Extensive Calcite Cementation Research Articles

Depositional Facies Model and Reservoir Quality of Paleogene Limestone in Labengki Island, Southeast Sulawesi

Eastern Indonesia has become an attractive venture for hydrocarbon exploration since 10 years ago. The discovery of hydrocarbon from Miocene carbonate of Tondo Formation has opened a new opportunity and hopes in the Southeast Sulawesi region. Can we find other potential reservoirs in Southeast Sulawesi? In this study, we assess and reconstruct the depositional model and the reservoir quality of the Paleogene Tampakura Formation in Labengki Island, Southeast Sulawesi. This field observation and petrographical study revealed that: (1) Tampakura Formation comprises mainly of grainstone, boundstone and floatstone with minor packstone and dolomitic wackestone/mudstone, (2) Tampakura Formation was deposited mainly in wide carbonate sand shoals and reef margin belt of rimmed carbonate shelf, (3) Boundstone and floatstone facies could be the best reservoir candidate in the region since they show extensive porosities development of cavernous and fracture porosity, (4) Dolomite cementation has deteriorated the reservoir quality of packstone which was deposited in platform interior-restricted marine, (5) Extensive calcite cementation in grainstone facies has reduced the reservoir quality of Tampakura Formation. However, locally, solution enlarged fracture porosity may have enhanced it. We suggest that post collision event of Late Oligocene - Early Miocene between Australian-originated microcontinent and ophiolite complex was highly responsible to create cavernous porosity. The collision resulted in the folding and uplifting of Tampakura Formation to the subaerial exposure. The carbonate strata were exposed to the surface developing a cavernous porosity and potentially becoming the best reservoir candidate for the next exploration target.Keywords: Carbonate facies, Kendari Basin, reservoir quality, Tampakura Formation.

Controls on near-surface and burial diagenesis of a syn-rift siliciclastic rock succession: A study from Permian Barren Measures Formation, Southern India

Distinct processes during the syn-depositional sedimentation and the post-depositional diagenesis determine the fabric of a clastic sedimentary rock. Temporal and spatial variations in any of these processes cause development of specialized characters in the sedimentary rocks. The Permian Barren Measures Formation in the Pranhita-Godavari Valley in southern peninsular India, is characterized by signatures of early to late diagenetic cementations within coarse-grained sandstones and mudstones. An integrated petrographic-geochemical study is carried out to understand the post-depositional near-surface and burial processes. Facies architecture manifests sedimentation in fluvial (channel, bar and overbank) to tide-influenced fluvio-tidal mixed depositional condition. Post-depositional near-surface early diagenesis changes include – (i) partial dissolution of detrital grains, (ii) biotite kaolinization, (iii) localized extensive ferruginous cementation, (iii) calcite cementation, and (iv) development of rhizocretions observed at different stratigraphic levels. These point to the role of meteoric water during the pre-burial stage that changed the pore water chemistry within the fluvial channel and bar sediments, along with onset of pedogenic (calcretization) processes in the vadose zone within the fine-grained overbank deposits. Marine (tidal) water influx in fluvial regime under a semi-arid climate modified the pore water chemistry. Shallow burial diagenetic changes, under elevated temperature and pressure include – (i) extensive calcite cementation in intergranular pore spaces and fractures, and (ii) late phase partial dissolution producing secondary porosity. A predictive model is derived to decipher the role of post-depositional changes in developing potential reservoir heterogeneity in a fluvio-tidal mixed depositional system.

Comparison of differential diagenesis of two oolites on the Lower Triassic platform margin, NE Sichuan Basin: Implications for the co-evolution of rock structure and porosity

Two oolites of the Lower Triassic Feixianguan Formation on the margins of the Kaijiang-Liangping (K-L) Trough in the NE Sichuan Basin were deposited in the upper parts of similar shallowing-upward sequences, and are capped by a subaerial exposure surface. However, they have contrasting diagenetic histories and reservoir qualities: the sour oolitic dolostone reservoirs on the eastern side of the K-L Trough have significantly higher porosities than the sweet oolitic limestone reservoirs on the western side. A petrologic and geochemical comparison of the two oolites was carried out so as to investigate the role of diagenetic processes on the co-evolution of oolite structure and pore network. On both sides of the K-L Trough, eogenetic meteoric digenesis led to an inversion of texture with ooid dissolution and interparticle cementation in the oolite layers close to the exposure surface. The layers relatively far from the exposure surface mostly underwent recrystallization in association with connate seawater, resulting in neomorphic fabrics in the ooid cortices and less interparticle calcite cement. During the subsequent dolomitization, the early-formed oolite structure were mostly inherited by the dolostones on the eastern side, with fabric-retentive textures in the layers close to the exposure surface and fabric-destructive textures in the layers far from the exposure surface. During deeper burial the oolitic limestones were subjected to extensive calcite cementation and porosity loss, whereas the dolostones generally retained more porosity and have better connectivity of the micropore network. The higher porosity of the dolostones is ascribed to lesser chemical compaction and associated cement precipitation, rather than the creation of new pores. Porosity enhancement is closely dependent on the absence of the calcite cement related to pressure solution. The heterogeneity of the dolostone fabrics and the porosity was further enhanced by the subsequent recrystallization and the dissolution-reprecipitation process induced by thermochemical sulfate reduction, during which small amounts of net porosity are calculated to have been created. The oolitic limestones are the best reservoirs close to the exposure surface, whereas the most porous dolostones with the highest degree of fabric destruction are commonly some distance from the exposure surface. These results provide some insights into the diagenetic controls on oolite structure and porosity, and have implications for predicting the porosity of deeply-buried oolitic reservoirs, so have global theoretical and practical significance.

Experimental Determination of Impure CO2 Alteration of Calcite Cemented Cap-Rock, and Long Term Predictions of Cap-Rock Reactivity

Cap-rock integrity is an important consideration for geological storage of CO2. While CO2 bearing fluids are known to have reactivity to certain rock forming minerals, impurities including acid gases such as SOx, NOx, H2S or O2 may be present in injected industrial CO2 streams at varying concentrations, and may induce higher reactivity to cap-rock than pure CO2. Dissolution or precipitation of minerals may modify the porosity or permeability of cap-rocks and compromise or improve the seal. A calcite cemented cap-rock drill core sample (Evergreen Formation, Surat Basin) was experimentally reacted with formation water and CO2 containing SO2 and O2 at 60 °C and 120 bar. Solution pH was quickly buffered by dissolution of calcite cement, with dissolved ions including Ca, Mn, Mg, Sr, Ba, Fe and Si released to solution. Dissolved concentrations of several elements including Ca, Ba, Si and S had a decreasing trend after 200 h. Extensive calcite cement dissolution with growth of gypsum in the formed pore space, and barite precipitation on mineral surfaces were observed after reaction via SEM-EDS. A silica and aluminium rich precipitate was also observed coating grains. Kinetic geochemical modelling of the experimental data predicted mainly calcite and chlorite dissolution, with gypsum, kaolinite, goethite, smectite and barite precipitation and a slight net increase in mineral volume (decrease in porosity). To better approximate the experimental water chemistry it required the reactive surface areas of: (1) calcite cement decreased to 1 cm2/g; and, (2) chlorite increased to 7000 cm2/g. Models were then up-scaled and run for 30 or 100 years to compare the reactivity of calcite cemented, mudstone, siderite cemented or shale cap-rock sections of the Evergreen Formation in the Surat Basin, Queensland, Australia, a proposed target for future large scale CO2 storage. Calcite, siderite, chlorite and plagioclase were the main minerals dissolving. Smectite, siderite, ankerite, hematite and kaolinite were predicted to precipitate, with SO2 sequestered as anhydrite, alunite, and pyrite. Predicted net changes in porosity after reaction with CO2, CO2-SO2 or CO2-SO2-O2 were however minimal, which is favourable for cap-rock integrity. Mineral trapping of CO2 as siderite and ankerite however was only predicted in the CO2 or CO2-SO2 simulations. This indicates a limit on the injected O2 content may be needed to optimise mineral trapping of CO2, the most secure form of CO2 storage. Smectites were predicted to form in all simulations, they have relatively high CO2 sorption capacities and provide additional storage.

Diagenesis of Oligocene continental sandstones in salt-walled mini-basins—Sivas Basin, Turkey

The recent discovery of Oligo-Miocene salt-walled continental mini-basins in the Sivas Basin (central Anatolia, Turkey) provides the opportunity to unravel the influence of halokinesis on the diagenesis of continental mini-basin infilling. In this study, petrographic and geochemical analyses are used to define the diagenetic sequences recorded by two mini-basins filled mainly by fluvial clastic sediments of the upper Oligocene Karayün Formation. The initial diagenetic features are those commonly encountered in arid to semi-arid continental environments, i.e. clay infiltration, hematite precipitation and vadose calcite cement. Other early cements were strongly controlled by sandstone detrital composition in the presence of saline/alkaline pore water. In feldspathic litharenites and lithic arkoses, near-surface alterations were characterized by the precipitation of analcime (up to 10%), albite and quartz overgrowths (<1%). These events were followed by extensive calcite cementation (up to 30%) during shallow burial diagenesis which prevented further mesogenetic alteration phenomena such as compaction. In feldsarenites, early diagenesis differs by (i) the absence of analcime, (ii) better developed albite cements, (iii) thin smectite–illite coatings forming pore linings and (iv) patchy calcite cementation (<5%). The limited development of calcite cement allowed mesogenetic alterations to occur, such as late quartz overgrowths, albitization of feldspar grains and chemical compaction. All these phases are responsible for the low porosity of feldsarenites (<2%). The greater abundance of carbonate cement in feldspathic litharenites and lithic arkoses is related to a greater proportion of detrital limestone in these sandstones. Early precipitation of analcime, albite, smectite–illite and quartz was likely triggered by the alteration of reactive grains by near-surface saline/alkaline brines originating from the dissolution of adjacent diapiric structures. Mini-basin confinement resulting from halokinesis was probably an important factor influencing surface and subsurface saline/alkaline fluid flow and related diagenesis. Despite the evident role of detrital composition in controlling the recorded diagenetic evolution pathways, the striking similarity between observations in the Sivas Basin and equivalent halokinetic settings of the Pre-Caspian domain suggests that continental clastic deposits of mini-basins may exhibit common diagenetic alteration effects, especially near-surface zeolite precipitation. These effects are linked to the close proximity of diapiric salt during deposition and burial, and result in a significant degradation of porosity.

Repeated injection of hydrothermal fluids into downdip carbonates: a diagenetic and stratigraphic mechanism for localization of reservoir porosity, Indian Basin Field, New Mexico, USA

AbstractA detailed study of the Pennsylvanian Indian Basin Field of New Mexico, USA is used to develop a conceptual model that predicts reservoir porosity in clean down-depositional-dip marine carbonate, where repeated fracturing allows for hydrothermal fluid flow. Strata updip probably experienced repeated events of subaerial exposure, resulting in mineralogical stabilization and extensive calcite cementation that prevented extensive hydrothermal alteration. Clean, carbonate sediment deposited downdip was more prone to alteration by hydrothermal fluids. In the reservoir, cement stratigraphy shows regionally persistent zones closely associated with fracturing. Fluid inclusion data show high temperatures and repeated rises and falls, indicating tectonic valving in a hydrothermal system. Salinity data support this and indicate a late event of meteoric influx. Sr- and O-isotope data indicate higher temperature and less rock–water interaction in fault damage zones, evidence for intensified fluid flow in such areas. Formation of vugs and molds was associated with the hydrothermal fluid flow, which was driven by convection, probably initiated during 40–30 Ma intrusive activity and continuing after the onset of Basin and Range uplift and unroofing.

Diagenesis of polymineralic temperate limestones in a cyclothemic sedimentary succession, eastern North Island, New Zealand

Temperate carbonate petrofacies (calcarenite and coquina) in the Pliocene‐Pleistocene Petane Group of eastern North Island, New Zealand, are dominated by aragonite faunas consisting primarily of bivalves and gastropods. Unlike calcite‐dominated temperate limestones, these polymineralic carbonates have undergone extensive early diagenetic alteration including extensive calcite cementation induced by aragonite dissolution. Marine cementation (type 1: pore‐lining, bladed calcite) was isolated to biogenic pores. It predated glauconite and may have been precipitated as low‐magnesium calcite, possibly in marine phreatic environments during sea‐level transgressions. Four phases of calcite cement with varying but definitive degrees of meteoric influence occur in the Petane Group. Type 2 (ferroan scalenohedral calcite) was the initial pore‐filling cement and precipitated from reduced pore fluids in a phreatic environment, possibly during or soon after the transition from marine to meteoric diagenesis. Type 3 (moderately ferroan drusy) calcite and type 4 (non‐ferroan drusy) calcite were sequentially precipitated during meteoric conditions from pore waters that changed from reducing to oxidising. Type 5 (sinter) cements comprise several forms precipitated during vadose meteoric diagenesis, the final meteoric phase of alteration in the Petane Group. Ferroan calcite cementation of silt matrix in coquina limestones overlain by terrigenous silt (type 6: matrix cement) probably occurred simultaneously with type 2/3 pore‐filling phases. A similar ferroan to moderately ferroan to non‐ferroan suite of drusy calcite cements also lithified concretions in non‐carbonate (siliciclastic sand) facies in the Petane Group, but only after the onset of compaction. Extensive skeletal diagenesis (stabilisation of magnesium calcite allochems, dissolution/recrystallisation of aragonite) occurred during type 3 and 4 cementation phases. Diagenesis in the Petane Group was stratigraphically influenced and ultimately controlled by uplift. Alternating sequences of porous and non‐porous formations developed a stacked sequence of confined aquifers that forced carbonate diagenesis to operate laterally.

Calcite cement distribution and its effect on fluid flow in a deltaic sandstone, Frontier Formation, Wyoming

Precipitation of extensive calcite cement during burial diagenesis can strongly modify the depositional permeability of a sandstone reservoir and affect fluid flow during production. To predict subsurface flow through cemented reservoirs, permeability distributions used in fluid-flow models must reflect this diagenetic overprint. Calcite cements in sandstones commonly occur as irregularly distributed concretions, which makes it difficult to predict diagenetic permeability modifications in the subsurface from typically spaced wells. Outcrops can provide a continuous image of heterogeneity produced by concretionary calcite cements. The size and distribution of calcite concretions were mapped in outcrops of the Frewens sandstone, Frontier Formation, in central Wyoming. Large, tabular calcite concretions in this deltaic sandstone generally follow basinward-inclined bedding. Median thickness of the concretions is 0.6 m, length is 4.2 m, and width is 5.3 m. The highest cement fraction is in the high-permeability facies at the top of the sandstone body. Concretion centers are approximately Poisson distributed within the sandstone. The upward-increasing cement fraction is caused by upward-increasing concretion size. Lateral variation in the fraction of the sandstone cemented by calcite has a normal distribution, with a mean of 12% (s = 5%). Spatial distribution of calcite cement in the Frewens sandstone was modeled using indicator geostatistics. Variograms were inferred from outcrop maps of cement. Indicator semivariograms of cement have a range of 30 m horizontally and 2.5 m vertically, dimensions that correspond approximately to the size of the largest concretions. Stochastic images of cement were created using indicator simulation with vertically varying cement proportion. Flow models indicate that concretions make flow paths more tortuous and retard flow in the coarser facies near the top of the sandstone. The fastest path through the sandstone is in the lightly cemented, high net-to-gross center of the sandstone body. Because the cement mainly occurs within the highest permeability facies in the sandstone body, a model based on depositional facies alone would overestimate upscaled permeability of the Frewens sandstone.

Diagenetic destruction of reservoir potential in shallow marine sandstones of the Broadford Beds (Lower Jurassic), north-west Scotland: depositional versus burial and thermal history controls on porosity destruction

The Broadford Beds are a shallow marine mixed siliciclastic-carbonate unit up to 140 m thick that crops out in the northern Inner Hebrides adjacent to a major Tertiary igneous centre. The formation can be divided into three lithological units: a lower limestone-dominated unit, a middle unit of muddy, calcareous sandstones and an upper unit of cleaner sandstones. The limestones underwent early meteoric diagenesis and early diagenetic processes in the overlying sandstones included the precipitation of berthierine, corrensite, phosphate and a small amount of Mg calcite. The most abundant burial cement is ferroan calcite, most of which precipitated during Mesozoic burial at depths of about 1 km. These cements were sourced through the chemical compaction of depositional carbonate within interbedded muddier horizons. Later calcite cements were precipitated in fractures from meteoric fluids during Late Jurassic — Early Cretaceous inversion and Early Tertiary uplift and a final stage of calcite cementation was associated with Tertiary hydrothermal activity. Other burial diagenetic phases include quartz overgrowths and chlorite. Quartz cementation was initiated before the main phase of calcite cementation, but the most significant quartz cementation accompanied the formation of chlorite during Tertiary hydrothermal activity. Overall the most significant factor leading to porosity loss in the Broadford Beds was the abundance of detrital clays and depositional carbonate in the original sediment. This led to extensive calcite cementation that both occuluded primary pores and inhibited the formation of secondary porosity.

Facies analysis and high‐resolution sequence stratigraphy of the Lower Eocene shallow marine Ametlla Formation, Spanish Pyrenees

ABSTRACTThe Lower Eocene Ametlla Formation of the Ager Basin, Spanish Pyrenees, is a rapidly deposited shallow marine unit formed in a setting characterized by syn‐sedimentary tectonic activity. Mapping of the formation over a distance of 25 km was conducted according to sequence stratigraphical principles with emphasis on facies analysis. Twelve facies, grouped in five facies associations, have been recognized in the Ametlla Formation. The studied succession records a vertical transition from deltaic systems prograding onto a sediment‐starved shelf, via estuarine deposits associated with incised valleys, to sandbar complexes in a tidal seaway.In terms of sequence stratigraphy, three scales of genetic sedimentary units were recognized.(1) At the regional scale, elements of two 3rd‐order composite sequences (sensu Exxon) have been recognized. These include a 3rd‐order highstand sequence set encompassing the lowermost part of the Ametlla Formation and the underlying Passarella Formation, and a 3rd‐order transgressive sequence set that constitutes the middle parts of the Ametlla Formation. The sequence sets are separated by an unconformity with up to 35 m of incision that is interpreted as a major sequence boundary. It is argued that the incised valleys associated with this unconformity were infilled during landward‐stepping of the shelfal depositional system. Basinwards, the unconformable surface becomes subhorizontal and is overlain by a 2 m thick oyster bed formed in a sediment‐starved setting subsequent to flooding of the incised valleys (which still acted as sediment conduits). Sandstones dominate the transgressive sequence set, whereas the highstand sequence set is dominated by siltstones, particularly in the lower part. In the transgressive sequence set, an upward increase in sand content and calibre is observed, relatable to punctuations of the transgressive trend by high‐frequency sea‐level fluctuations, and to downslope redistribution of sand.(2) At the subregional scale, detailed mapping indicates the presence of five 4th‐order sequences. The 4th‐order sequence boundaries are associated with sediment bypassing and minimal erosional relief, and were created by forced regressions during periods of relative sea‐level fall. Sharp‐based sandstones overlying these unconformities are believed to have accumulated during subsequent rise of relative sea‐level. Where 4th‐order maximum flooding surfaces can be recognized, the sequences may be subdivided into a sandstone‐dominated transgressive systems tract and a siltstone‐dominated highstand systems tract.(3) At the local scale, 2–9 5th‐order parasequences are present within the 4th‐order sequences. Superimposed parasequences are separated by flooding surfaces characterized by bioclastic accumulations, pervasive burrowing and extensive calcite cementation. The parasequences are commonly stacked in a landward‐stepping manner.

Kinetic Control on Clastic Diagenesis in the Los Angeles Basin as Determined by Apatite Fission Track Analysis: ABSTRACT

The relationship between diagenesis and thermal history in Miocene-Pliocene sediments of the Los Angeles basin were determined from analysis of illite/smectite ratios, apatite fission tracks, extent of feldspar dissolution, and carbonate cementation in argillaceous sediments and sandstones from subsurface and outcrop samples. The extent of the smectite-to-illite transformation and feldspar dissolution as a function of depth can be spatially related to principal faults and fault-bounded structural elements of the basin. The rate of smectite illitization and of feldspar dissolution increases near principal fault zones. Major cementation of sandstones by calcite and baroque dolomite cements is largely restricted to areas located along major fault zones; however, less extensive calcite cementation is found in the deeper parts of the basin central syncline. Time-temperature histories based upon track length histograms of fission tracks in apatite grains indicate that the more tectonically active flanks of the basin had higher thermal gradients during the latest Miocene to latest Pliocene compared to the central-basin structural block. For example, in the Brea Olinda field located along the Whittier fault zone, thermal gradients of 35{degree}C/km are interpreted since deposition of latest Miocene sediments whereas thermal gradients affecting a well 9 mi from the Whittier fault zone were muchmore » lower until 1-2 Ma, when the gradient increased to 35{degree}C/km. Taken together, these data indicate that the diagenesis of clastic sediments in the Los Angeles basin is strongly controlled by reaction kinetics and that the temperatures associated with structural domains in the basin have exerted the dominant control on reaction extent.« less

Calcite Cement Stratigraphy and Cementation History of the Burlington-Keokuk Formation (Mississippian), Illinois and Missouri

ABSTRACT Iron content and cathodoluminescence define six regionally extensive calcite cement zones over 100,000 sq. km of study area in the Burlington-Keokuk Formation of Illinois and Missouri. The regional cement stratigraphy is composed of mappable zones, analogous to lithostratigraphic units, which represent diagenetic episodes during which calcite with distinct cathodoluminescent properties precipitated in a given time interval relative to other diagenetic events such as cement fracturing and calcite dissolution. Individual cement zones may not be strictly isochronous throughout the study area but are not strongly diachronous. Two different, but related, cathodoluminescent sequences have been identified within pre-Pennsylvanian cements in the Burlington-Keokuk Formation: 1) within lower strata, early, nonferroan cement consists of both nonluminescent and luminescent calcite (zones II-IV); 2) within upper strata, early, nonferroan cement consists of entirely luminescent calcite (zone II). Petrographic relationships indicate that zone II is time equivalent with zones II-IV in lower Burlington-Keokuk strata. The occurrence of cement zones II-IV encased in Pennsylvanian shale and quartz sand within pre-Pennsylvanian karst cavities constrains the age of early, nonferroan calcite as pre-Pennsylvanian. In ll cases, pre-Pennsylvanian cements are followed by ferroan, dull to moderately luminescing calcite (zone V) which, in turn, is overgrown by nonferroan, moderate to brightly luminescing calcite (zone VI). Cross-cutting relations of Burlington-Keokuk syntaxial calcite with Mississippi Valley-type mineralization phases constrains the minimum age of cement zones V and VI as Early Permian. Pre-Pennsylvanian cements formed in a shallow, meteoric aquifer characterized by vertically segregated chemical facies. Pre-Pennsylvanian cements within lower strata precipitated from waters of varying Eh, while those in upper strata formed from reduced waters. Post-Mississippian/pre-Early Permian cements precipitated under less than 1 km of overburden from reducing meteoric waters or subsurface brines.

Geochemistry of Regionally Extensive Calcite Cement Zones, Mississippian of New Mexico: ABSTRACT

Carbonate cements from Mississippian skeletal limestones of southern New Mexico are dominated by echinoderm-syntaxial calcites that comprise 4 regionally extensive compositional zones. Previous petrography and cement stratigraphy proves that the oldest three zones (zones 1, 2, and 3) are pre-Pennsylvanian End_Page 513------------------------------ in age, and thus precipitated at temperatures around 25°C (77°F); they are interpreted as meteoric phreatic cements. The youngest cement, zone 5, is interpreted as a pre-Permian burial cement precipitated at temperatures less than about 80°C (176°F) and at burial depths less than about 1 km (3,300 ft). Originally defined on the basis of luminescence and staining, each zone has a distinctive assemblage of C13, O18, Mg, Mn, and Fe contents. The low MgO contents (less than 0.25 wt. %) in all zones indicates that sea water was insignificant in their precipitational waters over most of the region. Their FeO and MnO contents are compatible with their subsurface interpretations. The isotopically most positive values from bioherm muds and synsedimentary former high-Mg calcite cements (+4.5^pmil^dgrC13, -1.5^pmil^dgrO18 PDB) are interpreted as marine values, and offer a baseline with which to compare isotope values of non-marine cements. The pre-Pennsylvanian cements, zones 1, 2, 3, are markedly different from one another and show a progressive decrease in ^dgrC13 and ^dgrO18 with decrease age (^dgrC13 = +3.7, +2.4, -0.8^pmil PDB, respectively; ^dgrO18 = -1.3, -2.8, -3.7^pmil PDB, respectively). This decrease in ^dgrC13 is interpreted to reflect increased contribution from soil or atmospheric CO2 carbon. The decrease in ^dgrO18 is interpreted to reflect a decrease in O18 content of precipitational waters rather than an increase in temperature. The ^dgrC13 of all three zones is less than the interpreted marine values, which reinforces their fresh-water interpretation. The ^dgrO18 of zone 1 is greater and the ^d rO18 of zones 2 and 3 is less than the interpreted marine values. We propose a model in which zones 1, 2, and 3 precipitated in fresh phreatic groundwaters largely uncontaminated by sea water. Their chemistries reflect progressive stages in the chemical evolution of the water-rock system. This evolution resulted from either a progressive change from a rock-dominated to a water-dominated system, or may have involved a progressive climatic change from arid (zone 1) to wetter and more seasonal (zone 3). The carbon for these cements derived from Mississippian skeletal and lime mud components plus contributions of light organic carbon. Crinoids, the main skeletal component, could have been major sources only for zone 3 if they had C13 contents comparable to modern crinoids. More likely, they had C13 contents comparable to Mississipp an marine values and were major sources of carbon for all three zones via pressure solution and dissolution at the pre-Pennsylvanian unconformity. The post-Mississippian zone 5 has a wide range of ^dgrO18 values (mean = -7.4^pmil PDB), all less than those of the pre-Pennsylvanian zones. This light ^dgrO18 reflects elevated temperatures in the 40 to 60°C (104 to 140°F) range. Zone 5 has intermediate ^dgrC13 values which reflect complex, predominantly rock carbon sources, many of which were extraformational. End_of_Article - Last_Page 514------------

Facies Control of Cementation and Porosity, Pennsylvanian Fan-Delta Sandstones, Texas Panhandle: ABSTRACT

Interbedded fan-delta sandstones and limestones were deposited on a shallow carbonate shelf in the southern Anadarko basin during Missourian time. Hydrocarbon production from the fan-delta sandstones at Mobeetie field, Wheeler County, is controlled both by structure and facies-determined porosity distribution. Distal margins of some fan-delta lobes were reworked by marine processes, and carbonate fossil fragments and oolites were mixed with terrigenous clastics. Diagenetic history of the distal, marine-reworked sandstones was strikingly different from that of the more proximal, non-reworked sandstones. The first cement to precipitate in the reworked sandstones was a thin, isopachous rim of Mg-calcite cement that probably precipitated in the submarine environment soon after deposition. Next, establishment of a freshwater, phreatic environment in the sediments resulted in extensive calcite cementation in the calcareous sandstones. Dissolution of aragonitic oolites and fossils provided the source of the calcite that occluded primary porosity. In contrast, the non-reworked sandstones were not cemented because they lacked a calcite source, and so they retained high porosity. Rims of authigenic chlorite, which reduced porosity by only a few percent, were the earliest cements to precipitate in the non-reworked facies. With increasing burial, porosity in both the reworked and non-reworked fan-delta sandstones was reduced by precipitation of authigenic quartz, feldspar, kaolinite, Fe-calcite, and ankerite. These cements are generally minor in volume and do not influence porosity distribution. Generation of secondary porosity by dissolution of feldspars and rock fragments occurred in all sandstones but was more extensive in non-reworked facies. However, the main control of present porosity distribution is the presence or absence of early, freshwater calcite cement. End_of_Article - Last_Page 565------------

Diagenesis of Deltaic Sandstone: Olmos, San Miguel, and Upson Formations (Upper Cretaceous), Northern Rio Escondido Basin, Coahuila, Mexico: ABSTRACT

During the Late Cretaceous, the Rio Escondido basin was filled with deposits of lobate deltas that prograded eastward. Three distinct depositional environments can be recognized in the subsurface: delta plain (Olmos Formation), delta front (San Miguel Formation), and prodelta (Upson Formation). The sandstone of the Rio Escondido basin is predominantly feldspathic litharenite. Major framework grains are quartz (23%), plagioclase feldspar (35%), and volcanic and sedimentary rock fragments (42%). Most of the matrix was formed by compaction of sedimentary and volcanic rock fragments during early burial. Porosity was reduced to 19% by compaction before cementation. Chlorite, the earliest cement, formed thin rims around framework grains and replaced feldspar rock fragments, and matrix. Minor thin quartz overgrowths precipitated next. Extensive calcite cement occluded most remaining porosity by filling intergranular pores and replacing framework grains (chiefly feldspar). Subsequently, widespread dissolution of calcite created 5 to 10% secondary porosity. Kaolinite precipitated nex as a pore-filling cement and today is the chief occluder of secondary porosity. Ferroan dolomite replaced most remaining calcite; only remnant calcite is present in the sandstone today. Late-stage cements include local authigenic siderite and pyrite. A final dissolution event locally removed remaining calcite cement or framework grains (feldspar). Essentially all porosity present in the sandstone today is of secondary and not primary origin. Macroporosity in the sandstone is 5.0% and permeability is 2.6 md. Kaolinite cement, volcanic rock fragments, and matrix contain an estimated 6.5% microporosity. Porosity distribution is patchy and cannot be correlated with depositional facies. Porosity and permeability values do not indicate good reservoir quality in the northern Rio Escondido basin sandstone. End_of_Article - Last_Page 1564------------

Geochemistry of Regionally Extensive Calcite Cement Zones in Mississippian Skeletal Limestones, New Mexico: ABSTRACT

Petrography and cement stratigraphy of four regionally extensive cement zones in Mississippian crinoidal limestones indicate that these cements precipitated in meteoric phreatic environments. Each major zone has a distinct isotopic and trace element composition. Marine cements and lime muds in bioherms associated with the crinoidal sands have marine ^dgr13CPDB (+4.0 ppm). Phreatic cements become lighter in ^dgr13C in progressively younger zones 1, 2, and 3, representing a trend toward more contribution of organically derived carbon to precipitating waters. Since zones 1, 2, and 3 were all precipitated at shallow burial depths, their trend toward lighter ^dgr18O with decreasing age suggests increasingly light waters isotopically. At 25°C, the waters responsible for zone 1 and 3 cements are estimated as ^dgr18OSMOW= +1.5 and -0.9 ppm, respectively. The ^dgr13C of zone 5 cement is interpreted as a combination of rock-derived and organic-derived carbon, some of whic probably came from overlying Pennsylvanian strata. The distinctively light ^dgr18O of zone 5 is attributed to precipitation at somewhat elevated temperatures, averaging about 45°C, a value in agreement with estimated burial depths. Zone 1 cements appear to have formed in seaward parts of a freshwater phreatic system at shallow burial depths (relatively heavy ^dgr180). Zone 1 magnesium and carbon were derived from dissolution of skeletal high-Mg calcites (high Mg and heavy ^dgr13C). The driving force for zone 1 cementation was thus the solubility difference between high-Mg crinoidal calcite and low-Mg zone 1 calcite. Zone 2 also precipitated at shallow depths but in a more widespread groundwater system that contained some organic carbon (light ^dgr13C) and had a more landward recharge area (lighter ^dgr18O). Likewise, zone 3 is formed in a shallow-phreatic lens but with the most landward (freshest) recharge (lightest ^dgr18O). The inferred importance of organ c-derived carbon in zone 3 (light ^dgr13C) suggests degassing of CO2 as a driving force for precipitation. The light ^dgr18O of zone 5 reflects precipitation at elevated temperatures of deeper burial (750); its element composition (table) suggests a variety of intraformational and extraformational sources. Table End_of_Article - Last_Page 750------------

Diagenesis of Deep-Water Carbonate Turbidites, Upper Cretaceous Monte Antola Flysch, Northern Apennines, Italy

The Monte Antola Formation consists of a thick section of calcareous turbidites deposited at oceanic depths (below the calcium carbonate compensation level). Diagenetic features of the unit emphasize major differences between shallow and deep-marine diagenesis. The relatively stable original mineralogy and the absence of au early fresh water flush in the Monte Antola sediments allowed considerable compaction to occur before cementation, yielding a characteristic close-packed texture with relatively small volumes of cement. Early diagenesis consisted of remobilization of unstable opaline silica of sponge spicules and radiolarian tests. Late diagenesis included the precipitation of separate iron-free and ankeritic dolomite generations and extensive calcite cementation. Formation of recrystallization textures (microspar and pseudospar zones and veins) was controlled by original sediment grain-size and occurred after cementation, but before stylolitization. Weathering has resulted in oxidation of pyrite and associated dedolomitization (calcite after dolomite). Dedolomitization of ferroan dolomites results in the formation of ferroan calcites (maintaining original dolomite zonation) which retain ferrous iron in the lattice, not as ferric oxide/hydroxide coatings.