Permian Carbonate Rocks Research Articles

Cretaceous petroleum system of the Khasib and Tannuma oil reservoir, East Baghdad oil field, Iraq

Gas chromatography, palynomorph constituents, and maturation are analyzed for oil samples of the Campanian Khasib and Tannuma Formations in the wells of East Baghdad oil field for biomarker studies, while palynomorph constituents and their maturation, Rock Eval pyrolysis, total organic carbon (TOC) analysis are carried on for the Upper Jurassic and the Cretaceous Formations of core samples from the same wells for dating and evaluation of the source rocks. The gas chromatography of these oils have shown biomarkers of abundant ranges of n-alkanes of less than C22(C17–C21) with C19 and C18 peaks to suggest mainly liquid oil constituents of paraffinic hydrocarbons from marine algal source of restricted palaeoenvironments in the reservoir as well as low nonaromatic \( {\hbox{C}}_{15}^{+} \) peaks to indicate their slight degradation and water washing. Oil biomarkers of \( \Pr ./{\hbox{Ph}}{.} = {0}{.85,}{{\hbox{C}}_{31}}/{{\hbox{C}}_{30}} < 1.0 \), location is in the triangle of C27–C29 sterane, C28/C29 of 0.6 sterane, oleanane of 0.01, and CPI = 1.0, could indicate anoxic marine environment with carbonate deposition of Upper Jurassic–Early Cretaceous source. The recorded palynomorph constituents in this oil and associated water are four miospore, seven dinoflagellates, and one Tasmanite species that could confirm affinity to the Upper most Jurassic–Lower Cretaceous Chia Gara and Ratawi Formations. The recorded palynomorphs from the reservoir oil (Khasib and Tannuma Formations) are of light brown color of \( {\hbox{TAI}} = 2.8 - 3.0 \) and comparable to the mature palynomorphs that belong to Chia Gara and Lower part of Ratawi Formations. Chia Gara Formation had generated and expelled high quantity of oil hydrocarbons according their TOC weight percent of 0.5–8.5 with \({S_2} = 2.5 - 18.5\,{\hbox{mg}}\,{\hbox{Hc/g}}\;{\hbox{rock}} \), high hydrogen index of the range 150–450 mg Hc/g Rock, good petroleum potential of 4.5–23.5 mg Hc/g rock, mature (\( {\hbox{TAI}} = 2.8 - 3.0 \) and \( {\hbox{T}}\max = 428 - 443{\hbox{C}} \)), kerogen type II, and palynofacies parameters of up to 100 amorphous organic matters with algae deposited in dysoxic–anoxic to suboxic–anoxic basin, while the palynomorphs of the rocks of Khasib Formation are of amber yellow color of TAI = 2.0 with low TOC and hence not generated hydrocarbons. But, this last formation could be considered as oil reservoir only according their high porosity (15–23%) and permeability (20–45 mD) carbonate rocks with structural anticline closure trending NW-SE. That oil have generated and expelled during two phases; the first is during Early Palaeogene that accumulated in traps of the Cretaceous structural deformation, while the second is during Late Neogene’s.

Geochemical characteristics of the oceanic island-type volcanic rocks in the Chiang Mai zone, northern Thailand

The oceanic island volcanic rocks in the Chiang Mai zone, northern Thailand, are usually covered by Lower Carboniferous and Upper Permian shallow-water carbonate rocks, with the Hawaii rocks and potash trachybasalt being the main rock types. The alkaline series is dominant with sub-alkaline series occurring in few cases. The geochemical characteristics are described as follows: the major chemical compositions are characterized by high TiO2, high P2O5 and medium K2O; the rare-earth elements are characterized by right-inclined strong LREE-enrichment patterns; the trace element patterns are of the upward-bulging K-Ti enrichment type; multi-component plots falling within the fields of oceanic island basalts and alkali basalts, belonging to the oceanic island-type volcanic rocks, which are similar to the equivalents in Deqin and Gengma (the Changning-Menglian zone) of Yunnan Province, China.

EOR Field Experiences in Carbonate Reservoirs in the United States

Summary A considerable portion of the world's hydrocarbon endowment is in carbonate reservoirs. Carbonate reservoirs usually exhibit low porosity and may be fractured. These two characteristics along with oil-to-mixed wet rock properties usually result in lowered hydrocarbon recovery rates. When enhanced oil recovery (EOR) strategies are pursued, the injected fluids will likely flow through the fracture network and bypass the oil in the rock matrix. The high permeability in the fracture network and the low equivalent porous volume result in early breakthrough of the injected fluids. Infill drilling programs and well conformance strategies—mostly gas and water shutoff—have been effectively used to mitigate the early breakthrough and increase oil recovery. In most cases, however, 40 to 50% of the original oil in place (OOIP) is not produced. A large number of EOR field projects in carbonate reservoirs have been referenced in the literature since the early 1970s. These field projects demonstrate the technical feasibility of various EOR methods in carbonate reservoirs. However, because of the collapse in oil prices, most of the aforementioned project plans have been abandoned. This paper presents a comprehensive compilation of EOR (Gas, Chemical, and Thermal methods) field experiences in carbonate reservoirs within the US, as an attempt to identify key variables and project design parameters for future evaluation and revitalization of mature carbonate reservoirs. Carbon dioxide flooding [continuous or water-alternating gas (WAG)] is the dominant EOR process used in the US This is because of the high availability of low-cost CO2. CO2 EOR in particular represents the logical first step towards viable geologic carbon storage and sequestration. EOR chemical methods in carbonate reservoirs, especially polymer flooding, have been widely tested in US carbonate reservoirs. However, EOR chemical methods have made a marginal contribution, relatively, in terms of total oil recovered. Our study includes a brief overview of current laboratory (e.g. wettability changes and novel chemical additives) and field (e.g. injectivity enhancement) experiences in EOR chemical methods in carbonate formations. A brief discussion surrounding the screening methods used to identify viable EOR opportunities in carbonate fields based on past and present experiences is also included. Introduction Carbonate reservoirs are naturally-fractured geologic formations characterized by heterogeneous porosity and permeability distributions. In the case of low porosity and low permeability carbonate rocks (more specifically rock matrices), the fluid flow in the reservoir can be completely dependent on the fracture network while the matrix only plays a source role (analogous to tight sand formations and natural gas flow). In the case of porous carbonate rocks, fracture networks can cause uneven sweeping of the reservoir, leading to early breakthrough of injected fluids in the producing wells and resulting in low recovery factors. The abundance of carbonate reservoirs has been the subject of numerous studies attempting to characterize their heterogeneities, classify different types of fractured reservoirs, and determine how rock and fluid properties have an impact on ultimate recovery (Roehl et al. 1985; Allan and Qing Sun 2003; Carr et al. 2001; Grave et al. 2000; Benson et al. 1998; Wardlaw 1996). The TORIS database (maintained by the US Department of Energy) indicates 22% of the OOIP in the US is contained in shallow-shelf carbonate reservoirs. Currently in the US, these types of reservoirs exist in more than 14 states with over 70% of the OOIP located in reservoirs in Texas and New Mexico, mostly concentrated in the Permian Basin (Nuckols 1992, Xie et al. 2005). Over the last three decades, primary production, waterflooding, and CO2 floods, combined with infill drilling programs, have been the most widely used recovery methods. However, other EOR strategies have been tested in the past and there are currently several active research programs seeking alternatives to increase the recovery factor of these mostly light crude-oil reservoirs economically. Although the Permian Basin (west Texas and southeast New Mexico) can be considered mature, its potential for improved oil recovery is still very high. A recent study reports that there is an estimated 30 billion barrels of mobile oil in the Permian Basin, reiterating the strategic importance of EOR technologies for carbonate reservoirs and their impact on US oil production (Nuckols 1992; Xie et al. 2005; Seethepalli et al. 2004; Moritis 2004; Cole 2003; Moritis 2003; Dutton et al. 2004).

Formation Mechanism and Controlling Factors of Natural Gas Reservoirs of the Jialingjiang Formation in the East Sichuan Basin

Abstract: The Lower Triassic Jialingjiang Formation reservoirs are distributed widely in the East Sichuan Basin, which are composed mainly of fractured reservoirs. However, natural gas with high concentration of H2S, ranging from 4% to 7%, was discovered in the Wolonghe Gas pool consisting primarily of porous reservoirs, while the other over 20 fractured gas reservoirs have comparatively low, tiny and even no H2S within natural gases. Researches have proved the H2S of the above reservoirs are all from the TSR origin. Most of the Jialingjiang Formation natural gases are mainly generated from Lower Permian carbonate rocks, the Wolonghe gas pool's natural gases are from the Upper Permian Longtan Formation, and the natural gases of the Huangcaoxia and Fuchengzhai gas pools are all from Lower Silurian mudstone. The formation of H2S is controlled by the characteristics and temperature of reservoirs, and is not necessarily related with gas sources. The Jialingjiang Formation in East Sichuan is buried deeply and its reservoir temperature has ever attained the condition of the TSR reaction. Due to poor reservoir potential, most of the gas pools do not have enough room for hydrocarbon reaction except for the Wolonghe gas pool, and thus natural gases with high H2S concentration are difficult to be generated abundantly. The south part of East Sichuan did not generate natural gases with high H2S concentration because the reservoir was buried relatively shallow, and did not suffer high temperature. Hence, while predicting the distribution of H2S, the characteristics and temperature of reservoirs are the necessary factors to be considerd besides the existence of anhydrite.

Estimating permeability of carbonate rocks from porosity and vp ∕ vs

We present a method for predicting permeability from sonic and density data. The method removes the porosity effect on the ratio [Formula: see text] of dry rock, and it addresses the specific surface as an indirect measure of permeability. We look at ultrasonic data, porosity, and the permeability of 114 carbonate core plugs. In doing so, we establish an empirical relationship between the specific surface of the solid phase (as calculated by Kozeny’s equation) and [Formula: see text] (linearly transformed to remove the porosity effect). One must view the specific surface derived by using Kozeny’s equation as an effective specific surface because Kozeny’s equation only holds for homogeneous rock with interconnected pores. The ratio [Formula: see text] of dry rocks, on the other hand, seems to be controlled by the true specific surface, pointing to an inherent limitation in the method. The 114 carbonate plugs originate in three geological settings and comprise 83 calcitic and 31 dolomitic samples. Their depositional texture varies from mud-dominated to grain-dominated and recrystallized types. Our research applies the relationship to 137 carbonate samples from two different depositional settings. We find a reasonable match between predicted and measured permeability. The match is better for samples with carbonate mud-filled depositional textures than for carbonate mud-poor depositional textures. Diagenetic factors such as vuggy porosity decrease the predictability of permeability.

The eastern flank of the Caspian Basin: Sedimentary complexes and sedimentation conditions in the Early-Middle Carboniferous

Carboniferous and Lower Permian Carbonate and terrigenous rocks with the total thickness of >4000 m serve as the productive units in the Paleozoic subsalt complex at the eastern flank of the basin surrounding the northern area of the present-day Caspian Sea (hereafter, Caspian Basin in the broad sense). In recent years, several large oil and gas-condensate fields were discovered in these rocks. The complexity of geological evolution of this region, which is situated at the junction between the East European Platform and the Ural orogen, as well as multiple changes of sedimentation conditions during the Middle and Late Paleozoic, are reflected in the diversity of types of terrigenous and carbonate sediments and their facies alterations. Reconstruction of these environments makes it possible to elucidate specific features of the location of reservoir rocks in vertical and horizontal sections, as well as regularities of variations in their filtration-capacitive properties.

The first finding of carbonatite tuffs in Cenozoic basaltic volcano of Southeastern Primorye

The origin of carbonatites and carbonatite-type rocks remains a debatable petrologic problem. Therefore, any information on findings of these rocks is valuable for the better understanding of key mechanisms of their formation. We found carbonatite tuffs among eruption products of the early Neogene Amba Volcano in southwestern Primorye (Fig. 1). The volcanic edifice occupies an area of ~20 km 2 in the middle reaches of the Amba River at a distance of 28 km from Amur Bay. The basalts of the stratovolcano base overlie the Permian carbonate and volcanosedimentary rocks and the Triassic arkose sandstones. The basalts are, in turn, overlapped by the upper Miocene volcanosedimentary rocks of the Ust-Suifun Formation and basalts of the Shufan Plateau. The central vent of the Amba Volcano is filled with explosion breccia and basaltic lavas. Pyroclastic rocks, including layers of carbonatite tuffs, compose the peripheral facies of the volcano. The eruption is characterized by a sequential transition from olivine‐clinopyroxene to spinel‐Ti-augite‐ plagioclase varieties. Spinel and clinopyroxene megacrysts (3‐8 mm across) in basalts contain rounded carbonate inclusions. In addition, the basalt groundmass contains numerous late carbonate segregations in gas cavities. The segregations are especially abundant in the vent facies, indicating predominance of carbon dioxide among volcanic gases. The basalts are characterized by subalkali, high-Ti, high-Mg, and high-Ca composition with low content of silica, high contents of Sr, Ba, Nb, and Zr, and less common Ce, Nd, Sm, and Th (Table 1). According to their HFSE and REE contents, they correspond to withinplate alkali basalts with negative slope of PM-normalized REE pattern. The carbonatite tuff layers were found in the southwestern part of the volcanic edifice on the right wall of the Vtoroi Creek, the right tributary of the Amba River. The welded carbonatite tuffs occur as two layers (2.0‐ 2.5 m thick) at the base and top of the visible section. They are separated by interlayers of tuffaceous basaltic sandstones and gravelstones with a variable carbonate admixture. The tuffs are dark gray massive, severely welded rocks with psammitic and psephitic dimensions of crystal fragments and lithic clasts. The high degree of tuff welding is related to enrichment of the natural carbonate‐silicate cement in calcareous component. Tuffs contain fragments of tree trunks and xenoliths of intrusive and sedimentary rocks from the basement. The geological observations indicate that the bedded volcaniclastic member of carbonatite tuffs was formed as a result of multiple directed volcanic explosions with formation of gas-saturated pyroclastic flows deposited in depressions (often waterlogged) around the volcano. The welded tuffs in the section are supplemented with tufflavas represented by more compact carbonate‐ silicate aggregates (20‐25 cm in size) with large spinel and clinopyroxene phenocrysts. Walls of numerous gas cavities and pores in this rock are coated with pale crystalline carbonate. The carbonatite tuffs are composed of crystal and lithic clasts. The white or light gray lithic clasts (0.2‐ 0.5 cm in diameter) have irregular or rounded (ocellar) shape. They are largely composed of carbonate material with spinel and pyroxene phenocrysts and plagioclase laths (Figs. 2, 3). The lithic clasts have an unusually high content (up to 3‐5 vol %) of rather large (up to 5‐ 12 mm) crystals of aluminous spinel and their fragments. The fassaite clinopyroxene is less abundant (Table 2). They also contain relatively large (up to 0.5 cm) orange (less frequently, transparent white or cherrycolored) euhedral and rhombohedral Fe‐Mg-calcite crystals incorporated into the cement (Fig. 3, Table 2). Crystals of spinel (with faces of dissolution), clinopyroxene, and calcite occur in both the carbonatite clasts

The Effect of the Seawater Intrusion on the Robinzon Coastal Spring

The Robinzon Spring, located 70 m from the seacoast, is a freshwater,perennial spring with a yield ranging from the maximum 2 m3/s to theminimum 0.165 m3/s. The spring occurs at the contact between permeablecarbonate rocks and impermeable flysch deposits. This contactis deeply weathered, eroded and submerged below sea level. Suchconditions emphasize the delicate relationship between the fresh- andseawater. The objective of the study was to shed more light on thehydrogeological setting of the spring’s underground recharge area bymeans of borehole measurements and preexisting knowledge aboutthe dynamics of the fresh- and seawater acquired from similar cases.The spring’s underground area is divided into three zones: (i) zone ofgood water circulation (rockfall material and fractured dolomite), (ii)zone of poor water circulation (massive dolomites), and (iii) zone of‘trapped’ water (contact between the dolomite and flysch deposits).

Passive Treatment of Mn-Rich Mine Water: Using Fluorescence to Observe Microbiological Activity

Conventionally, limestones have been used in passive mine water treatment systems. Limestones with the highest proportion of calcite are recommended since they have the greatest long-term alkalinity generating potential. Manganese is present in mine waters and needs to be removed in order to comply with environmental quality standards. This paper compares seven different Permian carbonate rocks, both limestone and dolomite, in their ability to promote manganese oxidation in real mine waters over an 8-h period. The substrates are characterised using thermogravimetric analysis, X-Ray diffraction and scanning electron microscopy. Fluorescence spectrophotometry is used to monitor any changes in the dissolved organic matter concentration in the water as manganese is removed. We determine that there is no statistically significant correlation between manganese removal and the proportion of calcite or between manganese removal and substrate surface roughness. Fluorescence spectrophotometry demonstrates that there is a distinct change in the observed spectra in the water during manganese removal. There is a positive and statistically significant correlation between manganese removal and the production of a tyrosine-like substance (up to ∼150 ppb in 8 h), which fluoresces at 270–280 nm excitation wavelength and 300–310 nm emission wavelength, suggesting that microbial activity is an important factor in promoting manganese removal within dolomite passive treatment systems. It may be possible to use fluorescence spectrophotometry to monitor for microbial activity in passive treatment systems.

Carbon isotope variability and sedimentology of the Upper Permian carbonate rocks and changes across the Permian-Triassic boundary in the Masore section (Western Slovenia)

Carbonate and total organic carbon stable isotope analyses of the Upper Permian and Lower Triassic succession in the Masore section in western Slovenia indicate a high storage of organic matter during the Upper Permian, as well as the well known worldwide “light carbon isotope event” across the P/Tr boundary. The perturbations in the global carbon cycle observed in the investigated section span an approximately 50 cm thick interval (from −11 cm below to +41 cm above the lithostratigraphically determined P/Tr boundary), and coincide more or less with changes in lithology, as well as with an abrupt disappearance of Upper Permian marine fauna. In this section changes in the sedimentary environment are most probably related to Upper Permian—Lower Triassic sea level changes. The carbonate and organic carbon negative peak anomaly could be explained by accelerated changes in the end Permian carbon cycle, due to some co-occurring events, such as pronounced erosion and oxidation of organic carbon, a possible release of methane from stored hydrates, and volcanic activity, as well as by a sudden reduction in primary productivity triggered by not yet completely satisfactorily explained mechanisms.

Carbon isotope compositions of the Late Permian carbonate rocks in southern China: their variations between the Wujiaping and Changxing formations

The carbon isotope compositions of the Upper Permian marine carbonate rocks in southern and eastern Guizhou and central Guangxi of southern China have been investigated. The results reveal a δ 13C anomaly at the boundary between the Wujiaping and Changxing Formations of the Upper Permian. The δ 13C values in limestones of the Wujiaping formation are mostly within the range of 3.5 and 5.0‰, whereas the δ 13C values in limestones of the Changxing Formation are mostly less than 3.0‰, ranging from 0.2 to 3.1‰. The carbon isotope negative excursion at the boundary between these two formations is helpful to differentiate the two stages (Wuchiapian and Changxingian) of the Late Permian in southern China if substantiated by sufficient fossil materials. This also means that the δ 13C values of the carbonates were decreasing throughout the Late Permian, and this change was accompanied by some rapid falls, such as those in the Permian–Triassic boundary and possibly in the Wujiapingian and Changxingian boundary. During the Late Permian, the consumption of organic carbon by the oxidation of the previously formed coals, as a result of dry climates and tectonic uplifting, must have exceeded the burial of organic carbon (as in coal). In addition, lack of coal deposits during the late Tatarian on the Gondwana fragments suggests a global reduced coal accumulation. Consequently, more 12C-rich CO 2 was released into the atmosphere, which might contribute to the δ 13C depletion seen in Upper Permian marine carbonates.

ABSTRACT: Carboniferous-Permian (Late Paleozoic) Hydrocarbon System, Rocky Mountains, Great Basin U.S. Region -- Major Historic Exploration Objective

ABSTRACT Carboniferous-Permian rocks make up an important closely-related regional hydrocarbon source-reservoir System covering most basins of the Rocky Mountain and Great Basin Region. The System includes oil and gas accumulations in the Williston Basin and Sweetgrass arch areas in Montana and North Dakota: the Bighorn, Wind River, and Powder River basins of Wyoming; the Uinta, Sand Wash, and Piceance basins of northeast Utah and northwest Colorado; the Paradox Basin of the Four Corners area; and the basins of east-central Nevada. More than 400 hydrocarbon accumulations, at least 20 greater than 100 MM barrels each, are present, includ­ing Rangely, the largest Rocky Mountain field, and the central Utah partly-eroded Tar Sand Triangle stratigraphically trapped heavy oil and tar accumulation, arguably the largest (proba­bly 30 billion barrels oil equivalent or more) original in-place hydrocarbon accumulation of the United States. Regional and local detailed palinspastic, stratigraphic, paleotectonic source rock burial depths, and maturity projections aid in geologic analyses of these extensive hydrocarbon deposits. The estimated volume of total original hydrocarbons generated and accumulated in these Late Paleozoic rocks elevates the importance of this System to a major position in the Rocky Mountains-Great Basin oil and gas province.

Megaclasts from the Carboniferous‐Permian Rocks in the Langkawi Islands, Malaysia, and Their Implications

The Ninth Regional Congress on Geology, Mineral andEnergy Resources of Southeast Asia was held in KualaLumpur of Malaysia in August of 1998. After the Con-gress, the author participated in the geological fieldexcursion and investigation in the Langkawi Islands,and noted that the Carboniferous-Permian rocks con-tain megaclasts of metamorphic and pegmatitic rocks,which indicate the existence of pre-Carboniferousmetamorphism and magmatism. The observation resultsare as follows:

The Effect of Asphaltene Precipitation on Carbonate-Rock Permeability: An Experimental and Numerical Approach

SummaryA series of experiments was carried out to determine the effect of asphaltene precipitation on the intrinsic permeability of carbonate rocks. Experiments indicated the existence of three different flow regimes, namely monotonous steady state, quasisteady state, and continuous plugging. The numerical model helped identify mech- anisms involved in these flow regimes. The existence of a contin- uous plugging regime at a high flow rate is of particular concern in the petroleum industry because of increased probability of plugging near the wellbore. The numerical model couples the surface excess theory for adsorption with the mechanical entrapment theory. The mathematical model accurately predicts permeability damage from mechanical trapping and adsorption. The model predictions agree favorably with experimental results for different flow rates.

Fracture control of regional ground-water flow in a carbonate aquifer in a semi-arid region

We integrate fracture mapping and numerical modeling to assess the role of fractures in regional round-water flow. Although the importance of fractures in ground-water flow and solute transport is accepted generally, few studies have addressed quantitatively the regional hydrogeological implications of fractures. The field-study area in west Texas and southeastern New Mexico consists primarily of subhorizontal Permian carbonate rocks cut by extensional faults and fractures. Air-photo analysis and field mapping reveal a broad fracture zone extending from the Sacramento Mountains of New Mexico to the Salt Basin near Dell City, Texas. Most fractures are subparallel to major normal faults. The most intense fracturing coincides with a prominent trough in the potentiometric surface and an apparent “plume” of relatively fresh ground water. Flow models, corroborated by geochemical data, indicate that fracturing has created a high-permeability zone that funnels recharge from the Sacramento Mountains at least 80 km southeastward to its discharge zone. A steady-state finite-element flow model uses fracture data to predict the spatial transmissivity distribution. Given the probable range of recharge, discharge, and other hydrologic parameters, fractures are the most important factor affecting the potentiometric surface configuration. Our study implies that: (1) fractures can control ground-water flow over large (>1000 km 2 ) areas; (2) effective recharge areas and regional ground-water chemistry trends are strongly influenced by fractures; and (3) a priori inferences about aquifer properties and regional flow are possible by means of fracture studies. This study demonstrates that the timing and nature of fracturing can affect regional subsurface fluid flow, as well as related processes such as hydrothermal mineralization, diagenesis, and hydrocarbon transport and entrapment.

Authigenic potassium feldspar: a tracer for the timing of palaeofluid flow in carbonate rocks, Northern Calcareous Alps, Austria

Abstract Feldspar is a common authigenic constituent in Permian carbonate rocks which occur as tectonically isolated blocks within the evaporitic Haselgebirge mélange in the Northern Calcareous Alps (NCA). Coexisting with pyrite, anhydrite, (saddle) dolomite, magnesite, fluorite and calcite, K-feldspar and minor albite record an event of regionally extensive interaction of hot brines with carbonate rocks. Detailed petrographic, crystallographic and geochemical studies reveal a variability in crystal size and shape, Al-Si ordering, elemental and stable isotopic compositions of the K-feldspar, which is only partially consistent with the traditional view of authigenic feldspar as a well-ordered, compositionally pure mineral. 40 Ar- 39 Ar step-heating measurements of authigenic potassium feldspar from several localities yield two age populations, an older one of 145–154 Ma, and a younger one of c. 90–97 Ma. Most age spectra reflect cooling through the argon retention temperature interval, which was rapid in some localities (as indicated by plateau ages) and slower in others. Rb-Sr isotope data are more difficult to interpret, because in many K-feldspar samples they are controlled largely by Sr-bearing inclusions. The Jurassic 40 Ar- 39 Ar dates are interpreted as minimum ages of feldspar growth and hence imply that fluid-rock interaction is likely to be simultaneous with or to slightly predate mélange formation. Deformation associated with the closure and subduction of the Meliata-Hallstatt ocean south of the NCA during the Upper Jurassic is regarded as the principal geodynamic driving force for both enhanced fluid circulation and mélange formation. Some localities were reheated beyond the argon retention temperature for microcline during mid-Cretaceous nappe stacking of the NCA, thus obliterating the older signal.

Hydrogeological Exploration of the Rjecina River Spring in the Dinaric Karst.

The Rjecina spring is one of the major springs in the Dinaric Karst. It appears as the contact between permeable carbonate and impermeable clastic rocks, with a discharge of up to 120 m3/s but it dries up during the dry summer seasons. The spring occurs close to the town of Rijeka, 325 m above sea level and offers an outstanding opportunity to cover gravitationally the public water demand of a town of about 200,000 inhabitants, and the touristic needs of the whole region. This hydrogeological research project is a pan of efforts to solve the problems of water deficiency during the dry summer seasons up to a maximum of three months. It was necessary to enter the parts of a karst aquifer that are active even in time of any outflow from the Rjecina spring by complex geological, hydrogeological and geophysical exploration accompanied with deep exploratory boreholes. During earlier exploration, it was determined that there are no active innows in the immediate hinterland of the spring and that it is necessary to discover the inflows from other karst structures, that behave as retentions of karst springs in the zones of permanent discharge. The presence of multiple overthrusted structures in the zone around the spring site suggest the existence of deep zones of water retention, which may be reached by an access gallery from the Rjecina canyon. This work represents a substantial change in the exploration methodology for Dinaric Karst aquifers, because it directs the researchers toward deep, unknown retention spaces which contain large reserves of high-quality groundwater outside urban areas.

Dynamics of O re‐Forming Processes of the Stratabound Skarn Copper Deposits of Tongling, Anhui Province

Abstract The skarn and ore bodies of the stratabound skarn copper deposits of Tongling, Anhui Province, are both controlled by definite stratigraphic horizons, and they are concordant with the strata. They occur as layers and layer‐like bodies in permeable carbonate rocks of the Middle‐Upper Carboniferous Huanglong and Chuanshan Formations which are underlain by impermeable shale or siliceous rocks of the Upper Devonian Wutong Formation. The authors study the dynamics of ore‐forming processes of the ore deposits with the dynamic model of coupled transport and reaction, and the following results are obtained: The salinity gradient and flow rate of the ore‐forming fluids can both promote the mixing and reaction of juvenile water and formation water, and the permeable strata are favourable sites for the intense transport‐reaction of mixing and the formation of deposits. (2) As isothermal transport‐reaction took place along the bedding of strata, the moving transport‐reaction front formed at the contact between the ore‐forming fluids and the rocks advanced slowly along the permeable strata, and then stratiform skarn and ore bodies concordant with the strata were formed. (3) The gradient transport‐reaction taking place across the isotherms in the cross‐bedding direction caused the mineralogical composition to alter gradually from magnesian skarn to sulphide ore bodies.

Fluid inclusions in the Xichang Basin and their applications in oil-gas exploration

Fluid inclusions are commonly observed in various stages during the diagenetic evolution and the generation, migration and accumulation of hydrocarbons in Lower Permian carbonate rocks in the Xichang Basin, providing direct physico-chemical evidence for the process. Altogether seven stages can be recognized, i.e., the penecontemporaneous, early and mid-late diagenetic stages, the epidiagenetic stage and the early, middle and late reburying stages. Karst and dissolution pores and structural fissures developed during the epidiagenetic and reburying stages constitute the major reservoirs for hydrocarbon accumulation. Oil generation and migration began in the early reburying stage in Late Permian time and developed to its climax in the middle reburying stage in the Triassic period, followed by extensive generation and migration of natural gases during the late stage of reburying in the Jurassic and later periods. The generation, migration and accumulation of hydrocarbons are closely related to Hercynian basalt eruption and Indosinian and Yenshanian tecto-magmatism.

Adsorption of organic compounds on carbonate minerals: 3. Influence on dissolution rates

This is the third in a series of papers to investigate the role of organic compounds on wettability, porosity and permeability of carbonate rocks. These are important reservoir characteristics that influence the performance of petroleum reservoirs. This paper reports dissolution studies to address diagenetic effects on carbonate porosity and permeability. Comparative dissolution measurements were performed to assess the influence of adsorbed organic material on the reactivity of calcite and dolomite in the geologic environment. Organic compounds that adsorb strongly on carbonate minerals (fatty acids and carboxylated polymers) were found to dramatically inhibit dissolution, while weak adsorbates and nonadsorbates showed little or no effect on the dissolution rates. Experiments were performed at 20°, 50° and 80°C to examine the role of organic adsorbates at reservoir temperatures, and the strong adsorbates inhibited dissolution at all temperatures. Experiments from various initial pH's confirmed that strong adsorption is an ionic interaction and demonstrated that the inhibition of dissolution is limited to pH's above 4.5–5.0