters such as lithology, texture, facies and porosity has been investigated on the amount of horizontal and vertical permeabilities and their changes as well as the ratio of horizontal to vertical permeability in these formations in the central part of Persian Gulf. For this purpose, statistical methods have been used on the data obtained from the study of plugs and microscopic thin sections prepared from the 402 meters of cores. The results showed that the mean value of horizontal permeability is closed to the mean value of vertical permeability in the two types of dolomite and limestone lithology as well as crystalline carbonate and mudstone textures. By increasing porosity without considering other geological factors, permeabilities increase and the ratio of horizontal to vertical permeability decreases. Also, the two factors of heterogeneity and increase in the amount of grains (from mudstone to grainstone texture) are more effective in comparison with micrite and cement in the changes of permeabilities. The presence of grains increases heterogeneity. Horizontal permeability will be higher than vertical permeability in energetic sedimentary environments and in grain-supported facies. It was also observed that dolomitization increases the ratio of horizontal to vertical permeability due to increased mineralogical heterogeneity. The ratio decreases in very high amounts of this mineral (more than 90 percent) due to the homogeneity of rock texture. Therefore, this ratio depends on the degree of heterogeneity of rock, the amount of grain, and lithology. Increasing porosity and permeability, homogeneity of the sample both in terms of the type of mineral forming the rock and in terms of texture, causes closer horizontal and vertical permeabilities.

The Asmari Formation, in the Aghajari Oilfield (located in the Dezful Embayment) with about 400-meter thickness, was deposited in a homoclinal carbonate ramp and affected by various diagenetic processes. Core studies, microscopic thin sections and Scanning Electron Microscope images (SEM) from the 5 wells indicate that dolomitization is one of the most effective diagenetic processes of the reservoir quality of the Asmari Formation, and dolomitization mainly affects the upper parts of this Formation. In addition, five types of dolomites including (1) Dolomicrite (62 micron), (4) floated dolomites in the fine grain matrix, and (5) dolomite cements are recognizable. Each type of these dolomites have different effects on the trend of mercury injection curves (Pc), and therefore on the reservoir quality of the Asmari. Moreover, dolomicrospars are the most common type and have high porosity and permeability (average 16% and 35 mD respectively) due to the presence of intercrystaline porosities. Therefore, they can be saturated by mercury under low entry pressures (about 5 psi), and then show high-steep capillary pressure curves. Dolomicrites due to fine-grain nature have initially high porosity and relatively low permeability (11.5% and 4 mD respectively). But in along with dissolution, they show reservoir characteristics similar to Dolomicrospars, and their Pc curves shift to the left. Floated dolomites in matrix have no considerable role in the reservoir quality. Moreover, dolospars due to over-dolomitization and locking dolomite crystals, and dolomite cements due to disconnecting pore spaces led to decreasing in porosity and permeability. It seems zones 1 to 3 of the Asmari Formation in the Aghajari Oilfield have more potential for drilling and well-completion in the future because of more concentration of dolomicrospars and vuggy dolomicrites.

In this paper, we aim to recognize the different types of Jahrum dolomite and also to discuss the effects of dolomitization on the reservoir quality of the Jahrum formation in Bandar-Abbas region. For these purposes, we used a combination of two subsurface well sections at the Sarkhun-14 and West-Namak-01 gas-fields along with two outcrops (Finu and Khush anticlines). The lower stratigraphic intervals of the Jahrum formation were strongly altered by dolomitizing fluids. Based on petrographic observation by using polarized light, cathodoluminescence, and scanning electron microscopes, four different types of dolomite were distinguished. These data indicated that, when pervasive dolomitization occurred at the lower stratigraphic interval of Jahrum formation, this could be considered as suitable reservoir rocks and the effects of Mg-fluids on previous carbonate rocks formed abundant biomouldic porosity as a result of the dissolution of large benthic foraminifera.