Karl Ramseyer

Diagenesis and Fluid Evolution of Deeply Buried Permian (Rotliegende) Gas Reservoirs, Northwest Germany

Depositional environment and tectonic setting were important in the diagenesis and evolution of reservoir properties in the Rotliegende sequence of the North German Basin. Facies belts paralleling the edge of a central saline lake controlled the distribution of early and shallow burial cements. Lake shoreline sands with radial chlorite cement show the best reservoir properties in the study area. Juxtaposition of Rotliegende deposits against either Carboniferous Coal Measures or Late Permian (Zechstein) evaporites by faulting resulted in cross-formational fluid exchange. The introduction of fluids from Carboniferous Coal Measures into Rotliegende reservoirs produced intense clay cementation, significantly reducing rock permeabilities. Influx of Zechstein fluids favored pre ipitation of late carbonate and anhydrite cements. Cross-formational and fault-related fluid flow was enhanced during periods of fault activity.

Nature and origin of organic matter in carbonates from speleothems, marine cements and coral skeletons

Organic matter in speleothem calcite, marine carbonate cements and aragonitic coral skeletons was studied to determine its location, molecular structure, functionality and effect on mineral growth. SEM analyses showed that inorganically precipitated carbonates incorporate, during growth, adsorbed organic matter between submicroscopic subunits of the crystals whereas biologically secreted carbonates incorporate biogenic tissue between the crystals. Molecular fluorescence spectroscopy indicated that low molecular weight fulvic acids are the most important constituents of the organic matter. The fulvic acids are probably derived from soils (speleothem calcite), dissolved organic matter (marine carbonates) and biological decay products (aragonitic coral skeleton).

MIXED-LAYER ILLITE/SMECTITE MINERALS IN TERTIARY SANDSTONES AND SHALES, SAN JOAQUIN BASIN, CALIFORNIA

The southern San Joaquin Valley contains more than 7 km of sedimentary fill, largely Miocene and younger in age. Ancient depositional environments ranged from alluvial fans at the basin margins to turbidite fans toward the basin center. Mixed-layer illite/smectite (I/S) dominates the <2-μm fraction of Miocene shales, and kaolinite is abundant in Miocene sandstones. I/S from carbonate-cemented sandstones contains 5–20% more smectite layers than I/S from uncemented sandstones. The timing of cementation correlates with the proportion of smectite layers in the I/S, suggesting that cementation slowed the illitization process. Smectite and I/S with > 80% expandable layers occur at present burial temperatures of 120°-l 40°C in Miocene sandstones and shales. This highly expandable I/S is restricted to areas covered by thick deposits (1000-2500 m) of Pleistocene sediments. Rocks of similar age and at equivalent temperatures, but covered by <900 m of Pleistocene sediments, contain I/S having low expandabilities (<30%).Microprobe analyses of 16 discrete smectite and smectite-rich I/S clays indicate an average montmo-rillonite composition of:

Mn2+-activated luminescence in dolomite, calcite and magnesite: quantitative determination of manganese and site distribution by EPR and CL spectroscopy

\left( {C{a_{0.2}}{K_{0.15}}N{a_{0.1}}} \right)\left( {A{l_{3.0}}F{e^{3 + }}_{0.35}M{g_{0.75}}T{i_{0.03}}} \right)\left( {S{i_{7.7}}A{l_{0.3}}} \right){O_{20}}{\left( {OH} \right)_4} \cdot n{H_2}O.

Diagenetic Carbonate in Miocene Sandstone Reservoir, San Joaquin Basin, California

Smectite in I/S-rich clays of Gulf Coast shales has a similar composition except for lower octahedral Al/ Fe ratios (Al/FeVI = 3.1), compared with the San Joaquin samples (Al/FeVI = 8.6).Residence time at different temperatures appears to be an important influence on the percentage of smectite layers in I/S from the San Joaquin basin. Areas containing I/S with high expandabilities (e.g., 95% smectite layers) have a time-temperature index (TTI) of 4.0-4.5 at 120°C, whereas areas containing I/S with low expandabilities (e.g., 30% smectite layers) have a TTI of 5.0. Present data suggest that highly expandable I/S changed to slightly expandable I/S over a narrow temperature interval (10°-20°C). Differences in the potassium availability from detrital components and in the K+/H+ activity ratios of pore water do not appear to be related to the differences in the percentage of smectite layers of these I/S clays.

Mechanism of Plagioclase Albitization

Cathodoluminescence (CL) examination of Recent biogenic carbonates shows that they are often luminescent regardless of their mineralogical composition (calcite v. aragonite), habitat (marine v. fresh water), way of life (sessile v. vagile) or environment (hyper- v. hyposaline water). Thus, the presence of luminescence in biogenic particles is not a reliable indicator of diagenetic alteration as some authors have suggested. In addition, CL can reveal variations in the mineralogy of shell material (e.g. regenerated calcitic v. primary aragonitic) and can highlight growth-related structures. Manganese (Mn 2+ ) is the most likely activator of this luminescence, and its content in the shells of benthic organisms seems to be linked to growth rate, ontogeny, open sea conditions, bathymetry and salinity. In neritic environments the Mn 2+ content and the CL of molluscs and foraminifera appear to increase with decreasing salinity. This study indicates that CL may be an important tool for the determination of environmental and ontogenetic parameters in biogenic carbonates in addition to its current use indiagenetic studies.

Petrological modifications in granitic rocks from the siljan impact structure: evidence from cathodoluminescence

Abstract Dolomite, calcite and magnesite were studied by EPR and CL spectroscopy. Mn2+-activated luminescence in calcite and magnesite is characterized by single Gaussian peaks at 16,250 cm−1 (615 nm) and at 15,300 cm−1 (654 nm), respectively. In calcite the Mn2+ content and the normalized EPR area are linearly correlated when the Mn2+ content is Deconvolution of corrected CL spectra from dolomite shows two overlapping peaks at 17,315 cm−1 (578 nm) and 15,270 cm−1 (655 nm). In rare cases only the 15,270 cm−1 (655 nm) Gaussian peak is present. The CL peak at 17,315 cm−1 (578 nm, Ia) is due to Mn2+ in the Ca site and the peak at 15,270 cm−1 (655 nm, Ib) to Mn2+ in the Mg position. The Mn2+ content in dolomite is linearly correlated with the normalized EPR areas of the two sites. The distribution ratio (KD) of Mn2+ in the two sites of dolomite, i.e. the Ca and the Mg position, was determined by EPR and CL spectroscopy. A linear correlation exists between KD from EPR and the intensity ratio ( I b I a from CL. CL spectroscopy is thus an appropriate method to determine KD in dolomite. Evaporitic dolomites may be differentiated from non-evaporitic dolomites by their KD-values.

Evidence of growth and sector zoning in hydrothermal quartz from Alpine veins

In the fluvio-deltaic, quartz-arenitic sandstones of the Jurassic Khatatba Formation in the Salam field (Egypt9s Western Desert), diagenesis and sedimentary facies control reservoir quality. Fluvial channel sandstones have the highest porosities (10-15%) and permeabilities (100-600 md), in part because of siderite cementation, which (1) inhibited compaction and quartz cement and (2) was later dissolved, creating intergranular secondary porosity (1/4 of total porosity). Fluvial crevasse-splay and marine sandstones have the lowest reservoir quality because of an abundance of depositional kaolin matrix and pervasive, shallow-burial Fe-dolomite cement, respectively. Siderite precipitation was multiphase and separated by distinct dissolution events. The earliest siderite precipitated near surface, within suboxic tropical coastal swamps containing predominantly meteoric waters. Some influence of marine waters is indicated by local enrichments in Mg and Ca. The next major siderite generation shows a trend to decreasing Mn and Ca contents, and is of shallow-burial origin. The last major siderite phase is Mg rich and interpreted as deeper-burial in origin. Some dissolution occurred during shallow burial related to climatically controlled meteoric water fluxing under unconformities. The most important dissolution, however, occurred during deep burial, resulting in (1) a major corrosion surface predating the last Mg-rich zone, (2) selective dissolution of some earlier zones, and (3) secondary porosity. This burial dissolution is interpreted to have been caused by cooling of compactional waters expelled from the basin along major faults. Other diagenetic phases observed include early-diagenetic pyrite, kaolin, quartz, bitumen, and late-diagenetic barite and illite. Kaolinite precipitated at shallow depths ( 130°C) during and/or after rapid Late Cretaceous burial. Quartz developed in two phases, separated by oil migration. This paper illustrates that, in fluvio-deltaic quartz arenites deposited under the influence of humid tropical climate, reservoir quality can be largely controlled by the contrasting pathways of carbonate diagenesis followed by the different sedimentary facies. This paper also documents a case in which siderite dissolution generated significant secondary porosity in reservoir sandstones, and where both siderite cementation and dissolution took place in multiple phases during different diagenetic stages, including early, shallow-burial, and deep-burial diagenesis. The present study also shows that, in multiphase siderite cements, the earlier growth zones can be selectively dissolved and replaced by later siderite zones (i.e., recrystallized) during burial diagenesis. These findings contrast with the general thought that siderite cements are not susceptible to generation of significant secondary porosity by dissolution and that earlier-formed siderites are essentially stable during diagenesis.