Early Calcite Cement Research Articles

Diagenetic Variability in Middle Frio Formation Gas Reservoirs (Oligocene), Seeligson and Stratton Fields, South Texas

ABSTRACT Two distinct reservoir types (type I and type II), differentiated on the basis of framework mineralogy, diagenetic history, and reservoir quality, are present in the Oligocene middle Frio Formation in Seeligson and Stratton fields, South Texas. Type I reservoir sandstones are feldspathic litharenites to litharenites having an average composition of Q30F25L45. Lithic fragments are predominantly volcanic and carbonate rock fragments. These sandstones underwent early calcite precipitation, after which secondary porosity developed (dissolution of feldspar, rock fragments, and calcite cement). Kaolinite or chlorite and pyrite postdate secondary porosity and locally influence porosity and permeability trends. Porosity averages 20% and ranges between 7% and 28%. Permeability averages 400 md and ranges between 0.007 md and 4,000 md. The large variation in porosity and permeability in these gas reservoirs is directly related to the irregular distribution of calcite cement and the development of secondary porosity. Type II reservoir sandstones, which occur in the medial third of the 2,000- to 2,500-ft thick (600- to 760-m) middle Frio Formation, are lithic arkoses to feldspathic litharenites having an average composition of Q28F31R41. Lithic fragments are predominantly volcanic rock fragments; carbonate rock fragments are rare. These sandstones contain abundant unabraded to slightly abraded glass shards and volcanic ash matrix. Devitrification of volcanic glass detritus has resulted in analcime precipitation, which precedes and overlaps in time with an early calcite cement phase similar to that found in type I reservoirs. Secondary porosity, which developed through the preferential dissolution of volcanic glass and feldspar, postdates these cementation events and has resulted in the development of moldic and intragranular porosity that can be as high as 18%. Mixed-layer illite-smectite lines much of the secondary pores. Porosimeter porosity averages 18% and ranges between 5% and 27%. Permeability averages 5.5 md and ranges between 0.005 md and 78 md. Although average porosity in type II reservoirs is similar to that in type I reservoirs, much of it is ineffective. The precipitation of calcite and analcime in intergranular pores and illite-smectite in secondary pores, combined with abundant volcanic ash matrix, results in poor communication between pores and a corresponding decrease in reservoir quality.

Calcrete Profiles and Porosity Development in the Wagon Wheel (Pennsylvanian) Field, Ward County, Texas: ABSTRACT

The Cisco and Canyon formations in the Wagon Wheel field, located on the western edge of the Central Basin platform, contain 10-15 previously unrecognized calcrete profiles, providing excellent evidence of repeated Late Pennsylvanian subaerial exposure. Evidence for calcretes includes (1) rhizoliths, (2) alveolar texture, (3) circumgranular cracking, (4) tangential needle fibers, (5) calcrete glaebules, (6) light shifts in {delta}{sup 13}C profiles, (7) autobrecciation, and (8) laminated crusts. Extreme lateral variability of calcrete profiles makes correlation of subaerial exposure surfaces difficult. Porosity within calcrete profiles is occluded, providing a seal for underlying reservoir facies. Porosity in the Wagon Wheel field is strongly facies dependent. Porous intervals consist of lenticular skeletal grainstone-packstone facies, typically capped by calcrete profiles. Repeated meteoric phreatic lenses established during Late Pennsylvanian exposure events are interpreted to have formed moldic porosity through selective dissolution in strata containing mineralogically metastable allochems. Secondary porosity development, however, is often balanced by precipitation of eogenetic calcite cement. Therefore, meteoric diagenesis associated with Wagon Wheel calcrete development commonly occludes and only rarely enhances primary porosity. Two stages of calcite cementation are recognized: (1) an early pore-rimming nonferroan nonluminescent calcite cement with thin moderate to brightly luminescent microzones, and (2) a later ferroan, dullymore » luminescent calcite cement with broad, indistinct zones. Truncated cements in Cisco- and Canyon-derived lithoclasts indicate nonluminescent cement was precipitated from oxidizing meteoric phreatic waters. Microzones were precipitated during brief periods of stagnation in the phreatic lenses.« less

Meteoric Alteration of Early Formed Dolomite During Long-Term Subaerial Exposure: Mississippian Mission Canyon Formation: ABSTRACT

Dolomite from the Mississippian Mission Canyon Formation in south central Montana has been petrographically and geochemically analyzed to constrain the effects of long-term subaerial exposure on the formation of well-ordered and stoichiometric dolomite. Samples from five measured stratigraphic sections were analyzed for stoichiometry and for carbon and oxygen isotope composition. Dolomite {delta}{sup 18}O values from all sections vary from {minus}11.2 to +7.5{per thousand}. {delta}{sup 13}C ratios vary from +0.4 to +5.5{per thousand} but are relatively consistent within individual sections. Stoichiometry of the dolomite varies from 49.9 to 55.7 mole % CaCO{sub 3}. Measured sections from the northern portion of the study area dominantly contain {sup 18}O-depleted dolomite (+0.8 to {minus}11 2{per thousand} {delta}{sup 18}O). The most {sup 18}O-depleted dolomite, however. typically occurs in the upper 100 m of these sections. These {sup 18}O-depleted dolomites have homogeneous {delta}{sup 13}C ratios and are stoichiometric. Dolomite from the upper 100 m of these sections predates early calcite cement and locally is calcitized or partially replaced by Fe-oxides. These data suggest that early formed dolomite may have been recrystallized by meteoric water under very high water-rock ratio conditions. Decreasing amounts of water-rock interaction with depth is suggested by more heterogeneous {delta}{sup 18}O and {delta}{supmore » 13}C ratios downsection. Measured sections in the southern portion of the field area contain greater amounts of {sup 18}O-enriched dolomite (+7.5 to -7.1{per thousand} {delta}{sup 18}O). Dolomite from these sections also has more heterogeneous {delta}{sup 13}C values and variable CaCO{sup 3} content. These data suggest that the dolomite probably has not been extensively altered and may more closely represent the original geochemistry of the early formed dolomite.« less

Diagenesis of Cretaceous Sandstone Reservoirs of the South Gabon Rift Basin, West Africa: Mineralogy, Mass Transfer, and Thermal Evolution

ABSTRACT The off-shore Gabon basin contains large hydrocarbon reserves in Lower Cretaceous argillaceous sandstones deposited in the rifting stage of the opening of the South Atlantic. The study area (5 10 km) includes a N30°E-trending fault system a few kilometers wide. In the rocks of this fault system, the diagenetic mineralogy consists of early siderite and abundant calcite cements, followed by late quartz, minor dolomite, and chlorite. The abundant calcite cement occludes most of the porosity. In the region adjacent to the fault system, the diagenetic mineralogy consists of early siderite and minor calcite, followed by late quartz, abundant dolomite, and kaolinite. Amounts of diagenet c cement in both regions range from ten to more than forty percent of the bulk rock volume. Mass balance based on petrographic evidence and point-counting indicates that such quantities of cement cannot have been generated locally, which thus implies considerable large-scale mass transfer. 18O values for coexisting (though not clearly cogenetic) dolomite and kaolinite from the kaolinite zone suggest temperatures < 100°C in late diagenesis. Published evidence from fission tracks in apatites and from fluid-inclusion homogenization temperatures indicates that the temperature during early diagenesis ( 130-120 Ma) was 200-250°C or even higher. We hypothesize that both the required diagenetic mass transfer and the observed distribution of diagenetic minerals were brought about by hot water flow. This circulation, perhaps convective, was driven by the high early geothermal gradients directly related to the opening of the South Atlantic, and was especially focused along the tensional fracture systems involved in the rifting. The abundant early calcite cement that occurs in the fracture system within the study area represents the diagenetic mineral signature of the high geothermal gradient that prevailed early in the history of the Gabon rift basin.

Diagenetic Influences on Reservoir Characteristics of Terry Sandstone, Spindle Field, Colorado: ABSTRACT

The Upper Cretaceous Terry sandstones of Spindle field, Weld County, Colorado, were deposited as offshore bars in a shallow-marine environment. The Terry Sandstone Member of the Pierre Shale is presently buried at depths of 4500-5000 ft (1375-1525 m). The Terry sandstone reservoir was diagenetically created by selective dissolution of calcite cement and detrital material in downdip sands. Hydrocarbon entrapment occurred as a diagenetic/stratigraphic trap through preservation of cement updip and by shale boundaries laterally and vertically. The arkosic litharenites of the Terry sandstone have experienced four major stages of diagenesis: (1) early compaction and calcite cementation, (2) cement and grain dissolution and renewed compaction, (3) chlorite authigenesis, and (4) late illite and smectite authigenesis. In the study area, samples which have experienced dissolution of calcite exhibit porosities of up to 20%. However, permeabilities are low (0.1-11.0 md) due to the presence of pore-lining chlorite and the development of a pseudomatrix related to postdissolution compaction of glauconite pellets. The authigenic chlorite and glauconite pseudomatrix reduced porosity and blocked pore throats. Semilogarithmic plots indicate a logarithmic decrease in permeability with increased chlorite and glauconite content. Terry sandstone reservoir quality has been strongly modified by the diagenetic processes of cementation and compaction. Thesemore » diagenetic processes resulted in the destruction of primary porosity, the formation of secondary porosity, and the subsequent partial occlusion of secondary porosity. Variations in reservoir properties and production potential within the sandstones can be ascribed to both primary lithologic contrasts and subsequent diagenetic modifications.« 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.

Regional Cementation from Unconformity-Recharged Aquifer and Burial Fluids, Mississippian Newman Limestone, Kentucky

ABSTRACT The Mississippian Newman Limestone in Kentucky was studied to assess the effects of unconformities within and above the sequence on calcite cementation. Early cements are low in Fe and show a nonluminescent to dull to nonluminescent zonation. Early nonluminescent cement mainly formed in updip areas from oxidizing, meteoric waters in a regional paleoaquifer that formed during regional regression in the Late Mississippian. During Late Mississippian transgression, waters became reducing to form early dull cement (up to 600 ppm Mn, up to 2,700 ppm Fe but averaging 570 ppm). The aquifer underwent later recharge during Mississippian-Pennsylvanian regression to form a second nonluminescent cement. This event continued into the Pennsylvanian when the sequence underwent increasing burial. Earl cements have 13O values that become lighter downdip, perhaps reflecting a downdip temperature increase due to burial. 13C values are lightest in the north (postulated recharge area) and reflect input of organically derived CO2 in waters there. Most calcite cementation appears to have been associated with two post-Newman unconformities which generated regional aquifers and reflects a shift to a wetter climate in Late Mississippian-Pennsylvanian time. Early cement abundance decreases away from postulated recharge areas. Later iron-rich burial cements (up to 2,200 ppm Mn, up to 11,000 ppm Fe) fill remaining pore spaces, compaction-induced fractures, spalled cement rims, and tectonic fractures lined with saddle dolomite and pyrite. They formed above 60°C at depths of 1 to 3 km. 13O values are compatible with derivation by pressure solution of preexisting calcites in isotopically heavy waters highly modified by burial diagenesis. Light 13C values reflect isotopically light organic carbon in the deltaic waters entering the subsurface, or being expelled from shales or buffering of waters by light C, early calcite cements. The 13C calcite cements become heavier downdip by equilibration of waters with host limestone. Newman sediments may have been subjected to late, postcementation(?) migration of warm fluids based on secondary fluid inclusions. These late-stage fluids most likely were chemically complex, warm brines (7-17 equiv. wt. % NaCI, 50° to 160°C) that resided in the formation for sufficiently short times that conodont CAI values were not affected.

Secondary Porosity and Hydrocarbon Reservoirs in Lower-Middle Miocene Sandstones, Southern San Joaquin Basin, California: ABSTRACT

Immature lower to middle Miocene marine sandstones constitute important reservoir rocks in many southern San Joaquin basin oil fields. Surface samples from the east and west margins of the basin and subsurface samples from Round Mountain, Belridge, and Coalinga fields were examined. These localities have undergone recurrent uplift since middle Tertiary time and maximum burial probably did not exceed 2500-3000 m. Diagenetic features common to east- and west-side sandstones include phosphatization, early calcite cementation, pressure solution and replacement of silicate grains by calcite cement, framework grain dissolution and creation of secondary porosity, and replacement of biotite and hornblende by chlorite. Differences include recrystallization and dolomitization of early calcite on the west side, and massive carbonate dissolution followed by extensive crushing and pressure solution of silicate grains and late replacement of plagioclase by calcite and calcite by hematite on the east side. Replacement of biotite by chlorite occurred only in the deepest samples on either side of the basin. Basinwide differences in diagenesis reflect different tectonic evolutions between east and west sides of the basin. Local variations in diagenetic patterns are pronounced in all areas and are controlled by initial sediment composition. For example, in one core from Coalinga early calcitemore » cement, recrystallized calcite cement, and dolomitized calcite cement are interbedded over the 60-m interval sampled. Hydrocarbons in all samples reside mainly in secondary pores created by cement and framework-grain dissolution, underscoring the importance of diagenesis in creating reservoirs in this basin.« less

Influence of Fluid Chemistry and High Geothermal Gradient on Sandstone Diagenesis: Pattani Basin, Gulf of Thailand: ABSTRACT

Miocene fluvial sandstones have undergone extensive diagenesis and material transfer in the presence of low salinity, high carbon dioxide fugacity pore fluids, and high geothermal gradient. Present-day pore waters are sodium-bicarbonate-chloride-acetate waters with total dissolved solids of 3600 to 7200 mg/L and oxygen isotopic compositions of 0.2 to -4.4 per mil (SMOW). Carbon dioxide content of produced gas increases with depth and averages approximately 12 mole %. The importance of fluid chemistry as a control on diagenetic pathways is demonstrated by feldspar diagenesis in the Pattani basin. Low concentrations of sodium and high CO/sub 2/ fugacities caused extensive feldspar dissolution rather than albitization, even though temperatures are well into the range where albitization has been documented by increased molar sodium/chloride ratios in the pore water (avg. = 4.10) relative to either sea water (0.85) or the probable depositional water (1.4 to 1.7). Dissolution of K-feldspar is complete by 160/sup 0/C, while plagioclase dissolution occurs over a wider range of temperatures and is essentially complete by 180/sup 0/C. Kaolinite and illite, reaction products of the feldspar dissolution, occur within dissolution pores and indicate relative immobility of aluminum in the system. Carbon isotopic compositions of early calcite cement and dissolved carbon dioxidemore » indicate that the prime source of carbon dioxide in the pore fluids was the maturation of organic matter. Rapid burial and a high geothermal gradient promoted rapid production of carbon dioxide from kerogen. Consequently high carbon dioxide fugacities were produced and promoted extensive feldspar dissolution.« less

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

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

Limestone diagenesis of Upper Devonian Nisku carbonates in the subsurface of central Alberta

Limestone diagenesis of the Upper Devonian Nisku Formation in the subsurface of central Alberta began syndepositionally. Most early calcite cements are fibrous, bladed, and columnar and form isopachous precipitates in primary voids. Partial syndepositional induration of the lime matrix (sand and mud) is indicated by angular limestone intraclasts in deposits flanking some buildups, hardgrounds, and neptunian dikes in the reefal facies. Some buildups contain intertidal fades, possible vadose cements, and some early dissolution molds and vugs in their upper parts, suggesting shallow influence of meteoric pore waters towards the end of Nisku time. Postdating these features, precipitation of fibrous and subequant calcite cements, neomorphism, mechanical compaction, and incipient stylolitization resulted in complete induration and lithification of the buildups and associated marly strata.Trace-element compositions of the various calcites are variable within individual petrographic groups and may have originated from meteoric and (or) marine-phreatic diagenesis. However, isotopic and petrographic data of most cements suggest marine-phreatic diagenesis. Therefore, most cements were probably formed and (or) recrystallized in pore waters of marine parentage. The oxygen-isotope data suggest that limestone diagenesis and lithification were essentially completed at a depth of about 300 m. This burial depth was achieved near the end of Famennian or in Early Mississippian time.

Light- and Heavy-Mineral Diagenesis in Cambrian Sandstones of Michigan's Upper Peninsula: ABSTRACT

Sandstones of the Cambrian Munising Formation have a complex history of postdepositional alteration, including primary porosity reduction and intrastratal framework grain dissolution leading to secondary porosity formation. Primary porosity loss by carbonatization is evident in calcite- and dolomite-cemented samples. Feldspar overgrowths on detrital potassium feldspars effectively reduce primary pore space in 5-10% of the samples. Similarly, multiple generations of quartz overgrowths, distinguished by opaque inclusion rings, reduce porosity further. Authigenic pore-filling clays occur in more lithic-fragment-rich sands. Cemented samples clearly show that secondary porosity is associated with diagenetic alterations of feldspar and garnet. Feldspar dissolution occurs in detrital feldspar cores and as slightly corroded grain boundaries. Intrastratal dissolution of garnet is also evident. Thin sections show oversized pores associated with the garnet, and grain mounts show delicate imbricate wedge marks on garnet surfaces. Calcite also occurs as a fracture filling in, and replacement mineral on, garnet. Other heavy minerals (zircon and tourmaline) appear unaffected by diagenesis. The textural relationships of garnet dissolution vary with respect to the formation of early calcite and late dolomitic cements. The association between garnet and other diagenetic events constrains the chemistry of the intrastratal solutions, which affected garnet and formed overgrowths on feldspar and quartz.

Sedimentology and Diagenesis of a Rift Basin Lacustrine Sandstone: Pematang Group, Central Sumatra, Indonesia: ABSTRACT

The Central Sumatra basin is a back-arc basin that formed during the Paleocene as a series of half-graben structures. These early formed half-graben structures were filled with nonmarine clastics and lacustrine sediments of the Pematang Group, sourced from local highland areas. The Pematang Group consists of the Lower Red Beds, Brown Shale, Coal Zone Member, and Lake Fill Formation (in ascending order). The Pematang Group sedimentology is intimately related to the regional and basinal tectonic development and history. Sandstones of the Pematang Group are predominantly sublitharenites and litharenites; feldspars are rare. Sandstone diagenesis is relatively uniform regardless of the depositional environment. Diagenetic modifications include compaction, early calcite and dolomite cementation, quartz overgrowth formation, unstable rock-fragment dissolution, kaolinite precipitation, siderite formation, and local illite formation. Secondary porosity accounts for up to 50% of the effective porosity and was created by rock-fragment dissolution. This dissolution is probably caused by the organic acids released during kerogen maturation. Stable isotope data from diagenetic siderite are presented.

Depositional Influences in Sandstone Diagenesis of Lower Cretaceous Hosston Formation, Marion and Walthall Counties, Mississippi: ABSTRACT

ABSTRACT Poor reservoir quality is a significant impediment to exploration success in the deep Hosston gas/condensate trend of south-central Mississippi. Regionally, depositional lithology of the Hosston exerts significant influence on cementation and porosity reduction. The primary variables are depositional carbonate content, quartz sand-grain size, and the presence of matrix clay. Feldspar content is a variable of lesser significance. In Hosston fluvial and deltaic facies, carbonate content is low, and coarser sediments are present in channel sandstones. These coarser sandstones preserve commercial porosities, whereas finer-grained and shaley sandstones offer marginal porosities. Quartz overgrowths, pressure solution, and authigenic kaolinite are responsible for most diagenetic porosity reduction in this facies. In Hosston marine, prodeltaic sandstones, depositional carbonate is more common. Carbonate cements, dominantly ankerite and dolomite, are recrystallized from shell fragments and carbonate grains to occlude porosity. Quartz overgrowths, pressure solution, and stylolitization are also common in these finer sandstones. In distal, marine shelf sandstones, depositional carbonate is abundant, and quartz sand is very fine-grained. Porosities are very low in this lithofacies because of pervasive early carbonate cementation and later quartz overgrowths in intervals not cemented by carbonate. The first stage of Hosston sandstone diagenesis was early calcite or quartz cementation. Calcite was later replaced by ankerite or dolomite. Deeper burial initiated plagioclase feldspar dissolution and the development of pore-filling kaolinite. Hydrocarbons subsequently accumulated in structural and stratigraphic closures and retarded further diagenetic porosity reduction in these sandstones. Deeper burial caused thermal degradation of oil reservoirs to dry gas and pyrobitumen. Minor galena, sphalerite, pyrite, and barite are present in open, tensional fractures, associated with stylolites and as intergranular cements. These sulfides and sulfates were probably precipitated from metalliferous brines common in the Hosston.

Depositional Environments and Diagenesis of D Sandstone, Wild Horse Field, Weld County, Colorado: ABSTRACT

Wild Horse field is located on the gentle east flank of the Denver basin, northwest of Fort Morgan, Colorado. Production was established during 1981 from a 12-ft (3.7 m) thick porous sandstone in the lower part of the D sandstone of Cenomanian age. To date, 10 producing wells have been completed. Proven productive area is 1,800 ac and estimated reserves are 1.2 million bbl oil equivalent. D sandstone facies form complex reservoirs associated with regressive deltaic and marine sandstone deposited during a regional sea level drop 95 Ma. Traps are primarily stratigraphic in nature although fracturing has enhanced production. The D sandstones in the Wild Horse field area are interpreted to be the product of an episodic northwestward progradation of a lobate river-dominated delta system. On the basis of differences of the internal structure, textures, mineral composition, and trace-fossil content, the D sandstone is divided into five lithofacies: shoreface, prodelta platform, delta front, delta plain, and transgressive marker sandstone. The producing sandstones occur in overlapping bars proximal and lateral to principal channels in a bay-filled sequence. Calibration of well logs using cores allows detailed subsurface mapping of producing facies for more efficient development of the field. Original porosity has been reduced mainly by quartz overgrowth cementation. Porosity interconnection is present due to dissolution of framework grains and of the early diagenetic calcite cement. Pore-filling cements such as chlorite, kaolinite, and pyrite may produce formation damage if not treated during completion operations. End_of_Article - Last_Page 871------------

Lithofacies and Diagenetic Controls on Reservoir Development and Production, Goose Lake Field, Eastern Montana: ABSTRACT

Goose Lake field is located on a north-northwest-oriented structural nose on the western side of the Williston basin and produces from carbonate island facies in the Mississippian Ratcliffe zone. Island facies are concentrated on the structural nose and include low-energy, fenestral, pisolite-algal lime boundstones (supratidal) and high-energy, evenly laminated, pellet-oolite lime grainstones (beach and storm deposits). These beach grainstones occur preferentially along a narrow zone on the western side of the field, suggesting that major wave and wind approach was from the west. Development of distinct porosity systems within these rocks and in encasing offshore facies accounts for variations in permeability, oil and water production ratios, and overall cumulative production. Both reservoir facies contain porosity as high as 20%, whereas permeability is highly variable. In wells with high cumulative production (i.e., above 200,000 bbl), permeability is normally 10-20 md. Grainstones are typified by primary intergranular, oomoldic, and microvuggy porosity, whereas algal boundstones have micro-intercrystalline, fenestral, and microvuggy to mesovuggy porosity. Porosity occlusion in these facies is due to a variety of early calcite cements, late-stage anhydrite, and baroque dolomite. The encasing offshore marine facies may contain high micro-intercrystalline poro ity but lack effective permeability and serve as the updip seal. Porosity in these fine-grained facies and supratidal units is probably due to an early freshwater leaching episode that affected islands situated on paleohighs during rising sea level and less arid climatic conditions. End_of_Article - Last_Page 848------------

Neptunian Dikes and Sills in Devonian Reef Complexes of Canning Basin, Western Australia: ABSTRACT

Neptunian dikes and sills are conspicuous features of early-cemented limestones in Devonian reef complexes of the Canning basin. They are sedimentary fillings of fissures formed by fracturing of the limestones soon after deposition, and are most abundant in the reef subfacies, but also occur in some back-reef and marginal-slope deposits. The dikes and sills were filled with contemporary sediment (including oolite, peloid grainstone, biomicrite, and calcareous sandstone), encrusting organisms (Renalcis and stromatolites), and early radiaxial calcite cement. Some of the sediment is dolomitized or mineralized. Some dikes contain concentrically coated balls of radiaxial calcite up to 0.3 m (1 ft) across. Sediment in the dikes and sills was cemented rapidly, and repeated refracturing and infilling occurred during brief intervals. The largest dikes are up to 6 km (3.5 mi) long, 20 m (65 ft) wide, and originally extended more than 80 m (260 ft) below the surface. However, most are less than 100 m (330 ft) long and 5 cm (2 in.) wide. Neptunian sills are up to 0.8 m (2.5 ft) thick. The principal strike directions of major dike approximately parallel the original reef fronts of underlying basement ridges. Lateral extension through the development of fissures amounted to as much as 6.5%, and averaged about 3% in measured reef sections. The fissures allowed circulation of large volumes of seawater to considerable depth within the reef complexes, and must have played a significant role in early diagenesis. Fissuring probably resulted from earthquake shaking, differential compaction of underlying sediments, and slippage along bedding planes, especially in the steeply dipping marginal-slope deposits. End_of_Article - Last_Page 517------------

DEFOSITIONAL FRAMEWORK AND DIAGENESIS OF THE LATE PERMIAN GAS RESERVOIRS OF THE BONAPARTE BASIN

Late Permian sandstones form the reservoir of the Tern and Petrel gas fields in the offshore Bonaparte Basin. The producing reservoirs of the Petrel field were deposited in various environments associated with a major northwesterly trending deltaic system. The producing sands in the Tern field were deposited in the shoreface environment of a barrier-bar system.The reservoir quality of the sands is controlled by the diagenesis, which is facies dependent. In the Petrel field, sandstones deposited in the upper delta plain and along the shoreline are clean, medium-to coarse-grained and highly quartzose but have very low porosity and permeability due to extensive quartz diagenesis. However, sands deposited in delta front and lower delta plain environments are medium to fine grained, argillaceous and have fair to good reservoir potential. In these sands, the dispersed clays formed coats and rims on quartz grains during early diagenesis and inhibited quartz overgrowth. In the Tern field, sands of the upper shoreface have poor reservoir quality due to early calcite cementation. However, finer-grained sandstones of the lower shoreface facies have good reservoir quality. The porosity in these sands is mainly primary and preserved due to low carbonate and high clay content. The processes of quartz and calcite cementation which drastically reduced the reservoir quality of the coarse-grained sands occurred early and were influenced by the texture of the sands and probably also by the chemical character of the formation waters.

Distinguishing Diagenetic Environments of Equant Calcite Cementation: Example from Lower Cretaceous Pearsall Formation in South Texas: ABSTRACT

Equant calcite is the most common cement in limestones. It occludes more pore space, and hence has more control over reservoir quality than any other carbonate cement. It is important, therefore, to be able to delineate the environments of precipitation of equant calcite cements. The Lower Cretaceous Pearsall formation in south Texas has a history of equant calcite cementation that began early in the shallow-subsurface meteoric environment and is continuing today in the deep-subsurface basinal environment. Trace-element analysis of Mg+2 and Fe+2 along with morphological characteristics of the Pearsall equant calcite cements delineate environments of precipitation. Cements precipitated in the early, shallow-subsurface meteoric environment are generally very fine to medium-crystalline equant (less commonly bladed) calcites. The very fine to fine-crystalline calcite cement generally rims grains and is gradational into the medium-crystalline equant calcite cement that fills intergranular and moldic porosity. Early cements commonly have irregular crystal boundaries as seen in thin section. Late, deep-subsurface precipitated cements are generally coarse to very coarse-crystalline equant calcites. Commonly they have straight crystal boundaries and form one or several large crystals in a pore space. There is generally a sharp change in crystal size with the fine to medium-crystalline equant calcites precipitated in the shallow-subsurface meteoric zone. Trace-element analysis (electron microprobe) shows a statistically valid difference of Mg+2 and Fe+2 content between the early and late equant calcite cements. The early calcite cements have a higher Mg+2 content (1.8 ± 0.3 mole % MgCO3) and a lower Fe+2 content (785 ± 184 ppm) than the late calcite cements (1.3 ± 0.3 mole % MgCO3 and 2.618 ± 1,952 ppm Fe+2). Also, semiquantitative probe analysis, due to low count rate, indicates that the early calcite cement is richer in Sr+2 (738 ± 286 ppm) than the late calcite cement 472 ± 189 ppm). Magnesium in the fine-crystalline rim cement shows a pronounced trace-element distribution pattern. In the incipient crystal growth next to the grain, a high Mg+2 peak usually occurs, followed by a decrease in Mg+2 in the late rim cement and coarser equant calcite cement. This initial high Mg+2 peak is attributed to early meteoric diagenesis as described by Benson in 1974 in meteoric cements from Barbados. The Fe+2 trace-element distribution pattern shows an opposite trend from that of the magnesium. The early cements show low Fe+2, whereas the late cements are Fe+2 rich, indicating a reducing environment of precipitation for the late cements. Early, shallow-subsurface equant calcite cements can be distinguished from late, deep-subsurface equant calcite cements by relative position in pores, crystal size, straightness of crystal boundaries, gradation between crystal sizes, and by trace-element content and distribution patterns. These parameters may be valid environmental indicators of equant calcite precipitation in other limestone formations. End_of_Article - Last_Page 506------------

RELATIONSHIP BETWEEN DIAGENESIS AND PORE FLUID CHEMISTRY IN NIGER DELTA OIL‐BEARING SANDS

The sandstones of the Niger delta oil reservoirs are predominantly quartz arenites, yet silica cementation in the form of quartz overgrowth is at a minimum. This apparent lack of cementation is attributed among other things to the low concentration of silica in formation water, which results in low bulk‐volume reduction, little pressure‐solution and consequently, under‐compaction.Under‐compaction is aided by early calcite cementation which probably occurs at the boundary between the fresh waters of the Benin formation and the brackish to saline pore‐waters of the paralic sequence. Early cementation may have aided the discriminate entry of hydrocarbons into the reservoirs of the area.Entry of hydrocarbon into reservoirs appears to inhibit diagenetic processes as significant differences in mineral assemblages exist between hydrocarbon and non‐hydrocarbon reservoirs. Kaolinite is the dominant authigenic mineral in most hydrocarbon reservoirs whereas kaolinite, siderite, pyrite, calcite as well as minor amounts of smectite and illite occur in many non‐hydrocarbon reservoirs. The presence of mainly kaolinite in most reservoir rocks is suggestive of early entry of hydrocarbons into them. This deduction is at variance with the presently‐accepted theory of hydrocarbon generation within the deeply buried section of the paralic Agbada formation and the overpressured marine Akara shales. Instead, it suggests generation and migration of hydrocarbons from the paralic shales adjacent to the reservoirs.Lack of cemetation in most hydrocarbon reservoirs results in the production of very fine materials (“fines”) with the oil. It is proposed that most of the kaolinite present in many petroleum reservoirs is produced with such fines. The removal of kaolinite, in this way, reduces the hazard of permeability loss which would have occurred due to blocking of throat‐passages by floating kaolinite platelets.