Phase Of Diagenesis Research Articles

Understanding pore characteristics through core-based petrographic and petrophysical analysis in a heterogeneous carbonate reservoir: A case study from the Mumbai Offshore Basin, India

Carbonate rocks exhibit complex and heterogeneous pore structures; such heterogeneity is manifested by the occurrence of a wide variety of pore types with different sizes and geometries as a result of depositional and diagenetic processes. These complications substantially increase the uncertainty of predicted rock hydraulic parameters because samples with comparable porosities might have very different permeability values. In this study, small-scale characterisation of porosity and permeability in heterogeneous Eocene limestone samples from the Bassein Formation of the B-X structure of the MK Field in Mumbai Offshore Basin, India, was carried out, employing an integrated framework that incorporates thin-section petrography, routine core analysis, mercury injection capillary pressure and nuclear magnetic resonance data. The pore characteristics of these carbonates range from poor to excellent. The studied samples exhibited large ranges of porosity, permeability and other associated petrophysical attributes. The pore types, as well as their orientations and connectivity, are the primary factors causing the heterogeneity. Because of the complexity of the pore networks, a simple lithofacies classification alone would have been insufficient to link porosity and permeability. The reservoir characteristics in the study area are strongly linked to the development and/or destruction of reservoir porosity–permeability during different phases of diagenesis. Twenty-four carbonate core samples from the limestone unit were studied and classified into microfacies and pore type classes, producing an accurate assessment of reservoir attributes. The comprehensive workflow incorporates the pore volume distributions and pore throat attributes for each rock type. Three carbonate microfacies were identified by petrographic analysis and their petrophysical characteristics, such as porosity, permeability, pore throat size, pore volume and fluid flow factors, were measured. The study demonstrates how macroporosity, mesoporosity and microporosity are associated with various rock types and how they affect permeability and cementation exponents. The results of this study provide a comprehensive experimental framework for geological and geophysical interpretation that can be applied to identify potential reservoir facies and strengthen our understanding of heterogeneous carbonates. The framework can also be used to guide reservoir evaluation of similar heterogeneous formations in other areas.

Regional variation of evaporite diagenesis in Xingouzui sandstones in Jianghan Basin, China: Implications for the evolution and prediction of saline reservoirs’ quality

Diagenetic processes and associated pore evolution are essential factors that control the development and distribution of porosity and permeability in saline reservoirs. The goal of this study is to systematically describe the comparative diagenetic processes, porosity evolution, and diagenetic facies of sandstone reservoirs in the Eocene Lower Xingouzui Formation (LXF) in two depressions in the Jianghan Basin, China. To achieve these goals, an integrated approach of petrographic, mineralogical, geochemical, and petrophysical analyses such as thin-section observations, X-ray diffraction (XRD), and mercury intrusion porosimetry (MIP) was applied to sandstone samples obtained from the Qianjiang and Jiangling sags. Diagenetic studies showed that the LXF samples had experienced several phases of diagenesis, including compaction, non-ferroan carbonate/sulfate cementation, feldspar and carbonate dissolution, quartz overgrowth cementation, ferroan carbonate and sulfate cementation, cubic pyrite precipitation, and limited gypsum dissolution. Although the LXF samples from both the Qianjiang and Jiangling sags experienced a noticeable porosity reduction induced by the compaction, the differences in diagenesis between these two sags were primarily due to complex and diverse cementation types. For instance, both early carbonate and sulfate cementation were recognized in Qianjiang samples, while only early carbonate cements were present in Jiangling samples. Based on a quantitative evaluation of the relative importance of diagenetic processes, diagenetic facies were defined in the LXF sandstones. Additionally, the pore evolution study indicated that two (upper and lower) zones of abnormally high porosity and permeability (AHPP) were developed in both sags. In particular, the upper AHPP zones occurred in sandstones in the Qianjiang and Jiangling depressions at depths of 1300 and 1800 m, respectively, while the lower AHPP zones in both depressions were located at a depth of 3300 m. The differences in the development mechanisms between the upper and lower AHPP zones were determined. This work provides an overview of the evolution and evaluation of sulfate- and carbonate-bearing sandstone reservoirs, such as the LXF sandstone, which have experienced strong diagenetic modifications during their geological histories.

Diagenetic evolution and associated dolomitization events in the middle Jurassic Samana Suk Formation, Lesser Himalayan Hill Ranges, NW Pakistan

The Jurassic carbonates of the Samana Suk Formation are extensively exposed in the foreland areas of Himalayas and form major reservoir of the upper Indus basin. These carbonates are composed of oolitic, pelitic fossiliferous and micritic limestone units which have been extensively modified by diagenetic alterations, particularly dolomitization. Field observations show two distinct types of dolostone geobodies (i) bedding parallel stratiform, and (ii) patchy dolostone units respectively. Bedding parallel stratiform dolostones are present in the basal part of the formation, while patchy dolostones are present at the middle and upper parts of the Samana Suk Formation. The dolomitization intensity of both geobodies increases from NW to SE in the study area. Petrographic studies reveal six phases of dolomites and three phases of calcites based on texture, crystal size and morphology. These phases are: matrix replacive dolomites (MD-I to MD-III); cementing dolomites include, replacive cementing dolomite (RD), saddle cementing dolomite (SD) and late stage cementing dolomites (CD); and calcite phases include CC-I and CC-II. XRD analyses reveal that stratiform matrix dolomites (MD-I, MD-II) are stoichiometric (51.08–51.86 mol percent of CaCO3) and contain up to 95% of the mineral dolomite. The patchy dolomite cement is non-stoichiometric (33.39–55.08 mol% of CaCO3) and contains around 65% of the mineral dolomite, whereas saddle dolomites is also non-stoichiometric (51.57 to 53.50 mol% of CaCO3) in origin. Stable isotope studies reveal non-depleted δ18O and δ13C values of matrix dolomites (MD-I, MD-II) represents coeval sea-water signatures of Jurassic carbonates, hence may have been formed by evaporative process. Dolomite cements (RD, SD) shows depleted δ18O values which represent elevated temperature, related to hydrothermal fluid source for their formation. The fracture filling calcite (CC-II) exhibits less depleted values indicative of meteoric fluids affected by shallow to moderate burial. The dedolomites shows depleted δ13C values suggests their formation from the meteoric water. Field, petrographic and geochemical studies suggest that diagenetic evolution of the Samana Suk Formation is the multistage process. In the first phase, marine diagenetic processes including marine cementation, stratiform dolomitization may have formed due to surface processes of marine water in peritidal to intertidal settings, while the second phase of diagenesis is due to burial associated processes which includes hydrothermal dolomitization occurred due to movement of magnesium rich fluids along weak planes such as fractures, faults, bedding planes and stylolites. Last stage includes formation of hydrothermal fracture filling calcites, replacive pyrites and dedolomites due to the uplift related processes.

Diagenesis and evolution of the lower Eocene red-bed sandstone reservoirs in the Dongying Depression, China

Abstract The diagenetic evolution of the lower Eocene red-bed high-quality reservoirs in the Dongying Depression was systematically investigated through an integrated petrographic, petrophysical, fluid evolution and thermal history analysis. The reservoirs experienced several phases of diagenesis, including compaction, carbonate cementation, gypsum and ankerite cementation, feldspar and carbonate dissolution, quartz overgrowth cementation, and clay-mineral cementation and transformation. An early alkaline diagenetic environment caused significant cementation of calcite and gypsum in the reservoir sandstone units near the sandstone-mudstone contacts. Acidic formation fluids caused widespread dissolution of the feldspar and carbonate cements, leading to synchronous occurrences of primary intergranular pores and acidic dissolution pores. From the middle parts of the reservoir sandstone units to the sandstone-mudstone contacts, dissolution wanes gradually with ferro-carbonate progressively diminishing. Migration of exogenetic thermal fluid along faults may have contributed to the gradual decrease in dissolution and the gradual increase in cementation from the lower part to the upper part of fault blocks. The alternating alkaline and acidic diagenetic environments and the distribution of diagenetic products caused the development of the high-quality red-bed reservoirs in the middle part of the thick-layered sandstone units in the lower part of fault blocks. Because of the spatial and temporal relationships between the diagenetic stages and their products, lithological traps of a diagenetic origin were developed in the red-bed reservoirs.

Heterogeneous diagenetic patterns in the Pleistocene Ironshore Formation of Grand Cayman, British West Indies

The Ironshore Formation on Grand Cayman consists of six unconformity-bounded units (A to F) that developed in response to repeated transgressive–regressive cycles during the Middle to Late Pleistocene. Corals and matrices in limestones from the Rogers Wreck Point (RWP), offshore George Town (GT), and western onshore (WO) areas are characterized by complex diagenetic fabrics that reflect marine (bioerosion, micrite envelopes, internal sediments, fibrous high-Mg calcite, acicular aragonite, isopachous prismatic calcite cements), freshwater phreatic (circumgranular cements, even/random blocky calcite cements), and vadose (meniscus calcite cements, blocky calcite cements) diagenesis. Throughout these limestones, the matrices have undergone more meteoric diagenetic alteration than the corals. Overall, however, no systematic stratigraphic patterns exist to the distribution of these diagenetic fabrics and it is generally impossible to link the different phases of diagenesis with specific unconformities that cap each unit. These heterogenous patterns of diagenetic features can be attributed to many intrinsic and extrinsic factors, including the original compositions of the different components in the limestones, the porosity and permeability of the substrate, the nature of the diagenetic fluids, climate, and the duration of exposure during each lowstand. Integration of available data, however, indicates that maximum diagenesis took place during the lowstands denoted by the unconformities at the top of Unit C (Marine Isotope Stage 7) and Unit D (Marine Isotope Stage 5e) when long periods of exposure were accompanied by wet climates with high annual rainfalls.

Diagenetic history of tight sandstones and gas entrapment in the Yulin Gas Field in the central area of the Ordos Basin, China

The Ordos Basin is a large cratonic basin with an area of 250,000 km 2 in central China. Upper Paleozoic coals and shales serve as gas source rocks with peak generation and migration at the end of the early Cretaceous. Recent exploration has verified the huge gas potential in the “basin-centered gas accumulation system” (BCGAS). However, the mechanism for the gas accumulation is controversial. With an integrated approach of thin-section petrography, ultra-violet fluorescence microscopy, fluid inclusion microthermometry, Raman microspectrometry, scanning electron microscopy, and X-ray diffractometry, we identified diagenetic trapping and evaluated the diagenetic history of sandstone reservoirs in the Yulin Gas Field in the central area, where structural, stratigraphic and/or sedimentary lithologic traps have not been found. It was revealed that three phases of diagenesis and hydrocarbon charging occurred, respectively, in the late Triassic, late Jurassic and at the end of the early Cretaceous. In the first two phases, acidic water entered the reservoir and caused dissolution and cementation, resulting in porosity increase. However, further subsidence and diagenesis, including compaction and cementation, markedly reduced the pore space. At the end of the early Cretaceous, the bulk of the gas migrated into the tight reservoirs, and the BCGAS trap was formed. In the updip portion of this system, cementation continued to occur due to low gas saturation and has provided effective seals to retain gas for a longer period of time than water block in the BCGAS. The mechanism for the gas entrapment was changed from water block by capillary pressure in the BCGAS to diagenetic sealing. The diagenetic seals in the updip portion of the sand body were formed after gas charging, which indicates that there is a large hydrocarbon exploration potential at the basin-centered area.

From deep basin gas to diagenetic trap—An example from the Yulin Gas Field in the Ordos Basin

In the updip portion of the Yulin Gas Field in the Ordos Basin, there are no any structural, stratigraphic and sedimentary lithologic seals. Using thin-section petrography, ultraviolet fluorescence microscopy, micro-thermometry and Raman microspectrometry, this paper finds out the diagenetic trap and studies the diagenetic history of this field. It was revealed that three phases of diagenesis and hydrocarbon charging happened in late Triassic, late Jurassic and the end of early Cretaceous respectively. In the first two phases, acid geofluid entered the reservoir and caused dissolution and cementation. Although the porosities had been increased, further compaction accompanying re-subsidence resulted in tight sandstone and conglomerates. Till the end of the early Cretaceous, bulk of gas migrated into the tight reservoir. Cementation, however, kept on in the updip portion of this field due to low gas saturation and formed the diagenetic trap. The mechanism for gas accumulation was changed from deep basin gas to diagenetic trap, which offers a sealing condition that can retain gas for much longer time.

Alkenones as paleoceanographic proxies

Alkenone‐derived estimates of temperature and CO2 are contributing to our understanding of Earth's climate history. In order to increase confidence in alkenone‐based climate proxies we recommend the following actions. First, the sedimentary component or fraction containing alkenones should be identified in order to assess the impact of horizontal advection and vertical mixing on alkenone‐derived temperature and CO2 estimates. Second, differential mixing rates of alkenone‐containing particles and sand‐sized foraminifera should be quantified by independent dating of the two phases. Until that is accomplished, apparent temporal offsets of climate proxies in the two phases should be interpreted cautiously. Third, the stability of the unsaturation ratio and carbon isotopic compositions of alkenones during all phases of diagenesis should be confirmed. Both field and laboratory observations are required. Fourth, future alkenone investigations should be coupled with other paleoclimate proxy measurements at high‐deposition‐rate‐sites in a variety of oceanographic settings. In upwelling regions and in the vicinity of river plumes, salinity and nutrient proxies should be measured since changes in these parameters may affect alkenone biosynthesis.

Diagenetic History of Givetian Winnipegosis Formation in Southeastern Saskatchewan: ABSTRACT

A paragenetic sequence, established for the Winnipegosis Formation in the Tableland area of southeastern Saskatchewan, indicates that the main phases of diagenesis occurred in early marine and hypersaline fluids and late pressure-solution fluids. Initially, marine and possibly meteoric fluids precipitated several generations of fibrous-isopachous and equant-mosaic cements into primary porosity. Prior to extensive compaction and lithification of the sediments, increasingly hypersaline pore fluids resulted in the formation of CaSO/sub 4/ nodules and laminations and subsequently, dolomitization. The examination of incompletely dolomitized locations indicates that dolomitizing fluids came from above since dolostones overlie limestones. The dolostones are associated with common-to-abundant, early CaSO/sub 4/ precipitation. In addition, dolomitization and lithification of the sediments must have occurred prior to compaction since early CaSO/sub 4/ laminations, isolated in mudstones, are unbroken. Secondary porosity creation accompanied and postdated early dolomitization. Leaching was related to variations in the solubility of the substrate and, possibly, freshening of the pore fluids. Locally, leaching was accompanied by partial replacement of earlier dolomites by poorly consolidated very finely crystalline sucrosic dolomites. Pressure solutioning led to the precipitation of zoned overgrowth cements into secondary and remnant primary porosity. The zoned cements are most abundant in lithologies rich in stylolites, and the more » cement composition varies with the substrate composition (i.e., calcite in limestones and dolomite in dolostones). Pressure solution also caused local obliterative recrystallization and, possibly, a minor phase of cannibalization-dolomitization. « less

Diagenesis in Upper Jurassic marine sandstones from the North Sea Well 14/26-1 and its significance

AbstractA tightly cemented series of marine sandstones, interbedded with shales and mudstones, occurs in the lower part of the Kimmeridge Clay Formation (Kimmeridgian to Ryazanian) of Arco Well 14/26-1 (Core No. 5, 8067–8085 ft). The well is on the northern flank of the Fraserburgh Spur Basement Ridge. The sediments are in the immature stage of hydrocarbon generation and are now at their maximum temperature and burial depth: the bottom hole temperature is 75°C. The sandstone diagenesis was complex, essentially of an intrinsic type, and took place under considerable overpressures. Initially a series of early cements was precipitated (minor chalcedony, quartz, feldspar, ferroan calcite, non-ferroan dolomite and major ferroan dolomite). A later phase of diagenesis embraced two periods of intrastratal solution (each affecting both the silicate and the carbonate components of the sediment) separated by a phase of calcite precipitation and followed by a phase of kaolinite cementation. The early cements are interpreted as being of the intrinsic miagenetic type. The later phase of diagenesis (alternating intrastratal solution and cement precipitation) resulted from the interaction of (i) the biotic breakdown of organic matter by thermophyllic micro-organisms and (ii) the abiotic thermal alteration of organic matter with the mineral components of the sediment: of particular importance were varying PCO2 and the generation of carboxylic and phenolic acids. The diagenetic pattern is closely comparable to that known from various Upper Jurassic sandy shelf sediments in other parts of the northern North Sea which have very different burial histories.

HYDROCARBON OCCURRENCE AND DIAGENETIC HISTORY WITHIN PROTEROZOIC SEDIMENTS, McARTHUR RIVER AREA, NORTHERN TERRITORY, AUSTRALIA

The stratigraphy of the Proterozoic in the McArthur River area of Northern Territory consists of the basal, non-economic Tawallah Group, overlain unconformably by dolomitic carbonates and clastics of the McArthur Group, in turn overlain disconformably by Roper Group clastics. Several shows of tarry to brittle bitumen have been reported in sandstones of the Roper Group and in dolomites of the McArthur Group.In thin sections, the bitumen commonly displays shrinkage cracks, apparently associated with the loss of volatiles. Secondary minerals are observed infilling some of the cracks, indicating those phases of diagenesis which occurred subsequent to breaching of the hydrocarbon bearing reservoir. Additionally, the contact relationships of bitumen with the secondary minerals indicate a relatively early migration of hydrocarbons into the reservoir rocks.The inferred sequence for the McArthur Group dolomites is: early dolomitization and silicification, formation of vuggy (vadose) porosity, authigenic deposition of chalcedony at shallow burial depth, cementation by quartz at deep burial depth, migration of hydrocarbons (contemporaneous with sulphide formation), breaching of the reservoir, degradation of hydrocarbons, and deposition of sparry dolomite cement. The inferred sequence of diagenesis for Roper Group clastic reservoirs in this area is: authigenic deposition of minor quartz and illite cement, migration of hydrocarbon, breaching of the reservoir, major authigenic deposition of quartz and illite, degradation of hydrocarbon, and cementation by dolomite, hematite, and kaolinite.

Carbonate Pore Systems: Porosity/Permeability Relationships and Geologic Analysis: ABSTRACT

The porosity/permeability relationships of the common carbonate rock types have been studied, with emphasis on the variety of pore types in upward-shoaling grainstone sequences including: Smackover, Lansing, Salem, and San Andres Formations. A result of these studies is an improved conceptual understanding of permeability gained from the cross-plotting of porosity and permeability data from plugs and whole cores accompanied by textural and fabric analyses of rock samples, thin-sections, serial sections, and pore casts. Importantly, many depositional rock types cross-plot as distinctly different populations, commonly yielding linear trends on semi-log paper. These trends indicate a degree of order in the seemingly chaotic pore systems of carbonate rocks which have undergon cementation and/or compaction. For grainstone samples, there appears to be a change in the slope of this trend between the compaction and cementation phases of diagenesis. Once the depositional texture and fabric of the rock are defined in terms of porosity and permeability, the evaluation of fractures and secondary porosity can be addressed. The secondary porosity is observed to be as high as 14% of the rock volume in the Smackover example and 21% of an oolitic sample from the Lansing Formation. Pore casts and serial sections reveal that the grain-moldic porosity is poorly connected to the intragranular pore system and contributes little to the permeability of the rock. This insight allows quantitative estimates of this type of secondary End_Page 513------------------------------ porosity using standard porosity and permeability data. The geologic and quantitative analysis of the various pore types and porosity/permeability relationships also aided in the interpretation of the log data from the reservoirs studied. End_of_Article - Last_Page 514------------

Diagenetic Sequence Related to Structural History and Petroleum Accumulation: Spindle Field, Colorado

Two Upper Cretaceous marine sandstones have produced 36 million bbl of oil and 164 bcf of gas at Spindle field in the Denver basin of Colorado. The sandstones are offshore bars enclosed in finer grained rocks of the middle Pierre Shale. They were deposited in response to western interior sea-level fluctuations that affected water depths, energy levels, and sediment distribution across structurally controlled sea-floor topography during several intervals of Cretaceous sedimentation. Some of the structures that influenced Pierre deposition are believed to have controlled later emplacement of oil and gas. Preliminary petrographic analysis of the sandstones indicates that four phases of diagenesis have occurred. Early diagenetic events of compaction and formation of authigenic chlorite rims, silica overgrowths, and calcite cement within the sandstones (phases I and II) largely destroyed initially good primary porosity and permeability in these well-sorted sandstones. These early events approximately coincide with the energy-of-formation (^Dgr G°f) related paragenetic sequences in fine to medium-grained, well-sorted marine rocks as reported by other workers. However, later diagenetic events (phases III and IV) clearly record separate phases with no sequential relationship to earlier events. Later diagenetic events of calcite solution to form secondary porosity and partial occlusion of the secondary porosity by authigenic kaolinite and minor illite reflect a change in pore-fluid chemistry believed to be related to vertical migration of acidic fluids from below. Acid water, resulting from dissociation of the CO2 gas produced during degradation of organic matter in the source rocks, is considered a likely solvent for the calcite. Secondary porosity has created the reservoir at Spindle field; elsewhere, thick, originally well-sorted sandstones are tight. Both the acidic fluids and the oil and gas are postulated to have migrated vertically, under pressure related to petroleum generation, into the reservoir along tensional faults and fractures related to a paleostru tural element at depth. Spindle field hydrocarbons represent a combination of stratigraphic and diagenetic trapping. Diagenetic sequences record aspects of the total burial history of a rock that may be combined with other geologic information to give a better understanding of the total burial history. Predictability of timing and mechanism of hydrocarbon emplacement may be enhanced by relating diagenetic sequences to the structural features and history of an area. The diagenetic sequence observed in the sandstones at Spindle field is recognized as a response to structural influence on fluid migration.

Evaporitic Environments and Their Relationship to Porosity of Associated Carbonates in Williston Basin (Mississippian): ABSTRACT

Evaporite morphologies, indicative of specific environments of deposition, have been identified in late Cenozoic sediments, and are now recognized in the Williston basin (Mississippian, Little Knife field). The evaporitic environments which are represented include the supratidal (sabkha), the intertidal, and the shallow subaqueous. The development of the sabkha facies exerts a major control on porosity production in associated marine carbonates. Those evaporites forming subaqueously in related lagoons and other water bodies may occlude porosity within similar carbonate sediments. However, subaerial and subaqueous evaporites are now seen in the form of massive to nodular anhydrite and are usually classified together (in cores and well logs), but in fact they contain relic orphologies that permit more precise definition and separation of original facies. Subsequent porosity occlusion and/or creation may also be affected by later deformation of the regional structure and its effect on fluid migration. The recognition of the various evaporitic morphologies leads to a new understanding of porosity development in sediments of varied origins and may aid in distinguishing early from late phases of diagenesis. End_of_Article - Last_Page 626------------

Exogenic uranium mineralization in the sediments of the molassoid associations in intermontane basins

A study of uranium deposits associated with Cenozoic molassoid associations within one of the median massifs of the Alpine geosynclinal belt is presented. It was found that the early diagenetic, predominantly lean uranium mineralization is confined to the upper subaqueous parts of the sedimentary rhythms, consisting of siltstones and silt-pelites of the stagnant facies. Redistribution of uranium, with the formation of micro-concentrations and uranium-bearing carbonate nodules, probably took place under the influence of buried sedimentation waters. The concentrations of uranium have also been redistributed by the ground waters. Within the intermontane basins, there was a constant flow of water through the marginal conglomerates into all the water-bearing horizons of the thick aquifer complex. This ensured the inflow and redistribution of uranium during all the phases of the sediment and rock. In particular, during the phase of diagenesis, more complex and richer orebodies evidently formed in the zones where the limonitization thins out. The principal source of granite during the period of humid environment was decomposing granites. During periods of arid climate, the primary concentrations of uranium developed from the precipitation of a high evaporative concentration from the waters. (JMT)

Dolomitization and the mineralization of the Marl Slate (N. E. England)

The Marl Slate, the English equivalent of the Kupferschiefer, has been studied with particular reference to the relationships between dolomitization and the origin of the metal sulphides. Dolomite occurs as: 1) discontinuous lenses of ferroan dolomicrite, 2) micronodules of finely crystalline dolospar in association with length-slow chalcedony and 3) discrete laminae of ferroan or non-ferroan dolospar. The ferroan dolomicrite has excess CaCO3, and is more abundant in the lower, sapropelic facies of the Marl Slate. It is considered to have formed by the penecontemporaneous alteration of calcium carbonate under hypersaline conditions. Small micronodules (typically about 0.3 mm in diameter) are also more abundant in the sapropelic Marl Slate. These frequently contain cores of length-slow chalcedony (quartzine) fibres and sometimes quartz megacrysts. Textural observations clearly indicate that this silica is of authigenic origin and the dolomite/chalcedony micronodules are interpreted as diagenetic replacements of a calcium sulphate mineral such as anhydrite. The discrete laminae of finely crystalline dolospar are often inter-laminated with calcite in the upper part of the Marl Slate. This dolomite is also calcium rich and represents a replacement, possibly of anhydrite, during a later phase of diagenesis. Metal sulphides occur in two distinct forms: as disseminated framboidal pyrite and as discrete lenses of pyrite, chalcopyrite, galena, sphalerite and rarer sulphides. The framboidal pyrite originated during early diagenesis by reaction of sulphide, produced by reduction of sulphate by organic material and micro-organisms, with iron also released in the reducing environment. The sulphide lenses are often in intimate association with dolospar, length-slow chalcedony and authigenic quartz megacrysts. This indicates that the lenses were produced during diagenesis by the reduction and replacement of calcium sulphate (anhydrite). Various sources, such as co-precipitation with dolomite precursors and the underlying Yellow Sands, may have supplied metals which were mobilized and transported by connate brines as diagenesis progressed.

Contrasts in Cementation, Dissolution, and Porosity Development Between Two Early Cretaceous Reefs of Texas: ABSTRACT

The influence of cementation and dissolution on porosity is investigated by comparing two reefs with significantly different diagenetic histories. Reefs of the subsurface Sligo Formation in South Texas are buried to depths of 15,000 to 20,000 ft in a narrow belt along a shelf edge of regional extent. Cores from five wells distributed along 225 miles of the shelf edge show that porosity in the reef and backreef facies is persistently occluded by (1) radiaxial fibrous calcite, a cement whose origin appears to be related to the replacement of a synsedimentary marine cement, and/or (2) coarse calcite mosaic, a cement introduced later in the diagenetic sequence during a time of basin subsidence. Calcite cements of this type and sequence have also been reported to be detrimental to porosity in other ancient shelf-margin reefs. Therefore, the creation of porosity in shelf-margin reefs may depend on processes (dolomitization, fracturing, dissolution) which can offset the influence of cementation. In contrast to the Sligo, large patch reefs in the outcropping lower Glen Rose Formation in south-central Texas record a simpler diagenetic history which preserved porosity. One reef, selected for detailed study, has experienced only one phase of diagenesis—namely, early subaerial exposure which created moldic porosity and precipitated a single generation of calcite cement. After this exposure, no additional cements were introduced, despite burial beneath a few thousand feet of younger Cretaceous marine sediments and reexposure of the reef to subaerial weathering from Miocene to Recent time. Thus, the key to understanding porosity preservation in theGlen Rose reef lies in the role of paleohydrologic and geochemical conditions during burial and reexposure of the reef. Whether this can be predicted is a matter for future research. 'Atlantic Richfield Company, Dallas, Texas LOWER CRETACEOUS STRATIGRAPHIC MODELS FROM TEXAS AND MEXICO T. A. Bay, Jr

Transformation of sapropelic disseminated organic matter during the phase of diagenesis of sediments

Transformation of sapropelic disseminated organic matter during the phase of diagenesis of sediments