Carbonates and Evaporites最新文献

Al-Coeffiah karst terrain is located in Benghazi plain which is a part of Al-Jabal Al-Akhdar anticlinorium, northeast Libya. The exposed stratigraphic sequence of the area is largely represented by the Middle Miocene limestone rocks of Benghazi Formation. The area is affected by abundant karstic features such as caves and giant dolines with many subsurface voids and canal networks which extends for several kilometers. This research is based on field observations combined with analysis of high-resolution satellite imagery of Al-Coeffiah karst terrain. The main aim is to assess potential geohazards provided by the karst features, including (i) rock failure, (ii) groundwater pollution, and (iii) geochemical processes. The potential geohazards have been found to lies in the large caves at shallow depths such as Al-Jebah, Habibi, Merisi, and Boukarmah doline-caves. In addition, there is a greater geohazard in fractured karst terrains which are created by dolines in soft sediments. Chemical and bacteriological analyses indicate that the groundwater pollution in Al-Coeffiah area and its surrounding regions is considered as one of the most serious problems largely due to the seepage of sewage water through karst phenomena particularly during the last 10 years.

Abstract

Upper Cretaceous organic-rich carbonate rocks of the Abu Roash F (AR-F) Member represent both source rock and reservoir units and are considered as an unconventional reservoir rock at the Abu Gharadig Field (AGF), Western Desert, Egypt. This study aims to investigate the diagenetic history and its impact on the porosity and permeability of the AR-F unconventional reservoir rock using the sedimentological, petrographic, and petrophysical results for 24 core samples of the AR-F carbonate Member from well AG-121X, located in AGF, Western Desert, Egypt. The petrographic results show that the AR-F Member is composed mainly of mudstone, wackestone/packstone, and packstone microfacies. These microfacies and the richness of such marine assemblages, including planktic forams, calcispheres, pelagic bivalves, echinoderms, with rare pelecypods and ostracods suggest that the AR-F carbonate facies were deposited in mainly deep sea to deep shelf and slope marine settings. The studied AR-F carbonate facies have relatively low total porosity in the range of 0.95–2.33%, which includes mainly grain-based (intergranular and intragranular) and non-grain-based (fenestral, fracture, and dissolution) pores. The core laboratory measurements also show that the AR-F carbonate facies is impermeable rock, with values of up to 0.0039 mD. Therefore, the existence of different types of porosities and permeabilities pointed out that the reservoir quality of the AR-F carbonate facies at the AGF ranges from poor to very poor quality. This could be attributed to different stages of diagenesis, including eogenesis and mesogenesis. These deformation stages, on one hand, contributed to reducing and destroying the reservoir porosity and permeability by a series of processes like micritization, cementation, mechanical and chemical compaction, and less pronouncing pyritization and kaolinitic precipitation. On the other hand, dissolution and fracturing enhanced, in a limited way, the porosity. According to the findings of this study, the AR-F carbonate succession is suitable for further exploitation as an unconventional tight resource of oil reservoir potential.

Abstract

The Shunbei ultra-deep carbonate strike-slip fault-controlled reservoir in Tarim Basin is rich in reserves. The strike-slip faults and the natural structural fractures are the main storage space and flow channels of hydrocarbon resources. Therefore, studying the natural fracture development characteristics in this kind of reservoir is of great significance. The lithology of the Middle and Lower Ordovician strata in the Shunbei area is limestone and dolomite, including packstone, wackestone, grainstone, boundstone, lime mudstone, silicified limestone and silty-fine crystalline dolomite, medium-coarse crystalline dolomite. Based on the statistics of fracture density and brittleness index of core samples, it is found that in limestone, lime mudstone has the highest brittleness index, while boundstone has the lowest; in dolomite, the brittleness index of medium-coarse crystalline dolomite is higher than that of silty-fine crystalline dolomite. The natural structural fracture density has an obvious positive correlation with the rock brittleness index. The brittleness index of dolomite is generally higher than that of limestone. When the brittleness index is the same, dolomite has superior porosity and permeability, whereas limestone has a stronger capacity to fracture. Those kinds of highly brittle layers in carbonate reservoirs are more likely to be geological sweet spots in the Shunbei area.

The middle Jurassic Samana Suk Formation is well exposed in Himalayan foreland fold and thrust belt forming a good hydrocarbon reservoir of the Indus Basin; however, the combined sedimentological and geochemical studies are not conducted so far. An integrated approach using field, petrographic, geochemical, and isotopic studies was used to better understand the depositional and diagenetic processes within the formation. The formation is predominantly composed of thin to medium-bedded limestone with intercalation of shale. Field observations reveal sedimentary and diagenetic features such as cross bedding, sole marks, ripple marks, convolute bedding, stylolites, dissolution marks and patchy dolomitization. Microfacies associations include mudflat microfacies associations (mudstone MF-1, dolo-mudstone MF-2), lagoonal microfacies associations (siliciclastic bio-packstone MF-3, peloidal bioclastic packstone MF-4, bioclastic wackestone MF-5, and peloidal wackestone MF-6), barrier/shoal microfacies association (peloidal grainstone MF-7, ooidal–peloidal bioclastic grainstone MF-8, ooidal grainstone MF-9, and bioclastic peloidal grainstone MF-10). The above-mentioned microfacies associations suggest the deposition in the ramp settings (mudflats, lagoonal and shoal). The diagenetic features include: micritization, mechanical/chemical compaction, dissolution, neomorphism, cementation, dolomitization and fracturing. Selective replacement of grain dominated facies represents fabric retentive replacive dolomite RD-I formed at the early phase, followed by matrix replacive dolomite RD-II. Late-stage diagenetic alteration is marked by fabric-destructive dolomite RD-III. Geochemical data show a consistent decrease in salinity from the early to late diagenetic phases characterized by elevated Na and K concentration and reduced Fe and Mn concentration. Furthermore, stable isotopic data of limestone and dolomite phases show non-depleted δ¹³ C values ranging from + 0.26 to + 1.86‰ VPDB suggesting no external supply of carbon after the deposition of the carbonate units. The non-depleted δ¹⁸ O values ranging from − 1.96 to − 0.45‰ VPDB of dolomite phases represents seawater signatures, and hence may have formed in surface processes of marine water in mudflat settings/evaporitic conditions. Paleogeographically, Samana Suk Formation exhibits similar depositional conditions with the western coastline of the Tethys.

Abstract

The heterogeneity of carbonate reservoirs, influenced by sedimentary environments and diagenetic processes, leads to the development of microfractures and vugs, posing significant challenges for reservoir evaluation. This study investigates the complex pore structures of Y-type carbonate reservoirs in the Middle East. We assess the performance of classical permeability prediction models and introduce a novel approach that incorporates a bimodal pore size distribution (PSD) derived from High-Pressure Mercury Injection (HPMI) measurements, accounting for gas slip flow effects. Our results, based on CT imaging, categorize carbonate rock core samples into matrix, fracture, and vuggy. Matrix-type carbonate rocks show a strong correlation between permeability and pore-throat radius, while fracture-type carbonate rock cores exhibit weaker correlations. Traditional model permeability predictions without core categorization yield suboptimal results, with the Winland model achieving the highest R² of only 0.365. However, after categorization, classical model permeability predictions significantly improve in accuracy. Notably, the introduced bimodal Gaussian PSD model outperforms traditional permeability prediction models, with an R² of 0.8138, providing a valuable tool for predicting the permeability of carbonate rocks characterized by complex pore structures.

Karst desertification areas exhibit high spatial heterogeneity, extensive rock exposure and systems with a complex hydrological structure. Understanding runoff generation mechanisms is important for the interpretation of hydrological processes in karst critical zones. A typical karst desertification catchment in Southwest China was selected for a 2-year hydrological observation study of rainfall events, and rainfall–runoff characteristics and hydrological responses were studied. The results showed that very high rainfall infiltration occurs in the catchment, with an average runoff coefficient of 3.6%, and the runoff coefficient during most rainfall events ranges from 0.01 to 0.05%. The rainfall–runoff process is comprehensively affected by the landform characteristics, rocky desertification environment and epikarst infiltration system in the catchment. The rocky desertification hillslopes in the upper and middle reaches of the catchment are characterized by high permeability and low water-holding capacity because of the large areas of exposed rocks and fissures, which amplify the epikarst precipitation infiltration rate. Depressions in the lower reaches result in an increase in the retention time of water in the epikarst, which exhibits low permeability and high water-holding capacity. In this study, it was revealed that antecedent hydrological conditions significantly impact runoff generation processes in catchments. Runoff in the catchment mainly comprises old water (> 50%), and continuous rainfall events lead to a gradual increase in the proportion of new water.

The development of silica has played an important role in both the preservation of Ediacaran macroscopic life and the destruction of reservoir porosity and permeability. Extensive cherts of the late Ediacaran in South China were widely developed, typically in the 4th member of the Dengying Formation (refer to as D4 Formation below). Nevertheless, the characteristics and genesis of these cherts in the D4 Formation remain poorly described and constrained. Here, we have performed a careful petrographic description of siliceous rocks in the D4 Formation and geochemical modeling. It was found that (1) the silica is widely filled in primary matrix pores in the form of early-stage cement; (2) siliceous replacement of carbonate rocks can be occasionally observed under microscope, and residual carbonate inclusions exist in the cherts; (3) regular “adatom islands” structure was observed on the silica surface under scanning electron microscopy (SEM), and no luminescence was observed under cathodoluminescence (CL). The above phenomena indicate that the silica was formed in a low-temperature environment, and the solution precipitating silica was supposed to be several orders of magnitude supersaturated with respect to amorphous silica (SiO₂(am)) to replace carbonates and form the “adatom islands”. Therefore, directly nucleating silica at the ambient solution seems impossible because the aqueous silica concentration (SiO2(aq)) of late Ediacaran seawater is around 2 mmol/kg and is not supersaturated enough. Further geochemical modeling in a solution condition with organic matter (Catechol) shows that the formation and destruction of the organosilicon complex can lead to the supersaturation of SiO₂(aq) concentration with respect to SiO₂(am), and the decrease of pH value can facilitate the destruction of organosilicon. Combined with the widely developed microbialite in the D4 Formation, a comprehensive mechanism with microbial mediation for silica formation is presented here. First, organic matter on the surface of microorganisms was capable of being compounded with aqueous silica (SiO₂(aq)) in seawater, forming abundant organosilicon complexes. Later, the early degradation of organic matter in early diagenesis leads to the destruction of the organosilicon complex and the release of a large amount of SiO₂(aq) in the pore solution, a closed system, which led to the supersaturation with respect to SiO₂(am) and promotes the nucleation and growth of SiO₂(am). The degradation of organic matter also leads to the decrease of pH, which further promotes the destruction of organosilicon and leads to an unsaturated solution with respect to carbonate.

Geochemistry of the sediments deposited during the Archaean Era is potentially useful to decipher the Earth’s pristine seawater chemistry and the depositional environment. This study highlights the geochemical evolution, redox conditions, and the sedimentary environment based on carbonate sediments of the Tanwan group rocks, Bhilwara supergroup, Rajasthan, India, deposited during 2.9–2.5 Ga. The petrographic features and variation in major elements of these carbonate sediments indicate extensive dolomitization during this time. The elemental ratios such as Mg/Ca, Fe/Al, Fe/Sr, and Mn/Sr ratios and their mutual relationships suggest anoxic conditions during precipitation of the carbonates. The anoxia is well supported by the redox-sensitive element ratios such as Ni/Co, V/Cr, V/V + Ni, V/V + Cr, and Th/U, which show reducing/anoxic sedimentary environments during 2.9–2.5 Ga. The post-Archaean Australian Shale (PAAS) normalized anomalies, such as Ce anomaly, Gd anomaly, and Eu anomaly, showing average 1–1.48 values, indicate minor positive anomalies in the Archaean seawater. The rare earth elements and yttrium (REY) content in these sediments show no significant contamination, mixing, and alteration because Dissolution II (carbonate leachate) is characterized by Zr from 0.015 ppm to below detection limit (BDL), Th < 1 ppm, Sc < 1.5 ppm, and Al₂O₃ < 1%. The Y/Ho ratio > 26 represents insignificant detrital input during the precipitation of the carbonates. The Y/Ho ratio versus Nd also supports this contention. The positive anomalies for Y (i.e. Y/Ho > 26), Gd anomaly (Gd/Gd*), and La anomaly (La/La*) confirmed the preservation of anoxic pristine marine seawater signatures in these carbonates of the Bhilwara supergroup that were mainly deposited during the Neoarchaean Era.

The tectonic deformation by the Himalayan orogeny resulted in the development of sedimentary basins with multiple petroleum plays, especially in the Eocene rocks. Previous studies on the Eocene rocks broadly discusses the depositional environment, diagenesis, and outcrop features. However, the present work includes both outcrop and wireline log data to elucidate microfacies, diagenetic control, and sequence stratigraphic patterns used for reservoir characterization of the Sakesar Limestone, both at surface and sub-surface level. Field features include light to medium grey, massive, nodular, fossiliferous, fractured limestone which is mainly classified into wackestone to packstone microfacies. Several diagenetic features such as cementation, neomorphism, compaction, and calcite-filled fractures are observed and they have reduced the pore network. However, secondary dissolution in the bioclastic wackestone facies and fracturing have enhanced the pore network and its connectivity. Wireline logs also show sonic porosity of 5.3%, and 3.16% effective porosity is sufficient for carbonate reservoirs. The bulk porosity may reach up to 14%. Sequence stratigraphic study show that coarsening upward and massive carbonate beds characterized by packstone faices are interpreted as highstand systems tract in the Sakesar Limestone, which suggests the prograding depositional pattern. The outcome of this work demonstrates that potential reservoir intervals in the Sakesar Limestone are mainly fractures and solution enhanced pore spaces in the wackestone microfacies which can be targeted for enhancement for the exploration and production in these carbonate rocks .

Regional tectonic uplift information is recorded by knickpoints distributed in multiple rivers. The changing base level (mainstream) caused by tectonic uplift also controls karst development. Groundwater should drain as springs near the base level due to karstification usually occurring near the base level, but some drains were higher than the present river in the canyon. The assumption is proposed that the spring is more elevated than the current river formed at the older intermittent stage in uplift processes and that knickpoints at the elevation can correspond to the springs. A typical basin south of Daba Mountain is selected to identify the assumption. Three knickpoints with 11 karst landforms suggest that knickpoints can indicate the karst development elevation in the basin. This method is promoted to 29 basins around Daba Mountain which are separated into DTB (North Daba Mountain Pre-Land Thrust Fault Belt) representing north part and DFB (South Daba Mountain Pre-Land Fold Belt) representing south part to verify the effectiveness of the assumption. The result discovers that the knickpoints and karst landforms have a similar high-frequency elevation interval in DFB and little variance in DTB. The reason may be that DFB has better karstification conditions, including precipitation and carbonate rock cover area, than DTB. Knickpoints is an effective indicator for karst development in area that has sufficient karstification condition like DFB. This study may provide a new perspective on the relationship between karst development and tectonic uplift and a method to find the karst groundwater drain elevation.