Tectonic and sedimentary evolution of the Korean peninsula: a review and new view

The spatial and temporal distribution of diagenetic alterations in siliciclastic sequences is controlled by a complex array of interrelated parameters that prevail during eodiagenesis, mesodiagenesis and telodiagenesis. The spatial distribution of near-su

Spatial and temporal distribution of diagenetic alterations in siliciclastic rocks: implications for mass transfer in sedimentary basins

Experiments on the role of water in petroleum formation

Models and concepts of sandstone diagenesis developed over the past two decades are currently employed with variable success to predict reservoir quality in hydrocarbon exploration. Not all of these are equally supported by quantitative data, observations, and rigorous hypothesis testing. Simple plots of sandstone porosity versus extrinsic parameters such as current subsurface depth or temperature are commonly extrapolated but rarely yield accurate predictions for lithified sandstones. Calibrated numerical models that simulate compaction and quartz cementation, when linked to basin models, have proven successful in predicting sandstone porosity and permeability where sufficient analog information regarding sandstone texture, composition, and quartz surface area is available. Analysis of global, regional, and local data sets indicates the following regarding contemporary diagenetic models used to predict reservoir quality. (1) The effectiveness of grain coatings on quartz grains (e.g., chlorite, microquartz) as an inhibitor of quartz cementation is supported by abundant empirical data and recent experimental results. (2) Vertical effective stress, although a fundamental factor in compaction, cannot be used alone as an accurate predictor of porosity for lithified sandstones. (3) Secondary porosity related to dissolution of framework grains and/or cements is most commonly volumetrically minor (2%). Exceptions are rare and not easily predicted with current models. (4) The hypothesis and widely held belief that hydrocarbon pore fluids suppress porosity loss due to quartz cementation is not supported by detailed data and does not represent a viable predictive model. (5) Heat-flow perturbations associated with allochthonous salt bodies can result in suppressed thermal exposure, thereby slowing the rate of quartz cementation in some subsalt sands.

Sandstone diagenesis and reservoir quality prediction: Models, myths, and reality

Recent investigations of oil reservoirs in a variety of locales have indicated that these habitats may harbor active thermophilic prokaryotic assemblages. In this study, we used both molecular and culture-based methods to characterize prokaryotic consortia associated with high-temperature, sulfur-rich oil reservoirs in California. Enrichment cultures designed for anaerobic thermophiles, both autotrophic and heterotrophic, were successful at temperatures ranging from 60 to 90°C. Heterotrophic enrichments from all sites yielded sheathed rods (Thermotogales), pleomorphic rods resembling Thermoanaerobacter, and Thermococcus-like isolates. The predominant autotrophic microorganisms recovered from inorganic enrichments using H2, acetate, and CO2 as energy and carbon sources were methanogens, including isolates closely related to Methanobacterium, Methanococcus, and Methanoculleus species. Two 16S rRNA gene (rDNA) libraries were generated from total community DNA collected from production wellheads, using either archaeal or universal oligonucleotide primer sets. Sequence analysis of the universal library indicated that a large percentage of clones were highly similar to known bacterial and archaeal isolates recovered from similar habitats. Represented genera in rDNA clone libraries included Thermoanaerobacter, Thermococcus, Desulfothiovibrio, Aminobacterium, Acidaminococcus, Pseudomonas, Halomonas, Acinetobacter, Sphingomonas, Methylobacterium, and Desulfomicrobium. The archaeal library was dominated by methanogen-like rDNAs, with a lower percentage of clones belonging to the Thermococcales. Our results strongly support the hypothesis that sulfur-utilizing and methane-producing thermophilic microorganisms have a widespread distribution in oil reservoirs and the potential to actively participate in the biogeochemical transformation of carbon, hydrogen, and sulfur in situ.

/pdf/culture-dependent-and-culture-independent-characterization-2nt2mt3dto.pdf

Culture-Dependent and Culture-Independent Characterization of Microbial Assemblages Associated with High-Temperature Petroleum Reservoirs

Water-rock interaction in tertiary sandstones, San Joaquin basin, California, U.S.A. : diagenetic controls on water composition

The southern San Joaquin Valley contains more than 7 km of sedimentary fili, 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 80% expandable layers occur at present burial temperatures of 120~176 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 tem- peratures, 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: (Cao.2Ko.~sNao.0(A13.oFe3+o.35Mgo.TsTio.03)(Si7.7Alo.3)O2o(OHL �9 n H20. Smectite in I/S-rich clays of Gulf Coast shales has a similar composition except for lower octahedral A1/ Fe ratios (A1/Fe vl = 3.1), compared with the San Joaquin samples (A1/Fe w = 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~ 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~ Dif- ferences 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.

Mixed-layer illite/smectite minerals in Tertiary sandstones and shales, San Joaquin Basin, California

Observations of closely spaced porous and cemented sandstones from the San Joaquin basin, the Los Angeles basin and the Gulf Coast reveal that albitization of plagioclase occurs during fracturing of the detrital plagioclase grains. Albitization is found to be a chemically selective hydrous reaction in which An-rich plagioclase is preferentially albitized first relative to An-poor plagioclase. The observed non-positive volume change during albitization indicates that aluminum mobility takes place on a scale at least as large as a thin section. These findings are supported by thermodynamic stability calculations for plagioclase with different compositions and degrees of aluminum-silicon ordering at 25°C in a representative sea water and at 100°C in an oil field water. The calculations predict that 1) An-rich plagioclase is less stable than An-poor plagioclase, and 2) plagioclase with a random Al/Si distribution is less stable than plagioclase with the same composition but an ordered Al/Si distribution. Any composition and structural state of plagioclase, including albite, is unstable in sea water at 25°C. The width of the albitization zone may be controlled by the degree of aluminum-silicon ordering. This agrees well with the observed smaller temperature interval of albitization in sandstones containing only volcanically derived plagioclase.

Mechanism of plagioclase albitization

Stevens arkosic sandstones, deposited about 7 Ma in a deep marine environment, form the North Coles Levee reservoir. Dolomite and calcite cements occur as zones averaging 1 ft thick and having less than 300 ft of lateral continuity. Based on ^dgr18O measurements and estimated pore fluids between 0 and 2.8, the dolomite formed at 30°-40°C, an early calcite cement formed at 50°-60°C, and a late pore-filling and plagioclase-replacing calcite formed at 70°-80°C. Corresponding to these temperature estimates, dolomite formed in the zone of methanogenesis (^dgr13C = 4 to 8) and calcites in the zone of decarboxylation (^dgr13C = -3 to -12). Dolomite has between 4 and 12 mole % Fe, and calcites contain up to 8 mole % o Fe + Mg + Mn with a characteristic (Fe + Mn)/Mg ratio of about 3:1. The latest calcite is the most Ca rich, due in part to Ca sourced from plagioclase dissolution. Calcite cements in nonmarine sections have markedly high Mn contents (to 5 mole %) compared with the marine section at North Coles Levee. The volume of cement systematically decreases from about 40-20% over the cementation temperature interval, suggesting progressive compaction of the sandstones. Present-day mean porosity of the reservoir (15%) appears to be largely a result of mechanical compaction. A very late dolomite cement with high Fe content (13 mole %) and light carbon (^dgr13C = -14) is found in the cleavage flakes of crushed biotite. The dolomite is calculated to have formed from a fluid with a ^dgr18O of 0.9 at present reservoir temperatures of 105°C, suggesting precipitation in the past 20 years during release of the gas cap and injection of isotopically light water. Calcite also forms thick scales in production tubing primarily as a result of pressure drops from 4,900 to 400 psi in the tubing string. The calcites have high Mg contents (7 mole %) due to the composition of the injection water. Calculated distribution coefficients for Fe and Mn are similar to published values, but Mg is two times higher than published values at 90°C. Similar results are obtained for late calcite cements. A time-temperature plot for North Coles Levee constrains the timing of dolomite cementation between 5 and 6 Ma and the calcite cementation between 4 and 2.5 Ma. Plagioclase dissolution and hydrocarbon emplacement occurred within the past few million years, suggesting that the presence of the hydrocarbons and associated byproducts of kerogen maturation enhance porosity. Despite the narrow window for diagenesis, minimum fluid fluxes of only 0.02 cm3/yr per 100 cm3 rock are required to account for the volume of dissolved plagioclase.

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

The Moeraki boulders are large (to 2 m) calcite concretions with septarian veins of calcite and rare late-stage quartz and ferrous dolomite. The concretions are enclosed by Paleocene marine mudstones with vitrinite reflectance values of 0.29% R suggesting maximum burial temperatures of 25-30°C. Microprobe analyses of these concretions indicate a Ca enrichment-Mg depletion trend with growth. Calcite in the core contains about 88 mole % Ca and about 93 mole % Ca in the rim. Both rim and cores contain less than 1 mole % Fe and Mn. Septarian veins also show a systematic calcium enrichment with growth of the vein. The initial vein fills are similar in composition to the outer rim of concretionary bodies, suggesting septarian cracks form contemporaneously with attainment of final concretion size. The later-stage calcite vein-fills have less than 1.5 mole % Mg and up to 11 mole % Fe + Mn. These elements systematically decrease both into and outward along the vein, with increasing calcium substitution. The above compositional trends in veins suggest continued crystallization during propagation of the fractures. The carbonate composition trends reflect interaction between the growing concretion and the enclosing mudstone pore-fluid system. The observed Fe, Mn, Mg depletion trends probably reflect depletion of elements released by short-time-scale diagenetic events of finite size. The growth time of the larger concretions is estimated at about 4 million years based on published diffusion growth models. Extrapolation of compositional trends versus growth time from these concretion bodies suggests that septarian veins form on a time scale of several million years.

J. R. Boles

Papers

Water-rock interaction in tertiary sandstones, San Joaquin basin, California, U.S.A. : diagenetic controls on water composition

Mixed-layer illite/smectite minerals in Tertiary sandstones and shales, San Joaquin Basin, California

Mechanism of plagioclase albitization

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

The Moeraki Boulders--Anatomy of Some Septarian Concretions