Late Diagenetic Stage Research Articles

Origin of quartz cement in the Lower Ordovician Dongjeom formation, Korea

This study deals with the origin of quartz cement developed in deeply buried mature sandstones intercalated in a thick carbonate sequence. The Dongjeom Formation (Early Ordovician), Korea, is composed of three coarsening-upward sequences and is divided into two facies according to depositional conditions with respect to changes in relative sea-level: transgressive facies and regressive facies. The transgressive facies are characterized by calcite-cemented allochem-bearing sandstones, whereas the regressive facies is characterized by non-calcareous quartzose to arkosic sandstones. Quartz overgrowths occupy significant amounts of pore space (average 16%) in the regressive facies sandstones. Fluid inclusions reveal that quartz overgrowths precipitated between 110 and 165 °C. Petrographical study and homogenization temperatures indicate that quartz overgrowths precipitated mostly during and after chemical compaction during the late diagenetic stage. Considering the δ 18 O values of detrital quartz grains and the volumes of quartz overgrowth, the δ 18 O values of quartz overgrowth are estimated to range from +14.4 to +19.0‰ SMOW on the basis of mass balance. Silica for quartz overgrowths was principally derived from chemical compaction (pressure solution) of detrital quartz. Quartz overgrowths in the Dongjeom sandstones precipitated from formation waters (−2.1 to +2.5‰ SMOW) that evolved from contemporary meteoric waters by diffusion.

Smectite‐Zeolite Envelope Surrounding the Tsukiyoshi Uranium Deposit, Central Japan: A Natural Analogue Study

Abstract. The Tsukiyoshi uranium deposit in Gifu Prefecture is the largest one in Japan. It is embedded in lower part of the Mizunami Group of Miocene age. Relating to the existence of this uranium deposit, the constituent minerals in sediments were studied by XRD and SEM, using many drilling cores. The most abundant authigenic mineral is smectite. The amount of smectite increases generally from upper to lower horizons, and a highly smectitized zone is situated around the uranium deposit. Smectitization predominated in mafic glassy grains of sediments, which was probably formed in early burial diagenesis. Zeolites including clinoptilolite‐heulandite, mordenite, analcime, chabazite and philipsite are secondly abundant authigenic minerals. They seem to have been formed at early to late diagenetic stages. Opaline silica is rather rare. Carbonate minerals, including cal‐cite, dolomite, siderite and rhodochrosite are common. They may be formed by diagenesis as well. Gypsum and pyrite occur in upper horizons and lower horizons, respectively. In particular, a highly smectitized zone including pyrite probably played an important role for retarding the migration of uranium and as a result keeping the uranium deposit for past one million years. This smectite‐zeolite envelope surrounding the Tsukiyoshi uranium deposit is regarded as a natural analogue of the buffer materials surrounding the high‐level radioactive waste repository.

Isotopic Composition (δ 13C, δ 18O) and Origin of Manganese Carbonate Ores from the Early Oligocene Deposits, the Eastern Paratethys

Abstract Carbon and oxygen isotopic composition of the manganese carbonate ores and rocks was studied by the author in the Early Oligocene deposits and ore occurrences in the Ukraine, Georgia, and Kazakhstan. The δ 13C and δ 18O values were registered in the Nikopol (respectively, –24.6…–4.9‰ and 26.4…31.8‰), Mangyshlak (–32.9… –1.4‰ and 15.8…30.9‰), and Chiatura (–34.5…–8.3‰ and 18.9…30.5‰) deposits. Their distribution offers to distinguish two genetic types: the first one was formed at the early diagenetic stage and the second one – was typical of sediments at the late diagenetic (catagenetic) stage. Formation of manganese carbonates at the late diagenetic (catagenetic) stage (the Chiatura deposit) was generally a multistage process resulting from discharge of catagenetic waters of different nature.

Kinetic modeling of diagenesis of Eogene lacustrine sandstone reservoirs in the Jianghan Basin, southeastern China

In the Tuoshi oilfield, located in the Cenozoic Jianghan Basin of southeastern China, there have been found hydrocarbon reservoirs hosted in lacustrine sandstones of the Eogene Xingouzui Formation. The main diagenetic features identified in these sandstones include the dissolution of detrital K-feldspar and albite grains, the precipitation of quartz as overgrowths and/or cements, and the precipitation and/or transformation of clay minerals. These diagenetic features were interpreted to have occurred in early, intermediate and late stages, based on the burial depth. The kinetics of fluid-mineral reactions and the concentrations of aqueous species at each stage of diagenesis were simulated numerically for these lacustrine sandstones, using a quasi-stationary state approximation that incorporates simultaneous chemical reactions in a time-space continuum. During the early diagenetic stage, pore fluid was weakly acidic, which resulted in dissolution of K-feldspar and albite and, therefore, led to the release of K+, Na+, Al3+ and SiO2(aq) into the diagenetic fluid. The increased K+, Na+, Al3+ and SiO2(aq) concentrations in the diagenetic fluid caused the precipitation of quartz, kaolinite and illite. At the beginning of the intermediate diagenetic stage the concentration of H+ was built up due to the decomposition of organic matter, which was responsible for further dissolution of K-feldspar and albite and precipitation of quartz, kaolinite, and illite. During the late diagenetic stage, the pore fluid was weakly alkaline, K-feldspar became stable and was precipitated with quartz and clay minerals. When the burial depth was greater than 3000 m, the pore fluids became supersaturated with respect to albite, but undersaturated with respect to quartz, resulting in the precipitation of albite and the dissolution of quartz. The diagenetic reactions forecasted in the numerical modeling closely matched the diagenetic features identified by petrographic examination, and therefore, can help us to gain a better understanding of the diagenetic processes and associated porosity evolution in sandstone reservoirs.

Microtextural Characteristics and Origin of Dolomites in the Tepearasi Formation, SW of Beysehir‐Konya, Turkey

Abstract The Tepearasi Formation of the autochthonous Geyikdagi Group in the Central Tauride Belt, SE of Beysehir, is Dogger in age and consists dominantly of massive limestones and greyish dolomites occurring within the middle to upper sections. The total thickness of the dolomitic levels ranges from 100–300 m and laterally extends 500–700 m. Three types of dolomite were distinguished through petrographic analyses: homogeneous, mottled (saddle‐crystalline) and joint‐filling dolomite, which were interpreted to have formed in two different stages, early diagenetic and late diagenetic. The homogeneous dolomite of the early diagenetic stage is light‐coloured and monotonous‐textured and shows the form of a dolosparite mosaic. The mottled dolomite formed in the late diagenetic stage is light‐ to dark‐coloured and coarsely granular idiomorphic. The other type of late diagenetic dolomite, described as the joint‐filling type, presents a crystal growth pattern from the joint walls towards the centre of the joint space. It is associated with coarse calcite crystals as well as primary dolomite crystal clasts which were formed in the early diagenetic stage. In addition to these characteristics, cataclastic texture indicating the influence of tectonism is also observed. Microtexture‐oriented scanning electron microscopy (SEM) studies indicate that mottled dolomites show zonal structures and contain secondary dissolution vugs. SEM studies also revealed the existence of some remains (calcite, clay etc.) in joint‐filling dolomites. Analyses by energy dispersive spectrometry (EDS) indicate the existence of clay minerals (likely to be illite) in the pores of dolomite. Isotope studies conducted to shed light onto the origin of the dolomites of the Tepearasi Formation yielded the results of δ18O=‐2.48 to −3.87%‰ and δ13C=0.93 to 1.12%‰ for the early diagenetic homogeneous dolomites. Mottled and joint‐filling type dolomites of the late diagenetic stage, on the other hand, gave the results of δ18O = −5.42 to −7.12%‰, and δ13C=‐2.29 and −5.70 respectively. X‐Ray diffractometry (XRD) and energy dispersive analysis X‐Ray (EDAX), atomic absorption spectrometry (AAS) analyses, results of petrographic data, and δ18O and δ13C values suggest that the early diagenetic dolomitization in the Tepearasi Formation occurred under the control of hypersaline (rich in Mg) and fresh water mixing zone. The late diagenetic dolomites partly developed under the control of tectonic thrusting.

The development of Nummulites Banks within the Compressional Tectonically Controlled Tertiary Basin of Gürün Autochthone, Eastern Taurus, SW Sivas (Turkey)

The investigated area is located at nortwestern part of Eastern Taurus, on the Gürün autochthone. The limestones described a Nummulites banks and contemporaneous rock units situated on Tertiary uplift and in the Tertiary basin at behind of uplift. Tertiary basin, developped locally over rock units of Akdere basin which deposited on Mesozoic carbonate platform during Late Cretaceous-Early Paleocene and folded, uplifted during late Paleocene-Early Lutetian. This Nummulite bank Tertiary (Lutetian) unit (Demiroluk Fm.) is represented by tectonically controlled shallow-marine, shoeline and offshore facies. The conglomerate/breccia is at the lower part and transsionally Nummulit-Alveolina bearing limestone at the upper part. The three have types of facies assemblages has distinguishe; Facies-1: Conglomerate/breccia (Bio-litoclastic packstone-grainstone), Facies-2: Nummulit-Alveolina bearing wackestone-packstone, Facies-3: Pelagic mudstone-wackstone. In addition, Facies-2: is divided into two microfacies; Facies-2a and Facies-2b comprising Nummulited bearing wackstone-packstone and Alveolina bearing wackstone-garinstone respectively. The facies 1, 2 and 3 deposited in fore-bank slope, fore-bank, bank-rim and back-bank local basins. The Nummulites banks and comtempraneous underlying and overlying rock units undergone pre-and post-depositional different diagenetic events. the calcit and micritic cement developed within and intergrain areas and the syntaxial overgrowth has been developed on the echinoids which characterize the marine and fresh water pheratic environments. During the late diagenetic stage, the pressure solutions formed and caused the rock to have stylotic-fractured structure.

Black sandstones in the Midland Valley of Scotland: thermally metamorphosed hydrocarbon reservoirs?

ABSTRACTCarboniferous sandstones from within the thermal aureoles of igneous intrusions in the Midland Valley of Scotland sometimes have a strong, black colour and are termed ‘black sandstones’. Tait (1926) concluded that the black sandstones are the products of the thermal metamorphism of petroliferous sandstones, a theory which is discussed here in the light of modern petrological and geochemical techniques.The black colour of the sandstones is due to an amorphous, opaque bitumen which usually coats illitic clays, fills porosity and was mostly emplaced at a fairly late diagenetic stage. This solid bitumen has a low reflectance (c. 0·15% R0), no fluorescence under blue-light excitation, is insoluble in organic solvents, is isotopically heavy and has a very low H/C atomic ratio. These data, together with the field relationships of the black sandstones and igneous intrusions, suggest that the bitumen was formed by the thermal alteration of hydrocarbons, as described by Tait (1926), rather than by other possible mechanisms such as the deasphalting of an oil, or the generation of hydrocarbons from organic-rich rocks heated by igneous intrusions, followed by fractionation during migration. This conclusion suggests that by the time of emplacement of the quartz-dolerite intrusions, and some of the alkali-dolerite sills, there had been widespread generation and migration of hydrocarbons which possibly could have been preserved to the present day where not thermally altered.

Silicification and dolomitization of the Lower Eocene carbonates in the Eastern Desert between Sohag and Qena, Egypt

The Lower Eocene carbonates in the Eastern Desert between Sohag and Qena show remarkable diagenetic silicification and dolomitization beside other minor features of solution, cementation, syntaxial rim cement, compaction, recrystallisation and authigenic gypsum, anhydrite and halite formation. Two stages of silicification are recorded in the studied sequence, namely the syngenetic stage and the replacement stage. The first one is confined to the studied sequence, namely the syngenetic stage and the replacement stage. The in the open shelf lagoon facies as hard, dark silicified limestone concretions. The very early diagenetic chert formation was interrupted by the introduction of dolomite suggesting that dolomization postdates silification. The mode of occurrence of the syngenetic chert in the study area seems to be controlled by: the volume of the influx of hydrous silica in the interstitial water and the rate of lime mud deposition. Dolomites which are recorded here for the first time, represent about 40–60% of the carbonates and are confined mainly to the lime muds of the tidal flats and oyster bioclastic nearshore facies. The dolomitization phenomenon is considered to be of late diagenetic stage. Small elongate evaporite laths are scattered in the dolomitic lime mudstones. These authigenic evaporates associating the dolomitic facies of the tidal flats may suggest that hypersaline brines, rich in magnesium, were probably adjacent to these carbonates.

Thermo-Chronological Approach to Reservoir Diagenesis in the Offshore Angola Basin: a Fluid Inclusion, {40}Ar-{39}Ar and K-Ar Investigation (1)

Microthermometric analyses of fluid inclusions and geochronological surveys of authigenic silicates in Neocomian sandstones and Albian dolomitic reservoirs from the offshore Angola basin have allowed determination of the age and the physical-chemical conditions for fluid migration in a late diagenetic stage. The {40}Ar-{39}Ar laser probe dating of adularia overgrowths from the Albian reservoirs shows that the trapping of primary aqueous and oil inclusions occurred during the early Miocene (25.6 + or - 3.2 Ma). The K-Ar age of authigenic illites from the Neocomian sandstones is slightly younger (19 + or - 1 Ma), and the crystallization of regular illite-smectite mixed layers is related to a previous Eocene to Paleocene diagenetic stage (51-58 Ma). There appears to have bee both a regional and vertical variation in the salinity of aqueous fluids during the Miocene. The fluid inclusion trapping temperatures were estimated by correcting the homogenization temperatures of aqueous fluids, assuming that pressures during the Miocene were close to the present reservoir pressures, and by intersecting the isochores of aqueous and oil inclusions, assuming they were trapped at strictly similar pressure and temperature conditions. The fluid inclusion temperatures are consistent with those derived from the tetrahedral aluminum content (AlIV) of authigenic chlorites: 168-206 degrees C in the Albian reservoirs at depths between 2350 and 3580 m, and 175-237 degrees C in the Neocomian sandstones at depths between 2770 and 4470 m. In both reservoir formations, Miocene tempe atures were higher than the present ones and related either to deeper burial or higher geothermal gradient.

Assessment of Thermal Evolution Stages and Oil-Gas Migration of Carbonate Source Rocks of Early Tertiary in Eastern Sichuan, China, by Organic Inclusion Analysis: ABSTRACT

The Jialinjiang Formation of early Tertiary in Sichuan, China, is a series of limestone and dolomite sediments deposited in a platform shoal environment. The diagenetic sequence and organic inclusions trapped in minerals of 95 samples from 20 drillings have been studied. At the late diagenetic stage, pale yellow organic inclusions consisted of liquid hydrocarbons disseminated in pore-infiltrating dolomite, and the homogeneous temperature of contemporaneous saline liquid inclusions possessing a low gas-liquid ratio was 86/degree/C. This indicates the evolution of the organic matter had gone over the oil generating threshold and oil formation had initiated. In the limestone formed at the late diagenetic stage, more brown-yellow organic inclusions were scattered and/or developed along with fissures, comprising 60-70% liquid hydrocarbons and 30-40% gaseous hydrocarbons. Contemporaneous saline liquid inclusions with gas-liquid ratios of 5-10% had homogeneous temperatures of 90/degree/-130/degree/C. These findings show that the organic material had entered a high evolution stage and oil migration had taken place on a large scale.

Carbonate Replacements After Sulfate Evaporites in the Middle Miocene of Egypt

ABSTRACT Massive zones of diagenetic carbonate which replace sulfate deposits are observed in the Middle Miocene evaporites in several outcrops along the Egyptian coast of the Gulf of Suez. These carbonates display marked similarities of morphology, lithofacies, and stable isotope composition to diagenetic carbonates described in some portions of Messinian Gessoso Solfifera Formation of Sicily and in the Castiles of the Upper Permian Castile Formation of the Delaware basin. The mechanism of mineral transformation is basically bacterial sulfate reduction because the low 13C values of these carbonates argue for the organic origin of carbon. The 18O values fall i to two distinct groupings, indicating that the carbonates were crystallized during two different diagenetic stages. The early diagenetic stage occurred soon after deposition, in pore waters of marine origin and at different levels of evaporation; organic matter used by bacteria was produced in situ. The late diagenetic stage occurred when the tectonic or halokinetic uplift of the sequence caused flushing of the sediments by groundwaters of the phreatic aquifer and hydrocarbon migration. Furthermore, to explain the very low 18O values of these carbonates, it is inferred that locally ambient temperatures reached more than 35°C due to the large quantities of energy released during the bacterial sulfate reduction.

Organic Geochemical Investigations of Eastern U.S. Early Mesozoic Basins: ABSTRACT

Shales rich in organic matter and coalified plant fragments (phytoclasts) in the early Mesozoic basins of the Newark Supergroup of the eastern United States are the topic of a current multidisciplinary study to understand their burial history, their role in ore-forming processes, and their hydrocarbon potential. Samples from the Hartford, Newark, Culpeper, Richmond, Taylorsville, and Deep River basins were analyzed using elemental analysis, nuclear magnetic resonance spectroscopy (NMR), Rock-Eval pyrolysis, stable isotope mass spectrometry, pyrolysis-gas chromatography, and gas chromatography-mass spectrometry. The composition of the preserved organic matter in these samples is highly variable. Most sedimentary rocks of Triassic to Jurassic age in the Hartford, Newark, Culpeper, Richmond, and Deep River basins are in a catagenetic stage of thermal alteration. Samples from the Lower Jurassic Towaco Formation of the Newark basin are the least thermally altered samples analyzed and are apparently at a late diagenetic stage. Most of the older Triassic samples and a few of the latest Triassic to Jurassic samples, however, are highly thermally altered. Phytoclasts from the Lower Jurassic Feltville and Portland Formations, from the Newark and Hartford basins, respectively, are essentially aromatic; however, phenolic groups were observed in the NMR spectra. The initial organic geochemical results imply that the organic matter basically exists in two populations, one with a low to medium rank or level of maturation and the other representing a much higher maturation level. No gradual change in maturation from one rock unit to the next was observed for the samples analyzed, and the distribution of maturities apparently is neither stratigraphically nor temporally controlled. The presence of phenolic groups in the phytoclasts from the Feltville and Portland Formations suggests that carbonization rather than coalification processes may have occurred. End_of_Article - Last_Page 1439------------

Fluid Movement and Diagenesis in Fine-Grained Geopressured Sediments of Frio Formation (Oligocene), Kaplan Field, Southwestern Louisiana: ABSTRACT

Investigation of structure, temperature, pressure, salinity, and core samples at Kaplan field yields information on diagenesis of fine-grained sandstones deposited in an outer shelf/upper slope depositional environment. Cross sections and structural maps reveal a domal structure at 15,000 ft (4,572 m) of depth and a northeast-striking growth fault. Post depositional faults occur at shallower depths (11,500 ft; 3,505 m). A large growth fault forms the northern border of the study area. The shallow occurrence of End_Page 446------------------------------ geopressure is related to structure and a high shale/sand ratio. Low isothermal surfaces in the down fault blocks accompanied by anomalous high temperatures in the upthrown blocks indicate vertical leakage of fluids along growth faults from underlying geopressured aquifers. The association of low salinity fluids (less than 60,000 ppm) with leakage zones affirms structural control of fluid movement through the Anahuac and Frio formations (Oligocene) at Kaplan field. The Frio Formation core samples from 16,700 to 19,600 ft (5,090 to 5,974 m) of depth, representing channel and channel-edge turbidite sandstones, were examined petrographically and by SEM. The arkosic composition of late stage diagenesis sandstones at Kaplan field suggests an original arkose or lithic arkose composition (classification of McBride). Nonferroan calcite cementation, chlorite rims and cement, and quartz overgrowths characterize early diagenesis. At a middle stage of diagenesis secondary porosity is developed by dissolution of unstable grains and calcite cement. Samples flushed by geopressured waters from greater depth show kaolinite pore-fill and quartz overgrowths, chlorite (polytype IIb) and illite cement, and feldspar overgrowths in the late diagenetic stage. Premetamo phic textures are apparent in the deepest section at 338°F (170°C). The low permeability of sandstones with extensive early chlorite cement (channel-edge sandstones) precludes development of extensive secondary porosity. In contrast, sandstones with little early chlorite cement develop and maintain secondary porosity through the late diagenetic stage. Restriction of fluid movement by early chlorite cement has ramifications for migration of hydrocarbons or geothermal waters, and for gas production at Kaplan field. End_of_Article - Last_Page 447------------

Origin of Precambrian iron-formations in the Lake Superior region

The Precambrian banded, cherty iron-formations in the Lake Superior region were deposited on broad continental shelves, and deposits of the supratidal, intertidal, and subtidal zones can be recognized in the well-known facies of the iron-formations. The supratidal deposits were primarily calcitic, aragonitic, and sideritic muds. Other minerals, in order of decreasing abundance, may have been gypsum, anhydrite, ankerite, dolomite, quartz, apatite, and halite. Intraclasts were produced by desiccation-granulation, and some of this material was redeposited in the intertidal and subtidal zones. The intertidal zone was characterized by the development of extensive algal reefs and digital-sized and larger stromatolitic mounds. Desiccation-granulation and tidal rip of mineralized stromatolites further added to the accumulation of intraclasts. The intertidal zone was the domain of granules that were derived by abrasion and rounding of intraclasts. The original mineralogy of the intertidal deposits was similar to that of the supratidal zone; however, because of photosynthesis, the sea water was oxygenated. Lozenge-shaped grains are considered to be secondary, silicified, and iron-oxide replicates of original gypsum crystals, and, by comparison with recent evaporites, some anhydrite and halite may also have been present. The deposits of the intertidal and subtidal zones are considered to be comparable to the present-day Abu Dhabi complex in the Persian Gulf. Spherical microstructures are abundant and well preserved in the chert of the subtidal deposits. Pyrite, which characterizes these deposits, probably was formed largely by the bacterial reduction of sulfate derived from the intertidal zone. The laminations in the iron-formations resulted from several causes that were controlled by chemical and mechanical processes, such as the rates and localization of the precipitation of calcitic, aragonitic, and sideritic muds and the mechanical breakup and redistribution of these sediments as sand, silt, and mud. These processes resulted in compositional and textural laminations. The diversity of the banded, cherty iron-formations, recognized in the different facies, then, can be explained largely in the diagenetic changes. The large-scale pervasive diagenetic changes were the replacement of calcite and aragonite by chert and the formation of iron silicates by the reactions between ionic silica and siderite and iron-bearing dolomite. Vein-like structures and the development of magnetite along stylolites indicate mobility of iron in late diagenetic stages, and iron-organic chelates are suggested to account for the mobility. Depending on the composition of the laminae and the chemical environment, silicification during diagenesis produced the hematite, magnetite, siderite, pyrite, and mixed-mineral facies.

Peculiarities of Petroleum Formation in Highly Bituminous, Siliceous, Shaly, Carbonaceous Facies, Timan-Pechora Basin, USSR: ABSTRACT

Over 50 oil and gas fields with total reserves of about 6 × 109 BOE have been discovered in the Timan-Pechora basin, one of the most important Russian frontiers. Almost all the sequence is productive, although major reserves are confined to two stratigraphic intervals beneath regional seals. Principal source rocks are the so-called Domanik facies, 25 to 150 m (82 to 492 ft) thick, represented by rapidly alternating black shales, chert, marls, and siliceous and organic limestones. Exclusively sapropelic organic matter averages 5 to 7% and reaches 20% or more. Soluble bitumen is very abundant (1 to 2 wt. %) and contains all the components characteristic of crudes: from light oils to heavy tars and typical high-molecular asphaltenes. These characteristics exi t even on the basin's periphery where Domanik facies are only marginally mature. In other areas, Domanik facies are mature; they are probably overmature in the deepest troughs. Outside the area covered by Domanik facies, pools and even significant shows are absent. Deposition of this prominent facies began during the end of early Frasnian time in a wide stagnant sea that covered the eastern edge of the Russian platform. Beginning in the late Frasnian, shallow-water carbonate sedimentation resumed along the basin's edges and on uplifted blocks. Condensed Domanik deposits continued to form in the gradually deepening sea on the east side of the basin. Barrier reefs and clastic terraces that prograded basinward formed along the northern and western boundaries. The deep-water trough was finally filled by thick clastics at the end of early Tournaisian time. The unusual composition of Domanik facies and their exceptional enrichment by sapropelic organic matter result in their peculiarities as petroleum source rocks. Lithology of the rocks, particularly the abundance of huge carbonate concretions fully or partly replaced by silica, suggests a long delay in lithification and the relative importance of the late diagenetic stage of oil generation. This explains the presence of immature oils in underlying Devonian clastics and their absence elsewhere in the sequence. On the other hand, Domanik facies, owing to significant silicification that trapped giant amounts of bitumen in the rocks, became a natural repository of oil during geologic history. This oil migrated because of fracturing, especially during stages of tectonic activity. Spatial istribution of oil types and deposits of solid bitumen in traps having different ages of formation, clearly shows predominance of pulse-like vertical migration. Migration of oil from the Domanik continued during late stages of geologic history along with block uplifting, cooling of the sedimentary cover, and absence of sedimentation. Thus, methods of applied geochemistry that invoke models of heating should not be applied to Domanik-type rocks and more geologic data are required to assess their role as oil sources. End_of_Article - Last_Page 561------------

ORGANIC MATTER AS INDICATOR OF THE DEGREE OF METAMORPHISM OF THE CARBONIFEROUS ROCKS IN THE SOUTH WALES COALFIELDS

Isoreflectance maps of vitrinites in coals and phytoclasts (coaly inclusions) in the Coal Measures of S. Wales show a progressive increase in vitrinite reflectance with increase in coal rank and grade of metamorphism towards the anthracite area in the west, without significant variation in reflectances of coals with present depth. Differences between values of vitrinite reflectance in coals and phytoclasts in the same profile are recorded and attributed to variations in depositional environments, and to susceptibility to alteration of the host rocks. Spores in the Coal Measures are progressively carbonized (from golden brown to black) towards the west, and this is not a function of present depth of burial. Reflectance of vitrinites in coals, degree of spore carbonization and coal rank are used in definition of three different diagenetic zones in the coalfield. The Coal Measures could have acted as a source for accumulations of gas in the Permo‐Triassic sequence of the south Irish Sea, if necessary geologic conditions are present. Burial metamorphism (application of Hilt's law) could have caused the coalification of S. Wales coals up to the high and medium volatile bituminous stages (up to the late diagenetic stage). A relationship is established between the lateral variation in coal rank in the coalfield and the regional variation in coal rank in the southern British Isles, where two regions of markedly different geothermal history are recognised. The Diagenetic Zone in the E and S parts of the coalfields marks the final stage in the thermal history of the cold region (the W margin of London Stable Massif), whilst the Anchimetamorphic Zone in the NW part (the anthracite area) represents the final stage in the thermal history of the hot region (the S margin of St. George's Land).Phytoclasts and spores are recorded for the first time from the Dinantian massive limestones of S. Wales.

Migration and Accumulation of Oil and Gas

Primary and secondary migration of hydrocarbon are generally recognised among petroleum geologists. The former is defined as an expulsion of hydrocarbon from source to reservoir rocks although its concept is rather complicated in conjunction with petroleum genesis. The latter is the movement of hydrocarbon within reservoir rocks.On hydrocarbon genesis, opinion seem to have recently converged to the diagenetic theory. As the depth range of petroleum genesis is relatively long, some geologists favor early diagenetic stage of primary migration and the others emphasize late diagenetic stage. Therefore, the mechanisms for primary migration should guite differ between the two ideas, because rock properties considerably vary with depths. In this point of view, the theory of primary migration is not said to be established yet.The author has studied on the timing of primary hydrocarbon migration in the Japanese oil-bearing Neogene system mainly from the geohistorical aspect, and arrived at the conclusion that its principal timing is during the early mature stage, that is to say, within mature range of 0.5 to 0.7% in vitrinite reflectance Ro, and also that hydrocarbon generated in the late diagenetic stage is not able to migrate any more.Porosity of muddy sediments during primary migration is estimated to be from 15 to 25%, and media with which hydrocarbon migrates is pore water then exulsed from muddy sediments by compaction.Although many oil fields are often said to be well explained when assuming late diagenetic stage migration, it is noteworthy many writers recently intends to explain the formation of these oil fields in a light of the theory of early stage migration. Why the timing of primary migration should be seriously discussed is due to not only academic interest but also its technological and economical influence to practical exploration procedures.On the contrary to the complicated problems on primary migration, secondary one has been a well difined physical process which is quantitatively dealt with through experiments in laboratories. Its main force is bouyancy, and resistant force for secondary migration is capillary pressure. These are relatively easily calculated using simple equations immediately after obtaining unknown physical qauntities related to the equations.A better understanding of principles of bouyancy and capillary pressure on secondary migration will lead to the effective development of oil fields, and particulary to the discovery of unusual accumulations such as permeability traps.

Diagenesis of the black river (middle ordovician) limestones in Southern Ontario, Canada

The diagenetic history of the Black River Limestone Group is discussed with special reference to solution (leaching), cementation, neomorphism, authigenesis, pressure solution, compaction and dedolomitization. Formation of recrystallization textures (microspar and pseudospar) was controlled by original sediment grain size and occurred during both early and late stages of diagenesis. Dolomite, celestite and pyrite were the chief developments of late diagenetic minerals in the Black River carbonates. Pressure-solution (stylolitization) phenomena occurred at a very late diagenetic stage. Compaction played a minor role in the diagenesis of the rocks. Dedolomitization is a surface phenomenon related to weathering.

Fossil porphyrins: uncomplexed chlorins in a triassic sediment

The Triassic oil shale of Serpiano (Switzerland) contains, besides the well-known tetrapyrrole pigments of the porphyrin class, some small amounts of chlorins. These pigments have now been isolated and identified as desoxomesoetiopyropheophorbide a and etiochlorin III, with their structure confirmed both by dehydrogenation to the corresponding porphyrins and by synthesis. These ancient chlorins are not direct descendants of chlorophyll; rather, they are secondary, synthetic chlorins, formed by hydrogenation of meso- and desoxophyllerythroetioporphyrin under extreme reducing conditions, probably in a late diagenetic stage in the sediment.