Late Carbonate Cements Research Articles

Oligocene to Holocene hydrocarbon migration and salt-dome carbonates, northern Gulf of Mexico

The geochemistry and geology of Damon Mound salt dome in Brazoria County, Texas provides an insight to the timing of hydrocarbon migration and carbonate caprock development in the Houston salt-dome province. Reservoir rocks flanking the salt dome received a charge of crude oil from deeply buried Eocene source rocks during Early Oligocene-Early Miocene time. The light δ 13C of the carbonate caprock at Damon Mound (−24.8 to −31.7‰ PDB) is consistent with an origin from bacterial oxidation of crude oil with similar δ 13C (−27.1 to −28.8‰ PDB). A minimum weight estimate of carbonate caprock at Damon Mound is 32.7 × 10 6 metric tons, suggesting bacterial oxidation of about 34.2 × 10 6 barrels of crude oil to form the carbonate. Late Oligocene-Early Miocene erosional exposure of the carbonate caprock provided a hard substratum to initiate the development of a coral reef in an otherwise unfavourable mud-dominated environment. Although the bulk of the reef carbonate displays normal marine δ 13C, some late carbonate cements show light δ 13C values (−22.1 to −29.6‰) from bacterial hydrocarbon oxidation. Buried carbonate caprocks and coral reef facies serve as carbonate reservoir rocks over Gulf of Mexico salt domes, lending support to the hypothesis that intense hydrocarbon oxidation can, in itself, give rise to some carbonate reservoir rocks. Ongoing hydrocarbon migration from deeply buried Cretaceous source rocks and the development of embryonic carbonate caprocks over shallow salt domes on the present Gulf of Mexico continental slope offer analogies and contrasts to the processes that gave rise to carbonates at Damon Mound in the geological past.

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.

Predicting Carbonate Mineral Precipitation/Dissolution Events during Progressive Diagenesis of Clastic Rocks

There is an observable, regular progression of early and late carbonate cements that is separated by carbonate mineral dissolution in many sandstones during progressive burial and diagenesis. The distribution of early cements is a function of incipient hydration of framework grains, sulfate reduction, and bacterial methanogenesis. These early cements typically precipitate from the sediment water interace to burial depths corresponding to about 80C. The distribution of late carbonate cements is a function of the relationship of organic acid anions, aluminosilicate reactions and CO{sub 2} in formation waters. Elevated PCO{sub 2} in a fluid where the pH is buffered by organic acid anions or aluminosilicates results in precipitation of a late carbonate cement (typically ferroan); these late carbonate cements generally form over the temperature interval of 100-130C. The late and early carbonate cement events are separated in time by a period of carbonate mineral dissolution or nonprecipitation. This dissolution is related to the increase in concentration of carboxylic acid anions resulting from the thermocatalytic cracking of oxygen-bearing functional groups from kerogen and/or redox reactions involving kerogen. Examples of the importance of early carbonate cementation/decementation to hydrocarbon reservoirs include the Campos basin of Brazil and the U.S. Gulf Coast. An example ofmore » late carbonate cementation/decementation includes the Norphlet Formation. This observed sequence of cementation and decementation can be modeled, and the modeling results can be used to predict enhanced porosity in the subsurface.« less

Burial Diagenetic Geothermometer and the Classifying Characteristics of the Burial Diagenetic Zones of Tertiary Clastic Rocks in Shengli Oil Field, East China

It is very important to study burial diagenesis for reservoir assessment. During the Cenozoic Era, the studied area was a downfaulted depression basin with continuous subsidence that was filled by thick clastic sediments. Based on the petrological data from 66 wells in this basin, the present geotemperature corresponding to the occurring depth of the burial diagenetic event approximates or is equal to the initial temperature of the diagenetic event. Therefore, the initial temperature of the burial diagenetic event can be calculated by this equation: T = G(H {minus} h) + 14.5C, where T is the initial temperature of the burial diagenetic even; G is geothermal gradient; H is the shallowest depth for this diagenesis occurrence; and h is the depth of constant temperature zone. The annual average surface temperature of the studied area is 14.5C. The different burial diagenetic geothermometers calculated by the equation are as follows: 65 {plus minus} 5C for minor quartz overgrowth; 90 {plus minus} 5C for intensive quartz overgrowth; 75 {plus minus} 5C for epidote dissolution; 92 {plus minus} 5C for authigenic garnet; 95 {plus minus} 5C for late carbonate cement; 140 {plus minus} 10C for transformation from kaolinite to illite. Based on all the burialmore » diagenetic geothermometers, the burial diagenetic sequence can be divided into shallow ( 130C) zones. It is demonstrated that these zones can be used to classify and assess the reservoir rocks and source rocks.« less

Diagenesis and Preservation of Porosity in Norphlet Formation (Upper Jurassic), Southern Alabama

Sandstones of the Norphlet Formation (Upper Jurassic) in the vicinity of Lower Mobile Bay, Alabama, have porosities greater than 20% and permeabilities up to about 1 darcy at depths of more than 20,000 ft (6,096 m). Typically, pre-Tertiary sandstones buried to such depths have approximately 6% (±2%) porosity; thus, deep Norphlet sandstones contain up to 14% excess porosity. This porosity is largely intergranular and appears to be preserved primary porosity. When the diagenetic scenario for the Norphlet is reconstructed in a time-temperature framework, it is apparent that porosity preservation is not the result of any single diagenetic event, but is due to a continuous series of diagenetic conditions that overlapped in time. The most important of these circumstances were the following. (1) The lack of pervasive early calcite or anhydrite cements found in the large eolian dunes (10-300 ft or 3-91 m high) that comprise the best Norphlet reservoirs. This lack of cement may have set the stage for subsequent events by keeping open pathways through which fluids were later able to efficiently contact the sandstone. (2) Early grain-coating clay/iron oxide rims reacted to form chlorite in the 176°-248°F (80°-120°C) thermal wind w. The grain-rimming chlorite may have inhibited subsequent quartz cementation. Up to 10.5% porosity may be due to the porosity preserving effect of chlorite. (3) Migration of hydrocarbons and development of geopressures, beginning at temperatures as low as 230°F (110°C), retarded further cementation. Up to 4.5% porosity may be due to the combined effects of hydrocarbons and geopressures. (4) A series of CO2-generating reactions occurred, including decarboxylation of organic acids, thermal cracking of liquid hydrocarbons, and thermochemical sulfate reduction. As a result of these reactions, late carbonate cements were not abundant. Thermochemical sulfate reduction (275°-356°+F or 135°-180°+C) has been of particular importance in the deep Norphlet. Reaction of hydrocarbons with anhydrite has resulted in the local removal of nodular anhydrite and has also affected gas quality (i.e., H2S content). The preserved porosity in Norphlet sandstones is the result of a combination of circumstances. The fact that these conditions overlapped in time-temperature space may be as important as any single factor.

Aspects of the diagenesis of the Sherwood Sandstones of the Wessex Basin and their influence on reservoir characteristics

Summary The Sherwood Sandstone Group sandstones of the Wessex Basin are important potential reservoirs for both hydrocarbons and geothermal brines. Analyses of conglomerate, sandstone and siltstone samples from outcrop and deep boreholes indicate a complex diagenetic history that has had important positive and negative effects on the reservoir properties. Over most of the basin, early diagenesis (eodiagenesis) is dominated by calcrete development but, towards the basin centre, non-ferroan dolomite associated with early evaporitic sulphate cements that appear to have been deposited in an inland sabkha or playa environment are important. Later diagenesis (mesodiagenesis) is characterized by selective framework grain dissolution of feldspars, and anhydrite cementation and subsequent dissolution—processes that have locally yielded significant secondary porosity. However, in some cases this secondary porosity has been destroyed by the precipitation of late manganiferous ferroan calcite and ferroan dolomite. Quartz cements are locally important. Rocks near the present-day outcrop have been affected by weathering processes (telodiagenesis) that have resulted in decalcification, and the precipitation of kaolinite, illite and iron hydroxides. The key factors determining the permeability of the sandstones are the original grain size and degree of sorting, the precipitation and subsequent removal of anhydrite, and the extent of early and late carbonate cementation, framework grain dissolution and overgrowth.

Thermal and Diagenetic History of Pleasant Bayou-Chocolate Bayou Area, Brazoria County, Texas: ABSTRACT

Studies of the Pleasant Bayou 1 and 2 test wells and of data from the Chocolate Bayou oil and gas field have yielded the most complete picture of a geopressured geothermal aquifer system yet obtained from the Texas Gulf Coast. The principal geothermal reservoir, the C (Andrau) sandstone, has outstanding porosity and permeability owing to (1) initially high porosity resulting from deposition in a winnowed distributary-mouth bar complex, (2) enhancement of porosity by secondary leaching by acid waters, and (3) isolation from late carbonate cementation. The maturity profile obtained for the test well is anomalous, but can be modeled using various time-paleotemperature and burial-history configurations. These models fall into two groups. The first assumes the present geothe mal gradient, modified by cooling of the lower Frio section by water flow. In the second, anomalously cool paleogeothermal gradients must be enhanced by the passage of warm waters through the middle Frio section. Both models indicate a fairly late origin of secondary porosity. End_of_Article - Last_Page 1211------------

Mineralogy and Diagenesis of Low-Permeability Sandstones of Late Cretaceous Age, Piceance Creek Basin, Northwestern Colorado

This report presents preliminary results of a mineralogic and diagenetic study of some low-permeability sandstones from measured surface sections and cores obtained from drill holes in the Piceance Creek Basin of northwestern Colorado. A documentation of the mineralogy and diagenetic history will aid in the exploration for natural gas and in the development of recovery technology in these low-permability sandstones. These sandstones are in the nonmarine upper part of the Mesaverde Formation (or Group) of Late Cretaceous age and are separated from overlying lower Tertiary rocks by a major regional unconformity. Attention is focused on the sandstone units of the Ohio Creek Member, which directly underlies the unconformity; however, comparisons between the mineralogy of the Ohio Creek strata and that of the underlying sandstone units are made whenever possible. The Ohio Creek is a member of the Hunter Canyon Formation (Mesaverde Group) in the southwestern part of the basin, and the Mesaverde Formation in the southern and central parts of the basin. The detrital mineralogy is fairly constant throughout all of these nonmarine Cretaceous sandstone units; however, in the south­ eastern part of the basin, there is an increase in percentage of feldspar, quartzite, and igneous rock fragments in sandstones of the Ohio Creek Member directly underlying the unconformity. In the southwestern part of the basin, sandstones of the Ohio Creek Member are very weathered and are almost-entirely comprised of quartz, chert, and kaolinite. A complex diagenetic history, partly related to the overlying unconformity, appears to be responsible for transforming these sandstones into potential gas reservoirs. The general diagenetic sequence for the entire Upper Cretaceous interval studied is interpreted to be (early to late): early(?) calcite cement, chlorite, quartz overgrowths, calcite cement, secondary porosity, analcime (surface only), kaolinite and illite, and late carbonate cements. Authigenic high-iron chlorite, which occurs on grain rims and in pore throats, is primarily responsible for the low-permeability of the subsurface sandstones of the Ohio Creek Member in the center of the basin. Kaolinite is the most abundant pore-filling authigenic clay in these sandstone from the southwestern part of the basin and is responsible for their distinctive white-weathering color in outcrop. In the sandstones below the Ohio Creek Member, however, chlorite and kaolinite occur locally, and authigenic calcite and illite are more abundant. The occurrence and distribution of secondary porosity is one of the most important aspects of the diagenetic history of these sandstones. It is present as moldic intra- and intergranular porosity, as well as microporosity among authigenic clay particles. Although present locally in most sandstone units, secondary porosity is particularly common in the uppermost sandstone units and is interpreted to have formed primarily as a result of weathering during the time represented by the Cretaceous-Tertiary unconformity.

Mineralogic Composition and Diagenesis of Tertiary Sandstones along Texas Gulf Coast: ABSTRACT

The upper Gulf Coast Wilcox Formation is composed dominantly of quartz with feldspar, metamorphic and volcanic rock fragments, and mud clasts. The lower Gulf Coast Wilcox Formation contains more quartz and volcanic rock fragments. Lower Gulf Coast Yegua and Jackson Formations are composed of quartz, volcanic, and lesser carbonate rock fragments. Northward they become more quartz-rich and contain metamorphic rock fragments and mud clasts. Lower Gulf Coast Frio and Vicksburg Formations are similar to the Yegua and Jackson. Northward, they become more quartz-rich, but carbonate rock fragments are absent. Despite variations in composition, Tertiary sandstones exhibit a similar diagenetic sequence idealized as follows. Surface to shallow subsurface diagenesis (0 to 4,000 ft; 1,200 m) begins with pedogenic clay coats and with leaching and calcite replacement of feldspar. Minor kaolinite, feldspar overgrowths, and iron-rich carbonate material are also precipitated. Porosity is commonly reduced by compaction from the original 40% to less than 30%. Moderate subsurface diagenesis (4,000 to 11,000 ft; 1,200 to 3,300 m) involves leaching of early carbonate cements and subsequent cementation by quartz overgrowths and later carbonate cement. These stages commonly reduce porosity to 10% or less, but this trend may be reversed by late leaching of feldspar, rock fragments, and carbonate cements. Restoration of porosity to more than 30% can occur, but this may be reduced by late cementation by kaolinite and iron-rich dolomite and ankerite. Deep subsurface diagenesis (11,000 ft; 3,300 m) is a continuation of late iron-rich carbonate cements. End_of_Article - Last_Page 440------------