Carbonates and Evaporites最新文献

Previous research shows that the origin and distribution of formation overpressure are governed by several dominant factors including disequilibrium compaction, hydrocarbon generation, tectonic compression and diagenesis, influenced by salt components and their concentration, but it is unclear how salts affect formation overpressure in saline lacustrine basins. This paper investigated the effect of salts on formation overpressure based on organic geochemistry, rock mineralogy, logging curve comparison and wave velocity-density cross-plot by combining the sedimentary and structural background. In the Western Qaidam Basin, the proportion of abnormally high pressures rises from the Neogene to the Paleogene. The top surface of the overpressure is between 2300 and 2500 m deep. As the subsidence and sedimentary centers of the basin moved eastward, the centers of the overpressure migrated from west to east. In the Upper Oligocene, the overpressure is developed in the deep and semi-deep lacustrine facies, and the pressure coefficient is 1.8–2.0. Disequilibrium compaction is the primary control factor with a contribution rate of more than 60% in the intersalt and subsalt strata, followed by tectonic compression with a contribution rate of 20–30%. Fracture reducing by salt filling and fluid volume expanding by gypsum dehydration increase the fluid volume in the formation, which promotes formation overpressure. The gypsum salt rocks also have strong plasticity and sealing effect, thus providing a closed environment for the formation overpressure. Through providing the primary migration driven force and sealing conditions for oil and gas, the overpressure is meaningful to petroleum accumulation and preservation in saline lacustrine formation of the Paleogene and Neogene in the Western Qaidam Basin, NW China.

Magnetite mineralization accompanied by minor hematite, pyrite, chalcopyrite, tetrahedrite, tennantite, and goethite, occurs in the Guydash iron skarn deposit in East Azarbaijan province, Iran. Geologically, it is located in the northwestern part of the Sanandaj–Sirjan zone. The skarn was formed by the intrusion of igneous bodies, especially porphyritic diorites, in contact with Middle-Upper Jurassic limestones and lesser Eocene pyroclastics. During skarn formation, four paragenetic stages of mineralization are distinguished: the prograde, retrograde, sulfidic and supergene stages, with magnetite deposited in the retrograde stage. Microthermometric data from fluid inclusions in calcite and quartz showed that the retrograde mineralization stage occurred at low to moderate temperatures (159.7–299.5 °C), a maximum pressure of 95 bar, and a maximum depth of 1 km. The fluids responsible for mineralization in this stage were aqueous and had low to high salinity (2–34 wt% NaCl equivalent). Fluid inclusion data indicate that the mineralizing fluid in the Guydash deposit was derived from a mixture of magmatic, meteoric, basinal, and metamorphic waters. The δ¹⁸O values in magnetite range from + 5.8 to + 10.2‰. The δ¹⁸O values of water in equilibrium with magnetite at an average homogenization temperature of 230 °C were calculated to range from -2.43‰ to 1.97‰. The O isotope values in magnetite revealed that the mineralizing fluids were mainly from magmatic waters. The δ³⁴S values in pyrite from sulfidic stage range from + 10.2 to + 12.6‰, indicating that the sulfur was supplied from seawater sulfate source. Geological, mineralogical, fluid inclusion and isotopic data suggest that the Guydash deposit is a typical calcic-type Fe skarn deposit related to the intrusion of dioritic rocks into the Jurassic limestones.

For carbonate reservoirs primarily composed of micrite particles, fractures serve as the main channel for fluid flow, determining the storage capacity and development efficiency of oil and gas. However, controlled by various formation factors, these fractures are typically of a small scale and have an irregular distribution, making them difficult to be accurately characterized and predicted by conventional methods. Therefore, this study proposed a discrete fracture network (DFN) modeling method based on radial basis interpolation, which enhanced the comprehensiveness of modeling restraint condition and significantly improved the accuracy of fracture distribution prediction. First, a model of the primary spatial distribution of fractures was derived from the difference calculation of statistical parameters of fractures based on radial basis function (RBF) interpolation. Next, an RBF-driven interpolation was programmed as an auxiliary tool for the DFN model by taking into account both stochasticity and uncertainty. The primary and secondary restraint maps used for DFN model were generated under the control of geological parameters and seismic attributes, respectively. Finally, the stochastic DFN model was sequentially modified based on different levels of constraint conditions, and its accuracy was validated using actual data. The results indicate that the DFN model with RBF is more reliable and highly consistent with the actual geological conditions and well production data.

Two stratigraphic sections are investigated, and a diverse larger benthic foraminiferal assemblage is recorded from the upper Paleocene–lower Eocene Southern Galala Formation at the Galala Plateaus, north Eastern Desert, Egypt. Twenty-eight larger foraminiferal species, belonging to thirteen genera, are identified and their comparative stratigraphic range with the Tethyan zonations of Hottinger (1960) and Serra-Kiel et al. (1998) is documented. Nine shallow benthic zones (SBZ3-6 and SBZ8-12) are designated in the studied interval, involving index zonal markers, e.g., Glomalveolina primaeva (Reichel), G. levis Hottinger, Nummulites atacicus (Leymerie), Alveolina vredenburgi Davies, and A. ellipsoidalis Schwager. Fifty-four thin sections are prepared and analyzed, yielding seven microfacies types in the present study, with larger benthic foraminifera as the dominant biotic components. These microfacies types indicate a deposition in restricted tidal flat inner ramp to open-marine middle ramp environments. The inner ramp environments are dominated by alveolinids, orbitolitids, and in part by miliolids and orthophragminids, reflecting euphotic to mesophotic, meso-oligotrophic, and normal to hyper salinity conditions. The middle ramp environment is characterized by nummulitids, implying meso-oligophotic, oligotrophic, and normal salinity conditions. The studied successions at the Galala Plateaus are devoid of corals in platform stages II and III, similar to the Pyrenean strata from middle latitudes at the northern Tethys, except for small coral patches in both stages in the latter due to the cooler temperature. The Early Eocene (Cuisian) Nummulites accumulations suggest a development on paleohighs in a distal inner ramp environment, and then a transportation by wave and current actions into the surrounding proximal middle ramp environment.