Abstract The subsurface hydrologic system of large, actively filling sedimentary basins includes meteoric, compactional, and thermobaric regimes. Boundaries between the regimes and their contained flow systems evolve as basin filling proceeds. As a result, sands are continuously flushed by a succession of fluids of varying origins and chemistries. Careful examination of the existing hydrologic setting and a reconstruction of generalized hydrologic history and its relationship to observed diagenetic features within a depositional sequence may serve to validate interpretive diagenetic models and may explain or predict paragenetic relationships, regional diagenetic variations within the same depositional episode, and differences in diagenetic products in different episodes in the same or similar basins. The Frio/Catahoula Formations of the Texas Coastal Plain provide an example that both illustrates the coexistence of hydrologic regimes and relates associations of diagenetic features with existing regimes or with the important mixing zones that occur between regimes. Hydrocarbon geochemistries indicate meteoric flushing to depths approaching 2000 m. The deeper geopressured section is coincident with a thermobaric regime in which clay dewatering recharges the hydrologically restricted portions of the basin fill.

Abstract Reservoir quality trends in Lower Tertiary sandstones along the Texas Gulf Coast are a product of regional variations in intensity of diagenesis. The major controls on diagenesis were detrital mineralogy and regional geothermal gradient. Porosity and permeability in sandstones shallower than 3350 m (11,000 ft) are generally adequate for hydrocarbon production, whereas reservoir quality in deeper sandstones in the onshore Lower Tertiary section is highly variable. Many of these sandstone reservoirs have permeability values of less than 1 millidarcy (md), but in a few areas permeability values are higher than 1000 md. Wilcox sandstones are poorly to moderately sorted, fine-grained, quartzose lithic arkoses, becoming more quartz-rich from the upper to the lower Texas Gulf Coast. Most rock fragments are metamorphic or volcanic in origin. Wilcox sandstones exhibit no systematic regional reservoir quality trends. Along the lower and parts of the middle and upper Texas Gulf Coast, deep Wilcox sandstones are tight, but in other parts of the middle and upper Texas Gulf Coast, porosity exists at depth. Vicksburg sandstones are poorly sorted, fine-grained lithic arkoses. Rock fragments are mainly volcanic clasts with lesser carbonate and minor metamorphic clasts. The deep Vicksburg Formation has low-quality reservoirs. Frio sandstones range from poorly sorted, fine-grained, feldspathic litharenites to lithic arkoses along the lower Texas Gulf Coast to poorly sorted, fine-grained, quartzose lithic arkoses to subarkoses along the upper Texas Gulf Coast. Volcanic and carbonate rock fragments are common in the lower Texas Gulf Coast and decrease in abundance up the coast. Frio sandstones show a systematic improvement in reservoir quality from the lower to the upper Texas Gulf Coast that is related to grain composition and geothermal gradient. Reservoir quality trends in Tertiary sandstones have been substantiated by acoustic log analysis. In spite of variations in composition, Lower Tertiary sandstones exhibit similar diagenetic sequences generalized as follows: Surface-to-shallow-subsurface diagenesis (0 to 1200 m ±; 0 to 4000 ft ±) began with the formation of clay coats on framework grains, dissolution of feldspar, and replacement of feldspar by calcite. Minor amounts of kaolinite, feldspar overgrowths, and Fe-poor calcite was locally precipitated. Porosity was commonly reduced by compaction and cementation from an estimated original 40% to less than 30%. Intermediate subsurface diagenesis (1200 to 3400 m ±; 4000 to 11,000 ft ±) involved dissolution of eary carbonate cements and subsequent cementation by quartz overgrowths and later by carbonate cement. Cementation commonly reduced porosity to 10% or less, but this trend could be reversed by later dissolution of feldspar grains, rock fragments, and carbonate cements. Restoration of porosity to more than 30% occurred, but some porosity was later reduced by kaolinite, Fe-rich dolomite, and ankerite cementation. Deep subsurface diagenesis (>3400 m ±; >11,000 ft ±) was a continuation of late Fe-rich and Fe-poor carbonate cement precipitation. Plagioclase was albitized during this stage. Differences in intensity of diagenetic events and depths at which they first occurred correspond to the chemical and mechanical stability of the original detrital mineralogy and to regional variations in geothermal gradient.

Abstract Burial diagenesis of Frio sandstones deduced from detailed study of one small area of the northern Texas Gulf Coast (Brazoria County, Milliken et al, 1981) is regionally valid with only minor modifications. Quartz is most commonly the first cement of volumetric significance to form, and constitutes 2.5% of the average sandstone volume. The average δ5 18 O of quartz cement is +31 o/oo± 1.5 o/oo (SMOW), indicating precipitation at considerably cooler temperatures than most clay mineral transformation takes place. Calcite is the dominant cement in Frio sandstones, constituting about 5% of the total sandstone volume, and most commonly postdates quartz precipitation. Calcite more depleted than −10 o/oo (PDB) is uncommon, and most calcite has a δ 18 O of −7.2± 2 o/oo (PDB). δ 13 C values cluster closely around −4 ± 2 o/oo (PDB). Because of relatively constant isotopic composition, and relatively invariant iron and manganese content in calcite, both areally and with depth, both quartz and calcite cements appear to have been emplaced under relatively invariant chemical conditions prior to hydrocarbon migration. Detrital K-feldspar is essentially absent below 12,000 ft, and the zone of plagioclase albitization extends between about 9000 and 12,000 ft. Virtually no unaltered detrital feldspars are present below 12,000 ft in any of the samples examined, K-feldspar having been mostly dissolved and plagioclase albitized. The volume of water required to precipitate quartz and calcite cements far from the apparent sources of material, generate secondary porosity and alter all detrital feldspars regionally in this thick sandstone sequence far exceeds the volume of pore water deposited with, near, or beneath the sands. Active thermally driven convection is a plausible (though unproven) mechanism for moving such large masses of dissolved components (and hydrocarbons) through the sandstones.

Abstract Sequences of diagenetic minerals associated with secondary porosity show striking similarities. The formation of quartz overgrowths on detrital quartz grains is generally followed by carbonate cementation. The dissolution of this carbonate is the main secondary porosity-forming event, which commonly precedes kaolinite precipitation and iron-rich carbonate cementation. In the Texas Gulf Coast, oxygen isotopic data provide temperature estimates of authigenic phases that predate and postdate secondary porosity development: quartz, ⩾ 80° C; kaolinite, ⩾ 70° C; albite, 100–150° C; late carbonate, > 100° C. These data suggest that secondary porosity in the Tertiary Gulf Coast forms at temperatures of about 100 ± 25° C. Correlations among calcite saturation indices in pore fluids, abnormally high permeabilities, and mole percent CO 2 in natural gases of the Eocene Wilcox Group imply a strong interrelationship between carbon dioxide and secondary porosity development in clastic reservoirs. The CO 2 content of gases varies systematically with both the reservoir age and temperature, which suggests a kinetic control on generation. The amount of CO 2 in natural gases increases rapidly at approximately 100° C; this coincides with a rapid increase in the ratio of secondary to total porosity in associated sandstones. Stable isotopic analyses of carbonate cements indicate a strong component of organically derived carbon and therefore cycling of carbon between inorganic and organic systems. The type, amount and distribution of organic matter, and early carbonate in both shales and sandstones control the quantity of CO 2 available for generating secondary porosity.

Abstract Oxygen- and carbon-isotope compositions have been determined for clay and carbonate minerals from the Upper Cretaceous clastic rocks of the Milk River and Lea Park Formations. These units contain the Milk River aquifer and the southeastern Alberta Milk River Gas Pool, respectively. The stable isotope data provide important information concerning the diagenesis and paleohydrology of the study area. Authigenic minerals from sandstones in the Milk River aquifer are characterized by low δ 18 O and δ5 13 C values: clay minerals (<2 μm), dominated by authigenic kaolinite, δ 18 O = +11.3 to + 14.2 (SMOW); authigenic calcite, δ5 18 O = +15.3 to +18.5 (SMOW), δ 13 C = −9.9 to −2.6 (PDB). The authigenic minerals with the lowest δ5 18 O values occur within a zone of local recharge in the aquifer. Here the authigenic clay minerals and calcite closely approach isotopic equilibrium with existing meteoric water at low temperatures (<+15°C). Low δ 13 C values forthe calcite indicate incorporation of organically derived CO 2 , probably from decaying plant material in the overlying soil and till. Close agreement between actual formation temperatures and those calculated from isotopic data disappears in downdip portions of the aquifer, mostly because of the drastic enrichment in 18 O of the formation water in this direction (−20 to −6, SMOW; Schwartz et al, 1981). Authigenic minerals from these locations have retained isotopic signatures characteristic of 18 O-poor meteoric water no longer present in the system. This water was displaced by 18 O-rich formation fluids that are themselves now being flushed from the aquifer by modern-day ground water. Authigenic minerals from sandstones in the southeastern Alberta Milk River Gas Pool are more 18 O-rich than those from the aquifer: clay minerals, dominated by illite, +14.7 to +15.8, SMOW; calcite, +19.3, SMOW. Such compositions are compatible with mineral crystallization at low temperatures (+15 to +20° C) from formation fluids similar in δ5 18 O to other Cretaceous oil and gas pools that occur in Alberta. The low- 13 C nature of authigenic calcite and some dolomite (−7.6 to −3.0, PDB) from the Milk River Gas Pool may be related to the production of biogenic methane in this reservoir. Of all clay minerals analyzed, the illite-dominated mixtures from argillaceous rocks of the Milk River aquifer and the Milk River Gas Pool have the highest δ 18 O values (+16.0 to +19.0, SMOW). Such compositions reflect a detrital origin rather than diagenetic processes. Isotopic exchange between these clay minerals and formation water is insignificant. Most dolomite from the sandstones and the argillaceous rocks is not in equilibrium with the authigenic calcite. The dolomite is much richer in 18 O (+24.4 to +28.3, SMOW) and 13 C (−2.7 to +1.0, PDB); such values are typical of platform carbonate rocks. No evidence for extensive isotopic exchange between formation water and the dolomite can be demonstrated.

Abstract A diagenetic model based on convective fluid flow has been analyzed for typical reservoir conditions. Calculations based on the model suggest that a significant fraction of the inorganic diagenesis observed in sandstone reservoirs can be attributed to the presence of slowly circulating aqueous fluids. Stability considerations indicate that static pore fluids do not exist in porous bodies of geologic dimensions and that pore fluids will convect at a rate of about 10 −8 m per sec (~1 m per yr) in the presence of a normal geothermal gradient (25° C per km). If it is assumed that the pore fluid maintains chemical equilibrium with the rock matrix, it follows that mass must be transferred as the fluid crosses isotherms. Minerals such as quartz, which have prograde solubilities under normal reservoir conditions, will move from hot source zones to cooler sinks. Minerals such as calcite, which have retrograde solubilities, will move from cool sources to hot sinks. The net effect is a continuous transfer of rock matrix in the reservoir for as long as the fluid circulates. Because the temperature field can change sharply along a streamline, convection can localize precipitation and dissolution zones. In anticlinal structures, the fluid flow is most likely a modified torus in which warm fluid flows up the base of the ascending limb while cooler fluid flows down along the upper surface. The regions of most rapid heating and cooling of the fluid occur at the synclinal troughs and at the anticlinal crests. This flow pattern will produce zones of intense diagenesis at the crests and troughs of the structure. Zones of secondary porosity produced by the dissolution of framework grains or previously deposited cements are also predictable and the model provides explicit conditions for isomorphic replacement. Since hydrocarbon solubilities are similar to quartz solubility in the temperature range 60–150° C, hydrocarbon transfer and accumulation should closely approximate that of quartz. Calculations suggest that convection can transfer significant quantities of hydrocarbons in molecular solution and exsolve them in traps in relatively short geologic times. The convection model thus links inorganic and organic diagenesis and provides reasonable explanations for such observed phenomena as secondary porosity and thermal anomalies.

Abstract Methanic diagenesis commonly dominates pore-water chemistry in organic carbon-rich sediments from depths of tens of centimeters to 1000 m or more, and is coincident with the depths of principal sediment dewatering. As a result, methanic diagenesis in organic carbon-rich mudstones may control early diagenesis in adjacent sand and sandstone and can result in the accumulation of economic quantities of biogenic methane. Chemical aspects of methanic diagenesis in ancient marine sediments can be reconstructed by analogy with processes in modern diagenetic environments, on the basis of the mineralogy, texture, and isotopic composition of concretionary carbonate cements and other related authigenic minerals. This sort of diagenetic reconstruction is well illustrated by means of examples from the Upper Cretaceous Gammon Shale from southeastern Montana. Bioturbated mudstones of the Gammon accumulated in oxic, open marine waters, but dissolved oxygen was probably depleted from pore waters a few tens of centimeters beneath the sediment/water interface. Sulfate reduction took place beneath this depth and was most important in a zone of mixing at the base of bio-turbation, where isotopically light (δ 34 S ≃ −25 o/oo) iron sulfides accumulated. Organic matter oxidized during sulfate reduction gave rise to isotopically light calcite (δ 13 C ≃ −21 o/oo) that formed discrete concretions and that formed the interior portions of zoned calcite-siderite concretions. Sulfate was exhausted at depths of about 5–10 m, and CO 2 reduction (methanogenesis) became the dominant form of anaerobic respiration. Carbonate precipitation accelerated as pH increased because of CO 2 removal, while continued anaerobic oxidation of organic matter maintained bicarbonate activity at high levels. In the absence of dissolved sulfide, increased Fe +2 activity favored siderite over calcite as the principal authigenic carbonate. During the early stages of methanogenesis, kinetic fractionation caused δ 13 C of CH 4 to change from-90 to-70 per mil and δ 13 C of bicarbonate to change from −22 to approximately zero per mil over a depth interval of a few meters in the sediment column. Interpretation of methanic diagenesis in the Gammon Shale illustrates only part of a single diagenetic pathway for one type of organic carbon-rich mudrock. And yet, the implications are clear: Early diagenesis of muds is dominated by processes involving organic matter and by the products of organic matter decomposition. Because of the economic significance of organic carbon-rich mudrocks as source beds for hydrocarbons and because their diagenesis probably controls mineral precipitation and dissolution in many reservoir rocks, it is of the utmost importance that diagenesis in ancient mudstones be understood.

Abstract The development of secondary porosity (porosity enhancement) in many sandstones is the result of aluminosilicate and/or carbonate dissolution. The dissolution of aluminosilicate minerals and subsequent porosity enhancement is a problem of aluminum mobility. Our experimental data demonstrate that it is possible to increase significantly the mobility of aluminum and to transport it as an organic complex in carboxylic acid solutions. These same carboxylic acid solutions have the capability of destroying carbonate grains and cements. Carothers and Kharaka have shown that concentrations of carboxylic acid anions range up to 5000 ppm over a temperature range of 80–200° C in some oil field formation waters. Our experiments show that acetic acid solutions at the same concentrations and over the same temperature range can increase the solubility of aluminum by one order of magnitude, whereas oxalic acid solutions increase the solubility of aluminum by three orders of magnitude. The textural relations observed in the experiments are identical to those observed in sandstones containing porosity enhancement as a result of aluminosilicate dissolution. A natural consequence of the burial of sedimentary prisms is the maturation of organic material. These maturation reactions result in the evolution of significant amounts of organic acids and carbon dioxide. The experiments suggest that the enhancement of porosity in a sandstone as a result of aluminosilicate or carbonate dissolution is the natural consequence of the interaction of organic and inorganic reactions during progressive diagenesis. The degree to which porosity enhancement develops depends on the ratio of organic to inorganic matter, the initial composition of the organics, the sequences, rates and magnitude of diagenetic reactions, fluid flux, and sand/shale geometry.

Abstract Petrographic, SEM, and chemical analyses of closely spaced samples from a core of sandstone and shale (Oligocene Frio Formation, Brazoria County, Texas) reveal a mechanism for secondary porosity development. Maturation of organic and inorganic materials in the shale produced a solvent solution which, upon expulsion, resulted in zoned reservoir quality in the adjacent sandstone. Framework grain dissolution (secondary porosity) originated at the sandstone/shale contact zone (near the solvent source). Aluminum in this zone was not conserved by the process but instead was removed by mobile, shale-derived organic complexers. The production of these complexers (ligands) appears to be essential to the process of framework grain dissolution. Aluminum removal elevated the silica activity and resulted in precipitation of authigenic quartz cement. Secondary porosity was developed to a lesser extent farther away from the shale. Imported aluminum from the contact zone and a failure to complex aluminum adequately resulted in kaolinite precipitation. This sink for silica prohibited quartz precipitation. This general process of framework grain dissolution is probably common in sandstone/shale sequences. In summary, secondary porosity development is accentuated by: (1) high initial permeability, (2) increased relative thickness of shale to sandstone, (3) increased organic content in the shale, and (4) abundant soluble grains (potential secondary pores).

Abstract Framework grain dissolution (FGD) involving feldspars and rock fragments was found to be significant to reservoir properties in sandstones with more than 10% soluble grains. FGD porosity ranged up to approximately 70% and averages about 30% of the visible porosity in a study of some reservoir sandstones. FGD does not appreciably increase reservoir permeability. However, the amount of FGD porosity developed was found to be a function of the sandstone’s initial permeability. We propose that clay and organic maturation in shales produce the necessary water, acid, and complexing agents for FGD. The FGD solvent is expelled into the sandstones where feldspars and rock fragments are dissolved, and the resulting aqueous aluminum is complexed for transport out of the sandstone.

Abstract The Plio-Pleistocene nonmarine volcanic sandstones of the Cagayan basin, Philippines, have been significantly altered by early dissolution and cementation processes. The amount and type of alteration vary by formation, depth, and age of the deposit. Plio-Pleistocene fluvial sandstones (litharenites and feldspathic litharenites) buried to depths of 400–900 m, are only slightly compacted, but contain significant amounts of authigenic pore-lining clay and zeolites. Dissolution of plagioclase, heavy minerals, and volcanic rock fragments has occurred in nearly all samples, dissolving up to one-half the framework grains and increasing thin-section porosity to as much as 40%. The overlying Pleistocene sandstones are compositionally different (lithic arkoses and arkoses) and have not been as extensively affected by diagenetic processes. The more extensive alteration of the Plio-Pleistocene sandstones reflects increased diagenetic alteration with burial depth and time as a result of relatively high porefluid flow rates in shallow alluvial deposits. The diagenesis of the Cagayan basin Plio-Pleistocene sandstones indicates that significant secondary porosity can develop in nonmarine volcaniclastics as a result of early silicate dissolution during shallow burial diagenesis. Early dissolution and secondary porosity development have important implications for studies of nonmarine volcaniclastics. Early dissolution processes distort provenance, tectonic setting, and depositional environment interpretations based on the detrital mineralogy of older volcaniclastic sediments. Secondary porosity increases the reservoir quality of volcaniclastics prior to more extensive compaction and cementation. Recognition of similar shallow volcaniclastic reservoirs in the past may have been limited because of low resistivity sand identification problems caused by authigenic smectite.

Abstract Principles of equilibrium thermodynamics were applied and found useful in evaluating aspects of Frio diagenesis. Solutionmineral equilibria as predictors of reservoir quality and diagenetic history were tested by comparing formation waters from regions of good and poor reservoir quality, the upper and lower Texas coast, respectively. Comparison among waters from these regions was made on activity diagrams of 16 diagenetic reactions such as calcite = ferroan calcite, kaolinite = chlorite, and Ca-montmorillonite = Na-montmorillonite. Relative position of tested waters, with respect to the stability field of authigenic minerals occluding permeability and porosity, was related to reservoir quality. Solution-mineral equilibria are indicators of reservoir quality; equilibria in hydropressured waters best reflect reservoir quality. Activity indices favoring chlorite and ferroan calcite stability and large log([Ca +2 ] .16 /[Na + ] .33 ) ratios are the best indicators of reservoir quality in deep Frio sandstones. Change in ionic strength, analytical molality, and activity indices is correlated with geopressuring. Variation in mole ratios and activity indices with depth is largest between 8000 and 11,000 ft (2440 and 3355 m), the transition zone between the hydropressured and geopressured intervals. The variation is attributed to more active water-rock interaction, or diagenesis, in the transition zone. Diagenesis in the Frio Formation is a function of temperature, pH, activity, and pressure. Predictions made from solution-mineral equilibria add new insight into relative mineral stabilities and in situ pH and are consistent with the diagenetic sequence developed from petrographic data. Calcite equilibrium favors precipitation of calcite early in the burial history. Two stages of chlorite formation are postulated, one early in the hydropressured interval at the expense of smectite grain coats and another late in the geopressured interval at the expense of kaolinite cement. Chlorite and illite are the stable layer silicates in deep Frio sandstones. Albitization of feldspar is initiated in the hydropressured interval at temperatures less than 100°C.

Abstract Secondary porosity in Miocene Stevens sandstones of the North Coles Levee Field results from dissolution of calcite, ferroan dolomite, and calcic plagioclase (An30). Kaolinite is a leach product of plagioclase, and mass balance calculations indicate that alumina is conserved on a thin-section scale. Iron released from dissolution of Fe-carbonate is possibly conserved in late-stage pyrite. Detrital K-feldspar and early formed albite fracture filling in plagioclase are unaffected by the leach fluids, suggesting that these components are stable with respect to the leach fluids, whereas the anor-thite component is unstable. Thermodynamic considerations indicate present-day pore waters at 100°C, 260 bars fluid pressures, are nearly at equilibrium for the reaction: CaCO 3 + CaAI 2 Si 2 O 8 + H 2 O + 3H + = 2Ca +2 + HCO 3 − + AI 2 Si 2 O 5 (OH) 4 and for a similar reaction involving dolomite. Compaction following or perhaps contemporaneous with leaching has resulted in at least two diagenetic events. One involves albitization of plagioclase at stressed grain contacts with quartz, possibly as a result of higher silica activities stabilizing albite at these contacts. The other involves crushing of detrital biotite resulting in crystallization of carbonate adjacent to it. This phenomenon is due to depletion of H + in the pore water adjacent to newly formed mica surfaces as H + exchange occurs between pore water and mica.

Abstract Thermodynamic calculations indicate that, at diagenetic temperatures, laumontite is stable only in the presence of fluids of high pH and low . This is supported by experimental dissolution studies that suggest that laumontite is soluble in the presence of carboxylic acids. As both CO 2 and carboxylic acids are produced prior to and during hydrocarbon generation, laumontite is unlikely to form in sandstones plumbed to source rocks during the maturation process. It is more likely that early formed laumontite cements will be destroyed and secondary porosity created by the processes associated with maturing kerogen. Thus potential reservoir rocks may be found in or below laumontite-bearing sandstones. Laumontite in hydrocarbon environments would most likely have formed late relative to hydrocarbon maturation. The sedimentary basins of California may demonstrate such a late-stage, hydrothermal origin for laumontite. The concept of laumontite as an economic basement for hydrocarbon exploration must be carefully evaluated on a case-by-case basis.

Abstract The stratigraphic and lateral distributions of authigenic minerals in feldspar-rich Paleogene sandstones of the Santa Ynez Mountains, California, are important in determining their reservoir quality. The sandstones were deposited in an east-west elongate basin during two regressive episodes. Deep-water turbidites were overlain by shallow-water traction deposits and eventually by continental fluvial deposits as the basin was progressively filled from the east. Modal analyses document a common provenance for all the Paleogene sandstones consisting primarily of acidic to intermediate plutonic rocks, with minor volcanic, metamorphic, and sedimentary components. The average detrital mode of 27 sandstones is Q 37 F 54 L 9 , and the average partial mode including only the monocrystalline mineral grains is Qm 39 P 40 K 21 . Textural relationships and the stratigraphic distribution of diagenetic minerals delineate the paragenetic sequence: (1 ) syndepositional to very early pyrite; (2) early concretionary calcite cement; (3) incipient dissolution of detrital heavy minerals and feldspars; (4) clay pore linings and pore fillings; (5) formation of sphene and anatase; (6) incipient albitization of detrital plagioclase; (7) quartz, plagioclase, and K-feldspar overgrowths; (8) dissolution of feldspar creating secondary porosity; (9) local precipitation of pore-filling kaolinite; (10) laumontite cementation and replacement of plagioclase; (11) barite cementation and replacement of detrital grains; and (12) late-stage calcite replacement of detrital grains and earlier cements. Organic metamorphism, as expressed by vitrinite reflectance (R O ), provides a means to correlate mineral diagenesis in the sandstones with the thermal history of the Santa Ynez basin. In the eastern end of the basin (Wheeler Gorge) incipient albitization is first recognized at 0.5% R O corresponding to a paleotemperature of 110°C (4572 m burial depth), with complete albitization first occurring at a reflectance of 0.90% R O corresponding to a paleotemperature of 165°C (5425 m burial depth). The first occurrence of laumontite is in the turbidite beds of the basal Matilija Formation (5669 m burial depth) at approximately 1.0% R 0 reflectance (173°C). Further to the west, at Point Conception (Gerber No. 1 well), the first occurrence of laumontite is at an estimated burial depth of only 2515 m, corresponding to approximately 0.5% R 0 and a paleotemperature of 110°C. In this well, incipient albitization begins at 0.35% R O (77°C), with complete albitization occurring at roughly the same burial depth (2515 m) and reflectance (0.5% R O ) as the first occurrence of laumontite. The top of the laumontite zone occurs at greater burial depths and paleotemperatures in the eastern portion of the Santa Ynez basin than in the west. Laumontite distribution appears to be controlled by porefluid chemistry and post-compaction permeability variations, which are responsible for creating differences in fluid pressure between petrologically similar sandstones. “Dynamic” overpressuring may have occurred in the turbidite facies of the Juncal and lower Matilija Formations, whereby pore fluids enriched in Na + from the dewatering of smectite-rich shales permeated into the turbidite sandstones at a faster rate than they were expelled. Under these conditions, a continuous supply of Na + would have been delivered to the sandstones to allow albitization of calcium-bearing plagioclase, which in turn supplied Ca +2 necessary for the formation of laumontite. The authigenic minerals in the lower Paleogene sandstones of the Santa Ynez Mountains render them ineffective as reservoirs. Better reservoir prospects occur in the upper Paleogene and Neogene sandstones, particularly in the western part of the basin where they have not been subjected to deep burial, and secondary porosity is well developed.

Abstract Secondary porosity in sandstones is created by subsurface dissolution of grains or cement by pore water that is undersaturated with respect to one or more of the major mineral phases. Such undersaturated pore water may be derived from: (1) meteoric water driven by a hydrostatic head; (2) compactional pore water containing CO 2 released from maturing kerogen; (3) clay minerals reactions including the transformation of kaolinite and smectite to illite; and (4) reactions between clay minerals and carbonate releasing CO 2 . Calculations of the CO 2 generated from different types of kerogen suggest that few basins will generate enough CO 2 to produce large-scale leaching in thicker sandstones. Dissolution of minerals and removal of aluminum and silica in solution requires that very large volumes of pore water flow through the sandstone. Because leaching often enlarges primary pore space, it is very difficult to estimate the percentage of the pore space that is secondary. Leaching and formation of secondary pore space may also be accompanied by reprecipitation of other minerals so that the net gain in porosity is less than the observed secondary pore space.

Abstract Many of the major factors that control diagenesis, such as detrital composition, fluid composition, and fluid flux, can be related directly or indirectly to physical and biological processes operating at the time of deposition. Each depositional environment produces a lithofacies with a specific limited range of physical and compositional characteristics that affect diagenesis. The concept of diagenesis as a function of facies is well illustrated by the second Frontier sandstone of the Moxa Arch, Wyoming. The lower Frontier Formation on the Moxa Arch comprises sandstones and mudstones deposited in a delta/strand plain system on the western edge of the interior Cretaceous seaway. Depositional environments represented by the rocks include: marine shelf with sand ridges, marine shoreline, fluvial channels, and fluvial flood plain. Marine sandstones are significantly more quartzose than fluvial units because of sorting within the delta and wave abrasion on beaches. Substantial input of silica-rich water expelled from the underlying Aspen Shale caused nearly complete cementation of the cleaner beach and backshore sandstones by quartz overgrowths. Fluvial sandstones contain less quartz and more chert grains and rock fragments than the marine sandstones, and as a result, were less affected by quartz cementation. In addition, temporary filling of pores by calcite prevented further irreversibly destructive diagenesis. As a result, fluvial sandstones are better reservoirs, even though they are compositionally less mature. Clay-rich sandstones of the lower shoreface, lower sections of the sand ridges, and muddy fine-grained fluvial sandstones have poor present-day porosity and permeability primarily because of compaction. Fluid flux also appears to have played an important role in determining present-day porosity and permeability profiles. Because of a very low sandstone/shale ratio, fluid channeling in fluvial sandstones on the southern end of the Moxa Arch seems to have caused extreme leaching. The sandstone section here, although it is very thin, is very permeable. To the north, a higher sandstone/shale ratio appears to have permitted a lower fluid throughput per unit volume of sand. As a result, fluid channeling was not as severe, detrital and authigenic clays are more common, and the sandstone section is more homogeneous and of lower overall permeability.

Abstract Petrographic and geochemical data from cores in the Wyoming thrust belt are used to relate maturation and migration of Phosphoria Formation organic material to the evolution of porosity in the Tensleep and Madison Formations. Observed paragenetic sequences for each formation indicate that all three formations exhibit two distinct phases of diagenesis. These two phases are delineated chronologically by the simple designations of “early” and “late.” The period of early diagenesis was responsible for the pore network present in the Tensleep and Madison during migration of Phosphoria-sourced hydrocarbons. This early diagenetic phase exhibits a strong dependence on depositional environment, and is distinct for each formation. In contrast to this early diagenesis, the late diagenesis is consistent and similar for all three formations. This is interpreted to be the result of a relatively uniform sequence of fluids migrating through a pervasive Sevier fracture system that penetrated the entire 2000-ft thick stratigraphic interval of interest. The late diagenesis determined the present porosity configuration in these formations. The sequence of organic reactions related to the maturation and migration of Phosphoria organic material also influenced late diagenesis in all three formations. Generation of hydrocarbons was the first event in the late diagenetic period of each formation. Abundant organic material in the Phosphoria Formation entered the liquid window just prior to the emplacement of the Darby thrust during the Paleocene. Fracturing associated with thrusting during the Sevier orogeny provided conduits for the migration of Phosphoria hydrocarbons. Subsequent deep burial of the source and reservoir rocks beneath the Darby plate resulted in thermal degradation of the hydrocarbons, and liquid hydrocarbons were altered to solidified bitumen, methane, CO 2 , and H 2 S. Both the solidified bitumen as well as late dolomite cement severely damaged the early pore systems in the Tensleep and Madison. However, additional by-products of the thermal degradation process (CO 2 and H 2 S) dissolved remnant fossil fragments in the dolomite facies of the Madison to produce late, moldic porosity. The end result of this complex sequence of events is that at present the Phosphoria and Tensleep are tight and extremely well indurated, while the Madison possesses late moldic porosity from which to produce the methane reserves in the Tip Top field.

Abstract The Permo-Triassic Ivishak Formation is the main reservoir interval of the Prudhoe Bay field, North Slope, Alaska. Studies of cores from the field area reveal that porosity development within the Ivishak Formation has a complex relationship dependent on both depositional (lithofacies) and post-depositional (diagenetic) history. Four dominant lithofacies are identified: (1) interbedded very fine sandstones and mudstones; (2) parallel laminated carbonaceous fine sandstones; (3) multistory upward-fining medium sandstones; and (4) conglomerates. These lithofacies occur everywhere as upward-coarsening to conglomerate sequences. In the main field area the coarsening sequence is overlain by a gross upward-fining sequence of gravelly to medium-grained multistory sandstones, which thins dramatically to the north. Consideration of lithofacies and thickness variation leads to an interpretive model concerning evolution of the basin with respect to tectonics and sedimentation. Thus initial progradation of an active alluvial fan-delta system from the northeast was replaced by progressive transgression from the south of more distal upon proximal facies. Petrographic characteristics of the rocks reveal that porosity development is related to the diagenetic history of each lithofacies. Porosity within the medium-grained sandstones is predominantly secondary because of dissolution of grains and early grain-replacement calcite. Porosity within the conglomeratic intervals appears to be much more of a primary (textural) origin.