Abstract The business environment of the petroleum industry over the past 10 years has exerted increasing pressure on companies to improve exploration success with a corresponding reduction in costs and cycle time. Companies have responded by establishing processes for geologic risk assessment, volumetric estimation, and economic analysis. The ‘hydrocarbon system’ concept is integral to all of these processes because it focuses evaluation drilling programs to address fundamental uncertainties of the petroleum geology. In this manner, companies can significantly cut the cycle time to key decisions, move quickly into appraisal or to other opportunities, and thereby reduce costs and allow evaluation of additional prospective areas. The hydrocarbon system concept was used in a frontier exploration program in the Rawson Basins, offshore Argentina. The focus of the postulated play was a test designed to prove or disprove the presence of a hydrocarbon system in any of the basins, rather than the more classic approach of testing the most economically viable (i.e., largest) prospect. The exploration program commenced with acquisition and interpretation of seismic and potential field data. A geologic model was developed assuming a hydrocarbon system composed of a lacustrine source rock associated with early rifting of the Atlantic Ocean, interbedded shales and fluvial–deltaic sandstones for seal and reservoir rocks, and structural closures for trap. An evaluation drilling program was developed to test the presence of an active hydrocarbon, and a single obligation well was positioned to test for a lacustrine source rock in the basin center, migration pathways, and reservoir and seal. Drilling encountered a sequence of red beds that lacked discrete reservoir sandstones or organic shales. The well penetrated the ‘basement’ Paleozoic strata and was abandoned without any drill-stem testing. The absence of a hydrocarbon system was clear, and the decision to abandon the area was validated. The appraisal of the Rawson Basins was done quickly, had clear objectives for evaluation (to prove or disprove the hydrocarbon system), and resulted in a sound and timely decision, based on clear, definitive data, to pursue other opportunities elsewhere.

Abstract In the annals of carbonate sedimentology, few fields have undergone more discussion than the area of carbonate cements. From early discussions by Sorby and other European geologists, through such descriptions as provided by Cullis (1904), into the explosion of cement papers in the late fifties and sixties, carbonate cements have received disproportionate attention. This interest was further expanded with the discovery of a diversity of modern submarine cements (Friedman. 1964; Milliman, 1966; Cifelli et nl., 1966: and many others) culminating in the appearance of two landtilark publications edited by Fiichtbauer (1969) and Bricker (1971). Why this interest? Studies of carbonate cemcnts provide visual gratification to carbonate petrographers and mineralogists. Economic geologists exploring for minerals and hydrocarbons womy about their effect on porosity occlusion or preservation. Inorganic geochemists are now able to precipitate carbonate cements under controlled conditions, and organic geochenlists can observe their interactions with living or fossil organic matter. Students of modern carbonate environments of deposition can ohserve almost instantaneous cementation processes in a diverse group of environments ranging from fresh water to the deep sea floor.

Abstract An accurate knowledge of the potential reaction paths and sequences followed during early diagenesis is central lo models of limestone textural development and the chemical evolution of rock-water systems. The lithification of chemically metastahle carbonate sediments involves extensive dissolution of aragonite and magnesian calcite and the ultimate rock texture and chemistry is controlled to a large extent by the relative rates of these dissolution reactions. This paper summarizes results from a laboratory investigation of the dissolution rates of various biogenic carbonates and evaluates the roles mineralogy, grain microstructurc, and solution saturation state play in determining the relative stability of aragonite and magnesian calcite during the dissolution phase of diagenesis. Dissolution experiments were performed in seawater and meteoric type solutions using crushed samples of different skeletal grain types composed of calcite, aragonite, and magnesian calcite at a common grain size (37-125 microns). The results indicate that microstructural complexity can control the relative reactivities of carbonate grains and override differences in mineralogic stability. Aragonite grains having more complex microstructures can dissolve more rapidly than the reportedly less stable magnesian calcites inferring that the difference of their reactive surface areas apparently is greater than that of their thermodynamic stabilities. The degree of microstructural control on dissolution rate is related to the saturation state of the solution. The dissolution rate data at various saturation states have been used to delineate regions of microstructural versus mineralogic control. In solutions undersaturated with respect to calcite, aragonites having complex microstructures can dissolve more rapidly than magnesian calcites. In solutions supersaturated with respect to calcite but still undersaturated with respect to aragonite, mineralogic control becomes more significant and magnesian calcites containing > 12 mole% MgCO, dissolve at rates similar to or greater than aragonites. In solutions supersaturated with respect to both calcite and aragonite, only magnesian calcites containing > 12 mole% MgCO 3 actively dissolve. Dissolution experiments using untreated natural carbonate sediments in solutions undersaturated with respect to calcite reveal that the amount of Sr 2+ released during dissolution can only be accounted for by preferential dissolution of aragonites. Thus, the commonly accepted sequence of diagenetic alteration (magnesian calcites > aragonite > calcite) is only strictly observed in experiments conducted at or near aragonite saturation. The composition of natural diagenetic fluids may sometimes fall below calcite and aragonite equilibrium, the region where grain microstructure has the greatest impact on relative dissolution rates. Because dissolution rates increase exponentially as saturation state decreases, more diagenetic “work” may be accomplished by short times spent in undersaturated solutions than long times spent closer to equilibrium. Therefore, the microstructural detail of component sediment grains may be a more important control on the sequence of diagenetic alteration than mineralogic stability in certain diagenetic settings.

Abstract Seawater is a complex solution of inorganic ions and organic molecules in contact with solid phases. Because of its reactivity and sorptive properties, some of the organic matter (OM) will directly affect the kinetics of inorganic reactions by modifying the rates and perhaps the types of reactions occurring between the inorganic ions in solution and the solids. The interaction of natural OM with calcium carbonate systems occurs in two ways: (1) adsorption of the OM to calcium carbonate surfaces and (2) complexation or chelation of free cations by dissolved or adsorbed OM. Both processes involve polar functional groups on the organic molecules, with the carboxylate anion (RCOO~) being the most likely interacting species, although other functional groups may also be important. OM associated with a variety of skeletal and non-skeletal calcium carbonate, including skeletal organic matrix, OM within ooids, and OM extracted from carbonate grain surfaces, was studied for chemical characterization, adsorption phenomena, and cation-binding ability. Skeletal OM is largely protein while ooid and adsorbed OM are humic substances with proteinaceous components comprising about one-third or more of the composition. Aspartic acid is the most abundant amino acid in both skeletal protein and humic substances. Conversely, OM associated with non-carbonate sediments is poor in proteinaceous constituents and relatively depleted in aspartic acid. Aspartic acid-rich protein and humic substances bind or complex with metal ions in proportion to the concentration of carboxyl groups present. Blockage of carboxyl groups to make them inactive destroys the ability of the OM to bind metal ions. Many, if not most, of the carboxyl groups available for metal ion complexation in both calcified protein and aspartic acid-rich humic substances are on aspartic acid. Thus, this amino acid provides a significant portion of the metal-binding ability of the different types of OM. Aspartic acid-rich OM is preferentially adsorbed by calcite compared to quartz. Again, the carboxyl group is the likely function to be involved in this adsorption. Blockage of carboxyl groups significantly reduces the ability of humic substances to adsorb to mineral surfaces. The similarity in geometry, charge, and composition enables the carboxylate anion to substitute for the carbonate anion in complexing calcium ions or in adsorbing to calcite surfaces. Competition between organic and inorganic ions for dissolved species and surface adsorption sites is driven by the requirement of the system to remain electroneutral. Concentration variations in dissolved organic and inorganic ions in the pore waters brought about by bioturbation and organic and inorganic diagenesis result in variations in the tendency of OM to affect the chemistry of the system. Most of the calcium carbonate formed in the marine environment consists of skeletal material. This calcium carbonate contains protenaceous OM that is thought to be involved in formation of the mineral phase (biological calcification). By analogy, naturally-occurring OM of somewhat similar composition and properties and with identical functional groups may also be involved in the precipitation of calcium carbonate in the sedimentary environment (geological calcification)

Abstract Modern aragonite cements have a variety of morphologies, predominantly laths or needles. Needle fringes are commonly somewhat splayed (except where developed as epitaxial continuations of well-ordered substrate crystals, e.g., in molluscan shells). Similar needle fringes preserved as aragonite engulfed in second generation calcite spar have been found in Pennsylva-nian to Triassic rocks. Many other isopachous fibrous calcite cements with regular optic orientation have been commonly inferred to be the product of pseudomorphic replacement (paramorphic calcitization) of an aragonite precursor cement. Alteration textures in calcitized skeletons, ooids, and cements known to have been originally aragonite do not support that inference. Even if pseudomorphism were supportable, morphologies of aragonite and calcite cements fringes are dissimilar. Botryoidal aragonites, now calcitized to various irregular mosaics, occur in Mississippian to possibly lower Jurassic rocks, often with an elevated Sr 2+ content and/or rare oriented relics of the original aragonite fibers. Morphologically similar botryoids have been reported from the lower Cambrian and from the middle Precambrian. Comparable botryoids and engulfed aragonite needle fringes have not been encountered in rocks of intervening ages. In some Pleistocene cement fringes from Japan, mixed radiaxial fibrous calcite (RFC) and fascicular-optic calcite (FOC) overlie both aragonitic skeletons and first-generation aragonite cement. This occurrence and other properties of RFC-FOC refute earlier interpretations of genesis of those cements by replacement of a metastable fibrous precursor (possibly aragonite) at a distally migrating alteration front. That RFC-FOC and other cements with undulose extinction overlying still-aragonitic cements suggest that undulose extinction is not a secure criterion for replacement origin. Similar reservations apply for non-planar crystal boundaries when the cements are composite crystals (as in the RFC-FOC fringes).

Abstract Radiaxial fibrous calcite, a common cavity-fill in ancient reefs and mud mounds has previously been identified as a replacement of acicular carbonate cements. Diagnostic features of the calcite were suggested to be a combination of those inherited from the original cement and those introduced during isomorphism. A more rigorous examination of the proposed replacement theory, and of the evidence employed to support it. raises doubts as to its correctness. New observations upon cavity-filling radiaxial calcites from Western Australia suggest that most of the features originally considered of neomorphic origin are instead primary. The peculiar, and diagnostic, fabric of distally-convergent fast-vibration directions within each crystal (contrasting with a distally-divergent pattern of subcrystals) is now reinterpreted to have formed by a process of asymmetric growth within calcite crystals undergoing split-growth. Radiaxial crystals are composite and are attempts by these crystals to assume a spherulitic growth form in diagenetic environments which favor the growth of length-slow calcite. The changes from split-crystal spherulitic growth to more 'normal' unit-crystal growth (and visa-versa) that can be observed in the Australian cavity-fills were probably induced by changes in the chemistry of the cavity fluids. Implications of this study also include (1) that split-crystal growth is a common phenomenon in calcite cements, and (2) calcite, probably both high-and low-magncsian calcite, grew in the form of large, sparry crystals within the marine phreatic environment—a habit and mineralogy not common in present-day comparable settings.

Abstract Porosity prediction in subsurface carbonates remains difficult although our understanding of cementation in carbonate rocks has greatly increased during the past few years. The diagenetic setting is inherited from the depositional setting but continually modified during burial. In the marine setting, cementation occurs in the deep sea, in reefs, on shallow water platforms, and along shorelines. Carbonate and evaporite cementation typifies supratidal flats. During shallow burial, carbonate rocks contact fresh waters and additional cementation occurs along with solution, mineralogical stabilization and possibly dolomitization. Continued burial results in physical and chemical compaction, pressure solution and additional cementation. Several factors play a role in cementation in carbonates, i.e., the rate and total volume of water movement and timing of cementation relative to hydrocarbon generation; fluid chemistry; and the effect of pH, CO, activity, temperature and pressure on mineral solubilities. Petrographic studies coupled with measurements of stable isotopic ratios, trace elements and fluid inclusions are made to assess various stages of cementation and their relation to burial history and fluid flow and composition. Such integrated studies are providing a more detailed understanding of diagenetic events, especially cementation, and ultimately will increase the ability to predict porosity distribution.

Abstract By the time a carbonate unit has been buried to the depths of most petroleum reservoirs, the significant question is often not “how did the pores originate?”, but rather, “why are they still there?” Preservation of porosity, regardless of its origin, is a consequence of one or more of the following mechanisms: (1) minimal burial, (2) reduced burial stress, generally due to overpressured pore fluids, (3) increased framework rigidity, which prevents compaction, (4) exclusion of pore waters by petroleum entry, (5) stable mineralogy, (6) permeability barriers, isolating porous intervals from external diagenetic fluids, and (7) pore resurrection, a consequence of the temporary filling of pores with cement that is subsequently removed. Examples from the stratigraphic record demonstrate that each of these pore-preserving mechanisms may control reservoir quality.

Abstract The origin of peloidal textures common to submarine substrates lithified by magnesium calcite has been a subject of controversy. A review of the proposed origins of this intriguing and diagnostic feature of magnesium calcite submarine cements reveals four suggested sources for these peloids—algal, sediment, replacement and precipitate origins. A reconsideration of both the petrographic and chemical characteristics of these peloidal textures indicates that they probably originate primarily in chemical processes that may involve repeated nucleation around centers of growth. Several factors support this hypothesis: the peloids have a generally limited size range; their shape is typically spherical; the best-developed peloidal textures occur in restricted microcavities; peloidal textures exhibit well-developed zonation in many cement crusts; peloids and associated magnesium calcite rim cements have a similar chemical composition; and textural and depositional fabric characteristics are similar in magnesium calcite peloids and in precipitated spheroidal aggregates of pyrite. The pelletizing action of cryptic organisms and calcification of algal filaments also play a role—although a somewhat more limited one—in the formation of a peloidal texture in these magnesium calcite cements. With the recognition that some calcite peloidal textures can be formed chemically, caution is urged in interpreting the origin of “pelleted” limestones.

Abstract Classifications and “energy” interpretations of limestones are based on the ideas that, in calm waters, micron sized particles of lime mud are (1) available and (2) able to settle on the bottom and remain there, whereas in agitated waters, such tiny particles remain in suspension and are not deposited. Lithified, mechanically deposited lime mud is known as micrite, a contraction of “microcrystalline ooze” which has been inferred to represent a clay-size matrix whose presence signifies a lack of vigorous currents (Folk, 1962, p. 67). Micrite is usually equated to original matrix, hence a mechanically deposited material between particles, as distinct from precipitated cement. As thus interpreted, the loosely termed “micrite cement,” used in the literature is a contradiction. Accordingly, physically deposited interparticle material cannot be both micrite and cement. Submarine cryptocrystalline high-magnesium calcite cement in reefs (and beachrock) that is precipitated within millimeters to centimeters of the surfaces of reefrock looks just like micrite. Misidentification of this cement for micrite or non-critical use of the term micrite leads to error in interpretation of depositional facies. Case histories abound where unwary geologists have confused reefrock with a supposed “low-energy” lime-mud facies. Study of reefrock in thin section under an electron beam in which cryptocrystalline high-magnesium calcite cement luminesces and aragonitic particles appear in black presents a dilemma: where the cement : particle ratio is high it is easy to misidentify reefrock, even when one is taking great care.

Abstract Dredging of an offshore pipeline trench across the southeast Florida shelf exposed a well-preserved, porous relict Acropora palmata coral reef whose exposed upper framework surface is highly altered into a dense submarine-lithilied pavement. Five distinct facies were identified from samples obtained from an excavation ditch perpendicular to the shelf edge 40 km north of Miami: (1) back-reef coral head; (2) back-reef Acropora cervicornis; (3) Acropora palmata framework; (4) fore-reef coral head; and (5) fore-reef nibble. Reef facies zonation is similar to that of modern Caribbean reefs. Radiocarbon dates of A. palmata framework samples range from 7,145 ± 80 to 9,440 ± 85 years BP. Internal structure reveals a growth history of rapid reef-framework accumulation (ave. 6.6 m/1000 yrs) in response to rising sea level, and the transition from a fringing reef to an extensive shelf-edge barrier reef following the submergence of the Florida shelf. Occasional submarine cements formed during accumulation of reef facies are predominantly microcrystalline magnesium calcite and occur as rim cement, peloidal infill, coatings on grains, and as dense crusts on the surface of corals and reef debris. Syntaxial acicular aragonite cement is present in minor amounts within coral skeletal cavities and predates magnesium-calcite cement where present together. Magnesium-calcite submarine cements generally formed in areas of high agitation (e.g. wave energy) and/or low sedimentation (facies accumulation), preserving most internal skeletal porosity in A. palmata and fore-reef coral heads through the formation of dense surficial crusts and thin infill rims around coral skeletons. Submarine cements more easily filled and occluded primary porosity in the delicate branching coral A. cervicornis and reef rubble because of smaller sample cross-sectional area. Because reef growth was mostly vertical and somewhat landward, intermittent layers of well-cemented and encrusted rubble formed within fore-reef facies as a result of low sedimentation rates or hiatuses in talus slumping. Magnesium-calcite cements provide an effective control on primary porosity within the reef deposits, for their limited and crust-like distribution acts to preserve remaining porosity from further cementation and occlusion. Rapid reef growth during a time of rapid sea-level rise (Holocene transgression) prevented reef-framework deposits from being subjected to prolonged periods of submarine cementation, thereby significantly limiting the degree of porosity reduction. The demise of the once flourishing barrier reef was brought about by changing shelf water conditions coincident with the rapid flooding of extensive, relatively flat areas of the back-reef incipient shelf. Following this, the inactive barrier reef became submerged by the continuing rise in sea level, during which time encrusting organisms, and extensive reiterative submarine cementation, bioerosion, and sediment fill transformed the exposed upper reef-framework into a dense, micritic zone of low permeability. This pavement-like reef-framework crust has protected internal reef deposits from additional alteration and cementation, and as an early diagenetic seal may act to further preserve porosity in a potential reservoir facies.

Abstract Four shallow cores taken at Hogsty Reef atoll, southeastern Bahamas, reach the Early Pleistocene at 30 m below sea level. Unlike in other banks of the Bahamas, Hogsty Reef limestones consist of carbonate grains and cements that have maintained their original mineralogy. Five types of cement are found in the Pleistocene limestone of Hogsty Reef: (1) micritic Mg-calcite, which is predominant in all four cores; (2) bladed Mg-calcite, which occurs in several thin (<1 m) intervals where it is associated with (3) fibrous aragonite, which is also found in Holocene cemented crust; (4) blocky low-Mg calcite, which occurs in several thin intervals underlying subaerial exposure horizons; and (5) blocky Mg-calcite, an unusual type of cement which consists of clear anhedral crystals 25-100 p.m in size and contains less than 10 percent MgC03. It is found together with bladed Mg-calcite and aragonite. Preservation of original mineralogy of cements and grains may be attributed to a minor influence of fresh water, resulting from the semi-arid climate of the southeastern Bahamas, and the unlikely formation of a freshwater lens during periods of emergence.

Abstract Multiple horizons within the Redwater Shale Member of the Oxfordian Sundance Formation of southeastern Wyoming exhibit megascopic features recording repeated submarine cementation on the Jurassic seafloor. Some limestone units were bored by endolithic molluscs and encrusted by oysters and serpulid worms prior to the deposition of overlying units. Other sandstone and limestone layers were lithified, fragmented, reworked, and abraided at the sediment surface, giving rise to laterally-continuous layers of sandstone and limestone cobbles which were also densely bored and encrusted by benthic invertebrates. Still other encrusted and bored units contain lithified limestone clasts demonstrating multiple episodes of cementation during the deposition of individual hardground layers. Marine cement, truncated at lithoclast surfaces or crosscut by endolithic borings, consists of equant calcite crystals, acicular isopachous crusts, and epitaxial rims, but is predominantly equant inclusion-free spar, identical in habit to Holocene meteoric phreatic low-magnesium calcite. Its presence in Jurassic marine grainstones as a synsedimentary marine phase demonstrates that generalizations which relate cement morphology and composition to cementation environments in modern systems may not be valid when applied to ancient limestones. The variable morphology of early cements, even within individual hardground units, also suggests that calcite crystal habit may more frequently record the effect of kinetic factors, such as rates of crystal growth during cement precipitation, than the often-implied effect of Mg/Ca concentrations in ambient pore fluids.

Abstract Devonian (Givetian and Frasnian) reef reservoirs in Alberta and British Columbia contain 60% of the conventional, recoverable oil and 20% of the recoverable gas in the Western Canada Sedimentary Basin. Although the depositional history of these reefs is well understood, it is the diagenetic “overprint” that is often responsible for their reservoir quality. This paper discusses diagenetic facies common to many Devonian reefs and assesses the role of cementation in controlling reservoir quality. Frasnian (Woodbend and Beaverhill Lake Group) reefs are characterized by stromatoporoid and coral knoll reef belts deposited near moderately sloping bank edges. Bank interiors are commonly extensive and characterized by cyclic deposition of grainstone shoals and lagoonal and tidal flat sediments. Certain Givetian reefs found in evaporite basins usually occur as areally small, “pinnacle” reefs with steep (>20°) margins and only minor bank interior development. Givetian reefs studied include the “Presqu'ile” reef complex in northeast British Columbia and Rainbow Member “pinnacle” reefs in northwest Alberta. Frasinian reefs examined in Alberta include: (1) Swan Hills, Judy Creek, Carson Creek, North (Beaverhill Lake Group); (2) Golden Spike, Redwater, Strachan, Ricinus (Woodbend Group); and (3) Nisku reefs at West Pembina (Winterburn Group). Most reefs have been subjected to diagenesis in essentially three environments: (1) submarine (syndepositional, marine to hypersaline pore waters); (2) subaerial (fresh to marine pore waters); and (3) subsurface (well below phreatic aquifers, saline to brackish pore waters). Fibrous and bladed calcite cements, submarine “cracks”, micrite cements, and bored hardgrounds are typical submarine diagenetic fabrics, particularly at bank margins. Submarine cements in reef interiors occur as micro-crystalline or finely crystalline, fibrous, or drusy calcite rims on carbonate grains or as isopachous linings of fenestral pores. Subaerial disconformities are common in most reefs, and associated vadose diagenesis produced localized paleosols, micro-stalactitic cements, and zones with abundant solution porosity. Phreatic cements are predominantly non-ferroan, clear spar calcite with varied Fe2+ and Mn:+ concentrations. Subsurface cementation produced ferroan and non-ferroan calcites and dolomites which in many cases can be related to stylolite formation. Other subsurface diagenesis includes dolomite and anhydrite replacement, sulfide mineralization, and bitumen formation. The major diagenetic influences on limestone porosity are cementation and solution. Submarine cements are commonly extensive in reef margins and in Golden Spike and Rainbow reefs accounting for over 70% of the total cement volume. As submarine cements decrease in importance, porosity generally improves. Alternating cementation and solution associated with fresh water vadose/phreatic environments and localized cementation associated with stylolites is characteristic of most reef interiors. These processes enhance depositionally-controlled layering and produce interbedded porous and well cemented units. This type of “diagenetic stratigraphy” results in stratified reservoirs with numerous permeability barriers which can seriously hamper hydrocarbon recovery. In the subsurface environment, chemical compaction (pressure solution) and associated calcite and minor dolomite cementation are the most important processes affecting reservoir quality. In Swan Hills and “Presqu'ile” reefs, subsurface cements account for over 55% of the total cement volume. In contrast, subsurface cements represent less than 15% of the total cement volume in Rainbow reefs and certain Leduc reefs (Golden Spike, Redwater). The relative lack of subsurface cements helps to preserve reef porosity that survived early cementation.

Abstract Carbonate cements from Mississippian skeletal limestones of southern New Mexico are dominated by echinoderm-syntaxial calcites that comprise four regionally extensive compositional zones. Previous petrography and cement stratigraphy proves that the oldest three zones (zones 1,2,3) are pre-Pennsylvanian in age and were precipitated at temperatures around 25°C; they are interpreted as meteoric phreatic cements. The youngest cement, zone 5, is interpreted to be a pre-Permiau burial cement precipitated at burial depths less than 1 km and temperatures less than about 60°C. Originally defined on the basis of luminescent and staining characteristics, each zone has a distinctive assemblage of δ 13 C, δ 18 O, Mg 2+ , Mn 2+ and Fe 2+ contents. The low MgO contents (less than 0.25 wt.%) in all zones indicates that seawater was an insignificant component of their prccipitational waters over most of the region. Their FeO and MnO contents are compatible with their subsurface interpretation. Mississippian marine carbonate δ 13 C(~ + 4.0%c PDB) and δ 18 O(— 1.5%c PDB) are estimated from analysis of micrite, synsedimentary marine cement, and crinoid skeletal carbonate. In addition to providing a base line with which to compare non-marine cements, this estimate implies that the composition of Mississippian epeiric seawater was enriched in 13 C and comparable in 18 O to modern seawater. The pre-Pennsylvanian cements (zones 1, 2 and 3) are markedly distinct from one another and show a progressive decrease in δ 13 C and δ 18 O through time (δ 18 O= −1.3,-2.8, 3.7%o PDB respectively; δ 13 C= +3.7, +2.4, −0.85k PDB respectively). We propose a model in which zones 1, 2 and 3 precipitated in meteoric phreatic groundwaters largely uncontaminated by seawater. The distinctive trend of pre-Pennsylvanian cements reflects progressive stages in the chemical evolution of the water-rock system. This involved a decrease in the degree of rock-water interaction, which in turn resulted from depletion of metastable mineralogies through recrystallization or through isolation by cementation. Thus, the progressive decrease in 13 C reflects equilibration of porewaters with Mississippian skeletal carbonates during early cementation (zones 1 and 2) and increased contribution from soil-gas derived carbon during late cementation (zone 3). Similarly, the progressive decrease in l8 O in zones 1 through 3 reflects decreasing contribution of 18 O from Mississippian marine carbonates and progressive equilibration with meteoric porewaters. Hence, the isotopic composition of zone 3 cements reflects equilibrium precipitation with meteoric waters. Importantly, decreasing rock-water interaction during pre-Pennsylvanian cementation is supported by decreasing Mg 2+ contents among zones. High magnesian calcite crinoids are the dominant skeletal component in the Mississippian limestones and likely were the major contributors of carbonate for cementation during diagenesis of all three zones. A progressive decrease in Mg 2+ content is compatible with a model of diminishing rock-water interaction. Zones 1 and 2 have the heaviest isotopic compositions and highest Mg 2+ contents. The very low Mg 2+ content of zone 3 is predicted from very light δ 13 C compositions which predicates only minor contribution from crinoidal calcite sources. Post-Mississippian zone 5 cement has a wide range of δ 18 O values (mean= −7.49k PDB), all more depleted than any pre-Pennsylvanian cement. This light δ 18 O probably reflects elevated temperatures in the 40-60°C range. Zone 5 has intermediate δ 13 C values which reflect complex predominantly “rock” carbon sources, many of which were probably extraformational as suggested by stratigraphic variations in Mn 2+ and Fe 2+ .

Abstract The Stuart City Trend is a shelf-edge buildup of Lower Cretaceous bioclastic and reefal carbonates, now buried at depths from 3300 to 5000 m. Compaction and cementation have generally reduced rock porosities to less than 9%. Sediments were cemented in the marine environment by finely crystalline, bladed, isopachous Mg-calcite and the volumetrically important (14 vol%) coarse to very coarsely crystalline, fibrous to bladed, isopachous, Mg-calcite cement. These cements have been neomorphically altered to low Mg-calcites, forming unusual radial and radiaxial textures. Evidence for a marine origin consists of a relative 1 mol% Mg 2+ memory, a marine-like isotopic character (δ 18 O ≈ − 2.5 and δ 13 C ≈ +2.0) and early relative timing of precipitation. Diagenetic alteration by interaction with meteoric water in lenses formed within topographic highs along the shelf margin changed the initial marine chemical, isotopic, and textural character of the sediments. Secondary porosity formation, mineral stabilization, aggrading neomorphism and equant spar calcite cementation are the important products of meteoric diagenesis. The equant spar calcite cements make up approximately 16% by volume of the limestones studied. They are iron and manganese poor and the majority have a δ 13 C composition which falls in the range of modern marine carbonates; i.e., 0.0%c to +3.5%c The δ 18 O compositions range from − 1.3%c to −6.6%o relative to the PDB standard. Oxygen stable isotope and petrographic data suggest that over 7 volume % of the porous sediments was filled by equant spar calcite cements formed in a near-surface meteoric environment. An additional 9 volume % of the pores were filled by equant spar calcite cements formed at shallow to intermediate burial depths (<2,000 meters) in a water-limited system. Thermally induced δ 18 O depletion of the equant spar calcites, indicating fluid flow, was of minimal importance. Pyrobitumen pore fillings and inclusions in the outer 0.1 mm-thick rims of the very coarsely crystalline, equant spar calcite cements indicate that only minor amounts of cementation have occurred since the introduction of hydrocarbons. Deep burial diagenesis, i.e., post-hydrocarbon migration, consisted of the precipitation of minor amounts of galena, fluorite and Sr 2+ -rich equant spar calcites, i.e., <1%. These diagenetic products can be directly related to the present-day formation water. The chemical, isotopic, and textural characteristics of the Stuart City Trend limestones contain the imprints of their initial marine composition and shallow diagenetic alterations in a hydrodynamic system. Burial diagenesis has not significantly altered these limestones. Fault and fracture control on the movement of formation waters in this system determine the location and intensity of late stage diagenetic events.

Abstract Lower Cretaceous platform carbonates and shales, buried to depths in excess of 450 meters by the end of Eocene time, have been locally affected by late-stage diagenesis. In the South Texas study area, the rocks are presently buried to depths between 600 and 2800 meters. Burial diagenetic cements include ferroan and non-ferroan calcite, ferroan baroque dolomite, anhydrite, kaolinite, Ba-rich celestite, galena and sphalerite. A subsurface origin is evidenced by the absence of these minerals in outcrop, their occurrence in fractures, and their chemistry. Carbonate cements are chemically and isotopically zoned; the iron content of baroque dolomite cement varies by as much as 10 mole percent from the center to the edge of single crystals. The lack of stratigraphic or regional trends in the distribution of iron in baroque dolomite suggests that the iron is derived from nearby shales. A strong negative correlation between δ 13 C and average iron content of baroque dolomite suggests the simultaneous reduction of iron oxides and oxidation of organic material. The late subsurface carbonate cements commonly have depleted δ 18 O values which must either have formed by precipitation from hot brine or from isotopically depleted (meteoric) water. Both possibilities require the vertical movement of water along faults. Galena and sphalerite occur as minor components in some cores. Brines moving up faults after albitizing feldspars in more deeply buried formations could be the source of lead and zinc for these minerals. 87 Sr/ 86 Sr ratios for late diagenetic phases are similar to ratios of Edwards Formation brines in this area today, confirming that basinal brines have contributed to late-stage diagenesis. Most diagenetic phases can be qualitatively accounted for by the injection of basinal brines along fractures and faults into a meteoric-dominated aquifer.

Abstract The timing of petroleum migration in samples of fractured Cretaceous reservoir limestones from Oman and the United Arab Emirates is determined from observations of hydrocarbon fluid inclusions in calcite cements. Petrography and geohistory analyses of four wells that were variably affected by formation of the Oman Foredeep reveal five stages of diagenesis, fracturing, and fluid migration. (1) Quiet shelf deposition: early cementation associated with regional unconformities: (2) pre-orogenic shelf emergence: fractures cutting Stage 1 cements are healed by very cloudy, cleaved, and twinned calcite containing microfractures with yellow-white Hourescent, hydrocarbon fluid inclusions; (3) initial foredeep downwarp: fractures cross cutting Stage 2 fractures are healed with cloudy, cleaved, and sometimes twinned calcite containing dull-blue fluorescent, hydrocarbon fluid inclusions; (4) rapid subsidence and filling of foredeep with sediments including flysch. exotic blocks, and thrust toes: burial and tectonic stylolites crosscut Stage 2 and 3 fractures; and (5) uplift of the Oman Mountains: fractures cross cutting all diagenetic features are filled with clear, untwinned and uncleaved calcite containing only non-fluorescent, aqueous fluid inclusions. By correlating stylolite formation with overburden of approximately 2500 ft. (750 m), the hydrocarbon inclusions in Stage 2 fractures must predate all of Stage 4 and most of Stage 3. In the deepest portions of the foredeep, close to the Oman Mountain front, this correlation limits the presence of oil in fracture porosity to late TUronian-early Campanian time. Farther to the west, in the shallower parts of the foredeep. the timing constraint relaxes, and oil migration occurred as late as early Tertiary.