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Depositional Facies (depositional + facy)
Selected AbstractsLower Carboniferous peritidal carbonates and associated evaporites adjacent to the Leinster Massif, southeast Irish MidlandsGEOLOGICAL JOURNAL, Issue 2 2005Zsolt R. Nagy Abstract Analysis of a 275,m-thick section in the Milford Borehole, GSI-91-25, from County Carlow, Ireland, has revealed an unusual sequence of shallow subtidal, peritidal and sabkha facies in rocks of mid?-late Chadian to late Holkerian (Viséan, Lower Carboniferous) age. Sedimentation occurred on an inner ramp setting, adjacent to the Leinster Massif. The lower part of the sequence (late Chadian age) above the basal subtidal bioclastic unit is dominated by oolite sand facies associations. These include a lower regressive dolomitized, oolitic peloidal mobile shoal, and an upper, probably transgressive, backshoal oolite sand. A 68,m-thick, well-developed peritidal sequence is present between the oolitic intervals. These rocks consist of alternating stromatolitic fenestral mudstone, dolomite and organic shale, with evaporite pseudomorphs and subaerial exposure horizons containing pedogenic features. In the succeeding Arundian,Holkerian strata, transgressive,regressive carbonate units are recognized. These comprise high-energy, backshoal subtidal cycles of argillaceous skeletal packstones, bioclastic grainstones with minor oolites and algal wackestones to grainstones and infrequent algal stromatolite horizons. The study recognizes for the first time the peritidal and sabkha deposits in Chadian rocks adjacent to the Leinster Massif in the eastern Irish Midlands. These strata appear to be coeval with similar evaporite-bearing rocks in County Wexford that are developed on the southern margin of this landmass, and similar depositional facies exist further to the east in the South Wales Platform, south of St. George's Land, and in Belgium, south of the Brabant Massif. The presence of evaporites in the peritidal facies suggests that dense brines may have formed adjacent to the Leinster Massif. These fluids may have been involved in regional dolomitization of Chadian and possibly underlying Courceyan strata. They may also have been a source of high salinity fluids associated with nearby base-metal sulphide deposits. Copyright © 2005 John Wiley & Sons, Ltd. [source] AN INTEGRATED STUDY OF DIAGENESIS AND DEPOSITIONAL FACIES IN TIDAL SANDSTONES: HAWAZ FORMATION (MIDDLE ORDOVICIAN), MURZUQ BASIN, LIBYAJOURNAL OF PETROLEUM GEOLOGY, Issue 1 2009A. Abouessa Studies of the impact of diagenesis on reservoir quality in tidal sandstones can be of great importance in successful hydrocarbon exploration. The study reported here shows that diagenetic alterations and bioturbation have induced considerable deterioration and heterogeneity in the reservoir quality of the sand-dominated tidal deposits of the Middle Ordovician Hawaz Formation in the Muruq Basin, Libya. Comparison is made between the diagenetic evolution of samples from the subsurface (present-day depth 1500 m) and from surface outcrops in order to study the impact of burial and uplift on the spatial and temporal distribution of reservoir quality in the Hawaz Formation sandstones. Eogenetic alterations, which were mediated by meteoric water circulation, included kaolinitization and dissolution of framework silicates and mechanical compaction. Mesogenetic alterations (T > 70°C; depth > 2 km) included pressure dissolution of quartz grains and concomitant quartz cementation, conversion of kaolinite into dickite, illitization of kaolinite and of grain-coating clays, and the precipitation of Mg-rich siderite cement. Reduction of intergranular porosity was due more to compaction than to cementation, yet quartz overgrowths are up to 16% in some of the sandstones. Bioturbation has resulted in a greater reduction in sandstone permeability in the lower part of the formation than the upper part. A higher ratio of dickite to kaolinite in subsurface samples than in outcrop samples is attributed to the longer residence time of the former sandstones under mesogenetic conditions. Telodiagenesis has not resulted in enhancement of reservoir quality of the Hawaz Formation Sandstones but in pseudomorphic calcitization of siderite and oxidation of pyrite to goethite. This study shows that the reservoir-quality evolution of tidal sandstones can best be elucidated when linked to depositional facies and distribution of diagenetic alterations. [source] RESERVOIR POTENTIAL OF A LACUSTRINE MIXED CARBONATE / SILICICLASTIC GAS RESERVOIR: THE LOWER TRIASSIC ROGENSTEIN IN THE NETHERLANDSJOURNAL OF PETROLEUM GEOLOGY, Issue 1 2008D. Palermo The Lower Triassic Rogenstein Member of the Buntsandstein Formation produces gas at the De Wijk and Wanneperveen fields, NE Netherlands and consists mainly of claystones with intercalated oolitic limestone beds. The excellent reservoir properties of the oolites (,= 20-30%; k = 5-4000 mD) are predominantly controlled by depositional facies. Oolitic limestones are interpreted as the storm and wave deposits of a shallow, desert lake located in the Central European Buntsandstein Basin. The vertical sequence of lithofacies in the Rogenstein Member indicates cyclic changes of relative lake level. The reservoir rock is vertically arranged in a three-fold hierarchy of cycles, recognised both in cores and wireline logs. These cycles are a key to understanding the distribution of reservoir facies, and are used as the basis for a high-resolution sequence stratigraphic correlation of the reservoir units. Slight regional-scale thickness variations of the Rogenstein Member (in the order of tens of metres) are interpreted as the effects of differential subsidence associated with the inherited Palaeozoic structural framework. The depositional basin can be subdivided into subtle palaeo-highs and -lows which controlled facies distribution during Rogenstein deposition. Oolitic limestones show their greatest lateral extent and thickest development in the Middle Rogenstein during large-scale maximum flooding. Potential reservoir rocks (decimetre to metres thick) are present in the NE Netherlands, in particular in the Lauwerszee Trough and the Lower Saxony Basin, where abundant gas shows of 200 - 4000 ppm CH4 have been recorded. Preserved primary porosity is interpreted to be a result of rapid burial in subtle depositional palaeo-lows in this area. The thickest, amalgamated oolite intervals (tens of metres thick) occur in the eastern part of the Central Netherlands Basin. Because of poor reservoir properties, other areas appear to be less promising in terms of Rogenstein exploration potential. [source] POROSITY DESTRUCTION IN CARBONATE PLATFORMSJOURNAL OF PETROLEUM GEOLOGY, Issue 1 2006S. N. Ehrenberg The important thing to understand about carbonate diagenesis is not how porosity is created, but how it is destroyed. Detailed core observations from two deeply-buried carbonate platform successions (the Finnmark platform, offshore north Norway; and the Khuff Formation, offshore Iran) show that in both cases most vertical porosity variation can be accounted for by only two or three factors, namely: (1) stylolite frequency, (2) proportion of argillaceous beds, and (3) anhydrite cement. The spatial distribution of these factors is determined by the depositional distribution of clay minerals (important for localizing chemical compaction) and the occurrence of hypersaline depositional conditions and associated brine reflux (important for localizing anhydrite precipitation and dolomitisation). However, the intensity of chemical compaction and consequent porosity loss in adjacent beds by carbonate cementation also depend upon thermal exposure (temperature as a function of time). Evidence from the Finnmark platform and other examples indicate that the stratigraphic distribution of early-formed dolomite is also important for porosity preservation during burial, but this factor is not apparent in the Khuff dataset. Insofar as the Finnmark and Khuff platforms can be regarded as representative of carbonate reservoirs in general, recognition of the above porosity-controlling factors may provide the basis for general models predicting carbonate reservoir potential both locally (reservoir-model scale) and regionally (exploration-scale). Distributions of clay, anhydrite, and dolomitization should be predictable from stratigraphic architecture, whereas variations in thermal exposure can be mapped from basin analysis. In the present examples at least, factors that do not need to be considered include eogenetic carbonate cementation and dissolution, depositional facies (other than aspects related to clay and anhydrite content), and mesogenetic leaching to create late secondary porosity. [source] Applicability of Carbazole Migration Indices in Continental Rift Basins: A Case Study of Western Lujiapu Depression in Kailu Basin, NE ChinaACTA GEOLOGICA SINICA (ENGLISH EDITION), Issue 3 2010Shuqing ZHOU Abstract: Kailu Basin in which the Western Lujiapu Depression is located is a typical continental rift basin. Biomarker parameters of the oils indicate that depositional facies and environments vary between the Bao 1 and Bao 14 fault blocks with a higher saline environment in the Bao 1 fault block, but such difference has no significant impact on carbazole abundance and distribution. Maturity and migration distance are the main controls on carbazole abundance and distribution in the Western Lujiapu Depression. The commonly used migration indices, such as ratios of nitrogen shield isomers to nitrogen exposed isomers (1-/4-methylcarbazole ratio, 1,8-/2,4-dimethylcarbazole (DMC) ratio and half-shield/exposed-DMC ratio), absolute concentrations of alkylated carbazoles and BC ratio (= benzo [a]carbazole/ (benzo[a]carbazole + benzo[c]carbazole)) increase at the low mature range and decrease at a higher mature range with increasing maturity. At relatively low maturity stage (Rc<0.77%), maturation has reversal effects with migration on the ratios of nitrogen shield isomers to nitrogen exposed isomers, which may cover migration influence and makes these parameters fail to indicate migration effects. Valid migration indicators at this maturity stage are concentrations of alkylated carbazoles and BC ratios, which can provide ideal tools for migration direction assessment even within short migration distance. Maturity effects should be taken into account when carbazole compounds are applied to indicate migration direction, and at different maturity stages, these commonly used parameters have different validity in tracing migration direction. Coupled with our previous study in the Eastern Lujiapu Depression, a conceptual model of the variation of nitrogen migration indices can be established for terrestrial rifted basins, that is, strong fractionation lateral migration model through sandy beds, weak fractionation vertical migration model along faults, and maturity impacts on migration assessment. [source] | ||||||||||