Mineralogical, Geological and Geochemical Heterogeneities of Carbonate Reservoirs

Thin layers and high-yield dolomite reservoirs were recently discovered in the Permian Guadeloupian Maokou Formation. The genetic mechanism of this reservoir is controversial because of its complex sedimentation and diagenesis in the Maokou Formation. Traditionally, the genesis has focused on sedimentation, karst, and fracture, whereas the influence of the Emeishan mantle plume activity (EMP) has been ignored. In this study, we enumerated petrographic (grouped into micritic bioclastic limestone, limy dolomite, grain dolomite, dolomite cement, calcite cement, and saddle dolomite) and geochemical data (δ¹³C, δ¹⁸O, REE, and ⁸⁷Sr/⁸⁶Sr) from a microscopic perspective to support the impact of EMP on reservoirs. We conclude that EMP activity altered the sedimentary environment and induced a complex diagenesis. The paleogeomorphic reconstruction data indicate that the EMP caused an uplift zone in the NE–SW direction, depositing advantageous high-energy beach facies. In terms of diagenesis, the abnormally high ⁸⁷Sr/⁸⁶Sr ratios and REE with positive Eu anomalies suggest that dolomitization was influenced by both seawater and hydrothermal fluids. Based on the above evidence, we established a reservoir genetic model for the Maokou Formation related to the intensity of the EMP. This study provides a new perspective on the mantle plume activity for the study of carbonate reservoir genesis. Full article

The Chiltan formation is a potential hydrocarbon-producing reservoir in the Indus Basin, Pakistan. However, its diagenetic alterations and heterogeneous behavior lead to significant challenges in accurately characterizing the reservoir and production performance. This manuscript aims to utilize six carbonate core samples of the Chiltan limestone to conduct an in-depth analysis of the diagenetic impacts on reservoir quality. The comprehensive formation evaluation was carried out through thin-section analysis, SEM-EDS, and FTIR investigation, as well as plug porosity and permeability measurements under varying stress conditions. In result, petrography revealed three microfacies of intraclastic packestone (MF1), bioclastic pelliodal packestone (MF2), and bioclastic ooidal grainstone (MF3), with distinct diagenetic features and micro-nano fossil assemblages. The MF1 microfacies consist of bioclasts, ooids, pellets, and induced calcite, while the MF2 microfacies contain micrite cemented peloids, algae, and gastropods. Although, the MF3 grainstone microfacies contains key features of bioclasts, milliods, bivalves, echinoderms, and branchiopods with intense micritization. Diagenesis has a significant impact on petrophysical properties, leading to increased reservoir heterogeneity. The specified depositional environment exposed the alteration of the Chiltan formation during distinct diagenetic phases in marine, meteoric, and burial settings. Marine diagenesis involves biogenic carbonates and micro-nano fossils, while meteoric diagenesis involves mineral dissolution, reprecipitation, secondary porosity, compaction, cementation, and stylolite formation. Pore morphology and mineralogy reveal a complex pore network within the formation, including a micro-nano pore structure, inter–intra particle, moldic, vuggy, and fenestral pores with variations in shape, connectivity, and distribution. Various carbonate mineral phases in the formation samples were analyzed, including the calcite matrix and dolomite crystals, while silica, calcite, and clay minerals were commonly observed cement types in the analysis. The core samples analyzed showed poor reservoir quality, with porosity values ranging from 2.02% to 5.31% and permeability values from 0.264 mD to 0.732 mD, with a standard deviation of 1.21. Stress sensitivity was determined using Klinkenberg-corrected permeability at increasing pore pressure conditions, which indicated around 22%–25% reduction in the measured gas permeability and 7% in Klinkenberg permeability due to increasing the net confining stress. In conclusion, the Chiltan formation possesses intricate reservoir heterogeneity and varied micropore structures caused by diagenesis and depositional settings. The formation exhibits nonuniform pore geometry and low petrophysical properties caused by the diverse depositional environment and various minerals and cement types that result in a low-quality reservoir. Stress sensitivity further decreases the permeability with varying stress levels, emphasizing the need of stress effects in reservoir management. The results of this study provide a solid foundation in reservoir characterization and quality assessment that has implications for predicting fluid flow behavior, providing insight into geological evolution and its impact on reservoir quality and leading to improving resource exploration and production strategies. Full article

Despite the discovery of high-producing natural gas reservoirs in the low-permeability dolomite-mottled limestone (DML) reservoir of the fourth Member (Ma4) of the Majiagou Formation in the Ordos Basin, the current understanding of the processes responsible for reservoir formation are still superficial, which extremely restricts the effectiveness of deep petroleum exploration and development in the basin. Therefore, this study analyzed the paragenesis process of the DML reservoir through systematic petrographic and geochemical measurements. The DML consists of burrows and matrix. The burrows are mainly filled with dolomite with a small amount of micrite, calcite cement, and solid bitumen. The matrix mainly consists of wakestone or mudstone. The DML has experienced multiple diagenetic events, including seepage-reflux dolomitization, compaction, calcite cement CaI cementation, micrite recrystallization, dissolution, hydrocarbon charging, calcite cement CaII cementation, and dolomite progressive recrystallization. Dolomitization is critical to the DML reservoir formation. The pore created by dolomitization is the hydrocarbon-migrated pathway and storage space. Due to the difference in Mg²⁺-rich fluid supply, the degree of dolomitization decreases from west to east, which causes the difference in diagenetic evolution of the western and eastern parts of the study area. The high dolomitization degree led to strong anti-compaction ability in the west, contrary to the east. Thus, the reservoir quality of the west is better than the east. Full article

In recent years, Ordos Basin has been the largest petroleum field in China and represents significant potential for gas exploration in the Lower Paleozoic carbonate reservoirs. Taking the central-eastern Ordos Basin as an example, this work studies the reservoir characteristics, origin, and controlling factors of the crystalline dolomites of the Sanshanzi Formation by means of petrological and geochemical studies. The Sanshanzi Formation dolostones comprise three kinds: crystalline dolostone with grain-shadow (Rd1), very fine–fine crystalline dolostone (Rd2), and medium-coarse crystalline dolostone (Rd3). The pore spaces include intercrystalline pores and intercrystalline solution pores, residual intergranular pores, vugs, and fractures. The medium-coarse crystalline dolostone is the best reservoir. The average porosity is 4.61%, and the average permeability is 0.91 mD. The sedimentary environment in the upper Cambrian was a large area of tidal flats. Under the influence of penecontemporaneous meteoric water leaching, dissolution developed at the top of the dolomite flat–lagoon depositional cycle, with vugs developing in the very fine–fine crystalline dolostone. Penecontemporaneous dolomitization enhanced the compaction resistance and effectively preserved the pore space of the reservoir. In addition, recrystallization improved the crystalline texture, turning the intergranular pores into residual intergranular pores and intercrystalline micropores into intercrystalline pores. In this case, the very fine–fine crystalline dolostone was transformed into medium-coarse crystalline dolostone. The results of this study provide a case for the in-depth understanding of the development and preservation mechanism of ancient dolomite reservoirs and also point to the research pathways for oil and gas exploration in the Cambrian in the Ordos Basin in the future. Full article

In the Cenomanian, the southern passive margin of the Neotethys Ocean was dominated by a giant carbonate factory. This succession is known as Sarvak Formation, a significant reservoir in Iran. This study focuses on a detailed analysis of facies variations and paleoenvironmental reconstruction, including the interpretation of the platform types, during this time interval. Based on field observations and petrographical studies, 12 facies have been recognized and ascribed to six facies belts on a carbonate ramp. Sub-environments include the outer ramp and basin (distal open marine), talus and channel (mid-ramp) and lagoon and shoal (inner-ramp). The frequency of the facies and isochore maps indicate the paleoenvironmental conditions and their spatial variations in the study area. Based on all data and analyses, the suggested conceptual model for the Sarvak Formation in the Lurestan Zone is an isolated platform surrounded by two ramps. The upwind and downwind parts of these ramps were located in the central and northern sub-zones of the Lurestan Zone. This model can be used as a template for isolated platforms worldwide. Full article